Abstract:

Gas exchange and chlorophyll fluorescence quenching kinetics were investigated to explore the effect of salt stress on carbon assimilation and photosystem Ⅱ photochemical efficiency in sweet sorghum seedlings in this study. Chlorophyll content, relative water content and membrane lipid peroxidation were not significantly changed in sweet sorghum seedlings when salt (NaCl) concentration was below 100 mmol L^-1; while, 200 mmol L^-1 NaCl treatment induced an obvious decrease in chlorophyll content, relative water content and an increase in membrane lipid peroxidation. Net photosynthetic rate (P_n) and stomatal conductance (G_s) began to decline strongly at NaCl concentration higher than 50 mmol L^-1, whereas, the intercellular CO₂ concentration (C_i) increased strikingly. During this process, the stomatal limiting value (L_s) decreased with the increase of salt treatment. More important, the depression of carbon assimilation under salt stress could not be restored by the enhancement of CO₂ concentration. The measurement of chlorophyll fluorescence showed that the initial fluorescence yield (F_o), the maximum fluorescence yield (F_m) and the maximal quantum yield of photosystem Ⅱ photochemistry (F_v/F_m) significantly went down in 200 mmol L^-1 NaCl treated seedlings. Moreover, chlorophyll fluorescence quenching kinetics parameters were not affected by 50 mmol L^-1 NaCl treatment. When NaCl concentration was above 50 mmol L^-1, the efficiency of open centers of photosystem Ⅱ (F_v’/F_m’)，photochemical quenching (q_p) and actual photosystem Ⅱ efficiency (Φ_PSⅡ) decreased sharply, while non-photochemical quenching (NPQ) increased gradually. Therefore, we suggest that the decreased net photosynthetic rate (Pn) in salt-stressed sweet sorghum seedlings is mainly due to non-stomatal limitation; the depression of carbon assimilation capacity significantly alters the excited energy distribution. Under moderate salt stress (100 mmol L^-1 NaCl), it is the enhancement of thermal dissipation that dissipate excess excited energy protecting sweet sorghum seedlings against photodamage; however, severely salt treatment (200 mmol L^-1 NaCl) causes the reduction of light absorption and improvement of thermal dissipation for keeping the balance in light capture and utilization.

Key words: Salt-stress, Photosynthesis, PhotosystemⅡ, Sweet sorghum