Abstract
Brassinosteroids (BRs) elicit diverse physiological responses and ameliorate various biotic and abiotic stresses. With an aim to further explore and elaborate their role in plants subjected to abiotic stress, more specifically the heavy metal stress, the seedlings of mung bean were grown in a plant growth chamber under controlled conditions, on a sandy substratum. The seedlings were subjected to aluminium (0.0, 1.0 or 10.0 mM) stress, at 1-week-old stage and were sprayed with 0 or 10(-8) M of 24-epibrassinolide (EBL) or 28-homobrassinolide (HBL) at 14-day stage. The analysis of the plants at the completion of 3 weeks of growth revealed that the presence of aluminium in the nutrient medium caused a sharp reduction in growth (length, fresh and dry mass of root and shoot), the activity of carbonic anhydrase (E.C. 4.2.1.1), relative water content, water use efficiency, chlorophyll content and the rate of photosynthesis. However, the activity of antioxidative enzymes [catalase (E.C. 1.11.1.6), peroxidase (E.C. 1.11.1.7) and superoxide dismutase (E.C. 1.15.1 1)] in leaves and the content of proline, both in leaves and roots increased in the aluminium-stressed plants. The spray of EBL or HBL, in absence of aluminium strongly favoured the above parameters and also improved them, in the plants grown under aluminium stress. Moreover, it is also noteworthy that EBL and HBL caused a further stimulation of antioxidative enzymes and proline content, which were already enhanced by aluminium stress. This led us to the conclusion that the elevated level of proline in association with antioxidant system, at least in part, was responsible for the amelioration of Al stress in mung bean seedlings. (c) 2007 Elsevier B.V. All rights reserved.