Brassinosteroids Confer Tolerance to Salt Stress in Eucalyptus urophylla Plants Enhancing Homeostasis, Antioxidant Metabolism and Leaf Anatomy

Abstract

The sodium ion (Na+) is potentially toxic to plants because it can cause K+/Na+ imbalances, oxidative stress and negatively affect growth. Brassinosteroids (BRs), including 24-epibrassinolide (EBR), is an organic substance, biodegradable and positively contributes to plant metabolism. The aim of this research was to investigate whether EBR application via the leaves can enhance homeostasis and to examine the impacts of EBR on the anatomical, physiological, biochemical and morphological behaviours of young Eucalyptus urophylla plants exposed to salt stress. The experiment had four treatments: two salt conditions (0 and 250 mM NaCl, described as − Na+ and + Na+, respectively) and two concentrations of 24-epibrassinolide (0 and 50 nM EBR, described as − EBR and + EBR, respectively). The results suggest that EBR mitigated the deleterious effects caused by salt stress in young E. urophylla plants, thereby improving homeostasis related to the K+/Na+ ratio and increasing the nutrient contents of the tissues. Plants exposed to both Na+ and EBR showed increases in photosynthetic pigments and photochemical efficiency. This result may be a result of the antioxidant system, specifically, the significant increases in the CAT (20%) and APX (51%) enzymes, which were comparable to plants receiving equal treatment without Na+. Additionally, this steroid had benefits for gas exchange and growth that were associated with leaf anatomy and were confirmed by increases in stomatal density (23%), palisade parenchyma (14%) and spongy parenchyma (25%). Therefore, our results confirm that the exogenous application of EBR resulted in tolerance to salt stress.