**3. Structural and physiological responses of the cotton plant under drought condition**

Drought leads to a variety of changes in cotton plant growth and function. Drought, for example, seriously impedes many physiological processes regulating fiber quality and foliage output [45]. Mechanisms for drought resistance in plants consist of four groups, namely recovery, prevention, tolerance, and drought exhaust [75]. Water stress avoidance supports essential functions, such as stomatal control, in the event of moderate drought. The capacity of flora to undergo serious dehydration with osmotic adjustments and osmoprotectants is drought tolerance [76]. Plants have been developed to control the time of development to prevent moisture stress [77]. The capacity of plants to resume their development following drought damage is the recovery of drought. In the cotton area, biochemical, physiological, and molecular drought stress methods are examined in the preceding sections, as shown in **Figure 2**.

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

#### **Figure 2.**

*Drought tolerance mechanism of potassium [12].*

Plant roots are important for sensing and responding in direct contact with soil water and nutrients to different external environmental stimulation systems. Because the root structure configuration of dry soils is difficult to gather, little data on change in root systems are available in drought, and most research on cereal crops is done. Plant roots respond to the surface moisture changes, that is, water shortage at the top of the ground leads to deeper root penetration, whereas the upper layer of surplus water lowers the root penetration [78] by up to 3 m.

The growth of root rates for predicting agricultural production losses in cotton crops is widely used. Inadequate soil humidity inhibits root growth and development and so affects the working of the aerial portions [78]. Water shortage in the topsoil results in deeper root penetration for a more extensive moisture and nutrient exploration, whereas excess water in the top floor produces a decreased root penetration [78]. Drought decreases the formation of over ground biomass via a decrease in the density of root, root mass, and root lengths [79]. However, characteristics such as hydraulic conductance and plant allometry are of significant interest to scientists in the process of drought resistance. The rooting system with many short and thin side roots enables the acquisition of oxygen and soil nutrients to a wide root surface area, compared to the dispersed root system [80]. The Fine Root System drives soil activities such as carbon cycling, sequestration, nutrient flows, structural stability, and soil microbial activity [81]. For adjusting to drought, increased length of root and soil proliferation are desired characteristics. The growth and penetration of roots depend on external partial oxygen pressure inside the root region [82]. Mild drought stress may increase root elongation during the early stage but root morphological and physiological activity is severely impeded by long-term water stress disruptions [79]. Finally, deeper root insertion makes it possible for the plant to explore deeper water and nutrients in the soil. Therefore, it is necessary to promote the dispersion of vertical roots to improve crop growth and dry stress development. All root characteristics may be relevant for drought stress, but researchers have been particularly interested in hydraulic conductance and plant algometry. Different researchers have examined the probable origins of drought stress [6]. More abundant (greater radicular density) and deeper soil root systems often serve as desired features to adapt to drought. For one case, [79] reported that mild and initial drought stress improves cotton root length, but a longterm water shortfall reduces root activity in comparison to control plants. Transgenic

cotton plants, with a stronger root structure than in the natural, were more resistant to drought stress in another trial [83]. Arabidopsis also harbored transgenic cotton plants that had a highly developed root gene, in addition to other characteristics tolerant to drought 1/homodomain glabrous 11 (AtEDT1/HDG11) [84].

#### **3.1 Cotton lint yield**

Lint yield is a complex integration of many physiological processes in cotton crops; most of them are severely affected by water stress. The development of new nodes in a cotton plant is dependent upon the availability of water because of the indefinite habit of growth. Duration and intensity of stress and plant development are linked to the detrimental effects of moisture stress on yield. Drought terminal significantly restricts the production of cotton by limiting carbon absorption and the buildup of biomass [85]. Inhibited synthesis of carbohydrates together with the depletion of the storage reserves (i.e., starch) owing to continuous breathing [86]. Ref. [87] reported that as a result of drought, reproductive structures and boll size decreases are induced [88]. Increased fruit and leaf abscissions might be linked to a final output loss for drought-stressed cotton crops [60]. In short, the loss of cotton production is closely linked to physiological and morphological plant stress processes.

#### **3.2 Fiber quality**

The cotton farmers are mostly focused on fiber quality, both as the fiber characteristics directly influence the fiber output and enhance the spinning processes [89]. The quality of the fibers is combined with the length of the fiber, fiber fineness of the wall, the strength of the fiber, the elasticity of the body fiber, naps (fiber nodules), short fiber index, uniformity index (fibers fitted in the spinning process), color grade and reflectiveness (fibers brightness) [19]. Fiber quality features are numerical and regulated by a variety of phenotypically significant and minor genes [90]. Lint quality is directly affected by water during fiber cell formation [91]. As a complex phenomenon with various morphological and physiological characteristics is connected with drought tolerance in the plant [92], the breeding of enhanced fiber quality characteristics under moisture stress is difficult [93]. The discovery for irrigated and water deficiency environments of stable quantitative traits loci (QTL) might therefore allow the molecular breeding of cotton genotypes with enhanced fiber quality and yield parameters. The use of abundant DNA markers for the cotton genome is necessary for the QTL, genetic diversity, and structural analysis [94].

Several QTLs have been highlighted in upland cotton on fiber yield features [95, 96]; however, there has been less emphasis given to identifying QTLs in terms of fiber quality under drought [97]. The inter-specific cotton plants F2 and F3 [98] are generated from a crossing between inbred lines *G. Siv'on* and *G. hirsutum* cv. CV F-177 barbecue. QTL (13 and 33) reported for the 16 QTLs, covering plant production, physiology, and fiber quality, under situations of well-watered and water deficiency. Ref. [28] discovered 79 QTLs combined with fiber quality features in the F2 and F3 generations which are derived from Siv'on and *G. hirsutum* cv. Barbadense cv F-177 underwater conditions are irrigated and deficient. Seventeen of the 79 QTLs discovered were moisture-specific conditions whereas just two were well-watered. In the F2 population, the mapped physiological, yield, and plant structural characteristics in the cross between G were created [97]. Hirsutum cv. Hirsutum cv FH-901 (sensitive to drought) and *G. hirsutum* cv. Hirsutum cv RH-510 RH-510 (drought-tolerant).

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

A total of seven QTLs, three and two of which are water-limited and well-watered, were identified accordingly. These germplasm panel QTL analyses included a large range of relevant alleles that may be detected on *G. hirsutum* lines with various genetic information. In this work, 177 single SSR markers were utilized in a panel of 99 upland genotypes of cotton for the detection of significant quantitative trait loci (QTL) associated with 11 quality fibers and structural plant characteristics. The quality of fiber and the structural features of plants under water and water deficits were examined in another investigation. GLM analysis revealed that 74 and 70 QTLs were found under well-watered and limiting water situations, respectively. The MLM found 7 and 23 well water and water shortage QTLs, respectively [19].

For example, efforts have been undertaken to discover the particular fiber gene and its activities for improving fiber quality for characteristics of significant interest in cotton fiber. Cotton genomics promises to increase the understanding and systematic utilization of fundamental plant biology to improve cotton fiber quality, and cotton functional genomics. However, it is laborious to determine the activities of cotton genes that a quick pace has not been evaluated [15]. Actin cytoskeleton [99], polysaccharide biosynthesis, [2]), and the related genes are expressed in distinct routes for fiber formation. Of these, few are mostly present in the production of fiber [100], secondary biosynthesis cell wall [25], and fiber-extension [101]. A protodermal cotton gene 1 (GbPDF1) has now been expressed through the HDZIP2ATATHB2 core element at the fiber initiation stage [102]. In developing fibers, alpha expansion genes (GhExp1) encode a cell wall protein and govern the loosening of the cell wall [103]. Ref. [104] demonstrated the interruption of the fiber elongation and SuSy's role in osmosis control by the antisense removal of the sucrose synthase (SusSy) gene. On the other hand, GhPRP5 was a negative regulator in fiber formation, with proline-rich protein genes coding [105]. During secondary cell wall biosynthesis, cellulose synthesis is an important step in the formation of cells. Many types of research have been carried out to examine how cotton fiber controls and maintains the strong irreversible carbon sink that has a secondary synthesis of wall cellulose [25]. The subsequent discovery of a novel isoform Sus (SusC) during the secondary wall development of cellulose in fiber was followed [25]. Most of the expressed genes are associated with fiber maturity with cellular respiration [106]. Many transcription factors encode genes, that is, the fiber development phase included featured families from MYB, C2H2, bHLH, WRKY, and HD-ZIP. Past studies reveal that fiber formation in upland cotton has demonstrated significant expression in MYB-related genes [107]. Expression analyses from six MYB genes indicated that GhMYB6 was high in fiber, with R2R3 MYB gene encoding factor "GhMYB109" expressed in the fiber elongation and initiation [61]. The RAD-like GbRL1 is significantly expressed in cotton ovules at fiber commencement [108].

The identification of fiber-related loci markers can have useful impacts on genetic adaptation required under scarce water situations to create sufficient fiber. Many types of research into gene expression were conducted to understand the formation of cotton fibers, which offers problems. First, in comparison analysis, the bulk of the differentially expressed genes are connected to differences across species rather than to fiber-related features. Secondly, G protein-coding gene sequences are being used. G and Ramondii. Arboreum may not be precise enough for tetraploid cotton genetic analysis. Thirdly, it is unknown if any of the expressed genes found in previous research have changed the sequence between a mutant in cotton fiber and its natural type. In this context, the viable candidates for innovative cotton research are only differentially expressed genes with sequence variants and co-location with desired fiber properties.
