Modulation of radish metabolism by zinc phytotoxicity

The effect of excess zinc on radish (Raphanus sativus L) cv. Jaunpuri was examined by growing plants in refined sand for 24 days in complete nutrient solution; on 25^th day zinc as zinc sulphate was added at 0.1 and 0.2 mM (excess) levels. A set of radish plants was also maintained at 0.001 mM Zn and served as control. After 10–12^th day of metal supply, the visible symptoms as depression in growth and irregular interveinal chlorosis of mature young leaves of zinc toxicity were observed with 0.2 mM Zn. With ageing, growth almost ceased and chlorosis intensified covering entire lamina, later brown necrotic spots appeared on chlorotic areas, enlarged, coalesced and whole leaf turned necrotic and limped down. These effects were more severe at 0.2 than 0.1 mM Zn. After 40 days of treatments, excess zinc reduced the root development, fresh weight, biomass of radish, concentration of total as well as active iron, chlorophylls a and b and activity of catalase, peroxidase, acid phosphatase and starch phosphorylase in leaves. In radish the accumulation of zinc was more in tops than in roots with application of zinc.