Effects of Exogenous Silicon on Levels of Osmotic Substances and Na/K Homeostasis in Cotton under Salt Stress
The salt-tolerant'ZhongJ9807'(Z9807)and the salt-sensitive'Zhong J0102'(Z0102)of Gossypium hirsutum L.were used in greenhouse hydroponic experiments to investigate the effects of exogenous silicon on cotton under salt stress.Eight treatment combinations were tested:two NaCl concentrations(0 and 150 mmol·L-1)and two silicon concentrations(0 and 0.4 mmol·L-1 Na2SiO3).After 14 days of treatment,the root phenotype indicators,leaf osmolyte content,sodium(Na+)and potassium(K+)content,and expression levels of genes associated with root phenotype in the cotton seedlings were analyzed.The results showed that salt-induced stress significantly reduced the total root length,total surface area,total volume,in both cotton cultivars.However,exogenous sil-icon markedly improved root growth.The application of silicon alleviated salt-induced osmotic stress by significantly increasing the levels of soluble sugar and soluble protein in the seedlings.Salt-induced stress caused considerable accumulation of Na+in the cotton roots and aboveground parts,significant-ly reduced the K+content,and,consequently,increased the Na+/K+ratio.Exogenous silicon reduced the Na+content and Na+/K+ratio and thus mitigating ionic imbalance,particularly in the salt-toler-ant Z9807 cultivar.In the salt-sensitive Z0102 cultivar,silicon treatment enhanced ionic homeostasis and salt tolerance by significantly upregulating SOS1 and NHX1 genes,which promoted Na+efflux and compartmentalization,thereby decreasing Na+accumulation in shoots.Conversely,in the salt-tol-erant Z9807,downregulation of HKT1 expression limited Na+transport to the aboveground parts,while the upregulation of HAK5 expression enhanced K+uptake in high-Na+conditions,lowering the Na+/K+ratio.These finding suggest that exogenous silicon enhances salt tolerance in cotton seedlings by regulating the osmolyte content and expression of key genes that are responsible for K+absorp-tion,Na+efflux,and compartmentalization.
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