Stomatal and non-stomatal regulations of photosynthesis in response to salinity, and K and Ca fertigation in cotton (Gossypium hirsutum L cv.)

To study the response of photosynthesis of cotton plants to salinity, and K and Ca fertigation, leaf gas exchange, light and CO₂ response curves, chlorophyll fluorescence parameters, leaf N, P, Mg, K, Na and Ca concentrations, and antioxidant enzyme activities were measured. Cotton plants were grown at either 0 or 150 mM NaCl salinity (C and S treatment, respectively), and fertigated with or without K and Ca addition (O and K+Ca treatment, respectively). The results showed that 150 mM NaCl salinity decreased stomatal conductance (g_s), transpiration rate (T_r) and chloroplast CO₂ concentration (C_c). Yet it increased light saturated photosynthetic rate (A_L), CO₂ saturated photosynthetic rate (A_C), mesophyll conductance (g_m), maximum electron transport rate (J_max), maximum Rubisco carboxylation rate (V_cmax) and triose phosphates use rate (TPU), resulting in a similar net photosynthesis rate (A_n) to that of C plants. K+Ca treatment enhanced A_n, g_s, T_r and C_c, particular under S condition, while it had no significant effects on g_m, J_max, V_cmax and TPU. Salt-induced increases in J_max, V_cmax and TPU were associated with higher [N]_leaf and [P]_leaf, whereas the J_max/V_cmax ratio decreased with increasing [N]_leaf and [P]_leaf, and C_c decreased with increasing [N]_leaf. Salt-induced improvement in A_C was linked to lower J_max/V_cmax ratio and C_c, while the increase in A_L could be ascribed to the lower non-photochemical quenching and higher photosystem II efficiency, which could be partially attributed to the salt-caused improvement in leaf superoxide dismutase, peroxidase and catalase activies. In conclusion, 150 mM NaCl salinity increased stomatal limitation but decreased non-stomatal limitation on A_n, resulting in sustained A_n and significantly lowered g_s and T_r, and hence an improved water use efficiency. K and Ca addition could alleviate the salinity-induced decrease in g_s and increase in stomatal limitation.