24-Epibrassinolide Simultaneously Stimulates Photosynthetic Machinery and Biomass Accumulation in Tomato Plants under Lead Stress: Essential Contributions Connected to the Antioxidant System and Anatomical Structures

Abstract

Several toxic pollutants are released into the atmosphere through human activities. Among these pollutants, lead (Pb) is a non-biodegradable element that can cause reduced cell division, impact negatively on the biosynthesis of photosynthetic pigments, and lower biomass accumulation, which can lead to plant death. 24-epibrassinolide (EBR) is a plant growth regulator with broad benefits on physiological functions and biochemical responses, conferring tolerance to plants against several biotic and abiotic stresses. The experiment was randomized with four treatments, two lead concentrations (0 and 200 µM Pb, described as −Pb and +Pb, respectively) and two EBR (0 and 100 nM EBR, described as −EBR and +EBR, respectively). We detected a negative impact of Pb stress in tomato plants; however, the exogenous application of EBR induced protection on leaf anatomy and photosynthetic apparatus, mitigating the Pb impacts on growth. This steroid enhances the root and leaf structures (in root tissue, the epidermis thickness; and in the leaf, palisade parenchyma, and spongy parenchyma), improving the membrane selectivity, light energy absorption, and CO2 fixation. Applying 200 µM Pb and 100 nM EBR caused an increase in superoxide dismutase, catalase, ascorbate peroxidase, and peroxidase activity (by 26%, 18%, 25%, and 20%, respectively). Moreover, the improvements obtained on photosynthetic pigments, electron transport rate, the effective quantum yield of photosystem II photochemistry, and net photosynthetic rate prove the benefits and protection of photosynthetic apparatus, resulting in increased biomass accumulation, with increases of 95%, 115%, 74%, and 92% in leaf, root, stem, and the whole plant, respectively. Taken together, our findings confirm that EBR alleviates the damages provoked by Pb stress in tomatoes.