Oxidative damage caused by an excess of copper in oat leaves

The relationship between the toxicity of Cu2+ ions and oxidative reactions in plant cells was studied. Segments of leaves from 6- and 9-day-old oat seedlings were incubated in solutions that contained Cu2+ ions at various concentrations for 24 h in the light. High concentrations of Cu2+ ions caused the breakdown of chlorophyll and carotenoid, as well as an increase in membrane permeability and rates of lipid peroxidation. These effects were more pronounced in older leaves than in younger ones. Scavengers of free radicals, such as Tiron, sodium benzoate and mannitol, prevented the increases in these parameters of senescence. While Tiron was more effective in this regard in younger leaves, sodium benzoate was more effective in older ones. The treatment with Cu2+ ions enhanced the activity of superoxide dismutase, especially in the younger leaves. By contrast, Cu2+ ions decreased the activities of catalase and ascorbate peroxidase in both old and young leaves. Scavengers of free radicals protected these enzymes against inactivation. These results indicate that an excess of Cu2+ ions causes rapid senescence in plant leaves via oxidative reactions in the light. The reactions involve formation of ‘O−2, H2O2 and HO’ and a subsequent decrease in antioxidant defences which in turn enhances the efectiveness of toxic species of oxygen.