**3. Water-use efficiency**

The water-limited productivity of plants depends on the total amount of water available and on the water-use efficiency of the plant. Any plant capable of acquiring more water or that has a higher water-use efficiency will resist drought better. When water shortage develops slowly, it is sufficient to permit changes in developmental process as water stress has more than a few adverse effects on plant growth. In this situation, compatible solutes like proline, glycine-betaine, and trehalose are produced to counter the unfavorable cellular conditions. Osmotic adjustment (OA) is a net increase in these solute contents per cell, and it develops slowly in response to tissue dehydration and maintains turgor and osmotic pressure of effected plant species. Osmotic potential fluctuation by the soil solution creates the stress in plants by water ultimately leading to plant death as a result of growth arrest and molecular damage. Osmotic adjustment in plant cells helps to maintain plant water balance to carry on regular life processes [9].

Most plants take up CO<sup>2</sup> from atmosphere while limiting water loss. The cuticle covers exposed plant surfaces as an effective barrier to water loss that protects the plant from desiccation. These plants cannot prevent outward diffusion of water without excluding CO<sup>2</sup> from the leaf. The concentration gradient of CO<sup>2</sup> uptake is much smaller than the concentration gradient that drives water loss. If water reservoir is higher than usual, this triggers regulation of stomatal apertures at day time and remained close at night. There is no photosynthesis in the night, so no demand for CO2 inside the leaf; therefore, stomatal apertures are kept small, 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 inside the leaf is large, and the stomatal pores are wide open to claim more CO2 . Through transpiration, water loss is substantial under these conditions, but since the H2 O is plentiful, 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 its stomata closed in dry conditions [9].

compete more effectively for soil nutrients, while those with a higher proportion of shoots can collect more light energy and perform function accordingly. Root length is a better measure than the surface area of the absorbing ability of roots. Water moves slowly in soil so that a small root is almost as effective as a larger one in absorbing water and nutrients. According to Fahad et al. [19], the large proportions of shoot production are characteristic of vegetation in early succession phases, while high proportions of root production are characteristic of climax vegetation phases. Except for injury to the roots, a reduction in the rootshoot ratio is almost always in response to more favorable growing conditions. An increase in the rootshoot ratio

Role of Osmolytes and Antioxidant Enzymes for Drought Tolerance in Wheat

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

55

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

would indicate that a plant was probably growing under less favorable conditions.

**4. Drought stress**

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 maintain cellular milieu.

#### **3.1. Root-shoot ratio**

Crops of tomorrow are expected to grow under huge levels of atmospheric CO2 . Basic crop 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 compete more effectively for soil nutrients, while those with a higher proportion of shoots can collect more light energy and perform function accordingly. Root length is a better measure than the surface area of the absorbing ability of roots. Water moves slowly in soil so that a small root is almost as effective as a larger one in absorbing water and nutrients. According to Fahad et al. [19], the large proportions of shoot production are characteristic of vegetation in early succession phases, while high proportions of root production are characteristic of climax vegetation phases. Except for injury to the roots, a reduction in the rootshoot ratio is almost always in response to more favorable growing conditions. An increase in the rootshoot ratio would indicate that a plant was probably growing under less favorable conditions.
