**4. Drought stress**

of stomatal apertures at day time and remained close at night. There is no photosynthesis in

preventing unnecessary water loss. When water supply is abundant on a sunny morning, the solar radiation incident on the leaf favors a high photosynthetic activity, the demand for CO2

it is beneficial for the plant to trade water, the product of photosynthesis, which is essential for growth and reproduction. Mild water deficits also affect the development of the root system. Root-to-shoot biomass ratio appears to be governed by a functional balance between water uptake by the root and photosynthesis by the shoot. A shoot will grow with maximum water uptake by the roots and becomes limiting to promote growth until their demand for photosynthate from the shoot equals the supply. This functional balance is shifted if the water supply decreases. On the other hand, when soil water is less abundant, the stomata will open less or even remain closed on a sunny morning. Thus, plants avoid dehydration by keeping

Many organisms accumulate intracellular lowmolecularweight compounds due to water deficit to maintain equal water potential with the external conditions. Osmotic adjustment is contributed by many compounds which besides providing protection to macromolecules such as enzymes, proteins, electrolytes and temperature. Plant cells generally accumulate the inorganic ions mostly present in the soil environment, but in high concentration these become harmful to cellular integrity [15]. The organisms usually accumulate specific types of organic molecules called as compatible solutes required for maintaining the cytoplasm osmotically balanced. The main function of formed osmolytes is to maintain osmotic balance within the cell, and even their high concentrations may not impair the normal physiological function of the cell. As plant life savers, organic osmolytes facilitate osmotic adjustment normally to

Crops of tomorrow are expected to grow under huge levels of atmospheric CO2

growth parameters will be affected and major among those is carbon allocation. The ratio of root to shoot is dependent upon the separation of photosynthate which might be influenced by environmental stimuli. The upper layer of the soil gets dry without water, but the root growth started more to qualify moist zones under the soil. Deeper root growth into wet soil can be considered a second line of defense against drought. Better root growth into moist soil zones during stress requires osmolytes indirectly to maintain osmotic potential in order. During water shortage, the root growth is less prominent in reproductive plants as compared to vegetative plants. Therefore, the plants are more sensitive to water stress during reproduction period.

According to Iqbal et al. [16], the water deficit situation in wheat germplasm has shown detrimental developmental processes which effects plant growth ultimately. To counter this situation, compatible solutes like proline, glycine-betaine, and trehalose protect cellular milieu from dehydration. An increase in root growth in different plants under drought stress was also shown by Tahir et al. [17] and Jaleel et al. [18]. Plants with a higher proportion of roots can

inside the leaf is large, and the stomatal pores are wide open to claim more CO2

transpiration, water loss is substantial under these conditions, but since the H2

inside the leaf; therefore, stomatal apertures are kept small,

. Through

O is plentiful,

. Basic crop

the night, so no demand for CO2

54 Global Wheat Production

its stomata closed in dry conditions [9].

maintain cellular milieu.

**3.1. Root-shoot ratio**

Drought is the lack of inadequate moisture level in soil, leading toward water stress which adversely affects crop productivity. Indeed, it is hypothesized that differences in drought and salt tolerance arise because of changes in the regulation of a basic set of drought and salt tolerance genes. Attempts to improve the drought tolerance of crops by conventional breeding programs have shown very limited success because of the complexity of the trait. Drought tolerance is a complex process genetically and physiologically [20]. The components of drought resistance in plants include both avoidance and tolerance to water stress and desiccation. Early maturity mechanism helps drought resistance in wheat before the period of drought, deeper root system to efficiently utilize the available moisture, and prolonged closing of stomata during drought stress to decrease water loss. The development of wheat cultivars for drought stress tolerance commonly has narrow leaves and lower shoot/root ratios and may have a low yield potential than varieties developed for irrigated areas. Jaleel et al. [21] has reviewed the drought stress as a changed physiological situation caused by the trend to disturb equilibrium. The damage in physical and chemical change shaped the stresses in plants exposed to drought, oxidative stress, low and high temperature, salt, flooding, and heavy metal toxicity. Drought stress tolerance is observed in most of the plants but its extent varies from species to species and even within species. Water deficit and salt stresses are global issues to ensure the survival of agricultural crops and sustainable food production. A ramified root system is established during drought tolerance and high biomass production primarily due to its ability to extract more water from soil and its transport to above ground parts for photosynthesis. When the beginning of stress is in rapid state or the plant has reached its full leaf area before initiation of stress while on the other side, protective mechanisms started in the plant against immediate desiccation. Under these subnormal conditions, stomata closure reduces evaporation from the leaf surface area. At this stage, the stomatal closure is considered to be an important line of defense against drought. Uptake and loss of water in guard cells change their turgor and modulate stomatal opening and closing. The guard cells are located in the leaf epidermis which can drop turgor pressure as a result of a direct water loss by evaporation to the atmosphere. Hydropassive closure of stomata is due to the decrease in turgor that operates in air of low humidity and when direct water loss from the guard cells is too rapid to from adjacent epidermal cells. Secondly, hydroactive closure mechanism closes the stomata when the whole leaf or the roots are dehydrated and depends on metabolic processes in the guard cells. The reduced solute contents in the guard cells results in water loss and decreased turgor for closing stomata. The hydraulic mechanism of hydroactive closure is a reversal of the mechanism of stomatal opening. The loss of solutes from guard cells can be activated by a decreased water content of the leaf where abscisic acid (ABA) plays an important role in this process. Abscisic acid is synthesized continuously at a low rate in mesophyll cells and tends to accumulate in the chloroplasts. When the mesophyll becomes mildly dehydrated, firstly the ABA stored in the chloroplasts is released to the apoplast (the cell wall space) of the mesophyll cell [22]. The pH gradients redistribute ABA molecule within the leaf, making it possible for the transpiration stream to carry some of the ABA to the guard cells. Secondly, leaf apoplast is saturated with ABA synthesized at a higher rate, and this higher ABA concentration appears to enhance or prolong the initial closing effect of the stored ABA, leading to the mechanism of ABA-induced stomatal closure. Leaf dehydration can vary widely both within and across species due to stomatal responses. The drought tolerant species like cowpea (*Vigna unguiculata*) and cassava (*Manihot esculenta*) are more responsive to stomatal conductance and leaf water potential may remain nearly constant during drought due to less transpiration activity. Chemical signals from the root system may affect the stomatal responses to water stress. The stomatal conductance is more closely related to soil water status than to leaf water status because the average root system is directly affected by soil water status. In fact, dehydrating only part of the root system may cause stomatal closure even if the wellwatered portion of the root system still delivers ample water to the shoots.

the plant body to the atmosphere includes diffusion and osmosis. The plant water can be considered incessant hydraulic system connecting through water in the soil with the water vapors in the atmosphere. Guard cells regulate transpiration through the control of stomatal

atmosphere. Large negative pressures (or tensions) in the apoplastic water is generated by water evaporation from the cell walls of the leaf mesophyll cells. Xylem conduits hold these negative pressures, but when transpiration is high, negative pressures in the xylem water may cause cavitations (embolisms) in the xylem that can block water transport and lead to severe water deficits in the leaf. Water deficit plants adapt responses that modify the physiology and development of plants. Water move through soils by bulk flow driven by a pressure gradient, and plants absorb water from soil through roots. The rate of water flow in soils depends upon the factors like pressure gradient through soil and hydraulic conductivity of the soil. As the water content of the soil decreases, the hydraulic conductivity decreases drastically. In very dry soil, water potential may fall below the permanent wilting point. At this point, the water potential of the soil is so low that plant cannot regain turgor pressure which means that the water potential is less than or equal to the osmotic potential of the plant. Different plant species behave differently in soil, and the permanent wilting point is clearly not a unique property of the soil. Water uptake decreases when roots are subjected to low temperature or anaerobic conditions, or treated with respiratory inhibitors like cyanide. The anaerobic roots

transport less water to the shoots which suffer net water loss and begin to wilt [15].

should be oxygenated for running drought-stress experiments [16].

Polyethylene glycol (PEG) is a polymer used to modify the osmotic potential of nutrient solutions cultures to induce plant water deficit in experimental protocols. PEG 8000 (18%) is used as an osmoticum to induce drought in wheat genotypes after 1 week of plantation. Water stress greatly suppresses cell expansion and cell growth due to low turgor pressure. Osmotic regulation can enable the maintenance of cellular turgor for plant survival. The reduction in plant height was associated with a decline in the cell enlargement and more leaf senescence. The plants grown in nutrient culture containing PEG suffered from hypoxia, and such system

In plants, there are effective mechanisms of osmotic adjustment based on the synthesis of osmolytes which are lowmolecularweight compatible solutes. Osmolytes are frequently used by cells to accommodate osmotic pressure within the effected cells to avoid cellular injury due to oxidation phenomenon. They are highly soluble organic molecules that are synthesized in many organisms in response to different environmental conditions leading to osmotic stress [7]. They accumulate in the cytosol without interfering with the cellular metabolism even at high concentrations. Osmolytes have additional functions during the stress response and act as osmoprotectants by directly stabilizing protein and membrane structures under dehydration conditions. They have a diverse chemical nature, and apart

uptake while limiting water loss to the

http://dx.doi.org/10.5772/intechopen.75926

57

Role of Osmolytes and Antioxidant Enzymes for Drought Tolerance in Wheat

pore size to meet the photosynthetic demand for CO2

**4.2. Polyethylene glycol application**

**5. Osmolytes**

#### **4.1. Osmotic adjustment**

Water plays a crucial role in plants' life as approximately 500 g of water is absorbed by the roots for every gram of organic matter made by plant. Imbalance in water flow can cause water shortage that lead to malfunctioning of major cellular processes. The balancing of water uptake and loss is a crucial challenge for photosynthetic plants to utilize CO<sup>2</sup> from atmosphere, and by doing so, plant exposes to water loss and the next threat of dehydration. A main difference between plant and animal cells that affects almost all aspects of their relation with water is the existence in plants of the cell wall. The internal hydrostatic turgor pressure is a result of their normal water balance inside the cell wall. Turgor pressure is essential for many physiological processes including cell enlargement, gases exchange in the leaves, transport in the phloem, and various transport processes across membrane. Turgor pressure also contributes to the rigidity and mechanical stability of non-lignified plant tissues.

Water is essential to land plants to avoid lethal desiccation by water loss to the atmosphere. The large surface area of leaves, their high radiant-energy gain, and their need to have an open pathway for CO2 uptake may aggravate water loss. Water conservation and the need for CO<sup>2</sup> assimilation are a constant situation in plants for survival. Water makes up most of the mass of the plant cells, as each cell contains large water-filled vacuole whereas water typically constitutes 80–95% of the mass of the growing plant tissues. Seeds with a water content of 5–15% are among the driest of plant tissues that also absorb a considerable amount of water before germination. Plants continuously absorb and lose water during transpiration means and dissipate heat because the escaped water molecules have higher than average energy, breaking the bonds holding them in a liquid form. The transport of water bulk flow from the soil through the plant body to the atmosphere includes diffusion and osmosis. The plant water can be considered incessant hydraulic system connecting through water in the soil with the water vapors in the atmosphere. Guard cells regulate transpiration through the control of stomatal pore size to meet the photosynthetic demand for CO2 uptake while limiting water loss to the atmosphere. Large negative pressures (or tensions) in the apoplastic water is generated by water evaporation from the cell walls of the leaf mesophyll cells. Xylem conduits hold these negative pressures, but when transpiration is high, negative pressures in the xylem water may cause cavitations (embolisms) in the xylem that can block water transport and lead to severe water deficits in the leaf. Water deficit plants adapt responses that modify the physiology and development of plants. Water move through soils by bulk flow driven by a pressure gradient, and plants absorb water from soil through roots. The rate of water flow in soils depends upon the factors like pressure gradient through soil and hydraulic conductivity of the soil. As the water content of the soil decreases, the hydraulic conductivity decreases drastically. In very dry soil, water potential may fall below the permanent wilting point. At this point, the water potential of the soil is so low that plant cannot regain turgor pressure which means that the water potential is less than or equal to the osmotic potential of the plant. Different plant species behave differently in soil, and the permanent wilting point is clearly not a unique property of the soil. Water uptake decreases when roots are subjected to low temperature or anaerobic conditions, or treated with respiratory inhibitors like cyanide. The anaerobic roots transport less water to the shoots which suffer net water loss and begin to wilt [15].
