Exogenous 24-Epibrassinolide Alleviates Effects of Salt Stress on Chloroplasts and Photosynthesis in Robinia pseudoacacia L. Seedlings

The brassinosteroids (BRs) constitute a recently defined class of plant hormone that can enhance the resistance of plants to multiple stresses. However, the effects of BRs on salt-stressed woody plants, notably on photosynthesis and chloroplast ultrastructure, have received little attention. Black locust (Robinia pseudoacacia L.) seeds and seedlings were pretreated with 1.04 mu mol L-1 24-epibrassinolide (24-epiBL) by soaking and root dipping, respectively, and grown under non-saline or saline conditions (0, 100, 200mmol L-1 NaCl). Salinity stress decreased photosynthesis, chlorophyll concentration, transpiration, and stomatal conductance but also decreased the water-use efficiency, while chlorophyll fluorescence indicated a decrease in photochemical quenching and in maximum potential quantum efficiency. Indicators of oxidative stress (for example, H2O2 and antioxidant enzymes), membrane leakage, and amounts of Na+ ions in leaves and chloroplasts were increased and, at the highest stress, chloroplast ultrastructure was severely disrupted. Exogenous 24-epiBL improved membrane stability and reduced foliar Na+ levels, while substantially alleviating stress-induced changes in photosynthetic gas exchange. Improvements in chlorophyll content and indicators of oxidative stress were not as large but were still highly significant. Thylakoid membrane structure was protected. Both methods of applying 24-epiBL were effective, but root-dipped seedlings performed marginally better. The results suggest that treatment of black locust seedlings with 24-epiBL prior to planting may improve performance and aid establishment on salt-affected soils.