The influence of varied Mg supply (10-1000 micromolar) and light intensity (100-580 microeinsteins per square meter per second) on the concentrations of ascorbate (AsA) and nonprotein SH-compounds and the activities of superoxide dismutase (SOD; EC 1.15.11) and the H202 scavenging enzymes, AsA peroxidase (EC 1.11.1.7), dehydroascorbate reductase (EC 1.8.5.1), and glutathione reductase (EC 1.6.4.2) were studied in bean (Phaseolus vulgaris L.) leaves over a 13-day period. The concentrations of AsA and SH-compounds and the activities of SOD and H202 scavenging enzymes increased with light intensity, in particular in Mg-deficient leaves. Over the 12-day period of growth for a given light intensity, the concentrations of AsA and SH-compounds and the activities of these enzymes remained more or less constant in Mg-sufficient leaves. In contrast, in Mg-deficient leaves, a progressive increase was recorded, particularly in concentrations of AsA and activities of AsA peroxidase and glutathione reductase, whereas the activities of guaiacol peroxidase and catalase were only slightly enhanced. Partial shading of Mgdeficient leaf blades for 4 days prevented chlorosis, and the activities of the°2-and H202 scavenging enzymes remained at a low level. The results demonstrate the role of both light intensity and Mg nutritional status on the regulation of 02-and H202 scavenging enzymes in chloroplasts.Photoreduction of molecular 02 in chloroplasts is unavoidable and leads to the production of superoxide anion radicals (02 -) and H202 (1, 29). In nonstressed chloroplasts, photoreduction of 02 is estimated to be between 5 and 27% of the total electron flow (2, 3). Superoxide anion radicals, H202, and their derivatives hydroxyl radicals (OH,) and singlet oxygen ('02) are highly toxic, resulting in destructive effects on the functional and structural integrity of chloroplasts (8,17). As protection against these toxic 02 species, chloroplasts are equipped with several antioxidants and defense enzymes.SOD,2 mostly localized in chloroplasts in leaves (19)