Brassinosteroids (BRs) have been reported to counteract various stresses. We investigated effects of exogenously applied brassinosteroid, 24-epibrassinolide (EBR), and brassinosteroid-mimic compound, 7,8-dihydro-8-20-hydroxyecdysone (DHECD), on the photosynthetic efficiency and yield of rice (Oryza sativa L. cv. Pathum Thani 1) under heat stress. Solutions (1 nM) of EBR and DHECD were separately sprayed onto foliage of individual rice plants during their reproductive stage. Five days after the application, the plants were transferred to the day/night temperature regime of 40/30ºC for 7 days and then allowed to recover at normal temperature for 7 days. We demonstrated that both DHECD and EBR helped maintain the net photosynthetic rate. The DHECD and EBR application enhanced stomatal conductance, stomatal limitation, and water-use efficiency under the high-temperature regime. DHECD-and EBR-treated plants showed an increase in the nonphotochemical quenching that was lower than that in the control plants. Moreover, DHECD and EBR treatments maintained the maximal quantum efficiency of PSII photochemistry and the efficiency of excitation capture of the open PSII center. Furthermore, the treatments with DHECD or EBR resulted in higher chlorophyll content during the heat treatment compared with the control plants. The paddy field application of 1 nM EBR and/or 1 nM DHECD at the reproductive stage during the hot season could increase the rice yield, especially, the number of filled seeds. DHECD and EBR enhanced total soluble sugar and reducing sugar in straw and more starch was accumulated in rice seeds. Consequently, our results confirmed that DHECD showed biological activities mimicking EBR in the improvement of photosynthetic efficiency and in rising the rice yield under heat stress.

The application of brassinosteroids (BRs) has been reported to alleviate heat stress. This study investigated the effect of 7,8-dihydro-8a-20-hydroxyecdysone (DHECD)-a BR mimic-by comparison with 24-epibrassinolide (EBR) on the changes in photosynthetic performance, lipid peroxidation, and rice seed set. The results demonstrated that 10 28 M EBR and 10 27 M DHECD had the best actions to counteract the lethal heat temperature of 47°C for 2 h indicated by a reduction in the number of wilted leaves and an increase in the relative water content and leaf greenness. Moreover, plants treated with EBR or DHECD were exposed to high day/night temperatures of 40/30°C for 7 days. EBR-treated and DHECD-treated plants showed a high shoot fresh weight, leaf area, chlorophyll content, and carotenoid content. High temperature significantly decreased the leaf net CO 2 assimilation rate as well as increased lipid peroxidation. The application of EBR and DHECD maintained the high level of the net CO 2 assimilation rate by increasing the stomatal conductance and photochemical quenching. On the other hand, EBR and DHECD decreased the intracellular CO 2 content and nonphotochemical quenching leading to enhance photosynthesis under heat stress. EBR-treated and DHECD-treated plants significantly reduced their malondialdehyde and hydrogen peroxide contents as well as increasing their total soluble sugar contents. Moreover, BR treatments increased the filled seed of rice. This study confirmed that DHECDa BR mimic-has activities of heat stress alleviation similar to EBR.

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