Mechanisms of Plant Responses to Biotic Stresses

#### **Chapter 11**

## Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield under Stress Environments

*Partha S. Basu, Sushil Kumar Chaturvedi, Pooran Mall Gaur, Biswajit Mondal, Surendra Kumar Meena, Krishnashis Das, Vaibhav Kumar, Kalpana Tewari and Kusum Sharma*

#### **Abstract**

Reduction in biomass and pollen fertility are the two major constraints resulting in poor grain yield in major pulses grown under rainfed agrosystem. Generally, pulses are encountered into both heat and drought stresses during terminal reproductive stages. Though pulses have many adaptive features to counter the adverse effects of various abiotic stresses but yield is substantially reduced when the magnitude of these stresses is very high. The factors have been identified to enhance grain yield under stress environments which include promotion of biomass in the above ground part enabling crops to reserve a maximum amount of photosynthesis and water in the plant system itself before the onset of drought and heat stresses during reproductive stages. Various physiological mechanisms and fertility enhancement components including genetic diversity in key traits have been discussed here to improve yield of pulses under stressed conditions.

**Keywords:** pollen fertility, root, antioxidant, osmotic adjustment, photosynthesis, remobilization, chlorophyll fluorescence, pod

#### **1. Introduction**

The earth's climate system is experiencing a warmer phase. Increase in temperature and atmospheric CO2 concentration are the major effects of climate, besides increase or decrease in the local rainfall. The climate modeling systems envisage that as the twenty-first century progresses, there will be rise in temperature from 2 to 6°C. Rainfed agriculture accounts for more than 40% of total food grain production, and nearly 90% of it is occupied by cultivation of pulses. Thus, country like India which is a major producer and consumer of pulses, there are about 100 million hectare cultivated areas dependent on rainfall. In general, crop productivity is adversely affected by high temperature and drought [1]. The rise in global mean temperature

and drought has affected agricultural productivity worldwide [2]. According to IPCC (2013), decreasing water availability and increasing temperature are posing a great threat to food security. Therefore, it is an urgent need to identify tolerant plant species for these stresses [3]. The pulses or food legumes are rainfed crops grown under diverse soil types and agro-ecosystems in low rainfall areas of semi-arid regions including India, Pakistan, Bangladesh, Myanmar, and Nepal. The pulses find an extremely important place in the agricultural system as they require less chemical fertilizers and limited moisture. The legumes or pulse crops include chick pea (*Cicer arietinum* L.), pigeon pea (*Cajanus cajan* L.), green gram (*Vigna radiata* L.), black gram (*Vigna mun*go L.), lentil (*Lens culinaris* L.), as major pulse crops (**Figure 1**) while field pea (*Pisum sativum* L.), kidney bean or rajmash (*Phaseolus vulgaris* L.), lathyrus, cowpea, horsegram, etc. are considered as minor pulses because of their lesser consumption by people. These pulses are broadly categorized into cool-season legumes constituting chick pea, lentil, field pea, rajmash, and lathyrus while warmseason legumes include pigeon pea, green gram, black gram, cowpea, horsegram, etc. The pulse seeds have high nutritive values with a low glycemic index comprising of approximately 18–30% protein which varies among legume species. The pulses also supplement essential amino acids, resistant starches, complex oligosaccharides, fibers, minerals, vitamins, phenolics, tannins, phytic acids, antioxidants, and folic acids which have enormous health benefits [4]. Additionally, pulses improve soil health by contributing soil nitrogen through fixing atmospheric nitrogen by a symbiotic association of N2 fixing *Rhizobium* in their root nodules and host a number

#### **Figure 1.**

*Major pulses of cool season (A–F). Chick pea leaf with pods (A), chickpea seeds (B); lentil leaf with pods (C), lentil seeds (D); field pea leaf bearing pods (E), field pea seeds (F): major pulses of summer season (G–L). Green gram leaf with podding cluster (G), green gram seeds (H); black gram leaf with podding clusters (I), black gram seeds (J); pigeon pea leaf with pods (K), pigeon pea seeds (L).*

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

of beneficial microbes in their rhizosphere such as phosphate solubilizing bacteria (PSB) that solubilize insoluble phosphates in the soil. They are hardy crops thriving well with their vigorous root system and have strong ability to proliferate root system into the deep soil layers. Keeping in view of the growing demand, additional lands are required for expanding the areas of cultivation of pulses. The productivity of pulses had been always low as they are threatened by great challenges of climatic aberrations such as drought, heat, salinity, frost, and cold. Among several abiotic stresses, drought and heat are considered to be the major yield limiting factor followed by unprecedented high temperature during the reproductive phase [5, 6]. High temperature stress affects various physiological processes and alters the plant-water relationship [7]. The rise in temperature beyond 35°C may cause irreversible damage to growth and development of plants [8]. It is therefore imperative to analyze critically the implications of climate change on pulses. There are several knowledge gaps in our understanding toward low productivity of pulses particularly when drought and heat are combinedly imposed on crops [9]. Efforts are needed to make pulses more climate-resilient and productive [10]. Our current understanding is not sufficient to counter the challenges imposed by climate adversities [11]. Therefore, systematic approaches are to be taken to hit the primary targets of achieving higher productivity with limited resources and sustain our ecosystem, soil health, protecting environment keeping in view of the fact that pulses play a central role in sustaining our agro-ecosystem under the climate change scenario.

#### **2. Effects of major abiotic stresses on pulses**

#### **2.1 Drought**

Yield reduction in pulses has been directly correlated with the intensity of drought [5]. Among rainfed pulses, *Vigna* species green gram and black gram being warm season crops require four irrigations till crop maturity. Sources of drought tolerance in *Vigna* crops are very rare, though green gram is relatively better adapted to warm climates. The highest yield reduction was observed during the reproductive stage of many pulses. However, yield reduction in pigeon pea under drought was relatively lower than that of lentils, groundnut, cowpea, and green gram. In extra-short-duration pigeon pea, the impact of drought stress is evident at the vegetative, flowering, and pod-filling stages [12]. The germination and seedling growth, seedling vigor, hypocotyl length is adversely affected by drought stress in pigeon pea. Tolerance to drought in short-duration pigeon pea has been ascribed to the crop's ability to maintain total dry matter, a small pod size, few seeds in the pod, high seed mass, and low flowering synchronization [13]. The important mechanisms of drought tolerance in pigeon pea were included with high root resistance to water flow; slow shoot development; limited initial root development at depth; partitioning of assimilates into vegetative parts; leaflet movement during water stress; dehydration tolerance; and osmotic adjustment [14]. The lethal leaf water potential, i.e. the lowest water potential experienced by the last viable leaf, was a key measure of dehydration tolerance. The pigeon pea has more dehydration tolerance than others. The ability of cells to continue metabolism at low leaf water status is termed dehydration tolerance [15]. Membrane disorder is often measured as a leakage of solutes from the cell [16]. Water status parameters like relative water content may be a good indicator of drought tolerance in pigeon pea under semi-arid conditions [17]. Accumulation of proline in

cell in response to water deficit is another mechanism protecting protein structures as cell dehydrate, and as an organic nitrogen source. The relative drought tolerance in a range of pulses is based upon the physiological traits such as osmotic adjustment, root system, and lethal leaf water potential. Thus, the order of drought tolerance in different pulses could be Pigeon pea > chick pea > lentil > black gram > green gram.

#### **2.2 High temperature**

High temperature results in an overall reduction in plant growth including roots, leaf area, and dry weight [18]. It has been predicted that an average 1°C increase will reduce yield by at least 3–4% in many crops [19]. The average surface temperatures are expected to rise by 3–5°C by the end of this century, posing a major threat to the production of many food crops including legumes worldwide, especially in the semiarid tropics [20]. Moreover, increase in temperature will have more adverse effects on cool-season pulse crops (e.g. chick pea, lentil, and field pea) than the summer and rainy-season crops like green gram, black gram, and pigeon pea [21]. A temperature increase of 1–2°C above the threshold level is sufficient to reduce yield in many leguminous crops such as cowpea [22], groundnut [23], common bean [24], lentil [25], and chick pea [26, 27]. Heat stress causes considerable reduction in biomass production and grain yield in several crops [28]. A rise in temperature may limit the development of various yield components [29]. The majority of the food legumes including chick pea, lentil, pigeon pea, green gram, and black gram are grown by developing countries of south Asia under rainfed situation. They are often heavily influenced by climate variables. Crops grown at lower latitudes are often exposed to high temperatures above 40°C. Grain yield reduction in heat stress has been reported to be associated with a decrease in photosynthetic capacity because of altered membrane stability [30, 31] and enhanced maintenance respiration [32] along with a reduction in radiation-use efficiency. Photosynthesis is the most sensitive physiological process impaired by heat stress [33] which could be due to structural and functional disruptions of chloroplasts, reduction of chlorophyll, inactivation of chloroplast enzymes [34], or both stomatal and nonstomatal limitations [8]. High temperatures adversely affect starch and sucrose synthesis through a reduction in the activity of sucrose phosphate synthase and ADP-glucose pyrophosphorylase [35]. Crops exposed to high temperature are often subjected to oxidative stress-producing reactive oxygen species (ROS), which are highly toxic to cellular functions in plants because they damage nucleic acids and cause protein oxidation and lipid peroxidation; this oxidative damage eventually causes cell death [36, 37]. ROS toxicity during various stresses is considered to be one of the major causes of low crop productivity worldwide [38]. An increase in the activity of antioxidant enzymes, such as guaiacol peroxidase (GPX) and catalase (CAT), plays a significant role in minimizing the toxic effects of stressinduced ROS production [39]. High temperatures adversely affect nitrogen fixation in chick pea [40]. Increased day temperatures ranging from 32 to 35°C have shown reduction in nodule formation and nitrogen fixation in chick pea (**Figure 2**). It has been reported that high temperature delays nodulation, retards nitrogen fixation, and impairs nodule function and structure in chick pea [41–43].

#### *2.2.1 Effect of high temperature on reproductive and seed development in pulses*

The reproductive phase of major pulses is highly sensitive to temperature extremities [44]. High temperature inhibits flower set and flower retention, impair the

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

**Figure 2.**

*Effect of high temperature on chick pea cv K 850 nodulation. (A) Normal sown chickpea nodule (25/15°C max/min), (B) late sown chick pea nodule (40/27°C max/min).*

normal development of male and female gametophytes leading to ovule abortion, reduce pod settings, and impair grain filling which eventually results in significant yield loss. Several reports have indicated that heat stress caused reproductive failure due to impaired sucrose metabolism in the leaves, developing grains, and inhibition of sucrose transporters those results in decreased carbon-source to the anthers and developing pollen grains [45]. The heat stress results in drastic yield losses due to the decline of relative tissue water content (RWC) and leaf water potential (LWP) [46] pollen or ovule inactivity, flower abortion, and postfertilization impaired the growth and development of embryos or seeds in many pulses [47]. Ultrastructural studies revealed that pollen sterility could be due to degenerated tapetum owing to heat stress [48]. Temperature extremities have specifically detrimental effects on male gametophyte causing disrupted meiosis, tapetal hypertrophy, stunted development of pollen grains, anther protein degradation, pollen sterility, and pollen tube deformation [49] while heat stress adversely affects female gametophyte causing reduced size of style and ovary, disrupted meiosis, reduced stigma receptivity, callose deposits in style, damaged embryo sac components, and fertilization arrest. Flowering is either early or delayed, flowers abscised and become distorted and shedding occurs. Grain-filling process is impaired due to altered source-sink relations which lead to seed abortion and yield loss. However, the relative heat sensitivity varies for different crops [50]. High temperatures reduce yield and yield attributes such as dry matter accumulation and partitioning [51], pod set, pod weight, and harvest index in snap beans [52]. Temperatures above 40°C resulted in reduced pod set, seed production, and yield in soybean [53]. The water scarcity in floral parts and leaves due to high temperature driven increased transpiration causes heavy yield losses in snap bean (*P. vulgaris*) [54]. High temperature also adversely affects pollen germination and pollen tube growth. In cool-season legume lentils, pollen germination and pollen tube growth have been found to be retarded above 35°C [55]. Stressful temperatures often lead to impaired microsporogenesis and megasporogenesis at the pre-fertilization stage in various legumes such as chick pea (*C. arietinum*) [56]; and *P. vulgaris* [48, 57]. Loss of pollen viability and pollen germination have been reported at high temperatures in *C. arietinum* [58]; and *P. vulgaris* [57]; *Arachis hypogea* [59], loss of stigma receptivity, loss of ovule viability and flower abscission in *C. arietinum* [58]; *P. vulgaris*: [48]. Most of the pulse crops such as chick pea and lentil are sensitive to heat stress when

day temperature exceeds above 35°C resulting in reduced pollen germination, and decreased pod number and seed size. Arrest of fertilization and reduced embryogenesis have been reported at extreme temperatures in chick pea [60] and *Glycine max* [61]. The pigeon pea often experiences high temperature during the reproductive phase which results in pollen sterility, retards germination and pollen load on stigma, reducing the pollen receptivity on stigma and length of pollen tubes [58]. Consequently, heat stress has detrimental effects on both microsporogenesis and megasporogenesis causing incomplete cell division and eventually loss of viability of pollens and ovules [45]. Failure of anthesis would be a likely cause of flower abscission and also discontinuity of sustained photoassimilate mobilization to the developing grains results in pod abortion [62]. Identifying mechanisms of reproductive temperature tolerance in various legumes could be achieved through screening diversified germplasm available in gene banks. Warm-season legumes such as green gram, pigeon pea, black gram, and cowpea, though having higher temperature tolerance limits, are also affected negatively leading to a reduction in pod set in response to moderately-high night temperatures [63]. Thus, various legumes are sensitive to temperature extremities to different degrees from the vegetative to reproductive stage, resulting in metabolic and reproductive dysfunction and finally result in low yields.

#### **2.3 Effect of combined stresses of drought and heat**

There is a need to identify the tolerance mechanism of pulses in response to simultaneously occurring heat and drought stresses [64]. It is considered that the demand of water will proportionately increase with rise in temperature due to increase in soil-plant evapotranspiration water loss. To improve pulses for dual tolerance, traits that confer the tolerance to both heat and drought must be studied. The photosynthetic electron transport rate has been drastically declined when drought and heat superimposed simultaneously in chick pea. A high irradiance level had been found to be more deleterious for photosynthesis in chick pea under combined drought and heat stresses; however, genotypic variation was evident in light response of photosynthesis when different chick pea genotypes were subjected to combined stress (**Figure 3**). Similarly, it is urgently needed to inherit combined tolerance in pulses also. When both high temperature and drought stresses are superimposed, their combined effects are more detrimental and negative. The combined effects could be deleterious additive effects on phenology, growth, chlorophyll content, photosynthesis, grain number, fertility, grain-filling duration, and grain yield in several crops [65]. For example, the combined drought and heat stresses decrease leaf chlorophyll content by 49% while drought or heat alone reduce it by 9% or 27%, respectively [66]. Reproductive stages are more sensitive to combined heat and drought stresses than individual ones [1]. However, in some ways, drought and heat stresses are interlinked. Both the stresses significantly reduce photosynthetic efficiency, stomatal conductance, leaf area, and water-use efficiency in many crops, e.g. wheat, rice, chick pea [67]. Heat stress increases evapotranspiration from soil and plant canopy that leads to a disruption of water relations and plants often are succumbed to drought stress driven by high temperatures [68]. Crops may respond to drought and heat stresses individually or combinedly in a different manner [69] which eventually lead to yield reduction. Pigeon pea, chick pea, lentil, and field pea are often exposed to drought and high temperatures simultaneously particularly during reproductive stages [6]. The basic physiological mechanisms of tolerance to drought and heat may differ or be common with respect to some traits. For example, reduction of photosynthesis is a common

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

#### **Figure 3.**

*Genotypic variation in the light response of different chick pea genotypes under combined stress of drought and heat.*

phenomenon under drought and heat; however, photosynthetic inhibition due to drought is largely attributed by stomatal closure, reduced biomass production, and decreased leaf size. Drought limits gaseous exchange by triggering stomatal closure and in this way modifies plant-water relations whereas heat stress inhibits photosynthetic electron transport and PSII activity, conformational changes in thylakoid proteins, and distortion of membranes, and alters Rubisco enzyme and change in the solubility of CO2 and O2 inside mesophyll cells. Heat stress has detrimental effects on reproductive parts such as microsporogenesis and megasporogenesis of male and female gametophytes, loss of pollen viability and retards pollen germination and pollen tube growth, decreased stigma receptivity and pollen load on stigma, cell division, and accelerated rates of respiration and inhibition of carbohydrate metabolism in developing grains. However, there are common tolerance or avoidance mechanisms against both heat and drought stresses such as generation of harmful reactive oxygen species (ROS) and to counteract the deleterious effects of ROS, both the stresses are induced to produce antioxidative enzyme complex that enables scavenging of superoxide radicals being generated when plants are exposed to high temperatures in combination with drought and heat. Several heat shock proteins (HSPs) are expressed in response to high temperatures that helps to protect the membrane and vital molecules from denaturation and allows cellular machinery to operate normally. Both heat and drought stresses adversely affect water relation characteristics of plant which include decline in leaf RWC, water potential, osmotic, and turgor potential.

#### **3. Response of major food legumes to abiotic stresses**

#### **3.1 Cool-season legumes**

#### *3.1.1 Chick pea*

Chick pea (*C. arietinum* L.) is the second largest grown food legume of the world after beans. India, Australia, Pakistan, Turkey, Myanmar, Ethiopia, Iran, Mexico, Canada, and USA are the major chick pea producing countries. Chick pea is a good source of protein (20–22%) and rich in carbohydrates (~60%) [70], dietary fiber, and minerals [71]. The demand of chick pea is increasing keeping in view of its enormous

health benefits which include prevention of cardiovascular diseases, type 2 diabetes, digestive diseases, and even cancer [72]. Chick pea fixes atmospheric nitrogen through symbiotic nitrogen fixer rhizobium resulting in lesser dependence on chemical fertilizers and residual nitrogen left in the soil after harvest benefits the subsequent crops. Drought and heat are the major abiotic stresses and constraints limiting chick pea production globally which together account for about 50% of the yield losses [73]. Chick pea is largely grown under rainfed on residual soil moisture after withdrawal of monsoon. The crop often experiences terminal drought if winter rain fails and the soil moisture starts receding and reaching below to a critical level at grain-filling stages causing heavy yield losses primarily due to water limitation. Exposure to heat stress (≥35°C) at flowering and podding in chick pea results in drastic reductions in seed yields [74]. High temperature adversely affects seed germination, photosynthesis, respiration, membrane stability, fertilization, fruit maturation, quality of seeds, nutrient absorption, protoplasmic movement, transport of materials and also modulated levels of hormones, and primary and secondary metabolites [8, 75]. It has been observed that lower grain yields with greater exposure to hot days (30–35°C), during the reproductive period [7]. Heat stress at reproductive stages is thus increasingly becoming a serious constraint to chick pea production due to climate change. The optimal temperatures for chick pea growth range between 15°C and 30°C [76].

#### *3.1.2 Lentil*

Lentil (*Lens culinaris* Medik) is another cool-season food legume grown widely and consumed for edible purposes and intensifying the cereal-based cropping systems. Lentil often experiences high temperatures (>35°C) during flowering and pod filling stages, which leads to forced maturity and consequently affects seed yield and quality [77]. The delayed sowing of lentil coincides with terminal heat stress. Consequently, a large portion of cultivated areas (~11.7 million ha) in India remains fallow after the late harvest of rice [78]. In Australia, ~70% yield losses in lentil were observed due to a 6-day heat wave with a maximum temperature of 35°C or above [77]. In recent years, heat stress as a result of global warming has become a major challenge to crop production and productivity in general [79]. Development of heattolerant lentil cultivars is required to sustain production and productivity of lentil for semi-arid regions. These issues could be addressed by distinguishing the heat-tolerant and sensitive lentil genotypes at critical temperature [80]. Thus, identification of key physiological traits that impart heat tolerance can help to facilitate a breeding program for developing heat-tolerant lentil cultivars, leading to a reduction the yield losses under a changing climate scenario [81]. High and low temperatures cause photo damage to PSII [82] which could be due to damage of proteins that are involved in a photo damage-repair cycle [83]. However, cyclic electron transport around PSII constitutes an effective protective mechanism against photo-inhibitory damage [83], and some phenolic compounds have been identified in this protection [84]. In lentil, pollen and leaf traits could also be helpful in identifying heat-tolerant genotypes [47].

#### **3.2 Warm-season legumes**

#### *3.2.1 Pigeon pea*

Pigeon pea (*C. cajan* L.) Millsp. is a major grain legume of the arid and semi-arid regions of the world [85]. Drought and high temperature during the reproductive

#### *Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

stage are becoming a recurrent phenomenon in these regions resulting in significant yield loss in pigeon pea. Among pulses, pigeon pea is the hardiest crop which is cultivated in a wide range of climatic conditions from tropics to subtropics between 30°N and 30°S latitude. It is well adapted to semi-arid, arid, and marginalized regions receiving even less than 600 mm rainfall. This crop even tolerates as high as a temperature of 35°C if soil moisture is optimally available. Among pulses, the pigeon pea is an inherently drought and heat-tolerant crop to some extent [86]. High temperature leads to excessive water loss from crop canopy and soil through increased evapotranspiration. The decrease in soil moisture below a certain threshold level and the rise of temperature exceeding 35°C or more during the grain-filling stage often lead to poor yield in pigeon pea [87]. The high temperature causes oxidative damage, affects cell division, and may cause severe damage to the membranes and proteins and their synthesis, along with inactivation of major enzymes [88]. Even exposure to the high temperature for a shorter period during the seed filling can result in accelerated filling and eventually it results in incomplete grain development, poor quality, and reduction in the yield. High temperature beyond 35°C often leads to flower shedding, pod abortion, and incomplete grain development, inhibits photosynthesis [89] with increased respiration which cumulatively imbalance the source-sink relation [6]. Drought tolerance in pigeon pea is due to its deep-rooting tap root reaching up to 6 feet (2 m) in depth that helps to improve water infiltration into the deep soil [90]. Inherent drought tolerance in pigeon pea could be associated with the prolific root system with a higher number of thin lateral roots, higher hydraulic resistance to restrict flow of water or tends to conserve available water more efficiently, smaller but high stomatal density that regulates transpiration water loss to minimum level but maintain photosynthesis with lower order but not completely inhibited. Among all legumes, drought tolerance characters of pigeon pea also involve high osmotic adjustment and very low (more negative) lethal leaf water potential indicating a higher degree of dehydration postponement and dehydration tolerance. Genes expressing heat shock proteins (HSPs), dehydration responsive element-binding DREB, and cyclophilin have been found to be responsive to combined stresses of drought and heat in pigeon pea [91, 92]. Signaling effects of abscisic acid and reactive oxygen species (ROS), calcium, calcium-regulated proteins have been well characterized for their role in signal transmission under stress [93].

#### *3.2.2 Black gram or urdbean*

Black gram (*V. mungo* L. Hepper) is a popular food legume grown in many Asian countries including India, Pakistan, Myanmar, Bangladesh, Thailand, and China. India is the largest producer and consumer of black gram. It is a warm-season food legume, which requires 25–35°C temperature along with a high humidity for its normal growth and development. However, prevailing high temperature (*>*40°C) during flowering results in deformation of flower parts or flower drop leading to negative impact on yield. Nutritionally, urdbean is dense with protein (21–28%), dietary fiber (161–187 g/kg), iron (16–255 mg/kg), zinc (5–134 mg/kg), and other micronutrients like other pulses [94]. Urdbean is grown in different ecological conditions and seasons across the growing regions. In India, it is grown mainly in the rainy season (July-October) and in the southern part it is also cultivated as a winter season crop (November-February). However, its cultivation is not wide in the summer season due to excessive heat stress and a lack of humidity in the atmosphere. Thus, availability of heat-tolerant cultivars can bring more areas under urdbean cultivation. Urdbean is a close relative of mungbean, which is

extensively cultivated in identical ecological conditions. In this crop as well as in another *Vigna* pulse crop, e.g. cowpea, sources of heat tolerance have already been identified [49]. Knowledge of genetics underlying key traits imparting heat tolerance helps the breeder to make genetic improvements more precisely. In recent years, molecular markers helped to decipher the genetics of complex key morpho-physiological traits imparting heat tolerance in several crops [95].

#### *3.2.3 Green gram or mungbean*

Among all food legumes, green gram (*V. radiata* L. Wilczek), also known as mungbean, contains high amount of easily digestible seed proteins ranging between 24 and 28% which is higher than chick pea, lentil, and pigeon pea. The crop is grown during summer or rainy season and thrives most effectively at temperatures between 30°C and 40°C; however, significant flower shedding occurs at temperatures beyond 40°C [47]. It has been reported that the abscission of reproductive organs is the primary determinant of yield under heat stress in mungbean [24]. This is a short duration crop with a yield potential of about 1200–1500 kg/ha. During the reproductive stage, high temperatures cause flower drop, induce male sterility, impair anthesis, and shorten the grain-filling period. The productivity and adaptability of mungbean are seriously affected by a range of abiotic stresses including heat and drought. In mungbean, high temperature increases flower shedding [96] pollen sterility and dehiscence of anthers [22]. Being a summer crop, it is often exposed to temperature exceeding 40°C resulting in high turnover of sterile pollens, infertility, and flower abortion, and grain filling is adversely affected. High night temperature also affects grain filling and carbon partitioning. Further increase in the day maximum temperature to 44°C or above causes production of smaller and hard seeds. The pollen viability and germination were extremely sensitive to high temperature (>40°C) in mungbean, though a wide genotypic variation in the pollen germinability was observed. Seed size reduces; however, a majority of genotypes had reduced, shriveled, or deformed grains at high temperatures exceeding 40°C. The critical temperature range for damage of reproductive organs was found somewhere in between 40 and 45°C; however, sensitivity varied among genotypes. Earlier reports suggest that brief exposure of plants to high temperatures during seed filling accelerates senescence, diminishes seed set and seed weight, and reduces yield [97]. In mungbean, remobilization of pre-anthesis reserve carbohydrates and nitrogen in leaves, podwall, and stems contribute significantly towards grain filling. In the changing scenario of climate, sudden rise in the temperature beyond 35°C causes increase in the respiration rates and unusually high degradation of stored starch as major chloroplasts carbon source was observed. As a result, failure to set pods, reduced or incomplete grain development at high temperature could be partly due to inadequate supply of carbon and nitrogen from leaves or by decrease in the activity of sucrose synthase, the key enzyme playing a crucial role in grain development. Poor partitioning of carbon and nitrogen at high temperature leads to low harvest index and low productivity in mungbean. The productivity and adaptability of green gram are adversely affected by several abiotic stresses including heat, drought, salinity, and water-logging, which affect crop growth and development by altering physiological processes and the plant-water relationship [98]. Several studies have reported a reduction in the growth and development of legumes because of high-temperature stress [99].

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

#### **4. Mechanism of drought and heat tolerance in pulses**

Knowledge of key traits imparting heat and drought tolerance can help to improve the grain yield of food legumes [81]. Therefore, understanding of physio-biochemical mechanisms associated with these key traits imparting tolerance is essential for large-scale phenotyping of pulse germplasm under both field and controlled conditions [100]. In several crops, various physiological and biochemical traits such as accumulation of phenolic compounds, organic acids, photosynthetic activity, wateruse efficiency, canopy temperature, rooting length, osmotic adjustment, membrane stability, and pollen viability [47, 101] have been used to identify heat and drought tolerant genotypes and a significant genetic variability has been reported for key physiological traits under stress conditions [102].

Under rainfed or water-limiting regions, pulses experience drought, heat stress, or both at the terminal end that is at the reproductive stage. There is enough scope to improve grain yield in pulses when drought or heat appears particularly during reproductive or grain-filling stages. This type of terminal stresses is better defined as there was no moisture limitation at initial growth stages or temperature remains conducive to support normal physiological activities. The crop productivity is largely determined by initial crop vigor, biomass, number of podding nodes, water retention capacity in the tissue through regulated ways of gaseous exchange making a balance between photosynthetic carbon gain and minimum water loss through transpiration. However, if drought or heat during germination or intermittent drought accompanied by unusual temperature prevails at initial growth stages, it could be more detrimental for the crop to cope up the situation even most tolerant species introduced under such conditions. Therefore, drought or heat tolerance in pulses is practically limited to the conditions when they appear at the later or terminal end of the crop. There are two types of mechanisms that are operating in pulses toward improving yield in pulses under terminal drought or heat. One of the important requirements of the plant is to avoid terminal stresses or to develop a mechanism to postpone dehydration with depletion of soil moisture. Another strategy is to develop cellular resistances to counter the stresses such as stable membrane system, higher water-use efficiency, efficient photosynthetic system, expression of heat shock proteins, etc. Different models have been proposed for drought resistance of pulses which includes traits associated with productivity such as dry matter, harvest index, and water-use efficiency and other drought resistance mechanisms comprising morpho-physiological traits conferring resistance to drought [103, 104].

#### **4.1 Matching phenology**

Matching the phenology to the water supply is the primary way in which chick pea crop yield can be improved in water-limited environments [105]. This involves the identification of plant with an early flower and pod initiation, rapid but short growth period.

#### **4.2 Early biomass accumulation**

Early extensive biomass accumulation and setting of reproductive organs before the onset of terminal drought are largely considered as drought escape mechanisms. In environments in which terminal drought is likely, selection for shorter time to

**Figure 4.** *Biomass-yield relationship of chick pea under rainfed (stress) and irrigated (non-stress) conditions.*

flowering has been highly successful [106] . Vegetative biomass in most of the pulses has been found to be the most sensitive to water stress. The leaf expansion is affected even under mild water stress with a leaf water potential declined to ≤1.2 MPa. The biomass showed a linear relationship with yield under rainfed indicating sources (leaves & stems) are the major limiting factor for low yield. Contrary to this, increasing biomass beyond a certain limit did not further increase in the grain yield in irrigated fields that suggests the sink limiting condition of irrigated crops (no stress) **Figure 4**.

#### **4.3 Importance of plant types to improve yield under stress**

Higher biomass does not always lead to contribute higher yield in pulses. Two distinct plant types in chickpea had shown significant yield difference in chick pea. The plant with multiple shoots having uniform height emerged at the seedling stage leads to profuse primary and secondary branches with a higher number of podding nodes (**Figure 5**). On the contrary a different plant type with a single erect branch,

#### **Figure 5.**

*Two distinct plant types in chick pea. High yielding plant type (left) with multiple primary branches and low yielding plant type with single stem (right).*

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

with suppressed lateral branches at the base, has been observed to be a poor yielder due to lack of podding branches although these types of plant had shown vigorous root system and are better adapted to drought conditions (**Figure 5**). Therefore, it is essential to tag both attributes together that plant types with profuse branching and extensive root system.

#### **4.4 High biomass with profuse branching leads to higher yield**

Terminal drought markedly reduces leaf photosynthesis during grain filing in chick pea. Prestored assimilate reserve in the stems and leaves during the vegetative stage plays a major role in contributing carbon (C) and nitrogen (N) to the developing grains. The storing capacity of photosynthates (C and N reserve) is thus directly proportional to the biomass which includes stem and leaf reserves. The carbon and nitrogen fixed during vegetative growth is a potential source of assimilates for the developing seeds as photosynthetic gain during reproductive stages is substantially reduced due to stomatal closure or other nonstomatal factors as a result of progressive increase in the water stress. Thus, grain yield in chick pea and pigeon pea under terminal drought is largely influenced by remobilization of prestored carbon and nitrogen in the stems and leaves. Depending upon the cultivars, about 10–20% carbon and more than 60% nitrogen (N) in the seeds of water-stressed plants have been reported to be remobilized from stems and leaves of chick pea [107]. The soluble sugars including transportable sucrose and amino acids in leaves of water-stressed plants markedly increased as a result of starch and protein breakdown. The significant increase in the activation state of sucrose-phosphate synthase in leaves of water-stressed plants had been shown to faster conversion of sucrose in the leaves [108]. The increase in the low-molecular weight compounds like sugars, amino acids, organic acids, and sugar alcohols as a result of degradation of macromolecules, e g. starch and proteins in water-stressed plants, also plays a significant role in enhancing low-molecular weight osmotic solutes which could likely to help in maintaining transient water balance during stress.

#### **4.5 Role of pod wall to contribute photoassimilate under water stress**

Increasing photosynthetic ability as a whole is essential for acquiring higher crop yields. Non-leaf green organs (NLGOs) make important contributions to photosynthate formation, especially under stress conditions [109]. The pod wall of pea (*P. sativum* L.) was shown to contain two distinct photosynthetic layers. The outer, comprising chlorenchyma of the mesocarp, captured CO2 from the outside atmosphere; the inner, a chloroplast-containing epidermis lining the pod gas cavity, was involved in photoassimilation of the CO2 released from respiring seeds [110]. The respiratory CO2 released from the embryo of developing grains of legume pods is refixed by a layer of cells on the inner pod wall [111]. This refixation of respiratory CO2 plays a significant role to gain carbon by developing seeds under drought when leaf photosynthesis is drastically declined. It has been reported that the excised embryo, seed coat, and pod wall in chick pea are all photosynthetically competent, but the pod wall alone is capable of net O2 evolution over and above respiration. More than 80% of CO2 is fixed by this tissue when provided to the pod interior. Moreover, seed coat cell turgor in chick pea is independent of changes in plant and pod water potential [112] which indicates that seed coat water status is less influenced under drought though photosynthesis of pod wall and leaves may be affected markedly but

seed coat photosynthesis maintains normal photosynthesis and significantly contributes photoassimilates to the growing seeds.

#### **4.6 Remobilization of carbon and nitrogen from leaves and stems**

The onset of pod filling of chick pea occurs when net photosynthesis and nitrogen fixation are low as a result of the onset of terminal drought. Remobilization of carbon (C) and nitrogen (N) from vegetative parts to developing seeds may be an important alternative source of C and N for seed filling. Genotypic variation in remobilization ability has been observed in desi and kabuli cultivars. The better drought tolerance in desi genotypes could be a consequence of better remobilization and higher pod number. Thus, C and N assimilated prior to podding can supplement the supply of current assimilates to the filling seed in both well-watered and water-stressed chick pea. Remobilization of pre-podding N is an essential source of N for seed filling irrespective of environmental stress. Terminal drought decreases the rate of net photosynthesis of leaves and N fixation during seed filling [41, 113]. A high demand for assimilate from filling seeds when the supply of current assimilate is decreasing often results in an assimilate shortfall [114]. Consequently, alternative sources of assimilate are required to maintain seed filling and seed size. In a number of species, the remobilization of pre-anthesis stored reserves of C and N can be an important assimilate source for seed filling, particularly when plants are subjected to water deficit [115–117]. In cereals, remobilization has been reported to contribute between 3% and 64% of seed dry matter (DM). The contribution of remobilized DM in pulses ranges from 2 to 42% [118, 119]. In chick pea, a decrease of DM in the stems, leaves, and pod walls of field-grown plants suggests that DM remobilization is important as an alternative assimilate source to current photosynthate [120]. Using this method, it was estimated that one-third of the pod DM was derived from the remobilization of DM from vegetative tissue in field-grown chick pea [120]. Chick pea has an indeterminate growth habit with flowers, pods, and seeds developing sequentially along branches [120]. Under terminal drought, pods and seeds that form later in the upper parts of the canopy are likely to be subject to a more severe water deficit during seed filling than pods formed early in the lower parts of the canopy. Consequently, the contribution of remobilized C and N is expected to be higher in seeds filling later when competition for assimilates is high and current C assimilation is very low. There have been no studies examining how remobilized assimilates are distributed among seeds of different ages and this requires investigation. Similar to other indeterminate species, chick pea responds to water deficit by reducing stem elongation, the rate of leaf emergence and expansion, and the establishment of new reproductive nodes [121, 122]. In a previous study by Davies et al. [107], terminal drought led to decreased rates of flower and pod production with a significant decrease in pod and seed number and seed yield in both desi and kabuli chick pea. However, desi chick pea tolerated drought better than kabuli chick pea and was better able to maintain yield relative to the irrigated plants. As there were no substantial genotypic differences in the decrease in photosynthetic rate with water deficit, this difference in drought tolerance may be accounted for by variation in remobilization of assimilates between desi and kabuli chick pea types.

#### **4.7 Root characteristics**

A wide genetic variability among root characters such as density and depth, total root biomass has been reported in chick pea genotypes (**Figure 6**). Chick pea lines

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

#### **Figure 6.**

*Genotypic variability in root biomass in chick pea.*

with increased drought tolerance through increased root biomass have identified by several workers [123]. A drought resistant genotype ICC 4958 had 30% higher root dry weight than the standard control cultivar "Annigeri" which is relatively more sensitive to drought stress [124].

Although vigorous root system plays a major role toward drought adaptation, however root architecture differs across the locations depending upon the drought situations. In a case study, majority of the chick pea grown under cool environment, root system tends to proliferate just 30 cm below the surface of ground allowing to trap surface moisture received by dew precipitation during winter as well as glandular leaf trichomes use to trap dew water quite heavily. As a result, root length density (RLD) at different depths of the soil profile varies in chick pea depending upon the environmental conditions. In chick pea cultivars adapted to warmer environment roots usually go into the deep soil for mining water efficiently due to lack of surface soil moisture as a result of high temperature-dependent faster evaporative water loss coupled with a very negligible amount of dew precipitation. Therefore, root architecture is very important to consider to make a breeding strategy in developing drought tolerant cultivars. Near infrared reflectance (NIR) pattern in an RIL (recombinant inbred lines) population derived from ICC 4958 (high root) × Annigeri (low root) cross revealed a significant NIR reflectance depicting different amount of water content in leaves (**Figure 7**).

#### **4.8 Water-use efficiency, canopy temperature, and transpiration under stress**

The specific leaf area is negatively associated with SPAD Chlorophyll meter reading (SCMR) and water-use efficiency measured by Carbon isotope discrimination ( 13C/12C ratio). Therefore, SLA represents an indirect way of measuring water-use efficiency of plants, the low SLA indicated higher WUE and SCMR which could be useful to identify efficient genotypes with high water-use or transpiration-use efficiency (**Figure 8**) which is defined as more photosynthetic gain per unit loss of water through transpiration, or transpiration efficiency is a ratio between biomass and transpiration. The transpiration-use efficiency could be one of best strategies toward improved adaptation under stress environment. Improving water-use efficiency is associated with stomatal density, efficiency of RuBisco, and other physiological

#### **Figure 7.**

*(A) Chick pea genotype with high root length density (RLD) at the top soil surface layer adapted to cooler environment. (B) Chick pea genotype with high root length density (RLD) at the bottom or deep soil layer, adapted to warmer environment. (C) Different NIR (near infrared reflectance) values shown by RIL population having different levels of tissue water content which is likely to be associated with different amounts and depths of root biomass.*

#### **Figure 8.**

*Associations of specific leaf area (SLA) and water-use efficiency, 13C/12C ratio (WUE).*

parameters of mesophyll cells of leaves. Significant genotypic variation has been observed in chick pea and pigeon pea in water-use efficiency [125]. A decrease in transpiration rate due to drought may result in an increase of leaf internal temperature due to decreased evaporative cooling of leaf and therefore drought stressed plants show higher leaf or canopy temperatures than well-watered plants [126]. Cool canopies are associated with better yield output as a result of higher transpiration and photosynthesis [127]. To maintain canopy cool under drought and heat, plants should have attributed with deeper root system to access water from the deep soil layers. Key physiological traits like osmotic adjustments and root architecture need to be modified to make canopy cool under heat and drought stresses. Osmolyte accumulation also alters photoassimilate allocation between roots and shoots. Drought induced osmolyte accumulation tends to allocate more photosynthates toward roots; as a result, plants are able to adapt under drought by extracting more water from soil.

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

#### **Figure 9.**

*Plant microclimate under drought: A. Pigeon pea dwarf/closed canopy; B. Thermal image of A; C. Pigeon pea open canopy and D. Thermal image of C; E. Semi-erect dense chick pea canopy; F. Thermal image of E; G. Spreading open chick pea canopy; H. Thermal image of G. Canopy temperature can be assessed by different colors as shown by the thermal scale given at the right side.*

#### **4.9 Plant architecture for conserving moisture**

Conserving moisture is one of the strategies of plants to withstand drought. The plant architecture plays an important role in configure plant microclimate for conserving moisture and to make the canopy cool. Evaluation of contrasting pigeon pea genotypes by a thermal imaging technique showed that dwarf and closed canopy of pigeon pea (**Figure 9A** and **B**) remained cooler as compared to tall and open canopy (**Figure 9C** and **D**). Similarly dense and semi-spreading chick pea (**Figure 9E** and **F**) may have the advantage to keep the canopy cooler than spreading and open chick pea plant type (**Figure 9G** and **H**). However, plant types with deeper root system maintain the canopy cool by means of extracting soil moisture efficiently from deeper soil layers which in turn allowing plant to transpire under drought conditions. The transpiration cooling under limiting soil moisture prevents stomatal closure to some extent and simultaneously restores photosynthesis to sustain dry matter production.

#### **5. Dehydration postponement**

The second mechanism is the postponement of drought by reducing water loss primarily by stomatal control of transpiration, by increasing water use through deep and vigorous root system, or by the accumulation of solutes to maintain turgor as water shortages develop (osmotic adjustment).

#### **5.1 Stomatal mechanism**

The open stomata result in transpirational cooling of leaf hence canopy of a particular genotype having higher stomatal conductance should show lower temperature than those having close stomata. Techniques are now available to measure canopy temperature by infrared thermometry; hence, screening of pulse germplasm with varying stomatal responses to drought is possible. The reduction in the stomatal density and an increase in the leaf reflectance through production of glandular hairs in chick pea are considered to be adaptive traits through which leaves reduce water loss and intercepted non-photosynthetic radiation.

#### **5.2 Osmotic adjustment**

Osmotic adjustment (OA) is the process of active accumulation of solutes in leaves in response to increasing water deficits in the soil thereby maintaining turgor. Osmotic adjustment has been shown to maintain stomatal conductance and photosynthesis at low leaf water potential in many crops [128], delay leaf senescence, reduce flower abortion, and improve root growth and water extraction from the soil [129]. Intra and interspecies difference in osmotic adjustment and its range in grain legumes are given in **Table 1**. From the table it is clear that among pulses chick pea, pigeon pea, and peanut are tolerant to drought as compared to others. The genotypic variation in OA ranging from 0.45 to 1.25 MPa was significant at LWP (leaf water potential) below −2.0 MPa in chick pea and maximum was noticed in cultivar Tyson. The significant genetic variation in OA from 0 to 1.3 MPa has been reported earlier in chick pea at very low LWP. Comparative evaluation of two cultivars such as Tyson with high OA (1.25 MPa) and Kaniva with low OA (0.45) revealed that relative water content (RWC) varied periodically with progressive increase in water stress as crop growth advances toward maturity while RWC declined linearly in Kaniva with less OA (**Figure 10**). The expression of high OA in chick pea cultivars was usually observed in selective chick pea having inherent ability to osmotically adjust when crop was subjected to severe stress with decline in leaf water potential to −3.0 MPa and above, and this characteristic enables the cultivar


#### **Table 1.**

*Range of osmotic adjustment in grain legumes.*

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

#### **Figure 10.**

*Linear decrease of RWC in chick pea cv Kaniva having low OA while periodical changes of RWC in chick pea cv Tyson having high OA were observed with progressive increase in drought as crop growth advances. (B) High expression of OA in different chick pea cultivars occurred when the drought level was intensified at the terminal end of crop growth and OA declined after reaching a threshold level.*

to survive even under severe moisture limiting conditions. The mild water stress helps in conversion of starch into reducing sugars followed by increase in sucrose phosphate synthase (SPS) activity in leaves which led to transient accumulation of sucrose contributing osmotic solutes. The non-transportable carbohydrate, starch, is converted to transportable form of sugar such as sucrose in the process of osmotic adjustment, thereby facilitates remobilization of prestored photosynthates from leaf to various sinks. When sucrose accumulated a threshold level, it increases the water flux into the leaf cells to regain the turgidity and improve LWP or RWC. The leaf carbohydrates or sucrose and others as osmotic solutes started remobilizing as a consequence of regaining the leaf turgidity. Subsequently partial recovery of RWC (stress relief) led to decrease in the SPS activation state, partial restoration of photosynthesis, and reactivation of starch synthesis. Thus, it is evident that osmotic adjustment prevents lowering of RWC below a critical level by efficient water uptake, restores photosynthesis, and maintains positive carbon balance for longer stress periods. The increase in cellular osmolarity allows influx of water into cells and maintains the necessary turgor for cell expansion, maintains membrane integrity and prevents protein denaturation under adverse environmental conditions such as drought, high or low temperatures [130], and confers protection against oxidative damage [131]. The accumulation of mannitol in chloroplasts leads to increase resistance to oxidative stress in tobacco [132]. The accumulation of osmolyte in response to drought has a critical role in the mitigation or avoiding/ delaying the adverse effects of stress. The accumulation of osmoregulatory solutes has been considered as a unique biochemical trait which could be manipulated by breeding or transformation technologies.

#### **5.3 Dehydration tolerance**

#### *5.3.1 Lethal leaf water potential*

Sinclair and Ludlow [133] considered that the lethal leaf water potential, i.e. the lowest water potential experienced by the last viable leaf, was a key measure of dehydration tolerance. The lethal leaf water potentials in a range of grain legumes have been worked out, which shows that soybean has more dehydration tolerance than others.

**Figure 11.**

*Significance of osmotic solutes in imparting drought tolerance by improving membrane stability (MSI) and chlorophyll content (SPAD).*

#### *5.3.2 Membrane stability*

The drought tolerant mechanisms involve stability of the membrane, which could be assessed through electrolyte leakage from desiccated tissue (**Figure 11**). The ability of cells to continue metabolism at low leaf water status is termed dehydration tolerance. Membrane disorder is often measured as leakage of solutes from the cell [134]. Electrolyte leakage measured by a conductivity meter has been used as a screening technique for heat and dehydration tolerance.

#### *5.3.3 Proline accumulation*

Accumulation of proline in cell in response to water deficit is another mechanism protecting protein structures as cell dehydrate, and as an organic nitrogen source. Neither membrane injury index nor proline accumulation is being used as a screening technique against dehydration tolerance.

#### **6. Pollen and ovule fertility at high temperature**

Cool-season pulses such as chick pea, lentil, field pea, French beans, etc. are often exposed to very high day temperature during reproductive stages exceeding 40°C. The upper limit of cool-season legumes has been reported to be within the range of 30–35°C. The heat stress in cool-season legumes has been considered as major abiotic stress leading to substantial yield loss, reduction in seed size, flower abortion, leaf senescence, forced maturity, failure of pod setting, retarded pollen germination,

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

#### **Figure 12.**

*(A) Forced maturity of chick pea at high temperature with distinct genotypic variation; (B) normal pollen tube growth in chick pea at 250 ; (C) reduced and abnormal pollen tube emergence in chick pea at high temperature 41°C; and (D) reduction in the seed size in chick pea when day temperature exceeding >40°C.*

and ovule deformation (**Figure 12**) [135]. Though significant genotypic variation has been reported in all these reproductive traits. While warm-season pulses such as green gram, black gram, cowpea, and pigeon pea are less affected as their upper temperature limit of tolerance is relatively higher ranging between 35 and 45°C. Many legumes exhibit a high sensitivity to heat stress during flowering. One of the major yield determinants in food legumes is pollen fertility and flower shedding at high temperature. The pollen sap may be altered and becomes more viscous by progressive increase in the temperature beyond 37°C. The transformation of pollen sap into a dense and viscous fluid probably hinders the smooth movement of male gametes. A reduction in the pollen tube length was observed in the heat-tolerant chick pea ICCV 92944 and green gram genotype EC 398889 at 40°C and beyond. In contrast, multiple pollen abnormalities including emergence of multiple tubes, single or coiled forms, bursted pollen tubes were observed in heat-sensitive green gram LGG 460 at high temperatures. Earlier reports on rice have also indicated that an increase in temperature could limit yield by affecting pollen germination and grain formation [136]. The male gametophyte is particularly sensitive to high temperatures at all stages of development, while the pistil and the female gametophyte are considered to be more tolerant [137]. The pollens are most sensitive to high temperature; the crop yield is affected when temperature rises during pollen development [138]. High temperature decreases pollen viability and leads to sterile pollens and decrease of pod set and yield [139]. In legumes, heat stress during post-anthesis results in poor pollen germination on the stigma and reduced pollen tube growth in the style [140]. Under high temperatures (>30°C), flower sterility has been correlated with diminished anther dehiscence, poor shedding of pollens, and germination of pollens on stigma [6]. The reduction in photosynthesis under high temperature may also restrict supply of photoassimilates such as sucrose, hexoses, and starch in the developing pollens resulting in decrease in pollen fertility [49]. The role of sugars and invertase/sucrose synthase activity in anther development and pollen germination has been reported in several crops [141]. Some of the thermotolerant food legumes flowered and set pods at high temperature. Therefore, assessment of pollen viability and pollen germination at high temperature proved to be potential screening tools for heat tolerance.

#### **6.1 Acquired thermotolerance**

Acquired thermotolerance is a mechanism naturally occurring in plants and has been extensively used in thermotolerant line identification [142]. The cell viability after heat shock was tested using 2%Triphenyl tetrazolium chloride (TTC). Cells were considered viable if tissue turns purple color when treated with TTC; on the

#### **Figure 13.**

*Non-viable (TTC negative) and viable seedlings (TTC positive) of two contrasting chick pea genotypes after heat shock at 52°C. Acquired thermotolerance (ATT) in green gram in HT line EC 398889 (A–C) top and lack of ATT in HS green gram LGG 460 (C–E).*

other hand faint purple color or if tissue does not take stain, it is considered as dead. Thus, TTC (triphenyl tetrazolium chloride) test for tissue viability and chlorophyll accumulation after heat shock appeared to have some promise to identify thermotolerant genotypes having ability to acquire thermotolerance. Heat-sensitive green gram genotype LGG 460 lost cell viability after heat shock and thus identified as TTC negative. Thus, higher membrane thermostability and cell viability after heat stress could be monitored by the TTC test and the technique has been widely used for assessment of heat tolerance [143]. The TTC reduction assay measures the level of mitochondrial respiration activity, which serves as an indicator of cell viability [144]. Variability was detected among the 56 green gram genotypes for acquired thermotolerance ranging from 14.1% to 61.3% while it ranges from 20 to 40% in chick pea **Figure 13**.

#### **6.2 Membrane stability (MSI)**

Under stress conditions, a sustained function of cellular membranes is considered crucial for maintaining cellular processes such as photosynthesis and respiration [145]. The integrity and function of cell membranes are sensitive to high temperatures, as heat stress alters structures of membrane proteins leading to increased permeability of membranes resulting in increased loss of ions or solutes. The increased solute leakage is closely associated with cell membrane thermostability [146], and various attempts have been made to use this method as an indirect measure of heat tolerance in diverse plant species such as food legumes [147], soybean [81], potato, cotton, and tomato [148], and wheat [149]. Study conducted using green gram germplasm for assessing membrane stability index (MSI) and chlorophyll content or greenness index showed that both MSI and chlorophyll remained higher in heattolerant green gram line EC 398889 as compared to sensitive line LGG 460 when plants of these two contrasting genotypes grown under high thermal regimes 42/28°C maximum/minimum temperature [49].

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

#### **6.3 Photosynthesis and chlorophyll fluorescence**

Photosynthesis is the most thermosensitive plant function [150] which can occur optimally at wide temperature ranges between 15°C and 35°C, although adversely affected at temperatures ranging 35–40°C and above. The cultivars can be distinguished based on the photosynthetic performance under high temperatures. Photosynthesis at high temperatures can be assessed through gaseous exchange or the chlorophyll fluorescence imaging technique. The fluorescence imaging technique visualizes the activity or effects of stress on PSII, photosynthetic membrane system, and electron transport rates (ETR). The ETR in pretreated leaves (40°C) of heattolerant green gram EC 398889 was less affected at high irradiances whereas heatsensitive genotype LGG 460 with similar treatment showed complete reduction of photosynthetic ETR (**Figure 14**). Reduced electron transport and damaged photosystems caused by high temperature have been reported in poplar [89]. Chloroplast stroma and thylakoid membranes are damaged by high temperatures [151]. Photosystem (PS) II in the light reaction [152] and Rubisco (ribulose1, 5-bisphosphate carboxylase/oxygenase) activase in the Calvin cycle [153] are both thermolabile. Heat stress thus impairs the electron transport chain and affects the activation and activity of the enzyme Rubisco [154]. Although PSI and PSII are both adversely affected by high temperatures, PSII is more sensitive to heat stress than is PSI [155]. Chlorophyll fluorescence is a rapid and non-invasive, high-resolution technique to determine changes in photochemistry through monitoring the fluorescence emission of photosystem II (PSII) *in situ* [156]. High temperature affects membrane stability, cell viability, and the quantum efficiency of photosystem II, as measured by chlorophyll fluorescence [157]. Chlorophyll fluorescence as affected by heat stress causes a decrease of *F*v/*F*m ratio in susceptible wheat compared with tolerant lines [158]. The quantum yield is one of the powerful non-destructive parameters to differentiate

#### **Figure 14.**

*A. Heat-tolerant green gram genotype IPM 02-3 abled to set pods at high temperature 43/28°C whereas heatsensitive genotype LM 95 failed to set pods or flower at the same temperature regime. B. Light response of photosynthetic electron transport rate (ETR) at different temperatures in heat-tolerant (HT) green gram exotic accession EC 398889. High photosynthesis (ETR) detected even at 40°C coupled with high irradiance levels; however, ETR was inhibited beyond 40°C. C. Light response of photosynthetic electron transport rate (ETR) at different temperatures in heat-sensitive (HS) green gram exotic accession LGG 460. Photosynthesis (ETR) inhibited even at 40°C coupled with high irradiance levels; however, ETR was inhibited completely at 45°C at all irradiances. D. Large-scale phenotype of heat tolerance in green gram genotypes based on heat treatment of leaves at 40°C for 1 h followed by capturing quantum yield images. Deep blue color represents high values of quantum yield (Fv/Fm) and indicated tolerant lines. Whereas color deviated to green or pale green represented reduction in the quantum yield or heat-sensitive lines. E. Fluorescence imaging (quantum yield; Fv/Fm) of high temperature treated (40°C for 1 h) and light-adapted leaves distinguished HT (EC 398889) and HS (LGG 460) green gram accessions.*

tolerance levels of a large number of genotypes with respect to particular abiotic stress. QTLs have been reported for chlorophyll fluorescence in drought- or heatstressed wheat. The fluorescence parameters such as minimal Fo, maximal fluorescence, Fm, and variable fluorescence Fv change when plants are subjected to various abiotic stresses and eventually quantum yield of PSII (ratio of variable to maximum fluorescence, Fv/Fm) is affected. Using a large number of different fluorescence data, modifications of photosynthetic process under stresses can be assessed precisely, which includes thylakoid membrane organization, electron transport, and carbon assimilation [159]. The images captured for effective PSII quantum yield (YII) in high temperature treated leaves under high irradiances would be able to distinguish heattolerant and susceptible genotypes. The numerical values of different fluorescence parameters such as Fo, Fm, and Fv/Fm were converted to image format to visualize the adverse effects of heat on photosynthetic machinery at the chloroplast level. The image transformation with specific color code was shown in **Figure 7**. The intensity of the color depicts increase or decrease in the numerical values of fluorescence parameters. Similarly, the light response of electron transport rate (ETR) based on calculation using quantum yield (Fv/Fm) and PAR was also able to distinguish the genotypes based on their sensitivity to heat stress. Overall, this technique involving chlorophyll fluorescence imaging has proved to be effective and a precise phenotyping method for screening germplasm of pulses in a big way for stress tolerance. One of the mungbean line EC 398889 has been identified as highly heat tolerant based on chlorophyll fluorescence imaging and pollen fertility, and this line was used as one of the donor parents for developing short duration mungbean variety 'Virat'. The first distinct change in both the structure and function of photosystem II (PSII) reported to be occurred at 40–50°C in barley [160]. The first temperature induced transient changes had been shown at 42–48°C with a disruption of the PSII donor side and corresponding loss of oxygen evolution [161] followed by changes in thylakoid membranes at about 60°C and loss of electron transport through PSII [162] representing a denaturation of the PSII reaction centers. At about 75°C, a denaturation of light-harvesting complex of PSII (LHCII) has been observed [162]. The modification of chlorophyll florescence in response to heat stress has been reported in numerous crops, and heat tolerance of plant species can be quantified by measuring chlorophyll florescence [163]. When leaves are exposed to high temperatures and high light intensity simultaneously, the combined effects are more detrimental to photosynthesis. The relative assessment of fluorescence images for quantum yield (Fv/Fm) in high temperature treated in several green gram genotypes revealed that light-adapted leaves of the heat-tolerant green gram genotype EC 398889 exhibited higher quantum yield than the heat-sensitive genotype, LGG 460. The photosynthetic system partially or completely collapsed in light-adapted leaves of LGG 460. The fluorescence images combined with the light curve of ETR strongly could precisely differentiate varying sensitivity of photosynthesis to heat stress in the two contrasting genotypes, e.g. heattolerant green gram EC 398889 and sensitive ones LGG 460 [49]. The differential degree of membrane thermostability may distinguish the genotypes toward different sensitivity to heat stress. Chloroplast-targeted AtFtsH11 protease plays critical roles for maintaining the thermostability and structural integrity of photosystems under high temperatures [164]. Therefore, the photosynthetic efficiency may be modified under heat stress by improving FtsH11 protease in photosystems, hence, to improve plant productivity. Molecular characterization of heat tolerance in green gram using specific marker CEDG 147 could also be able to distinguish heat-tolerant green gram genotypes from sensitive ones (**Figure 15**).

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

#### **Figure 15.**

*Molecular characterization of heat tolerance in green gram. Marker CEDG147 (C) differentiated heat-tolerant (HT) and heat-sensitive (HS) green gram genotypes, L- 100 bp ladder, 1 (HUM12), 2 (Ganga-8), 3 (EC398889), 4 (IPM-02-3), 5 (IPM-02-14), 6 (LGG460), 7 (Kopergaon), 8 (NSB 007).*

#### **6.4 Oxidative stress**

The plants produce antioxidant enzyme systems as a defensive mechanism which involve superoxide dismutase (SOD), catalase (CAT), and peroxidase (POX) for scavenging ROS under stressed conditions [165]. Heat-sensitive chick pea and black gram genotypes were induced to express more SOD and POX under heat treatment as a defensive mechanism of protection from ROS; however, heat-tolerant genotype is inherently tolerant to stress and therefore producing less ROS (**Figure 16**). During prolonged stress exposure, photosynthetic activity is further inhibited by excessive accumulation of reactive oxygen species (ROS), causing damage to the membranes, proteins, and chlorophyll molecules of the photosynthetic apparatus [2, 166]. Plants use a complex antioxidant system to regulate ROS levels and avoid toxicity, but

#### **Figure 16.**

*Superoxide radical SOD in heat-tolerant (HT) and sensitive (HS) chick pea (A) and black gram (B) and peroxidase (POX) in black gram (C) when both HT and HS grown under high temperature regimes 43/25°C max/min day/night.*

changes in redox status are also perceived by plants as a signature of a specific stress that will result in a corresponding acclimation response [167]. ROS scavenging is commonly induced under drought and heat stresses through enhancing antioxidant activities, and this is correlated with tolerance to stress [168]. In some wheat genotypes, tolerance to drought or heat stress was associated with increased antioxidant capacity and reduced oxidative damage in some wheat genotypes [169]. Under prolonged stress exposure, photosynthetic activity is further inhibited by excessive accumulation of ROS, causing damage to the membranes, proteins, and chlorophyll molecules of the photosynthetic apparatus [166].

#### **6.5 Trait introgression for combined tolerance: Use of wild accessions**

The pulses have narrow genetic diversity to accept the challenges of global warming and associated changes in the water availability [170]. Therefore, it is necessary to exploit wild species and land races for extensive gene mining for useful adaptive traits/genes to incorporate into our present cultivars. The wild species are the rich sources of many useful genes [171] as they have evolved under natural selection to survive climatic extremes and can potentially provide further genetic gains [172]. Therefore, wild species need to be exploited in genetic improvement programs to alleviate the challenges of global warming and its related effects in pulses. Two wild accessions of *Vigna* were identified as photo-thermo insensitive. These include one accession each of *V. glabrescens* (IC 251372) and *Vigna umbellata* (IC 251442) based upon viable pollen and normal pollen tube formation, podding, and seed set at high temperature up to 44°C and low temperature up to 4.4°C. Distant hybridization programme for climate resilience in mungbean was initiated using these wild *Vigna* species. Wild relatives of pigeon pea are known to have many useful traits for tolerance to various abiotic stresses. The wild species of pigeon pea like *Cajanus sericeus, C. scarabaeoides,* and *Cajanus acutifolius* showed a high degree of osmotic adjustment, OA (−2.5 to −5.0 MPa) imparting drought and heat tolerance based on the chlorophyll fluorescence imaging and pollen fertility test. These wild species showed fertile pollens and pod setting under severe drought. The photo-thermo insensitive accession of wild *C. scarabaeoides* ICP 15671 demonstrated high OA and normal pollen germination at temperature as high as 47°C. The heat-tolerant ICP 15671 showed normal pollen fertility and ability to set pods even at 47°C (**Figure 17**). The accumulation of osmolytes with higher order plays a major role as thermo and desiccation protectant preventing damage of membrane and vital macromolecules. This wild species is photo-thermo insensitive as well as heat and drought tolerant, and therefore, this wild *Cajanus sps* (ICP 15671) appeared to be a potential reservoir of genes for multiple abiotic stress tolerance. Many wild derivatives developed from crosses *C. cajan cajan × C. scarabaeoides* (ICP 15671) showed significant genetic variations in the combined

#### **Figure 17.**

*Combined tolerance (heat and drought) in pigeon pea wild accession Cajanus scarabaeoides (L.). One of the accessions of C. scarabaeoides ICP 15671 flowered and set pods even temperature as high as 47°C with normal pollen germination.*

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

tolerance to both drought and heat stresses. The pre-breeding line ICP 711 developed from a cross between cultivated vs. wild chick pea *C. arietinum × C. judaicum* showed an increased number of primary branches, pods per plant, and green seeds for further use in chick pea improvement program [173].

#### **7. Conclusion**

Drought and heat stresses or combined effects of both are the major challenges for pulses which adversely affect grain yield to the extent of about 60%. Underlying mechanisms of drought tolerance in major pulses involve extensive and deep root systems, osmotic adjustment, high water-use efficiency, remobilization of prestored photosynthates, stomatal control for regulated water loss, and internal moisture conservation, etc. While stable photosynthetic system, membrane stability, and high pollen fertility have been considered as potential physiological considerations for heat tolerance. However, the fact is that very little success has been achieved so far to improve grain yield through introgression of these traits. The major yield limiting factors for low yield in pulses under these stresses could be due to poor biomass accumulation which is the most sensitive component as affected by stresses. No potential traits have so far been identified by which biomass enhancement can be achieved, if biomass accumulation is adequate at initial stages of growth, the adverse effects of these stresses can be minimized. Therefore, plant type concept has been emerged as a potential strategy to improve yield under abiotic stresses. At the initial growth stage, plants must be configured to achieve more branches and pod bearing nodes to store more moisture in the vegetative parts as it is an inherent character of pulses to retain water for longer time. Pods are semi-autonomous organs to photosynthesize and gaining carbon and capable of refixing respiratory release of CO2 internally, and moreover the water status of pod wall/seed coat is partially independent of water status of plants. High biomass also contributes proportionately higher carbon and nitrogen mobilization to developing sinks. The early, phenology, quick biomass accumulation with profuse branching and podding nodes are considered to be important in determining yield under drought and heat. The physiological trait introgression needs to be considered important on the background of high yielding plant types. Extensive efforts have been made to identify genes/QTLs in chick pea, pigeon pea, and other pulse crops [174]. Recently, genomics tools are becoming an integral part of the current conventional breeding which could be applied for genetic improvement for climate-smart pulses [175]. Genome sequences of major pulses including pigeon pea and chick pea are now available [176].

#### **Acknowledgements**

Authors are grateful to the Indian Council of Agricultural Research, New Delhi, India for funding this work under the project National Innovations on Climate resilient Agriculture (NICRA). The facilities provided by Director, ICAR-Indian Institute of Pulses Research, Kanpur, India to carry out this investigation are thankfully acknowledged.

*Advances in Plant Defense Mechanisms*

#### **Author details**

Partha S. Basu1 \*, Sushil Kumar Chaturvedi2 , Pooran Mall Gaur3 , Biswajit Mondal1 , Surendra Kumar Meena4 , Krishnashis Das1 , Vaibhav Kumar1 , Kalpana Tewari1 and Kusum Sharma1

1 ICAR-Indian Institute of Pulses Research, Kanpur, Uttar Pradesh, India

2 Rani Lakshmi Bai Central Agriculture University, Jhansi, Uttar Pradesh, India

3 International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Hyderabad, India

4 ICAR-IIPR, Arid Pulses Research Centre, Bikaner, Rajasthan, India

\*Address all correspondence to: psbsu59@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Physiological Mechanisms of Tolerance to Drought and Heat in Major Pulses for Improving Yield… DOI: http://dx.doi.org/10.5772/intechopen.106054*

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#### **Chapter 12**

## Role of Microorganisms in Alleviating the Abiotic Stress Conditions Affecting Plant Growth

*Talaat El Sebai and Maha Abdallah*

#### **Abstract**

Agriculture is one of the main sectors that participate in building up world economy, and offers the main source of food, income, and employment to their rural populations. Despite the necessity of doubling agricultural production, quantitatively and qualitatively, to cope with the worsening increase in the global population and to meet the increasing humanitarian needs, the agricultural sector faces many abiotic stress conditions. Additionally, the great climate changes lead to an increase in the negative impact of these stressors. There are many conventional and nonconventional ways that could directly or indirectly mitigate the adverse effects of these stressors, each of them has its advantages and disadvantages. The biological tool is one of the promising methods; it depends on the effective use of beneficial microorganisms to alleviate stress conditions that affect plant growth, development, and therefore productivity. This method is economically inexpensive and eco-friendly toward the environment. Beneficial soil microorganisms such as PGPRs and AMF colonize the root zone of many plant species and help to enhance plant growth and development. Thus, this chapter is aiming to highlight the role of microorganisms in alleviating the abiotic stress conditions affecting in plant growth.

**Keywords:** environmental stress, mitigating, plant productivity, PGPR, sustainable agriculture, climatic changes

#### **1. Introduction**

Agriculture is the backbone of developed and particularly developing countries, with more than 60% of the population of the developing countries depending on it for their livelihood. Increasing food production to fulfill the needs of increasing world population becomes a major concern. By the year 2050, it is expected that the human population will rise up to 10 billion. Hence, it is necessary to produce 70% more food for meeting the need of additional population. Furthermore, fighting poverty and hunger, consuming limited natural resources with more efficiencies, and acclimatizing to global warming must be taken into account to attain sustainable development [1]. Therefore, to make sure nourishment security, crop production will have to be doubled, and produced in more environmentally sustainable means [2]. However, improvements in the agriculture production process, land and water use are essential

to realizing food security, poverty reduction, and total sustainable development. This can be realized by increasing cultivable land area and/or by increasing efficiently the productivity of land and water units. Really, several other factors cause a further reduction in crop productivity resulting in a lack of food security, particularly in developing countries. Of them, the availability of agricultural land, freshwater resources, ever-increasing abiotic and biotic stresses, and low economic activity in agricultural sector are the main factors. Moreover, Agriculture sector is categorized as one of the most exposed sectors to climate change. Plant productivity, principally in arid and semi-arid zones is fronting growing stresses triggered by natural and human's activities issues. Augmented occurrence of both abiotic and biotic stresses has become the principal cause for declining productivity in main crops. There is evidence of yield drops in several crops in many parts of the world due to increasing drought, salinity stress, reduction in precipitation rate and elevated air temperature. Abiotic stresses can directly or indirectly disturb the physiological status of an organism by changing its metabolism, growth, and development. It is generally thought that abiotic stresses are considered to be the main source of yield reduction [3]**.**

Abiotic stresses affect plants in various ways and are causes of reducing crop productivity (**Figure 1**). To enhance plant production, it necessities to apply

#### **Figure 1.**

*Adverse effects of abiotic stress on plants and the role of PGPRs in alleviation of these stresses. This figure illustrates an overview of mechanisms in microbial phytohormone-mediated plant stress tolerance. Several root associated microbes produce cytokinin (CK), gibberellin (GB), indole-3-acetic acid (IAA), salicylic acid (SA), and abscisic acid (ABA), which help plants to cope with stress by improving its antioxidant potential, by up-regulation of the antioxidant system and by accumulation of compatible osmolytes therefore reducing oxidative stress-induced damage; improving photosynthetic capacity and membrane stability; promoting cell division and stomatal regulation; stimulating growth of root system, and acquisition of water and nutrients. (Adapted from [4]).*

#### *Role of Microorganisms in Alleviating the Abiotic Stress Conditions Affecting Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.105943*

cost-effective technologies to control stress conditions. Soil microorganisms, living in the soil under normal and harsh conditions, have shown great properties, which, if exploited can help agriculture for improving and sustaining crop productivity. Whereas it is well recognized that beneficial microbes can stimulate growth and increase productivity through mechanisms like increasing nutrient availability, hormone production and disease controlling, it is also becoming increasingly clear that their effects may be more far-reaching.

Soil microorganisms (SMs) are very important in naturally occurring populations that play a significant role in soil fertility, plant growth, and maintaining healthier environment. This microbial population may comprise number of microorganisms like bacteria, actinomycetes, cyanobacteria, and fungi. Some of these are considered efficient owing to their growth enhancing abilities. Among these naturally occurring populations, plant growth promoting rhizobacteria (PGPR) have been investigated widely due to their positive effect on plant growth and protecting the environment from various hazards. PGPR are free living bacteria that enhance plant growth by root colonization [5]. These are also noted as plant health promoting bacteria (PHPB) or nodule promoting bacteria (NPB) [6] and can be characterized as intracellular PGPR (iPGPR) and extracellular PGPR (ePGPR) on the basis of their proximity in related to the host plant [7]. **Figure 2** shows the degree of nearness and influence of the plantmicrobe interactions.

In the present chapter, we attempt an overview of current knowledge on how plant-PGPMs (Rhizobacteria, fungi, Arbuscular Mycorrhizal Fungi (AMF), Blue Green Algae or CyanoBacteria (BGA, CB), Actinomycetes or Actinobacteria, etc.) interactions help in alleviating abiotic stress conditions in different crop systems, which can be used for sustainable agriculture.

#### **Figure 2.**

*The extent of proximity and influence of the plant-microbes interactions, small colored shapes (blue, green, red, purple and yellow) represent soil microbes. Diversity and density of microbes are variable according to soil organic contents and types, distance from plant roots, plant species, and plant tissue. (Adapted from [8]).*

#### **2. Stress definition and types**

Stress conditions are a set of either abiotic or biotic factors that are unsuitable for plant growth of which the plant may be exposed during its various growth stages (one or more) from germination to fruiting, which may not only negatively affect its growth and productivity but may lead to entirety stopping its growth and thus its productivity. To which the plant may respond by making physiological and/or molecular and/or morphological changes or all of the previously. The plant stresses are defined as responses describing a suite of molecular and cellular processes prompted by the detection by a plant of some form of stress. These processes may be accompanied by the plant's induces for a reduction or an increase in some plant metabolites leading to an increase in plant resistance or tolerance. These stresses can be abiotic stress such as nutrient deficit, drought (water deficit or salinity), water-logging or flooding, extreme cold, frost, heat, sodicity, and metal and metalloid toxicity or biotic stress which are responsible for the damage done to an organism by other living organisms like herbivores or pathogens, bacteria, viruses, fungi, parasites, beneficial and harmful insects, weeds, and cultivated or native plants.

#### **3. Adverse effects of abiotic stress conditions on plant growth and productivity**

Various abiotic stress conditions such as salinity, drought, flooding, temperature (heat, cold), nutrient elements deficiency, alkalinity, organic and inorganic pollutants and heavy metals adversely affect crop plants growth, development and productivity [9] as shown in **Figure 3**.

#### **3.1 Adverse effects of salinity stress conditions**

**Excessive salinity** is one of the most important abiotic factors influencing the world's agricultural lands [11]. Also, it is one of the principle reasons that limit agricultural productivity [12]. It delays plant development by shifting numerous physiological, biochemical, and metabolic processes. Excessive accumulation of sodium chloride (NaCl) and other salts persuades water-deficient conditions owing to uncontainable stomata closure causing osmotic stress to plant roots. It results in ionic inequity which causes reduction in shoot and leaf growth, untimely leaf death, and necrosis [13, 14]. Reduced water absorption and augmented salts accumulation like Na+ , K+ , Mg+ 2, Ca+ 2, and Cl− inside the cell and as a result increased ion toxicity. The reduced growth of the plants under salinity is due to nutrient disturbances, affecting the availability, mobilization, and distribution of nutrients. This may be attributed to the competition of sodium (Na+ ) and chloride (Cl<sup>−</sup> ) with nutrients such as potassium (K+ ), calcium (Ca+ 2) and nitrate (NO<sup>−</sup> 3) [15]. Under higher accumulation of salts, the activity of nitrogenase enzyme encompassed in biological nitrogen fixation (BNF) is reduced then the nodulation process highly diminished [16, 17]. Currently, 50% of all irrigation patterns are impacted by salinity.

#### **3.2 Adverse effects of drought stress conditions**

**Drought stress** is one the greatest stressors for plants which can occur when the availability of water to the roots is insufficient or when the transpiration rate is too

*Role of Microorganisms in Alleviating the Abiotic Stress Conditions Affecting Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.105943*

#### **Figure 3.**

*Diverse abiotic stresses and the strategic defense mechanisms adopted by the plants. This figure shows diverse abiotic stresses and the strategic defense mechanisms adopted by the plants. although the consequences of salinity, heat, drought, and chilling are different, the biochemical responses seem more or less similar. High light intensity and heavy metal toxicity also generate similar impact, but submergence/flood situation leads to degenerative responses in plants where aerenchyma are developed to cope with anaerobiosis. It is, therefore, clear that adaptive strategies of plants against variety of abiotic stresses are analogous in nature. It may provide an important key for mounting strategic tolerance to combined abiotic stresses in crop plants. (Adapted from [10]).*

high. These two conditions regularly coincide with tropical (arid) and sub-tropical (semi–arid) climates. Water deficit restricted photosynthesis activity due to imbalance between light capture and its utilization as a consequence oxidative stress occurred [18]. Drought stress prompted a remarkable decreasing in photosynthesis, which is reliant on photosynthesizing tissue and photosynthetic pigments [19, 20]. Through stresses, active solute buildup (*i.e*., TSS, proteins, and FAAs) is claimed to be an effective stress tolerance mechanism [21]. Drought stress conditions lead to a decrease in the metabolic and physiological performance of plants and consequently the plant growth and productivity negatively affects. Additionally, drought stressor limits biological nitrogen fixation, and pigment content [13] as well as it reduces nutrients accessibility and their passage. Likewise, it greatly increases reactive oxygen species (ROS) concentration leading to an increase in oxidative stress, which take place because of an inequity created between the rate of electron transport and reducing power activity for metabolic consumption [22, 23]. Reactive oxygen species further prompt modifications in tissue construction and performance, enzyme stability, and lipid peroxidation [24].

#### **3.3 Adverse effects of temperature stress conditions**

Climatic changing conditions result in an increase of the intensity of heat and cold stress. The temperature stress causes alterations in membrane, water potential, and photosynthetic activity in plants. The optimum temperature for third carbon plants' (C3 plants') growth is stated 15–25°C by a number of scientists [25–27]. Up and down the optimum temperature, the plant performance was limited. Heat stress restricts cool-season plant development in summer in many positions of the world. Throughout the warm season, heat stress limited photosynthesis and carbohydrate buildup, augmented cell membrane damages triggered protein folding and even cell death in C3 plants [27]. The same damages have been recorded in warm-season plants, fourth carbon plants' (C4 plant species), in the winter. Also, the C4 species uptake less water and needed to alter themselves to be able to absorb mineral elements with low solubility [28].

#### **3.4 Adverse effects of nutrient element deficiency stress conditions**

Nutrient elements are considered fundamental for plant growth, development, and survival. 17 essential elements are necessary to maintain plant growth and development. Three of them (C, H and O) are derived from the air and water whereas the rest (N, P, K, Ca, Mg, S, Fe, Mn, Cu, Zn, Cl, B, Mo, and Co) are supplied either from soil or by adding fertilizers. Each of them plays a special role in plant life cycle and their necessity varies with the plant species and growth phases. Both the shortage and surplus of these nutrients lead to negative impacts on plant growth and development (**Figure 4**). Further, to make sure the efficient utilization of the nutrients, the environmental factors should be satisfactory. The plants absorb these elements in ionic form and its ability to absorb them is related to their quantities and distribution in the soil.

#### **3.5 Adverse effects of alkalinity stress conditions**

Alkalinity achieves its specific negative effect characteristics on crop plants in alkaline soils and disturbs plants at biological and physiological level. In addition to sodium chloride (NaCl) stress, there are other salts like sodium carbonate (NaCO3) and sodium hydrogen carbonate (NaHCO3) which are harmful to crops at excessive accumulations. High pH (more than eight) in alkaline soils diminishes the nutrient availability of crucial macro- and micro-nutrients, such as phosphorus (P), manganese (Mn), zinc (Zn), copper (Cu), and iron (Fe) causing nutrient deficiency and osmotic stress [29].

#### **3.6 Adverse effects of contaminants stress conditions**

Organic and inorganic pollutants are repeatedly being used in our environment via human interfering comprising industrial effluent discharge and agricultural practices, e.g., unreasonable and undue application of mineral elements and plant protective materials (pesticides) to soil. These chemical pollutants are causing major dangers to human health and their environment and may be directly or indirectly affecting on crop growth, development and productivity

*Role of Microorganisms in Alleviating the Abiotic Stress Conditions Affecting Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.105943*

**Figure 4.** *The signs of essential nutrient elements deficiency in plants.*

#### **4. Plant behavior under stress conditions**

A deficit of one or more of the vital nutrient elements caused several alterations that may be occurred at morphological, physiological, and also molecular levels of crop plants. The data presented in **Table 1** summaries these changes and in addition the symptoms that result from the deficiency of these essential nutrient elements on plants.

#### **5. Role of microorganisms in mitigating abiotic stress conditions**

The rhizosphere contains the tiny parts of soil inherent to roots of plants. The average count of microorganisms at the plant root region is very high as compared with the rest of the soil. So, it is clear that plant roots have an assortment of mineral, nutrient, and metabolite components, which are considered the principle factor for captivating microorganisms to assemble and link together. Root exudate of plants is a critical factor for microbial settlement in the rhizosphere. Shifting of microorganisms regarding the root exudates has an important role in pulling force of the microbial population to colonize the plant roots.

The interactions between microbial community and crop plants are vital to the modification and endurance of both in any abiotic environment. Induced Systemic Tolerance (IST) is the expression exploited for microbe-negotiated triggers of abiotic


*Role of Microorganisms in Alleviating the Abiotic Stress Conditions Affecting Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.105943*


#### **Table 1.**

*Summaries the plant response at physiological and morphological levels and some symptoms under nutrient deficiency stress.*

stress reactions. The duty of microorganisms in altering abiotic stresses in plants attracted the attention of several researchers [51–53]. The intrinsic metabolic of microbes and genetic aptitudes, participate to reduce abiotic environmental stresses in the plants [54]. The function of numerous rhizospheric microbes inhabitants with the genera *Azospirillum* [55], *Azotobacter* [56, 57], *Bacillus* [58–60], *Bradyrhizobium* [61], *Burkholderia* [62], *Enterobacter* [60], *Methylobacterium* [63], *Rhizobium* [60, 64], *Pantoea* [60, 65], *Pseudomonas* [60, 66], *Trichoderma* [67], and cyanobacteria [68] in elevation and control of growth in plant grown under different kinds of abiotic stresses has been reported.

In this regard, [69] reported that *Streptomyces* sp. strain PGPA39 alleviates salinity stress and stimulates the growth of "Micro-Tom" tomato plants and *Arabidopsis* [70]. *Burkholderia phytofirmans* strain PsJN overcome drought stress in maize [71] and wheat [72]. The data presented in **Tables 2** and **3** outline some examples of beneficial microorganisms that play a pivotal role in alleviation the adverse effects of abiotic stresses.

#### **5.1 Role of microorganisms in mitigating salinity stress conditions**

Endophytes and rhizobacteria as PGPB have potent in mitigating salinity stress. Their direct actions involve stimulation of phytohormones production, improvement of nutrient uptake, promotion of siderophore production, and nitrogen fixation. Some other indirect roles have resembled to actions in water-deficit stress as osmotic stability, which is pivotal in both conditions, such as accumulation of osmolytes


#### **Table 2.**

*Soil microorganisms (endophyte or rhizobacteria acting as PGPR and conferring the plants' abiotic stress tolerance.*

(glycine betaine, proline, trehalose, EPS, and volatile organic compounds accumulation). These compounds elevate plant growth via perpetuate ion homeostasis. PGPR improves plant tolerance to salinity stress via induced systemic tolerance (IST) [16, 122]. In this connection, [123] proved that the application of plant growth-promoting bacteria, PGPB, producing ACC deaminase enzyme or transgenic plants revealed the corresponding acdS gene, growth evolution, seeds productivity, and enhancement of *Camelina sativa* quality on plants grown in marginal land which not suitable for cultivation due to high salinity.

#### **5.2 Role of microorganisms in mitigating drought stress conditions**

Plant Growth Promoting Bacteria (PGPB) supports the antioxidant apparatus of plants via managing antioxidant enzyme level, consequently, increasing the plant


*Role of Microorganisms in Alleviating the Abiotic Stress Conditions Affecting Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.105943*

#### **Table 3.**

*Arbuscular Mycorrhizal Fungi (AMF) that act as PGP and conferring the plants abiotic stress tolerance.*

resistance to abiotic stresses [124]. Plant growth-promoting rhizobacteria mitigate the water deficit condition by altering several physiological and biochemical processes in plants via a rhizobacterial-induced drought endurance and resilience (RIDER). This procedure includes secretion of exo-polysaccharides (EPS), management of endogenous phytohormones and antioxidants, and coordinated organic solutes, e.g., sugars, amino acids, and polyamines, and/or fabricating of volatile organic constituents, dehydrins, and heat shock protein [125]. These techniques help plants to sustain water deficit by preserving plant growth, membrane stability, and enzyme constancy and effectively controlling the water and mineral uptake by increasing the surface area of root [16, 126].

#### **5.3 Role of microorganisms in mitigating temperature stress conditions**

Adapted microbes to high or low temperatures could alleviate their harmful effects. Microbes have explicit enzymatic structures that manage their metabolism to overcome the changing temperature and preserve their membrane and enzyme stability. Under these conditions, heat and cold shock proteins are established. These molecular chaperones contribute resistance to adjacent high-temperature stress [16, 127]. These severe conditions caused protein denaturation, which is handled with trehalose through formation of a gel-like web to save plants from dehydration [128]. Cold-adapted microbes found at high-altitude agro-ecosystem, have a

vast prospect to assist plants in alleviating unfavorable climatic conditions. In cold desert of the Himalayas, India psychrophilic and psychro-tolerant bacteria exhibited plant growth-stimulating characteristics, including *Arthrobacter, Aeromicrobium, Aeromonas, Bacillus, Bosea, Burkholderia, Brevundimonas, Citricoccus, Exiguobacterium, Janibacter, Janthinobacterium, Jeotgalicoccus, Kocuria*, *Methylobacterium, Pseudomonas, Providencia, Psychrobacter, Pantoea, Plantibacter, Rhodococcus, Sanguibacter, Sporosarcina, Staphylococcus, Sphingobacterium,* and *Variovorax* [129]. Correspondingly, the isolation of bacteria associated with heat-tolerant plants from wheat exhibited improvement in traits of plant growth and development under heat stress. They encompassed bacterial genera like *Alcaligenes, Arthrobacter, Bacillus, Delftia*, *Methylobacterium,* and a number of pseudomonads [130].

#### **5.4 Role of microorganisms in mitigating alkalinity stress conditions**

Application of encouraging phytoremediation technology depends on the integrated effect of plants and associated microbes. It has a valuable strategy to clean up the biodegradation of organic pollutants and heavy metal-polluted soils.

#### **5.5 Role of microorganisms in mitigating contaminants stress conditions**

Application of encouraging phytoremediation technology depends on the integrated effect of plants and associated microbes. It has a valuable strategy to clean up biodegradable organic pollutants and heavy metal-polluted soils [131]. PGPB responds to heavy metal stress via different mechanisms involving bioaccumulation, enzymatic detoxification, metal mobilization, immobilization, volatilization, and EPS complexation as well as accumulation of phytohormone, solubilization of phosphate, siderophore, ACC-deaminase, and NF [132, 133]. Metal solubility and accessibility in the soil were influenced by microbes. Any metal pollutants cannot be easily degraded, so they must be either stabilized or extracted from the soil. Metal-chelating siderophores and enzyme mechanisms involved in phosphate solubilization expedite heavy metal uptake under stress conditions [134]. Growth-promoting microbes build up chelating compounds such as siderophores which may decrease soil pH and promote metal solubility via complex formation. Also, the production of organic acids, such as citric, gluconic, and oxalic, may promote metal mobilization, and uptake consequently, accumulation in plant shoots, by phytoextraction. Redox processes promote bioavailability of metals as reduction of Mn (IV) to Mn (III) and Fe (III) to Fe (II) so, become less toxic. Moreover, the bioavailability could increase using bio-surfactants and phyto-chelatins via formation of the complex with heavy metals [134–138]. Phyto-stablization through growthenhancing bacteria and plant development may reduce metal availability in highly metal-polluted soils. This may occur via the formation of new specific metals, altered metal adsorption on plant cell walls, or ejection through downfall. Phyto-management is a combination of several phyto-technologies, a sustainable application and cost valid can contribute enormous assistance in repair of metal-polluted soils [139].

#### **6. Mechanisms of microorganisms for alleviating abiotic stress conditions**

The bio-fertilizers, bio-stimulators, and bio-control effects of PGPRs (**Table 4**) are contingent on their natural ability, as well as the interaction manner and militant endurance circumstances. GPB promotes plant proliferation with direct and/or

*Role of Microorganisms in Alleviating the Abiotic Stress Conditions Affecting Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.105943*


#### **Table 4.**

*Some mechanism of action of PGPRs that enhance the plant growth\*.*

indirect techniques [6, 145]. Concerning direct mechanisms, it involved the synthesis of compounds that expedite the uptake of crucial nutrients and micronutrients from the soil and accumulation of plant growth regulators, such as phosphorus and potassium solubilization, iron and zinc sequestration, siderophore and plant hormone accumulation, and atmospheric nitrogen fixation. Regarding the indirect techniques, it occurs through the accumulation of HCN and antifungal components, hostile activity regarding pathogenic organisms, and resistance to unfavorable stress conditions. Moreover, the bacteria can promote systemic resistance in plants via the accumulation of certain metabolites that provide extracellular signals and stimulate a series of internal processes. Ultimately, these signals are recognized by different plant cells responsible on the promotion of the defense system.

In addition to bacteria, fungi especially mycorrhizae are considered pivotal plant growth stimulators. Mycorrhizae are mainly divided into mycorrhizal fungi (MF) and vesicular-arbuscular mycorrhizal (VAM) fungi. These types of fungi are either still connected externally with the host plant (ectomycorrhizae) or they may organize endosymbiotic associations (VAM). They form extended networking of fungal mycelium, so, maximize nutrient uptake via roots. In this connection, [146] concluded that the endophyte root fungal of *Piriformos poraindica* promoted salt and drought tolerance in Chinese cabbage and barley, respectively. These stimulatory effects were achieved by promoting the concentration and activity of antioxidants and stimulating many other processes [147]. The possibility of microbial connections with the plants has several aspects. It starts with the induction of local or systemic stress mitigation techniques in plants to resist unfavorable stress conditions. Then, they assist plants to protect their growth, proliferation, and development via fixation, mobilization and/or accumulation of nutrients, hormones and organic phytostimulant components. These multipronged roles of microorganisms or their populations demonstrate their strength, achievable and critical options for different alleviation techniques for abiotic stress in plant crops.

Various suggested techniques explain the effect of microbes in mitigation of abiotic stress. Soil-dwelling microbes can be classified into genera *Achromobacter, Aeromonas, Azospirillum, Azotobacter, Bacillus, Enterobacter, Klebsiella, Pseudomonas,* and *Variovora which* exhibited enhancement of plant growth under different stress conditions [60, 75, 89, 122, 125, 148]. Several publications concerned with the role of microbes for alleviating abiotic stresses indicate the importance of microbes in this field (**Tables 5**–**9**). All soil-inhabiting bacteria are organized as plant growth promoters (PGP) if they are able to promote plant growth even under different unfavorable physicochemical conditions. There are several tools by which microbes promote plant growth as indole acetic acid (IAA), which is synthesized in the shoot apical meristem and gathered in the active root apical meristems. The auxins have growth-promoting roles in plant-involved cell elongation, consequently root growth induction and lateral root formation. In contrast, the high auxins concentrations, promote retardant effects on root growth [60, 186]. The same result was recorded as a result of high ethylene synthesis [186]. Results also concluded that the rhizosphere colonizing bacteria promote plant growth via phytohormones production [187]. Generally, agricultural practices observed that the PGPRs not only assist in alleviation of environmental stresses, but also increase the yield of several crop plants including barley, maize, rice, and soybean [174, 188, 189]. In this regard, *Pseudomonas* sp. PMDzncd2003 enforces salt tolerance on rice germinates under salt stress. It also has a high ability to root colonizing parallel to the ability to accumulate exo-polysaccharides (EPS) that



*Role of Microorganisms in Alleviating the Abiotic Stress Conditions Affecting Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.105943*


#### **Table 5.**

*List of some microorganisms that have the ability for mitigating drought stress condition through different mechanisms.*


*Role of Microorganisms in Alleviating the Abiotic Stress Conditions Affecting Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.105943*


#### **Table 6.**

*List of some microorganisms that have the ability for mitigating salinity stress condition through different mechanisms.*


#### **Table 7.**

*Some microorganisms that have the ability for mitigating temperature stress conditions through different mechanisms.*

promote salinity tolerant [190]. Also, inoculation of rice with *Bacillus pumilus* mitigates salinity and high boron stresses [191]. The reported technique for cell protection under stress conditions was high antioxidant enzyme activity accompanied by the presence of bacterial inoculant. More studies are needed to investigate the communication between plant and bacterial colonizers at the molecular level.

Finally, [192] have proved the duty of *Trichoderma harzianum* on alleviation of stress in different rice genotypes through adjustment of dehydrin, malonialdehyde and aquaporin, and genes parallel to several physiological traits. Rhizobacteria-promoted resistance to water deficit and resilience (RIDER) by altering the phytohormone levels, enzyme activities, defense-related proteins incorporation, antioxidant levels, and epoxypolysaccharide accumulation for plants. These strategies help plants to mitigate unfavorable conditions [122, 125]. Using stress tolerant microorganisms is a promising tool in improving the productivity of crop plants grown in stress-susceptible areas. Application of *Trichoderma harzianum* improved oil content in NaCl affected Indian


#### **Table 8.**

*List of some microorganisms that have the ability for mitigating flooding stress conditions through different mechanisms.*


#### **Table 9.**

*List of some microorganisms that have the ability for mitigating heavy metal stress conditions through different mechanisms*

mustard (*Brassica juncea*) via increasing the uptake of essential nutrients, promoting the accumulation of antioxidants and osmolytes, and decreasing NaCl uptake [67]. In addition to, up-regulation of monodehydroascorbate reductase in treated plants. It also alleviates salinity stress via accumulation of ACC-deaminase [193]. Moreover, inoculation of barley and oats, with *Acinetobacter* sp. and *Pseudomonas* sp. enhance the accumulation of IAA and ACC deaminase under saline soil [169].

#### **7. Conclusion**

Agriculture is the backbone of developed and particularly developing countries, with more than 60% of the population of the developing countries depending on it for their livelihood. Increasing food production to fulfill the needs of an increasing world population becomes of a major concern. Despite the necessity of doubling agricultural production, in terms of quantity and quality, to cope with the worsening increase in the global population and to meet the increasing humanitarian needs, the agricultural sector faces many abiotic and biotic stress conditions. Additionally, the great climate changes resulting from global warming lead to an increase in the negative impact of these stressors. Throughout this literature study, it is well established that the abiotic stress conditions (salinity, drought, high and low temperature, alkalinity, and organic and inorganic pollution have great side effects on plants (decreasing in plant growth and productivity, physiological changes, alteration in osmotic balance and ion cytotoxicity). Moreover, the side effects of abiotic stress conditions

#### *Role of Microorganisms in Alleviating the Abiotic Stress Conditions Affecting Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.105943*

have been expected to be increased because of the bad or nonsustainable agricultural practices, water scarcity and reduced arable land, soil degradation, human activity, and the climate change (global warming of the planet). Hence, it has become a necessity to reduce the different causes behind the increasing abiotic stress conditions. On one hand, these can be achieved through good and sustainable agricultural practices such as agricultural rotation system, integrated crop management, integrated nutrient management, and integrated pest management re-mapping of agricultural map in the light of climate change, soil fertility, *etc*. On the other hand, in order to increase crop productivity, it becomes necessary to develop low-cost technologies for abiotic stress management. Soil microorganisms, surviving in the soil under extreme conditions, have shown high properties, which, if exploited can serve agriculture by increasing and maintaining crop productivity. Our literature study has indicated the paramount importance of these beneficial microorganisms in the mitigation of the negative consequences resulting from different abiotic stress conditions. Where, it is well established that beneficial soil microorganisms can promote growth and increase productivity through different mechanisms such as increasing the availability of essential nutrient elements and enhancement of their uptake, phyto-hormones production, ACC-deaminase production, biological control agents' production, *etc.* Even though, more efforts should be given in this field like that, isolation and characterization worldwide benefit microbes from different biological niches and under various harsh conditions. Further researches will be required concerning the optimization of the mass production of these microorganisms, the best carrier that allow increasing the shelf life of beneficial microorganisms and par consequence increasing its storage ability, also, the better ways for its field application. The application of these beneficial microorganisms is still limited and how to increase their application rate should be taken into account.

#### **Author details**

Talaat El Sebai1 \* and Maha Abdallah<sup>2</sup>

1 Agricultural Microbiology Department, Agricultural and Biologyl Research Institute, National Research Centre, Giza, Egypt

2 Botany Department, Agricultural and Biologyl Research Institute, National Research Centre, Giza, Egypt

\*Address all correspondence to: tn.elsebai@nrc.sci.eg; talaatelsebai@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 13**

## Techniques against Distinct Abiotic Stress of Rice

*Ananya Prova and Md. Saeed Sultan*

#### **Abstract**

Plants cannot physically escape environmental stresses because they are sessile organisms, which can stunt their growth. As a result, plants have had to evolve distinct strategies to deal with abiotic stress. Indeed, responding to and eventually adapting to abiotic stress may be a driving force in speciation. Because of the complexity of stress, multiple sensors, rather than a single sensor, are more likely to be responsible for stress perception. Stress-induced gene issues can be divided into two categories: those involved in stress tolerance and those involved in signal transduction. Stress-tolerance genes help plants cope with stress in both short- and long-term responses. These can include the synthesis of chaperones and enzymes for osmolyte biosynthesis. And, as with cold stress, detoxification causes a change in the composition of membrane lipids. Gene products can also function as transcription regulators, controlling groups of stress-related genes, or as components in the production of regulatory molecules. It has been shown that multiple signaling pathways can be activated during stress, resulting in similar responses to different triggers.

**Keywords:** stemless, abiotic stress, resistance, membrane lipid, enzymes, defense, tolerance, toxicity

#### **1. Introduction**

Land plants have evolved to thrive in harsh environments since their inception. To address the various environmental constraints that affect their growth and development as stemless organisms, plants have evolved a variety of complex and efficient molecular and physiological mechanisms [1]. They are physically and chemically hostile to cold and heat, for example. Stress can be caused by a lack of or an excess of water, high salt levels, heavy metals, ultraviolet (UV) radiation, and other factors. These pressures, known as biological stress, pose a significant threat to agriculture and ecosystems, resulting in significant crop losses [2, 3]. Plants must adapt to their ever-changing environment because they lack stems. During development, plants are subjected to a variety of environmental influences, which can limit productivity. A yield gap is observed when the crop is grown under suboptimal environmental conditions, so the average yield achieved is much lower than the maximum potential yield of a particular crop [4]. Yield gaps for the three major grains: wheat, rice, and corn account for 40, 75, and 30% of the world's major growing regions, respectively [5]. The primary causes of crop yield gaps can be classified as some abiotic factors such as temperature, water, or minerals, or (ii) biological factors such as bacteria, fungi, or insect invasion [6, 7].

Rice cultivation and productivity are water-intensive, making them highly vulnerable to drought and flooding. Due to drought impairment, Asia's most pertinent riceproducing belts most pertinent rice-producing belts in Asia produce only 40% of total production efficiency [8, 9]. The overwhelming majority of strain-related studies in rice, as in other plant species, have primarily focused on single stresses, either abiotic or biotic. Despite undeniable progress in the overall field, this approach provides an oversimplified and unrealistic picture. Furthermore, changes in the global environment (GEC) endanger crop production. Droughts, floods, soil acidification, soil salt content, frigid and hot temperatures, and other harmful environmental conditions are all caused by GEC. All of these stressors have an impact on crop yield and quality, either directly or indirectly. Furthermore, with the world's population expected to reach 9 billion by 2050, crop production must be increased to feed an additional 2 billion people over the next 40 years. Rice (23%), wheat (17%), and corn account for roughly half of total human calories (10%).

Despite their immobility, plants must simply respond to and endure a variety of environmental and biotic stresses in the field. A biotic and abiotic factor contends both cause crop yield losses [10]. As a result, many crop improvement programs prioritize the development of stress-tolerant plant varieties [11–13]. Plants respond differently to different or concurrent stresses, and breeding for single stress (e.g., drought, salinity, pathogen) rather than multiple stresses (e.g., abiotic or biotic) may be risky. It is worth noting that increasing tolerance to one type of stress may reduce tolerance to another [14, 15]. Climate change is increasing the frequency of extreme weather events, and plants are subjected to a variety of stresses in the field, including additional pressure from plant diseases [16].

As a result, recognizing the similarities and differences among stress response pathways is essential for optimizing targeted crop improvement. Stomatal closure, reduced photosynthesis, increased reactive oxygen scavenging activity, reduced leaf growth, and increased root length are all indications of plant responses to abiotic stresses [17]. Plant pathogens, for example, cause stomata to close, reducing photosynthesis [18, 19]. The production of toxic compounds such as phytoalexins and reactive oxygen species, as well as the induction of localized cell death, are other pathogen-induced plant responses [20]. Many of these responses are governed by phytohormones [21, 22].

Hormones like abscisic acid (ABA) and jasmonic (JA) play a key role in regulating inanimate stress tolerance. For pathogen immunity, plants primarily rely on salicylic acid (SA), JA, and ethylene signaling. The abiotic stress response is regulated by many transcription factor (TF) families, both ABA-dependent and ABA-independent. ABAinduced basic leucine zipper (bZIP) transcription factors are among the aforementioned [23, 24]. These TFs are responsible for stoma closure, dehydration resistance gene expression, and other adaptive physiological responses [25–27]. ABA, on the other hand, frequently increases plant sensitivity to biological interactions [28–32] and frequently interacts detrimentally with SA [28–35]. As a result, plants must have evolved the ability to detect and respond to multiple environmental cues in varying combinations. Developing stress-tolerant plants and testing their performance against individually imposed stresses, according to this viewpoint, may be insufficient. Furthermore, plants exposed to multiple stresses activate a single response that is not simply additive but results from synergistic and antagonistic interactions, resulting in unpredictable effects when each stress is taken into account [11].

The presence of secondary stress, according to this theory, can either aggravate the negative consequences of primary stress or, on the other hand, contribute to a better response.

#### **2. Defenses against abiotic stresses in general**

#### **2.1 Cuticle**

The cuticle is a fine translucent lipid structure that seals the aerial surfaces of land plants' organs on the outside. The thin hydrophobic layer is essentially a cutin matrix filled with cuticular waxes and encapsulated with them. As the plant's primary interface with the environment, the cuticle cuticle, as the plant's primary interface with the environment, is critical for controlling liquid and gas fluxes, defending against pathogen and bug attacks, and resisting abiotic stresses. The ability of land plants to deploy an outer shield made of simple molecules is a brilliant innovation that is critical to their success in terrestrial colonization [36–38]. The cell wall, the second barrier that actively remodels in response to abiotic stresses [39, 40], is far more complex and poorly understood to order to counteract this [41, 42]. The cuticle is macromolecular polyester of C16 or C18 oxygenated fatty acids (FAs), which are exclusively produced by epidermal cells. Waxes, on the other hand, are a high-end blend of C24 to C34 FA derivatives such as alcohols, aldehydes, alkanes, esters, and ketones. The biosynthetic pathways of these organisms are nearly complete and well documented [10, 37, 43–45]. In a nutshell, C24 and C34 are constituted from acetyl coenzyme A (CoA) in plastids via de novo FA synthesis, adding the addition of two carbons in each recurring cycle. Until C16/C18 products emerge, they are transported to the endoplasmic reticulum (ER), they are oxidized and incorporated to form cutin precursors (monoacylglycerols) or elongated modified to include wax components. The alcohol-forming (or acyl-reduction) pathway for primary alcohols and esters and the alkane-forming (or decarbonylation) pathway for aldehydes, alkanes, secondary alcohols, and ketones, are two distinct modification pathways. These materials must be transported from the ER to the cytomembrane (PM), where cutin monomers polymerize and wax members crystallize, to form the apoplastic cuticle on the outer surface. Membrane vesicle trafficking [46] is one of the systems involved in intracellular shipments to the ATP-binding cassette (ABC) transporters that channel the PM [38, 43, 47].

#### **2.2 Unsaturated fatty acids**

C16/C18 FAs are not only important components of the cuticle, but also of membranes, which serve as basic biological barriers. Phospholipids and glycolipids with a glycerol core and two FA-derived "tails" are the primary components of botanic membranes. Membrane properties are greatly influenced by FAs. Their degree of unsaturation, in particular, is an important determinant of membrane fluidity. The UFA chain will kink at a cis-double bond, which will act as a steric hindrance in the intermolecular package, causing the intermolecular package to become more fluid [48, 49]. Membrane fluidity is vulnerable to abiotic stresses, particularly extreme temperatures. Cold-driven rigidification and heat-driven fluidization can cause biomembrane dysfunction, as exemplified by protein deactivation and ion leakage [50]. Cytoskeleton destabilization is also a direct consequence [51, 52].

C18 UFAs are used as a raw material in the production of a variety of aliphatic compounds in plants, including membrane glycerolipids, TAG, cutin/suberin, jasmonates, and nitroalkenes (NO2-FAs). All of these products, as well as C18 UFAs, help plants; defend themselves against biotic and abiotic stresses. Multiple mechanisms implicate C18 UFAs in stress defense, either directly and indirectly. Biomembranes are a functional platform for many cellular processes, including substance exchange, signal transduction, and many metabolic reactions, in addition to being a structural barrier for cells and intracellular organelles [53]. The signaling of Ca2+, a versatile second messenger involved in virtually every stress response in plants, is based on membrane isolation and transportation. With the help of its channels, such as the PM cyclic nucleotide-gated channels (CNGCs) and the tonoplast TWO PORE CHANNEL 1 (TPC1), the sharp influx of the cation ignites ca2+ signaling into the cytosol [51, 54, 55]. Furthermore, efflux through Ca2+-ATPases and Ca2+/H+ exchangers has quickly rinsed it [56, 57]. Other ion transporters, such as the K+ rectifier ARABIDOPSIS K+ TRANSPORTER 1 (AKT1) [54] and the Na+/H+ antiporter SALT OVERLY SENSITIVE 1 (SOS1) [26, 34], preserve a sufficient K+/Na+ ratio in the cytoplasm, which is necessary for salt tolerance [58, 59]. The electrochemical gradient created by transmembrane proton pumps, such as PM H+-ATPase, vacuolar H+-ATPase (V-ATPase), and vacuolar H+-translocating inorganic pyrophosphatase (V-PPase), energizes these secondary transporters [60–62]. The PM H+-ATPase, in particular, is a critical site that responds to salt and other stresses such as cold and heavy metals, as well as active transport across the PM [63].

Membrane fluidity is vulnerable to a variety of stresses, including extreme temperatures. Cold-induced rigidification and thermal fluidization, on the other hand, are harmful to membrane function, causing protein deactivation, electrolyte leakage, and perhaps even cytoskeleton destabilization [51, 52]. As a thermodynamic property, membrane fluidity could be used as a sensor in heat flux signaling. Interestingly, dimethylsulfoxide (DMSO) and benzyl alcohol (BA) can both mimic the effects of cold and heat at 25°C. Plants are poikilothermic organisms, emphasizing the significance of membrane remodeling, as well as the threat of climate change.

#### **2.3 Scavengers of reactive species**

The endless generation of noxious RS, particularly reactive oxygen species (ROS) such as superoxide (O2), oxide (H2O2), hydroxyl (OH), and singlet oxygen (O2), as well as reactive carbonyl species (RCS) such as malondialdehyde and methylglyoxal, is an inherent paradox in aerobic organisms' normal metabolism (MG; CH3COCHO). The two types of RS are inextricably linked. RCS can be caused by ROS-induced lipid peroxidation, while ROS are frequently raised by RCS activities. Almost all abiotic stresses can cause a surge of ROS and RCS, transforming their scavengers into particular defenses. Nonetheless, ROS and MG have been shown to perform a signaling role at low levels, which is tactically exploited to aid stress perception and elicitor retortion [64, 65]. As a result, it's critical to maintain the delicate RS homeostasis, which must be taken into account when trying to manipulate RS scavengers for multi-stress tolerance.

#### **2.4 Reactive oxygen species**

The active transport of chloroplasts imposes a greater burden of ROS on plant cells. When these small chemicals are overproduced, they attack a variety of

*Techniques against Distinct Abiotic Stress of Rice DOI: http://dx.doi.org/10.5772/intechopen.105808*

biomolecules such as carbohydrates, lipids, proteins, and nucleic acids, causing oxidative catastrophes such as increased photoinhibition inhibition and membrane damage, which can be measured by the amount of MDA generated per oxidation of UFA [64, 66, 67]. MDA is a potential RCS that attacks under acidic conditions, forming covalent adducts known as advanced lipoxidation end products (ALEs), usually causes protein dysfunction and its consequences. Plants have evolved sophisticated ROS scavenging systems that employ both non-enzymatic and enzymatic methods. Many metabolites, such as betalain, carotenoids, flavonoids, and vitamin E, have antioxidant properties [68, 69]. Superoxide dismutase (SOD), catalase (CAT), and various peroxidases are examples of special enzymes (POD). SOD converts oxygen to hydrogen peroxide (H2O2), which is then reduced to water by CAT and POD. Dehydroascorbic acid reductase (DHAR), monodehydroascorbic acid reductase (MDHAR), and glutathione reductase are all core component of the ascorbate glutathione (ASAGSH) cycle, which is required for ascorbic acid peroxidase (APX) (GR). The intervention of detoxifying enzymes can undoubtedly be used to achieve multiple stress tolerance. Transgenic plants have used APX, an important enzyme that makes sure the removal of H2O2, to combat drought, salt, and intense light [70–72]. It also revealed glutathione peroxidase (GPX) activity, as well as cold air, heat, ultraviolet light, and heavy metals [73–75].

#### **2.5 Carbonyl reactive species**

Methylglyoxal, a substantial type of RCS, is attracting more attention in stressful situations. Because of the non-enzymatic dephosphorylation of two intermediates, glyceraldehyde3-phosphate, and dihydroxyacetone phosphate, glycolysis is the primary source of this cytotoxin in plant cells. Even before MG increases to a dangerous level, it can harm a variety of biomolecules, particularly because of its aldehyde group. Plant defenses against abiotic stresses ALEs, MG can help accelerate the photoreduction of O2 to O2 in chloroplasts and consume GSH via spontaneous combination into hemithioacetal, actually results in a vicious cycle and eventual cell death.

#### **2.6 Chaperones at the molecular level**

Heat shock proteins (HSPs) are molecular chaperones that are articulated either induced or constitutively to assisted protein folding, assembly, transport, and degradation. HSPs' anti-stress role isn't limited to their definition. This large family of proteins is a universal rescue system used by nearly all living organisms to combat all dangerous factors can trigger protein damage. They work to prevent denatured proteins from accumulating, assist in their refolding, or present them to lysosomes or proteasomes for proteolysis, restoring cellular homeostasis [76, 77]. Furthermore, some unusual hydrophilic proteins, such as members of the late embryogenesis abundant (LEA) and cold-regulated (COR) families, may act as chaperones to protect proteins and membranes from stress injury [78, 79]. The five conserved HSP classes based on molecular weight are HSP100/Clp, HSP90, HSP70/DnaK, HSP60/Chaperonin, and small HSP (smHSP). The most widely conserved among species is HSP70, which consists of an N-terminal ATPase domain and a C-terminal substrate-binding domain. In response to stress, smHSPs accumulate quickly and are more likely to seize non-native proteins and transfer them to ATP-dependent chaperones like the HSP70 system for re-naturation [80, 81]. According to a new study, Arabidopsis transformed with HSP16.4 from pepper (*Capsicum annum*) was less

susceptible to drought, heat, and their combination, and ROS scavenging enzymes were more active under stressful conditions [82].

#### **2.7 Compatible solutes**

Conformance solutes are small organic compounds with electrical neutrality, high solubility, and low toxicity that can cause some problems when present in high concentrations in cells. Qualified molecules include sugar and amino acid derivatives, as well as their derivatives, such as ash, trehalose, inoside, mannit, proline (PRO), and glycine betaine (GB). Except for the protein and membrane RS and stabilizer, these metabolites begin under stressful conditions for dehydration and can be started. A compatible dissolved compatible lysis solute is a small organic connection with electrical neutrality, high solubility, and low toxicity that really can cause a variety of problems in a significant cell concentration. Qualified molecules include sugar and amino acid derivatives, as well as their derivatives, such as ash, trehalose, inoside, mannit, proline (PRO), and glycine betaine (GB).

#### **3. Plant responses by signal transduction**

Plant response to environmental changes has been related to changes in signaling molecules [e.g., sugars, hormones, calcium, reactive oxygen species (ROS), nitrous oxide (NO)] [54, 83], along with large-scale genomic restructuring, including transposon activation [84, 85], and rapid changes in gene expression patterns (e.g., genes encoding transcription factors [86, 87]. A percentage of transcription factors (TFs) from different crops have been discovered to play critical roles in abiotic stress responses. The ability of transcriptional regulators to act as master regulators has been hailed as a long-term solution for modifying complex traits in crop plants [85, 88]. Several transcription factor families, including AP2/ERF, bZIP, Zn-finger, NAC, MYB, and WRKY, have been implicated in abiotic stresses in past few decades [88–90]. Three major methodologies have been used to identify TFs associated with abiotic stress responses in rice: comparative genomics—abiotic stress-responsive genes from Arabidopsis and maize have been used to identify rice orthologs; forward genetics—genes related to traits like drought or hypoxia tolerance were identified through association mapping; genome-wide expression profiles—transcriptome analysis using microarrays was used to identify novel abiotic stress-response genes. Plant stress responses have also been interconnected to chromatin remodeling and nuclear organization. Salinity and heat-shock stresses, for example, caused decondensation of interphase ribosomal chromatin in rice and wheat [91, 92]. Heterochromatin maintenance mechanisms may repress transcription in normal circumstances, but they may fail to cause chromatin remodeling and novel gene expression profile in stressful situations [93, 94].

Changes in signaling pathways molecules for example, B. sugar, hormone, calcium, reactive oxygen species (ROS), basic oxide (NO)] are linked to plant response to the environmental changes. Scharf et al. [83] only Not even with the most thorough genomic reconstruction, which included transposon activation (for example, gene encoding transcription factor) (e.g., transcription factor) [75, 87]. Many transcription factors (TFS) derived from various plants were found to play a key role in their stress responses. TFS's ability was viewed as a long-term solution for reconfiguring different dynamics in crops, as well as a long-term solution as a master regulatory

*Techniques against Distinct Abiotic Stress of Rice DOI: http://dx.doi.org/10.5772/intechopen.105808*

authority [88]. TF families such as AP2 / ERF, BZIP, Zn-finger, NAC, MYB, BZIP, Zn-finger, NAC, MyB, WRY, and others have been implicated in the rise in stress resistance in recent decades [13, 88–90]. In rice, three effective interventions were used to identify TFS. By assigning associations, the forward genetics gene associated with characteristics like drought or hypoxia resistance was discovered. Microarrays were used to identify new and assistant voltage attractive genes using genomic effect expression profile transcript analysis. The plant's response to stress also involves chromatin remodeling and nuclear organization [95]. Salt and heat shock stress, for example, caused decondensation of interphase ribosomal chromatin in rice and wheat [91, 92]. Heterochromatin maintenance mechanisms can suppress transcription under normal conditions, but under stress, these mechanisms can disrupt, leading to chromatin remodeling and new genetic patterns [93, 94].

#### **4. Abiotic stress responses: epigenetic mechanisms and gene expression regulation**

The effects of stress on genomic epigenetic marks, which affect gene expression regulation, are referred to as environmental epigenetics [96, 97]. Epigenetic memory is achieved by interacting with a variety of molecular mechanisms, including DNA methylation, post-translational modification of the nucleosome core histone protein's N-terminal region, and chromatin remodeling [98, 99]. Many proteins, known as transcription factor-interacting proteins (TFIPs), have been found to regulate epigenetic responses to environmental stress, but only a few have been found in rice. Rice underlying genetic factors were discovered primarily through comparative genomics. Plant plasticity responses to unpredictable abiotic stresses rely heavily on epigenetic mechanisms.

#### **4.1 DNA methylation and abiotic stress**

Cytosine methylation is a conserved epigenetic mark that plays a role in genome defense against endogenous transposable elements and viral DNA, as well as gene regulation regulation throughout plant development. Methyltransferases catalyze the addition of a methyl group to cytosine residues (MTases). Furthermore, in plants, this can happen in both asymmetric (CHH) and symmetric (CG and CHG) situations. DOMAINS REARRANGED METHYLTRANSFERASE TFS AND EPIGENETIC MECHANISMS IN ABIOTIC STRESS RESPONSES 847 (DRM), METHYLASE 1 (MET1), CHROMOMETHYLTRANSFERASE (CMT), and DNA methyltransferase homolog 2 (Dnmt2) are four key families of plant MTases that seem to have distinct functions in de novo and/or maintenance methylation [100]. While methylated cytosines are replaced with unmethylated ones during DNA replication, active demethylation occurs without DNA replication throughout a base excision repair mechanism mediated by DNA glycosylases [101]. Other frameworks, such as the RNA-directed DNA methylation (RdDM) pathway, mediated by siRNAs [102], and chromatin remodeling factors, also impact DNA methylation [78, 103]. Overall, these regulatory pathways provide a dynamic platform for establishing DNA methylation patterns, which may be critical for epigenomic plasticity and rapidly respond to developmental cues and environmental stress. The technique of DNA methylation's implication on transcription is still unidentified. Several lines of evidence suggest that cytosine methylation has a broad array of functions that are likely individualized for different

genes [100, 104]. Methylated cytosines may attract methyl binding proteins, which in spin may attract histone modifiers and chromatin remodeling proteins, resulting in a complex that can disrupt transcription factor linkage [36, 104].

On either hand, high-resolution DNA methylation mapping has demonstrated some common aspects related to the H3K9me3 and H3K27me3 are infused in genes of euchromatic regions [105]. Biotinylation and sumoylation, two other histone modifications, have been interconnected to gene repression [106]. Large and powerful histone lysine acetylation has been linked toward a more open chromatin structure and thus enhanced transcription, whereas weak acetylation has been linked to chromatin compaction and gene silencing [107]. Histone acetyltransferases (HAT) and histone deacetylases enforce histone lysine acetylation (HDAC). In plants, there have been four major classes of HDAC encoding genes [108], also with HD2 class being the only one that exists [109, 110]. In hybrid rice, the OsHDT1 gene is involved in regulation of gene expression [111]. There are at least 19 HDAC genetic traits in the nucleotide sequence, and most of them are differentially regulated by different abiotic stress conditions [12, 112]. Most rice HDAC genes were exceptionally responsive to drought or salt stresses, primarily through transcriptional repression, as according microarray data [112]. As a result, abiotic stresses may start regulating the transcription of chromatin modifier enzymes. Down regulation of HDAC may be required in this case to allow the induction of stress-responsive genes [12, 113].

The highest density of methylated cytosines is reported in transcriptionally inactive heterochromatic regions, which contain countless transposable elements (TEs) and repetitive sequences. Lower but still significant cytosine methylation levels were observed in euchromatic regions. Surprisingly, DNA methylation related to active genes was more abundant in transcribed regions than in promoters in both Arabidopsis [114] and rice. The magnitude of methylation within the gene body was negatively correlated with transcript elongation performance in Arabidopsis [114]. It's reasonable to assume that rice has a similar principle. Abiotic stresses may end up causing changes in DNA methylation levels, which may be posted a link to chromatin remodeling and stress-responsive gene transcription regulation. Genomewide analyses in several plant species reveal a global methylation readjustment in response to stress, owing primarily to demethylation [115–117]. Because conserved patterns were observed between different genotypes or tissues, the AFLP-based methylation-sensitive approach (MSAP) demonstrated that some of these improvements (methylation/demethylation) are site-specific. This method is best suited to CG methylation analysis. Other studies, on the other hand, have found that stress induces transcriptional induction of silent loci without a loss of DNA methylation, but instead a decrease in nucleosome occupancy [118, 119]. Elevated expression of the AtHKT1 gene, which encodes for a vacuolar Na +/H+ transporter, was also linked to lower DNA methylation in the Arabidopsis met1-3 mutant. The methylation pattern of a putative small RNA target region in the AtHKT1 promoter is required for the differential expression of this gene in roots and leaves, which may influence salt sensitivity and response [120]. Two *Laguncularia racemosa* species that grow in salt marsh and riverside habitats had different global DNA methylation patterns [121]. It's possible that epigenetic variation plays a role in helping plants adapt to different environments under natural conditions. It's possible that epigenetic variation plays a role in helping plants adjust to different environments under natural circumstances. Several MTases have been defined in rice, with such microarray data indicating that some are found to be elevated preferentially even during the commencement of floral organs [81, 122].

#### *Techniques against Distinct Abiotic Stress of Rice DOI: http://dx.doi.org/10.5772/intechopen.105808*

Furthermore, during the booting and heading stages, increased levels of methylation in rice leaves were detected (as ascertained by MSAP) than during the tillering stage [116]. As a result, drought-induced demethylation levels were higher at the tillering stage than it is at the booting and heading stages [116]. Changes in DNA methylation may differentially modulate response of plants to abiotic stress across the whole of development, according to these studies.

#### **4.2 Abiotic stress and histone modifications**

Nuclear DNA is packed and organized in eukaryotic cells in affiliations with a histone protein core-forming nucleosome, is also one of the chromatin's structural units. Combinations of histone variants and covalent modifications of histone tails, also including acetylation, methylation, phosphorylation, ubiquitination, biotinylation, or SUMOylation, resulted in changes in nucleosome structure. Assemble an integrated histone code that has been linked to gene expression regulation [106, 123, 124]. Depending on which lysine is methylated and how many methyl groups are added, the methylation process of lysine residues on histone H3 has been linked to transcription activation or repression [96, 105, 106]. Histone H3 lysine four trimethylation (H3K4me3), for example, has been linked to euchromatin and gene activation in maize, revealing inactive gene sequences not found in transposons [125]. Histone H3 lysine 9 dimethylation (H3K9me2) has been accompanied with transposons in Arabidopsis as an indication for heterochromatin and repressed transcription [105, 126]. H3K9me3 and H3K27me3 are, on the other hand, abundant in euchromatic genes [105]. Biotinylation and sumoylation, two other histone modifications, have been linked to gene repression [106]. In terms of histone lysine acetylation, strong acetylation has been linked toward a more relaxed chromatin structure that promotes transcription, so even though weak acetylation has been linked to chromatin densification and gene silencing [107, 127].

Histone acetyltransferases (HAT) and histone deacetylases enforce histone lysine acetylation (HDAC). In plants, there are four major classes of HDAC encoding genes [128, 129], with the HD2 class being the only one [109]. In hybrid rice, the OsHDT1 gene is involved in genetic variations [111]. There are at least 19 HDAC genes in the rice genome, and most of them are differentially regulated by different abiotic stress conditions [12, 112]. Most rice HDAC genes were exceptionally responsive to drought or salt stresses, pretty much exclusively through transcriptional repression, thus according microarray data [112]. As a result, abiotic stresses may regulate the transcription of chromatin modifier enzymes. Down regulation of HDAC may be taken into account to allow the induction of stress-responsive genes [12, 130]. Gene expression regulation has been coupled to histone modifications and DNA methylation crosstalks. In Arabidopsis, for example, the loss of DNA methylation in the ddm1 mutants was associated with low levels of dimethylation of histone H3 at lysine 9 (H3K9me2) [127, 131, 132]. A SUVH [Su(var)3–9 homologs] protein that plays in H3K9 methylation has been discovered to directly bind to methylated DNA, revealing the existence of a self-reinforcing feedback loop for DNA and histone methylation preservation in this species [133]. Recently, several rice SUVH genes with a deduced role in heterochromatin formation were revealed [134]. Through DNA methylation and H3K9me3, some of these have been shown how to resolve retrotransposon repression [74, 135].

Furthermore, most protein-coding genes with methylated DNA in rice are associated with H3K4me2 and/or H3K4me3, and when H3K4me3 is present, the repressive impact of DNA methylation on gene expression is reduced. Due to the rapid

technological advances such as chromatin immunoprecipitation (ChIP) and genomewide sequencing, the consequence of abiotic stresses on the genome-wide landscape of histone modifications is beginning to be deciphered (ChIP-Seq). Submergence stress in rice resulted in a decline in H3K4me2 levels, an increase in H3K4me3 levels, and a gradual increase in H3 acetylation at the ALCOHOL DEHYDROGENASE 1 (ADH1) and PYRUVATE DECARBOXYLASE 1 (PDC1) genes' 5- and 3-coding regions [136]. These changes were associated with increased ADH1 and PDC1 expression as a result of stress [136]. Cold stress, on the other hand, lessened H3K27me3 in the promoters of two cold-responsive genes, COLD REGULATED 15A (COR15A) and GALACTINOL SYNTHASE 3 (ATGOLS3), whereas salt stress increased H3K9ac, H3K14ac, and H3K4me3, while depleting H3K9me2 at stress-responsive genes [60]. These observations show the importance of histone code plasticity in transcriptional regulation during plant responses to various abiotic stresses when taken together.

#### **5. Drought tolerance's molecular mechanism**

Drought stress could severely limit rice production, leading to significant financial losses. It has become a more serious issue as the world's temperature rises. In light of current and projected global food demand, it is critical to prioritize increasing crop productivity on drought-prone rainfed lands. Drought-tolerant rice varieties are intended to address the assembly target in rainfed areas, and genetic improvement of rice for drought tolerance should also be a high priority theme of research in the next two decades. Breeding for drought tolerance could be an interesting challenge. The present study would be severely hampered by the complex nature and multigenic control of drought-tolerant traits.

Environmental drought impulses are intercepted by membrane sensors, which are still heavily portrayed. The signals are then transmitted via various signal transduction pathways, resulting in the outflow of drought-responsive attributes with effective gene functions and drought tolerance [98, 137]. Drought is a complex phenomenon, making it difficult to comprehend [73, 138]. As a result, hybridization and selection strategies could not provide precise drought tolerance results. Using DNA markers in molecular studies, at the other hand, can append the procedure by providing precise outcomes. These molecular markers are also useful for identifying drought-tolerant germplasm in a mass and using it to improve crops. Many studies have been conducted in order to identify some qualitative trait loci (QTL) linked to various traits [139, 140]. DNA studies based on marker-based phenotyping were the very first methodologies used to distinguish genes associated in rice drought resilience. Despite the progress, only a few traits have been officially approved for drought resistance [138, 141]. Molecular breeding can improve crop varieties, and yield assortments, produce productive, safe harvests, and also have high agronomic credibility in this way.

#### **6. Rice drought tolerance genes and transgenic approaches**

Many remarkable genes are highly expressed in rice after exposure to drought varieties, with approximately 5000 genes upregulated and 6000 genes downregulated [142, 143]. These genes can be divided into three categories: membrane transport, signaling, and transcriptional regulation [140, 144]. Many important genes/transcription factors are expressed differently in rice and are used to create transgenic plants

*Techniques against Distinct Abiotic Stress of Rice DOI: http://dx.doi.org/10.5772/intechopen.105808*

for drought strains [73, 140]. The majority of the genes regulated by drought are ABA-independent, as are the ABA-independent regulatory requirements that manage rice's drought tolerance mechanisms [131, 145]. OsJAZ1 has also been shown to mitigate drought tolerance in rice by impairing ABA signaling, which synchronizes plant responses to expansion and success under drought stress [12]. Osmoregulation and late embryogenesis abundant (LEA) proteins, which confer terminal drought tolerance in rice, are also linked with a number of genes [137, 140]. In transgenic rice, the gene DRO1 causes root elongation and deeper rooting. In rice under water deficit conditions, other genes like as OsPYL/RCAR5 and EcNAC67 induced leaf water content, delayed leaf rolling, improved growth parameters mass, and stomatal regulation [146, 147]. Over expression of OsDREB2B, CYP735A, and OsDREB1F [82, 148] pronounced the DREB2-like gene OsDRAP1 conferring drought tolerance in rice accelerated root morphological diversifications in rice under drought strain. Increased grain yield in rice under drought is critical, and it can be achieved by using transgenic approaches to start introducing genes like OsNAC5 [112], OsbZIP71 [51], OsWRKY47 [149], OsbZIP46 [150], and OsNAC10 into the crops. The WRKY genes play an important role in plant improvement by responding to drought strains and can be used to create drought-tolerant transgenic plants [151, 152]. Several genes were investigated using transgenic approaches to confer drought tolerance in rice grown in a research lab or glasshouse conditions. However, those genes must be investigated further.

#### **7. The role of micro RNA in rice drought tolerance**

Micro RNAs (miRNAs) are small noncoding regulatory RNAs that modulate gene expression during abiotic stress, as has been acknowledged [151, 153]. These 20–24 nucleotide long proteins control gene expression at the post-translational level [140]. Several miRNAs have also been found to alter gene expression in rice by up- up-and down-regulation, which confers drought tolerance [134, 154, 155]. Arabidopsis [156] was the first to reveal the expression of miR393, miR319, and miR397 in response to drought, and rice control transcriptional factors OsAUX1 and OsTIR1 confer tiller number increment, early flowering, and auxin increased sensitivity [157].

Rice does have 30 miRNAs, 11 of which are down-regulated and eight of which are up-regulated under drought stress [126]. Under drought stress, MiR160 and MiR167 regulate the expression of the ARFs gene, that further regulates early auxin response [140]. Through ROS homeostatic genes, DST- amiRNA enhances drought resistance by increasing stomatal closure and decreasing stomatal density [155, 158]. Over expression of the UDP-glucose-4-epimerase gene, facilitated by OSA-miR169-3p and Osa-miR166e-3p, regulated root development and cell wall biogenesis, along with carbohydrate metabolism [154, 159]. Ten miRNAs (miR531, miR827, miR8175, miR977, miR6300, miR1861, miR440, miR9773, miR3982, and miR1876) were recently discovered to be regulated under drought stress and confer tolerance attributes in traditional rice land races [160]. Drought tolerance can be accomplished by gene manipulation of these miRNAs. As a consequence, miRNAs regulate many drought tolerance responses, potentially enhancing the development of drought-tolerant rice genotypes.

Natural rice genotypic variation could be investigated to seek novel genotypes with a drought-tolerant trait of interest and a gene/locus affiliated with them. Droughttolerant rice varieties can be developed and use these novel genotypes in traditional breeding programs using marker-assisted selection. The breeding program's aim is to create high-yield lines with improved performance parameters, as well as to

commercialize the cultivars. Numerous researchers have investigated the progeny of drought-tolerant genotypes in the past [139, 161, 162], but the overall performance has been far lower than expected due to the difficulty in finding suitable donors with a higher tolerance level, as well as the environment-specific nature of the genotypes. The majority of marker-assisted breeding approaches for improving drought-tolerant rice varieties have always been carried out at the International Rice Research Institute in the last decade [163]. In India [145, 160], the Philippines [159, 161], and Malaysia, several works on marker-assisted progression of popular varieties were carried out [164]. Several QTLs for drought tolerance in rice have been incorporated into leading cultivars using marker-assisted breeding techniques [162]. By using a marker-assisted backcrossing approach, they were able to successfully incorporate QTLs such as qDTY9.1, qDTY2.2, qDTY10.1, and qDTY4.1 in the high yielding IR64 variety [27]. They also developed the drought-tolerant elite Malaysian rice cultivar MR219 by pyramiding three QTLs, qDTY2.2, qDTY3.1, and qDTY12.1. They created TDK1 rice varieties with three QTLs for high yield in drought conditions (qDTY3.1, qDTY6.1, and qDTY6.2). Drought has only received attention as a constraint, and no effective methods for developing drought-tolerant rice varieties have yet been successful. Farmers prefer to grow high-yielding cultivars with better grain quality but are drought-prone, or traditional drought-tolerant varieties with low yield. As a result, more effort will be needed in the future to develop unique rice varieties that can produce high yields in drought and acclimate to a range of adverse climatic conditions.

#### **8. Salt tolerance**

Salt stress may cause progress in multiple physiological and metabolic pathways depending on the severity and duration of the stress, leading to a reduction in rice productivity [50, 120, 165, 166]. To estimate the phenotypic coefficient of variation (PCV), genotypic variance (GCV), broad-sense heritability, and genetic Advance, genetic characterization of salt tolerance-related traits is required (GA). Assume there is sufficient variation in the germplasm for salt tolerance-related attributes with greater heritability and genetic advance. As a result, by utilizing salt-tolerant landraces/germplasm in breeding programs, it may be possible to improve the personality characteristics associated to salt tolerance in rice. The low Na-K ratio was controlled by both additive and dominance gene effects, according to a genetic component analysis (GCA) study [89, 167]. The results of the amalgamating ability analysis show that both general combing ability (GCA) and specific combining ability (SCA) effects are important in understanding salt tolerance genetics. They also revealed that selection for common heritable traits like the Na-K ratio could have been built in subsequent generations under controlled conditions to reduce environmental effects. Additive gene action is linked to narrow-sense heritability [168, 169], and additive gene action may enhance or fix the action of the desired combination of genes. As a result, early generation preference for salinity tolerance is possible. Recognizing the gene action in rice that generates salt tolerance will support future breeding efforts [112, 170].

#### **9. Submergence**

Submergence is among the most important abiotic stresses in rice-growing areas prone to flash floods [171]. Submergence tolerance is a necessary trait for rice in

#### *Techniques against Distinct Abiotic Stress of Rice DOI: http://dx.doi.org/10.5772/intechopen.105808*

rain-fed lowland conditions (Oryza sativa). A significant gene known as Sub1 is mainly accountable for this trait. Indica cultivar FR13A is a highly tolerant rice variety that can withstand complete submersion for up to two weeks. Near the centromere of chromosome 9, they have a substantial quantitative trait locus known as submergence1 (Sub1) [157, 172, 173]. Background genetic information for submergence tolerance was well documented out of some research using QTL mapping and map-based cloning techniques [172, 174, 175]. Because of the great specificity of contemporary rice varieties, salt stress is a significant constraint in many rice-producing areas. One of the most severe abiotic stresses restricts rice growth and development of plants, resulting in yield reduction of more than 50% [176, 177]. Salinity tolerance is multifaceted, involving a range of biological mechanisms such as sodium exclusion from root system. Salinity is estimated to affect over 150 million hectares of current and potential rice land in tropical and subtropical regions of the world [176, 178]. Despite the fact that rice is the source of nutrition for half of the world's population, it is more susceptible to salt stress than other cereals [179, 180].

If rice plants are immersed in water for even more than five days resulting from environmental or abiotic stress, they become the deepest submergence-tolerant contributors and are widely used by rice breeders. The pyramiding of submergence and salinity tolerance is especially important in coastal areas where floodwaters are frequently saline. On chromosome 1, a major salinity QTL has recently been introduced and characterized [79, 170]. Although many QTLs may be necessary to accomplish adequate salinity tolerance in the field, additional QTLs for vegetative growth and reproductive-stage salinity tolerance may be compelled to provide salinity stress defense during in the rainy season. Using molecular marker technologies to stack multiple tolerance genes/QTLs into single rice varieties provides breeders with a once-in-a-lifetime chance to advance tolerant cultivars more faster for specific environments [172, 181]. There are various types of biotic stresses. The Sub1 gene, which is managed to find on chromosome 9 of rice, is wellknown for conferring submergence tolerance intolerant rice cultivar FR13A and its progenies [157, 173, 175].

#### **10. Modern breeding techniques**

Using a variety of innovative tools, genomic assisted breeding (GAB) is routinely used to improve the genetics of salt-tolerant rice. Genomic breeding, forward breeding, rapid breeding, and haplotype-based breeding are all examples of genomic breeding [115, 182]. 5G breeding methods are used to improve genotype productivity by improving genome sequence availability (genome assembly), characterization of germplasm at the genomic and morpho-agronomic level, genomic detection and understanding function, genomic breeding, and genome editing [118, 183, 184]. These could be used to improve efficiency and accuracy of breeding for complex abiotic stress tolerance traits. Through SNP-based speed breeding, SNP-assisted introgression of the hst1 (Salt-tolerant 1) gene enhanced salt tolerance in a high-yielding rice variety [41]. The emphasis in contemporary breeding is on data-driven parent selection. Genetic technique, trait categorization through diagnostic trait markers, genomic screening, and breeding value estimation are all applied to local and exotic germplasms. Native germplasm with low yield potential may contain traits of interest (ToI) like salinity and submergence tolerance, aroma, and resistance to disease. Then, in order to develop pre-breeding materials, ToI is first transferred to a privileged

background with a higher yield. The elite line with the desired traits is then used to accomplish the product profile for breeding [185].

#### **11. Underpinning defense systems is a regulatory network**

The five general defense mechanisms are coordinated by a delicate regulatory network composed of numerous signaling molecules and gene regulation indicators in the face of abiotic stresses. We'll look at some of the more well-known ones here. Stress hormones (ABA), reactive oxygen species (ROS), hydrogen sulfide (H2S), nitrogen oxides (NO), polyamines (PAs), phytochrome B (PHYB), and calcium interplay with others at various levels, synergistically or antagonistically, to establish a specific directive for downstream effectors, especially transcription factors (TFs), to alter gene expression and protein/enzyme activities in a specific pattern, thereby launching a proper response. Hormones of Stress Phytohormones like ABA, ethylene (ET), jasmonic acid (JA), and salicylic acid (SA) are important organizers of systemic stress defense, and they work together in the complex hormonal signalosome [3]. S Notably, melatonin, a universal multi-regulatory molecule across all life forms, is increasingly recognized as a potent stimulator against stress in the plant. One notable aspect of this yet-to-be-licensed phytohormone is that it operates as if a commander of other phytohormones [28, 186]. Nevertheless, ABA is the main stress hormone, which not only extensively interplays with other phytohormones but with all following signaling molecules. Particularly, components of all biochemical defenses remarked above can be mobilized by ABA, including cuticular waxes [187, 188], HSPs [95], Pro [120], antioxidants [189, 190], and RS detoxifying enzymes [137, 191]. Stress stimuli can rapidly trigger de novo synthesis of ABA from oxidative cleavage of β-carotene, with 9-cisepoxycarotenoiddioxygenase (NCED) is the rate-limiting enzyme.

ABA can also end up causing organic changes in order to cope with stressful situations. Probably one of the best is the closing of stomata, tiny pores formed by paired guard cells that allow gas exchange and thus minimize water loss from transpiration and thus mitigate dehydration. The activity of ion channels and aquaporins is modulated to achieve this movement. As a result, the outflow of K+ and anions pulls water out through osmosis, causing guard cell shrinkage, which is ensured by actin filament reshuffling [48, 192, 193]. Another unique feature is seed dormancy, which enables seeds to avoid existing stresses and wait for ideal germination conditions [194–197]. Endogenous ABA elevation and exogenous ABA addition both help plants cope with a wide assortment of stresses. For in-field applications, the development of ABA analogs with greater security is promising. Overloaded ABA signaling, such as that caused by over expressed NCED or constitutively active PYLs, can cause vegetative growth retardation and grain yield reduction [81, 198], while foliar ABA spraying can cause leaf senescence in rice (*Oryza sativa*) and maize (*Zea mays*). As a result, it's critical to gain a better understanding of ABA homeostasis, its wide range of biological effects, and crosstalk with other pathways in order to create crop stress tolerance strategies that don't sacrifice economic traits. ROS are continuously produced in plant cells as byproducts of aerobic metabolism in chloroplasts, mitochondria, and peroxisomes, among other cytoplasmic organelles. However, as noted previously, it is not only toxins that can only be removed but also signaling molecules that are required for a variety of physiological processes, including stress resistance. A ROS signal is shaped by a variety of factors, including dose, duration, source, and type

#### *Techniques against Distinct Abiotic Stress of Rice DOI: http://dx.doi.org/10.5772/intechopen.105808*

[199, 200]. The extensive crosstalk between H2O2 and other signaling molecules has been reviewed, including ABA, ET, JA, SA, NO, and Ca2+ [136]. Ca2+ influx, in particular, is a notable event in H2O2 signaling, which modulates H2O2 levels by activating producing (e.g., RBOHs) or scavenging enzymes. Because of the degradation of PAs, H2O2 is strongly intertwined to them [189]. Numerous enzymes in the apoplast can produce ROS on their own in response to increases stimuli. Respiratory burst oxidase homologs are the most common (RBOHs). These PM-localized NADPH oxidases are activated by Ca2+ binding to the EF-hand motifs in the N-terminal cytosolic region, in combination with phosphorylation by receptor-like cytoplasmic protein kinases, for example (RLCKs). The MAPK signalosome plays a key role in H2O2 intracellular signaling, which is triggered by metabolic disturbance and/or apoplastic discharge. It is a pertinent stress signaling divergent node. MAPK kinase (MAPKKK), MAPK kinase (MAPKK), and MAPK make up each phosphorylation cascade. Many MAPKs and cascades have been discovered to decode the H2O2 signal in different ways, but how the precision is determined is still unspecified. The MAPK pathway, on the other hand, functions upstream of ROS by modulating the activities of RBOHs, either positively or negatively [201, 202]. Two MAPKs, MPK3 and MPK6, are aligned in defensive response in Arabidopsis, and can eventually elevate the levels of defending factors such as GSTs and HSPs. H2O2 can activate them though the ANP1, a MAPKKK, and other kinases such as oxidative signal-inducible 1 (OXI1), which is required for full activation of MPK3/6, and NUCLEOTIDE DIPHOSPHATE KINASE 2 (NDPK2), which can interact with MPK3/6 and potentiate its activities. NPK1 (tobacco ANP1 ortholog) and AtNDPK2 imparted tolerance to a variety of stresses, which would include salt and extreme temperatures, on transgenic plants. MKK2, a MAPKK in another anti-stress cascade, MEKK1-MKK2-MPK4/6, did this very same [44, 203].

#### **12. Nitric oxide and hydrogen sulfide**

At low concentrations, toxic secondary gaseous molecules, H2S and NO, show impressive powers in providing protection against a wide range of stresses, just like H2O2. Many anti-stress mechanisms are shared by the two. Both, for example, can reduce salt toxicity by increasing Na+ exclusion by unlocking SALT OVERLY SENSITIVE 1 (SOS1), a PM Na+/H+ antiporter [155, 204]. The ability to combat oxidative stress should be their most prominent role, as both of them not only act as antioxidants in their own right but can also repress ROS production and activate ROS elimination. H2S can be assimilated into GSH as a source of sulfur, resulting in an increase in this important R.S. scavenger [20, 205]. The first recognized gasotransmitter, carbon monoxide (CO), is also an elicitor. CO research on this topic, however, is still in its infancy [114]. During various stresses, the three R.S., H2O2, H2S, and NO, are typically available and together demonstrate intricate interactions depending on the context. It can ablate NO accumulation and aid stomata opening with the antagonist's face [176]. NO, on the other hand, is a mediator of H2S in the promotion of adventitious root development [103, 206], which can boost O2 uptake and thus reduce hypoxia stress caused by waterlogging [33, 173]. H2S, on the other hand, is a NO mediator in maize heat tolerance and bermudagrass (Cynodon dactylon) cadmium resistance [116]. Furthermore, with the addition of another R.S. player, MG, the situation will become even more complicated. Their chemical reactions add to the complexity, blocking one another while also forming new compounds with physiological implications, such as

peroxynitrite (ONOO) formation by NO and O2 nitrosothiol formation by NO and H2S. There is even a competition between M.G. and GSH participants. Thiol modification, such as oxidation by H2O2, sulfhydration by H2S, nitrosylation by NO, glycation by M.G., and glutathionylation by GSH, can directly modulate protein function [152, 176]. Even though GSH is a derivative of H2S but a cocktail mixer of the other three, it provides additional pathway for their crosstalk. Besides this, the MAPK pathway is likely to be a point of convergence for the four signaling RS. Two gasotransmitters originate from various sources and are natural products of botanic metabolism. Cysteine desulfhydrases (DES) and sulfite reductase (SIR) for H2S, along with nitrate reductase (NR) and a nitric oxide synthase (NOS)-like a pathway for NO, though a truthful NOS has yet to be discovered in plants. PAs have long been acknowledged for their protective role in plant response to a variety of stresses [75, 165, 189]. Indeed, protein expression of every PA biosynthetic enzyme, such as arginine decarboxylase (ADC), spermidine synthase (SPDS), and S-adenosylmethionine synthase (SAMS), simultaneously breakthroughs stress tolerance mechanisms in various plant species, seeking to make exogenous PA application unnecessary [8, 75, 207]. Dissecting the mechanisms underlying PAs' anti-stress effects is complicated. Because of their polycationic nature, RS-scavenging property, and signaling function, it's conceivable that these multifaceted substances contribute to stress defense in a multitude of ways. Protonated PAs, for instance, not only take an active part in ion homeostasis at the physiological PH, but they can also bind to negatively charged molecules such as membrane lipids and integral proteins, which helps mitigate stress-induced membrane damage. PAs not only convey with ABA in stress signaling, but they can also induce rapid NO production. PAs are linked to other stress-related metabolites, such as Pro and ET, which are linked to PA anabolism and H2O2. Furthermore, GABA is produced as a result of PA catabolism [8, 75]. PAs also fall into the Janus category [168], which must be taken into account in their practical implementation due to the obvious specific link with H2O2. Phytochromes PHYB, a modest family of chromophore-containing proteins that serve as photoreceptors to perceive red (R) and far-red (FR) light, is emerging as a negative regulator in stress tolerance. PHYB's signaling activity undergoes reversible photoconversion, that also involves R activation and FR deactivation in response to protein aggregation. The Pr (R-absorbing) form of nascent PHYB is inactive. Dimeric PHYB will translocate into the nucleus once converted to the bioactive Pfr (FR-absorbing) form, where it can interact with and trigger the proteasomal degradation of phytochrome interacting factors (PIFs), a subfamily of basic helix-loop-helix (bHLH) TFs, to remodel the expression profile of thousands of light-responsive genes, guiding photomorphogenesis [131, 208–210].

PHYB was discovered recently to be a thermosensor [46, 71]. Warm ambient temperatures can effectively induce elongation development, which is phenocopied by shade avoidance and is controlled by the PHY-PIF cascade. Indeed, thermal (or dark) reversion, which is independent of light but sensitive to temperature, can end up causing to spontaneously revert to warm temperatures, especially at night, can relieve PIF4 repression by fastly unplugging PHYB and increasing PIF4 transcription, resulting in thermo morphogenesis. Because active PHYB was discovered to interact with PIF-binding sites (G-boxes) at PIF4-targeted promoters, it was tried to suggest that it could play a co-repressor or competitor role in gene regulation with PIF4 [211].

In case of light and temperature-induced growth, another cascade downstream of PHYB involves the RING E3 ligase CONSTITUTIVE PHOTOMORPHOG for degradation to depress the growth genes. ENIC 1 (COP1), and the TF ELONGATED HYPOCOTYL 5 (HY5), with COP1 ubiquitinating. Notably, COP1 can indirectly potentiate the activity of PIF4, thereby connecting the two branches [211, 212]. Plant

#### *Techniques against Distinct Abiotic Stress of Rice DOI: http://dx.doi.org/10.5772/intechopen.105808*

Defenses against diverse abiotic stresses, ROS and GOLS are essential for raffinose synthesis via binding upon oligomerization to the heat shock elements (HSEs) located in their promoter regions. Therefore, HSFs are capable of launching three general defensive systems. Surprisingly, plants have a powerful and relatively variable number of HSFs. Nonetheless, genetic manipulation of HSFs remains a viable option for conferring multiplex sensitivity to plants [83, 89]. It's also worth noting that HSFA6b and HSFA3 allow ABA to play a legitimate role in HS response. The former is stimulated directly by AREB1, whereas the latter is activated downstream of DREB2A, a destination that both AREB1 and HSFA6b share [95]. Another member of the class 2 DREB family, DREB2C, is also an HSFA3 activator [96]. It can also work against salt toxicity by trying to target chaperones like COR15A and DESICCATION-RESPONSIVE PROTEIN 29A (RD29A) [173, 213]. Interestingly, DREB2C from *Ammopiptanthus mongolicus* and evergreen broadleaf shrubs living in the desert was newly reported to up-regulate 11-pyrroline-5-carboxylate synthetase initiates Pro biosynthesis, as well as FADs that catalyze 18:3 production, thereby promoting Arabidopsis endurance to drought, freezing, and heat [188]. A safe conclusion can be drawn that this single TF governs all four cellular general defenses. Furthermore, a computational analysis of the Arabidopsis DREB2C promoter revealed a variety of elements that are sensitive to ABA (ABRE), MeJA, SA (TCA), heat (HSE), low thermal conductivity (LTR), and stress (TC rich) [206, 214], implying that TF is a central point in stress signaling. It is an ABA-inducible transcription factor that can delay seed germination by exerting good feedback on ABA biosynthesis by trans-activating NCED9. The enzyme genes in Arabidopsis are primarily controlled by TFs from two families: the AP2/ERF superfamily's SHINE 1 (SHN1), -2, -3, and DEWAX, as well as the R2R3-MYB family's MYB16, MYB30, and MYB106. MYB96, on the other hand, is a repressor of cutin synthesis, whereas MYB41 is an inducer of wax manufacturing [187]. MYB96, in specific, is a key player in ABA signaling, which helps regulate the whole of wax metabolism. Not only elongation and modification enzymes, but also ABC transporters and nsLTPs are directly or indirectly targeted by at least one isoform gene [103, 215]. MYB96 transgenesis continued to improve Arabidopsis drought and freezing tolerance, but it also caused significant dwarfism [99, 215]. Wax production 1 (WXP1), an ERF member from *Medicago truncatula,* may be a better candidate, as it was the one responsible for the previously mentioned observation that higher n-alkane and predominant alcohol contents results in improved viability under drought and freezing without interfering with transgenic Arabidopsis growth [99].

#### **12.1 E3-Ubiquitin ligases, water stress responses**

At some point during the plant growth cycle, climate change is threatening more than 20 million ha of rain-fed lowland rice (12 percent of total rice area and about 20% of global production) [216–218]. As extreme events become more familiar, water scarcity is expected to worsen, with yield losses of up to 81 percent [161, 216]. Drought has a particularly negative impact on seedlings (2 to 3 weeks old) and reproductive tiers (pollen-development stage) [207, 211], delaying flowering and lowering yields [186, 219]. Drought leads to inefficient water use, stomatal closure, photosynthesis impairment, and decelerated cell division and expansion [186]. As a proactive approach to cope with water deficit, plants directly influence divergent genetic and metabolic methodologies, such as cellular osmotic potential, stomatal aperture, impaired antioxidant, phytohormones, and chlorophyll content. This really is likely to result in the adaptation and maintenance of their physiological activity under drought

conditions [220]. As a result, it's critical to comprehend the molecular mechanisms underpinning rice drought response, particularly the role of ubiquitination, and to apply these skills to the development of drought-tolerant crop varieties. Over the last two decades, several rice E3-ubiquitin ligases and their interacting proteins have been linked to drought response in plants. To fully comprehend their interactome and function, however, more research is required.

#### **12.2 E3-Ubiquitin ligases in salinity responses**

High salinity, drought, and ABA induce Oryza sativa salt-induced RING finger protein 2 (OsSIRP2), which encodes a RING-type E3-ubiquitin ligase that binds specifically to the nucleus of rice protoplasts for both control and high-salinity conditions. OsSIRP2 been shown to confer tolerance to salinity and osmotic stresses in Arabidopsis when overexpressed [221]. OsSIRP2 had been shown to interact with the rice transketolase 1 (OsTKL1) in the cytoplasm, causing it to be targeted for degradation by the UPS. OsTKL1 is a member of the transketolase family, which is involved in the Calvin cycle's oxidative pentose phosphate pathway. It is observed in the chloroplast and is considered necessary for the regeneration of ribulose 1,5-bisphosphate [189, 191]. In tobacco, reduced TKL1 activity causes photosynthesis to be inhibited [165]. The enzymatic activity of transketolase is also compelled for the stress-induced manufacturing of cytosolic NADPH, which is an important component of a plant's defense against ROS-induced damage [214]. It is critical to perform functional characterization of these two proteins in rice to stronger understand the mechanism of OsSIRP2 and the physiological implying of the OsSIRP2–OsTKL1 interaction in salt (and drought) stress responses, which include photosynthesis performance. It's crucial to figure out if and how OsSIRP2's deleterious regulation of OsTKL1 promotes stress tolerance. Finally, because OsSIRP2 did not change specificity under salt stress, the translocation of OsSIRP2 from the nucleus to the cytoplasm to ubiquitinate OsTKL1 raises a question of the underlying mechanism driving this export.

#### **12.3 Response to low temperature**

OsPUB2 and OsPUB3 are homologous U-box type E3-ubiquitin ligases that have recently been identified as positive regulators of rice's cold stress response [222]. Minimum temperature, drought, and salt stress upregulate OsPUB2, whereas OsPUB3 expression is unaffected by any of the aforementioned stresses. Overexpression of OsPUB2 or OsPUB3 in rice plants, on the other hand, confers a cold-tolerance phenotype in the result of enhanced survival rates, total chlorophyll, and diminished ion leakage. Furthermore, gene expression analysis reveals that under both control and cold conditions, overexpression of the two OsPUB genes is linked to upregulation of cold stress-inducible genes also including glutamate decarboxylase (GAD), WRKY77, and multidrug resistance protein 4 (MRP4). Furthermore, both were crafted more stable by the cold. Both E3-ligases are found in small cytosolic punctate bodies in Nicotiana benthamiana leaf protoplasts' subcellular localization. However, it remains to see whether those two homologous E3-ubiquitin ligases collaborate to confer cold tolerance to rice plants and, if so, which target protein(s) are used to achieve this tolerance. Furthermore, the OsPUB2 mutants should really be studied for phenotypic analysis under a variety of stresses. The RING-type upregulation of osmotically responsive gene 1 (OsHOS1) is another E3-ubiquitin ligase that modulates rice plant response to cold stress [204]. OsHOS1 proteins bind to the nucleus's Inducer of CBF

Expression 1 (OsICE1) and direct it to be broken down by the UPS. Stress-responsive transcription factor dehydration responsive element (DRE)-binding protein 1A (OsDREB1A) transcript thresholds and protein levels of OsICE1, a master integrator of cold stress, are higher in OsHOS1-silenced (RNAi) lines.

#### **13. Conclusion**

Plants are tough and have evolved strategies accordingly to a variety of environments over the course of their evolution. As a result, understanding the molecular mechanisms that underlie stress tolerance is essential for improving crop stress tolerance as the impact of abiotic stresses grows as a result of global climate change. Distinctive cutting-edge/modern breeding strategies are aggregated in the holistic breeding approach. Genotypes could help the farmers cope with rising temperatures, increase varietal turnover, and help meet the challenges of abiotic stress-prone ecosystems by increasing productivity and ensuring food security. In addition, rice cultivation areas in slightly elevated abiotic stress-prone areas under which salt stress is extremely crucial for rice production during pre- monsoon season will be decided to expand in various rice-growing countries, including Bangladesh. Furthermore, the stress associated with HNT must be highlighted because this stress could pose a threat to food security in areas where rice is a staple food.

#### **Conflict of interest**

No conflict of interest.

### **Author details**

Ananya Prova1 \* and Md. Saeed Sultan<sup>2</sup>

1 Department of Plant Pathology, EXIM Bank Agricultural University Bangladesh (EBAUB), Chapainawabganj, Bangladesh

2 Department of Plant Pathology, Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU), Gazipur, Bangladesh

\*Address all correspondence to: provaananya16@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[220] Ding Y, Dommel M, Mou Z. Abscisic acid promotes proteasomemediated degradation of the transcription coactivator NPR 1 in Arabidopsis thaliana. The Plant Journal. 2016;**86**(1):20-34

[221] Cao S, Du XH, Li LH, Liu YD, Zhang L, Pan X, et al. Over expression of Populus tomentosa cytosolic ascorbate peroxidase enhances abiotic stress tolerance in tobacco plants. Russian Journal of Plant Physiology. 2017;**64**:224- 234. DOI: 10.1134/s1021443717020029

[222] Bernard A, Joubès J. Arabidopsis cuticular waxes: Advances in synthesis, export and regulation. Progress in Lipid Research. 2013;**52**:110-129. DOI: 10.1016/j.plipres.2012.10.002

Section 4

## Cross Cutting Mechanisms of Plant Responses to Stresses

### **Chapter 14**

## Interactive Effects of Salinity, Drought, and Heat Stresses on Physiological Process and Selection Criteria for Breeding Stress-Resistant Cotton

*Volkan Mehmet Cinar, Serife Balci and Aydın Unay*

#### **Abstract**

The cotton crop is adversely affected by the combination of salinity, drought, and heat stress during all growth stages in cultivated areas. The negative impacts of salinity together with water scarcity on osmotic stress dramatically increased the sensitivity of reproductive development. After membrane integrity and signaling networks are depressed under stress at the cell level, the metabolic and physiological processes are disrupted in the next stage. The restricted root growth, ion and water uptake, phloem, photosynthetic and respiratory capacity, incompatible hormonal balance, and reduction in yield due to lower boll retention are the most important symptoms. The seed treatments and foliar applications of osmoprotectant and fertilization appear to reduce multiple stress factors in possible climate change conditions. The osmotic adjustment, antioxidative ability, electrolyte leakage in the membrane, and chlorophyll fluorescence are evaluated as selection criteria for improving genotypes. Direct selection of plants with high yield under stress conditions may increase the success of cotton breeding. It is important to know the molecular approaches and gene functions responsible for abiotic stress. In this chapter, the effects of high temperature, salinity, and drought on cotton plants and characteristics associated with tolerance were focused on cotton improvement. The classical breeding methods and molecular approaches should be combined for breeding new cotton varieties.

**Keywords:** breeding, high temperature, saline condition, stress physiology, water deficiency

#### **1. Introduction**

Agricultural areas with saline soils are estimated at 1100 Mha in the World. These areas are classified as saline (60%), sodic (26%), and saline-sodic soils (14%). Areas suffering from salinity are mostly in the Middle East, Australia, North Africa, and Eurasia. However, 20 to 50 percent of irrigated soils in arid or semi-arid climates

**Figure 1.**

*Cotton plants under saline soil (A), stunted cotton plants under drought conditions (B), and cotton plants under alkali soil (C).*

are salt-affected [1]. A report produced by Cotton 2040 emphasized that cotton affected by heat stress and drought will reach 40% and 50%, respectively by 2040 [2]. Although heat stress is a problem along with drought [3], cotton-growing face many stress factors such as drought (**Figure 1B**), high temperature (**Figure 2C**), salinity (**Figure 1A**), alkalinity (**Figure 1C**), and heavy metal contamination, which are seen together in the same ecology. A single stressor can play a predominant or protective role depending on stress resources [4]. The long-term effect of climatic change resulted in higher pH values with drought, unfavorable soil organic C, total N and P, and available N [5], and soil salinity due to increasing the sea level [6].

Cotton's response to stress is different at plant growth and development stages. Germination and seedling growth are adversely affected by high salinity and low temperature at the early stages in cotton (*Gossypium hirsutum* L.) [7, 8]. Although progenitors of cotton have spread in adverse conditions, modern cotton varieties are non-resistant to stresses from the squaring stage to boll retention stages, in which yield formation occurs [9]. Water deficiency [10–13] and heat stress [13–15], especially during flowering and boll formation can reduce yield and fiber quality. In many

#### **Figure 2.**

*A picture of a plastic tunnel under heat stress at 11.00 am–17.00 pm (A), non-affected plants by heat stress (B), and aborted/shed bolls affected by heat stress (C).*

*Interactive Effects of Salinity, Drought, and Heat Stresses on Physiological Process and Selection… DOI: http://dx.doi.org/10.5772/intechopen.105576*

areas of the world where cotton is cultivated, as in the Aegean, Mediterranean, and Southeast Anatolia regions of Turkey, maximum temperatures during the reproductive stage in July and August are above 40°C with low humidity and precipitation. The boll components such as seed and boll weight cannot be affected by short-term temperature changes, whereas the effects of sudden temperature rises during anthesis on seed number per boll are very high [16].

This review presents the physiological mechanism of stress tolerance and principles of genotype improvement, both classically and by genetic engineering in cotton. Also, it discussed the effects of agronomic management such as seed and foliar treatments to alleviate drought, salt and heat stress.

#### **2. Roles of osmolytes**

The stress-induced protein breakdown and amino acid synthesis caused the accumulation of amino acids in cells under abiotic stress conditions. The overall accumulation of amino acids has two different symptoms: cell damage and the beneficial effect of specific amino acids such as proline during stress acclimation [17, 18]. Drought, salinity, and heavy metals caused an increase in the accumulation of proline in resistant plants. Although proline was not synthesized in tobacco exposed to heat stress [19], the combination of heat and drought stress [20], and drought stress [21] induced an increase in the proline content of some cotton cultivars. In contrast with this finding, genotypic differences were non-significant for proline content under drought conditions [22].

The first approach to stress resilience is to restrict ethylene synthesis, which triggers the abscission of leaves and all reproductive organs under stress conditions [23]. Boll retention and yield increased by application of ethylene inhibitors such as aminoethoxyvinylglycine (AVG) or by downregulation of genes responsible for ethylene synthesis [24]. Abscisic acid (ABA), another growth-inhibitory hormone, is an important regulator of abiotic stress tolerance. The main function of ABA is to stimulate stomatal closure and gene expression to respond to the drought, salinity, and excessive temperature in the adaptive mechanism of plants [25, 26]. Under stress conditions, plants can alter their metabolism such as the synthesis of compatible solutes. In cell metabolism, the different unfavorable conditions increased the concentration of γ-aminobutyric acid (GABA) through enhanced activity of enzymes involved in GABA biosynthesis [27]. Under alkali stress conditions, the accumulation of GABA and putrescine in young and old leaves of cotton increased [28]. Polyamines such as putrescine, spermidine, and spermine have a protective role against salinity, drought and heat stress, and putrescine and spermine could be evaluated as selection criteria for stress-tolerant genotype breeding in cotton [29]. Exogenously applied polyamines increased the stress tolerance and yield under drought and salinity stress conditions in cotton [30, 31].

The osmoprotectant solutes such as glycine betaine (GB) are evaluated in three different ways. Firstly, the increase in GB under stress conditions is analyzed as biochemical. Secondly, the selection of the plants with high GB levels is used for improving the stress-tolerant cotton genotypes in conventional breeding and genetic engineering. Thirdly, the success of seed treatment with GB and foliar application of GB were examined in stress conditions. The accumulation of GB increased in transgenic crops, whereas foliar application of GB or seed dressed are common applications in plants where a certain amount of synthesis is insufficient [32, 33]. Generally

localized in the chloroplast, GB has an important role in protecting photosystem II, stabilizing membranes, and alleviating ROS (reactive oxygen species) damage [34] and chilling damage [35], enhancing tolerance to lead (Pb) [36]. Cottonseed coating with GB enhanced seed cotton yield by approximately 20% [37]. Also, combined foliar application with GB and salicylic acid (SA) increased the tolerance to salt stress due to an increase in the leaf gas exchange with positively correlated stomatal properties and stimulate antioxidant enzyme activity in cotton seedlings [38, 39].

#### **3. Heat stress**

On the cell basis, heat stress firstly induced structural changes in chloroplast protein [40] and plasma membrane [41, 42], and these changes stimulated cell elongation, expansion, and differentiation. ROS, as a signaling molecule, induced excessive MDA (malondialdehyde) synthesis by increasing lipid peroxidation, and membrane structure disintegrated under heat stress conditions [43–45]. Also, molecular chaperones and heat shock proteins (HSP) accumulate to protect the membrane integrity by the expression of the sHSP coding gene in leaves under drought and heat stress conditions [46]. Heat stress caused the accumulation of specific isoforms of activase in cotton leaves [47, 48]. Although the efficiency of Rubisco activase decreased under heat stress, an enzyme can develop an alternative function by relocating on a thylakoid membrane [49], and this contributes to the acclimation of photosynthesis during high temperatures in cotton [50]. In addition, the synthesis of phytohormones such as SA and jasmonic acid (JA) to respond to heat stress is enhanced by ROS and Ca2+ [51]. The association between heat stress and plant nutrition demonstrated that the fertilization containing some macro and microelements such as K, Zn, and B could be used to alleviate the harmful effects of higher temperatures. These elements play an important role in chlorophyll synthesis and delay senescence in cotton [52, 53]. Zn spray eliminates the adverse effects of heat stress in cotton [54].

The reproductive period is synchronized with many processes in cotton. Although square retention is less affected by heat stress [55], sexual reproduction is defined as both sensitive to high temperature and thermotolerant during flowering – boll retention depending on the time, length, and severity of stress in cotton [56–58]. Heat stress during this period adversely affected the development of both sexual organs and caused flower abnormalities such as small flower, elongated stigmas (**Figure 3A** and **B**) [55], gametophyte, pollen germination, and pollen tube growth [59, 60]. Abiotic stress limited fertilization by preventing pollen tube development [61]. The tapetum, the innermost layer of somatic cells in anther lobes, is responsible for microsporogenesis and secretion of enzymes for the release of microspores from tetrads [62]. Programmed cell death (PCD) induced tapetal degeneration by heat stress, resulting in male sterility [63]. The regulation of tapetal PCD and anther dehiscence were controlled by the GhCKI gene (*G. hirsutum* casein kinase I) in the heat stress tolerance of cotton [64].

The decrease in the photosynthetic capacity of cotton at a temperature above 32°C reduces the accumulation of sucrose [65]. The decrease in sucrose accumulation adversely affected the fiber quality by inhibiting cellulose synthesis in higher temperature conditions [66]. Hereby, sucrose transport from subtending leaf of boll to developing boll negatively affected, and boll number per plant and boll weight reduced [67]. Plant growth is mostly regulated by sucrose, which is a source of substrates' energy production and biosynthesis by decomposing into hexoses. Furthermore, the plant responses are determined by ROS scavenging capacity and

*Interactive Effects of Salinity, Drought, and Heat Stresses on Physiological Process and Selection… DOI: http://dx.doi.org/10.5772/intechopen.105576*

**Figure 3.** *Elongated stigma affected by heat stress (A), and normal flower structure (B).*

the signal pathway of sucrose [68]. In many crops, drought increased carbohydrate deprivation and ABA levels, whereas the ability of reproductive sinks to use sucrose and starch was reduced [69].

Auxin, an essential hormone, is very important for stress tolerance [70] and plays a role as a coordinator of plant growth and development [71]. The inhibition of auxin synthesis by overexpressing miR157 caused the sensitivity to heat stress and anther sterility in the reproductive period of cotton [72]. The auxin signaling pathway is controlled by the suppression of the sucrose synthase gene, and the antagonistic relationship between auxin and sucrose regulated plant growth and development [73, 74]. The favorable balance between auxin and sucrose is indispensable for the response of anther to heat stress [75].

Shedding is the formation of the abscission layer by natural or stress between sympodial or monopodial branches and reproductive organs such as square, flower, and boll in cotton (**Figure 2B** and **C**) [76]. The amount of hormones and regulation among hormone-controlled shedding, as auxin inhibits, and abscisic acid (ABA) promotes. Shedding is induced by increasing ABA and decreasing auxin under drought conditions [77]. In the reproductive period, failed fertilization caused by non-available pollen in higher temperatures resulted in flower shedding.

#### **4. Salinity stress**

The physiological drought via osmotic stress and ion toxicity caused by Na<sup>+</sup> , Cl− , and SO4 2− are two major forms of damage. Although cotton is classified as moderately tolerant to salt stress (7.7 dS m−1), salinity caused a decrease in seed cotton yield [78], boll number per plant [79–81], and an increase in early maturity [82]. Seed germination and early seedling growth in cotton are the most sensitive stages [83]. Both forms of salt damage caused abnormal plant growth such as stunted root and shoot growth by reducing photosynthetic capacity (**Figure 1A**) [84]. The excessive salt accumulation under salinity caused cellular injury in transpiring leaves [85]. However, cotton was successfully cultivated as a monoculture crop in saline-alkali soil. Plant cells are affected by low amounts of available water, loss of membrane functions, and ionic toxicity under excess sodium (Na+ ). As with most abiotic stress, salinity contains an

osmotic component, and cellular dehydration causes and disrupts the internal balance (homeostasis). The plant's first reaction is to reduce the Na<sup>+</sup> level in the cytosol by restricting influx, increasing efflux, and accumulating Na<sup>+</sup> in the vacuole for maintaining the cell metabolism. Transporters such as antiporter, uniporter, and symporter localized in the membrane are responsible for reducing Na<sup>+</sup> [86–88]. In a study conducted by [89], the Na<sup>+</sup> /H+ antiporter gene (GhSOS1) was detected in the plasma membrane of cotton (*G. hirsutum* L.). This gene is a Salinity Overly Sensitive, which has an important role to synthesize protein in upregulating under stress conditions such as salinity and drought. The suberization and lignification may occur around endodermal cells to inhibit apoplastic absorption of toxic ions in cotton [90, 91].

Many researchers focused on a root-associated microorganism to alleviate salinity stress in cotton. The arbuscular mycorrhizal fungi (AMF) are capable of increasing P and Zn uptake and promoting leaf proline accumulation [92, 93] but it should be noted that *Glomus mosseae*, AMF species, isolated from saline soil found to be less successful in alleviating salt stress compared to that of non-saline soil [94]. The microorganisms with the PGPR effect such as *Pseudomonas fluorescens* have IAA producing ability involved in the synthesis of important compounds under salinity stress [95]. In addition, the higher rhizosphere colonization of PGPR induced moderate N application due to the signaling molecule role of nitric oxide (NO) in the denitrification process [96]. Also, melatonin, as an indole hormone, alleviated the adverse effects of salt stress to reduce ROS production and ion toxicity and increases proline content in cotton seedlings [97].

#### **5. Drought stress**

The increase in the tensile force of the xylem, due to drought, caused a greater probability of rupture in the water column and formation of gas emboli in the xylem [98], and embolism decreased carbon assimilation depending on the linkage between water and CO2 exchange. Furthermore, stomatal closure and decrease in the stomatal area take place [99], and desiccation of all plants is induced in the further process (**Figure 1B**) [100]. Thus, the most negative impacts occurred in photosynthetic capacity and transpiration rate [101]. The environmentally induced PCD (programmed cell death) occurred in response to drought, and PCD increased ROS accumulation, DNA fragmentation, organelle degeneration, and cytoplasm shrinkage [102]. The tylose formation and xylem inhabitation by wilt pathogens in waterlimiting conditions may cause drought sensitivity to stress the susceptibility [103]. Primarily, MAPK (Mitogen-activated protein kinase), and secondly, ROS play an important role in intracellular signaling [104, 105]. *GhMKK3* and *GhMPK2* from *G. hirsutum* increased root hair development and ROS production by regulating ethylene synthesis, respectively under drought conditions [106].

Arbuscular mycorrhizal fungi (AMF) have an important role to alleviate drought stress by spreading on the soil and water transport by hyphae [107]. Similarly, PGPR coated phosphorus exhibited high performance due to the increasing stomatal conductance, net photosynthetic capacity, and yield of cotton under osmotic stress [108]. As a result of studies investigating the relationship between nutrients and drought, it was suggested that high N concentrations may decrease the effects of drought through nitrogen metabolism, proline synthesis antioxidant capacity, and osmotic regulation in cotton [109, 110]. However, K application has osmotic regulation due to increasing the osmoprotectant and regulates N metabolism [53, 111, 112] and photo-assimilation

*Interactive Effects of Salinity, Drought, and Heat Stresses on Physiological Process and Selection… DOI: http://dx.doi.org/10.5772/intechopen.105576*

and translocation process [113] in drought-stressed cotton plants. Also, supplemental Zn alleviated the negative effects of drought stress by increasing antioxidant capacity and decreasing MDA content in cotton [112, 114]. The nanoparticles such as nano-TiO2 and nano-SiO2 could alleviate drought stress because their foliar applications increase photosynthetic pigments, antioxidant capacity, and proline content in cotton [115].

#### **6. Heritability and breeding for stress tolerance in cotton**

The combination of multiple stress factors and the identification of the model plant is complex at the physiological and genetic levels [116]. Growth chamber, greenhouse, and field studies were conducted by many researchers to screen the cotton genotypes in different stress studies. Stress tolerant genotypes can be bred through a selection of promising single plants under stress conditions followed by testing in either stress and non-stress conditions or selection under more optimum conditions for effective selection criteria [117]. Previous studies recommended the selection of cotton plants with higher seedling vigor, enhanced early root development, and lower root/shoot ratio for drought tolerance, pollen carrying a dominant heat-tolerance allele for heat stress, and salinity barrier for salt stress [118].

In stress-tolerant cotton breeding, genetic stock in strains with the D genome rather than those with the A genome should be considered. Transcriptomic analyses indicated that thick cuticles and a double layer palisade layer of D-genome species such as *G. harknessii*, *G. armourianum,* and *G. turneri* are important germplasm resources for water deficiency. Similarly, *G. gossypioides* and *G. thurberi* with aggressive and deep root structures for drought tolerance and *G. aridum*, *G. davidsonii,* and *G. klotzschianum* for both drought and salt tolerance can be used in cotton breeding [84, 119–122]. In support of this knowledge, [123] revealed that QTLs (quantitative trait locus) responsible for salt tolerance are usually localized on the D-subgenome. At the same time, novel genes and alleles in wild relatives are important to overcome the abiotic stress tolerance caused by narrow gene pools in cultivated cotton species [124].

The selection of plants with thick cuticle and waxy surfaces is important to reduce solar radiation in the breeding of heat tolerance [125]. The intense absorbency of cotton cultivars increased sensitivity to heat stress [126]. The most important cultural management is to arrange the sowing time to avoid the higher temperature in the reproductive period. However, planting cotton before the recommended time faced the problem of low temperature during the early growing stage. Therefore, breeding of high-temperature tolerant cotton varieties has been suggested as the best method [127].

A pre-screening of cotton germplasm and evaluation of hybrids by constructing a polythene tunnel at the reproductive stage is a very common method in cotton (**Figure 2A**) [128]. However, some researchers preferred delayed planting set of cotton plants to test genotypes in heat tolerance breeding [129–132]. In addition, some researchers have used tetrazolium chloride for identifying heat tolerance in both vegetative tissues and pollen viability in cotton [133–135]. Various studies have emphasized the success of wild cotton species, their stacking progenitor alleles, and *Gossypium tomentosum* (heat-resistant species) for stress tolerance [13, 136, 137]. The pollen characteristics, germination ability, and tube length have been screened to determine the tolerant and susceptible genotypes under higher temperatures in cotton [13, 138]. On the other hand, the increase in fiber wax content of susceptible cotton genotypes under heat and drought stress indicated that acceptable fiber wax levels could be used to improve tolerant genotypes in conventional breeding [139].

In quantitative genetic studies about heritability and gene action for different selection criteria of heat stress, multigenic inheritance and both additive and nonadditive gene action in controlling cellular membrane thermostability was found to be higher under heat stress conditions [140]. The results of the scaling test, which is an important biometrical analysis method, indicated the significant dominance, additive x dominance, and dominance x dominance referred to as non-additive gene action for relative cell injury under heat stress [141]. Similarly, non-additive gene actions were estimated for fiber quality characters and ginning out-turn in heat stress (~38–39°C) at peak flowering time under field conditions by arranged sowing time [142]. In contrast, high heritability associated with high genetic advance for hydrogen peroxide content, catalase activity, total soluble proteins, carotenoids, and chlorophyll contents were found to have significant additive gene action under heat stress conditions [143]. In conventionally breeding of drought and heat stress tolerance, it was emphasized that instead of single plant selection in the F2 generation, superior plants should be selected in further generations such as F4 [144, 145].

PEG 6000 (polyethylene glycol) is defined as a rapid and effective method to observe the response of cotton genotypes for selection in drought tolerance breeding [146, 147]. Drought-tolerant species are defined with low maximum transpiration and photosynthetic rate, stomatal conductance, specific leaf area, small leaf size high leaf longevity, root mass ratio, and small leaf size [148]. The higher chlorophyll stability and relative water content exhibited drought tolerance due to photosynthate, which results in higher biomass [149] and fiber quality [150]. Furthermore, the presence of late embryogenesis abundant proteins is important for drought tolerance of cotton [151]. The physiological characteristics used for drought tolerance should be combined with yield, yield attributes, and fiber quality parameters in the breeding of cotton varieties with high adaptability [152]. Drought responsive genes were classified as induced (stress-related, metabolism, transcription factor, proline, and cellular transport) and repressed (mainly comprising metabolism, cellular transport, and stress-related) [153]. Drought tolerance genes such as RD2 (rice drought-responsive), HAT22 (homeobox from *Arabidopsis thaliana*), PIP2, PIP2C (plasma membrane intrinsic proteins), and GaTOP6B (encoding DNA topoisomerase from *G. arboreum*) were associated with drought in cotton [104]. Similarly, QTL analysis revealed that genes responsive to drought tolerance are spread over nine chromosomes while one QTL hotspot is concentrated on the eighth chromosome [154]. Anwar et al. [155] evaluated the selected varieties by molecular under drought stress, and MNH-886 cotton cultivar with high boll retention percentage, photosynthesis rate, and stomatal conductance was recommended against drought stress. The ratio of general combining ability and specific combining ability variance in line x tester analysis showed significant non-additive gene effects for proline content, total chlorophyll, canopy temperature, and cell membrane stability in drought stress conditions [156].

NaCl treatment at different doses is the most used method for screening cotton genotypes in order to determine suitable genotypes. Many researchers revealed significant variations in morphological, physiological, and biochemical characters under salinity stress [157–159]. The performance of genotypes, depending on genetic factors, compared with environmental factors and genotype x environment interactions, and high genetic gain by suitable selection increased the breeding success because of high additive effects under saline [160] and drought conditions [161]. On the other hand, non-additive gene actions were found significantly higher for chlorophyll content, K+ /Na+ ratio and within boll yield components under saline conditions [144, 162]. At the same time, selection for K+ , Na+, and K+ /Na+ were recommended for

#### *Interactive Effects of Salinity, Drought, and Heat Stresses on Physiological Process and Selection… DOI: http://dx.doi.org/10.5772/intechopen.105576*

salinity tolerance according to the results of factor analysis in cotton [163]. In addition, a reciprocal effect found in some studies indicated cytoplasmic and cytoplasmic x nuclear genes interaction for salinity tolerance breeding [160, 164]. Besides, the fact that genes with pleiotropic effects are effective for both salinity and drought indicated physiological characters are used in indirect selection can increase the success of multi-stress breeding [165, 166].

In many studies about engineering abiotic stress-tolerant crops, genes responsible for multiple stress factors found to confer tolerance in different plants are defined [167]. Co-overexpressing of SUMO E3 ligase (OsSIZ) and Vacuolar H+− pyrophosphatase (AVP1) from Arabidopsis and vacuolar Na+ /H+ antiporter genes performed significantly in increasing tolerance to multiple stressors such as drought, salinity, and higher temperature [168]. Overexpression of mitogen-activated protein kinases (MAPK) from cotton GhMPK2 performed stress tolerance when induced by salt, ABA, and water scarcity [169]. Eight genes belonging to the GhHSP20 family are responsible for heat, drought, and salinity [170], and the expression of GHSP26 caused a significant increase in proteins under drought stress [171]. Similarly, it was verified that GhCIPK6a overexpressed cotton lines can reduce the negative effects of salinity, and the seeds of these lines exhibited higher water absorption capacity at the germination stage [172]. In another study, JAZ proteins were identified as inhibitors of JA, and overexpression of JAZ genes resulted in a higher performance to salt stress [173]. GaJAZ1 transgenic plants carrying genes transferred from *G. arboreum* to *G. hirsutum* were found to be different from wilt type in terms of salinity tolerance [174]. *GhPHD* genes (plant homeodomain genes from *G. hirsutum*) increased plant tolerance in adverse environmental conditions to alleviate abiotic and phytohormonal stresses [175]. Also, the overexpression of the 14–3-3 gene GF14λ from Arabidopsis resulted in a stay-green phenotype due to decreased wilting, delayed senescence, and higher photosynthetic capacity. It was suggested that this gene can be used successfully in drought resistance [176, 177].

#### **7. Inferences (future considerations)**

The nutrient relations of cotton plants were greatly affected by abiotic stress. The nitrogen, silicon, magnesium, and calcium uptake are limited under high temperatures, drought, and salinity. In addition, the architecture of cotton changed to capture the less mobile nutrients. Therefore, fertilization should be reconsidered under stress conditions. To reduce the negative effects of abiotic stress, the use of some osmoprotectant and hormones may be a solution. The sowing time and plant density should be rearranged especially for high temperatures and drought. Cluster-type cotton varieties suitable for High-Density Planting System (HDPS) should be developed. These varieties will also reduce the density of weeds and dry matter content that compete for available water under drought conditions.

The most effective way to overcome the adverse effects of multiple abiotic stress factors is to improve tolerant cultivars. The possible effects of drought, salinity, and high-temperature stress on physiological, morphological, and yield should be well resolved to mitigate stress and develop varieties. The crossing between standard cultivars and donor suitable parents is a basic stage for genetic variation in conventional breeding. Multiple stress factors are very complex and polygenic characters and are controlled by non-additive gene effects with low heritability. Therefore, bulk selection should be useful for traits with low heritability such as stress tolerance, and the selection of a single plant should be postponed to F4 or F5 generation. Drought and high temperature are effective stress factors from flowering and during the first boll formation period, whereas salinity is an important problem in the early development period. In areas where these conditions are created, indirect selection can be made in terms of physiological characteristics, whereas direct selection can be made for yield. The evaluation of genotypes to stress tolerance under the plastic tunnel at the reproductive stage and in saline and drought conditions is a more accurate approach compared to artificial conditions and delayed sowing date in cotton.

Genetic engineering studies are promising, but manipulation of a single gene does not seem to be sufficient as resistance to multiple stress conditions is controlled by multiple genes. However, future transgenic cultivar breeding may be a good approach for multiple stress tolerance. In both classical breeding and genetic engineering studies, tolerance to multiple stress factors should be pyramided with yield and fiber characteristics. The non-genetically CRISPR-Cas system could be evaluated for engineering multiple stress tolerance in future cotton cultivars for all cotton-growing regions.

It could be highlighted that temperature extremes and fluctuations in the cottongrowing season along with other stress appear to be major factors of future yield reductions.

### **Acknowledgements**

Volkan Mehmet CINAR thanks the Higher Education Council of Turkey (YOK) for 100/2000 PhD scholarship.

### **Conflict of interest**

The authors declare no conflict of interest.

#### **Author statement**

Volkan Mehmet CINAR: Conceptualization, Literature survey, Writing – Review & Editing. Serife BALCI: Writing – Review & Editing. Aydın UNAY: Conceptualization, Literature survey, Writing – Review, Supervision.

*Interactive Effects of Salinity, Drought, and Heat Stresses on Physiological Process and Selection… DOI: http://dx.doi.org/10.5772/intechopen.105576*

#### **Author details**

Volkan Mehmet Cinar1 , Serife Balci2 and Aydın Unay1 \*

1 Faculty of Agriculture, Department of Field Crops, Aydın Adnan Menderes University, Aydın, Turkey

2 Department of Plant Breeding and Genetics, Cotton Research Institute, Nazilli-Aydın, Turkey

\*Address all correspondence to: aunay@adu.edu.tr

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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### **Chapter 15**

Influence of Soil Moisture Stress on Vegetative Growth and Root Yield of Some Cassava Genotypes for Better Selection Strategy in Screen House Conditions and Different Agro-Ecologies in Nigeria

*Najimu Adetoro and Sikirou Mouritala*

#### **Abstract**

Cassava is a vital staple crop for many African populations particularly in Nigeria. This study was conducted to determine the effect of soil moisture on the performance of selected 12 cassava genotypes that were evaluated for yield and related traits under three percentages of field capacity (75% – control, 50%, and 25%) in the screen house and field conditions in three agro-ecologies (Ibadan-Derived Savanna, Mokwa-Southern Guinea Savanna, and Zaria-Northern Guinea Savanna) and randomized complete block design was used. Data were collected on plant height, stem girth, number of nodes and leaves, shoot weight, stomata conductant, stay-green, fresh root weight, and dry matter percentage and were analyzed using descriptive statistics and ANOVA. Genotypes differed significantly across and within locations. The higher stress level (25% field capacity – F.C.) resulted in a more significant reduction in vegetative growth than the moderate stress level of 50% F.C.; moisture levels were uniform over time for plant height and stem girth. The response to moisture levels varied widely among genotypes, indicating that they experienced a higher stress condition. Genotypes IITA-TMS-IBA980581, IITA-TMS-IBA010040, and IITA-TMS-IBA010034 were identified with good drought tolerance. Integrating physiological research with breeding efforts will help in the selection of suitable varieties for release.

**Keywords:** soil moisture, cassava, agro-ecologies, root yield, selection

#### **1. Introduction**

Cassava is a major staple food and widely grown across Nigeria owing to its wide adaptability, economic importance, and acceptance both in rural and urban regions being a common staple consumed by Nigerians. It is also increasingly becoming

raw materials for food, feed, and industrial applications. In 2018, worldwide production of cassava stood at about 278 million tons (t). In the same period, Nigeria produced about 60 million t [1], and Africa's total production was about 170 million t (about 56% of the world production) [1]. It is a source of calcium, vitamins B and C, and other essential minerals [2]. However, several biotic and abiotic constraints, such as drought, pests, diseases, low soil fertility, shortage of planting material, postharvest physiological deterioration, and access to markets, limit cassava production [3, 4].

A major impact of climate change is drought or water deficit, which imposes limited water environment on plants [5]. Global monitoring and analysis of climatic variables have provided evidence that the countries where cassava is cultivated are experiencing impacts of climate change [6]. Under drought conditions, water available for plant uptake for metabolic reactions falls below requirement, thus adversely impacting growth and physiological processes. The effects of water deficit on cassava plants are many and vary depending on length and intensity of drought and stage of growth of the plant [7].

Drought or water deficit remains the major impact of climate change, which imposes limited water environment on plants and seriously affected tuber yield [5]. Crops are dependent on rainfall, and so water scarcity is the primary productivity constraint in arid and semiarid tropical areas [8]. As a meteorological event, drought is a period in which the potential evaporation exceeds the rainfall. Agricultural drought is the result of water flow imbalance between the environmental demands of evapotranspiration and water transport in the soil-root system [9]. Water stress increases abscisic acid (ABA) concentration in plant, which in turn increases root resistance by affecting membrane permeability and root tuberization (**Figure 1**) [12, 13].

The morphophysiological responses to drought stress increases abscisic acid (ABA) concentration, ion transport, and the induction of the associated signaling pathway genes in plant, which in turn increases root resistance by affecting membrane permeability and root tuberization [14]. Under water deficit, cassava leaves rapidly accumulate large amounts of ABA and young leaves halt leaf expansion growth and transpiration rate decreases. Young leaves accumulate more ABA than mature leaves, but the high ABA levels under water deficit are completely reversed to control levels after one day of re-watering, corresponding with a rapid recovery of leaf area growth rate. The rapid reduction in leaf area growth and stomatal closure might be due to cassava's ability to rapidly synthesize and accumulate ABA at an early phase of a water deficit episode [15]. Plants have developed defense mechanisms, which enable them to adapt and survive under drought condition in their life cycle [16].

#### **1.1 Biochemical and molecular mechanisms known to contribute to water-deficit stress tolerance in cassava plants**

The defense strategies against drought environment also vary from different cassava cultivars. During a prolonged drought stress condition, reactive oxygen species (ROS) generate excessively and cause oxidative damage [17]. ROS can damage multiple cellular components such as proteins and lipids, and unlimited disruption will finally lead to cell death [18].

#### **1.2 Use of crop diversity in plant breeding for drought-tolerance traits**

Valuable genes from natural inter- and intraspecific diversity can be used to take advantage of several mechanisms of survival and coadaptation in plants produced by *Influence of Soil Moisture Stress on Vegetative Growth and Root Yield of Some Cassava… DOI: http://dx.doi.org/10.5772/intechopen.105526*

#### **Figure 1.**

*General description of physiological responses of plants to drought stress conditions. ROS: Reactive oxygen species and ABA: abscisic acid [10, 11].*

natural selection [19]. Some of these genes are conserved by farmers (in landraces) or are present in crop wild relatives and the narrow genetic base of modern cultivars; therefore, crop wild relatives have been extremely valuable in adapting crop varieties to changing climatic conditions [20].

Cassava grows and produces well in the Nigerian environment but shows different growth behavior and yields in different years due to differences in annual weather conditions. The water regime of an environment is an essential factor that affects the growth of crops. Differential soil water and nutrient regimes have been reported to affect yield stability in cassava [21]. Although it is incredibly tolerant to water stress, a long dry period has been reported to

decrease yields [22]. Similarly, prolonged moisture deficiency leads to a reduction in growth, development, and root yields [23].

In the past, decade the International Institute of Tropical Agriculture (IITA) had developed improved varieties, which were selected from diverse source crosses; that are resistant to the crops, major pests, and diseases. As these new genotypes are developed, there is need to evaluate their performance under different moisture regimes to identify those that are stable across varying moisture environments. This study evaluates the vegetative growth parameters and root yield of selected genotypes under different moisture conditions in the screen house and three different agro-ecologies. The objectives of the study are to determine the influence of soil moisture stress on vegetative growth and root yield of selected cassava genotypes.

#### **2. Materials and methods**

Twelve genotypes (IITA-TMS-IBA010040, IITA-TMS-IBA011086, IITA-TMS-IBA011663, IITATMSIBA020131, IITA-TMS-IBA30572, IITA-TMS-IBA91934, IITA-TMS-IBA920067, IITA-TMS-IBA920326, IITA-TMS-IBA950166, IITA-TMS-IBA980510, IITA-TMS-IBA980581, and TMEB 1) were selected based on their field performance for root yield, root numbers, and dry matter and were evaluated in the screen house at IITA, Ibadan. The plants were raised in large polythene bags of 36 cm length and 156 cm circumference for 6 months under three moisture conditions: 75% (control), 50%, and 25% F.C. using the procedure of Anderson and Ingram [24].

The F.C. moisture levels at 75% (well-watered), 50% (moderate), and 25% (severe) were used to simulate stress conditions in the field. The polythene bags were filled with 86 kg of topsoil (obtained from Ibadan) to a height of 36 cm and made firm by being doubled. The soil used was classified as Ferric Luvisol with sandy-loam texture (USDA); pH (water) was 5.4, organic carbon (C) 1.26%, and total nitrogen (N) 0.12%. Available phosphorus (P) was 34.4 mg/kg, calcium (Ca) 5.8 cmol/kg, and magnesium (Mg) 0.7 cmol/kg. The experiment was laid out in a 3 by 12 factorial arrangement (moisture level clones) in a completely randomized design (CRD) and replicated three times. Each of the replicates had a total of 36 bags in three rows of 12 bags per row. Healthy stakes of uniform length (25 cm) were planted vertically in the central portion of the bags. Holes at the bottom of the bags allowed easy draining. Following the procedure of Anderson and Ingram [24], plants were watered to field capacity for the first 4 weeks to ensure good plant establishment, after which moisture treatments were imposed by irrigation once a week with 5.58 liters for 75% F.C., 3.72 liters for 50% F.C., and 1.86 liters for 25% F.C. Measurements on vegetative traits were taken at 4, 8, 12, 16, and 20 weeks after planting (WAP) for plant height (cm), and stem girth (cm). Yield parameters were taken at 24 WAP.

These 12 genotypes were also evaluated on the field using healthy stakes of 25 cm length planted in a slanting position on ridges 30 cm high with two-thirds of the length buried in the soil. Mokwa [(Southern Guinea Savanna (SGS), Lat.9°29<sup>1</sup> N and Long. 5°041 E and 152 masl] and Zaria [Northern Guinea Savanna (NGS), Lat.11°11<sup>1</sup> N and Long.11°78<sup>1</sup> E and 610 masl]. The three locations represent different agroecologies with varying climatic and soil characteristics. Planting was done in each location when soil moisture was sufficient to sustain establishment. Weeding was

*Influence of Soil Moisture Stress on Vegetative Growth and Root Yield of Some Cassava… DOI: http://dx.doi.org/10.5772/intechopen.105526*

manual at 1 month after planting (MAP), and herbicides were applied at 3, 6, and 9 MAP. Harvesting was done at 12 MAP.

#### **2.1 Data collection**

Data collected in the screen house were **Plant height** at 4, 8, 12, 16, and 20 WAP; **Stem girth** at a uniform stem length of 50 cm from ground level; **Fresh shoot weight** was obtained in kg as the fresh weight of shoots per plot; **Fresh root weight** per plot and estimated in t/ha at 24 WAP; **Number of stems per plant; Number of leaves** per plant was counted per plot and Screening genotypes for resistant to Cassava Mosaic Disease (CMD), Cassava Bacteria Blight (CBB), and Cassava anthracnose (CAD). The incidence and severity of genotypes to the African cassava mosaic disease (CMD) were evaluated at 1, 3, and 6 months after planting (MAP) at 3 and 6 MAP for cassava bacterial blight (CBB) and CAD at 6 and 9 MAP; since the symptom would not have expressed at 1 month after planting. Disease incidence was taken as proportion of plants units that are visibly diseased relative to total number of plants, while disease severity = volume of plant parts affected compared with the whole plant unit.

Disease incidence = Number of infected plants/plot/Total number of plants/plot. Severity of genotypes to CMD was recorded based on a scale of 1–5 [25].

**Leaf Chlorophyll**: The leaf chlorophyll contents of four selected leaves in each plot were measured at 4, 8, 12, 16, and 20 WAP using Chlorophyll Meter Model SPAD-502 (Minolta Co. Ltd. Japan).

**Leaf stomata conductance:** A steady-state porometer (Licor Instrument Corporation, Model Li-1600) was used to measure diffusive resistance and transpiration rate on the abaxial surface of the uppermost fully expanded leaves of four plants per plot. The sensor head with a narrow leaf aperture (LI 1600–01) with an area of 1cm2 was used. Measurement was taken when sun was not too low or high in the morning (900– 1100 h) and afternoon (1330–1530 h) on a clear sunny day at 3, 6, and 9 months after planting (MAP) in all locations [26].

The dried sample was weighed, and root dry matter percentage was calculated as follows.

Percent root cortex DM ¼ dry weight*=*Fresh weight � 100

*Gari* **production**: Cassava roots were converted to fermented roasted granules called *Gari*. Ten kilograms of roots were taken from each genotype harvested, washed with water, and grated with a grating machine. The pulp was put in a jute bag, and pressure was exerted on it to remove water from the pulp. Dewatering took up to 3 days and the pulp was also undergoing fermentation while being dewatered. The pulp was sieved to remove chaff and toasted in a pot until gelatinized grains were formed.

#### **2.2 Statistical analysis**

All data obtained were analyzed using the Statistical Analytical System (SAS) (9.2 version). The model used was the factorial arrangement in a CRD. Least Significant Difference (LSD) at 5% probability was used to separate mean squares.


#### **Table 1.**

*Pooled analysis of variance for yield and yield related traits of 12 cassava genotypes evaluated in the screen house Ibadan Nigeria.*

#### **3. Results**

#### **3.1 Performance of selected genotypes in different moisture conditions in the screen house and field environments**

Mean squares (MS) from the combined analysis of variance (ANOVA) for root and shoot characters of the twelve cassava genotypes in the screen house at Ibadan are presented in **Table 1**. The result shows highly significant (p < 0.001) mean squares (MS) for all sources of variations. However, MS for all traits studied were significant for more than one source of variations. The mean squares (MS) for treatment were highly significant so also mean square for except for fresh root weight and fresh shoot weight. Whereas the interaction between replicate and the treatments was not significant except for fresh shoot weight as well as the interaction between treatment and genotype except for leaf chlorophyll. But the mean square for replicate was not significant for all the traits studied. Mean values at 75% and 25% FC differ significantly for all parameters at 24 WAP, except for leaf number, leaf chlorophyll content, and fresh root weight.

When mean values at 25% FC were compared with mean values at 75% FC, it was observed that mean plant height decreased by 29.33%, stem girth by 17.32%, root weight by 61.76%, and shoot weight by 29.27% at harvest (**Table 2**). There was no significant difference in mean values at 75% and 50% FC for plant height at harvest


#### **Table 2.**

*Mean and percentage differences of 12 cassava genotypes in the greenhouse at different moisture levels for different traits at 24WAP.*

*Influence of Soil Moisture Stress on Vegetative Growth and Root Yield of Some Cassava… DOI: http://dx.doi.org/10.5772/intechopen.105526*

24WAP. Similar growth pattern was observed for plant height and stem girth over time at 75% and 50% FC (**Table 3**).

IITA-TMS-IBA010040 had the highest fresh root weight of 70.2 g, followed by IITA-TMS-IBA920326(55.4 g) and IITA-TMS-IBA980581 had fresh root weight of 50.9 g; while IITA-TMS-IBA30572 had the least fresh root weight of 38.4 g (**Table 4**).


*Same numbers are not significant from each other while numbers together mean they are over lapse and a separate number is significantly different.*

*pltht: plant hight, stmgrth: stem girth, lfchorph: leaf chlorophyll, rtfrhwt: root fresh weight, shtfrhwt: shoot fresh weight, stmno: stem number.*

**Table 3.**

*Mean differences of 12 cassava genotypes in the screen house at different moisture levels for different traits.*


*\*\*\*, \*\*, \* means significant at p* ≥ *(0.001, 0.01, 0.05) and ns means not significant.*

*Mpltht: mean plant hight, mstmgrth: mean stem girth, mlfchorph: mean leaf chlorophyll, mrtfrhwt: mean root fresh weight, mshtfrhwt: mean shoot fresh weight, mstmno: mean stem number.*

#### **Table 4.**

*Overall means of yield and yield related traits of 12 cassava genotypes evaluated in the screen house at 24 WAP.*

The mean reduction in genotypes performance at moisture stress levels of 25% and 50% FC ranged from 15.9% (30572) to 44.3% (010040) for plant height, between 7.7% (020131) and 41.6% (920326) for stem girth, between 40.8% (011086) and 85.7% (30572) for root weight, and between 12.8% (TMEB 1) and 63.9% (011086) for shoot weight. The mean plant height was 125.1 cm with 011663 having the tallest while 980510 and 91934 being the shortest 102.8 cm. Stem girth ranged from 1 cm (91934) to 1.4 cm (980510); leaf number ranged from 34.8(011663) to 52 cm (91934). The coefficient of variation for the traits was quite low (**Table 5**).

#### **3.2 Overall fields disease means scores, yield, and yield-related traits of 12 cassava genotypes evaluated for 2 years at three locations in Nigeria**

There was significant (P ≤ 0.05) difference in genotypes performance with respect to stay green, mean diseases score, fresh root yield, harvest index, root yield and size, shoot weight, and gari yield except for dry matter content. 80% of tested genotypes performed better than the checks in terms of stay green, mean diseases score, fresh root yield, and shoot weight while 70% and 30% of the genotypes outperformed checks with respect to root size and gari weight. The genotypes with outstanding field performance across the three locations were IBA980581, IBA010034, and IBA010040 (**Table 6**).

#### **3.3 Morphological and physiological field performance of 12 cassava genotypes evaluated at three locations in Nigeria for 2 years**

There is significant difference (p ≥ 0.05) in genotypes morphological and physiological traits except for stem girth. More than half of tested genotypes perform better than checks in all traits except for stomata, level of branching, and number of leaves. The genotypes that recorded highest and least score for plant height were (IBA920067, IBA91934), number of nodes (IBA980510, IBA91934), chlorophyll content (IBA011663, IBA980581), and Leaf Area (IBA011663, TME 1). Genotypes IBA011663, IBA980510, IBA010040, IBA010034, IBA30572 were identified for being stable across three locations for physiological and morphological traits (**Table 7**).

#### **3.4 Location means recorded by 12 genotypes evaluated for 2 years at three locations in Nigeria**

Ibadan location recorded the highest score for level of branching, chlorophyll content, plant height, gari weight, number of leaf, fresh root yield, harvest index with least score for cassava bacteria blight severity (CBBS), Mokwa location had highest mean score for shoot weight, number of nodes, stem girth, stomata, and least mean score for cassava anthracnose disease severity (CADS) while Zaria location recorded highest mean score for stay green, dry matter content, and least mean score for cassava mosaic disease severity score (CMDS). Zaria and Mokwa location recorded the same mean score for Harvest Index while Ibadan and Mokwa location also recorded the same score for root size (**Table 8**).

#### **3.5 The correlation coefficients for traits measured for 2 years at three locations in Nigeria**

Most of the traits show significant correlation *inter se*. Notably, Fresh root yield was positively correlated garri, harvest index, and cassava mosaic disease, Fresh root


*Influence of Soil Moisture Stress on Vegetative Growth and Root Yield of Some Cassava… DOI: http://dx.doi.org/10.5772/intechopen.105526*

> **Table 5.**

*Overall mean performance and percentage difference among 12 cassava genotypes under different moisture conditions in the greenhouse for four traits.*


**Table 6.** *Overalldiseaseyield,andyield-relatedtraitsofgenotypesevaluatedforatthreelocationsinNigeria.*

 *mean scores,* 

 *12 cassava* 

 *2 years* 

#### *Advances in Plant Defense Mechanisms*



*\*\*\*, \*\*, \* means significant at p* ≥ *(0.001, 0.01, 0.05) and ns means not significant, Number in parenthesis represents the rank/position of each genotype Nnode: number of nodes, Lbrch: level of branching, Nleaf: number of leaves, LA: leaf area.*

#### **Table 7.**

*Overall means morphological and physiological traits of 12 cassava genotypes evaluated for 2 years in three locations in Nigeria.*

yield was however negatively correlated to cassava bacterial blight (0.28) and cassava anthracnose disease (0.29).

Chlorophyll content was positively correlated to fresh root yield (0.25), but negatively correlated to dry matter (0.14).

Stomata had significantly negative correlation with harvest index (0.11) and number of root (0.15).

Shoot weight correlated with chlorophyll content (0.19), and fresh root yield (0.79) but had negative correlation with dry matter (0.17) (**Table 9**).

#### *Advances in Plant Defense Mechanisms*


#### **Table 8.**

*Overall mean location values for morphological, physiological, and yield traits at three locations for 2 years in Nigeria.*

#### **4. Discussions**

As an important environmental limitation, drought has become a rising concern due to its harm to the development and productivity of crop plants [27]. Cassava is a major staple food to resource-limited people in marginal areas because of its ability to survive and produce in such poor land with infrequent rainfall and low fertility [28]. The present study by using 12 cassava genotypes can provide a fundamental basis for the identification of drought-tolerant germplasm resources.

Plants are known to respond to water deficit with some adjustment at morphological, physiological, cellular, and metabolic levels. These responses are, however, dependent upon the duration and severity of stress, the type of genotype/the stage of development, and the organ and cell in question [29]. Reduction in photosynthesis results in the inability of the genotypes to produce tuberous roots compared with conditions with relatively higher moisture levels. Any factor in the plant's environment that is not the optimum, being either deficient or in excess, will limit plant growth. Moisture stress at 25% F.C. in the screen house led to a reduction in root weight by over 61%, and this is indicative that severe moisture stress that occurs within a period of 8–24 WAP can lead to a very high level of yield loss [30]. Porto [30] also reported that water stress from 4 to 20 WAP led to a reduction of storage root


*Influence of Soil Moisture Stress on Vegetative Growth and Root Yield of Some Cassava … DOI: http://dx.doi.org/10.5772/intechopen.105526*

> **Table**

**9.** *Correlation coefficient for morphological, physiological, yield, and related traits of 12 cassava genotypes evaluated for 2 years at three locations in Nigeria.* yield by 32–60%. Shoot weight at 25% F.C. was also reduced by 22.4%. Reduction in plant height was 29.3%, leaf chlorophyll content was reduced by 17.7%, leaf number had a reduction of 28.9%, whereas stem girth had a reduction of 15.4%. The implication is that moisture stress could hinder the manifestation of the genetic potentials of cassava. Therefore, a variety that is the best for a trait offers the opportunity (genetic base) for improving such trait through selection or hybridization and further selection. Even with significant differences in the traits that should contribute to root yield, fresh root weight was still not significantly different among genotypes. This also goes along with the result of the screen house experiment by [7]. The presence of the tonoplast sugar transporter (for roots and tubers) to transport the assimilates from the source to the sink (roots) enhances root formation and development. If not, the yield would not be significantly different as shown in this result. Ludewig and Flügge [31] and Cho *et al.* [32] also report similar observations. There should be continued work on several of the implied and not yet confirmed transport steps within plants, which frequently turn out to be the rate-limiting step to production of valuable compounds in storage sinks.

Plants respond to drought conditions either by increasing or decreasing their root growth.

Relatively higher stomata conductance observed in most genotypes in this study implies that the genotypes would have high photosynthetic potential although not translated to the highest levels of dry matter and root yield. The essence is that this attribute alone might not be a strong contributor to high yield under water-limited conditions. Measurements of leaf diffusive resistance reflected bulk resistance to water loss, combining activities of both the stomata and genotypes. In the dry savannas, genotypes with the ability to optimize water use are desirable due to limited availability of cultivable soil during the dry season [33]. A major component parameter associated with high Water Use Efficiency (WUE) is rapid stomata closure. Generally, stomata resistance of all the genotypes was low in the morning but increased in the afternoon and showed an increase in response to light and vapor pressure deficit (VPD) [34]. The morphophysiological responses to drought stress increases abscisic acid (ABA) concentration, ion transport, and the induction of the associated signaling pathway genes in plant, which in turn increases root resistance by affecting membrane permeability and root tuberization [14]. The rapid reduction in leaf area growth and stomatal closure might be due to cassava's ability to rapidly synthesize and accumulate ABA at an early phase of a water deficit episode [15].

Plants obtained energy and virtually all their structural materials by photosynthesis and the leaves are the main photosynthetic organ. Jarvis and Morison [35] and Akparobi *et al.* [36] had similar results. This may indicate that the higher the number of photosynthetic organs, the higher the photosynthetic rate. In this study, correlation between leaf number and root yield was positive and significant. Also, chlorophyll, harvest index, and plant height correlated positively with leaf number.

However, genotypes IITA-TMS-IBA30572 and IITA-TMS-IBA980510 with higher leaf numbers yielded less than IITA-TMS-IBA980581, which had fewer leaves. High leaf number in these genotypes is largely due to their branching habit. Shoot apices and storage roots compete for available carbohydrate in cassava [37, 38]. The high number of branches (level 2) may have caused higher competition for available carbohydrate in IBA30572 than in the other improved genotypes, which probably reduced the root yield in the former genotype despite its high number of leaves. Experiments have shown that production (formation) and growth (elongation) of cassava's adventitious and lateral roots are suppressed by deficit in soil moisture [7, 39].

#### *Influence of Soil Moisture Stress on Vegetative Growth and Root Yield of Some Cassava… DOI: http://dx.doi.org/10.5772/intechopen.105526*

The higher leaf area observed in genotype IITA-TMS-IBA011663 did not translate to higher root and dry matter yield. This did not go along with the results of Enyi [37] and [40], who observed that high yielding cultivars had high leaf area but also validated the findings of (Ludewig and Flügge [31] and Cho *et al*. [32] that a plant with a good number of stems, better plant height, enough leaf chlorophyll content without the presence of the tonoplast sugar transporter (for roots and tubers) that transports the assimilates from the source to the sink (roots) will not translate to high yields as shown in this result. This might suggest that other parameters of canopy function such as leaf, carbon fixation, and assimilate use and partition could interact significantly in yield formation. Among the improved genotypes, IITA-TMS-IBA011663 possibly partitioned the highest amount of dry matter into leaves, petioles, stems, and fibrous roots and was not the highest in root yield; thus, partitioning of dry matter partially explained the lower yield of IITA-TMS-IBA011663 than in other improved genotypes even though its L.A. was higher.

A better value was observed for stay-green in the improved genotypes IITA-TMS-IBA010040 and IITA-TMS-IBA980581 compared with the value in the local variety TME 1. This implies that the improved genotypes would be expected to be more efficient in supporting photosynthesis, stomata conductance, and carbon fixation. The better stay-green values for IITA-TMS-IBA010040 and IITA-TMS-IBA980581 may be a contributor to their root yield being the best compared with TMEB1, which had the overall worst root yield and the worst stay-green value. However, this finding does not go along with the finding by Oluwafemi *et al*. [41], which reported that the local variety with high stay-green yielded better than the improved ones.

The lower values that were observed in CMD for new improved genotypes IITA-TMS-IBA980581, IITA-TMS-IBA010040, and IITA-TMS-IBA010034 compared with the checks (local variety TMEB1 and the old-improved varieties IITA-TMS-IBA30572) implied that the new improved genotypes would have healthy clean leaves for better photosynthesis compared with the local variety and the old-improved genotypes. This might be a contributing factor for these genotypes exhibiting better root yields than the old but improved varieties while the local variety had the worst root yield. Cassava mosaic disease was positively correlated with plant height, *Gari* weight, harvest index, root size, and fresh root yield, whereas CBB was negatively correlated with yield and yield-related traits such as harvest index, plant height, chlorophyll, stomata, root size, and root number.

The significant positive correlations that existed between storage root yields and harvest index indicated that these traits were important in improving root yield. Makame [42] also reported a strong association between storage root yield and root size, while the works of (Radhakrishnan and Gopakumar [43] and Rubaihayo *et al*. [44] also revealed a strong association between storage root yield and harvest index.

Dry matter in this study was not correlated with storage root yield, thus signifying that dry matter was not an important indicator of storage root yield. Similar observations were also made by Ntawuruhunga [45], Varma and Mathura [46] and Makame [42]. Stomata conductance also showed negative association with storage root yield, indicating this was not an important index of yield. Selections in favor of any of these traits would result in simultaneous decrease in root yield. Negative association between root yield and other traits has also been reported by Makame [42] and Rubaihayo *et al*. [44].

The high variation that was observed in this study for storage root yield across the different environments indicated that these environments differed greatly as reflected in each location being categorized as a unique agro-ecological zone. The expression of yield, therefore, depends on the genetic factors and on the environmental factors. The genetic factors, which promoted yield, will only be able to express themselves to full capacity when the environmental conditions needed for its expression are found and where this is limiting, yield will be reduced.

Furthermore, effect of storage roots on storage root yield appeared to be positively influenced by root size, root number, and harvest index. [47] made similar assertions and reported that yield increase was mainly due to increase in both numbers of storage roots and individual root weight. Mahungu [48], however, observed that number of tuberous roots contributed more to the final yield than root size. Radhakrishnan and Gopakumar [43] also observed that a high value for harvest index indicated a correspondingly higher yield for storage roots. These three characters can thus be regarded as the most reliable components of yield for selection of high yielding genotypes. A similar assertion was made by [47], who reported that storage root number was more closely correlated with root dry weight than with the individual weight of storage roots and consequently yield increase was mainly due to increase in both numbers of storage roots and individual root weight. Storage root yield, number of storage roots, and root sizes had the greatest contribution to total observed variability, thus suggesting that these characters were the most important of all that were evaluated and improvement on them will lead to improvement in yield and hence, bring progress in crop improvement. The plant breeders and agronomists are, however, interested not only in high yields but also on the ability of genotypes to produce such high yields across diverse environments over several years. Therefore, breeders are interested in identifying genotypes that are stable across environments as well as those that are well adapted and suitable for a specific agro-ecology [49, 50]. Results from the field experiment were able to identify IITA-TMS-IBA980581, IITA-TMS-IBA010034, and IITA-TMS-IBA010040 as having good drought tolerance.

This experiment explained the use of eco-physiological research to improve the genetic base and develop clones that are more adaptable to the drought environments of the tropics. Interdisciplinary research that combines physiology and breeding approach will help in selection of good varieties for release since the performance of a genotype is a function of its adaptability and the availability of conducive environment along with better genetic composition.

Further research is needed and investigation on nutrient transportation, the effect of source and sinks, and their relationship as these affect yield.

#### **Acknowledgements**

The authors thank the technical staff of International Institute of Tropical Agriculture (IITA) cassava breeding program that assisted with the fieldwork, screen house evaluation, and laboratory analyses. We also extend our appreciation to the Bill & Melinda Gates Foundation and the United Kingdom's Foreign, Commonwealth & Development Office (FCDO) for funding this research through the "Next Generation Cassava Breeding Project."

#### **Contributions**

ANA, conceived, designed the study, implemented field trials, performed analyses, and wrote the manuscript, ANA and SM, performed analysis. SM, edited the manuscript, generated, and curated data.

*Influence of Soil Moisture Stress on Vegetative Growth and Root Yield of Some Cassava… DOI: http://dx.doi.org/10.5772/intechopen.105526*

#### **Funding**

This research work was generously funded by the Bill & Melinda Gates Foundation and the United Kingdom's Foreign, Commonwealth & Development Office (FCDO) through the "Next Generation Cassava Breeding project" (award number OPP1048542) managed by Cornell University.

**Copyright:** ©2021 Adetoro et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

#### **Declaration of interest**

The authors have not declared any conflict of interest.

#### **Data availability statement**

All relevant data supporting findings in this study are available on request from the corresponding author.

#### **Author details**

Najimu Adetoro1,2\* and Sikirou Mouritala2,3

1 International Institute of Tropical Agriculture (IITA), Ibadan, Oyo State, Nigeria

2 International Institute of Tropical Agriculture (IITA), Bukavu, Democratic Republic of the Congo

3 International Institute of Tropical Agriculture, Kinshasa, Democratic Republic of the Congo

\*Address all correspondence to: n.adetoro@cgiar.org

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## Tolerance of Plant Cell Wall to Environment

*Olena Nedukha*

#### **Abstract**

Drought and flooding of soil are negatively factors for growth and development of plants. Exogenous factors, including moisture of soil, intensity of sun light, temperature, salinization, the content and diffusion rate of CO2 and O2 is main that influence terrestrial and flood plants. Cell walls actively participate in the mechanisms of plant adaptation to drought and flooding. It has been established that the resistance of plants to unfavorable environmental conditions is due to the plasticity of the structural, biochemical and functional characteristics of plant cell walls, that manifests itself in a change of ultrastructure cell walls, density of stomata and wax in leaf epidermis, compacting or loosening of cell walls, presence of cuticle pores, change of content of crystalline and amorphous cellulose, hemicellulose, callose and lignin and change in a ratio of syringyl/quajacyl monolignols and also expression of the specific genes.

**Keywords:** cell wall structure, lignin, cellulose, drought, flooding, stress, genes

#### **1. Introduction**

It is known that external environmental conditions provoke to the phenotypic and genetic plasticity of plant during vegetative and generative growth and lead to change of duration of ontogenesis of both individual species and populations [1]. Given that exposed to a specific exogenous factor, some plants experience stress, and for other plant species this factor is the optimal condition for life, the definition of stress for the plant is quite complex and problematic. According to many definitions, stress is a harmful adverse force or condition that inhibits the normal functioning of a biological system, such as a plant. According to [2], stress for a plant is its response to the action of adverse or even detrimental to growth and development of plant. For the plant, stress is measured by both signs of survival and signs of adaptation, yield, growth parameters and assimilation. External signals of the environment, such as light, temperature, water status of the soil–these are the most important signals that affect the growth of the plant. The perception of these signals and the plant's response to them affects a whole cascade of events that require knowledge of the signal and its transduction into a physiological response [3]. In the perception of signals of adverse abiotic stresses, primarily involved protein receptors of the cell wall, that send this signal to the transport system into the cytoplasm. Such receptors of ell wall appear to be arabinogalactan protein molecules that bind the cell wall to

the plasmalemma, cytoskeleton elements, and apoplast components. In addition to these proteins, stress receptors can be mitogen-activated protein, numerous kinases, and several transcription factors [4]. Stress is first perceived by cell wall receptors, which send a signal to the receptors of the cytoplasmic membrane, then the signal is reformed and reduced, and the result of this transformation is the participation of secondary mediators [3–6].

Determination of the plant state in a changing environment in conditions of increased anthropogenic pressure and global climate changes is becoming one of the main problems of plant biology and ecology. In natural conditions plants can be influenced by a complex of unfavorable environmental factors. Despite the long list of abiotic and biotic stresses, including: cold, high temperature, salinity, drought, floods, radiation, air and soil pollutants, pathogens and others, we will consider the most significant adverse environmental factors: drought and flooding, which negatively affect plants' growth, up to their death. The search for universal biomarkers that would make it possible to determine the state of plants regardless of nature and number of stress factors is urgent. The cell wall of plants can be such a marker, since it is the growth and differentiation of the cell wall during primary and secondary growth that undergoes significant changes under conditions of changes in the water balance of the plant. The basis of this section is the idea that the stability of ontogenesis under conditions of unfavorable climatic and anthropogenic changes in the environment is due to the plasticity of the structural and functional organization of plant cell walls. We put forward a hypothesis about the existence of a coordinated response of the structural and functional systems of the cell wall and the cytoplasm of plant cells, which is involved in the adaptation of the plant to the action of extreme natural factors—drought and flooding.

#### **2. Drought**

Drought is a deficit of water in the soil, which affects the growth and development of the plant. Drought stress is seen as a condition in which water potential and turgor of a cell are reduced, although the plant can function normally. Water stress is considered as the loss of water by the plant, which leads to the closure of the stomata and restriction of gas exchange by the plant. Wilting of plants is characterized by an intensive loss of water, which leads to next changes, including of plant metabolism and cell structure, to change of activation of catalytic enzymatic reactions, to inhibiting the process of photosynthesis and destructed metabolism, which can lead to cell death [7, 8]. Drought can be chronic or temporary. The latter is observed when the weather changes rapidly and unpredictably. Moderate drought is a phenomenon in which the plant begins to feel the effects of drought. Under such conditions, plants have developed specific mechanisms of acclimatization and adaptation in response to the short-term or long-term action of the factor [9, 10].

In this respect, the reaction of plants to drought is well studied in psammophytes growing on sand dunes has been better studied. Psammophytes develop mechanisms and specific features that ensure not only a normal state of life, but also functioning under stressful conditions. These mechanisms are reflected in the morpho-anatomical changes in the vegetative organs of plants [7, 11] that help psammophytes to adapt to environmental conditions, and manifested in a decrease in the size of leaf blades, the formation of water-retaining parenchyma, a change in the size of the leaf conducting system, twisting of leaf blades, a change in the cell wall structure, change of density

of stomata, an optimization of transpiration, enhanced synthesis of wax and lignin, the formation of trichomes and silicon inclusions in cell walls and formation a thick cuticle [7, 12].

#### **2.1 Growth processes during drought**

It is showed that even with a slight drought, the growth rate of plant organs decreases: roots and aboveground organs react very strongly to such stress, their growth reduce [13, 14] that connected with structural-functional changes of cell walls [15]. Drought cell growth decreases have been described for leaves for psammophytes, including *Zygophyllum album* and *Nitraria retusa* [16], *Tragopogon borysthenicus* and *T. orientalis* [11], *Helichrysum arenarium* and *H. corymbiforme* [17]. The mechanisms of this phenomenon in psammophytes have not been studied. However, in mesophytes and halophytes, cell and organ sizes have been shown to be mediated by the rate of cell division and stretching [7]. That is, the decrease in cell growth is mediated by changes in the synthesis of polysaccharides of cell walls. This phenomenon has been studied in *Sium latifolium* leaves [18]. When studying the development of the ephemeral desert plant *Gymnarrhena micranatha* for 50 days, it was found that its inflorescence is located in the depth of the soil. The apical bud is drawn from the soil surface into the soil to a depth of 10 mm. It is shown that the direct retraction of the main apex of the stem occurs from the soil surface into its depth, while the stem rotates, changes its direction of growth by 180o , continues to grow and develop. What processes take place in cells' walls? There was compression of cells in the endoderm, pericycle and primary phloem of the root, which decreased in length (by ½ initial lengths). This was due to the formation of radial cell walls. The shrunken and compressed cell walls had a wavy appearance. Partial disintegration of polysaccharides was observed in cell walls, including lysis of cellulose wavylike micro fibrils [19].

Under drought conditions in the roots there revealed a decrease in the size of the parenchyma [20]; in the endoderm, cell walls thicken, and additional layers of cells were with strongly suberinized cell walls are formed around the stele [21]. In the periderm, cell walls were also impregnated with suberin, which reduces the penetration of water through the cells of the cortex. Special lacunae for water storage were formed in root [22]. Whereas in leaves the effect of drought is manifested in the reduction of sugars in the fraction of cell walls, which should certainly be reflected in the composition of polysaccharides in the walls. Studies of the effects of drought on crops have shown that the cell walls of aboveground photosynthetic organs are also sensitive to this factor. Studies of polysaccharides of cell wall matrix in reduced coleoptiles of wheat seedlings under drought from 6 to 15 weeks shown that during the first week of drought exposure, drought-sensitive varieties showed a decrease sugar in the fractions of wall matrix: rhamnose, mannose, galactose, arabinose, xylose, and glucose and uronic acids [23]. In addition, in the hemicellulose fraction of drought-resistant variety was shown decrease in arabinose, mannose, galactose and increase in rhamnose, xylose, glucose, uronic acids in comparison with droughtsensitive variety. These changes were accompanied by an increase in the activity of glucoside hydrolysing enzymes: α-galactosidase, α-L-arabinofuranosidase and 1.3–1.4-β-glucanase in drought-resistant varieties. The observed changes in the matrix of cell wall of coleoptiles of two varieties of wheat under the action of drought reflect changes in cell metabolism, which directly affected the growth rate [23]. Similar changes in the content of sugars (glucose, fructose and sucrose) and the activity of

1.3–1.4-β-glucanase have been previously noted by other researchers in studying the effects of water and salt stress on wheat stalks [24].

#### **2.2 The role of wax and cuticle**

Wax and cutin are involved in the regulation of water and lipids transport through the cell wall [25]. Plant' wax is a mixture of aliphatic and cyclic hydrocarbons and their derivatives. The composition of waxes varies depending on the species and organs' plant. Cutin is involved in the regulation of the diffusion of gases and moisture in the main cells of the epidermis and the stomata. It is known that the cuticular membrane can be both hydrophobic and hydrophilic. If the cuticular membrane is hydrophobic, the functions of the cuticle are to reduce water loss by the organs; and if the cuticle is hydrophilic, then the function is to transport water, aqueous solutions, and lipids (waxes) [26]. It is known that the aboveground organs of plants that grow in dry climates synthesize a significant amount of wax and cuticle, which are a barrier to transpiration [27]. Wax and cuticle are the main barriers against "uncontrolled" water loss by leaves. Therefore, in the adaptive responses of above-ground bodies, to action of a drought, the strengthened synthesis of these two components of cellular components of epidermis plays a certain role.

Wax can be located the inside cutin layer, or be situated on top of the cuticle. A two-year study of the long-term effects of drought on pine needles (*Pinus pinaster*) showed that pine needles activated the synthesis of cuticular wax, which was accompanied by the expression of transcription factor (*SHINE*), which is involved in cuticle synthesis [28]. It was found that the content of newly deposited wax under such conditions depends on the duration of exposure and the plant state [29]. Thus, a study of the effects of drought on tobacco plants (*Nicotiana glauca* L.) showed an increase of one and a half to two times the wax content in the leaves during 3 days of drought compared with unstressed plants [30]. Under conditions of enhanced drought, *Arabidopsis thaliana* plants increase not only the wax and cutin content, but it was accompanied by an increase of almost 50% in the thickness of cell walls. Wax deposition and extension of the cuticle layer in the epidermis increased plant tolerance to prolonged drought. A similar phenomenon has been noted by other researchers in studying the effects of drought on the leaves [31]. Even in temperate climates (under conditions of natural water deficit in the soil) there is an acceleration of wax synthesis on the surface of leaf blades. Carrying out a comparative study of the ultrastructure of the epidermis of leaves *Alisma plantago-aquatuca*, which grew on the river bank and in conditions of moderate soil drought, the researchers found an increase in the density of wax on the upper epidermis 2.5 times, on the lower epidermis—eight times [32]. In roots, drought or salt stress causes an increase in the content of suberin in the cell walls of the exo- and endoderm, which was shown in the species *Pistacia integerrima* and a hybrid, *P. atlantica x Plagiochila integerrima* [33].

For plants that grow in drought conditions is characterized by the participation of cell walls of the epidermis of the leaves in the water intake. It is known that the above ground organs of desert plants can absorb water from the leaf surface, intercept precipitation and absorb fog, using an atmosphere saturated with water [34, 35]. To do this, plants use trichomes [36], the specialized glands [37], and also form a hydrophilic surface in specialized epidermal cells that contain water pores [38]. It is shown that the leaves of *Reaumuria soongorica,* a super-xerophytic desert plant, are characterized by the presence of water-absorbing cells in the epidermis, which are closed by scales. Such scales cover water-absorbing cells during the day

#### *Tolerance of Plant Cell Wall to Environment DOI: http://dx.doi.org/10.5772/intechopen.105452*

and open water-absorbing pore channels at night, rising above the surface of epidermis. During the day, the valves of the water-absorbing structure of the scales are compressed, leaving a small central hole. At night, when the humidity rises, the basal cells raise the upper cells of the lid; after which the cells of the porous channels (capillaries) expand, forming a hole through which they begin to absorb atmospheric water [39]. The depth of the stomata is also an important structural feature of plants that have adapted to drought. Slightly sunken, submerged stomata are a typical feature of numerous psammophytes growing on the Mediterranean coast, including the coasts of Crete, Lampione, Tavolari and Malta, where extreme values of annual temperature have been recorded [40]. The results indicate that modifications in leaf architecture, including deepening the stomata into the epidermis, are important anatomical and physiological strategies that help psammophytes reduce water consumption.

Another feature of leaf structure to optimize water balance is twisting and/or folding of leaves. Leaf twisting is designed to maintain the optimal water balance of plants growing in inadequate water supply conditions [40, 41]. The twisting of the leaves of many psammophyte grasses is due to the specialized structure of the epidermis of the leaf blades and the presence of bulliform (motor) cells, the cell walls of which function to enter and exit water, reducing leaf area affected by drought [41, 42]. The cell walls of the bulliform cells of the epidermis synthesize guajacyl monolignol and callose, which helps to quickly change the entry or exit of water from these cells [43]. Twisting preserves optimal heat transfer and optimal water-vapor density in leaf tissues [41].

The presence of trichomes and increased cuticle density in cell walls are typical features of the leaf blades of psammophyte plants growing on coastal dunes [44]. Psammophytes have two types of trichomes: glandular and non-glandular. Glandular trichomes were found in the leaves and stems of psammophyte *Silene thymifolia*, which grew in Romania [45], in the leaves of *Tragopogon borysthenicus* and *T. orienthalis*, which grew in the Black Sea Reserve (Ukraine) and sand terraces of the Desna River, Ukraine [11].

#### **2.3 Role of lignin, pectin and cellulose**

Lignin is a branched biopolymer that, together with hemicellulose and pectin, acts as an adhesive matrix for cellulose microfibrils. Lignin provides mechanical strength of tissues and organs, impermeability of water and aqueous solutions through the cell walls. Lignin is a complex of phenylpropanoids (monolignols) [46]. Early work (Barnett, 1976) on the effect of drought on wood lignification showed that the tracheid rings stuck together because the secondary walls of young trees did not contain lignin. The formation of false rings in drought-stricken trees is a well-known phenomenon [47]. According to Lloyd Donald [48], who studied the anatomy of wood and the characteristics of cell walls in *Pinus radiate* under conditions of water stress (drought), false growth rings with bundles of weakly lignified tracheids, were found. It has been shown that wood exfoliation is due to poor adhesion between the tracheids due to a decrease in lignin content in the middle plate. The author explains this phenomenon in such a way that, apparently, there is an abnormal lignification due to dehydration of the outer cell walls. Since the formation of lignin occurs with the movement of water, the emergence of water or its insufficiency in the cell walls should interfere with or prevent both the transport and inclusion of lignin precursors and the process of lignification of the wall.

It has been shown that even a slight drought (up to 12 days) caused an increase in lignin precursors (coumaric and caffeic acids) in xylem maize juice, and this was due to a decrease in anionic peroxidase activity, indicating the effect of drought on lignin biosynthesis [49]. Different areas of the corn root respond differently to drought: in the basal part of the roots, growth is inhibited compared to the apical part of the roots, which is associated with the expression of two genes involved in lignin biosynthesis: shinamyl-CoA reductase-1 and -2. Such decrease in growth is due to an increase in lignin deposits, which increase the stiffness of the cell wall and reduce the growth rate, which may also be due to changes in factors such as water, minerals and sugars.

It was shown that after 28 days of drought, *Trifolium repens* reduced growth and increased lignin synthesis in the leaves, accompanied by increased activity of guajacol peroxidase, syringaldazine peroxidase and coniferol alcohol peroxidase [50]. It has been found that the activity of enzymes associated with lignin synthesis changes in plant leaves during drought. Thus, in the study of prolonged (35 and 47 days) drought on the leaves of *Ctenanthe setosa*, it was found that in parallel with the increase in lignin activity of enzymes involved in its synthesis also increased, in particular: phenylalanine ammonium lyase, soluble covalently bound peroxidase and polyphenol oxidase [51].

It is established that the impact of drought depends on the duration of its action, the species of plants and the growth stage. It has been shown that even a slight drought (up to 12 days) caused an increase in lignin precursors (coumaric and caffeic acids) in xylem maize juice, and this was due to a decrease in anionic peroxidase activity, indicating the effect of drought on lignin biosynthesis [49]. Roig-Oliver et al. showed for the first time that during long-term water deficiency, changes in the content of lignin, cellulose and hemicellulose in the cell walls of *Helianthus annuus* were accompanied by a significant decrease in phenols associated with the wall (coumaric, ferulic and caffeic acids) and with a negative correlation with photosynthesis (conductivity of the mesophyll to CO2), and with a positive correlation with palisade mesophyll thickness [52, 53].

Abiotic stress, including drought, cause a change in the mechanical strength of the cell wall due to the synthesis of lignin and activation of several the types of reactive oxidative species (ROS). Cell walls become stiffer and the overall mechanical stability of tissues and cells increases provided of an increase of wall peroxidases activity, increase in H2O2 concentration and/or an excess of peroxidase substrates [54]. The resulting increase in mechanical strength of the cell wall is occurred the change of cell's turgor that enable plant cells to endure the osmotic stress caused by drought [55].

Cell walls not only change their structure in response to drought, to reduce water evaporation by cells, but also act as structures that, accumulate water for the needs of the cell. In particular, plants increase the content of pectins as a wet absorbing structure. This has been shown in the laboratory in the study of roots and stems of wheat seedlings (*Triticum durum*) of two varieties (drought-resistant and drought-resistant) [14]. Comparison between the two genotypes showed some differences in the content of polysaccharides of the wall matrix and the content of α-cellulose. It was found that the residues of xylose, glucose and arabinose in the matrix are more than 90 mole%; the level of xyloglucans—was 23–39 mole%, arabinoxylans—38–48 mol%, while the content of pectins and 1–3,1–4 β-D-glucans—was very low. It has been shown that in drought-stable plants the content of rhamno-galacturonans I and II significantly increased under conditions of water stress, while in the second genotype such an increase was not observed [14]. The obtained results indicate that in drought-resistant

#### *Tolerance of Plant Cell Wall to Environment DOI: http://dx.doi.org/10.5772/intechopen.105452*

wheat varieties adaptation to drought occurs due to the increase of pectin chains, which leads to an increase in pectin gel that is the wet absorbing structure of the wall.

The study of the effect of water deficiency on the content of pectins in sunflower leaves showed that this polysaccharide is the most sensitive to water stress, it is the first to react to stress, reducing its content after a short exposure to stress (5 hours), while hemicellulose and lignin changed its contents only after 24 hours of stressful influence [56]. Early was established that pectins are crucial to determine wall characteristics. Changes in pectin physicochemical properties during stress induce the rearrangement of cell wall compounds, thus, modifying wall architecture and influencing on photosynthetic characterization of leaves of *A. thaliana atpme17.2* (SALK\_059908) and *atpae11.1* (GK 505H02) mutants from wild-type Columbia (Col-0) [57]. This study provides insights on how different cell wall architecture could influence the photosynthetic efficiency in *A. thaliana atpme17.2* and *atpae11.1* mutants in comparison to wild-type Col-0. Thus, it was established that cell wall composition modification could lead to reduced photosynthetic traits in *atpme17.2* and *atpae11.1* mutants maybe because of alterations in However, there was a strong reduction in the cell wall pectin fraction, expressed by the amounts of galacturonic acid in both *atpae11.1* and *atpme17.2* mutants.

The use of a model object, in particular *Penium margaritaceum*, which is a unicellular zygnematophyte to study the effect of a hyperosmotic environment on the formation of cell walls, showed that rhamnogalacturan-1 is one of the first components of the wall, which take part in acclimatization to hyper osmotic stress. This phenomenon was established using the method the labeled antibody [33, 58]. The study of cell walls composition in six Antarctic mosses (*Brachytecium austrosalebrosum, Warnstorfia sarmentosa, Bryum pseudotriquetrum*, *Polytrichum juniperinum, P. alpinum*, and *Sanionia uncinata)* showed a clear positive correlation between cell wall thickness and pectin content, which apparently determines the porosity of cell walls in arctic species, and contributes to adaptation and optimal photosynthesis in such conditions. The investigators found that the less pectin in the leaves, the higher the values of CO2 assimilation [59].

#### **2.4 Role of silicon**

It is established that the resistance of plants to drought is due not only to changes in the structure of cell walls of epidermal tissue, but also the deposition of silica in cell walls in the form of amorphous or crystalline inclusions [60]. According to Wang [61] silicon inclusions in epidermal cells reduce the influence of thermal effect on the leaves by reflecting the heat flow in the far infrared region of the sun light flux. This provides a passive mechanism for cooling the leaves in high sunlight. Although the mechanism of this action is not yet known, these issues need further to study. Silicon can deposit in leaf epidermis trichomes giving these structures are hardness and rigidity, making the leaves inedible to animals [62]. As a rule, most silicon is contained in cell wall protopectin, a water-soluble pectin fraction [63].

It has been established that silicon decrease the cuticle transpiration of aboveground organs. This chemical element, which accumulates in the cells of the epidermis of leaves and stems, forms a thickened cuticle-silicon wall, which protects the plant from excessive moisture consumption by reducing the cuticle transpiration. In addition, the plant's walls can form hydrophilic silicate-galactose complexes that bind free water, thereby increasing the water retention capacity as in specific cells, as and in different tissues and in the organs of plant [64, 65].

Because of the density of cell walls and their ability to retain moisture, silicon compounds can significantly increase plant resistance to drought and protect plants from being lodged (fallen) [66]. Silicon reduces of water evaporation on the leaf surface, as has been shown, for example, on rice seedlings [67], on other crops, in particular in drought-resistant wheat [68] and sorghum [69]. Silicon can also influence water transport by regulating the osmotic potential of cells by increasing synthesis and accumulation of osmotic active substances (e.g., proline, sugars and inorganic ions) [70, 71].

#### **2.5 Involvement of genes in adaptation process to drought**

Over the years, significant progress has been made in discovering the cell wallspecific genes related to drought tolerance [72, 73]. These researches were carried out at rice in vegetative and reproductive stages [72]. In the reviews [72, 73] shown the major candidate genes underlying the function of quantitative trait loci directly or indirectly associated with the cell wall plasticization-mediated under drought tolerance or salinity stress of plants. On rice plant during of drought stress was identifying series genes, which take part in tolerance of this species to both drought or salinity stress: 1) drought inducible AP2/ERF family TF gene *OsERF48,* including cell wall related genes such as *OsXTH9*, *OsAGP24, OsEXPA4, OsEXPA8, OsEXPB2, OsEXPB3, OsEXPB6* and *OsAGP3,* which associated with cell expansion and cell wall plasticization-mediated root growth under abiotic stress; 2) a lignin biosynthesis gene *OsCCR10* (*Oryza sativa CINNAMOYL-COA REDUCTASE 10*) is also highly induced by drought in the roots of rice/ and 3) genes are associated with cell wall loosening (*OsEXP1*, *OsEXP2*, *EGase*, and two *XETs*), with lignin biosynthesis (*PAL*, *C3H*, *4CL*, *CCoAOMT*, *CAD*, and *peroxidase*), and with the metabolism of cell wall proteins (*GRP* and *UDP-GlcNAc pyrophosphorylase*) and polysaccharides (*OsCslF2*, *GMPase, xylose isomerase,* and *beta-1,3-glucanase*), and 4) genes, including drought responsive, ABA-responsive, superoxide dismutase and cell wall-related genes (*LOC\_Os01g64860; LOC\_Os01g72510; LOC\_Os05g35320; LOC\_Os12g36810*, etc. [72].

Molecular methods have shown that during drought, increased wax and cutin synthesis is accompanied by activation of genes (*Ltps* and *WAX9*) that express proteins involved in the synthesis and deposition of wax and cutin, as well as the synthesis of transport proteins [74]. In a study of prolonged drought on the model plant *A. thaliana* [75, 76] was identified the presence of 11 genes, which were divided into groups according to the functions of the proteins they encode:


An early response of the *pDr* (*mDr*- drought) genes to stress drought in *A. thaliana* seedlings was detected and it was shown that drought reduces the expression of four expansin genes on the first day of stress. The fifth gene, *EXLB1*, has been

#### *Tolerance of Plant Cell Wall to Environment DOI: http://dx.doi.org/10.5772/intechopen.105452*

deducting since the second day of stress [76]. It was shown that these genes take part in an early response to drought. Other researchers have found the expression of expansin genes (*Exp1, Exp5* and *ExpB8*) by reducing water potential in the apex of roots, corn leaves, and in the apex of tomato stems [77–79].

The physical properties of the cell wall are also play an important role in water deficiency [80]. Analysis of the *pDr* transcript showed repression of numerous genes involved in wall elongation stretching [81], whereas the action of mean osmotic stress induced elongating genes [82]. Microray analysis (qRT-PCR) revealed the regulation of cell stretching genes during *mDr* treatment. It has been established that most drought genes reduce their regulation during drought. The repression of four *EXLB* genes begins on the first day, while the fifth *EXLB1* gene peaks on the first day and decreases on the second day; it is on the first day that acclimatization to stress may begin, depending on the organ, species and tissue [29, 83]. These studies were performed on *A. thaliana* [76].

The study of physical properties, stiffness in particular, cell wall from the root elongation zone using atomic force microscopy in *A. thaliana* Columbia-0 (Col-0) wild-type and mutant plants (with *TETRATRICOPEPTIDE THIOREDOXIN-LIKE 1, ttl1* gene, cause root swelling and root growth arrest under NaCl and osmotic stress) revealed that root of mutant (with *ttl1* gene) increase of the stiffness of the cell wall in root elongation zone [84].

A study of the effects of drought on Arabidopsis mutant plants (with cellulose synthase genes—*AtCesA8 / IRX1*), which were resistant to drought, NaCl, mannitol and other osmotic stresses, showed that cellulose synthesis under drought and osmotic stress is due to the expression of cellulose-synthase [85]. These researchers showed the effect of drought, by the absence of watering for 2 weeks, on the genetic traits of the cellulose-synthesizing complex in the leaves of *A. thaliana*. It was found the next following physiological and molecular changes in the leaves of two mutants Arabidopsis (with genes withering, leaf wilting 2–1 and leaf wilting 2–2; genes: lew2–1 and lew 2–2), which were resistant to drought, salt salinity, mannitol and other osmotic stresses: the lew2 mutant was shown to accumulate more ABA, proline, and soluble sugars compared to the control (wild type). New alleles of the AtCesA8 / IRX1 gene encoding subunits of the cellulose-synthesizing complex have been discovered in this mutant. The data obtained suggest that cellulose synthesis is quite important for the response to osmotic stress and drought [85].

Rui and Finneny [86] proposed a model for regulating the cell wall response to stress; according to this model, certain aspects of the wall itself can act as growth-regulating signals. The molecular components of the signaling pathways that determine and maintain cell wall integrity are shown, including sensors that detect changes on the cell surface and downstream signal transduction modules. There are several cell wall receptors that sense stress, including drought or salinity. Such receptors, according to the authors, may be the receptor-like kinase THESEUS1 (THE1) and FERONIA (FER) localized on the plasma membrane or Ca2+.ATPase. Kinase THE1 has been identified by suppressor screening in a cellulose-deficient mutant background; and FER is widely expressed and serves as a signaling node that functions in a wide range of processes, including plant growth, vacuole morphology, mechanosensing, hormonal signaling, and others. In contrast, the FER protein exhibits defects in growth recovery under salt stress as a result of failure to reverse salt-induced softening of the wall and increased frequency of cell rupture.

Summarizing the above material of numerous experimental works, we can propose the following scheme of response of cell walls of plants growing in drought

#### **Figure 1.**

*Schematic representation of the main functional changes of plant cell wall during adaptation to drought.*

or deserts: perception of drought signal (high air temperature and low soil moisture) leaves and roots → stopping or inhibiting growth of root and leaves → reduction of cell size → closure of stomata in leaves → reduction of stomatal conductivity for CO2 (or cessation of stomata and shedding of leaves) → in the roots of the formation of water lacunae; in stems of succulents (during leaf shedding) water storage in specialized lacunae of the parenchyma → thickening of cell walls, their lignification and suberinization, intensified synthesis of wax, expression of genes associated with the synthesis of extensins, dehydrins and cellulose, activation of enzymes for synthesis of lignin, suberin due to changes in the expression of the corresponding genes (**Figure 1**).

#### **3. Flooding**

Flooding is a potentially detrimental stress for many terrestrial plants; flooding occur when water covers the area, caused by both natural (river floods, heavy rainfall, tides) and artificial causes (construction of reservoirs, ponds); it can be short-term, intermittent (during river floods) or long-term, in which many species may die. Peculiarity of flooding as a stress factor is a combination of significant changes in water availability of plant and oxygen respiration in the root system, and as a result there is inhibition of aerobic processes, impaired absorption of ions and nutrients,

#### *Tolerance of Plant Cell Wall to Environment DOI: http://dx.doi.org/10.5772/intechopen.105452*

changes in metabolism and growth processes [87]. The next factors are affected on the flood plant: a decrease in illumination and change in the light spectrum, a lack of acidity and CO2. It is known that water absorb flow of light and disperse of light [88, 89]. In flood conditions, the diffusion of gases is much slower than in air, and this is what limits normal photosynthesis and aerobic respiration [87]. Some plants that are resistant to flooding use the acceleration of stem growth to get out of the water and such a stem rises above the flooded part of the plant. The part of the plant that emerges from the water begins to come into contact with the air environment, renovating aerobic metabolism and photosynthesis [90]. Hydrophytes and wetland plants, which have adapted to both the lack of oxygen in the soil and the constant aquatic environment, have for millennia developed certain mechanisms of adaptation at different levels of the organization. The main signs of rearrangement are a decrease in the thickness of the leaf blade, rearrangement of the mesophyll, the presence of chloroplasts in the epidermis and changes in the structure of cell walls [91]. Cell wall of the epidermis of flooding plants is the first to react to the water environment, changing their structural- functional characteristics to optimize the water balance of plants. Therefore, the analysis of comparative structural and functional studies of flooded and above-water leaves is important for understanding the role of cell wall in the adaptation of plants to the aquatic environment.

#### **3.1 Structural and functional changes of cell walls**

The greatest stress for under-water plants is the weakening of gas exchange, which causes a decrease in oxygen in the stem and root, and also [92, 93] can induce enhanced growth by elongation, which promotes the release of leaves from the water to the surface and accelerates their contact with air [94]. Modification of cell walls for underwater growth and elongation requires energy, but, as a rule, such plants are characterized by limited aerobic metabolism. It is studied the structural changes in *Rumex palustris* stems and leaves that were induced by flooding [95]; authors have shown the decrease of size leaf and cuticle thickness in comparison with that in above-water leaves.

In cell walls of flooding leaves is occurred in protein synthesis. Under-water growth of rice is characterized by more elastic cell walls, which are usually characteristic of walls with increased synthesis of expansin [96–98]. In the cell wall noted protein modification, including expansins, which are activated at acidic pH [99, 100]. Rapid regulation of apoplastic pH provides a rapid way to regulate and modify apoplast expansin activity. The association between decreased cell wall elongations has been attributed to decreased tissue sensitivity to expansins [101]. Changes in the composition and nature of cross-links between cell wall polysaccharides may be limited by the mobility of expansins or their availability to the substrate polymer. The study showed a change in the ability of expansions to bind to cellulose depending on the properties of the hemicelluloses that cover the microfibrils [102].

In low-growing rice, flood resistance is explained by the activation of two genes: *SUB1A-1* gene and gene *ERF* (ethylene response factor) [103]. The *SUB1A-1* gene exhibits prolonged resistance to flooding associated with anaerobic metabolism and suppression of genes associated with high-energy processes, such as growth involving expansin [104]. The expression of *SUB1A-1e* is induced by ethylene, which is synthesized in a plant growing under water. Restriction of expansin transcripts by induction of *SUB1A-1* occurs as a result of suppression of stem tension and dormancy of the plant. Modification of the growth rate is mediated by the modification of the cell wall

in the plant organs that is not only under water but also above water. In particular, when the internode of rice came out of the water into the air, the rate of growth decreased.

The composition of the wall can also determine the effectiveness of expansins to elongation of a wall under conditions of flooding the plant. The decrease in the elongation of cell walls in the segments of underwater rice stalks at the exit from the water to the air correlates with the changes in the composition of walls: an increase in xylose and pectic acids, such as ferule acid [101], which has the ability to form cross-links between polysaccharides of a cell wall [105]. Deposits of xylose-enriched polysaccharides can change the composition of the cell wall by limiting the action of expansin. It was found that the composition of polysaccharides of flooded plants differs from that of surface organs, as shown by Little [106] in stems of *Ludwigia repens*: along with a significant decrease in cellulose content, the content of hemicelluloses and lignin in underwater stems increased.

The outer cell walls of the epidermis of submerging and the above-water leaves are the first barrier, the first transport route of CO2 and water, as well as the point of contact of plant organs with the environment. Cell walls of flooding leaves became thinner and their structure is characterized by loosening. Regarding the loosening of the cell wall, there are many models of this process. The first hypothesis about the acid-induced loosening mechanism was proposed by Cleland [107, 108]. It was later shown that the hydrolysis of polysaccharides during loosening is a complex process in which the enzymatic hydrolysis of polysaccharides of the wall matrix occurs with the participation of endoglucanases and expansins. The latter shown that hydrolyse polysaccharides induce cell expansion and increase the plasticity of the wall depending on the pH of the apoplast [109, 110]. The mechanism of formation of thin cell walls in various plant tissues is explained by changes in cell turgor and a decrease in the activity of enzymes involved in the synthesis of wall polysaccharides [109].

In submerged plants in the epidermis is also synthesized and deposited cuticle in the periclinal walls, and cuticle structure change [111]. In the cuticle of leaves and stems, which grow rapidly by elongation under water, there is an accelerated hydrolysis of cutin polymers [112]. In aquatic plants (hydrophytes) the cuticle of the epidermis of leaves and stems causes the presence of super hydrophobicity; it is this property that prevents the formation of an aqueous film on the surface of the organs submerged in water, which greatly reduces the gas exchange between the surface of the leaf and the gases dissolved in water. Despite the fact that CO2 absorption for photosynthesis is reduced in flooded plants, the air layer or gas film on the surface of underwater leaves continues to exchange O2 and CO2 through the cuticle from the surrounding water layer, and therefore underwater photosynthesis and underwater respiration occur in epidermal cells [113].

Most underwater leaves of hydrophytes have no stomata. Transport functions mainly fall on the cell walls of the epidermis and pores in epidermis. Cuticular pores were revealed on the cross-sections of epidermal cells of underwater *Sagittaria sagittifolia, Trapa natans, Myriophyllum spicatum, Potamogeton pectinatus* and *Potamogeton perfoliatus* [113, 114]. It was shown that average high of the cuticular pores in the cells of the epidermis *of M. spica*tum was about 130 nm; pore density ~ 12–15 per 1 μm of cuticle length; in *P. pectinatus* cells—the pore length depending on the plane of the section is ranged from 80 to 100 nm, the average pore density ~ 20 per 1 μm of cuticle length; in *P. perfoliatus*, the cuticular pores had low contrast and were barely visible in the form of rounded electronically transparent structures with a diameter near ~4–5 nm. Besides, author shown the absence of stomata on both surfaces of flooding

#### *Tolerance of Plant Cell Wall to Environment DOI: http://dx.doi.org/10.5772/intechopen.105452*

leaves, the decrease of high of cuticular ridges in anticlinal walls and absent of wax in cell walls. The mechanism of the absence of wax on the surface of the periclinal wall of the underwater leaves can be explained by the next, it is established that genes (*Ltp, LTPs* and *WAX9*) that are responsible for the transcription of lipid-transporting proteins have recently been identified in wax-enriched epidermal cells. In the case of expression of the corresponding genes in the cells of the epidermis, the effect of the accumulation of the corresponding mRNA was found [115, 116], and possible that absent of wax in the epidermis of underwater leaves may be a consequence of genetic plasticity, which lead to the inhibition of the synthesis of precursors of wax (C12-, C14- and C16-ω-hydroxy fatty acids) and wax synthase activity (fatty alcohol acyltransferase) [117, 118].

#### **3.2 The role of cellulose**

Under-water leaves are characterized by the increase of amorphous cellulose and the decrease of its crystalline form. It is known that water is adsorbed by amorphous zones of cellulose, which are dominated by hydrogen bonds [119]. The crystalline component of cellulose micro fibrils is not involved in the transport or absorption of water molecules [120]. Given the above literature, we hypothesized that one of the adaptive features of the plant to flooding should be not only differences in cellulose content, but also advantages in the synthesis of its amorphous form. An optimal example of such adaptation to flooding can be the data of comparative structural and functional studies of cellulose in plants with underwater and above-water (surface) leaves, in particular in *Sium latifolium, T. natans* and *S. sagittifolia*, as well as leaves of hydrophytes growing only under water: *Potamogeton perfoliat, P. pectinatus* and *Myriiophyllum spicatum*. The above-water leaves of studied species, particular *S. latifolium, T. natans* and *S. sagittifolia* contained twice more of crystalline cellulose [18, 121, 122]. Considering the data on the identification of *PhEXP1* gene (in *Petunia hybrida* mutant) responsible for the synthesis of amorphous cellulose [123], we believe that increased content of amorphous cellulose in underwater leaves, due to genetic differences, is an adaptive change of flooded plants. Literature data on the cellulose content in flooded plants are different. Métraux and Kende [124] found no differences in cellulose content in flooded and above-water internodes of rice stems, whereas Little [106] showed a 1.5-fold decrease in cellulose content in flooded shoots of *L. repens* compared to above-water shoots. It is possible, that it depends on the species, tissue and stage of development of the plant. We hypothesize that the decrease in cellulose content in the underwater leaves of the studied species may be due to inhibition of thegene *CesA* (cellulose synthase catalytic subunit) encoding cellulose synthesis enzymes in both primary [125] and secondary cell walls [126].

#### **3.3 The role callose, lignin and pectin**

Callose—a polysaccharide of the cell walls, formed by glucose residues, connected at the base of β-1-3-glucoside bonds and in the lateral branches—1-6 connections. It is known that β—1,3—glucan plays a key role in intercellular water transport, cell growth and differentiation, osmotic stretching of cells, plant protection under biotic and abiotic stresses [127] and increases the elasticity and flexibility of leaves and stems [128–130]. It was established the effects of natural flooding on callose content in *Alisma plantago-aquatica* leaves with laser confocal microscopy. It was shown that the content of callose in the cell walls of submerged leaves was more in three times in

epidermis walls and in 8 times—in mesophyll cell walls in comparison with abovewater leaves [131]. Increase of callose content in cell walls of other species was noted also: in submerged leaves of *S. sagittifolia—*in six times, in floating leaves of *T. natans*in 1.4 times in comparison with above-water leaves. It was established that increase of callose content in submerged leaves are accompanied by change of calcium ions content in walls [132].

Lignin is a polymer of aromatic alcohols, which is synthesized in the cell walls, is completed the growth by tension, and it is involved in the adaptation of plants to flooding and in the change of the structure of the matrix of cell wall, providing obstruction of water and aqueous solutions through the cell walls and also form the barrier for pathogens. Lignin is a complex of monolignols formed from p-hydroxyphenyl, guajacyl, syringyl and H-phenylpropanoids components [133], which are involved in the polymerization of lignin, and they differ in the degree of methoxylation [46]. Flooding and siltation affect the lignification of cell walls. The study of mechanisms of adaptation of the root system of rice to flooding and siltation shown the main effect is the deficiency of oxygen, resulting in roots forming aerenchyma for storing of oxygen [134]. Lignin deposition, which counteracts the penetration of ions such as Fe2+, Cu2+ and NaCl [135] has been observed during of flooding roots. It is considered that lignin and suberin can form a barrier to the penetration of oxygen and ions.

The effect of flooding on the lignification of rice stems was found by comparing the stems of three varieties of rice. It was found that the lignin content in rice stems and the activity of two enzymes of the lignification (coniferol alcohol dehydrogenase (CAD) and phenylalanine ammonium lyase (PAL) were reduced after flooding in the flood-sensitive variety and in control. Lignin and the activities of the studied enzymes were interrelated. According to researchers [136, 137], underwater plant organs are stressed due to the tension of the water column and the mechanical action of waves, which should cause stress in flooded organs. Lignin of dicotyledonous plants consists of guajacyl (G), syringyl (S) and phenylpropanoids (H) components, Lignin of most monocotyledons have G and S units, the content of which is almost the same, they may also contain H units [138].

The question of the distribution of lignin in various tissues of submerged plant organs, the role of monolignols and their ratio in cell walls in the process of natural adaptation of plants to flooding has remained open until recently. Recently it was established that in floating leaf walls underwater leaves of *T. natans* the level of lignin fluorescence intensity increased 1.52 times in the anticlinal walls of the epidermis and 1.2 times—in the periclinal walls, and decreased 1.6 times in the cell walls of photosynthetic parenchyma compared to the corresponding cell walls of floating leaf [18]. Cytochemical studies of monolignols, their localization and content in the leaves of the four studied species of hydrophytes (*Sagittaria sagitifolia, T. natans, M. spicatum; and P. perfoliatus*) showed both common and different features. Common features were: (1) the presence of syringyl and guajacyl in the studied species, regardless of the conditions of leaf growth; (2) almost identical (low) values of the S/G ratio in the cells of the vessels of the above-water leaves of *Sagittaria sagitifolia* and water nut; (3) the highest S/G values for *M. spicatum*; and (4) a certain polarity S/G, which is characteristic of each species. Distinctive features were: (1) the relative content of syringyl and guajacyl, as well as the ratio of S/G in the cell walls of underwater and abovewater leaves of arrowroot and water nut; and (2) high S/G ratio in the periclinal walls of floating leaves compared to those in submerged leaves of water nut [132].

#### *Tolerance of Plant Cell Wall to Environment DOI: http://dx.doi.org/10.5772/intechopen.105452*

Similar to the increase in lignin in flooded stems *Ludviga repens* was established [106]. Little S. showed that when *L. repens* stems were flooded, the lignin content became 1.6 times higher than in stems that came out (from) of the water. Why is this happening? Lignin is known to be a highly branched polymer of phenylpropanoid components synthesized in a complex cycle [139], the passage of which depends on numerous endogenous (phytohormones) and exogenous factors, including exposure to light, temperature, different gases and biotic stresses [140]. It is believed that the functional value of lignin is the mechanical support of tissues, which allows the plant to stay upright relative to the Earth and not fall [141]. At the same time, underwater organs of flooded plants (stems and leaves) must withstand water pressure (its weight) and the action of waves [136, 137] which affects the growth and structural and functional parameters of tissues and cells as this is described for flooded leaves of *Veronica anagallis*-*aquatica* [141]. On the other hand, it is known that underwater leaves are devoid of trichomes, stomata and thick cuticle) [132], so the surface of submerged plant organs becomes more sensitive to pathogen invasion. It has been shown that leaves and stems synthesize lignin in response to attack by pathogens (bacteria and fungi) [142]. It is possible that the above exogenous parameters and some endogenous factors cause increased synthesis of lignin in the underwater leaves of the studied species of *T. natans* and *S. sagittifolia* [132].

At flooding of terrestrial plants leads to the formation of aerenchyma in roots, nodules, stem or submerged leaves. Aerenchyma helps the plant to survive in conditions of hypoxia by reducing the number of oxygen-consuming cells in vegetative organs [143, 144]. It was established that at lysigenous type of formation of an aerenchyma occurred the lysis not only of cytoplasmic organelles in tissues, but also lysis of their cell walls. The increases in aerenchyma air volume may enable prolonged functioning of aerobic metabolic processes in tissues exposed to low-oxygen conditions. Cellulose, hemicellulose and pectin lysis are occurs during aerenchyma formation. Probably, that modification of the pectin homogalacturonan backbone structure through de-methyl-esterification appears to be one mechanism by which cell walls and middle lamella of tissues is degradate of pectin and enable cavity formation of aerenchyma in roots [143]. Additionally, presence of fully and partially de-methylesterified homogalacturonan residues in cell walls of forming tylose-like cells suggests these pectin structures are essential to development of the cells that occlude aerenchyma of *P. sativum*, *P. coccineus* and *C. arietinum*. The investigators think that aerenchyma formation may depend on activity of cellulase, xylanase working together to achieve cell wall degradation. Specifically, xylanases and cellulases may degrade xylan and cellulose polysaccharides in advance of de-methyl-esterification of pectin by PME enzymes and subsequent degradation by pectinases [143].

That is, the constant aquatic environment is one of the main exogenous factors of increased synthesis of lignin in the studied hydrophytes. In addition, we see that the presence of syringyl and guajacyl monolignols, as well as their relationship in the cell walls of the epidermis, mesophyll and leaf vessels of hydrophytes is similar to that described for dicotyledonous angiosperms [138, 145]. We do not rule out that the cell walls of the underwater leaves of the studied plants contain a third monolignol phenylpropanoid (*p*-hydroxyphenyl) which will need to be investigated by other methods. Summarizing the whole section on the impact of flooding on the structural and functional changes of cell walls, in particular on the synthesis of cellulose, callose and lignin, we can schematically present the course of major events occurring in the apoplast of most cells (**Figure 2**).

**Figure 2.** *Schematic representation of the main functional changes of plant cell wall during adaptation to flooding.*

#### **4. Concluding remarks**

The results of researches concerning on the role of cell walls in plant response to natural unfavorable conditions influences show that cell wall is one of the compartments of a plant cell that responds to drought and flooding. In most wild species and in cultivated species, cell walls stand a marker of such influence. The inhibition of plant growth, the change of plant morphological and anatomical signs, change of cell wall ultrastructure, its composition is occurred under prolonged drought or flooding. Changes in the structural and functional characteristics of cell walls allow plants to survive. Plant adaptation to these factors is depended on species, stage of growth plant and influence duration. Numerous studies have shown that drought effects negatively on cell walls. The main mechanisms of plant adaptation to the effects of drought involve a decrease in the intensity of transpiration, an increase in the synthesis of wax, suberin, and lignin, as well as the compaction of the walls of the epidermis tissues for preservation of optimal water balance. Upon exposure to flooding, adaptation mechanisms are expressed in the next: decrease stomata density

#### *Tolerance of Plant Cell Wall to Environment DOI: http://dx.doi.org/10.5772/intechopen.105452*

and wax in leaf epidermis; a loosening of cellulose micro fibrils in walls of epidermal tissue and a present of cuticle pores; the decrease of common cellulose content and crystalline form of cellulose; an increase of content of amorphous cellulose, hemicelluloses in a cell wall; an intensification of callose synthesis; the change of a ratio of monolignols (syringyl and quajacyl) in walls; the activation of peroxidase and expansin, an intensification of ethylene synthesis and a change of calcium balance in apoplast. However, the sequence of these processes has not been fully disclosed. The question of the launch of adaptative processes also remains open. These issues require further research. The question of the relationship between the water balance of the cell, photosynthesis and the values of energy of light photons on the surface of the leaves, which launch an adaptive response in the plant under adverse natural changes or under stress, also remains open.

### **Competing interests**

The author declares that there is no conflict interest.

### **Author details**

Olena Nedukha Department of Cell Biology and Anatomy, M.G. Kholodny Institute of Botany, of National Academy of Sciences of Ukraine, Ukraine

\*Address all correspondence to: o.nedukha@hotmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 17**

## Climate Change and Abiotic Stresses in Plants

*Ananya Baidya, Mohammed Anwar Ali and Kousik Atta*

#### **Abstract**

Any altered physiological conditions which can hamper the growth and development of crop plants that is denoted as stress. The challenges of abiotic stress on plant growth and development are evident among the emerging ecological impacts of climate change. In a compatible environment for one genotype may not be fitable for another. The field of plant abiotic stress encompasses all studies on abiotic factors or stresses from the environment that can impose stress on a variety of species. Abiotic stress induces redox imbalance during reproductive growth. These studies indicate that stress induced over accumulation of ROS leads to pollen abortion and programmed cell death of microspores in developing anthers consequently resulting in male sterility. With these changing climatic conditions climate resilient crops and crop varieties have been recommended as a way for farmers to cope with or adopt to climate change. Integrated physiological and molecular approaches are important for combating multiple abiotic stresses.

**Keywords:** abiotic stress, climate change, ROS

#### **1. Introduction**

Plants encounter changing environments during their entire life cycle, from seedling to reproductive stage, that are often unfavourable to their growth and developmental processes, and they create unique mechanisms to cope with these challenges. There are primarily two types of negative environmental variables. The first are biotic variables, such as disease and herbivore attacks, and the second are abiotic factors, such as drought, heat, cold, nutrient inadequacy, and heavy metal build-up in the soil. Salt, drought, and temperature, for example, have an impact on the geographical distribution of plant species and disturb plant metabolism. As a result, they reduce the quality and quantity of food production in agriculture, lowering the food supply for a growing population, and tolerance mechanisms in plants have been thoroughly explored to overcome these negative impacts. Various environmental variables (biotic and abiotic) activate stress tolerance genes in plants, causing them to become resistant.

#### **2. Heat stress**

The average temperature has been determined to be increasing by 0.2°C per year, and it will have to grow by 1.8°C 4°C by the end of the year 2100, making temperature

**Figure 1.**

*Effect of low and high-temperature stress in plants. Source: Tiwary et al. [9].*

one of the most damaging stresses [1]. Temperature-related climate change is a global concern that has affected plant physiological and biochemical activity, lowering crop output [1, 2]. Plants are subjected to heat stress as a result of rising temperatures, which is dependent on the quality, intensity, and duration of light.

All environmental conditions (biotic and abiotic) contribute to the production of reactive oxygen species (ROS), including heat stress, which damages macromolecules such as DNA, proteins, and lipids [3], and plants are under oxidative stress. Heat stress also changed the expression of genes involved in the creation of osmo-protectants, detoxifying enzymes, transporters, and regulatory proteins [4]. Heat stress, on the other hand, inhibits protein folding, alters membrane (lipid bilayer) fluidity and cytoskeleton arrangement, and has an impact on vegetative and reproductive tissue [5, 6]. A rise in temperature up to a certain point is helpful to plants, since it governs plant circadian rhythms, plant movements (corolla opening/closing), and impacts the geographical dispersion of plants in nature [7]. High temperatures increased the susceptibility of plants to pathogens. When the ambient temperature rose, the infection capability of tobacco mosaic and tomato-spotted wilt viruses increased, causing viral illnesses in tobacco (*Nicotiana tabacum*) and pepper (*Capsicum annuum*), respectively [8]. Wheat genotypes were shown to be more sensitive to *Cochliobolus sativus* (which causes spot blotch) when night time temperatures rose (**Figure 1**) [10].

#### **3. Cold stress**

Apart from heat stress, a drop in ambient temperature causes chilling stress in plants, which has a significant effect on cell physiology. Chilling, according to Ruelland et al. [5], promotes cell death by suppressing enzymatic activities,

#### *Climate Change and Abiotic Stresses in Plants DOI: http://dx.doi.org/10.5772/intechopen.105575*

rigidifying biological membranes, stabilising nucleic acids, generating reactive oxygen species (ROS), and impairing photosynthesis. Low temperature causes flowering in plants, which is known as vernalization [11], and upregulates metabolic processes that confer the tolerance strategy of plants, known as the cold-hardening process [12], which results in the accumulation of compatible solutes (sugar), membrane composition changes, and increased synthesis of dehydrin-like proteins [13]. Plants had long- and short-term responses to temperature stress (heat or cold stress). Long-term effects included morphological and phenological adaptations, whereas short-term effects included leaf orientation changes, increased transpiration, and changes in membrane lipid content.

Heat stress causes a reduction in water loss by closing the stomata, as well as increased stomatal densities and larger xylem vessels [14], allowing plants to thrive in these harsh conditions. ROS production is an unavoidable by-product of aerobic activity, and its toxicity is determined by its concentration. It functions as a signalling molecule at low concentrations, but at greater concentrations, it becomes poisonous and causes cell death [15]. Under various stress situations, particularly heat stress, ROS such as H2O2, O2, and 1 O2 are produced [16]. Every plant contains an assortment of antioxidant systems to deal with the detrimental effects of ROS. These systems help to lessen the negative effects. Enzymatic antioxidants such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and glutathione S-transferase (GST), as well as non-enzymatic antioxidants such as cysteine, proline, non-protein thiols, and the synthesis of molecular chaperones known as heat-shock proteins (HSPs), are part of the antioxidant system [17]. HSPs are important proteins that are activated by heat stress and target HS-responsive transcription factors, which regulate protein quality by re-energising proteins [18].

#### **4. Salt stress**

The impact of salt stress in plants has become an important issue in modern agricultural development, climate change and global food crisis. Survival of plants under adverse environmental conditions relies on integration of stress adaptive metabolic and structural changes into endogenous developmental programs. Almost all crops are sensitive to high salt (NaCl) levels [19], however the degree of sensitivity varies substantially between species and marginally between cultivar types within a species [19]. The plant response to high salt concentrations is complex. The effect of excessive NaCl concentrations on plants results in osmotic stress and creates an ionic imbalance due to accumulation of toxic ions like Na<sup>+</sup> and Cl<sup>−</sup> . The plant's developmental growth stage [20] as well as external environmental elements influence the plant's sensitivity to salinity [21]. Salt stress is a major issue for agriculture systems since it reduces crop yield potential [22]. Excess salt in the soil affects plant growth by lowering seed germination, plant height, root length, and fructification [19], and it has also negative impact on mineral homeostasis, in particular Ca2+ and K+ . But it also has indirect impacts on the food web segment that depends on that host plant [22]. Salinity induced imbalance of cellular ion homeostasis is coped with regulated ion influx and effect at plasma membrane and vacuoler ion sequestration [23]. All of this occurs as a result of salt-induced oxidative stress [24], ion toxicity, and decreased photosynthetic rate in plants, all of which led to a considerable reduction in overall crop production [25].

Despite this, plants may naturally live and complete their life cycle under extreme salt stress [26]. They have well-developed biological, chemical, and physiological mechanisms [19], which could result in the synthesis of products and the start of processes that improve plant resistance to soluble salts [26]. Depending on the types of modifications they must perform in response to plant salt stress, these systems could be complex or simple [26]. Plant stress sensing and signalling machinery are critical components of their salt stress tolerance network, according to several studies [27]. The most common salt tolerance systems in plants are salt excessively sensitive (SOS) signalling pathways, hyperosmotic sensors, gene regulation in roots, and plant membrane Na+ and K+ transporters [27]. Plants' biological salt tolerance adaptations include osmoregulation and hormonal alterations. Other options for improving plant tolerance to salt stress include the use of plant growth-promoting rhizobacteria (PGPR), plant fungal associations, and the application of organic and inorganic amendments [28]. To address this problem, scientists used salt tolerant engineered plants or transgenic salt resistant cultivars, as well as a potential physiological method [27]. However, because the salt tolerance mechanism in plants is genetically complex, it was not very successful [19].

#### **5. Drought stress**

Drought is one of the key factors affecting crop output around the world, as crop growth and yield are both influenced by this stress [29]. Drought stress is caused by a lack of rainfall, salt buildup in the soil, significant temperature swings, and excessive light intensity. Due to climate change, climate simulation models that take previous year data and estimate the future have indicated that this stress would become more severe in the near future. Drought stress affects plant growth, water retention, and water efficiency [30, 31], as well as causing changes in physiological, biochemical, morphological, and molecular features [30, 31]. Drought-tolerant/resistant plants have evolved a more efficient drought resilience mechanism to withstand drought stress, however these mechanisms are not well-organised or investigated. Plants, in general, have a mechanism for maintaining cell homeostasis that involves increased water transport into the plant cell. Drought resistance is a cellular defence mechanism that allows cells to survive long periods of drought [32]. Plants, in addition to drought tolerance, tend to undergo a number of metabolic changes in response to drought stress, including decreased ribulose bisphosphate (RuBP) and adenosine triphosphate (ATP) levels, as well as reduced Rubisco activity. Plants reduce substomatal CO2 conductivity and close stomata to avoid water loss up to their maximum capacity during drought. Water stress reduces the light saturation rate, decarboxylation velocity, ribulose 1,5-bisphosphate regeneration ability, photosystem II (PS-II) efficiency, and stomatal conductance in plants [33]. Drought has a negative impact on amino acids in plants, such as asparagine and glutamic acid, although plants can respond by boosting amino acids and soluble levels to temporarily relieve stress and manage osmotic potential [34].

#### **6. Conclusions**

Daily environmental fluctuations can have dramatic effects on plant vegetative growth at multiple levels, resulting in molecular, cellular, physiological and morphological changes. Environmental stress factors such as drought, elevated temperature,

#### *Climate Change and Abiotic Stresses in Plants DOI: http://dx.doi.org/10.5772/intechopen.105575*

salinity and rising CO2 affect to sustainable agriculture. Plants are even more sensitive to environmental changes during reproductive stages. Changing climate condition imposes different abiotic stresses to plant growth and development. This chapter will provide how different stress condition affects the plant growth and development and how they acclimatised with changing environment.

With this conclusion, there are some future scopes of research. Changing climate and Yield reduction in the late sown crop may be minimised with appropriate hormonal and nutritional interventions. Hormonal boosting may invigorate the plants under stress [35]. The stimulatory effect of hormones does not sustain for a long period when applied externally. Hormone based stimulatory physiology may be enhanced internally through nutritional treatment with boron and zinc which either enhances auxin stimulation or auxin biosynthesis. Smooth and healthy reproductive development is a precondition for the realisation of the yield potential of the crop.

#### **Author details**

Ananya Baidya1 \*, Mohammed Anwar Ali2 and Kousik Atta1

1 Plant Physiology, Bidhan Chandra Krishi Viswavidyalaya, India

2 Crop Physiology, Agricultural College Bapatla, Angrau, India

\*Address all correspondence to: theananya4@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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### *Edited by Josphert Ngui Kimatu*

Increasing human migrations, technological advances, agricultural activities, and climate change are forcing plants to adapt to new environments. This book highlights current morphological, anatomical, physiological, molecular, and genomic advances in plant defense mechanisms. These advances, including epigenetic mechanisms, have been linked to observed phenotypic plant plasticity. The book also outlines next-generation food systems, considering the resilience and sustainability of plant genomes and epigenomes.

Published in London, UK © 2022 IntechOpen © Jevtic / iStock

Advances in Plant Defense Mechanisms

Advances in

Plant Defense Mechanisms

*Edited by Josphert Ngui Kimatu*