Foliar application of brassinosteroids alleviates adverse effects of zinc toxicity in radish (Raphanus sativus L.) plants

Growth chamber experiments were conducted to investigate the comparative effect of 24-epibrassinolide (EBL) and 28-homobrassinolide (HBL) at 0.5, 1.0, or 2.0 mu M concentrations by foliar application on radish plants growing under Zn2+ stress. In radish plants exposed to excess Zn2+, growth was substantially reduced in terms of shoot and root length, fresh and dry weight. However, foliar application of brassinosteroids (BRs) was able to alleviate Zn2+-induced stress and significantly improve the above growth traits. Zinc stress decreased chlorophyll a, b, and carotenoids levels in radish plants. However, follow-up treatment with BRs increased the photosynthetic pigments in stressed and stress-free plants. The treatment of BRs led to reduced levels of H2O2, lipid peroxidation and, electrolyte leakage (ELP) and improved the leaf relative water content (RWC) in stressed plants. Increased levels of carbonyls indicating enhanced protein oxidation under Zn2+ stress was effectively countered by supplementation of BRs. Under Zn2+ stress, the activities of catalase (CAT), ascorbate peroxidase (APX), and superoxidase dismutase (SOD) were increased but peroxidase (POD) and glutathione reductase (GR) decreased. Foliar spraying of BRs enhanced all these enzymatic activities in radish plants under Zn2+ stress. The BRs application greatly enhanced contents of ascorbate (ASA), glutathione (GSH), and proline under Zn2+ stress. The decrease in the activity of nitrate reductase (NR) caused by Zn2+ stress was restored to the level of control by application of BRs. These results point out that BRs application elevated levels of antioxidative enzymes as well as antioxidants could have conferred resistance to radish plants against Zn2+ stress resulting in improved plant growth, relative water content and photosynthetic attributes. Of the two BRs, EBL was most effective in amelioration of Zn2+ stress.