Brassinosteroid enhanced the level of antioxidants under cadmium stress in Brassica juncea

The change in plant growth, photosynthesis, carbonic anhydrase, nitrate reductase and antioxidative enzymes resulting from the feeding of cadmium and/or 28-homobrassinolide (HBL) to Brassica juncea were studied in 60-day-old plants. One-week old seedlings were supplied with 50, 100 or 150 μM of cadmium along with the nutrient solution. Subsequent seedlings, at day 30, were sprayed with 0.01 μM of HBL to their foliage. The plants fed with cadmium alone exhibited a decline in growth, the levels of carbonic anhydrase (E.C. 4.2.1.1) and chlorophyll pigments and net photosynthetic rate. Moreover, nitrate content, the activity of nitrate reductase (E.C. 1.6.6.1) and the level of carbohydrate, both in the leaves and roots, decreased as the concentration of cadmium, in nutrient solution, increased. Compared with the leaves, roots possessed a larger quantity of nitrate. However, the trend was reversed in case of nitrate reductase and the level of carbohydrates in the two plant organs. The toxic effect, generated by cadmium was overcome if the stressed plants were sprayed with HBL.The activities of antioxidative enzymes [viz. catalase (E.C. 1.11.1.6), peroxidase (E.C. 1.11.1.7) and superoxide dismutase (E.C. 1.15.1.1)] and the contents of proline increased, over the control, irrespective of the treatment. Their level increased further, if the plants supplied with cadmium were also supplemented with HBL, both in the roots and the aerial parts. Nevertheless, the contents of proline in roots were higher than the leaves.