Interactive Effects of Salinity, Drought, and Heat Stresses on Physiological Process and Selection Criteria for Breeding Stress-Resistant Cotton

Abstract

The cotton crop is adversely affected by the combination of salinity, drought, and heat stress during all growth stages in cultivated areas. The negative impacts of salinity together with water scarcity on osmotic stress dramatically increased the sensitivity of reproductive development. After membrane integrity and signaling networks are depressed under stress at the cell level, the metabolic and physiological processes are disrupted in the next stage. The restricted root growth, ion and water uptake, phloem, photosynthetic and respiratory capacity, incompatible hormonal balance, and reduction in yield due to lower boll retention are the most important symptoms. The seed treatments and foliar applications of osmoprotectant and fertilization appear to reduce multiple stress factors in possible climate change conditions. The osmotic adjustment, antioxidative ability, electrolyte leakage in the membrane, and chlorophyll fluorescence are evaluated as selection criteria for improving genotypes. Direct selection of plants with high yield under stress conditions may increase the success of cotton breeding. It is important to know the molecular approaches and gene functions responsible for abiotic stress. In this chapter, the effects of high temperature, salinity, and drought on cotton plants and characteristics associated with tolerance were focused on cotton improvement. The classical breeding methods and molecular approaches should be combined for breeding new cotton varieties.

1. Introduction

Agricultural areas with saline soils are estimated at 1100 Mha in the World. These areas are classified as saline (60%), sodic (26%), and saline-sodic soils (14%). Areas suffering from salinity are mostly in the Middle East, Australia, North Africa, and Eurasia. However, 20 to 50 percent of irrigated soils in arid or semi-arid climates are salt-affected [1]. A report produced by Cotton 2040 emphasized that cotton affected by heat stress and drought will reach 40% and 50%, respectively by 2040 [2]. Although heat stress is a problem along with drought [3], cotton-growing face many stress factors such as drought (Figure 1B), high temperature (Figure 2C), salinity (Figure 1A), alkalinity (Figure 1C), and heavy metal contamination, which are seen together in the same ecology. A single stressor can play a predominant or protective role depending on stress resources [4]. The long-term effect of climatic change resulted in higher pH values with drought, unfavorable soil organic C, total N and P, and available N [5], and soil salinity due to increasing the sea level [6].

Figure 1.

Cotton plants under saline soil (A), stunted cotton plants under drought conditions (B), and cotton plants under alkali soil (C).

Figure 2.

A picture of a plastic tunnel under heat stress at 11.00 am–17.00 pm (A), non-affected plants by heat stress (B), and aborted/shed bolls affected by heat stress (C).

Cotton’s response to stress is different at plant growth and development stages. Germination and seedling growth are adversely affected by high salinity and low temperature at the early stages in cotton (Gossypium hirsutum L.) [7, 8]. Although progenitors of cotton have spread in adverse conditions, modern cotton varieties are non-resistant to stresses from the squaring stage to boll retention stages, in which yield formation occurs [9]. Water deficiency [10, 11, 12, 13] and heat stress [13, 14, 15], especially during flowering and boll formation can reduce yield and fiber quality. In many areas of the world where cotton is cultivated, as in the Aegean, Mediterranean, and Southeast Anatolia regions of Turkey, maximum temperatures during the reproductive stage in July and August are above 40°C with low humidity and precipitation. The boll components such as seed and boll weight cannot be affected by short-term temperature changes, whereas the effects of sudden temperature rises during anthesis on seed number per boll are very high [16].

This review presents the physiological mechanism of stress tolerance and principles of genotype improvement, both classically and by genetic engineering in cotton. Also, it discussed the effects of agronomic management such as seed and foliar treatments to alleviate drought, salt and heat stress.

2. Roles of osmolytes

The stress-induced protein breakdown and amino acid synthesis caused the accumulation of amino acids in cells under abiotic stress conditions. The overall accumulation of amino acids has two different symptoms: cell damage and the beneficial effect of specific amino acids such as proline during stress acclimation [17, 18]. Drought, salinity, and heavy metals caused an increase in the accumulation of proline in resistant plants. Although proline was not synthesized in tobacco exposed to heat stress [19], the combination of heat and drought stress [20], and drought stress [21] induced an increase in the proline content of some cotton cultivars. In contrast with this finding, genotypic differences were non-significant for proline content under drought conditions [22].

The first approach to stress resilience is to restrict ethylene synthesis, which triggers the abscission of leaves and all reproductive organs under stress conditions [23]. Boll retention and yield increased by application of ethylene inhibitors such as aminoethoxyvinylglycine (AVG) or by downregulation of genes responsible for ethylene synthesis [24]. Abscisic acid (ABA), another growth-inhibitory hormone, is an important regulator of abiotic stress tolerance. The main function of ABA is to stimulate stomatal closure and gene expression to respond to the drought, salinity, and excessive temperature in the adaptive mechanism of plants [25, 26]. Under stress conditions, plants can alter their metabolism such as the synthesis of compatible solutes. In cell metabolism, the different unfavorable conditions increased the concentration of γ-aminobutyric acid (GABA) through enhanced activity of enzymes involved in GABA biosynthesis [27]. Under alkali stress conditions, the accumulation of GABA and putrescine in young and old leaves of cotton increased [28]. Polyamines such as putrescine, spermidine, and spermine have a protective role against salinity, drought and heat stress, and putrescine and spermine could be evaluated as selection criteria for stress-tolerant genotype breeding in cotton [29]. Exogenously applied polyamines increased the stress tolerance and yield under drought and salinity stress conditions in cotton [30, 31].

The osmoprotectant solutes such as glycine betaine (GB) are evaluated in three different ways. Firstly, the increase in GB under stress conditions is analyzed as biochemical. Secondly, the selection of the plants with high GB levels is used for improving the stress-tolerant cotton genotypes in conventional breeding and genetic engineering. Thirdly, the success of seed treatment with GB and foliar application of GB were examined in stress conditions. The accumulation of GB increased in transgenic crops, whereas foliar application of GB or seed dressed are common applications in plants where a certain amount of synthesis is insufficient [32, 33]. Generally localized in the chloroplast, GB has an important role in protecting photosystem II, stabilizing membranes, and alleviating ROS (reactive oxygen species) damage [34] and chilling damage [35], enhancing tolerance to lead (Pb) [36]. Cottonseed coating with GB enhanced seed cotton yield by approximately 20% [37]. Also, combined foliar application with GB and salicylic acid (SA) increased the tolerance to salt stress due to an increase in the leaf gas exchange with positively correlated stomatal properties and stimulate antioxidant enzyme activity in cotton seedlings [38, 39].

3. Heat stress

On the cell basis, heat stress firstly induced structural changes in chloroplast protein [40] and plasma membrane [41, 42], and these changes stimulated cell elongation, expansion, and differentiation. ROS, as a signaling molecule, induced excessive MDA (malondialdehyde) synthesis by increasing lipid peroxidation, and membrane structure disintegrated under heat stress conditions [43, 44, 45]. Also, molecular chaperones and heat shock proteins (HSP) accumulate to protect the membrane integrity by the expression of the sHSP coding gene in leaves under drought and heat stress conditions [46]. Heat stress caused the accumulation of specific isoforms of activase in cotton leaves [47, 48]. Although the efficiency of Rubisco activase decreased under heat stress, an enzyme can develop an alternative function by relocating on a thylakoid membrane [49], and this contributes to the acclimation of photosynthesis during high temperatures in cotton [50]. In addition, the synthesis of phytohormones such as SA and jasmonic acid (JA) to respond to heat stress is enhanced by ROS and Ca²⁺ [51]. The association between heat stress and plant nutrition demonstrated that the fertilization containing some macro and microelements such as K, Zn, and B could be used to alleviate the harmful effects of higher temperatures. These elements play an important role in chlorophyll synthesis and delay senescence in cotton [52, 53]. Zn spray eliminates the adverse effects of heat stress in cotton [54].

The reproductive period is synchronized with many processes in cotton. Although square retention is less affected by heat stress [55], sexual reproduction is defined as both sensitive to high temperature and thermotolerant during flowering – boll retention depending on the time, length, and severity of stress in cotton [56, 57, 58]. Heat stress during this period adversely affected the development of both sexual organs and caused flower abnormalities such as small flower, elongated stigmas (Figure 3A and B) [55], gametophyte, pollen germination, and pollen tube growth [59, 60]. Abiotic stress limited fertilization by preventing pollen tube development [61]. The tapetum, the innermost layer of somatic cells in anther lobes, is responsible for microsporogenesis and secretion of enzymes for the release of microspores from tetrads [62]. Programmed cell death (PCD) induced tapetal degeneration by heat stress, resulting in male sterility [63]. The regulation of tapetal PCD and anther dehiscence were controlled by the GhCKI gene (G. hirsutum casein kinase I) in the heat stress tolerance of cotton [64].

Figure 3.

Elongated stigma affected by heat stress (A), and normal flower structure (B).

The decrease in the photosynthetic capacity of cotton at a temperature above 32°C reduces the accumulation of sucrose [65]. The decrease in sucrose accumulation adversely affected the fiber quality by inhibiting cellulose synthesis in higher temperature conditions [66]. Hereby, sucrose transport from subtending leaf of boll to developing boll negatively affected, and boll number per plant and boll weight reduced [67]. Plant growth is mostly regulated by sucrose, which is a source of substrates’ energy production and biosynthesis by decomposing into hexoses. Furthermore, the plant responses are determined by ROS scavenging capacity and the signal pathway of sucrose [68]. In many crops, drought increased carbohydrate deprivation and ABA levels, whereas the ability of reproductive sinks to use sucrose and starch was reduced [69].

Auxin, an essential hormone, is very important for stress tolerance [70] and plays a role as a coordinator of plant growth and development [71]. The inhibition of auxin synthesis by overexpressing miR157 caused the sensitivity to heat stress and anther sterility in the reproductive period of cotton [72]. The auxin signaling pathway is controlled by the suppression of the sucrose synthase gene, and the antagonistic relationship between auxin and sucrose regulated plant growth and development [73, 74]. The favorable balance between auxin and sucrose is indispensable for the response of anther to heat stress [75].

Shedding is the formation of the abscission layer by natural or stress between sympodial or monopodial branches and reproductive organs such as square, flower, and boll in cotton (Figure 2B and C) [76]. The amount of hormones and regulation among hormone-controlled shedding, as auxin inhibits, and abscisic acid (ABA) promotes. Shedding is induced by increasing ABA and decreasing auxin under drought conditions [77]. In the reproductive period, failed fertilization caused by non-available pollen in higher temperatures resulted in flower shedding.

4. Salinity stress

The physiological drought via osmotic stress and ion toxicity caused by Na⁺, Cl⁻, and SO₄²⁻ are two major forms of damage. Although cotton is classified as moderately tolerant to salt stress (7.7 dS m⁻¹), salinity caused a decrease in seed cotton yield [78], boll number per plant [79, 80, 81], and an increase in early maturity [82]. Seed germination and early seedling growth in cotton are the most sensitive stages [83]. Both forms of salt damage caused abnormal plant growth such as stunted root and shoot growth by reducing photosynthetic capacity (Figure 1A) [84]. The excessive salt accumulation under salinity caused cellular injury in transpiring leaves [85]. However, cotton was successfully cultivated as a monoculture crop in saline-alkali soil. Plant cells are affected by low amounts of available water, loss of membrane functions, and ionic toxicity under excess sodium (Na⁺). As with most abiotic stress, salinity contains an osmotic component, and cellular dehydration causes and disrupts the internal balance (homeostasis). The plant’s first reaction is to reduce the Na⁺ level in the cytosol by restricting influx, increasing efflux, and accumulating Na⁺ in the vacuole for maintaining the cell metabolism. Transporters such as antiporter, uniporter, and symporter localized in the membrane are responsible for reducing Na⁺ [86, 87, 88]. In a study conducted by [89], the Na⁺/H⁺ antiporter gene (GhSOS1) was detected in the plasma membrane of cotton (G. hirsutum L.). This gene is a Salinity Overly Sensitive, which has an important role to synthesize protein in upregulating under stress conditions such as salinity and drought. The suberization and lignification may occur around endodermal cells to inhibit apoplastic absorption of toxic ions in cotton [90, 91].

Many researchers focused on a root-associated microorganism to alleviate salinity stress in cotton. The arbuscular mycorrhizal fungi (AMF) are capable of increasing P and Zn uptake and promoting leaf proline accumulation [92, 93] but it should be noted that Glomus mosseae, AMF species, isolated from saline soil found to be less successful in alleviating salt stress compared to that of non-saline soil [94]. The microorganisms with the PGPR effect such as Pseudomonas fluorescens have IAA producing ability involved in the synthesis of important compounds under salinity stress [95]. In addition, the higher rhizosphere colonization of PGPR induced moderate N application due to the signaling molecule role of nitric oxide (NO) in the denitrification process [96]. Also, melatonin, as an indole hormone, alleviated the adverse effects of salt stress to reduce ROS production and ion toxicity and increases proline content in cotton seedlings [97].

5. Drought stress

The increase in the tensile force of the xylem, due to drought, caused a greater probability of rupture in the water column and formation of gas emboli in the xylem [98], and embolism decreased carbon assimilation depending on the linkage between water and CO₂ exchange. Furthermore, stomatal closure and decrease in the stomatal area take place [99], and desiccation of all plants is induced in the further process (Figure 1B) [100]. Thus, the most negative impacts occurred in photosynthetic capacity and transpiration rate [101]. The environmentally induced PCD (programmed cell death) occurred in response to drought, and PCD increased ROS accumulation, DNA fragmentation, organelle degeneration, and cytoplasm shrinkage [102]. The tylose formation and xylem inhabitation by wilt pathogens in water-limiting conditions may cause drought sensitivity to stress the susceptibility [103]. Primarily, MAPK (Mitogen-activated protein kinase), and secondly, ROS play an important role in intracellular signaling [104, 105]. GhMKK3 and GhMPK2 from G. hirsutum increased root hair development and ROS production by regulating ethylene synthesis, respectively under drought conditions [106].

Arbuscular mycorrhizal fungi (AMF) have an important role to alleviate drought stress by spreading on the soil and water transport by hyphae [107]. Similarly, PGPR coated phosphorus exhibited high performance due to the increasing stomatal conductance, net photosynthetic capacity, and yield of cotton under osmotic stress [108]. As a result of studies investigating the relationship between nutrients and drought, it was suggested that high N concentrations may decrease the effects of drought through nitrogen metabolism, proline synthesis antioxidant capacity, and osmotic regulation in cotton [109, 110]. However, K application has osmotic regulation due to increasing the osmoprotectant and regulates N metabolism [53, 111, 112] and photo-assimilation and translocation process [113] in drought-stressed cotton plants. Also, supplemental Zn alleviated the negative effects of drought stress by increasing antioxidant capacity and decreasing MDA content in cotton [112, 114]. The nanoparticles such as nano-TiO₂ and nano-SiO₂ could alleviate drought stress because their foliar applications increase photosynthetic pigments, antioxidant capacity, and proline content in cotton [115].

6. Heritability and breeding for stress tolerance in cotton

The combination of multiple stress factors and the identification of the model plant is complex at the physiological and genetic levels [116]. Growth chamber, greenhouse, and field studies were conducted by many researchers to screen the cotton genotypes in different stress studies. Stress tolerant genotypes can be bred through a selection of promising single plants under stress conditions followed by testing in either stress and non-stress conditions or selection under more optimum conditions for effective selection criteria [117]. Previous studies recommended the selection of cotton plants with higher seedling vigor, enhanced early root development, and lower root/shoot ratio for drought tolerance, pollen carrying a dominant heat-tolerance allele for heat stress, and salinity barrier for salt stress [118].

In stress-tolerant cotton breeding, genetic stock in strains with the D genome rather than those with the A genome should be considered. Transcriptomic analyses indicated that thick cuticles and a double layer palisade layer of D-genome species such as G. harknessii, G. armourianum, and G. turneri are important germplasm resources for water deficiency. Similarly, G. gossypioides and G. thurberi with aggressive and deep root structures for drought tolerance and G. aridum, G. davidsonii, and G. klotzschianum for both drought and salt tolerance can be used in cotton breeding [84, 119, 120, 121, 122]. In support of this knowledge, [123] revealed that QTLs (quantitative trait locus) responsible for salt tolerance are usually localized on the D-subgenome. At the same time, novel genes and alleles in wild relatives are important to overcome the abiotic stress tolerance caused by narrow gene pools in cultivated cotton species [124].

The selection of plants with thick cuticle and waxy surfaces is important to reduce solar radiation in the breeding of heat tolerance [125]. The intense absorbency of cotton cultivars increased sensitivity to heat stress [126]. The most important cultural management is to arrange the sowing time to avoid the higher temperature in the reproductive period. However, planting cotton before the recommended time faced the problem of low temperature during the early growing stage. Therefore, breeding of high-temperature tolerant cotton varieties has been suggested as the best method [127].

A pre-screening of cotton germplasm and evaluation of hybrids by constructing a polythene tunnel at the reproductive stage is a very common method in cotton (Figure 2A) [128]. However, some researchers preferred delayed planting set of cotton plants to test genotypes in heat tolerance breeding [129, 130, 131, 132]. In addition, some researchers have used tetrazolium chloride for identifying heat tolerance in both vegetative tissues and pollen viability in cotton [133, 134, 135]. Various studies have emphasized the success of wild cotton species, their stacking progenitor alleles, and Gossypium tomentosum (heat-resistant species) for stress tolerance [13, 136, 137]. The pollen characteristics, germination ability, and tube length have been screened to determine the tolerant and susceptible genotypes under higher temperatures in cotton [13, 138]. On the other hand, the increase in fiber wax content of susceptible cotton genotypes under heat and drought stress indicated that acceptable fiber wax levels could be used to improve tolerant genotypes in conventional breeding [139].

In quantitative genetic studies about heritability and gene action for different selection criteria of heat stress, multigenic inheritance and both additive and non-additive gene action in controlling cellular membrane thermostability was found to be higher under heat stress conditions [140]. The results of the scaling test, which is an important biometrical analysis method, indicated the significant dominance, additive x dominance, and dominance x dominance referred to as non-additive gene action for relative cell injury under heat stress [141]. Similarly, non-additive gene actions were estimated for fiber quality characters and ginning out-turn in heat stress (~38–39°C) at peak flowering time under field conditions by arranged sowing time [142]. In contrast, high heritability associated with high genetic advance for hydrogen peroxide content, catalase activity, total soluble proteins, carotenoids, and chlorophyll contents were found to have significant additive gene action under heat stress conditions [143]. In conventionally breeding of drought and heat stress tolerance, it was emphasized that instead of single plant selection in the F₂ generation, superior plants should be selected in further generations such as F₄ [144, 145].

PEG 6000 (polyethylene glycol) is defined as a rapid and effective method to observe the response of cotton genotypes for selection in drought tolerance breeding [146, 147]. Drought-tolerant species are defined with low maximum transpiration and photosynthetic rate, stomatal conductance, specific leaf area, small leaf size high leaf longevity, root mass ratio, and small leaf size [148]. The higher chlorophyll stability and relative water content exhibited drought tolerance due to photosynthate, which results in higher biomass [149] and fiber quality [150]. Furthermore, the presence of late embryogenesis abundant proteins is important for drought tolerance of cotton [151]. The physiological characteristics used for drought tolerance should be combined with yield, yield attributes, and fiber quality parameters in the breeding of cotton varieties with high adaptability [152]. Drought responsive genes were classified as induced (stress-related, metabolism, transcription factor, proline, and cellular transport) and repressed (mainly comprising metabolism, cellular transport, and stress-related) [153]. Drought tolerance genes such as RD2 (rice drought-responsive), HAT22 (homeobox from Arabidopsis thaliana), PIP2, PIP2C (plasma membrane intrinsic proteins), and GaTOP6B (encoding DNA topoisomerase from G. arboreum) were associated with drought in cotton [104]. Similarly, QTL analysis revealed that genes responsive to drought tolerance are spread over nine chromosomes while one QTL hotspot is concentrated on the eighth chromosome [154]. Anwar et al. [155] evaluated the selected varieties by molecular under drought stress, and MNH-886 cotton cultivar with high boll retention percentage, photosynthesis rate, and stomatal conductance was recommended against drought stress. The ratio of general combining ability and specific combining ability variance in line x tester analysis showed significant non-additive gene effects for proline content, total chlorophyll, canopy temperature, and cell membrane stability in drought stress conditions [156].

NaCl treatment at different doses is the most used method for screening cotton genotypes in order to determine suitable genotypes. Many researchers revealed significant variations in morphological, physiological, and biochemical characters under salinity stress [157, 158, 159]. The performance of genotypes, depending on genetic factors, compared with environmental factors and genotype x environment interactions, and high genetic gain by suitable selection increased the breeding success because of high additive effects under saline [160] and drought conditions [161]. On the other hand, non-additive gene actions were found significantly higher for chlorophyll content, K⁺/Na⁺ ratio and within boll yield components under saline conditions [144, 162]. At the same time, selection for K⁺, Na^+, and K⁺/Na⁺ were recommended for salinity tolerance according to the results of factor analysis in cotton [163]. In addition, a reciprocal effect found in some studies indicated cytoplasmic and cytoplasmic x nuclear genes interaction for salinity tolerance breeding [160, 164]. Besides, the fact that genes with pleiotropic effects are effective for both salinity and drought indicated physiological characters are used in indirect selection can increase the success of multi-stress breeding [165, 166].

In many studies about engineering abiotic stress-tolerant crops, genes responsible for multiple stress factors found to confer tolerance in different plants are defined [167]. Co-overexpressing of SUMO E3 ligase (OsSIZ) and Vacuolar H⁺⁻ pyrophosphatase (AVP1) from Arabidopsis and vacuolar Na⁺/H⁺ antiporter genes performed significantly in increasing tolerance to multiple stressors such as drought, salinity, and higher temperature [168]. Overexpression of mitogen-activated protein kinases (MAPK) from cotton GhMPK2 performed stress tolerance when induced by salt, ABA, and water scarcity [169]. Eight genes belonging to the GhHSP20 family are responsible for heat, drought, and salinity [170], and the expression of GHSP26 caused a significant increase in proteins under drought stress [171]. Similarly, it was verified that GhCIPK6a overexpressed cotton lines can reduce the negative effects of salinity, and the seeds of these lines exhibited higher water absorption capacity at the germination stage [172]. In another study, JAZ proteins were identified as inhibitors of JA, and overexpression of JAZ genes resulted in a higher performance to salt stress [173]. GaJAZ1 transgenic plants carrying genes transferred from G. arboreum to G. hirsutum were found to be different from wilt type in terms of salinity tolerance [174]. GhPHD genes (plant homeodomain genes from G. hirsutum) increased plant tolerance in adverse environmental conditions to alleviate abiotic and phytohormonal stresses [175]. Also, the overexpression of the 14–3-3 gene GF14λ from Arabidopsis resulted in a stay-green phenotype due to decreased wilting, delayed senescence, and higher photosynthetic capacity. It was suggested that this gene can be used successfully in drought resistance [176, 177].

7. Inferences (future considerations)

The nutrient relations of cotton plants were greatly affected by abiotic stress. The nitrogen, silicon, magnesium, and calcium uptake are limited under high temperatures, drought, and salinity. In addition, the architecture of cotton changed to capture the less mobile nutrients. Therefore, fertilization should be reconsidered under stress conditions. To reduce the negative effects of abiotic stress, the use of some osmoprotectant and hormones may be a solution. The sowing time and plant density should be rearranged especially for high temperatures and drought. Cluster-type cotton varieties suitable for High-Density Planting System (HDPS) should be developed. These varieties will also reduce the density of weeds and dry matter content that compete for available water under drought conditions.

The most effective way to overcome the adverse effects of multiple abiotic stress factors is to improve tolerant cultivars. The possible effects of drought, salinity, and high-temperature stress on physiological, morphological, and yield should be well resolved to mitigate stress and develop varieties. The crossing between standard cultivars and donor suitable parents is a basic stage for genetic variation in conventional breeding. Multiple stress factors are very complex and polygenic characters and are controlled by non-additive gene effects with low heritability. Therefore, bulk selection should be useful for traits with low heritability such as stress tolerance, and the selection of a single plant should be postponed to F₄ or F₅ generation. Drought and high temperature are effective stress factors from flowering and during the first boll formation period, whereas salinity is an important problem in the early development period. In areas where these conditions are created, indirect selection can be made in terms of physiological characteristics, whereas direct selection can be made for yield. The evaluation of genotypes to stress tolerance under the plastic tunnel at the reproductive stage and in saline and drought conditions is a more accurate approach compared to artificial conditions and delayed sowing date in cotton.

Genetic engineering studies are promising, but manipulation of a single gene does not seem to be sufficient as resistance to multiple stress conditions is controlled by multiple genes. However, future transgenic cultivar breeding may be a good approach for multiple stress tolerance. In both classical breeding and genetic engineering studies, tolerance to multiple stress factors should be pyramided with yield and fiber characteristics. The non-genetically CRISPR-Cas system could be evaluated for engineering multiple stress tolerance in future cotton cultivars for all cotton-growing regions.

It could be highlighted that temperature extremes and fluctuations in the cotton-growing season along with other stress appear to be major factors of future yield reductions.

Acknowledgments

Volkan Mehmet CINAR thanks the Higher Education Council of Turkey (YOK) for 100/2000 PhD scholarship.

Conflict of interest

The authors declare no conflict of interest.

Author statement

Volkan Mehmet CINAR: Conceptualization, Literature survey, Writing – Review & Editing. Serife BALCI: Writing – Review & Editing. Aydın UNAY: Conceptualization, Literature survey, Writing – Review, Supervision.