Concerted action of antioxidant enzymes and curtailed growth under zinc toxicity in Brassica juncea

Investigations were carried out to study the enzymatic detoxification of high concentration of oxygen species in shoots of Brassica juncea (Brassicaceae) seedlings raised on mineral growth medium with various concentrations of zinc sulphate (0.007, 0.05, 5 and 10 mM). Zinc at 0.05 mM promoted growth of seedlings, but at 5 and 10 mM caused significant reduction in growth. Moreover, zinc at and above 5 mM brought over 2.5-fold enhancement in malondialdehyde content (an index of lipid peroxidation) over that of control, indicating that free radical generation was accelerated in plants exposed to toxic levels of zinc. The shoots of seedlings raised in the presence of toxic levels of zinc showed an enzyme-specific increase in the activities of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), guaiacol peroxidase (EC 1.11.1.7), ascorbate peroxidase (EC 1.11.1.11), monodehydroascorbate reductase (EC 1.6.5.4), dehydroascorbate reductase (EC 1.8.5.1) and glutathione reductase (EC 1.6.4.2). Concentrations of both ascorbate and glutathione were significantly enhanced in seedlings raised at toxic levels of zinc. These results suggest that B. juncea seedlings try to counteract high concentrations of oxygen species produced under zinc toxicity through a co-ordinated increase in the activities of enzymes involved in their detoxification. (C) 1999 Elsevier Science B.V. All rights reserved.