Unraveling the roles of brassinosteroids in alleviating drought stress in young Eucalyptus urophylla plants: Implications on redox homeostasis and photosynthetic apparatus.

Water deficit is the most limiting abiotic stress to plants because it affects several physiological and biochemical processes. Brassinosteroids, including 24-epibrassinolide (EBR), are steroids that regulate growth and positively act on gas exchange. This research aims to determine whether EBR can attenuate the negative effects of water deficit, revealing possible contributions of this steroid on photosynthetic machinery of young Eucalyptus urophylla plants under water deficit. The experiment had a completely randomized factorial design with two water conditions (control and water deficit) and three levels of EBR (0, 50, and 100 nM EBR). Water deficit caused a decrease in the levels of total chlorophyll and carotenoids, but these photosynthetic pigments increased by 135 and 226%, respectively, in plants sprayed with EBR when compared to the water deficit + 0 nM EBR treatment. Regarding the antioxidant system, 100 nM EBR induced significant increments in superoxide dismutase (42%), catalase (52%), ascorbate peroxidase (147%), and peroxidase (204%). Steroid application in E. urophylla plants exposed to water deficit increased the effective quantum yield of the photosystem II (PSII) photochemistry and electron transport rate. However, interestingly, it decreased the nonphotochemical quenching and relative energy excess at the PSII level, indicating improvements related to PSII efficiency. This research revealed that application of 100 nM EBR attenuated the negative effects caused by water deficit, being explained by the positive repercussions on antioxidant enzyme activities, chloroplastic pigments, PSII efficiency, electron flux, and net photosynthetic rate.