**5. Osmolytes**

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

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

sphere, 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

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

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

uptake may aggravate water loss. Water conservation and the need for CO<sup>2</sup>

from atmo-

uptake and loss is a crucial challenge for photosynthetic plants to utilize CO<sup>2</sup>

contributes to the rigidity and mechanical stability of non-lignified plant tissues.

root system still delivers ample water to the shoots.

**4.1. Osmotic adjustment**

56 Global Wheat Production

pathway for CO2

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 from contributing to maintain osmotic balance, they do protect cell against oxidative stress as scavengers of "reactive oxygen species" (ROS) [23, 24].

its accumulation in the leaves of many halophytic higher plants grown in saline environment. Proline protects membranes against adverse effects of high concentration of inorganic ions and temperature extremes. It is also functional as a proteincompatible hydrotope and as a

Role of Osmolytes and Antioxidant Enzymes for Drought Tolerance in Wheat

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

59

Proline biosynthesis in plants is initiated with the ATP-dependent phosphorylation of the carboxy group of glutamate by glutamyl kinase (GK).The resulting glutamyl phosphate (GP) is reduced to glutamic semi-aldehyde (GSA) by GSA dehydrogenase and glutamyl kinase which forms obligatory enzyme complex [32]. The accumulation of proline under stress in many plant species has been correlated with stress tolerance, and its concentration has been shown to be generally higher in stresstolerant than in stresssensitive plants. In wheat, an assessment of the effects of drought stress on proline accumulation in a drought-tolerant and a droughtsensitive cultivar revealed that the rate of proline accumulation and utilization was significantly higher in the drought-tolerant cultivar [33]. Furthermore, in *B. juncea* plants grown under stress conditions, activities of proline biosynthetic enzymes P5CR and ornithine-aminotransferase (OAT) increased mainly intolerant lines though the activity of

Trehalose is a vital soluble sugar osmolyte frequently used by cells to accommodate osmotic pressure within the effected cells to avoid cellular injury due to oxidation phenomenon. According to recent research, sugarsignaling mechanism plays a vital role in accelerating the photosynthetic performance of plants to its maximum rate in association with trehalose metabolism. These positive effects of trehalose on gas exchange parameters are due to its role in osmoregulation which may affect the stomatal opening. It can be concluded that improvement in growth in wheat cultivars under waterstressed condition with trehalose application may have been due to the role of trehalose in osmotic adjustment. Different plant species respond differently on exogenous application of trehalose and proline. The plant development may be hampered by the external application of these compounds resulting in growth inhibition or yield reduction. The beneficial applications of these osmolytes on crop stress

tolerance must carefully be determined for appropriate plant developmental stages.

growth and final crop yield under stress conditions [30].

In plants, trehalose increased the biomass production in shoots and roots in all wheat cultivars under waterstressed conditions as an osmoprotectant under adverse environmental conditions. Exogenous applications of trehalose and proline to plants during or after stress exposure and the increase in the internal levels of these compounds generally enhance plant

Among the many quaternary ammonium compounds known in plants, glycine-betaine (GB) occurs most abundantly in response to dehydration stress. GB is abundant mainly in chloroplast where it plays a vital role in adjustment and protection of thylakoid membrane, thereby

prolinedegrading enzyme "proline oxidase" decreased in all lines.

hydroxyl radical scavenger [31].

**5.2. Proline biosynthesis**

**5.3. Trehalose**

**5.4. Glycine-betaine**

The osmoregulators such as protein, sugars, amino acids, and compounds of quaternary ammonium play a vital role in adjusting the osmotic pressure and stabilizing of plant cells and tissues [25]. Drought stress causes osmotic stress in plants which causes a reduction in growth, imbalance ion transport, and a decrease in transpiration rate and an increase in membrane permeability. Such effects result in less water-absorbing capacity of crop plants, and different plant species and genotypes within a species respond differently to adverse environmental conditions. In order to counteract unfavorable environmental conditions, plants accumulate different types of organic and inorganic solutes in cytosol to decrease osmotic potential by which they can maintain cell turgor.

Plant cells lose water and decrease turgor pressure under water-stress conditions. There is an increase in different plant hormones in case of water stress like abscisic acid, which has important roles in the tolerance of plants to drought, high salinity, and cold. Abiotic stresses, which cause depletion of cellular water, are responsible for the greatest agricultural losses. Upon exposure to these prevalent stresses, the accumulation of osmoprotectants is in sufficient quantity to facilitate osmotic adjustment. The increase in cellular osmolarity due to these compatible solutes is accompanied by the influx of water into the cells, providing the turgor necessary for cell expansion [7]. Water deficit develops slowly enough to allow changes in developmental processes as water stress has several adverse effects on plant growth. In this situation, compatible solutes like proline, glycine-betaine, and trehalose produce to counter the unfavorable cellular conditions. The osmotic potential fluctuation of soil solution creating a water stress in plants ultimately leads to plant death due to growth arrest and molecular damage. Osmotic adjustment of cells helps to maintain plant water balance to establish internal milieu [9].
