2. Impacts and Responses of Salt Stress on Cotton Plant
A potential avenue for improving cotton performance in saline environments could involve gaining an understanding of how cotton responds to salt, its resistance mechanisms, and effective management approaches. This knowledge could inform the development of strategies to enhance cotton growth and yield in such environments
[30].
Salt stress decreases biomass production, stem thickness, reduction in leaf area, root and shoot weight, and yield of seed
[31]. Cotton yield decreases at a salinity level of 7.7 dS m
−1, and a 50% reduction in output was noted at 17.0 dS m
−1 [32]. Under salt stress, fiber strength, length, and micronaire values decrease in both
Gossypium hirsutum and
Gossypium barbadense, but ginning out-turn increases. However, this increase is accompanied by a decrease in fiber strength, length, and micronaire values in both
Gossypium hirsutum and
Gossypium barbadense [31]. In addition, salt stress also decreases the photosynthetic activity and percentage of carotenoid contents, ultimately resulting in poor plant growth. Compared to later stages, salt stress is more detrimental to the germination, emergence, and seedling phases
[33]. Salt stress can lead to delayed flowering, a decrease in the number of flowers per plant, an increase in fruit shedding, and a reduction in boll weight. Under salt stress, the concentration of Na
+ and Cl
+ increases by decreasing the K
+, Ca
2+, and Mg
2+ concentration in cotton leaves. Increasing certain ions can decrease other ions due to competition for uptake by the plant. When a plant is exposed to high concentrations of some ions, such as sodium and chloride ions, they compete with other ions, such as potassium and magnesium, for uptake by the plant. This can result in decreased uptake of the other ions, leading to a decrease in their concentrations in the plant. This competition between ions for uptake is known as ionic competition and can have a significant effect on the overall ion concentrations in the plant
[34].
Na
+ exclusion has commonly been attributed to salt tolerance in cotton. Cotton is affected by high salinity, resulting in reduced uptake of potassium (K) and nitrogen (N), whereas low salt levels have minimal impact on their absorption
[31]. Reduction of metabolic enzyme activity, such as alkaline invertase, sucrose phosphate synthase, and acidic invertase results in low fiber quality under salt stress. For example, Peng and others in 2016 discovered that in two different cotton cultivars, high soil salinity hindered cellulose synthesis, decreased the rate of sucrose conversion, and affected the functions of sucrose-metabolizing enzymes
[35].
2.1. Impact of Salt Stress on Cotton Growth
To address the salt stress issue, it is important to comprehend how salt affects cotton at various growth stages.
2.1.1. Root and Shoot
Salt stress is more common in cotton at germination, emergence, and young seedling stages
[31]. However, salt stress is more sensitive in seedlings at germination stage than in seedlings at the juvenile stage
[30]. A significant reduction in cotton production occurs when there is a decrease in the plant population due to poor germination
[36]. The growth of roots is impeded by salt stress as it decreases the number of secondary roots and diminishes the length of roots
[37]. Primary root length is reduced with high salt concentration, while secondary root length is similarly slowed by modest salt stress
[38]. Root growth is variably reduced according to soil type as salt stress increases. The effects are more obvious in clay and loam soils than in sandy soils
[39]. High salt stress has a detrimental impact on vegetative development. Salt stress lowers the ratio of shoots to roots, indicating that shoot growth is more susceptible to salt stress as compared to root growth
[30]. Studies conducted at different stages of cotton growth have found that the six-leaf stage is particularly susceptible to the negative impacts of salt stress
[40].
2.1.2. Boll Development and Yield
As salt stress increases, cotton yields decrease drastically, which is evidenced by a decrease in the number of bolls and their weight. Furthermore, a reduction in the number of fruit-bearing positions, a delay in blooming, an increase in flowers shedding, and a decrease in the number of bolls per plant due to salt stress all contribute to a reduction in mature bolls
[30]. Detrimental impacts of high salt stress on vegetative development eventually delay flowering and might also cause a delay in flower blooming. Irrigating cotton with highly saline water during the budding stage can result in a yield reduction of approximately 90 percent
[30].
2.1.3. Fiber Quality
Fiber quality traits are genetically controlled but are influenced by the environment
[41][42]. Fiber length, strength, and maturity are all reduced under salt stress, whereas fiber fineness increases. It has been reported that when the Na
+ ion percentage is increased, it negatively affects the fiber length, strength, and micronaire values
[34][43]. In salt-sensitive cultivars, cellulose content and sucrose transformation rate both dropped considerably with an increase in NaCl level, resulting in fiber quality degradation. Sucrose is accessible in a saline environment, but due to reduced activity of metabolic enzymes such as sucrose phosphate synthase, acidic invertase, and alkaline invertase, it is not effectively transformed into cellulose
[35].
Table 1 represents the findings of salt stress effects during various growth stages in cotton.
Table 1. Impact of salt stress on different developmental stages of cotton.
2.2. Response of Cotton Plant to Salt Stress
Under conditions of salt stress, soluble salts accumulate in the root zone of cotton, leading to the development of osmotic and ionic stress, as well as disturbances in mineral balance
[65], which result in a severe decrease in crop quality and production
[66]. Because of osmotic, ionic, and oxidative stressors, salt stress severely affects cotton growth, development and production. As a result, identifying and developing cotton cultivars that can withstand salt stress is a major challenge for sustainable agriculture
[67].
Cotton’s most effective response to salt stress either excludes excess sodium or compartmentalization. There is significant potential to create salt-tolerant cotton cultivars by boosting enzymatic and nonenzymatic antioxidant gene expression. Additionally, priming seeds is an efficient method for enhancing cotton germination in saline soils
[30].
Seed priming is an economical method of hydrating seeds and promoting rapid, uniform germination. This technique results in reduced imbibition time, increased metabolic activity, and osmotic adjustment. It also triggers molecular changes such as DNA synthesis, protein production, and the accumulation of antioxidants. There are various types of priming methods, including hydropriming (presoaking in water with or without drying), osmopriming (soaking in osmotic solutions such as sugar or mannitol followed by air drying), and hormopriming (soaking in hormone solutions such as auxin or gibberellic acid). These methods have been reviewed in multiple studies
[68].
Zhang and fellows in 2021 divulged that melatonin priming can enhance the salt tolerance of
Gossypium hirsutum L. (cotton) seedlings under salt stress conditions. According to the study, seedlings that were cultivated from seeds primed with 25 mM melatonin displayed greater root and shoot biomass and increased ion accumulation in comparison to the control group. These results suggest that melatonin priming has a beneficial effect on salt stress tolerance. The study also concluded that melatonin-primed seedlings performed better under saline conditions compared to nonprimed seedlings, indicating the potential for melatonin priming to enhance salt tolerance in cotton plants
[66].
Shaheen and colleagues (2015) found that seed priming with KNO
3 (1.5%) was found to reduce salt stress in cotton seedlings, improving dry matter and nutrient uptake, as well as shoot and root lengths, biomass, and cation (Ca
2+, Na
+, and K
+) accumulation
[69]. Wang and others (2021) also demonstrated that Mepiquat chloride-priming positively improve cotton seed germination and seedling establishment when exposed to salt stresses
[70]. According to the report by Ahmadvand and fellows (2012), the priming of cotton seeds with KNO
3 resulted in improved germination and seedling growth even when subjected to salt stress
[71].
Utilizing marker-assisted selection (MAS) and exploiting the inter- and intravariation in cotton germplasm can be effective in generating salt-resistant variants. Additionally, a transgenic approach could serve as a crucial tool for cultivating cotton in saline conditions. Transgenic approaches involve transferring specific genes from one organism to another in order to achieve desired characteristics. Transgenic methods are quicker than traditional breeding techniques and can enable crossing of genera boundaries. Through the transfer of salt-responsive genes from other sources, transgenic approaches have been utilized to create salt-resistant plants. This technology has already demonstrated successful implementation in cotton
[30].
Research studies have shown that the introduction of
TsVP, a gene for H
+-PPase from
Thellungiella halophilla, into transgenic cotton plants can enhance their root and shoot growth, as well as their photosynthetic activity under high salt stress conditions
[72]. This is likely the result of
TsVP aiding the storage of Na
+ and Cl
− in the vacuoles, which leads to a decrease in membrane ion leakage and malondialdehyde levels
[72]. Expressing the
TsVP gene from
Thellungiella halophila can enhance cotton emergence, survival, and fiber quality under high saline conditions, while expression of the
AVP1 gene from
Arabidopsis thaliana improves growth and fiber yield in salt-stressed transgenic cotton. Co expression of
AtNHX1 and
TsVP genes in cotton also boosts emergence rate and yield under high saline environments
[73]. In the future, researchers may utilize a combination of conventional techniques and state-of-the-art molecular technologies to breed salt-tolerant plant varieties
[30].
There is substantial inter- and intraspecific variation in cotton salt tolerance, which is critical for selection and breeding regarding salt stress
[40]. In the context of saline stress, the process of ion exclusion, specifically the exclusion of Na
+/Clˉ, is accountable for the uptake and storage of detrimental ions within the tissues of cotton
[65]. Several studies have shown that increased levels of K
+/Na
+ and Ca
2+/Na
+ in cotton tissues are associated with greater tolerance to saline stress. For instance, Kumar and colleagues in 2020 observed varying levels of inorganic sodium (Na
+) accumulation in different cotton genotypes. The salt tolerant genotypes displayed higher potassium (K
+)/sodium (Na
+) ratios than their salt-sensitive counterparts
[67]. Zafar and others during 2020 and 2021 discovered that tolerant cotton genotypes were able to maintain a stable potassium-to-sodium ratio in comparison to salt-sensitive cotton genotypes
[27]. In 2003, Ahmad and colleagues conducted a study to investigate the effect of the calcium-to-sodium ratio for salt tolerance in plants. They reported that salt tolerant genotypes exhibited higher calcium-to-sodium ratios in their leaves than salt sensitive ones under saline conditions. The outcomes of the study indicate that calcium may have a pivotal function in the maintenance of proper membrane function and the regulation of its permeability, leading to normal growth in salt tolerant varieties in contrast to salt sensitive ones
[74].
Consequently, this parameter can serve as a selection criterion for screening salt tolerant varieties. Genotypes demonstrating elevated antioxidant activity under saline conditions can be considered more tolerant to salt stress
[75]. Genetic analysis of growth, fiber characteristics and yield under salt stress have shown to be genetically regulated via different quantitative trait loci (QTLs). Larger genetic additive variance of these traits can be utilized in cotton breeding programs for salt tolerance
[76].