Brassinosteroids-Mediated Amelioration of Iron Deficiency in Soybean Plants: Beneficial Effects on the Nutritional Status, Photosynthetic Pigments and Chlorophyll Fluorescence

Iron (Fe) is essential for chlorophyll biosynthesis and functions in chloroplasts. Fe deficiency provokes negative effects on photochemical efficiency and electron transport. 24-Epibrassinolide (EBR) is a natural molecule with potential advantages, including a natural origin, biodegradability and high plant steroid bioactivity, improving metabolism and inducing tolerance during stress. Present study was aimed to investigate whether pre-treatment with EBR can trigger protective roles in soybean plants cultivated under the conditions of Fe deficiency and to evaluate the responses linked to the nutritional status, photosynthetic pigments and chlorophyll fluorescence. The study was carried out using a completely randomized design with four treatments (0 nM EBR + 250 µM Fe, 0 nM EBR + 2.5 µM Fe, 100 nM EBR + 250 µM Fe and 100 nM EBR + 2.5 µM Fe). Results revealed that the exogenous EBR minimized the damage caused by Fe deficiency. This steroid maximized the Fe content in the leaf, stem and root, as well as improved the nutrient content and metal homeostasis, as confirmed by the increased detection of Fe2+/Mg2+, Fe2+/Mn2+ and Fe2+/Cu2+ ratios in plants under Fe deficiency. Additionally, plants under Fe deficiency and sprayed with EBR had improvements on chloroplastic pigments, with significant increases in chlorophyll a (14%), chlorophyll b (23%), total chlorophyll (15%) and carotenoids (28%). Steroid also increased the photochemical efficiency, positively regulating electron transport and reducing the negative impacts associated with photoinhibition in photosystem II. Therefore, pre-treatment with EBR improved the nutrient contents and physiological performance of soybean plants under the conditions of Fe limitation.