Effects of Salt Stress on Photosystem II Activity in Sweet Sorghum Seedlings Grown in Pots Outdoors

Abstract

The effects of the different NaCl treatments (100 mmol L-1, 200 mmol L-1) on photosynthetic characteristics and the photosystem Ⅱ (PSⅡ) activity in sweet sorghum seedlings grown in pots outdoors were carefully investigated in this study. The Na+ content and the ratio of Na+/K+ in leaves increased significantly with increases of NaCl concentration and treatment time. The net photosynthetic rate (Pn), the efficiency of excitation energy captured by open PSII reaction centers (Fv /Fm ), the photochemical quenching (qP) and the actual photosystem II efficiency (ΦPSII) all decreased while the non-photochemical quenching (NPQ) increased greatly with the increasing of NaCl concentration. In addition, Pn, Fv /Fm , qP, and ΦPSII had an ameliorative trend in the 100 mmol L-1 NaCl treatment, but this phenomenon did not appear in 200 mmol L-1 NaCl treatment. The maximum quantum yield of photosystem II photochemistry (Fv/Fm) was slightly affected in 100 mmol L-1 NaCl treatment, whereas distinct decreased in 200 mmol L-1 NaCl treatment. The chlorophyll fluorescence kinetic curves exhibited no changes at the beginning of NaCl treatment, while the rising speeds of O-K and O-J phases increased markedly in the 200 mmol L-1 NaCl treatment after five days. Furthermore, the photoinactivation rate of PSII did not change in both treatments while the repair rate was depressed significantly after five days treatment of 200 mmol L-1 NaCl. Therefore, we suggest that under salt stress the decrease of carbon assimilation in leaves of sweet sorghum seedlings grown in pots outdoors is mainly due to the accumulation of Na+, and salt stress alters the excited energy distribution. During salt stress outdoors, the decline of PSII activity is majorly attributed to the depression of the repair rate of the photoinactivated PSII, rather than the photoinactivation rate under high light. It is helpful to understand the mechanisms of photoinhibition in C4 crop outdoors under salt stress to some extent.