**2. Biochemical modifications in cotton plant under water stress**

In many plant activities, water is vital for the transport of nutrients, chemical and enzymatic reactions, cell growth, cell division, and transpiration [43]. Drought stress reduces plant development due to disruptions in the plant's main biochemical and physiological systems [44]. Drought stress affects root penetration, and stem elongation and increases water use efficiency. The leaf water potential, rate of transpiration, and leaf temperature are important traits that affect the growth of plants under water-deficit conditions. Although all stages of cotton development are affected by water stress, however, the reproductive phase, that is, flowering and boll development, are generally accepted as the most sensitive stages [45]. A very close relationship between nutrient uptake and water use efficiency has been observed in many crops. Therefore, nutrient uptake was reduced during drought stress, particularly N and K in cotton. Similarly, K contents in plant tissue also decrease due to drought stress and an overall reduction in nutrient uptake and there utilization [43].

*Enhancing Water Use Efficiency by Using Potassium-Efficient Cotton Cultivars Based… DOI: http://dx.doi.org/10.5772/intechopen.112606*

Root growth is important for plant development as water and nutrients are mainly taken up via roots, whereas root elongation is affected by drought stress which ultimately limits crop yield [20]. Under severe drought stress, the elongation of plants may be inhibited by interrupted water flow from the xylem to the surrounding cells, which impairs mitosis and cell expansion, resulting in a reduction of root growth, nutrient uptake, plant height, and leaf area as well as plant growth [46]. In cotton, under water stress conditions chlorophyll contents decreased which reduced photosynthesis rate, sugar production, and pigments including chlorophyll *a*, chlorophyll *b,* and carotenoids [47].

There is a root-to-leaf transduction of chemical signals caused by water-deficit stress, through the production of abscisic acid (ABA), which results in the closure of stomata. Under drought stress, ABA promotes stomata closure to reduce the transpiration rate [48]. When water potential is low in plants, ABA also stimulates root growth and inhibits shoot growth. Once the soil water availability is reduced, the amount of ABA in the xylem increases, and consequently, ABA concentration in different parts of the leaf is increased. As ABA is directly associated with stomatal conductance, it reduces net CO2 intake and decreased photosynthesis [48]. Existing cotton cultivars vary in tolerance to abiotic stresses, such as drought; therefore, maintenance of optimum plant cell turgor is an indication of the drought tolerance potential of a cultivar [49]. Similarly, a change in carbohydrate metabolism with an increase in glucose concentration in leaves and sucrose concentration in pistils of white flowers of cotton was observed under drought stress [50].

Ascorbate peroxidase activity is reported to increase whenever plants undergo in drought stress as reported in cotton grown under drought stress but glutathione metabolism levels were not changed [51]. However, the investigation of antioxidant activity in cotton plants exposed to drought stress is still controversial and is not well understood and superoxide radicals (SOD) or catalase (CAT) activities remained unchanged under drought stress [52]. It has been shown that CAT activity is not affected by drought stress, while, the Ascorbate peroxidase (APX) and SOD activity was increased [53]. In most plants, osmoregulation reduces the water potential of cells, thus increasing the gradient for water flow in the cell to maintain cell growth [54]. The maintenance of cell turgor contributes to ongoing physiological processes such as stomatal conductance and photosynthesis, as reported by Ref. [21, 55]; however, information on osmotic regulation in modern cotton varieties under drought stress is still lacking.

Yield is essentially the precise integration of the many systems of physiology. Drought stress negatively affects most of these physiological systems. The negative effects of shortage of water on yield depend mainly on the intensity of stress and the growth stage of plant life. In key crops, significant drought stress losses have been observed. The low moisture caused by parenthesis lowered the time to anthesis and reduced the time to fill the grain with cereal by anthesis [56]. Exposure of plants to drought stress can lead to total sterility of pearl millet (*Pennisetum glaucum* L.), typically owing to disrupted mobility in the assimilated ear [57]. The drought may be caused by many causes such as decreasing photosynthetic rate [58], disturbing assimilate partitioning [44], or inadequate growth of a prominent leaf [59]. Maize was significantly reduced in returns when exposed to drought conditions at the tasseling stage [14]. In the same way, the production and abortion of the developed bolls in cotton under drought circumstances were significantly reduced, which eventually affected the lint yield [60]. In drought environments, there was also a substantial drop in barley grain production (*Hordeum vulgare* L.), largely due to lower viable

grain tillers and grain with less than 1000 seeds [61]. Drought stress in the blooming stage produced more than a 50% decrease in the seed output by the exposition of pigeon peas (*Cajanus cajan* L.) [62].

Drought has a primary influence on the plants, namely poor germination and impaired planting. Several studies have demonstrated detrimental effects on germination and seedling development of drought stress [44, 63]. In major crops of fields, including pea (*Pisum sativum* L.), alfalfa (*Medicago sativa* L.), and rice (*Oryza sativa* L.) under drought stress, drought stress was observed to reduce germination, early sowing growth, root and shoot dry weight, hypocotyl ate length, and vegetative development [64–66]. Plant growth is achieved largely through the division of cells, expansion, and differentiation. Drought affects mitosis and cell elongation leading to poor growth [39]. Drought restricts the cell development process, largely owing to turgor loss [67]. Water limitation leads to cell elongation, largely because of the inadequate passage of water from the Xylem to the next cells [68]. Drought also reduces the number of leaves and the size of each leaf. The leaf growth usually depends on the turgor and the availability of assimilates. Reduced turgor and slower photosynthesis rate in circumstances of drought stress particularly restrict the growth of the leaf [59]. Fresh and dry weight in the water limitation conditions are likewise reduced significantly [69]. Plant height, leaf size, and, stem diameter were diminished significantly in maize under water-limiting conditions [70]. Ref. [71] found in another investigation that the bioaccumulation of maize has decreased considerably under dry conditions at different stages of growth.

Certain variables, such as the leaf water, leaf and canopy temperature, transpiration rates, and stomatal conductivity, impact water relations. Drought stress disrupts all of these processes in plants, but stomatal conductance is particularly impacted [44]. Drought conditions, which eventually raised the leaf and canopy temperature, showed a substantial decrease in the leaf water potential and transpiration rates [72]. The efficiency of the dry matter ratio accumulated with the water used is another essential characteristic of plant physiology control. Efficient wheat crops are more efficient at using water during drought conditions [73]. This improvement in the efficiency of water usage is large because the dry matter accumulates by absorbing less water due to the closure of the stomata and less transpiration. When subjected to an early season water scarcity, a decreased water efficiency in potatoes (*Solanum tuberosum* L.) was found, and eventually, biomass buildup and output were low [74].
