24-epibrassinolide induced antioxidative defense system of Brassica juncea L. under Zn metal stress

The present study deals with the effects of 24-epibrassinolide on growth, lipid peroxidation, antioxidative enzyme activities, non-enzymatic antioxidants and protein content in 30 days old leaves of Brassica juncea (var. PBR 91) under zinc metal stress in field conditions. Surface sterilized seeds of B. juncea were given pre-soaking treatments of 24-EBL (10(-10), 10(-8) and 10(-6) M) for 8 h. Different concentrations of zinc metal in the form of ZnSO4.7H2O (0, 0.5, 1.0, 1.5 and 2.0 mM) were added in the soil kept in experimental pots. Seeds soaked in 24-EBL for 8 h were sown in the earthern pots containing different concentrations of Zn metal. After 30 days of sowing, the plants were analyzed for growth parameters in terms of shoot length and number of leaves. Thereafter, leaves were excised and content of proteins, non-enzymatic antioxidants, malondialdehyde (MDA) and the activities of antioxidative enzymes (superoxide dismutase (SOD) (EC 1.15.1.1) catalase (CAT) (EC 1.11.1.6), ascorbate peroxidase (APOX) (EC 1.11.1.11), guaiacol peroxidase (POD) (EC 1.11.1.7) glutathione reductase (GR) (EC 1.6.4.2), monodehydroascorbate reductase (MDHAR) (EC 1.1.5.4) and dehydroascorbate reductase (DHAR) (EC 1.8.5.1)) were analyzed. It was observed that the growth of plants was inhibited under Zn metal stress. However, 24-EBL seed-presoaking treatment improved the plant growth in terms of increase in shoot length. 24-EBL also mitigated the toxicity of Zn metal by increasing the number of leaves. The activities of antioxidative enzymes (SOD, CAT, POD, GR, APOX, MDHAR and DHAR) and contents of proteins and glutathione were also enhanced in leaves of plants treated with 24-EBL alone, 10(-8) M concentration being the most effective. The activities of antioxidative enzymes also increased in leaves of B. juncea plants by the application 24-EBL supplemented Zn metal solutions. Similarly, the content of proteins and glutathione increased considerably in leaves of B. juncea plants treated with 24-EBL, whereas the level of MDA content decreased in 24-EBL treated plants as compared to untreated control plants thereby revealing stress-protective properties of the brassinolide.