Effects of NaCl stress on photosynthesis and antioxidant activity in Ficus concinna var. subsessilis

In this study, chlorophyll fluorescence, gas exchange and antioxidant activity in Ficus concinna var. subsessilis were investigated under NaCl stress. Under a low NaCl concentration (50 mmol/L) treatment for 7 d, net photosynthetic rate (Pn), stomatal conductance (Gs) and intercellular CO2 concentration (Ci) in the F. concinna var. subsessilis seedlings decreased by 24%, 31% and 30%, respectively, in comparison to the control. However, low NaCl concentrations did not decrease the efficiency of energy conversion of open PSⅡ(F′v/F′m) and the proportion of open PSII reaction centers (qP). These results indicated that the decline of Pn under low NaCl concentrations was due to the stomatal factors. Under a high NaCl concentration (200 mmol/L) treatment for 7 d, Pn, Gs, Ci, qP and F′v/F′m in the seedlings dropped by 81%, 54%, 51%, 72.7% and 87.6%, respectively, compared to the control. Additionally, high NaCl concentrations also decreased the F′v/F′m and qP, thereby resulting in an reduction in electron transport flow and Pn. The non-photochemical quenching (NPQ), MDA content, SOD and POD activities of the seedlings were significantly increased after high NaCl concentration stresses. These results showed that, though the light energy dissipation was increased, excessive excitation energy was still accumulated under high NaCl concentrations, which induces the increase of activities of antioxidant enzymes and the aggravation of membrane lipid peroxidation, while the decrease of Pn under high NaCl concentrations was due to both stomatal and non-stomatal factors.