24-Epibrassinolide Improves Root Anatomy and Antioxidant Enzymes in Soybean Plants Subjected to Zinc Stress

The aim of this research was to determine whether 24-epibrassinolide can mitigate oxidative stress in soybean plants subjected to different zinc levels; to examine this, we evaluated the possible repercussions on anatomical, nutritional, biochemical, physiological and morphological behaviours. The experiment followed a completely randomized factorial design with two concentrations of 24-epibrassinolide (0 and 100 nM EBR, described as - EBR and+EBR, respectively) and three zinc supplies (0.2, 20 and 2000 mu M Zn, described as low, control and a high supply of Zn). In general, low and high zinc supplies produced deleterious effects. However, plants exposed to high zinc +100 nM EBR presented increases of 25%, 7%, 9% 29% and 69% for root epidermis, root endodermis, root cortex, vascular cylinder and metaxylem, respectively, when compared to the same treatment without the steroid. The steroid spray alleviated the impact produced by zinc stress on nutritional status, and these results were intrinsically linked to incremental changes in root structure, mainly vascular cylinder and metaxylem. Antioxidant enzymes play crucial roles in the photosynthetic machinery of plants treated with 24-epibrassinolide and stressed by high and low zinc supply, modulating reactive oxygen species scavenging and protecting the chloroplast membranes, with clear positive repercussions on photosystem II efficiency and photosynthetic pigments. The stimulation induced by this steroid on gas exchange can be explained by the favourable conditions detected in stomatal performance and leaf anatomy, thus enhancing the diffusion of carbon dioxide.