Effects of 24-epibrassinolide on the photosynthetic characteristics, antioxidant system, and chloroplast ultrastructure in Cucumis sativus L. under Ca(NO3)2 stress

The effects of 0.1 μM 24-epibrassinolide (EBL) on plant growth (plant height, leaf area, fresh weight, and dry weight), chlorophyll content, photosynthetic characteristics, antioxidant enzymes, and chloroplast ultrastructure were investigated using cucumber seedlings (Cucumis sativus L. cv. Jinyou No. 4) with 80 mM Ca(NO3)2 to induce stress. The presence of Ca(NO3)2 caused significant reductions in net photosynthetic rate (PN), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr) of leaves. In addition, Ca(NO3)2 markedly reduced the chlorophyll content and inhibited photochemical activity, including the actual photochemical efficiency (ΦPSII). In contrast, EBL increased the chlorophyll content, especially chlorophyll b, and minimized the harmful effects on photosynthesis caused by the Ca(NO3)2. The application of EBL to the plants subjected to Ca(NO3)2-enhanced photochemical activity. EBL protected the photosynthetic membrane system from oxidative damage due to up-regulating the capacity of the antioxidant systems. Microscopic analyses revealed that Ca(NO3)2 affected the structure of the photosynthetic apparatus and membrane system and induced damage of granal thylakoid layers, while EBL recovered the typical shape of chloroplasts and promoted the formation of grana. Taken together, EBL compensated for damage/losses by Ca(NO3)2 due to the regulation of photosynthetic characteristics and the antioxidant system.