24-Epibrassinolide Positively Modulate Leaf Structures, Antioxidant System and Photosynthetic Machinery in Rice Under Simulated Acid Rain

Acid rain causes significant damage to rice plants, affecting the photosynthetic machinery and growth. 24-epibrassinolide (EBR) is an organic steroid, which, used at low concentrations, positively regulates plant growth and mitigates deleterious effects related to environmental changes. The aim of this study was to investigate whether exogenous treatment with 24-epibrassinolide can alleviate the negative effects of simulated acid rain (SAR) and the possible tolerance mechanism involved by evaluating chlorophyll fluorescence, gas exchange, the antioxidant system and leaf anatomical variables. The experiment was randomized with four treatments: two simulated acid rain treatments (0 and 0.5 M H2SO4, described as − SAR and + SAR, respectively) and two concentrations of brassinosteroids (0 and 100 nM EBR, described as − EBR and + EBR, respectively). Our results show that plants exposed to SAR suffered negative interferences; however, EBR-treated plants presented benefits related to chlorophyll fluorescence, alleviation of the photoinhibition in photosystem II and protection against damage caused by the imbalance of reactive oxygen species. Additionally, EBR promoted increases in gas exchange that were clearly linked to stomatal regulation, improving the uptake and distribution of CO2 in intercellular spaces. This research revealed that EBR attenuated the negative effects of SAR, increasing the activities of superoxide dismutase (12%), catalase (17%), ascorbate peroxidase (17%) and peroxidase (33%), reducing thylakoid membrane damage, as confirmed by increases in chlorophyll and carotenoids. Finally, the effects of EBR observed in plants under SAR demonstrate that this substance positively modulated important anatomical structures linked to leaf protection, increasing trichome density (8%), epicuticular wax (48%) and the aerenchyma area (100%). These results provide evidence that EBR confers tolerance to rice plants exposed to SAR.