Abstract: To understand the responsive mechanism of leaf photosynthesis of cotton to salinity stress, we investigated the effects of salt stress on leaf photosynthetic characteristics of cotton seedlings with the FvCB model under five levels of salt concentration, i.e., 0 (CK), 50, 100, 150 and 200 mmol·L^-1. Results showed that, compared with CK, the salt concentrations of 50 and 100 mmol·L^-1 increased the maximum carboxylation rate (V_{c max}) and the maximum electron transport rate (J_max), while the salt concentrations of 150 and 200 mmol·L^-1 significantly decreased V_{c max} and J_max. The net photosynthetic rate (P_n), mesophyll conductance (g_m) and dark respiration rate (R_d) gradually decreased with the increases of salt concentration. Compared with CK, the salt concentrations of 50 and 100 mmol·L^-1 did not affect g_m, but significantly decreased P_n and R_d. The salt concentrations of 150 and 200 mmol·L^-1 significantly decreased P_n, g_m and R_d, which were significantly different from the salt concentrations of 0, 50 and 100 mmol·L^-1. P_n of cotton seedlings under different salt concentrations was simulated by the FvCB model. Compared with the results from the FvCB model without considering g_m, the FvCB model with g_m improved the determination coefficient between the simulated and measured values and decreased the mean absolute error. The salinity threshold of cotton seedlings ranged between 100 and 150 mmol·L^-1. With the increases of salt concentration, the limiting factor of leaf photosynthesis changed from mesophyll conductance to impaired components of photosynthetic apparatus. The FvCB model combined g_m could improve the accuracy of photosynthesis simulation.

Key words: salt stress, FvCB model, mesophyll conductance, maximum carboxylation rate, maximum electron transport rate