Light‐dependence of paraquat‐initiated membrane deterioration in bean plants. Evidence for the involvement of superoxide

Treatment of primary bean leaves with 10 mg/I paraquat induces the formation of gel phase lipid in microsomal and chloroplast membranes and enhances the activity of superoxide dismutase, but only if the leaves are exposed to light. These light‐dependent changes in membrane lipid phase properties show a close temporal correlation with enhanced O 2 7 production by illuminated chloroplasts and the onset of lipid peroxidation. Malondialdehyde and ethane, which are both formed during lipid peroxidation, are produced in large amounts by paraquat‐treated leaves exposed to light, but not by those maintained in the dark. Electron spin resonance measurements indicate that production of O 2 7 by illuminated chloroplasts is more than 2‐fold greater in the presence of paraquat than in its absence. The identity of the radical formed by illuminated chloroplasts in the presence of paraquat was confirmed by using the diagnostic spin trap 5,5′‐dimethyl‐l‐pyrroline‐l‐oxide and by establishing that its formation is sensitive to superoxide dismutase. The observations collectively indicate that paraquat‐mediated membrane deterioration is light‐dependent and attributable to enhanced O 2 7 ‐production.