Anatomical Modifications Modulated by Pretreatment with 24-Epibrassinolide Alleviate Boron Stress in Soybean Plants: Valuable Repercussions on Nutrient Contents, Photosynthesis, and Biomass

Boron (B) is an essential micronutrient for vascular plants, and its availability in the soil represents a limiting factor for world agricultural production. 24-Epibrassinolide (EBR) is a biodegradable and eco-friendly plant steroid hormone, with multiple benefits connected to growth and development. We hypothesized that inadequate B supplementation could limit plant development, causing damage to leaf and root structures. Therefore, the objective of this research was to verify the possible contributions of EBR in root and leaf structures and biomass accumulation in soybean plants under inadequate B supplies (deficiency or toxicity). 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) combined with three B supplies (0.6, 30, and 1500 µM B, described as low, control, and high supply of B). EBR alleviated the damages occasioned by the inadequate B supplies on root tissues, specifically maximizing the vascular cylinder, metaxylem, and epidermis, improving the nutritional status. This steroid also minimized the harmful effects of B stress on leaf anatomy, stimulating the epidermis on both leaf sides, palisade parenchyma, and spongy parenchyma; structures intrinsically related to protection and carbon dioxide availability to the photosynthetic process. Concomitantly, this steroid had a positive impact on biomass accumulation. These results are explained by beneficial actions on leaf structures and photosynthetic machinery. Therefore, our results demonstrate that the EBR application can improve soybean plants’ tolerance under inadequate B supplementation.