Responses of Photosynthesis, Chloroplast Ultrastructure, and Antioxidant System of Morinda Officinalis How. To Exogenous 2, 4-Epibrassinolide Treatments Under High Temperature Stress

The present study attempts to evaluate the effects of EBR (2, 4-epibrassinolide) on photosynthetic parameters, biochemicals, antioxidant systems, and chloroplast ultrastructure in the leaves of Morinda officinalis under high temperature (HT).HT stress significantly reduced the net photosynthetic rate (PN), stomatal conductance (Gs), and transpiration rate) (Tr), photosynthetic pigments, and inhibited photochemical activity.Besides, the application of EBR alleviated the decrease in chlorophyll contents and the inhibition of photosynthesis induced by HT stress and improved photosystem II efficiency.Furthermore, HT stress markedly increased reactive oxygen species levels and lipid per-oxidation, while the application of 0.5-1.0mg L -1 EBR inhibited their increase and enhanced the activity of anti-oxidative enzymes.Microscopic analyses revealed that HT stress induced damages to chloroplasts and thylakoid membranes, displaying chloroplast envelopes disrupted, grana lamellae blurred and stroma lamellar disordered, while 0.5-1.0mg L -1 EBR treatment effectively protected the thylakoid membrane structure from HT stress, maintained the typical shape of chloroplasts, and promoted the formation of grana.It can be concluded that 0.5-1.0mg L -1 EBR can reduce the harmful effects of HT on M. officinalis seedlings by improving photosynthesis and protecting the photosynthetic membrane system from oxidative damage through up-regulating the capacity of antioxidant system.