Brassinosteroids alleviate heat-induced inhibition of photosynthesis by increasing carboxylation efficiency and enhancing antioxidant systems in Lycopersicon esculentum

To investigate the effects of exogenously applied brassinosteroids on the thermotolerance of plants, leaf CO2 assimilation, chlorophyll fluorescence parameters, and antioxidant enzyme metabolism were examined in tomato (Lycopersicon esculentum Mill. cv. 9021) plants with or without 24-epibrassinolide (EBR) application. Tomato plants were exposed to 40/30C for 8 days and then returned to optimal conditions for 4 days. High temperature significantly decreased the net photosynthetic rate (P (n)), stomatal conductance (G(s)), and maximum carboxylation rate of Rubisco (V-cmax), the maximum potential rate of electron transport contributed to ribulose-1,5-bisphosphate (RuBP), as well as the relative quantum efficiency of PSII photochemistry (phi(PSII)), photochemical quenching (q(P)), and increased nonphotochemical quenching (NPQ). However, only slight reversible photoinhibition occurred during heat stress. Interestingly, EBR pretreatment significantly alleviated high-temperature-induced inhibition of photosynthesis. The activities of antioxidant enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPOD), and catalase (CAT) increased during heat treatments, and these increases proved to be more significant in EBR-treated plants. EBR application also reduced total hydrogen peroxide (H2O2) and malonaldehyde (MDA) contents, while significantly increasing shoot weight following heat stress. It was concluded that EBR could alleviate the detrimental effects of high temperatures on plant growth by increasing carboxylation efficiency and enhancing antioxidant enzyme systems in leaves.