**4.2 Salinity effect on the crops**

The magnitude of agricultural estate affected by high salinity is increasing worldwide as a result of both natural and agricultural occurrences such as irrigation schemes. In plant growth, salinity presents two primary concerns: osmotic stress and ionic stress. It also manifests oxidizing stress. The detrimental effects of salinity alter different physiological and metabolic processes of plants. Often, the answers to these modifications are accompanied by various symptoms such as decreased leaf area, increased leaf density and succulence, leaf abscission, root and shoot necrosis, and decreased internode lengths. Salinity stress inhibits growth and increases cell senescence during extended exposure. Inhibition of growth is the major injury resulting in other symptoms, while programmed cell death may also happen under serious salinity shock [39]. Abscisic acid synthesis is induced under salt stress that closes stomata during transportation to guard cells. Due to stomatal closure and inhibition of photosynthesis and oxidative stress, photosynthesis reduces. Osmotic stress can directly or indirectly inhibit the development of cells through abscisic acid metabolism and translocation. Potassium is not received by plant root surface due to excessive sodium ions near the root zone. Due to the comparable biochemical behavior of sodium and potassium ions, sodium has a powerful repressive impact on root potassium

absorption. Deficiency of potassium predictably results in inhibition of growth as potassium maintains cell turgor, activity of enzyme and membrane potential as the most abundant cell cation. Once sodium enters the cytoplasm, the functions of many enzymes are inhibited. This inhibition also depends on the quantity of potassium present: the most deleterious is an elevated sodium/potassium ratio. Plant growth is decreased due to salinity related nutrient disturbances by altering accessibility, transport and partitioning of nutrients. High salt concentration can result in nutrient deficits or imbalances due to Na+ and Cl<sup>−</sup> competition with nutrients such as K+ , Ca2+ and NO3 <sup>−</sup>. Under saline circumstances there are specific ion toxicity of Na+ and Cl<sup>−</sup> and ionic imbalances influencing biophysical components and/or metabolism of plant growth. Most of the crops combat salinity stress by deposition of low molecular weight organic solutes like linear polyols (sorbitol, glycerol or mannitol), amino acids (proline or glutamate) and betaine (betaine glycine or betaine alanine), cyclic polyols (inositol and other derivatives of mono- and dimethylated inositol) [40].

#### **4.3 Cold effect on the crops**

Cold stress is a significant abiotic stress which affects growth and development of crops, leading to loss of strength and lesions on the surface. These symptoms are triggered by changes in the physical and chemical organization of cell membranes, among other metabolic procedures [41]. It is estimated that rises in extreme temperature frequency, severity, and duration are a prevalent feature of our setting. Climate change controls greater changes in temperature, leading in frequent cold periods. Susceptible plants with cold temperatures have reduced growth and growth, restricted use of precious varieties, and reduced yields. Plants use separate strategies to cope with stressful conditions and integrate a variety of physiological, metabolic and molecular adaptations. These methods initially generate modifications to safeguard the plant, followed by cold acclimatization, which increases the survival of the plant under cold stress [9]. While a lot of these mechanisms are facilitated by transcription factors (TFs) that stimulate gene expression associated to stress, the transcription network is not restricted to the reaction of plants to cold [41, 42]. As a foremost element of plasma and endo-membranes, lipids play a significant organizational role in mitigating the effects of cold temperatures [43]. Cold stress decrease plants growth and development that affects the physical and chemical structure of the cell membrane, causes leakage of electrolytes, and reduces protoplasmic streaming and changes in the metabolism of cell [44, 45]. Additional cold reactions comprise changes in nucleic acid and protein synthesis, water and nutrient equilibrium, enzyme affinity and conformation and deficiency in photosynthesis, specifically down-regulation and photo-damage of Photosystem II (PSII) [16].

Changes in the structure of proteins and lipid membranes assist restore homeostasis of metabolites and are regarded a mechanism by which cells feel cold temperatures. For its metabolic and physical function, the liquid state of the plasma membrane is a structural and functional asset. When low temperatures are present, the plasma membrane transitions from a liquid state at elevated temperatures into a stiff gel stage [46]. Low temperature-mediated changes in the physical conformation of the membrane are mainly because of enhanced levels of unsaturated lipids, which increase the fluidity and stability of the membrane, enabling cells to adapt mechanically to cold [47].

#### **4.4 Heat effect on the crops**

The sequence of modifications in morphology, biochemistry, and physiology arising from high temperature stress also considerably disturbs growth and

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*Effect of Abiotic Stress on Crops*

*DOI: http://dx.doi.org/10.5772/intechopen.88434*

temperature requirements are expected to rise [53].

A prevalent and serious problem is heavy metal atmospheric pollution through

human activities and/or natural processes. Often referred to as trace metals or heavy metals are potentially toxic elements. Trace metals are related to the trace quantities of components existing in soils. Heavy metals, a loosely specified group of components, constitute elements with an atomic mass exceeding 20 (excluding alkali metals) and specific gravity exceeding 5 [59]. Because of their differing solubility/bioavailability, heavy metals exist in different forms in soil. Many soil physicochemical characteristics change heavy metal geochemical conduct in soil, plant uptake, and effect on crop productivity. Excessive deposition of heavy metals in plant tissue is harmful to multiple biochemical, physiological, and morphological operations in plants either directly or indirectly and in turn affect crop productivity. Heavy metals decrease crop productivity by causing seed germination, accumulation, and re-mobilization of seed reserves during plant growth, germination and photosynthesis to deleterious effects on various plant physiological processes [60]. Heavy metal toxicity on the cell platform decreases the productivity of plants by forming reactive oxygen species, disrupting the redox equilibrium and causing oxidative stress. Metals mainly enter the plants through the root from the soil [61]. The cultivation of metals includes several processes, including desorption of metal from soil particles,, uptake of metals by roots, transportation of metals to plant

**4.5 Heavy metal**

development of plant [48]. As a result of increasing atmospheric temperatures, heat shocks are currently primary limiting factors for crop productivity globally. This increasing temperature may result in changes in the phases of growth and distribution of agricultural plants [49]. High-temperature stress can cause serious protein damage, interrupt synthesis of protein, inactivate critical enzymes, and damage membranes. High temperature stress can have significant effects on the cell division process [50]. All of these harms can substantially restrict plant development and also promote oxidative damage. In addition, short exposure to elevated temperature in seed filling can lead to rapid filling, leading to low quality and lower yield. Under a restricted supply of water, the temperature rise is fatal. Overall, water loss due to heat stress is predominantly due to enhanced transpiration rate during the day, which eventually damages certain physiological procedures in crops. Heat stress also decreases the amount, weight and root growth and eventually decreases the accessibility of water and nutrients to the plant parts above ground [51, 52]. Lightdependent chemical reactions that happen in the stroma in the thylakoid and the carbon metabolism are the primary places of harm owing to elevated heat stress. Increased adjustment of PSII thermo-tolerance of PSII leaf temperature and density of photon flux [53]. The PSII is extremely temperature sensitive and significantly affects and even partly terminates its activity under elevated temperature stress [54]. Oxygen-evolving complex also experiences severe harm at elevated temperatures, which can lead to imbalanced electron flow to the PSII acceptor site [55]. At higher temperatures, the proteins D1 and D2 also suffer from denaturation [56]. High heat stress has a significant impact on the activity of significant enzymes like sucrose phosphate synthase, invertase, adenosine diphosphate-glucose pyrophosphorylase, and starch and sucrose synthesis [57]. The reduced CO2 binding enzyme activation status, Rubisco, limits net photosynthesis in many species of plants. Although Rubisco's catalytic activity rises with greater temperatures, its low CO2 affinity and O2 binding ability limit the rise in net photosynthesis speed [58]. Despite all these negative photosynthesis impacts of elevated temperature, with elevated concentrations of CO2 in the atmosphere, optimum photosynthesis

#### *Effect of Abiotic Stress on Crops DOI: http://dx.doi.org/10.5772/intechopen.88434*

development of plant [48]. As a result of increasing atmospheric temperatures, heat shocks are currently primary limiting factors for crop productivity globally. This increasing temperature may result in changes in the phases of growth and distribution of agricultural plants [49]. High-temperature stress can cause serious protein damage, interrupt synthesis of protein, inactivate critical enzymes, and damage membranes. High temperature stress can have significant effects on the cell division process [50]. All of these harms can substantially restrict plant development and also promote oxidative damage. In addition, short exposure to elevated temperature in seed filling can lead to rapid filling, leading to low quality and lower yield. Under a restricted supply of water, the temperature rise is fatal. Overall, water loss due to heat stress is predominantly due to enhanced transpiration rate during the day, which eventually damages certain physiological procedures in crops. Heat stress also decreases the amount, weight and root growth and eventually decreases the accessibility of water and nutrients to the plant parts above ground [51, 52]. Lightdependent chemical reactions that happen in the stroma in the thylakoid and the carbon metabolism are the primary places of harm owing to elevated heat stress. Increased adjustment of PSII thermo-tolerance of PSII leaf temperature and density of photon flux [53]. The PSII is extremely temperature sensitive and significantly affects and even partly terminates its activity under elevated temperature stress [54]. Oxygen-evolving complex also experiences severe harm at elevated temperatures, which can lead to imbalanced electron flow to the PSII acceptor site [55]. At higher temperatures, the proteins D1 and D2 also suffer from denaturation [56]. High heat stress has a significant impact on the activity of significant enzymes like sucrose phosphate synthase, invertase, adenosine diphosphate-glucose pyrophosphorylase, and starch and sucrose synthesis [57]. The reduced CO2 binding enzyme activation status, Rubisco, limits net photosynthesis in many species of plants. Although Rubisco's catalytic activity rises with greater temperatures, its low CO2 affinity and O2 binding ability limit the rise in net photosynthesis speed [58]. Despite all these negative photosynthesis impacts of elevated temperature, with elevated concentrations of CO2 in the atmosphere, optimum photosynthesis temperature requirements are expected to rise [53].

#### **4.5 Heavy metal**

A prevalent and serious problem is heavy metal atmospheric pollution through human activities and/or natural processes. Often referred to as trace metals or heavy metals are potentially toxic elements. Trace metals are related to the trace quantities of components existing in soils. Heavy metals, a loosely specified group of components, constitute elements with an atomic mass exceeding 20 (excluding alkali metals) and specific gravity exceeding 5 [59]. Because of their differing solubility/bioavailability, heavy metals exist in different forms in soil. Many soil physicochemical characteristics change heavy metal geochemical conduct in soil, plant uptake, and effect on crop productivity. Excessive deposition of heavy metals in plant tissue is harmful to multiple biochemical, physiological, and morphological operations in plants either directly or indirectly and in turn affect crop productivity. Heavy metals decrease crop productivity by causing seed germination, accumulation, and re-mobilization of seed reserves during plant growth, germination and photosynthesis to deleterious effects on various plant physiological processes [60]. Heavy metal toxicity on the cell platform decreases the productivity of plants by forming reactive oxygen species, disrupting the redox equilibrium and causing oxidative stress. Metals mainly enter the plants through the root from the soil [61]. The cultivation of metals includes several processes, including desorption of metal from soil particles,, uptake of metals by roots, transportation of metals to plant

roots and shoots [62]. Transport of heavy metal to aerial components of the plant is via the xylem and is most probably encouraged by transpiration [63]. The metals, after joining the central cylinder, move towards the aerial parts of the plant where evaporation of water occurs and metal stacks up through the water stream of the vascular system [64]. Only a slight percentage of heavy metals are translocated in most crops to the shooting tissues. In some cases, only if the plant is a hyper accumulator or chelate-assisted, there is sequestration of 95% or more of the metal in the plant's roots [65, 66].

#### **5. Crops tolerance against abiotic stress**

Plant resistance to abiotic stress includes escaping stress avoidance and tolerance. Escape: Before extreme stress begins, dry escape depends on efficient reproduction. The plants integrate brief life cycles with high growth rates and gas exchange, using the highest existing resources while soil moisture lasts. It also relies on escaping the unfavorable environmental conditions by shedding off the leaves, no germination, nighttime closure of stomata, compact growth, that is, shortening of any plant part [67].

Avoidance: reversible physiological changes involve decreasing water loss (closing stomata, decreasing light absorption through rolling leaves and condensed canopy leaf region) and growing water absorption (increasing root investment, morphological changes occurring in crops to decrease transpiration, re-allocation of nutrients stored in older leaves and greater photosynthesis rates [68].

Tolerance: abiotic stress tolerance appears to be the consequence of cellular and molecular level coordination of physiological and biochemical changes. These changes may include osmotic adjustment, stiffer cell walls, or smaller cells [69]. Changes happening rapidly at the concentrations of mRNA and protein consequence in an intolerant state. Various morphological, physiological, biochemical and molecular modifications happen in crops in order to fight different abiotic stresses [70].

#### **5.1 Morphological changes**

Under stress, roots extend their length in the soil in order to seep water around themselves and absorb a sufficient amount of water to persist against stressed conditions. Due to an increase in length and more absorption of water through soil, roots biomass also increases in abiotic stress conditions. Shoot length is higher due to sufficient transpiration and translocation mechanism whereas in water deficit plants, shoot length observed was comparatively dwarf as plants need to overcome water and nutrient deficit conditions caused by drought. Shoot dry weight depends on the inner mass and tillers of wheat. Irrigated plants can accumulate water inside tissues due to a sufficient amount of landed water whereas water deficient cannot do so due to which inner mass decreases in case of water stress plants [23, 71].

#### **5.2 Physiological changes**

Plants facing abiotic stress, respond at the molecular, cellular and whole plant levels through a number of physiological modifications.

#### *5.2.1 Relative water content (RWC)*

A leaf's relative water content (RWC; or' relative turgidity') is measuring its real water content at complete turgidity relative to its peak water holding capacity.

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plants [30, 71].

*5.2.4 Osmotic adjustment*

cell's water potential balance [74].

*Effect of Abiotic Stress on Crops*

*5.2.2 Relative stress injury (RSI)*

*5.2.3 Water use efficiency (WUE)*

*DOI: http://dx.doi.org/10.5772/intechopen.88434*

potential reduced in drought-stressed plants [67].

protein concentrations that result in tolerance towards stress [71].

RWC provides a measurement of the decline in leaf water content and may involve a degree of stress in water deficit and heat stress. RWC includes leaf water potential (another helpful estimation of plant water status) with the impact of osmotic adjustment, a powerful mechanism for preserving cell hydration as a measure of plant water status. The development of leaves relies on the water content and the rate of transpiration. With the absorption of water from roots and passing on to leaves, plants will have high rates of transpiration and water content in leaves will elevate effectively in irrigated plants unlikely in water deficit plants and water

It is the relative injury caused to plants under stressed conditions. Relative stress injury is actuated under stressed conditions providing a measurement of injury caused to plants. Plants under such stressed conditions try to become resistant towards the extraneous factors where plants activate some genes and provide tolerance towards the environment. Abiotic stress tolerance appears to be the result of cooperation at the cellular and molecular levels between physiological and biochemical alternation. These modifications may include more rigid cell walls, osmotic adjustment, or smaller cells. There are rapid changes in the mRNA and

Efficiency in water use (WUE) is a crucial variable responsible for the productivity of plants under restricted supply of water. In agronomic terms, it is defined as the percentage of total dry matter (DM) generated (or harvested) to (or applied) used water. Physiologically speaking, nevertheless, WUE is well-defined as the proportion between the set carbon rate and the transpired water rate. The connection between water use and crop production rate is defined as water use efficiency. It is measured in terms of biomass generated by transpiration unit. Greater biomass generated by limited amount of water under stress circumstances is crucial for higher crop yield. Combined stress can also occur to the plant at same time, for example, water shortage can lead to drought and salinity stress simultaneously, uttermost significant factors limiting crop effectiveness and yielding worldwide. Drought resistance in plants can be improved by escaping or avoiding drought condition using WUE mechanism to maintain water level or growing drought tolerant

Osmotic adjustment (OA) is the net elevation of intercellular solutes in response

to water stress that allows turgor to be conserved at a lower water potential. OA has been considered as the primary mechanisms in adaptation of plant towards drought as it promotes the tissue's metabolic activity and enables for regeneration but varies considerably between genotypes. The efficiency of plants in arid conditions has been linked with OA in many species such as sorghum, wheat and oilseed brassicas. High levels of ions can critically inhibit cytosolic enzymes of plant cells [72]. Throughout osmotic adjustment, ion accumulation appears to be limited to the vacuoles where ions are kept out of contact with cytosol or subcellular organelles [73]. Because of this ion compartmentation, other solutes such as sugar alcohol, amino acid proline must be assembled in the cytoplasm in order to preserve the

#### *Effect of Abiotic Stress on Crops DOI: http://dx.doi.org/10.5772/intechopen.88434*

RWC provides a measurement of the decline in leaf water content and may involve a degree of stress in water deficit and heat stress. RWC includes leaf water potential (another helpful estimation of plant water status) with the impact of osmotic adjustment, a powerful mechanism for preserving cell hydration as a measure of plant water status. The development of leaves relies on the water content and the rate of transpiration. With the absorption of water from roots and passing on to leaves, plants will have high rates of transpiration and water content in leaves will elevate effectively in irrigated plants unlikely in water deficit plants and water potential reduced in drought-stressed plants [67].

### *5.2.2 Relative stress injury (RSI)*

It is the relative injury caused to plants under stressed conditions. Relative stress injury is actuated under stressed conditions providing a measurement of injury caused to plants. Plants under such stressed conditions try to become resistant towards the extraneous factors where plants activate some genes and provide tolerance towards the environment. Abiotic stress tolerance appears to be the result of cooperation at the cellular and molecular levels between physiological and biochemical alternation. These modifications may include more rigid cell walls, osmotic adjustment, or smaller cells. There are rapid changes in the mRNA and protein concentrations that result in tolerance towards stress [71].
