Improving the soil K+/Na+ ratio under moderate salt stress synergistically increases the yield and quality of cotton fiber and cottonseed

Cotton production areas have been shifted to saline-alkaline soils with high Na+ content, resulting in low soil K+/ Na+ ratios, leading to K+ deficiency in cotton, which is detrimental to the yield and quality of the fiber and cottonseed. It was hypothesized that improving the soil K+/Na+ ratio by applying potassium under moderate salt stress could affect the contents of K+ and Na+ in various organs of cotton bolls. This could affect the growth and biomass distribution of the cotton bolls, ultimately impacting the yield and quality of both the fiber and the cottonseed. To test this hypothesis, a pot experiment using the cotton cultivar CCRI-79 was conducted in 2021 and 2022, and the results showed that improving the soil K+/Na+ ratio under moderate salt stress (1) increased the cotton boll K+/Na+ ratio, accelerated boll growth, and increased the proportion of boll biomass distributed to the fiber and the cottonseed kernel. (2) increased fiber and cottonseed yield. Specifically, the boll retention rate and the number of fruit nodes increased, thereby increasing the boll number. The increase in boll weight can be explained by an increase in the components of fiber and cottonseed yield per boll. Compared with those in the control soil without added salts (C), the fiber and cottonseed yield at a K+/Na+ ratio of 1:9 (SK2) were only 7.1%-21.0% and 4.9%-17.0% lower, respectively. (3) increased fiber and cottonseed quality. SK2 can completely alleviate the effects of moderate salt stress on fiber uniformity, length, and micronaire. And the fiber strength of SK2 is only 5.0%-7.2% lower than C. The oil content and yield of cottonseed kernels increased, and the total unsaturated fatty acid (UFA) and unsaturated index (UI) increased. Although the protein content decreased, the protein yield increased, and the total amino acid (TAA) content increased. SK2 and C did not differ significantly in oil and protein contents. Taken together, these findings indicate that the optimal alleviation of moderate salt stress on the yield and quality of fiber and cottonseed was achieved at a K+/Na+ ratio of 1:9. These results will help to synergistically increase the yield and quality of cotton fiber and cottonseed in saline-alkaline soil and provide new clues for elucidating the mechanism through which the K+/Na+ ratio regulates salt tolerance in cotton.