Effects of deficient (20 mmol m-3 ) and sufficient (1000 mmol m-3) magnesium (Mg) supply and of varied light intensity (100 mumol m-2 s-1 to 580 mumol m-2 s-1) on paraquat-dependent chlorophyll destruction in bean (Phaseolus vulgaris) plants grown in nutrient solution were studied over a 12-d period using leaf discs or intact primary leaves. Treatment of leaf discs with 10 mmol m-3 paraquat for 15 h caused severe chlorophyll loss, especially with increasing light intensity. This chlorophyll destruction by paraquat was very much higher in Mg-sufficient than Mg-deficient leaves. The occurrence of paraquat resistance in Mg deficient leaves was already apparent after 6 d growth in nutrient solution, i.e. before any decrease in chlorophyll or growth by Mg deficiency was evident. Also, following foliar application of paraquat (10-140 mmol m-3) to intact plants, Mg-deficient plants were much more resistant to paraquat, even following longer exposure duration (72 h) and four to 14 times higher paraquat concentrations than those received by Mg sufficient plants. From experiments where exogenous scavengers of superoxide radical (O2.-), hydroxyl radical (OH.) and singlet oxygen (O-1(2)) were applied to leaf discs, it appears that O2.-, and partly, OH. are the main O2 species which contribute to chlorophyll destruction by paraquat. The results demonstrate that Mg-deficient bean plants become highly resistant to O2.--mediated and light-induced paraquat injury. The mode of this paraquat resistance is attributed to well-known stimulative effects of Mg deficiency on O2.- and H2O2 scavenging enzymes and antioxidants.