Na+ accumulation alleviates drought stress induced photosynthesis inhibition of PSII and PSI in leaves of Medicago sativa

To probe the mechanism of action of NaCl in regulating drought adaptation in Medicago sativa, we pretreated M. sativa with different concentrations (0, 50, 100 and 200 mM) of NaCl, then studied the effects of NaCl pretreatment on water content, chlorophyll content, photosynthetic function, and membrane peroxidation of M. sativa plants subjected to drought stress. Although pretreatment with different concentrations of NaCl significantly increased the Na+ content in the aboveground of M. sativa, it had no significant effect on the activity of PSII and PSI. However, different concentrations of NaCl pretreatment significantly increased the water content of the aboveground of M. sativa under drought stress, alleviated the degree of photoinhibition of PSII and PSI, and maintained the flow and stability of donor and acceptor electrons in PSII. NaCl pretreatment promotes the increase of non-photochemical quenching (NPQ), which relieves oxidative damage in the leaves of M. sativa under drought stress, playing an important role in maintaining the activity of PSII and PSI. In conclusion, pretreatment with a certain concentration of NaCl can help M. sativa leaves maintain relatively high-water content and alleviate the photosynthesis inhibition of PSII and PSI under drought stress, thereby improving the drought resistance of M. sativa.