Mechanisms of Plant Responses to Abiotic Stress

#### **Chapter 1**

## Impact of Abiotic Stress on Phytoplankton and Zooplankton with Special Reference to Food Web

*Golden Gokhale and Guru Dutt Sharma*

#### **Abstract**

In aquatic ecosystems, bacterial colonies constitute an important aspect of biological diversity and biogeochemical cycling. Phytoplankton is the primary producer of the food web and zooplanktons are an important part of freshwater food webs and biogeochemical cycles, as they serve as the main trophic connection between primary producers (phytoplankton) and fish. This chapter conducts abiotic stress effects on phytoplankton and zooplankton along with the impact of abiotic stress on their energy succession. Abiotic stress shows the decreasing supply of essential vitamins due to abiotic stress can have huge consequences for the aquatic food web. Abiotic factors had a significant impact on the biomass of phytoplankton and zooplankton communities exposed including increased temperature, acidification, nutrient enrichment and increasing ultraviolet (UV) environment of the aquatic ecosystem that significantly affect their survival, behaviour, nutritional procurement, reproduction and their overall population dynamic. Oxygen stress also is a widespread occurrence in freshwater environments, with the depletion of DO in the water layers under the epilimnion becoming increasingly common. At moderately high salinities, a decreased top-down control by zooplankton on phytoplankton may be an indirect result, leading to a worsening of eutrophication symptoms.

**Keywords:** abiotic stress, phytoplankton, zooplankton, antioxidants, UV radiation

#### **1. Introduction**

In aquatic ecosystems, planktons form the base of the food web. Planktons are microscopic organisms that flow with streamlined water. The term "Plankton" is used to describe all non-motile, water-current-resistant organisms that are present in both freshwater and marine environments. Water current carries them along. Its range in size is from 0.2 mm to more than 20 cm ranging from tiny microorganisms to enormous creatures like jellyfish. Their distribution changes depending on the amount of light and nutrients available. Planktons are a source of food for large aquatic species. It shows the dynamic nature of the aquatic ecosystem due to which they act as the most dominating group among all water bodies. Planktons survival depends upon the structure of the aquatic ecosystem and the availability of nutrients. They adapt themselves into strategic adaptations. Planktons are acts as an important biological

indicator of water quality and also determine the trophic status of an aquatic ecosystem. They are categorised into Phytoplankton and Zooplankton.

Phytoplankton also known as microalgae are chlorophyll-containing organisms that require sunlight to grow and survive. The majority of phytoplankton remains buoyant and floats in the upper layers of the ocean where sunlight can reach them. Additionally, phytoplankton needs inorganic elements like nitrates, phosphates and Sulphur which are essential for making proteins, lipids and carbohydrates. The phytoplankton plays important role in the production of oxygen by the process of photosynthesis in the presence of light, e.g. during the daytime, the water column forms the photic layer. Firstly, oxygen enters in water and then evaporates into the air from the water surface thus; it is contributing oxygen to the atmosphere. A decrease in the oxygen productivity by phytoplankton can disturb the balance between earth and life which further cause extinction in organisms. The phytoplankton forms the base of the food chain in an aquatic ecosystem. They also convert solar energy into chemical energy thus, they act as energy transducers. This converted energy is transferred by zooplankton to the higher trophic levels while providing the link between producers and consumers. The growth and morphological properties of phytoplankton are examined by the assessments of biological and physico-chemical parameters of an aquatic ecosystem.

Zooplankton is microscopic organisms that are present in the water column of almost all water bodies, such as lakes, ponds, and seas. However, they are largely unable to survive in rivers and streams. They may contain the larval stages of larger organisms like mussels and fish and range in size from a few millimetres to a few microns (1 μm is equal to 1/1000 of a mm).

Zooplanktons are microscopic organisms that are present in the water column of almost all water bodies, such as lakes, ponds and seas. However, they are unable to survive in rivers and streams. They may contain the larval stages of larger organisms like mussels and fish which range in size from a few millimetres to a few microns (1 μm = 1/1000 of mm). They also act as a food source for invertebrates and fishes. They play important role in the transmission of energy to higher trophic levels. The seasonal variations in an aquatic ecosystem can cause fluctuation in environmental characteristics to result from abiotic stress. This abiotic stress causes a greater impact on patterns of energy succession.

In an aquatic ecosystem, extrinsic and intrinsic interaction is observed. The intrinsic interaction is defined as competition, parasitism, predation and mutualism and the extrinsic is known as an interaction between aquatic organisms and their environment. They both cause an impact on the dynamic pattern of taxa because seasonal variations could vary the communities. The energy flow of the biotic factors ecosystem starts with photosynthetic organisms which use solar energy to convert inorganic compounds to organic compounds. The diatoms and dinoflagellates are more dominant species in the marine ecosystem, which are primary producers in the classical food web. The primary predators are Zooplankton for phytoplankton which is get affected by variations in environmental factors [1–7].

In this chapter, the impact of abiotic stress on phytoplankton and zooplankton energy succession has been discussed. Analysis, the regulation of zooplankton can be examined by phytoplankton composition [8]. In Mesotrophic Lake, Predation can cause the breakdown of blooms of bacteria-plankton and also varies the phytoplankton structure [9]. The interaction between phytoplankton and zooplankton is depending upon the trophic status of an aquatic ecosystem [10, 11] (**Figure 1**).

*Impact of Abiotic Stress on Phytoplankton and Zooplankton with Special Reference to Food Web DOI: http://dx.doi.org/10.5772/intechopen.106633*

**Figure 1.**

*Abiotic stress impact on food web.*

#### **2. Abiotic stress impacts α-tocopherol and β-tocopherol antioxidants**

In aquatic ecosystems, the phytoplankton is a major source of non-enzymatic antioxidants and their precursors for primary consumers and other organisms higher up in the food web [12–14]. Antioxidants can be categorised into two categories: (1) enzymatic antioxidants such as superoxide and (2) non-enzymatic antioxidants such as glutathione, α-tocopherol (vitamin- E) and β- carotene. In an aquatic ecosystem, the phytoplankton the antioxidants α-tocopherol (vitamin E) and β-carotene in phytoplankton depend on changes in abiotic factors.

The α-tocopherol plays important role in the protection and prevention of membrane lipids whereas β-carotene act as antennae of complex photosystem II. The maximum variations in antioxidants in the aquatic ecosystem are because of temperature, density and salinity fluctuations. The antioxidant value in phytoplankton directly depends upon the composition of species, physiological characteristics and their strategies to deal with oxygen species. The α-tocopherol and β-tocopherol are nonenzymatic antioxidants which are produced by photosynthetic organisms at higher trophic levels in the form of dietary intake due to system shifts toward the phytoplankton can cause environmental variations which cause a larger impact on aquatic food webs. The composition of species decreased due to unbalancing of tocopherols.

#### **3. Impact on vitamin B1 (thiamine)**

Vitamins play important roles for plants and animals. Thiamine (vitamin B1) compound is soluble in water. The structure of pyrophosphate is consisting of pyrophosphate ester and thiamine diphosphate (TDP) which act as a cofactor in many reactions of metabolic. It also plays role in acetyl-coenzyme A and it also has a role in the Kerb cycle. Thiamine monophosphate (TMP) is intermediate in thiamine metabolism. The ethology of thiamine deficiencies is not well known but some researchers illustrate that transmission of thiamine gets decreases from phytoplankton to other higher trophic levels in an aquatic ecosystem. This result that variation in thiamine causing an impact on the composition of species with lower thiamine content and also decreasing the thiamine cellular concentration. In a marine food web, the synthesis of thiamine is done by prokaryotes and phytoplankton which are further transmitted to higher trophic levels through energy succession. But only some species of phytoplankton can secrete thiamine but not all species synthesise thiamine. It has been observed that both thiamine autotrophs and thiamine producers can uptake external thiamine.

The salinity in the water caused a larger impact which was further followed by temperature and photon flux density were examined in phytoplankton species. This effect can cause a decrease in thiamine concentration availability for the aquatic food chain and food webs. The decrease in thiamine concentration shows a larger impact on the aquatic food webs. It is an unclear statement that vitamin variations are more efficient to cause an impact on grazers in climatic conditions. The mechanism for example selective grazing in zooplanktons also causes an impact on their nutritional habits. Vitamin B1 in diatoms increases at high temperature whereas it lowers at salinity.

The decreasing fatty acid level shows an adverse impact on growth and also affects the transmission of carbon in the food web [7, 15, 16]. The zooplankton fatty acid depends upon the fatty acid of phytoplankton and their composition but zooplankton causes greater inter-specific differences [15, 17]. The thiamin content of copepod is closely related to the thiamin concentration of micro, nano and picoplankton while there is some difference in thiamine content of copepod species. Hence, aquatic food web processes depend upon the concentration of thiamin at the higher trophic level.

#### **4. Eutrophication**

The assessment of physico-chemical parameters is necessary for examining the water quality and energy succession of an aquatic ecosystem. The abundance and diversity of phytoplankton and zooplankton depend upon the physico-chemical parameter of an aquatic ecosystem [18]. Eutrophication is defined as the process by which all nutrients are accumulated in all aquatic ecosystems. Eutrophication is a major problem for the aquatic food web. This is caused by the increase in the concentration of phosphorus and nitrate. The formation of algal blooms limits the intake of oxygen for phytoplankton and another organism which can cause the death of an organism. The water of algal bloom smells foul and it also causes the death of fish. The negative effects of eutrophication on water bodies include a decrease in biodiversity can also toxicity in water bodies and a change in species abundance (**Figure 2**).

#### **4.1 Classification of eutrophication**

#### *4.1.1 Anthropogenic eutrophication*

Anthropogenic eutrophication is occurring through human activity such as golf courses, lawns etc. in the form of fertilisers which get a transfer into water bodies.

*Impact of Abiotic Stress on Phytoplankton and Zooplankton with Special Reference to Food Web DOI: http://dx.doi.org/10.5772/intechopen.106633*

When it comes in contact with algae and plankton it causes eutrophication. The variation of nutrient which enters water bodies by human intervention increase eutrophication rapidly. This eutrophication process does not require decades. Cultural eutrophication begins with phosphorus which is present in fertilisers along with partially sewage treatment. Phosphorus is considered the strongest indicator of the growth of algae. Deforestation and soil erosion are the major cause of eutrophication.

#### *4.1.2 Natural eutrophication*

Natural eutrophication is defined as the enrichment of water bodies with help of natural calamities. This process takes a slow time as compared to anthropogenic eutrophication. It depends upon the environmental temperature. Natural eutrophication duration is up to 100 years because in this process organic matter takes a long time for the deposition. Natural eutrophication occurs in natural conditions like landslides and floods with environmental characteristics such as temperature, carbon dioxide and light also play important role in natural eutrophication. The natural eutrophication primary stage starts in oligotrophic water in which accumulation of nutrient take place. The accumulation of nutrients and their utilisation get continues which further causes eutrophication. The duration of eutrophication depends upon the variation of water quality (**Figure 3**).

**Figure 3.** *Types of eutrophication and their causes.*

#### *4.1.3 Effects of eutrophication*

During eutrophication, phytoplankton grows faster. These phytoplankton species are toxic. In eutrophic water, the gelatinous zooplankton bloom grows faster in this water. Biomass of algae increases in eutrophic water. The loss of transparency in water and water smell and colour. The shellfish and harvestable fish population get decreased. The concentration of dissolved oxygen decreases which causes the death of fish.

#### **5. Ocean acidification**

Ocean acidification is a steady decrease of the pH concentrations as a result of the release of carbon dioxide (CO2) from the atmosphere. Human use of fossil fuels is the barrier to ocean acidification. The CO2 concentration is obtained from the atmosphere which depends upon the interactions between biological activities and physico chemical parameters such as temperature, salinity, intensity and surface chemistry. The decrease in the pH concentration because of anthropogenic CO2 concentration causes ocean acidification [19, 20]. Ocean acidification is controversial that how it causes an impact on the carbon fixation by photosynthetic organism. The process of calcification in phytoplankton gets affected by the influence of ocean acidification. Ocean acidification caused due to global warming as well as it causes environmental factors on productivity and composition. Ocean acidification serious problem which affects the growth and development along with the nutritional quality of primary producers which further affects the higher trophic levels. The phytoplankton biochemical composition gets disturbed during acidification causing an impact on the taxonomic group. The sensitivity

*Impact of Abiotic Stress on Phytoplankton and Zooplankton with Special Reference to Food Web DOI: http://dx.doi.org/10.5772/intechopen.106633*

to pCO2 causes low food quality for higher trophic levels. Zooplankton is less affected by acidification but the acclimation process leads to an increase in the rate of respiration and also an increase in the rate of grazing. Because their shell is mainly composed of chitin which prevents them from acidification. At higher trophic levels organisms are more affected because the outer skeleton is made up of calcium carbonate.

#### **6. UV radiation**

During summer, growth and development decrease growth of phytoplankton when UV radiation are high this shows that phytoplankton species show complexity toward UV radiation and temperature. Moreover, they are dependent on UV radiation and temperature [21–23]. UV radiation also affects the growth and development of zooplankton and also changes their pattern of species composition. The cell size variation due to UV radiation causes an impact on zooplankton feeding habits. The phytoplankton gets more affected by high UV radiation compare to zooplankton. UV Radiation causes a moulting process for a few zooplankton present in the water bodies. The process of Moulting involves chitinolytic enzyme and apoptosis process done by caspase-3-activity. It has been observed that UV radiation cause an adverse impact on the moulting process as a result it decreases the growth and development of the population dynamic of plankton. The UV-B radiation can strongly cause impairment of photosynthesis whereas inhibition of calcification is done by UV-A radiation. The outer calcified scales form an exoskeleton. At maximum calcium concentration, the zooplankton more resists UV radiation rather than at limited calcium concentration (**Figure 4**).

**Figure 4.** *Factors affecting the phytoplankton and zooplankton.*

### **7. Conclusion**

This chapter suggests a well understanding of phytoplankton and zooplankton along with their role in the ecological succession and food web. The seasonal changes show an impact on the physicochemical parameters which changes the patterns of the composition of species. Mostly they depend upon the Temperature. These factors affect the growth and development of higher trophic levels in the food web The abiotic stress such as antioxidants, vitamins, eutrophication, acidification and UV radiation show an impact on phytoplankton and zooplankton. Antioxidants such α-tocopherol and β-tocopherol secretion in the phytoplankton depend upon the seasonal variations. The unbalancing in the α-tocopherol and β-tocopherol also disturbed the composition of species. The thiamine deficiency decreases the growth and development at a higher trophic level. The ethology of thiamine deficiencies is not well known but some researchers illustrate that transmission of thiamine gets decreases from phytoplankton to other higher trophic levels in an aquatic ecosystem. This result that variation in thiamine and also causes an impact on the composition of species with lower thiamine content and decreases the thiamine cellular concentration. Eutrophication is a major problem for the aquatic food web. This is caused by the increase in the concentration of phosphorus and nitrate the negative effects of eutrophication on water bodies include a decrease in biodiversity can also toxicity in water bodies and a change in species abundance. However, acidification affects phytoplankton biochemical composition which gets disturbed during acidification causing an impact on the taxonomic group and UV radiation shows an adverse impact on the phytoplankton which further limits the all type of nutrition for higher trophic levels. These abiotic stress show harmful effects on the food web and the composition of species.

#### **Author details**

Golden Gokhale\* and Guru Dutt Sharma Department of Life Sciences, Devi Alhiya University, Indore, Madhya Pradesh, India

\*Address all correspondence to: gokhalegolden1432@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.

*Impact of Abiotic Stress on Phytoplankton and Zooplankton with Special Reference to Food Web DOI: http://dx.doi.org/10.5772/intechopen.106633*

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**Chapter 2**

## Effects of High Temperature on Crops

*Theivasigamani Parthasarathi, Saiyyeda Firdous, Einstein Mariya David, Kuppan Lesharadevi and Maduraimuthu Djanaguiraman*

#### **Abstract**

The effect of high-temperature situations leads to a significant reduction in yield. The elevated temperature on crops is expected to have a widespread negative effect as a consequence of global warming. Meanwhile, the global population is rapidly increasing and is predicted to be 11 billion in 2100. An increase in 70% of global food production is a challenging task to feed the increasing population. Increasing the food crop yield is crucial to meet the global food demand and ensuring food security. An increase in high temperature every year due to global warming and an increase in greenhouse gases leads to a rise in temperature. The rise in temperature significantly affects the yield; so, it is important to understand the mechanism and how to counteract high temperature on food crops. It is also important to neutralize the effect of high temperature on food crops and to increase the yield by minimizing the effect of high temperature and developing heat resistant or tolerant variety. It is essential to develop heat-tolerant crops or transgenic food crops that can assure great yield and food security for future generations. It is essential to examine the metabolic, physiological, and molecular mechanisms of food crops to have an enhanced understanding of high temperature and their effects on crops.

**Keywords:** heat stress, high temperature, photosynthesis, pollen and root

#### **1. Introduction**

Agricultural productivity is prone to change in temperatures. Knowledge of climate change, specifically high temperatures, is essential for agronomists, decisionmakers, and crop producers to ensure food security across the globe [1]. Crops thrive at their optimum growth temperatures. However, elevation in temperature level predominantly influences plants' physiological processes, especially photosynthesis, transpiration, respiration, and yield. High temperature causes declining yields in major food crops, which is a major concern for depreciating agricultural productivity [2, 3]. Temperature is classified as minimum, optimum and maximum, and it has been predicted that the temperature will rise 2–5°C in the future climate in 2100 (IPCC, 2014) [4–6]. Agnolucci et al. [7] have demonstrated the significance and impact of climate change with different statistical patterns in 18 crops that contribute 70% of the land and 65% of calorific value. In conclusion, we emphasize the yield disparity in the primary crop is associated with high temperature [8].

The maximum threshold temperature for various crops differs. However, high temperatures above 35°C can cause damage to rice crops. Evident injuries were observed due to high temperatures in different developmental stages. Recent studies exposed that sorghum pistils and pearl millet both are similarly sensitive to high temperatures [9]. Moreover, Zhao et al. [1] have investigated and concluded that the rise in global temperature leads to global yield loss in four significant food crops. In addition, it also raises concern about the increasing temperature, and reduced yield should be neutralized with modern and sustainable modern agricultural techniques to fend off global hunger and to meet the prospective food requirement. Global warming due to greenhouse gas (GHG) emissions, is considered a significant threat to global agriculture productivity [10]. It is evaluated that without the use of CO2 fertilization, efficient solution, genetic transformation, each 1°C rise in the global

**Figure 1.** *High-temperature influences on crops.*

#### *Effects of High Temperature on Crops DOI: http://dx.doi.org/10.5772/intechopen.105945*

mean temperature reduces global maize yield by 7.4%, wheat yield by 6.0%, rice yield by 6.2%, overall milled rice by 7.1–8.0%, head rice by 9.0–13.8% and overall milling profit by 8.1–11.0% and soybean yield by 3.1% [1, 11]. According to Food and Agriculture Organization (FAO) data, the relative rates of increase in yield for major cereal crops are reducing. However, crop productivity must be increased as the population is projected to reach 11 billion in 2100 (UN Population Division report) [12] and about a 70% increase in global agricultural productivity is essential; increase in 2°C or increase in the average temperature could lead to 20–40% reductions in cereal grain output, notably in Asia and Africa [13]. The increasing global warming provokes the weather pattern, leading to an increase in global temperature by 2.0–3.5°C in all regions as reported in the fifth assessment report (AR5) by the Intergovernmental Panel of Climatic Change (IPCC, 2018) it will reach 2.5–5.8°C before the 2100 s [6, 14]. High temperature during grain-filling has a significant effect on sunflower seeds and oil constituents [15]. In addition, it also reduces the linoleic acid content in numerous oilseed oils [16]. It also reduces the oil content, seed yield, and speeds up seed maturity as a consequence, erucic acid over seed development was influenced **Figure 1** [17].

#### **2. Stage and intensity**

High temperature limits the yield and affects various growth stages in plants. The reproductive stage is the most vulnerable phase of the crop's entire lifespan; this vulnerability during the reproductive stage leads to significant depletion in seed set and crop yield [9, 18]. Numerous food crops such as rice, wheat, soybean, maize, cotton, sorghum, and tomato are tremendously vulnerable to high temperatures [19]. Intense high temperature causes pollen abortion resulting in incomplete pollination. The interactions and stability between pollen and pistil during high temperatures lead to successful reproduction. But pollen and pistil both are extremely susceptible to high-temperature [20]. Crops have a variety of alternatives for resisting, minimizing, and surviving high temperatures during flowering. Plants can survive in high temperatures by keeping a cooler canopy through enhanced transpiration [9]. An elevated temperature could disturb numerous metabolic processes that take place in guard cells, as a result of high temperature; the stomatal response is frequently influenced by transpiration rate, photosynthetic rate, plant water status, and vapor pressure deficit [21]. High temperature considerably influences the crops by affecting several physiological injuries like leaf abscission, leaf scorching, senescence, and root and shoot growth limitation that subsequently leads to a reduction in yield. Moreover, the impact of high temperature affects photosynthetic membranes followed by ion leakage, enlargement of grana stacks, and aberrant stacking. By downregulating particular genes in carbohydrate metabolism, high temperature alters the activities of carbon metabolic enzymes, starch accumulation, and sucrose production. High temperature increases certain essential phytohormones such as abscisic acid, ethylene, and salicylic acid and reduces a few like gibberellic acid, cytokinin, and auxin; it furthermore leads to the fabrication of reactive oxygen species [4]. In addition, the high temperature throughout the day is properly not reported. However, the progressive rise in night temperature needs to be scrutinized considering that it causes prior to time of day of anthesis in cereal grains like rice crop.

#### **3. Changes in mechanism under temperature stress**

#### **3.1 Effects of high-temperature stress**

Plants endure various factors during growth and development, and hightemperature stress is one of the major abiotic factors that adversely affect crop production. High-temperature stress causes risk at different growth stages and ultimately reduce the yield by affecting the physiological mechanism [22]. The seedling stage of crops is susceptible to high-temperature stress in rice, mungbean, wheat [23, 24] and the reproductive stage in rice, wheat, and other cereals [25, 26]. High temperature modifies cells' morphogenetic structure, leading to a decrease in cell size, enhancing stomatal density and cell membrane permeability, inflating xylem vessels, and impairing mesophyll cells [27].

High-temperature stress causes various physiological changes in crops during the different growth stages and germination is affected at the initial stage. The impact of temperature on seeds has decreased seed germination percentage, plant emergence, poor seedlings vigor, abnormal seedlings, and decrease radicle and plumule growth [4]. Seed germination is inhibited under high temperatures through the stimulation of abscisic acid biosynthesis. Exposure to high temperatures reduces plant height, total biomass, and the number of tillers. In wheat, the germination rate was completely inhibited and caused cell death and the embryo for seedlings establishment rate was additionally decreased. The morphophysiological characteristics such as phenology, plant water relations, dry matter partitioning, and shoot growth were restricted by heat stress in bean plants [28]. In some plants, it reduces total phenological duration, shorter grain filling period, and diminishes the germination period. The loss of cell water content due to high temperature eventually decreases the cell size and growth. In response to high temperature, the net assimilation rate was reduced, and it was directly associated with plant growth. Elevated temperature causes programmed cell death in specific cells or tissues as long as the denaturation of proteins. In addition, the high temperature for a prolonged period might cause gradual death; these injuries may lead to the shedding of leaves, abortion of flowers, or even death of the whole plant [29]. Furthermore, high temperature enhances the evapotranspiration during vegetative and reproductive stages ration which limits plant's water availability and uptake, which influences dehydration that reduces growth at the organ level as well as the whole plant level [30].

#### **3.2 Root physiology**

Root size and morphology play a vital role in plant water, and nutrient uptake, whereas the plant root system requires the optimal temperature to grow, if it exceeds the normal temperature it may change the uptake [31]. The temperature of the root system is lower when compared to the shoot. In addition, the optimum root temperature may vary across plant species [32]. Exposure to high temperatures (>29°C) leads to a decrease in primary root length, lateral root density, and root growth in sunflower crops [32, 33]. Even though species share the same environment, their root system architecture (RSA) may differ from species to species by the changes in soil temperatures [34, 35]. In some plants, the increased temperature may produce an expanded root system, whereas, in adult maize plants, it suppresses the lateral root growth by developing the long axial root to take up water from deep soil layers. Similarly, at increased temperature, the initiation and elongation of adventitious and lateral roots

#### *Effects of High Temperature on Crops DOI: http://dx.doi.org/10.5772/intechopen.105945*

were inhibited [36] in potatoes. Swelling of root cap meristem and blends in root tip were found on potatoes [37] with the rise in soil temperature. Root zone temperature in Sorghums causes a decreased rate of cell production and root elongation. Overall, these changes by the high temperature hinder the root growth with a decreased rate of cell division [38]. The impact of elevated temperature on the formation of root growth in lupine species was studied at the initial and lateral stages. Temperature modifies the growth through altering the root architecture [39]. On the other hand, root respiration could vary based on the temperature range. During this root respiration process, cells uptake the oxygen which is surrounded in the spaces among the soil particles at the root zone, increasing with every 10°C on soil temperature, where the solubility of oxygen is in contrast to temperature. This increased requirement for oxygen leads to root hypoxia [32]. Reactive oxygen species (ROS) produced in excess under high-temperature stress results in oxidative stress [40]. The increase in ROS production includes hydrogen peroxide (H2O2), superoxide free radicals (O2 − ), and lipid peroxidation, which leads to the enhancement of cell membrane damage [41, 42]. To prevent the effect of ROS, plants produce antioxidant enzymes, such as catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione reductase (GR), dehydroascorbate reductase (DHAR), and glutathione S transferase (GST), and nonenzymatic antioxidants such as anthocyanin, flavonoids, carotenoids, and ascorbic acid (AA) that protect crop growth from various other stresses also. Overall, the high temperature on root growth of plants either be promotive, inhibitory, or first stimulatory and then inhibitive once the optimum temperature is crossed [14].

#### **3.3 Leaf physiology**

High-temperature stress showed various morphological symptoms in shoots such as sunburns of leaves and twigs, scorching of leaves and stems, leaf senescence, root and shoot inhibition, fruit damage, and discolorization, which finally caused a decrease in crop productivity. In some cases, high temperature causes drying and rolling of leaves, scorching of leaf tips and margins, and necrosis was found in sugarcane. Leaf photosynthesis is considerably affected by extreme temperatures. The components of photosynthetic metabolism processes are sensitive to temperature [43]. Chloroplast is substantially affected by high temperature during the photosynthesis process including swelling of grana stacks and abnormal stacking [44]. However, chloroplast plays an important role to activate the adaptive process to these extreme conditions [45]. Studies reported that high temperature is associated with the upregulation of two hundred chloroplast-related genes in model rice plants [46]. To evaluate the heat-resistant crops, stay-green physiological traits were used to affirm the mechanism of heat damage interacting with Chl adaptability, antioxidant and photosynthetic capacity. Stay-green genotypes are highly associated with chlorophyll metabolism, these genes encode magnesium dechelatase, which is involved in the degradation of chlorophyll. A chloroplast targeted DnaJ protein (SlCDJ2) which is located in the thylakoids and stroma, protects the Rubisco activity and regulates the CO2 assimilation in tomato plants to cope with heat stress [47, 48]. In lettuce seedlings, the application of exogenous spermidine regulates the stability of mitochondrial and chloroplast structure. The stomatal opening and density are inhibited by exogenous spermidine, thus leading to an increase in the photosynthetic rate and biomass of lettuce. It significantly alleviates the high-temperature stress and protects the leaves from damage [49].

#### **3.4 Reproductive physiology**

The reproductive tissues of plants are more susceptible to elevated temperatures among the other plant tissues, which result in a yield penalty. At the time of reproduction in crops, a short period of extreme temperature could significantly increase flower abortion. Similarly, heat spells at reproductive stages may not produce flowers, or flowers do not produce fruit or seed [29]. Reproductive development in various species is reported to be sensitive under high-temperature stress that disrupts/affects meiosis in male and female gametes, pollen germination and pollen tube growth, pollen/pistil interactions, ovule viability, number of pollen grains, formation of endosperm and embryo development, fertilization, and post-fertilization processes. The effect of high-temperature day/night causes an impact on pollen viability in crops including sorghum, wheat, rice, canola, groundnut, common bean, and soybean [50]. Due to high-temperature stress ROS accumulation, membrane integrity, changes in protein, carbohydrate, and lipid mechanism, and alters in phospholipids profiles in the mature pollen causes loss of pollen viability [41, 51, 52]. High temperature (33°C) subsequently reduces anther dehiscence and pollen fertility rate; it causes a decrease in the number of pollen on stigma which leads to decreased fertilization, spikelet fertility, and sterile seed in rice [53]. The process of anther dehiscence prompts to dispense mature pollen grains from the locules of the anther is responsible for pollination. During this process, the anther wall is opened by degeneration of anther tissues called septum and stomium [54]. The proper release of pollen from the anther needs expanding of endothecial cells and also strengthening and thickening of cell walls. In common bean and rice plants, the anther indehiscence happens due to high temperature, thus disrupting the pollen release. In addition, the inhibition of thickening in the endothecium cell and dissolution of interlocular septa in common bean and tomato were observed, which lead to failure in anther dehiscence and pollination [55, 56]. Heat treatment during panicle initiation significantly reduced the yield through a decreased number of spikelets per panicle and seed setting percent in rice. However, the heat susceptible variety showed poor seed setting percent mainly associated with spikelet fertility which is induced by a decrease in anther dehiscence, reduced pollen viability, and poor shedding of anthers [57]. Exposure to heat stress for a short period alters pollen development during meiosis. Increased spikelet sterility might decrease pollen germination due to high night temperature in rice. Other intrinsic factors of pollen development are carbohydrate metabolism and partitioning [50]. Reduced pollen viability is directly associated with sugar level accumulation in developing anther and pollen walls.

Plant female reproductive organ pistil/carpel includes stigma, style, and ovary. Female gametophyte occurs in the ovary, and the nutritional support and shelter were provided by sporophytic maternal tissue. The development of female gametophytes occurs in two phases: megasporogenesis and megagametogenesis. Generally, it consists of three antipodal cells, two synergid cells, one egg cell, and one central cell. Male reproductive development is susceptible to high-temperature stress when compared to female reproductive development. However, heat stress affects female reproductive development it causes degenerated eggs and synergids, embryo sac malformations, less number of ovules, increase ovule abnormalities and abortion, decrease in the size of transmitting tissue present in the style, and dried and drooping stigma, style and ovary [41, 51]. Heat stress interrupts the pollen-pistil interactions by changing the structural positioning of anthers and stigma. In response to high temperature, the maturation of stigma/style changes it leading to modification

#### *Effects of High Temperature on Crops DOI: http://dx.doi.org/10.5772/intechopen.105945*

in cell division and elongation [58]. Heat stress affects reproductive function and has been reported in many studies. For example, in *Brassica* it decreases the fertility of microgametophytes, in tomato, it impaired meiosis in the male gametophyte which causes pollen germination and pollen tube growth. To counteract this problem, variations in genotypes of various pollen traits were selected to study the heat-tolerant genotypes. In recent study, 12 cultivars of *Brassica napus* L. were used to study heat tolerance based on pollen traits such as pollen germination, pollen viability, and pollen tube growth [59]. Abortion of flowers, young pods, and loss of seed numbers in soybean was observed under high-temperature stress [60]. Even mild heat stress (30°C/25°C) for 5 days, as long as the loss of tapetum differentiation and injuries to the microsporogenesis process was observed in Barley. The tapetum cells consist of enough mitochondria and a high metabolic activity under optimum growing temperature which in turn production of reactive oxygen species (ROS). But the plants exposed to high temperature provokes ROS homeostasis in the tapetum and microspores which cause accumulation of ROS initiation to lead to unplanned programmed cell death in tapetal cells through stimulating the membrane damage, lipid peroxidation, decreased transcription, and translation [61]. Extreme temperatures shortened the plant growing days, which resulted in early maturity, a lower life cycle, and accumulation of lesser biosynthetic products which eventually decreased grain development. The duration of grain filling decides the grain development which is related to grain yield. Crop yield is associated with temperature because even a 1.5°C increase in temperature might cause drastic yield loss in crops. The grain yield was mostly affected via the phenological development process. Many reports show that high temperatures cause a reduction in yield in various crops including pulses, cereals, and oil-yielding crops [29, 62]. Filled seed weight and size were ultimately reduced due to heat stress which directly affects grain yield in sorghum was reported. In addition, heat stress not only affects the yield, but it also correlates to reduce the quality of grain in barley. The concentration of non-structural carbohydrates, starch, raffinose, fructose, and lipids was reduced in barley grain, whereas in okra, the fiber content was decreased [63].

#### **4. Physiological traits associated with high temperature stress**

#### **4.1 Cell membrane integrity**

In response to heat stress, various physiological modifications might cause cell membrane damage. Exposure to heat stress causes cell membrane damage resulting from denaturing membrane protein, and inactivating enzyme, which leads to cell membrane permeability and integrity causing reduced ion flux, leakage of electrolytes, changes in relative water content, production of toxic compounds, and interruption in homeostasis that result in decreased cell viability. Reduced cell viability ceased plant growth and prompted leaf wilting, leaf abscission, and leaf area reduction [64]. Among the other physiological factors, membrane permeability and relative cell injury were affected at different growth stages under heat stress. Cell membrane stability differs with plant tissue, age, growing season, growth stage, plant species, and severity of heat stress. To measure electrolyte leakage from the cell membrane in an aqueous medium is used to determine the cell membrane malfunction in response to heat stress [65]. Leaf membrane stability index (MSI) was estimated in leaf tissues (100 mg) heated at 40°C for 30 min (C1) and 100°C for 10 min (C2) in water bath.

The conductivity of samples is measured by the using conductivity bridge and MSI was calculated using the following formula (MSI = [1 – (C1/C2)] × 100) [66].

#### **4.2 Leaf gas exchange parameters**

Transpiration is the physiological process that occurs in crops where the net radiation energy is converted into heat, underneath physiological control through alters in the stomatal aperture. In plants, photosynthesis and stomatal conductance play a vital role in all aspects. The leaf gas exchange parameters are commonly measured using a portable photosynthesis system including LI-6400 and LI-6800 (LI-COR®, Lincoln, USA), the CIRAS-3 (PP systems, Amesbury, USA), the GFS-3000 (Walz Gmbh, Effeltrich, Germany), and the iFL (Opti-sciences, Hudson, USA) [21]. However, decrease root hydraulic conductance in response to soil drying is an important control mechanism of stomatal closure. Low leaf water potential induced by high transpiration rates results in decreased stomatal conductance. The potential of crops to maintain CO2 assimilation rate and leaf gas exchange under high temperatures is accompanied by heat tolerance. The water status in the leaf, stomatal conductance, and intercellular CO2 concentration are affected by high temperatures [67]. Stomatal closure under high temperature is a distinct reason for impaired photosynthesis which disturbs the intercellular CO2. In many plant species, the stomatal conductance (gs) and net photosynthetic rate decline due to mild heat stress; it was caused due to reduced activation of Rubisco. High temperature affects vapor pressure density (VPD) but it may change hydraulic conductance and water supply to the leaf area [68]. Higher stomatal conductance accelerates transpirational cooling and canopy temperature depression (CTD). Increased stomatal conductance and related leaf cooling contribute to heat tolerance against high temperature; these conclude a positive interaction between stomatal conductance and the yield of wheat under extreme temperatures [69].

#### **4.3 Chlorophyll pigments**

Chlorophyll biosynthesis in plastids plays an important role in light-harvesting was high impact due to high-temperature stress. In addition, the high-temperature stress causes impairment and degradation of chlorophyll pigments in plastids. Chlorophyll biosynthesis was inhibited due to high temperature stress through the eradication of enzymes involved in the mechanism of chlorophyll biosynthesis. The enzymatic activity of 5-aminolevulinate dehydratase (ALAD), the first enzyme of pyrrole biosynthesis was decreased under high-temperature stress [70]. In some cases, the barley seedlings pre-treated with temperature for 4 h or 8 h prohibited the chl biosynthesis which result in reduced protochlorophyllide. Similarly in wheat, the protochlorophyllide (Pchlide) synthesis, Pchlide oxidoreductase, and porphobilinogen deaminase are eventually affected. These conclude that high temperature causes reduction in chlorophyll a, total chlorophyll content, sucrose content, and in contrast, it increases the reducing sugar content and leaf soluble sugar was observed in soybean [29]. However, in celery leaves, chlorophyll biosynthesis was inhibited under extreme temperatures due to the down-regulation of mRNA genes associated with biosynthesis [71]. Chlorophyll content was measured in the leaf using acetone by spectrophotometrically measuring the absorption at 663, 652, and 645 nm [72]. Nondestructively chlorophyll can be measured using the handheld devices such as SPAD chlorophyll meter (SPAD 502 Plus Chlorophyll Meter, Konica Minolta, Japan) [73],

CCM Chlorophyll content meter (CCM-200plus *Chlorophyll Content Meter,* Opti-Sciences, Inc., USA), CL-01 Chlorophyll Meter (Hansatech Instruments Ltd., United Kingdom). The NIR reflectance spectroscopy can be a promising methodology to measure/predict chlorophyll and other pigments under field conditions [74].

#### **4.4 Heat shock proteins (HSPs)**

Production of heat shock proteins in plants to protect the cell from various stress factors. These types of proteins were not found in non-stressed plants [75]. These proteins are categorized into high molecular (68kD to 110kD) and low molecular weight proteins (15kD to 27kD). Low molecular weight HSPs are found in higher plants which are plant-specific proteins whereas high molecular weight is found in all types of plants. Some heat shock proteins are known as molecular chaperones but not all HSPs are molecular chaperones [76]. Molecular chaperones are proteins that generate during high-temperature stress. Chaperones bind to denature proteins or unfolded proteins to make them stabilize and protect from thermal aggregation. Heat shock proteins regulate cellular homeostasis by eliminating harmful proteins which rise from aggregation and misfolding [69]. Some classical HSPs show chaperone activity to protect the protein denaturation from thermo-aggregation. The non-classical heat shock proteins consist of plastid protein synthesis elongation factor (EF-Tu) and peptidyl-prolyl cis/trans isomerases that produce low molecular weight proteins and provide heat resistance. Under heat stress, the EF-Tu gene in transgenic wheat exhibits reduced thermo-aggregation and decreased thylakoid membrane damage moreover increasing the photosynthetic system [77]. The chloroplast localized HSPs provide heat resistance to the photosynthetic electron transport chain in isolated chloroplast. Some studies revealed that small heat shock proteins pertain with thylakoid and protect O2 evolution and oxygen-evolving complex proteins of PSII from heat stress. The heat treatment in tomato leaves protects PSII from temperature-dependent oxidative stress by chloroplast heat shock protein HSP21 present in chloroplast. In addition, the chloroplast HSPs may not repair the stress-induced damage but they avoid damage [78].

#### **4.5 Optimal light and dark reaction**

High-temperature stress is directly associated with light intensity which damages the photosystem. Heat stress alters cell respiration and photosynthesis, which shortens the life cycle and reduces crop production [79]. Thermal stress modified the structural changes of chloroplast protein complexes and decreased the activity of enzymes. Light-dependent chemical reactions occur in the thylakoid membrane and carbon metabolism takes place in the stroma, which is an important portion, that is damaged in response to high temperature. Increased temperature of the leaf and photon flux density impacts the thermo-tolerance adjustment of photosystem (PS) II. For measuring the quantum yield of PSII mostly modulated fluorimeters are used such as the Mini-PAM II by Walz, FMS2 by Hansatech instruments (King's Lynn, UK), the OS5+ by Opti-Sciences, or the FluorPen FP 100-MAX of Photo Systems Instruments (Drasov, Czech Republic) [21, 80, 81]. PSII is particularly receptive to temperature and its activity is enormously impacted to some extent and terminated under high temperature stress. Oxygen developing complex is exposed to temperature causing damage; it concludes in an imbalance flow of electrons to the acceptor site of PSII. The synthesis of starch and sucrose was eventually affected being a decrease in the activities of some enzymes such as sucrose phosphate synthase, adenosine

diphosphate-glucose pyrophosphorylase, and invertase due to high-temperature stress. Net photosynthesis in many plant species is suppressed because of a decrease in the activation state of the CO2 binding enzyme, Rubisco. Although these negative impacts of adverse conditions on photosynthesis, the optimum temperature prerequisites for photosynthesis are familiar to rise with increased concentration of CO2 in the atmosphere [30]. The decrease in photosynthesis was due to damage to the chlorophyll pigments, reduced leaf nitrogen content, a hindrance to PSII reaction and electron flow, reduced quantum efficiency (Fv/Fm), and down-regulation of PSII photochemistry.

Many studies reported PSI is more stable than PSII towards heat stress. PSI activity increased the thylakoid proton conductance and cyclic electron flow under high temperatures. When the activity of PSII is reduced due to adverse conditions, the proton conductance and cyclic electron flow around PSI could be the favorable process that produces ATP. Further, heat stress enhances the dark reduction of plastoquinone and stimulates the thylakoid proton gradient which was elucidated to activate the cyclic electron flow around PSI [76]. There is an increase in ATP under mild temperature stress due to Rubisco activase and active photorespiration. The larger requirement of ATP under mild heat stress as long as a higher in NADPH/ATP ratio is helpful for non-photochemical reduction of the plastoquinone pool from stroma donors which is sequential to activate the NADH-mediated cyclic electron pathway. This process could distribute the energy and produce increased ATP to regulate active CO2 fixation.

#### **5. Future perspectives**

High-temperature stress eventually affects germination, vegetative stage, reproductive stage, and yield. Due to increased global warming, the temperature will increase in upcoming years which drastically affects crop production. Subsequently, the decreased yield may lead to food scarcity in the future. Even though many research projects work on abiotic and biotic stress to protect the plants from adverse conditions. We need to improve the crop from the seedling stage to the harvest stage. At each stage, optimum growth conditions need to be provided for the plants. To counteract the problem, researchers found stress-tolerant genotypes. However, these can act on particular metabolic pathways and regulate the mechanism to improve plant growth. Sometimes, the crops endure both biotic and abiotic stress which can significantly affect the yield. In the future, we need to target the multi-stress-tolerant approach to protect the plants from multiple or unprecedented stress conditions.

#### **6. Conclusion**

High temperature is a devastating environmental factor that influences crop growth and yield by affecting numerous crop mechanisms. High temperature is an alarming concern that needs to be considered since it directly affects yield in a situation where have to generate high yield to sort out global hunger and meet the demands of global food hunger and ensure food security. High-temperature tolerance is not only important for the current situation, but also for the future since the simulated predictions show the elevating high temperature that enormously affects yield. New possibilities to resist or enhance high-temperature tolerance need to be investigated. Even though there are several studies related to high temperature on crops that reveal

#### *Effects of High Temperature on Crops DOI: http://dx.doi.org/10.5772/intechopen.105945*

various factors that are up-regulated and down-regulated, there are abundant wild types that are not explored. Touching the untouched wild types can reveal amazing outcomes on how high temperatures can be managed with novel tolerant genes. Advancements in molecular techniques provide rapid detection of traits in wild types. It is also important to implement or follow advanced agricultural practices across the globe towards raising the global temperature, by implementing climate-smart practices, high-throughput phenotyping methods, and revealing the traits that are tolerant can protect, safeguard and defend tomorrow.

### **Author details**

Theivasigamani Parthasarathi1 , Saiyyeda Firdous1 , Einstein Mariya David1,2, Kuppan Lesharadevi1,2 and Maduraimuthu Djanaguiraman3 \*

1 VIT School of Agricultural Innovations and Advanced Learning (VAIAL), Vellore Institute of Technology, Vellore, Tamil Nadu, India

2 School of Bio Sciences and Technology, Vellore Institute of Technology, Vellore, Tamil Nadu, India

3 Department of Crop Physiology, Tamil Nadu Agricultural University, Coimbatore, Tamil Nadu, India

\*Address all correspondence to: jani@tnau.ac.in

© 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 3**

## Drought Stress in Millets and Its Response Mechanism

*Anjali Tiwari, Kapil Kesarwani, Arushi Sharma, Tapan Ghosh, Nisha Bisht and Shailja Punetha*

#### **Abstract**

Drought is a major abiotic stress that diminishes crop yield and weakens global food security, especially in the current emerging situation of climate change as well as increases in the prevalence and severity of stress elements. Millets are nutrient-dense and capable to resist variety of harsh environmental conditions, including lack of moisture. Millet's crop has evolved dynamically in terms of morphology, physiology, and biochemically that allow them to flee and/or adapt to adverse environmental situations. Drought stress has a significant impact on the vegetative and reproductive phases of plants. Millets generate a very low yield compared to main cereals like wheat and rice, despite their agronomic, nutritional, and health-related benefits. It is necessary to understand how these complex features are regulated and ameliorated the impact of droughts on millet productivity. Keeping this in view, the present work aims to understand the processes used for reducing the negative impacts of droughts in the production of millets varieties using advanced agronomic management strategies (use of information technology) and the biotechnology (improvements in crop genetics).

**Keywords:** millets, drought, abiotic stress, morphology, physiology

#### **1. Introduction**

Millets are the major cereal crop in the developing world, particularly in Africa and Asia's arid and semi-arid tropical regions, where they are utilised in food both for humans and cattle [1]. Millets also play a key role in the subsistence of people living in the mountains. In addition to addressing the issue of the food security at a global level, efforts need to be directed towards such indigenous crops that could flourish amidst the scenario of water scarcity. It is believed that the changing climate will have significant effects on the types of crops cultivated in the next century. Millets are known for their climate-resilient characteristics, such as their ability to adapt to a wide range of ecological conditions, reduced irrigation requirements, improved growth and productivity under low nutrient input conditions, reduced reliance on synthetic fertilisers, and low vulnerability to environmental stresses. Millets might provide alternative climate-smart crops as their adaptation to challenging environments is better than the current major crops of the world [2].

In line with Asia and Africa's food security, millets contribute to over half of the Africa's entire cereal production, together with sorghum. Millets are thus regarded as a poor man's crop because of their major contributions to the diets of resourceconstrained producers and consumers. Millets are the primary source of nutrition for small farmer communities in India, Africa, China, and parts of Central America, and they help to ensure food security in Asia and Africa's low-income countries. Millets are ancient food crops which are highly nutritious and can be grown under marginal environmental conditions. Millets are the major energy source and highly nutritious staple foods for sustenance. Millets, often known as 'miscellaneous or coarse cereals,' are a group of small edible grasses in the Poaceae family. In total, millets are distributed around 10 genera and 20 species [3]. Approximately 31,019,370 tonnes of Millets were produced globally in 2018 [4]. Among the summer crops millet's water requirement is much less than those of the two main crops, viz., wheat and rice. Millets are also known as famine crops because these are the only crops assuring yields in famine conditions. About 80% of the millets are used for food and the rest are stocked as feed. Millets have good grain quality that's why it is used in processing industries. Millets have many nutritional, pharmaceutical and nutraceutical properties. They are especially rich in fibre content and starch. They are used in reducing the risk of diabetes, help to lower the cholesterol and are rich in antioxidant activity [5].

Millets are considered to be the essential crops in world's agricultural system due to their resistance to pests and diseases, short growing season and capable to grow in extreme environmental conditions, and because of moderate productivity under enduring water stress conditions when major cereals cannot be relied upon to provide sustainable yields [6]. Millets will stand up to all unwell effects of temperature change apart from doable higher ozone concentrations around urban zones. However, millets would be least struck by increasing greenhouse gas levels within the native microclimate. Similar to maize and sorghum, millets possess a C4 photosynthesis system, hence they avoid photorespiration. As a result, they efficiently utilise the insufficient moisture present in the semi-arid regions. Since C4 plants are able to close their stomata for long periods, they can considerably reduce moisture loss through the leaves.

#### **2. An overview: small millets**

Millets are categorised by a collective term accustomed to see a various group of small-seeded annual C4 Panicoid grasses such as barnyard millet (*Echinochloa frumentacea*), finger millet (*Eleusine coracana*), foxtail (*Setaria italica*), pearl millet (*Pennisetum glaucum* (L.) R. Br.), kodo millet (*Paspalum scrobiculatum* L.), little millet (*Panicum sumatrense* Roth ex Roem. & Schult.) and proso millet (*Panicum miliaceum*) (**Figure 1**) [6]. These are extensively cultivated as food and fodder crops in temperate, sub-tropical and tropical regions across the globe [7, 8] and have outstanding biological process properties. Salient features of the millet's crops are described in **Table 1**.

#### **2.1 Importance of millets crop**

Millets can withstand extreme temperatures, droughts, and floods. Millets grow well in arid zones/rain-fed locations with marginal soil fertility and moisture. Because of its effective root system, millets require far less water than other cereal crops for their production. Some crops and their water availability are shown in **Table 2**. Millets are grown with organic inputs and do not require synthetic fertilisers or pesticides.

*Drought Stress in Millets and Its Response Mechanism DOI: http://dx.doi.org/10.5772/intechopen.105942*

**Figure 1.**

*Representation of different types of small millets grown in the semi-arid regions.*

Pesticides are avoided since millets are less susceptible to illnesses and pests. Millets contribute to climate change mitigation by lowering CO2 levels in the atmosphere [9]. They have a high capacity for carbon sequestration, which aids climate adaptation, particularly in light of global projections of rising methane emissions from rice fields. Millet diets are beneficial for persons with celiac disease and diabetes since they contain no gluten and have a low glycaemic index [1]. Millets crop like pearl millet is most recently used as a low-cost substitute for maize in poultry and animal feed. Millet crops are also used in fermented food and drinks products.

One of the major concerns of the global world is the abrupt changes in the earth's environment, which have had a devastating effect on the earth's ecology [10]. Currently, we are in the age of an agrarian crisis, which has necessitated for crop improvement to tackle the negative effects of climate change. Intensive agriculture of a few crops for food needs has resulted in poor nutrition and genetic degradation, as well as the negligence of locally nutritious crops. Agriculture is the largest consumer of water in India and in the world. In the Central Himalayan part of India, only about 10% of the cultivated land is under irrigation system. It is largely because the millet crops, unlike rice and wheat, have high water use efficiency and the major chunk of the cultivated land in the summer season is devoted to the millet crops. In contemporary times when water is increasingly becoming scarce, we should conserve and make economical use of water resources at each microand meso-scale (farmers' field to watershed scale). The millet crops hold great promise for the strategies of conserving water resources and fight against drought


*Salient features of millets crop [6].*

*Advances in Plant Defense Mechanisms*

**34**

*Drought Stress in Millets and Its Response Mechanism DOI: http://dx.doi.org/10.5772/intechopen.105942*


#### **Table 2.**

*Rainfall requirement for various crops [7].*

conditions. The millet crops can prove to be helpful in making strategies for conserving water resources and handling drought conditions. Millets are tolerant to the harsh climatic and soil conditions seen in Asia and Africa's semi-arid regions. Millets can be easily grown in harsh environments, particularly in those areas that have insufficient precipitation, low soil fertility, and are unsuitable for the world's primary crops.

Water deficit is one of the most environmental stresses affecting agricultural production and productivity around the world and may result in considerable yield reduction [11]. Hundreds of genes and their products respond to Water stress at transcriptional and translational level [12]. It can be asserted in the context of climate change, millets could be the answer to fighting climate change, poverty and malnutrition. Drought-proofing crops by producing heat-resistant cultivars is one aspect of this adaptation approach for millet crops [13, 14]. In comparison to most of other major crops such as rice and wheat, millets are highly flexible and stress resistant. Water stress affects these millet crops frequently and the problem appears to be becoming worse as weather and climate change is becoming more evident. Millets are agronomically advantageous because they are drought, heat, salt, and biotic stress tolerant, and they may live in marginal lands under rainfed circumstances. Drought-resistance mechanisms in many cereals have been studied by various researchers, but millets drought tolerance has been limited due to a number of factors. The response mechanism of plants in response to drought stress must be assessed. The goal of the present study is to evaluate changes in the millets crop's morphological, physiological, biochemical and molecular features as a result of drought stress, as well as plant tolerance mechanisms to the stress. Elements of drought stress in important millets crops such as finger millet, pearl millet, barnyard millet, foxtail millet, and proso millet were examined in this study. The current methods for discovering drought tolerance genes and metabolic pathways are also discussed. Recent advances in elucidating essential drought stress responses, phenotyping and QTL mapping for drought tolerance, genetic engineering of drought-tolerant crops, and crop management have also been reviewed.

#### **2.2 Drought stress**

Drought is outlined as "a temporary reduction in wetness accessibility, in which the amount of available water is significantly below normal for a specified period" and is one of the most common environmental stresses [10]. This condition negatively affects growth and productivity of crops. Water stress is a natural phenomenon in rain-fed (unirrigated) cultivated areas. In general, water stress often causes a series

of morphological, physiological, biochemical and molecular changes that unfavourably affect plant growth, development and productivity. Plants are subjected to biotic and abiotic stressors, both of which have a significant impact on their survival [10]. Abiotic stressors are important environmental factors that limit crop productivity and affect the quality and amount of crop yield. Particularly water stress directly affects the physiology of plants, especially photosynthesis. In mountains, summer (*kharif*) crops often encounter water stress. In the face of a global scarcity of water resources, drought has already become a primary factor in limiting crop production worldwide. Water-limited crop production depends on the intensity and the pattern of drought which varies from year to year [15]. The severity of water shortage imposed on field crops also depends on the susceptibility of crops during different stages of their development. The general effects of drought on plant growth are well known. When soil moisture is deficient, crop establishment may be reduced, limited growth of plants, normal development patterns disrupted and eventually, final yield is lowered [16, 17].

#### **2.3 Effects of water stress in millets**

Plant cellular activities, growth, development, and economic yield are all affected by water stress. It affects the structure of membranes and organelles at the cellular level, as well as the hydration and structure of proteins and nucleic acids, and the pressure differential across the membrane cell wall complex. Drought causes stomatal closure, which leads to an excessive accumulation of reactive oxygen species (ROS) and oxidative stress. Lipid peroxidation and damage to other biomolecules occur because of this stress [18]. Phytohormones including Abscisic Acid (ABA) and Ethylene (ET) are frequently engaged in drought stress signalling and tolerance. Plant tolerance to drought, salt, and heat stress is improved by salicylic acid (SA) and jasmonic acid (JA) [19]. Plant height, length, biomass, weight, and grain number have all decreased as a result of drought stress in finger millet [20]. Water stress result a decrease in chlorophyll, photosynthesis, and RWC, as well as an increase in proline concentration, in both barnyard and finger millets. Finger millet demonstrated higher levels of tolerance than barnyard millet [21]. Drought-induced oxidative stress caused droopy shoots, curling leaves, increased proline, catalase and malondialdehyde (MDA) content, electrolyte leakage, impaired membrane integrity, and a considerable rise in H2O2 in finger and barnyard millets [21]. In finger millet, see [18] found an increased activities of antioxidant enzymes such as glutathione reductase (GR), superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione peroxidase (GPX) and catalase (CAT) during drought stress. A low photosynthetic and stomatal conductance rates, reduced root respiration, accumulation of protective metabolites (serine, threonine, valine, fructose, glucose, maltose, isomaltose, malate, itaconate) in roots, and better utilisation of carbon and nitrogen were found in a high temperature tolerant variety of foxtail millet [22]. The effect of water stress on different millets crop is given in **Table 3**.

To cope with various environmental challenges, plants have evolved various morphological, biochemical, physiological, and molecular systems. Plant cells detect stress events through a variety of sensors, which activate a variety of signalling pathways. Plant hormones, secondary messengers, transcription regulators, and signal transducers are all involved. Drought stress has a variety of effects on plants vegetative growth, reproductive development and molecular level, and are all affected by these changes (**Figure 2**).

*Drought Stress in Millets and Its Response Mechanism DOI: http://dx.doi.org/10.5772/intechopen.105942*


#### **Table 3.**

*Effect of water stress on millets crop.*

#### **2.4 Morphological and physiological adaptations to stress in millets**

Drought causes distinct morphological and physiological changes in millets crops, which can be seen at various phases of plant growth. Millets have a short life cycle and plant heights, as well as small leaf regions, thickened cell walls, and dense root systems, all of which aid in stress resistance. The morphological response of millets can often be divided into two categories: shoot and root. Changes in leaf form, leaf expansion, leaf area, leaf size, leaf senescence, leaf pubescence, leaf waxiness, cuticle tolerance, and shoot length are all components of the shoot. Changes in root dry weight, root density, and root length are included in the lower root section. Several studies have found that the relationship between morphological and physiological features including grain yield per plant, grain spike per plant, spike fertility, plant height, root length, shoot length, harvest index, chlorophyll content and relative

**Figure 2.** *Effect of drought stress on millets crop and its possible responses.*

water content can be used to screen drought-tolerant crops. The various morphological and physiological adaptations of millets to water stresses are summarised in (**Table 4** and **Figure 3**).

#### **2.5 Biochemical adaptations to stress in millets**

Drought means water loss and dehydration at normal or even temperatures. Shrinking of cell leads to loss of turgor, osmotic stress and change of membrane potentials, upon severe water loss from the cells, membrane disintegration and abolition of metabolic processes occur [32]. In finger and barnyard millets, the biochemical adaptation response to water stress consists of increased proline content, reduced relative water content, and chlorophyll content [2, 6]. In barnyard and finger millets, some antioxidant enzymes represent adaptive mechanism against water stress. This consists of CAT, phenol and flavonoid content [2]. According to see [33], suggested that during drought stress condition in pearl millet, a higher expression of secondary metabolite genes associated with alkaloid, terpenoid, flavanols, lignin, wax, mevalonic acid (MVA), and Shikimic acid (SA) metabolic pathways were observed in flowering stage than vegetative stage. Following are some stress response mechanisms which takes place in millets during water stress condition (**Table 5** and **Figure 3**).

#### **2.6 Molecular response against water stress in millets**

The genome and transcriptome sequences of plants give crucial information for identifying the types of genes involved in the control of drought tolerance, especially in plants that are more resistant to water scarcity. In a study on transcriptome *Drought Stress in Millets and Its Response Mechanism DOI: http://dx.doi.org/10.5772/intechopen.105942*


#### **Table 4.**

*Morphological and physiological adaptations to stress in millets.*

sequencing in finger millet provides information of 2824 genes under water stress condition [39]. According to see [40], SiLEA14 gene from foxtail millet, increased the tolerance in transgenic Arabidopsis plants to salt and osmotic stress. The induction of AKR1 gene (Aldo Keto Reductases) in roots and leaves of finger millets is studied with increasing water stress and salt stress. The up-regulated AKR1 gene shows physical defence against oxidative stress (**Table 6** and **Figure 3**).

A single gene known as -carbonic anhydrase (PgCA) was continuously upregulated in pearl millet exposed to several abiotic stimuli such as drought, salinity, and heat [57]. There are other genes which are known to be involved in drought response or tolerance in millets were EcDehydrin 7 [49], Ec-apx1 [50], EcbHLH57 [51], EcbZIP60 [52], EcGBF3 [53], EcbZIP17 [54], mt1D [55], Metallothionein, Farnesylated protein ATFP6, Farnesyl pyrophosphate synthase and Protein phosphatase 2A, RISBZ4 [56] from finger millet, SiARDP [41], SiCDPK24 [42], SiLTP [43],

**Figure 3.** *Effect and response mechanism of drought tolerant millets.*

SiATG8a [44], SiNF-YA1 and SiNFYB8 [45], SiASR1 [46], SiASR4 [47] and SiMYB56 [48] wild foxtail millet and PgGPx [58], PgRab7 [59] and PgeIF4A [60] from wild foxtail millet.

#### **3. Management techniques for water stress in millets**

In order to manage water stress, genetic advancements must be combined with appropriate cultural behaviours. To cope water stress, several cultural practices should be done. In recent year, conventional and molecular breeding techniques have been evolved to improve stress tolerance in plants.

#### **3.1 Improvement of water use efficiency in millets**

To improve water, use efficiency of millets, multiple factors such as physiological characteristics, time of planting, soil characteristics, meteorological conditions frequency of tillage and application of herbicide, should be measured properly [20, 6].

#### **3.2 Conventional breeding**

In conventional breeding, old plant breeding techniques such as introduction, selection and hybridization were used for the identification of stress-tolerant genetic traits in crops [24]. The major phases in conventional breeding are the artificial introduction of water stress and the selection of stress tolerant genotypes. Through hybridization, stress tolerant genes can be transferred into commercially growing types for improved performance under water stress conditions. Drought resistance selection can be done in the field or in a greenhouse. The field environment is particularly ideal for selection work. When it comes to determining which features to utilise to improve selection efficiency, those that contribute to productivity will be more valuable than

*Drought Stress in Millets and Its Response Mechanism DOI: http://dx.doi.org/10.5772/intechopen.105942*


**Table 5.**

*Biochemical adaptations to stress in millets.*

those that contribute to survival. Using managed field-based stress around the blooming period, when crop is particularly susceptible to water stress has been the key to enhanced pace of advancement in breeding and selection.

#### **3.3 Molecular breeding**

Plant genome improvement is insufficient for the development of novel plant varieties using traditional breeding approaches. Since the 1990s, molecular markers have been utilised to identify superior hybrid lines to overcome this barrier in plant breeding procedures. Molecular breeding is evolving a new breeding technology, which has the potential to improve crops dramatically. The majority of attributes including stress tolerance, are quantitative and influenced heavily by the environment. With the help of molecular breeding techniques, many biofortified crops and plant types with high yielding variety with new characteristics like pest and disease resistant were developed. Marker-assisted breeding (known as gene stacking) is a more efficient and cost-effective method than traditional breeding, which makes it impossible to transfer many resistance genes into a single agricultural plant at


#### **Table 6.**

*Drought-tolerance genes and their key characteristics in millets.*

*Drought Stress in Millets and Its Response Mechanism DOI: http://dx.doi.org/10.5772/intechopen.105942*

the same time [61]. Every generation in conventional breeding necessitates timeconsuming and costly progeny testing [62]. Resistance genes were introduced into the recurrent parent from the donor parent using marker-assisted breeding. This method can also be used to find the recurrent parent genotype utilising markers that are dispersed over the genome and aren't intimately connected to the target trait [63]. With the use of molecular markers, genes driving the majority of variation in stressed situations like heat and drought have been identified, and field testing has been augmented with marker-assisted selection. The genomic regions that control stress tolerance in plants is known as quantitative trait loci (QTL), and their identification and mapping aids in the early selection or screening of genotypes with stress resistance.

Markers can be used for QTL identification, mapping, and screening as well. Different approaches in molecular breeding for producing water stress-tolerant genotypes include Marker Assisted Selection (MAS), Marker Assisted Backcrossing, Marker Assisted Stacking, and so on (**Figure 4**).

#### **3.4 Transgenic approach**

Transgenic techniques require modification of both qualitative and quantitative characters by transferring desired genes [42]. The identification of a specific gene of interest that contributes to water stress tolerance, as well as the transfer of that gene through transgenic breeding, expands the possibilities for crop improvement in drought conditions. Water stress tolerance is induced when a transgene is overexpressed. The engineering of genes that encode growth regulators, suitable solutes, and antioxidants involved in stress tolerance have received the most attention.

**Figure 4.**

*Schematic representation of the molecular breeding approach for drought tolerance in millets crop.*

#### **3.5 Microbe-plant interactions**

Microorganisms live in a wide range of environmental conditions, from sub-zero temperatures to desert extremes [64]. Plant-supported phylogenetically varied microbial communities to withstand drought by altering phytohormone levels in the rhizosphere and creating water-sequestering biofilms [65]. Rhizospheric bacteria that lives near plants roots produce a variety of biocontrol chemicals as well as plant growth promoters [66]. These microorganisms are also modifying soil structure, fertility, pH, and oxygen availability [67]. Some other examples of microbes assisted drought tolerance in plants have been listed in **Table 7**.

#### **3.6 Drought resistance in millets: Potential characteristics**


Water scarcity and recurrent drought spells in agricultural ecosystems have resulted in considerable yield losses for numerous crops around the world.

#### **Table 7.**

*Drought tolerance in plants were improved by microbes.*


#### **Table 8.**

*Potential traits/characters for screening millets crop for drought resistance.*

*Drought Stress in Millets and Its Response Mechanism DOI: http://dx.doi.org/10.5772/intechopen.105942*

Innovative research results and the rapid development of several unique tools and methodologies in drought-resistance breeding have resulted in significant improvement. However, our understanding of drought resistance in millets crop is still limited, and we know very little about the complex genetic architecture of drought tolerance. We need to uncover the genetic bases of any trait associated with drought resistance in crops that can be used in crop breeding. There are various traits have been used to screen for drought tolerance, including smaller leaf area, leaf area maintenance, water use efficiency, root and shoot biomass, osmotic adjustment, pod number per plant, grain weight, biochemical parameters in millets by different researchers **Table 8** [25, 33, 35, 37, 38, 71].

#### **4. Conclusions**

Abiotic stresses are a significant barrier to crop productivity around the world. Plants respond to drought in a variety of ways, the most common of which are changes in plant development and morphology. Crop yields in the past were drastically reduced, according to historical evidence. Drought stress slows crop growth and development, resulting in changes in the crop's morphological, physiological, and biochemical characteristics [10].

Millets play an important role in the livelihood of the developing world's people; on the other hand, have a wide range of coping mechanisms to deal with these challenges. Mostly foxtail millet, pearl millet, proso millet and finger millets perform better tolerance against drought stress [2, 6, 18, 20, 23, 24, 26, 27, 29, 37, 42]. So far, we have studied stress tolerance mechanisms, adaptations, genetic modification, targeted expression of enzymes and transporters, and the role of proline, among other things, in millets [3, 5, 7, 24]. Both traditional and new methods of improvement have yet to be fully adopted. Climate change is expected to have a substantial impact on the types of crops farmed in the coming century.

In future, the key to successful crop improvement will be the ability to identify and access genetic diversity including new or improved variability for target traits by selecting parental germplasm proven to be resilient under likely climate change, including extreme events such as high temperatures [72]. Understanding how millets crops respond to drought stress is therefore essential for drought tolerance breeding. The traditional breeding strategy has demonstrated its ability to sustain productivity growth in numerous crops over the previous century. Meanwhile, current technical breakthroughs have hastened the production of novel cultivars and their impact.

An understanding of the genetic basis of drought tolerance in millets is prerequisite for plant breeders to evolve superior genotypes by adopting biotechnological approaches. There is an urgent need to improve the efficiency of molecular breeding and transgenic approach to develop new and proficient varieties with boosted natural osmolytes and raised tolerance for crops. Additional molecular studies are required to enhance knowledge on proteomic and metabolic activities on millets crop in response to drought stress. The current study's goal was to combine multiple drought tolerance mechanisms and improve these processes in millets crop. Drought stress causes plants to go through morphological, physiological, biochemical, and molecular changes. Changes in leaf structure, root growth, and stomata regulation are all morphophysiological processes. Changes in phytohormonal levels, such as ABA, JA, Auxins, Ethylene, Gibberellins, Cytokinin, and Brassinosteroids, are biochemical processes. Plant phytohormone levels rise in response to drought, resulting in the activation of

morphophysiological and other biochemical processes [73]. Furthermore, emerging 'omics' sciences such as genomics, transcriptomics, proteomics, and metabolomics could greatly improve our current understanding of the underlying drought-tolerant candidate genes as well as deciphering the complex gene networks and signalling pathways involved in drought tolerance in millets crop. Importantly, novel strategies like as GE tools and 'speed breeding' will aid in a better understanding and effectively speed up the creation of DS-resistant millets crop to reduce the risk of global food instability.

### **Acknowledgements**

The authors are thankful to the Director, GBP-NIHE (Almora, Uttarakhand) for providing the necessary support to conduct this study. The work was supported by project no. GBPI/In-House Project-03/2020-2021/9094 funded by Ministry of Environment, Forest & Climate Change, Govt. of India (New Delhi).

### **Author details**

Anjali Tiwari, Kapil Kesarwani\*, Arushi Sharma, Tapan Ghosh, Nisha Bisht and Shailja Punetha G.B. Pant National Institute of Himalayan Environment, Almora, Uttarakhand, India

\*Address all correspondence to: kapilgbpi@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.

*Drought Stress in Millets and Its Response Mechanism DOI: http://dx.doi.org/10.5772/intechopen.105942*

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#### **Chapter 4**

## Abiotic Stresses and Their Management in Vegetable Crop Production

*Khursheed Hussain, Sameena Lone, Faheema Mushtaq, Ajaz Malik, Sumati Narayan, Majid Rashid and Gazala Nazir*

#### **Abstract**

The stress concept, first proposed by Hans Selye in 1936, has also been applied to plants to describe adverse and environmental restrictions. The notion of plant stress, differs significantly from that of animals and humans. Due to ever fluctuating climatic circumstances and variables, the crop-environment interaction in horticultural crops leading to losses in yields and quality of produce occurs and thus climate change with respect to horticulture industry is attracting more attention. Abiotic stress is the leading cause of crop yield loss globally, lowering average yields by more than half for most main crop plants. Abiotic stressors are highly correlated and connected, causing morphological, biochemical, physiological and molecular changes in vegetable crops, leading in a significant profit drop. Water stress is the most common abiotic stress that causes significant losses in vegetable production, especially because it is often coupled by additional stresses like as salt, high temperatures, and nutritional deficits. Increased CO2 and temperature in the atmosphere, variation in amounts of precipitation causing more frequent droughts and floods, widespread runoffresulting in soil nutrient leaching and a loss in fresh-water availability are all contributing factors. Efforts to mitigate various pressures should be focused both throughout the growing season and after harvest. Stress-tolerant cultivars are being developed using a variety of methods, including traditional breeding and transgenic technology. Instead of genetic engineering, using vegetable breeding procedures or directed breeding is one the best options to improve stress tolerance in vegetables. Besides, post-harvest treatments, application of growth regulators, antioxidants, germplasm and *in vitro* selection, and modified environment packaging with different plastics may all help to improve tolerance and hence increase the shelf and nutritive life of vegetables.

**Keywords:** abiotic stress, vegetable crops, yield, growth regulators, tolerance

#### **1. Introduction**

Crops are increasingly commonly subjected to abiotic stressors in today's climate change scenarios. Abiotic stress, such as drought, salt, and severe temperatures, which typically cause main crop losses around the world, are predicted to produce a yield loss of more than 50% in agricultural crop plants [1]. We should also focus on increasing the food production and supplies by two-fold till 2050 so as to fulfill the requirement and demand of human population. This can be achieved by a basic comprehension of mechanisms underlying abiotic stresses. It is for this reason that development of stresstolerant plants has received gotten a lot of attention in recent years for these reasons. During production, processing, storage, and distribution, harvested vegetables might be subjected to a variety of abiotic stressors. When there is a moderate or severe abiotic stress, quality losses nearly always occur at market [2, 3]. Moreover, there are abiotic stressorswhich ultimately decreases the defense mechanisms of plants and increases their susceptibility to infection by pathogens. Understanding the nature andorigins of abiotic stressors that impact vegetables is critical. In addition, increased understanding opens up possibilities for better control or resistance [3, 4]. As a result, as postharvest problems limit the storage and shelf-life potential of vegetables, understanding the effects of field abiotic stresses on postharvest stress susceptibility will become increasingly important [4]. Why there are studies in relation to advances in physiology, molecular biology, and genetics is because of the fact that our comprehension of plants' responses to various stresses, as well as the basis for varietal tolerance variances is a big solution to the very big problem of abiotic stresses.

As the best way to solve these issues is to focus on both pre-harvest and postharvest abiotic stress reduction, it's crucial to understand the relationship between pre-harvest and post-harvest abiotic stresses that occur during vegetable crop production and handling, storage, and distribution, respectively.

#### **2. Pre-harvest stresses during vegetable production**

#### **2.1 Temperature extremes**

Plants are vulnerable to low temperature stress if the temperature dips below 15°C, and high temperature stress if the temperature rises over 45°C. Plants are affected by high temperature stress in a variety of ways, including physiology, biochemistry, and gene regulation mechanisms. High temperatures during the reproductive period of plants can increase senescence, diminish fruit set and lower yield. Furthermore, temperature stress makes the plant vulnerable to pests and other environmental issues besides limitingor preventing seed germination, depending on the species and stress level. Exposure to high temperatures throughout the growing season might also affect antioxidants in vegetable crops. Susceptibility to postharvest chilling injury can be exacerbatedif the preharvest temperature causes chilling induced harm in the field. As a result, the magnitude of the preharvest temperature extreme will determine whether the exposure has a favorable or negative impact on postharvest stress sensitivity.

#### **2.2 Drought**

We know that a third of the world's population resides in areas that are having water-stress condition which may become more severe due to increasing carbondioxide concentrations in the atmosphere. The climatic changes are therefore more expected with severe droughts in furture. Water scarcity is expected to remain a major abiotic issue influencing worldwide crop output. Reduced canopy absorption of photosynthetically active sunlight, decreased radiation-use efficiency, and reduced harvest indexare all effects of soil moisture deficit on crop output. Drought *Abiotic Stresses and Their Management in Vegetable Crop Production DOI: http://dx.doi.org/10.5772/intechopen.105453*

circumstances during the development of vegetable crops are becoming increasingly common as a result of climate change patterns [5]. In root crops, field water deficiency (stress) has been found to have both positive and negative effects. Water stress prior to harvest (Irrigating to 25–75 percent of soil water field capacity) may weaken the cells, resulting in increased membrane leakage (cell damage) and, as a result, more weight loss in storage for root crops like carrots.In response to mechanical stresssuch as bruisingpotato cells undergo decompartmentationresulting in black spot conditions [6]. Water stress, especially during the tuber-forming stage, can make potatoes more susceptible to the black spot condition after harvest [6].

#### **2.3 Light**

Tomatoes grow smaller when cultivated in low light environments, such as early spring in northern latitudes [7], and because theratio of surface area to volume is higher in smaller fruits, vulnerability to postharvest desiccation stress increases [8]. It has been reported that when lettuce is cultivated under less intense lights, due to less number of photons, photosynthetic efficiency as well as quality traits like vitamin C is suboptimal and thus decreasing shelf life after harvest.

#### **2.4 Salinity**

Excessive quantities of soluble salts in the soil water (soil solution) are known as saline soil, and they can significantly affect plant growth, resulting in lower crop yields and even plant mortality in extreme cases. Salts are substances that dissolve into ions, such as NaCl, MgSO4, KNO3, and sodium bicarbonate. Electrical conductivity (ECe), exchangeable sodium percentage (ESP) or sodium adsorption ratio (SAR), and pH of soil paste (saturated) extractare used to calculate it. As a result, saline soils have saturated soil paste extracts with an ECe of more than 4 dSm−1, an ESPof less than 15%, and a pH of less than 8.5 [9]. Tomato crop grown inhigh salinity generate smaller fruits with a greater soluble solids content. It is a matter of understanding that fruits having smaller size have more surface area than their volumes (Known as Surface-area to Volume ratios), making them more vulnerable to postharvest water loss (desiccation stress) [8].

#### **2.5 Flooding stress**

Crops are also subjected to severe physiological stress as a result of sudden inundation following heavy rainfall events. Plants must adapt to a distinct, but equally challenging, flooding environmentthat occurs in a more regular cycle of seasonal fluctuations in river levels and concomitant slow flooding of crop lands. Waterlogging is the term for soil condition when there is flooding that creates hypoxia which also affects stems causing wilting with other physiological conditions.

#### **2.6 Plant nutrition**

When plants are unable to complete the reproductive stage of their life cycle due to a shortage of mineral components, they are considered essential. In several crops, calcium supplementation during production has been associated to postharvest issues [10]. Calcium has been proposed as a possible signaling molecule involved in the development of abiotic stress cross tolerance [11]. As a result, the effect of preharvest calcium

nutrition on postharvest stress resistance is likely to be multifaceted, and it will depend on whether the vegetable is also exposed to abiotic environmental difficulties. Preharvest nitrogen levels are frequently linked to poor postharvest vegetable quality. Excessive nitrogen fertilization causes large zinc and aluminum accumulations in cabbage, as well as nitrate-induced manganese deficiency [10]. Nitrogen fertilizer affects black spot susceptibility in potatoes. Nitrogen deficit or lower-than-recommended nitrogen treatment rates, on the other hand, will almost always result in higher vitamin C concentration in plants. Vitamin C concentration has been connected to storage life potential [12], which is likely due to the antioxidant nutrient's usefulness in preventing oxidative damage, which leads to quality losses in storage. Potassium deficit in carrots is linked to increased weight loss during storage. At potassium levels below 1 mM in the soil media, weight loss was directly linked to increased membrane leakage (i.e., damaged cells) in carrot tissues.

#### **3. Post-harvest stresses during handling, storage and distribution of vegetable crops**

#### **3.1 Temperature extremes**

The relevance of temperature in determining a harvest index cannot be overstated. Many products, particularly those delivered by air or ocean container, face persistent postharvest temperature abuse during distribution [13]. Heat treatments limit respiration and ethylene generation, diminish protein synthesis, and accelerate protein degradation in the short term. Fruit vegetables, root and tuber crops are all susceptible to chilling [14]. The ability to produce flavor has been shown to be a sensitive early indicator of cold stress effects, andChilling injury is related with visible (surface pitting, interior browning) and texture (accelerated softening and development of mealiness) alterations.

#### **3.2 Oxygen and carbon-dioxide level**

The difficulty is worsened when processing products in modified atmosphere (MA) containers rather than controlled atmosphere (CA) systems, because temperature is generally not as easily managed in MA packages as produce goes through a distribution chain [4]. High CO2 stress can cause a diverse array of physiological problems, such as black heart in potatoes and brown stain in lettuce [15]. Chilling stress, ethylene-induced diseases, and vulnerability to pathogenic attack can all be influenced by high CO2 levels [15].

Low oxygen levels are known to cause stress-induced changes in metabolism and metabolite accumulations [16], but acute low oxygen injury does not show up until the tissue is re-aerated and an uncontrolled oxygen burst (consisting of hydrogen peroxide and other radicals) occurs, causing lipid peroxidation, protein denaturation, and membrane injury [17]. Varied vegetables have different low O2 stress thresholds depending on architecture, temperature, physiological age, the presence of supplementary gases (e.g., CO2, CO, SO2), and the duration of exposure.

#### **3.3 Mechanical injury**

Injury from impact is linked to product loading for transportation, incidents during transportation, unloading, and throughout the packaging and processing lines. Cuts can produce brief increases in respiration, ethylene synthesis, phenolics generation,

and cell degradation near the injury site [18]. The severity of the reaction to cutting is highly dependent on tissue properties, the maturity of the vegetable of interest, the coarseness or sharpness of the cutting object employed, and the cutting temperature.

Fresh-cut vegetable products have the highest rate of cut injuries. Many vegetables suffer from cut injuries during the harvesting process, which are more severe in machine-harvested produce than in hand-harvested produce. Cut-edge browning or blackening is the most typical symptom of cutting-related diseases, however yellowing in green tissues and whitening on carrots can also occur [18]. The degree and amount of bruising received is influenced by maturity, tissue or cellular orientation at the region of the injury, water potential, form of the object imparting the bruising force, energy and angle of impact, and product temperature. Internal black patches appear in potato tubers as a result of impact trauma.

#### **3.4 Desiccation**

Water loss in vegetable tissues causes degradation, which is a significant problem inpostharvest processing and distribution [19]. Water stress can cause rapid senescence, which manifests as tissue weakening, membrane degradation, and yellowing in addition to wilting [19]. The vapor pressure deficit, which is the connection that explains the difference in water activity of the vegetable and the water activity of the atmosphere surrounding it [19], is thedriving factor for water loss. The more of a vapor pressure deficit there is, the more water is lost. For limiting water loss in any vegetable, there are a few postharvest handling principles to follow:


#### **4. Abiotic stress response Mechanismsat the biochemical and molecular levels**

Acute and sub-acute reactions to abiotic stresses exist; acute responses reflect circumstances where cell death is a direct resultof the stress, whereassub-acute responses represent cases where the stress induces adaptive changes in biochemistry and gene expression [6]. Many reactive oxygen species (ROS), especially hydrogen peroxide, operate as signaling molecules that cause biochemical changes in gene expression. Abiotic stresses disrupt vegetable cellular homeostasis, resulting in increased production of reactive oxygen species (ROS) in the apoplast, mitochondria, peroxisomes, cytoplasm, chloroplasts, and endoplasmic reticulum [20]. The cell's ability to cope at first will be largely determined by its endogenous free radical scavenging capacity [20]. When free radical production surpasses endogenous scavenging capability, ROS interact with sensors whose full natureis unknown, triggeringmitogen activated protein kinase (MAPK) cascade events and up-regulating transcription

factors and calcium/calmodulin kinases directly. The MAPK cascade reactionactivates a number oftranscription factors that enable de novo ROS formation, ROS scavenging systems, heat shock protein accumulation, and NADPH supply modulation in the cell [20]. Some MAPK cascade pathways have also been connected to ethylene production particularly, which is likely why ethylene production appears to be intrinsic to most stress reactions. Heat shock proteins (HSPs) accumulation, whichismediated by transcription factor activation downstream of the MAPK cascade [20], has been shown to improve long-term stress resistance in afflicted tissues [21]. Under normal, stress-free conditions, HSPs are thought to be an essential element inprotein folding, assembly, translocation, and degradation [22]. HSPs have also been linked to protein stabilization, membrane stability, and protein refolding under stress. As a result, HSPs are assumed to play akey role in protecting plant tissues from stress by ensuring cellular homeostasis. Metabolic Responses to Abiotic Stress Signaling, physiological regulation, and defense responses are all heavily influenced by metabolism when the environment is hostile and plant development is harmed. Abiotic stressors alter the production, concentration, transport, and storage of primary and secondary metabolites in a feedback loop.

#### **5. Drought stress and plant metabolomics**

Water deficiency stress causes a variety of physiological and biochemical changes in plants, including cell development and photosynthesisarrest, as well as increased respiration. Thegenome's expression is modified extensively, activating and suppressing a wide range of genes with various roles. Abscisic acid (ABA) accumulates inhydric-stressed plant tissues and increases stomatal closure, which reduces transpiration. Plants use this technique to reduce water loss and reduce stress injury.

#### **6. Temperature extremes and plant metabolomics**

In sensitive vegetables, chilling stress causes the formation of lipid peroxidation products, superoxide anions, and hydrogen peroxide, as well as a reduction in flavor volatile synthesis. It interferes with ethylene metabolism [23], causing the softening process to speed up. Cell wall metabolism is also altered by chilling stress, with upregulation of cell wall breakdown enzymes as pectin methyl esterase and endopolygalacturonase. Heat stress causes metabolic changesthat lead to the buildup of heat shock proteins, which are known to give long-term stress resistance in heat-exposed animals [22]. It can alsostop lycopene from being produced and accumulated.

#### **7. Wounding stress and plant metabolomics**

The upregulation of phenylalanine ammonia lyase (PAL) by wounding stress causes phenolic buildup [24]. The upregulation of PAL was linked to the synthesis of ethylene by wounds. Theinitial reaction to wounding stress is characterized by a progressive accumulation of ACC synthase, ACC, and ethylene synthesis in tomatoes, which can last up to 2 hours, butethylene production reduces if the sliced tomatoes are kept for longer periods of time. ACC synthase and ACC, on the other hand, continue to accumulate, showing that there is a capability to generate ethylene, but ACC to

ethylene transition is prevented or inhibited. Other metabolites, such asisocoumarin in carrots [25], anthocyanins in red-pigmented lettuce midribs [25], methanethiol, allylisothiocyanates, and dimethyl disulfide in cabbage [26], and six-carbon aldehydes and alcohols in cut peppers [27], rise in response to wounding stress.

#### **8. Oxidative stress and plant metabolomics**

When exposed to anaerobic or anoxic environments, the body goes through two separate phases. The first phase is characterized by a metabolic shift caused by a limitation of the principal electron acceptor, molecular O2, in the mitochondrial electron transport chain. ATP levels, pyruvate dehydroxylase activity, and cytoplasmic pH have all decreased as a result of this impairment [15]. The activity ofpyruvate decarboxylase, alcohol dehydrogenase, and lactate dehydrogenase also rise under these conditions [15]. Anaerobic respiration is induced, and acetaldehyde, ethanol, ethyl acetate, and/or lactateaccumulate. Damage to the mitochondrial electron transport chain also causes electron leakage in the cells, resulting in the production of superoxide anions and hydrogen peroxide, which are destroyed by existing cellular antioxidant systems. During an anoxic or hypoxic incident, ascorbate and glutathione levels can also rise. Any or all of these alterations in vegetable tissues are signs of oxygen stress and have an impact on qualitative qualities. However, it isnot until the second phase, when the vegetable is returned to greater O2 atmospheres, that true tissue harm occurs. When cells are exposedto aerobic environments, rates of oxygen radical production in the impaired electron transport chainincrease, resulting in largeaccumulations of superoxide anion, hydrogen peroxide, and hydroxyl radical that cannot be fully decomposed by existing antioxidant protection systems, resulting in membrane damage, enzymatic browning, and cell death [17].

#### **9. Salinity stress and plant metabolomics**

Salt stress causes the production of abscisic acid, which is transferred to guard cells and seals stomata, resulting in impaired photosynthesis, photoinhibition, and oxidative damage. This results in an instantaneous halt tocell expansion, which manifests as slowed plant growth, rapid development, and senescence. Plants use strategies including reduced photosynthesis, stomatal conductance, and transpiration rates to cope with salt stress. Because sodium ion has the same chemical structure as potassium ion, it competes with potassium uptake and suppresses it. Potassium deficiency inhibits growth since it is involved in the capacitance of a wide range of enzyme activities, as well as controlling membrane potential and cell turgor.

#### **10. Desiccation stress and plant metabolomics**

Under extreme handling conditions, desiccation stress increased the osmotic potential of carrots, which is a function of free sugars in the roots. (i.e., at 13°C). The elevation in osmotic potential in response to water loss was most likely explained by increased polysaccharide hydrolyzing enzyme activity in response to stress. As a result, enzymes such polygalacturonase and pectin esterase may become more active, resulting in a loss of cell wall integrity and a rise in soluble sugars. This could account for at least some of the loss of stiffness observed in carrots as they lose water.

#### **11. Management of Abiotic stresses**

#### **11.1 Abiotic stress tolerance and the role of plant growth regulators**

Applications of growth regulators may also improve stress resistance, particularly invegetables that are prone to rapid senescence in response to stress. As a result, anti-ethylene products such amino vinyl glycine (AVG) and 1-methylcyclopropene (1-MCP) may help to extend storage or shelf life if ethylene synthesis in reaction to stress is a major problem. Bell peppers and zucchini squash can benefit fromother growth hormones, such as methyl jasmonate (which increases leaf senescence). In some crops, abscisic acid has been shown to decrease chilling-induced damage. Other growth regulators (e.g. 2, 4-D) have been proposed for use in avoiding senescence in leafy vegetables, but their practical utility is limited.

#### **11.2 Postharvest treatments to enhance stress resistance**

A variety of postharvest treatments have been tested to improve vegetable abiotic stress tolerance [3]. Temperature manipulation (including intermittent warming), extreme atmospheres (high O2, CO2, and low O2), growth regulators, antitranspirants, antioxidant dips, growth regulators, nitric oxide, and ethanol haveall been put to the test [3]. In fresh-cut items, hot or warm water treatments have been demonstrated to reduce cutting-induced damage. Treatments like these can also be utilized to prevent chilling harm by inducing heat shock proteins [21]. In cut and packed lettuce, a warm water treatment has also been demonstrated to lower irradiation susceptibility [28]. Gradual cooling (2°C per day) has been reported to lower the tomato chilling injury susceptibility, most likely by enabling the intrinsic stress resistance systems to mature before true chilling conditions [29]. Atmospheric treatments, such as modified or controlled atmospheres, have been demonstrated to aid in the reduction ofchilling injury in a variety of vegetable crops.

#### **11.3 Use of molecular probes for marker-assisted breeding**

As there are many genes and proteins linked to stress tolerance in plants, applying the stress of interest and doing quantitative trait loci (QTL) analysis is the best way to find stress tolerant lines. This method canbe employed with intact plants and/or harvested plant parts, with the plant component of interest in the breeding improvement strategybeing used in most cases [30]. The method necessitates the examination of adaptive changes in QTL expression rather than constitutive expression. To distinguish between resistant and susceptible lines, a stress protocol must be created to which the target vegetable will be exposed. However, because the stress response is complicated, successful use of QTLs will necessitate an interdisciplinary effort that integratesbiochemistry, gene mapping, and phenotyping activities to allow for reliable interpretation and successful application of adaptive QTLs for stress resistance selection.

#### **11.4 Molecular engineering**

Because of two major factors, molecular engineering for stress resistance in vegetables is limited:


#### **11.5 Germplasm selection**

Germplasm selection and cultivarsdeveloped through breeding programmes will be more resistant to postharvest stress and so have improved storage capacity. Hodges *et al*. [12] were able to establish that variations in the balance of antioxidant systems in the tissues caused larger accumulations of ROS, notably hydrogen peroxide, in a cultivar that was more prone to yellowing. They theorized that greater ROS levels were directly responsible forthe yellowing of the chlorophyll in spinach leaves. *In vitro* selection isa technique in which plant cells from a target vegetable are tissue-cultured and subjected to a stressor, with the surviving cells used to regenerate new plants with higher stress resistance [31]. It is a far less expensive technique than molecular engineering, and laboratories may be built up practically everywhere in the globe with basic utilities and utilizing low-cost technology. This method has proven to be particularly effective in regenerating germplasm from a variety of crop plants that can be regenerated using tissue culture techniques.

#### **11.6 Postharvest handling**

Simple changes to postharvest handling methods can occasionally result in a significant reduction in stress exposure, allowing for longer storage and/or shelf life. Because most produce is refrigerated as a required step to minimize rotting and protect food safety, avoiding low temperature stress is typically impossible. While quick cooling is normally suggested to maintain quality, delaying or gradually chilling sensitive crops to allow them to acclimate to storage and handling conditions may be beneficial. Slow cooling of tomatoes from 12–4°C at a rate of 2°C per day has recently been proven to decrease chilling harm when stored at the lower temperature [19]. In many circumstances, modified environment packaging is thought tohelp minimize moisture loss in fresh-cut and whole vegetables by controlling humidity surrounding the product. Many vegetables have improved their shelf life and quality by using plastic film packaging or wraps to prevent desiccation [19]. Anti-transpiration coatings have also been demonstrated to be useful in preserving quality by reducingwater loss. A single stress resistance-enhancing treatment may notprovide enough resistance to all postharvest stresses [3]. To obtain optimal levels of resistance, it may be beneficial to explore using a combination of two or more stress tolerance increasing treatments.

#### **12. Conclusion**

Drought, excessive watering, severe temperatures, salt, and mineral toxicityall have a negative impact on thegrowth, development, yield, and quality of vegetables on and off the farm until they reach the customer. Furthermore, climate change has introduced new environmental variablesthat may influence the vulnerability of vegetables to postharvest stress. Crop management can have a substantial impact on stress susceptibility. Adapting horticulture crops to changing surroundings could be the single most essential action we can take to prevent climate change's negative consequences. The management steps must be followed from the field circumstances to the point where the product reaches the consumer. While many crops are being bred for stress resistance to helpthem adapt to climate change, it is unclear if in the field breeding for stress resistance will also transmit stress resistance qualities to the harvestedcomponent. To properly evaluate the benefits that abiotic stress during production may offer for postharvest abioticchallenges, it's critical tounderstand the basis of molecular and biochemical response networks to diverse stresses faced in the field and throughout the postharvest continuum. Theuse directed plant breeding to improve the toleranceto stress in vegetablesshould probably be the focus of attention. Temperature modulation, usage of growth regulators, anti-transpirants, antioxidants, and other sorts of postharvest management can improve tolerance and hence extend the keeping quality of vegetables. The use of various plastic sheets to generate tailored environment packaging is one of the best promising technologies. As a result, new vegetable types that are resistant to abiotic stressors are urgently needed to assure food security and safety for many years to come.

### **Author details**

Khursheed Hussain\*, Sameena Lone, Faheema Mushtaq, Ajaz Malik, Sumati Narayan, Majid Rashid and Gazala Nazir Division of Vegetable Science, SKUAST-Kashmir, Shalimar (J&K), India

\*Address all correspondence to: khussainskuast@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 5**

## Copper Toxicity in Plants: Nutritional, Physiological, and Biochemical Aspects

*Flávio José Rodrigues Cruz, Raphael Leone da Cruz Ferreira, Susana Silva Conceição, Edson Ugulino Lima, Cândido Ferreira de Oliveira Neto, Jessivaldo Rodrigues Galvão, Sebastião da Cunha Lopes and Ismael de Jesus Matos Viegas*

#### **Abstract**

Copper (Cu) is an essential micronutrient for plants because it participates in several redox reactions and the structural constitution of the Fe–Cu cluster. Although it is required in small concentrations at toxic levels, Cu triggers physiological and biochemical disorders that reduce plant growth. In higher plants, the normal range of Cu concentration is in the range of 2–20 mg Cu kg−1 DW. Above the upper limit of this range, Cu toxicity may occur if the plants are not tolerant to the stress caused by toxic levels of Cu. In view of the growing agricultural and industrial activity that are the main sources of Cu addition in nature, understanding the physiological and biochemical mechanisms of Cu toxicity in plants constitutes an important tool for the selection of more tolerant genotypes based on biochemical and physiological indicators to heavy metal stresses. In this chapter, we propose a systematic review of plants grown under toxic levels of Cu, based on the responses of physiological, biochemical, and nutritional variables. Understanding these responses will contribute to improving the understanding of the basic mechanisms of stress tolerance by toxic levels of Cu in higher plants, providing valuable information for the improvement of genotypes resistant to toxic levels of Cu in the plant culture medium.

**Keywords:** nutritional disorder, gas exchange, micronutrient, plant growth

#### **1. Introduction**

The micronutrient copper (Cu) is a transition metal with atomic number 29, an atomic mass of 63.5 g mol−1, and a density of 8.96 g cm−3. It is the 25th most abundant chemical component in the Earth's crust and the third most used worldwide [1].

Cu occurs naturally in soils with contents ranging from 60 to 125 mg kg−1 [2]. It is an essential micronutrient for plant development and, under physiological conditions, it exists in the form Cu+ and Cu2+. Cu acts as a structural element in regulatory proteins and participates in the electron transport chain of photosynthesis and

respiration, oxidative metabolism, cell wall metabolism, and hormonal signaling [3, 4]. Among the proteins, ascorbate oxidase, Zn/Cu superoxide dismutase, and Cu amino oxidase are those that have more than one Cu atom in their structures (8, 2, and 2 Cu atoms, respectively) [5].

Cu is absorbed in the form of Cu2+ or Cu chelate and, despite being poorly mobile in plants, it can be translocated from old leaves to new leaves. Its concentration in the dry mass of plants is small and generally ranges from 2 to 20 mg kg−1. However, concentrations in the dry mass of plants varying between 20 and 100 mg kg−1 are toxic to most plants [6].

Soil contamination by Cu is mainly caused by human action represented by industrial, mining, and agricultural activities. Intensive use of Cu-containing agrochemicals or swine manure is the main source of Cu entry into the agricultural soils [7, 8]. This scenario is worrying because the world population is expected to reach nine billion inhabitants in 2050 [9], which suggests an increase in the area of agricultural crops to meet the world demand for plant and animal foods with the consequent increase in the consumption of Cu-containing agrochemicals to phytosanitary purposes.

At toxic levels in the soil, Cu reduces the absorption of water and mineral nutrients [10], promotes oxidative stress [11] and affects photosynthesis [12], causing reduced growth [13] and plant production [14].

This chapter aims to address the impact of copper toxicity on plant growth, emphasizing key physiological, biochemical, and nutritional variables in studies of heavy metal toxicity in higher plants.

#### **2. Copper absorption mechanism**

Mineral nutrients are absorbed from the soil matrix by plants through the cells of the root epidermis and then transferred to its center through the parenchyma, endoderm, and xylem. This unidirectional pathway of transition-metal absorption is supported by different metal transporters that act in coordination with other metal transport molecules that sequester/chelate so that adequate absorption and transport of ions occurs in all plant tissues throughout the metagenesis of plants [15].

Cu absorption occurs through three types of transporters present in the plasma membrane of root cells—P-type ATPase copper transports, COPT copper transports, ZIP family transports, and NRAMP family transports. P-type metal transporters (P-type ATPase copper transports) are responsible for the transmembrane transport of toxic metals, such as divalent cations (Cu2+, Zn2+, Cd2+, and Pb2+). These transporters use ATP to pump charged molecules across cell membranes [15, 16]. The family of proteins responsible for the transport of Cu in the reduced form is the COPT transport protein (copper transporter protein). These proteins are formed by five members, that is, COPT1, COPT2, COPT3, COPT4, and COPT5 [17]. The zinc–ironregulated transporter-like protein (ZIP) are involved in the absorption of Cu ions. Depending on the concentration of Cu in the plant-growth medium, ZIP2 or ZIP4 can act as a Cu carrier [18]. The natural resistance-associated macrophage protein (NRAMP) is responsible for the reallocation of ions, such as Fe, Ni, Mn, Zn, and Cu, from the root and shoot against cellular and vacuolar membranes [19].

After being absorbed by the roots, Cu can be transported in the xylem to the shoot in the form of Cu<sup>+</sup> and Cu2+. But usually, the transport of Cu from the root to the shoot is done in the form of Cu<sup>−</sup> complex. In plants, the xylem is the main source of

*Copper Toxicity in Plants: Nutritional, Physiological, and Biochemical Aspects DOI: http://dx.doi.org/10.5772/intechopen.105212*

#### **Figure 1.**

*Variation in Cu content in different plant structures. Because it is not very mobile in plants, Cu accumulates to a greater extent in the root, stem, and leaves.*

Cu for the shoot [20, 21]. However, Cu is poorly mobile in plants, accumulating to a greater extent in the root system compared to the shoots of plants [3, 22] (**Figure 1**).

#### **3. Effect of copper toxicity on antioxidant metabolism**

Cu is an essential micronutrient for plants because it is a component of several enzymes that act in electron transport and catalysis of redox reactions in mitochondria and chloroplasts [3]. However, at toxic levels, Cu generates oxidative stress that damages cellular structures and molecules, such as DNA, proteins, and lipids. Cu has specific chemical characteristics that generate oxidative stress through the catalysis of oxidation–reduction reactions that form reactive oxygen species, such as singlet oxygen (O2 − ), hydrogen peroxide (H2O2), and hydroxyl radical (OH− ). These reactive oxygen species promote lipid peroxidation [23].

The biochemical process of free radical formation triggered by Cu involves three mechanisms—participation in Fenton-type reactions (1), reduced glutathione depletion (2), and substitution of Fe in the Fe–S cluster (3) [24].

$$\rm Cu^{\cdot} + H\_{2}O\_{2} \rightarrow Cu^{2+} + OH^{\cdot} + OH \tag{1}$$

$$\text{2Cu}^{2+} + \text{2GSH} \rightarrow \text{Cu}^{+} + \text{GSSG} + \text{2H} \tag{2}$$

$$\left\langle \right\rangle^{3} \Big\rangle\_{\text{fb}} \Big\langle \underbrace{\text{co}}\_{\text{g}} \Big\rangle\_{\text{gb}} \xrightarrow[\text{eq}]{} \text{co} \Big\langle \underbrace{\text{co}}\_{\text{g}} \Big\rangle\_{\text{c}} \Big\rangle\_{\text{c}} \tag{3}$$

However, the production of free radicals can be potentiated by the Haber–Weiss reactions (4) and (5) in which superoxide and hydroxyl radicals are formed [25].

$$\mathrm{H}\_{2}\mathrm{O}\_{2} + \mathrm{OH} \rightarrow \mathrm{H}\_{2}\mathrm{O} + \mathrm{O}\_{2}^{-} + \mathrm{H}^{+} \tag{4}$$

$$\rm H\_2O\_2 + O\_2^- \rightarrow O\_2 + OH^- + OH \tag{5}$$

Plants under heavy metal stress conditions have enzymatic (superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase) and nonenzymatic (reduced glutathione and ascorbate) mechanisms that minimize the negative impact of oxidative stress triggered by free radicals.

Cu toxicity (100–500 μM Cu) increases the activity of important antioxidant enzymes, such as superoxide dismutase, ascorbate peroxidase, catalase, and glutathione reductase [26, 27], because, in excess, Cu increases the production of O2 − and H2O2 by modulating the Haber–Weiss reaction [25]. In addition, nonenzymatic mechanisms are involved in the attenuation of oxidative stress triggered by Cu toxicity, such as reduced glutathione (GSH) and ascorbate. GSH is a tripeptide widely distributed in plant tissues. It reacts with harmful oxidants to protect thiol groups of proteins [28, 29]. The concentration of GSH can be reduced in plants treated with toxic levels of Cu because it is used to neutralize free radicals directly or indirectly [28, 30].

Iron–sulfur (Fe–S) groups are versatile cofactors formed by inorganic iron and sulfide atoms because various metabolic pathways and proteins require Fe–S groups for proper functioning [31–33]. The main function of Fe–S-containing proteins is the transfer of electrons to produce redox potential in chloroplasts and mitochondria [34]. Under Cu toxicity conditions, iron (Fe) can be replaced by Cu in Fe–S groups, impairing the functioning of the electron transport chain in mitochondria and chloroplasts and, consequently, reducing plant growth [5].

#### **4. Effect of copper toxicity on gas exchange**

Cu is an essential micronutrient element in the transport of electrons between photosystems II and I because it enters the structural constitution of plastocyanin, an important component of the electron transport chain along photosystems [3–5]. However, under toxic levels of Cu, photosystem II is the most sensitive site of Cu action [5, 35]. The most apparent toxic effect of Cu is the inhibition of the oxygen evolution complex and the quenching of photochemical variables [36–38]. Cu2+ ions inhibit both the acceptor and the donor site of photosystem II. However, the oxidizing site of photosystem II is more sensitive to toxic Cu [39, 40].

Cu at toxic levels affects photosynthesis because it decreases Rubisco enzymatic activity and stomatal conductance, in addition to increasing the intercellular concentration of CO2 in plants [12, 41, 42]. Therefore, Cu can affect photosynthesis due to stomatal (reduction in stomatal conductance) and non-stomatal factors (damage to Rubisco and the electron transport chain). For example, toxic levels of Cu (100– 1000 mg L−1) impose a stomatal limitation on photosynthesis because the decrease in photosynthetic activity and intercellular concentration of CO2 occurs in response to the decrease in stomatal conductance caused by Cu [43, 44]. On the other hand, Cu toxicity can promote non-stomatal limitation of photosynthesis due to a decrease in photosynthesis and stomatal conductance in parallel with an increase in the intercellular concentration of CO2 [12]. This suggests biochemical damage to enzymatic

components of photosynthesis, such as Rubisco. Toxic Cu (700 mg kg−1) negatively affects photosynthesis, transpiration, stomatal conductance, and internal cell concentration in plants [45]. Toxic levels of Cu (800 mg kg-1) cause a considerable reduction in photosynthesis and stomatal conductance, with deleterious effects on plant height and stem diameter [13].

#### **5. Effect of copper toxicity on nutritional status**

Depending on the concentration and plant species/genotype, heavy metals can induce toxicity that manifests itself through a decrease in chlorophyll concentration, reduced nitrate reductase activity, nutritional disorder, and, consequently, reduced plant growth [11, 46]. Cu at toxic levels promotes changes in root cell membrane permeability, expression of phosphorus membrane transporters (P), volume, and root area, which result in lower P absorption [47].

The negative impact of toxic levels of Cu on the mineral metabolism of plants may originate from morphological changes in the root system that decrease the surface for nutrient uptake. Thus, there is a close relationship between nutritional disorders and plant root growth. Toxic levels of Cu (50 μM) reduce the diameter, length, area, and root biomass, coinciding with lower levels of P, Ca, Mg, Mn, and S in plants [47].

Toxic levels of Cu negatively affect the nitrogen metabolism of plants by decreasing the reduction of nitrate and its assimilation into organic compounds. The toxicity of 10.3 μM reduces the activity of nitrate reductase in roots and leaves, contributing to a decrease in root nitrate content [48]. High concentrations of Cu (5–20 μM) inhibit the activity of nitrate reductase and genes encoding the synthesis of low-affinity nitrate transporters (NRT.1), resulting in lower nitrogen uptake and accumulation in plants [42]. Cu (20–100 μmol) considerably reduces the activity of nitrate reductase and the accumulation of nitrate in leaves and roots of seedlings, with negative repercussions on plant growth [49].

Regarding sulfur metabolism in plants, Cu toxicity (5 –10 μM) induced an increase in total sulfur and glucosinolate levels as a defense against Cu-induced stress (antioxidant role and/or Cu chelating agent) [50]. The concentration of thiols and the activity of the enzyme O-acetylserine (thiol)lyase are increased in the aerial part of plants in the presence of 20 μM of Cu [51]. These changes suggest that Cu at toxic levels modulates changes in the concentration and activity of components of sulfur metabolism in plants to minimize the deleterious effects of Cu.

Despite the toxic effect of Cu, root architecture can be remodeled by plants. For example, the total density of lateral roots, the density of lateral roots less than 0.3 cm, and the density of lateral roots greater than 0.3 cm show an increase in their values when subjected to toxic levels of Cu (10–75 μM). Despite this, root remodeling lacks molecular studies to highlight the mechanisms involved in this process. Lateral root elongation and primary root mitotic activity are inhibited in the toxicity range of 50–75 μM Cu. These changes in the root system occur together with a reduction in the concentration of essential nutrients, such as P, K, Ca, Mn, and Fe, in the aerial part of plants [52].

Plant roots show a reduction in the concentration of essential nutrients (Ca, K, and F) and a decrease in root length and biomass in the presence of toxic levels of 4–80 μM of Cu [53], suggesting that morphological changes in roots and shoots induced by Cu are linked to nutritional disorders in higher plants exposed in the medium and long term to toxic levels of Cu.


#### **Table 1.**

*Ranges of Cu concentration in roots and shoots of plants grown under copper toxicity.*

The Cu is poorly mobile in plants, with higher levels in the root, stem, and leaves, respectively. This pattern of Cu accumulation is evident in studies of Cu toxicity in plants, in which the highest ranges of Cu contents are observed in the root system followed by the shoot (**Table 1**).

#### **6. Effect of copper toxicity on growth**

Cu is an essential oligonutrient for plant growth, but it is lethal when it exceeds the permissible limit, leading to poor plant-growth performance with loss of production. Thus, the reduction in plant growth induced by heavy metals is a final consequence of changes that initially occur at the biochemical, physiological, and mineral levels of plants.

The physiological functions of essential plant nutrients are disturbed when the concentration of these nutrients is below an adequate limit due to the presence of toxic levels of Cu in the culture medium. Thus, the reduction in the concentration of Fe, Zn (in leaves), and Mg (roots) modulated by toxic levels of Cu coincides with the decrease in leaf area, root length, and SPAD index in plants. This suggests that Cu at toxic levels affects the physiological functions of these nutrients, consequently decreasing plant growth [54]. Natural populations of plants not tolerant to the toxicity of 100 μM of Cu (*Rumex japonicus*) have a reduction in shoot and root dry mass together with a decrease in P, Mg, and Fe contents in the root and P and Fe in the shoot [55]. Thus, disorders in the mineral metabolism of plants are accompanied by a reduction in plant growth and/or production when the plant culture medium has toxic levels of Cu.

#### *Copper Toxicity in Plants: Nutritional, Physiological, and Biochemical Aspects DOI: http://dx.doi.org/10.5772/intechopen.105212*

In the context of net carbon assimilation, photosynthesis is a vital process, because it allows the carboxylation of CO2 and the synthesis of phosphate trioses that will constitute the different structural and nonstructural components of plants. However, this process is affected under toxic concentrations of heavy metals due to damage to the photochemical and/or biochemical apparatus, culminating in reduced plant growth. Cu toxicity (700 mg kg−1) has a negative impact on gas exchange (photosynthesis, transpiration, stomatal conductance, and internal cell concentration), chloroplast pigments (chlorophylls a and b, and carotenoids), and photochemical parameters (Fv/Fm, qP, and ETR). These changes promoted by Cu contribute decisively to the reduction of vegetative growth [45]. Furthermore, Cu has a specific action in inhibiting the oxygen evolution complex in photosystem II, which is associated with the oxidation of cytochrome b559 [5, 38]. In soils naturally rich in Cu (3050 g g−1), height and seed production are strongly reduced under wheat-growing conditions. These results were accompanied by lower photosynthetic activity and lower concentration of chlorophylls [14]. All these disorders in the photochemical apparatus of photosynthesis have the ultimate effect of reducing shoot growth, roots, and plant production.

#### **7. Morphological symptoms of copper toxicity in plants**

Toxicity due to toxic levels of Cu manifests itself in the root system, which tends to lose its vigor with longer exposure to Cu, acquiring a dark color and thickening, culminating in reduced growth. In the aerial part of the plants, the morphological symptoms of Cu toxicity are evidenced by the chlorosis of the leaves and a marked reduction in growth (leaf area, height, and stem diameter). In advanced stages, leaf edges may become necrotic [10, 56]. **Figure 2** shows the general aspects of Cu toxicity

#### **Figure 2.**

*Overview of copper toxicity in plants at the level of gas exchange, oxidant-to-antioxidant metabolism, mineral metabolism, and growth.*

in plants, which affects photosynthetic, antioxidant, and mineral metabolism, culminating in reduced plant growth.

### **8. Conclusion**

Contamination of soil and plants with trace elements is one of the most severe ecological problems in many industrialized countries due to industrial, mining, and agricultural activities. However, important progress has been made in understanding the biochemical, physiological, nutritional, and morphological mechanisms of Cu toxicity associated with higher plants. Cu toxicity reduces plant growth because Cu at toxic levels acts as a prooxidant, increasing the production of free radicals that cause damage to cellular and subcellular structures, causing protein oxidation and lipid peroxidation. In another toxic mechanism, excess Cu can replace S in F-S groups, forming Fe–Cu, which affects electron transport in chloroplasts. In addition, Cu toxicity affects the oxygen evolution complex and cytochrome b559. These changes together imply a reduction in the synthesis of phosphate triose and, consequently, in the production of dry matter in plants. In the context of mineral metabolism, Cu has a strong impact on the reduction of P, Ca, and Fe nutrient concentrations. Despite the existence of numerous studies involving the toxicity of Cu, little is reported in the literature about the prooxidant role of Cu at the physiological and molecular levels. For example, plants accumulate most of Cu in the root, but there is a need for further understanding of whether enzymatic and nonenzymatic antioxidant mechanisms in the root contribute to the tolerance of accumulator plants to toxic levels of Cu. Furthermore, understanding how toxicity modulates sulfur metabolism is crucial because sulfur is a key element in the antioxidant activity of glutathione in plants. This suggests the need for further studies to demonstrate the toxic role of Cu and its relationship with the production of oxidative stress in plants because antioxidant metabolism is one of the key mechanisms in inducing tolerance to trace element toxicity.

*Copper Toxicity in Plants: Nutritional, Physiological, and Biochemical Aspects DOI: http://dx.doi.org/10.5772/intechopen.105212*

#### **Author details**

Flávio José Rodrigues Cruz1 \*, Raphael Leone da Cruz Ferreira1 , Susana Silva Conceição2 , Edson Ugulino Lima3 , Cândido Ferreira de Oliveira Neto2 , Jessivaldo Rodrigues Galvão2 , Sebastião da Cunha Lopes4 and Ismael de Jesus Matos Viegas2

1 Federal Institute of Education, Science and Technology of Amapá, Laranjal do Jari, AP, Brazil

2 Federal Rural University of Amazonia, Belém, PA, Brazil

3 State University of Pará, Belém, PA, Brazil

4 Federal University of Pará, Belém, PA, Brazil

\*Address all correspondence to: fjrc@bol.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 6**

## Metal Nanoparticles and Abiotic Stress Tolerance

*Maryam Dahajipour Heidarabadi*

#### **Abstract**

Nanotechnology opens a large scope of novel applications in the fields of biotechnology and agricultural industries. Nanoparticles (NPs) are described as particles with at least one dimension in the 1–100 nm size range. They have unique physicochemical properties, i.e., high surface area, high reactivity, tunable pore size, and particle morphology. Abiotic stresses like drought, salinity, flooding, heat, heavy metals, etc. are major constraints that affect the growth and productivity of plants. To overcome the impact of these abiotic stresses, many strategies could be considered to support plant growth including the use of metal nanoparticles. Several metal nanoparticles (such as Zn, Fe, Ti, Ag, Mn, Cu, and Mo NPs) are being studied to assess their potential in protecting plants from abiotic stresses, improving plants, and modulating various plant processes. The present review has investigated the beneficial role of metal nanoparticles in alleviation of various abiotic stresses in some plants.

**Keywords:** abiotic stress tolerance, antioxidant activity, metal nanoparticles, secondary metabolites

#### **1. Introduction**

Nanotechnology, the fourth wave of the industrial revolution, is one of the new technologies that is developing rapidly [1]. In the last decade, nanotechnology has been considered as an important tool to increase agricultural production. Nanotechnology has the potential to transform the agricultural industry through the development of new formulations for pesticides and fertilizers, the identification and diagnosis of plant diseases, water supply for agriculture, and soil management [2]. The agricultural industry is considered as one of the important economic pillars of developed countries. As the world's population grows, the need for food and agricultural products is increasing. Factors such as climate change, limited soil and water resources, increased environmental pollution, and plant diseases create problems in agriculture and the production of sufficient and healthy food [3]. In general, nanotechnology can make a significant contribution to the growing prosperity of the industry by optimizing the consumption of agricultural inputs such as water, fertilizers, and pesticides and reducing effluents and pollution [4]. Nanotechnology can improve the overall use efficiency of agricultural inputs such as water, light, and chemicals. Health and functions of both soil and plant improve through microbiome

enhancement and decreasing losses by managing crop disease better, which leads to less collateral damage to the environment. Therefore, nanotechnology has a promising potential to develop sustainable agriculture [5].

The environmental factors that restrict plant growth, vitality, and fertility are known as abiotic stresses. Plants are naturally exposed to a variety of abiotic stresses such as drought, salinity, heavy metals, chilling, and heat [6]. Plants as sessile organisms have numerous mechanisms to cope with changes in their growth conditions to show the necessary flexibility in responding to environmental stresses, without affecting cellular, physiological and developmental processes [7]. One of the main concerns in sustainable agriculture is increasing tolerance to abiotic stresses. Based on this, researchers have been able to take a big step in global sustainable agriculture by reducing the harmful effects of abiotic stresses [8]. When plants are exposed to abiotic stresses, reactive oxygen species (ROS) accumulate at the toxicity level in the cell. Overproduction of ROS causes the degradation of membrane lipids and proteins, cell toxicity, and reduction of plant growth. The antioxidant defense system scavenges ROS to alleviate oxidative stress [9]. In the last years, the use of nanoparticles in technology has been considered due to their properties such as small size, high surface area, higher solubility, and reactivity compared to bulk materials [10]. Metallic nanoparticles (MNPs) including Zn, Fe, Ti, Ag, Mn, Cu, and Mo NPs have earned significant attention due to their environmentally friendly implementations in the agricultural sector [11]. They have recently been used for seed germination, plant growth, and stress tolerance of a number of plants [12, 13]. The goal of this review was to better understand the stress resistance mechanisms and MNP-mediated plant tolerance increase via antioxidant activity regulation.

#### **2. Types of nanoparticles**

Nanoparticles (NPs) have dimensions between 1 and 100 nm. They have unique physical and chemical properties such as high surface vitality, large surface-to-volume ratio, and high reactivity [14]. NPs are generally classified into different groups: metal-based NPs, metalloid NPs, metal magnetic NPs, metal oxide NPs, dendrimers, and carbon-based NPs (**Table 1**). In the last decade, metal and metal oxide-based NPs are comprehensively studied in agriculture fields for the improvement of crop productivity and increasing the plant flexibility and tolerance under abiotic stress


#### **Table 1.** *Types of nanoparticles [15].*

conditions [16]. Metal-based NPs and their oxides including nanomaterials of gold, silver, copper, aluminum, iron, titanium dioxide (TiO2), cerium oxide (CeO2), iron oxide (FeO), aluminum oxide (Al2O3), and zinc oxide (ZnO) are gaining so much attention of scientists to modulate abiotic stress [12, 13, 17].

#### **3. Effect of metal nanoparticles on antioxidant defense system improving of plants during abiotic stress exposure**

Reactive oxygen species (ROS) are generated in various plant cell compartments such as plasma membranes, endoplasmic reticulum, peroxisomes, chloroplasts, mitochondria, and cell wall in natural and stress conditions [18]. ROS such as singlet oxygen (1 O2), superoxide (O2−), hydrogen peroxide (H2O2), and hydroxyl radicals (OH. ) are accumulated in all the abiotic stresses that result in oxidative stress. Increased ROS act as a signal, and ROS scavengers are one of the defense mechanisms in plants [8]. As the level of ROS is elevated in plants in response to abiotic stress exposure, an antioxidant defense system that is capable of scavenging ROS is activated [19]. Antioxidant defense system of plants includes a number of antioxidant enzymes such as catalase (CAT), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), guaiacol peroxidase (GPX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), and superoxide dismutase (SOD) and non-enzyme antioxidant compounds such as proline, glycine betaein, anthocyanins, flavonoids, carotenoids, glutathione, and ascorbate [20–22]. As shown in **Figure 1**, MNPs enter the plant cell through penetration or transportation via specific channels in the plasma membrane. Then, as stress signaling molecules, by inducing the expression of regulatory factors in the activation of the defense system, they result in stress tolerance.

#### **Figure 1.**

*Antioxidative mechanism of action of MNPs in plants under abiotic stress (NPs: Nanoparticles; MDHAR: Monodehydroascorbate reductase; SOD: Superoxide dismutase; APOX: Ascorbate peroxidase; DHAR: Dehydroascorbate reductase; GR: Glutathione reductase; ROS: Reactive oxygen species) [17].*

In addition, MNPs can activate the plant defense system under stress by maintaining ROS levels to induce the ROS signaling network. MNPs treatment alters biological pathways involved in defense mechanisms by upregulation of genes that encode proteins that play a key role in ROS balance such as peroxidases (POX), NADPH oxidase, glutathione S-transferase (GST), and superoxide dismutase (SOD) [23]. MNPs upregulate the genes responsible for the activation of antioxidant enzymes. For instance, Laware and Raskar experiments on onion seedlings showed that TiO2 NPs enhanced SOD enzyme's activity in NP-supplemented plants [24]. Also, results from transcriptomic studies showed that the expression of Cu/Zn SOD, Fe/Mn SOD, catalase, and ascorbate peroxidase in plants that were treated with ZnO NPs under drought was notably enhanced [25]. Concrete evidence provided by Thakur et al. showed that an increase in GPX and SOD activities in wheat plants treated with ZnO NPs under heat stress improved heat tolerance by further reducing H2O2 levels and establishing membrane stability [26]. Studies have also reported that MNPs participate in the induction of Ca2+-binding protein expression, resulted in stress tolerance by launching a cascade of intracellular signaling and upregulation of associated genes [27].

#### **4. Secondary metabolites role induced by MNPs in abiotic stress tolerance**

Metabolites are essential molecules for growth, adaptation to stress, and defense of a living organism. Metabolic pathways leading to the synthesis of molecules including carbohydrates, proteins, amino acids, fatty acids, and nucleotides are considered as the primary metabolism, and the compounds produced in these pathways, which are necessary for plant survival, are called primary metabolites [28]. Primary metabolites are involved in various life functions in plants, such as cell division, growth and development, photosynthesis, respiration, and reproduction [29]. Plants produce a diverse group of organic compounds called secondary metabolites that do not have a role in processes such as photosynthesis, respiration, metabolism, protein synthesis, and nutrient accumulation directly [30].


#### **Table 2.**

*MNPs as an elicitor for secondary metabolites.*

*Metal Nanoparticles and Abiotic Stress Tolerance DOI: http://dx.doi.org/10.5772/intechopen.105211*

A significant number of secondary metabolites, such as terpenoids, steroids, phenolics, flavonoids, and alkaloids, by the removal of ROS in cellular stress and defense response, function as an adaptation mechanism to stress conditions [31]. Evidence has shown that secondary metabolites are involved in the non-enzymatic defense of plants against stress [32]. The MNPs are commonly found in agrochemicals such as pesticides, fungicides, herbicides, and fertilizers [33]. A few studies have demonstrated that treatment of plants with MNPs resulted in increased production of secondary metabolites, which might act as antioxidants to scavenge the ROS [34, 35]. Several studies have shown that MNPs have the potential to induce plant secondary metabolites' production (**Table 2**).

#### **5. Alleviation of abiotic stress by metal-based and metal-oxide NPs application**

Abiotic stresses including drought, salinity, heat, chilling, heavy metal toxicities, etc. are major obstacles to plant growth and productivity [7]. Plants adapt to and alleviate abiotic stresses by alterations in morphological, physiological, biochemical, and molecular levels. Researchers have shown that MNPs help plants to overcome abiotic stresses by their concentration-dependent impact on plant growth and development [15]. MNPs can be supplied to plants in form of seed coating, soil, or foliar application according to their mode of action. Extensive researches have elucidated the positive effects of some MNPs on some plant species under different abiotic stress conditions (**Table 3**). For instance, MNPs were effective in ameliorating the detrimental effects of abiotic stresses by increasing flavonoid, anthocyanin, phenolic, and photosynthetic pigment contents, upregulating the antioxidant enzymes, reducing the stress markers (MDA and H2O2), water balance, ion accumulation, improvement of the nutrient absorption, and the Na+ /K+ ratio [17, 44–60].



*Metal Nanoparticles and Abiotic Stress Tolerance DOI: http://dx.doi.org/10.5772/intechopen.105211*


#### **Table 3.**

*Application of some metal NPs induce abiotic stress tolerance in different plant species.*

#### **6. Conclusion**

Plant production globally is subjected to various environmental stress challenges. Today, the application of nanotechnology in various scientific fields is expanding. Recent studies have highlighted the potential applications of nanotechnology in improving plant growth and performance. MNPs due to their small size and having large surface area, as compared to their bulk chemical forms penetrate and absorb in relatively shorter period of time into plant cells. It has been found that MNPs have a multitude of beneficial effects on morphological, physiological, and biochemical characteristics of plants and enhance their tolerance under a variety of abiotic stresses. Accordingly, the application of MNPs in abiotic stress improvement has been noticed by agricultural researchers. MNPs enhance ROS level in plants that is associated with the amplification of a stress signal that can efficiently activate defense systems of them. It may be concluded that MNPs alleviate the abiotic stress-caused damage by activating the defense system in plants. In addition to, MNPs can regulate photosynthetic efficiency, water balance, nutrient absorption, and osmolytes accumulation, thereby enhancing growth and productivity of plants. However, to investigate the exact action of MNPs in improve plant stress, further research is needed at molecular and subcellular levels. Although MNPs have many advantages that deserve to be explored for alleviation of abiotic stress in plants; it must be noticed that its application without care can lead to a series of issues to the plants, animals, and finally to humankind. The toxicity level of MNPs is related to their concentration, size, number, surface activity, modification, and aggregation. Thus, ambiguities about the risk of use and fate of MNPs in plants and soil, as well as their interaction with the environment, should not be overlooked. In addition, green synthesized MNPs in comparison with chemically synthesized MNPs can be efficiently used due to the lack of limitation of use and toxicity in modulating various abiotic stresses in plants. Finally, developing a comprehensive database, an alert system, as well as international cooperation in regulation and legislation is essential for the use of this technology.

*Advances in Plant Defense Mechanisms*

#### **Author details**

Maryam Dahajipour Heidarabadi Department of Genetics and Plant Production, Agriculture College, Vali-e-Asr University of Rafsanjan, Rafsanjan, Iran

\*Address all correspondence to: m.dahaji@vru.ac.ir

© 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.

*Metal Nanoparticles and Abiotic Stress Tolerance DOI: http://dx.doi.org/10.5772/intechopen.105211*

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### Section 2
