Abstract: To understand the relationships between leaf K⁺, Na⁺ concentration and photosynthetic parameters (net photosynthetic rate, maximum carboxylation rate, maximum electron transport rate, mesophyll conductance) in cotton seedlings under salt stress, we measured the concentrations of K⁺ and Na⁺ and photosynthetic parameters in cotton seedling leaves under four levels of salt stress (50, 100, 150 and 200 mmol·L^-1) and control (0 mmol·L^-1). The results showed that there was a significantly positive correlation between foliar Na⁺ concentration and salt level, with a determination coefficient (R²) of 0.958. Foliar K⁺ concentration decreased with increasing salt levels, with R² of 0.871. Moreover, the Na⁺/K⁺ ratio was positively related to salt levels. There was an extremely significant negative correlation between K⁺ and Na⁺ concentration in the leaves. Foliar Na⁺ concentration was negatively correlated with net photosynthetic rate, mesophyll conductance, maximum carboxylation rate, dark respiration rate and maximum electric transport rate, with the correlation coefficients of -0.878, -0.861, -0.835, -0.837, and -0.813, respectively. Foliar K⁺ concentration was positively correlated with net photosynthetic rate, mesophyll conductance, maximum carboxylation rate, dark respiration rate and maximum electron transport rate, with the correlation coefficients of 0.718, 0.735, 0.645, 0.856 and 0.603,respectively. Under the salt stress of 150 mmol·L^-1, foliar K⁺ and Na⁺ concentrations were 11.54 and 3.32 mg·g^-1, which was the threshold of potassium and sodium ions affecting photosynthetic parameters. The results could provide an important reference for alleviating the ion damage on plants under salt stress.

Key words: K⁺ concentration, Na⁺ concentration, maximum carboxylation rate, maximum electron transport rate, mesophyll conductance, dark respiration rate.