The effect of phosphorus (P), potassium (K), and magnesium (Mg) deficiency on the development of leaf symptoms (chlorosis and necrosis) and activities of ascorbate-dependent H2O2 scavenging enzymes (ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase) was studied in bean (Phaseolus vulgaris) plants over a 12 d period of growth in nutrient solution. With increasing plant age Mg- and K-deficient leaves developed severe interveinal chlorosis and, accordingly, chlorophyll concentrations were reduced. However, in P-deficient leaves neither chlorosis nor necrosis appeared; the leaves remained dark green and even at an advanced stage of P deficiency, chlorophyll concentrations were still higher than those of control plants. In K- and, particularly, Mg-deficient leaves with an increase in severity of leaf chlorosis, activity of ascorbate-dependent H2O2- scavenging enzymes was progressively increased. In contrast, in P-deficient leaves, as in leaves of the control plants, activity of H2O2-scavenging enzymes remained at a low level over the 12 d period. Accordingly, compared with P-deficient and control plants, Mg- and K-deficient leaves with elevated anti-oxidative potential showed much higher resistance to chlorophyll destruction by the herbicide paraquat. Elevated levels of H2O2-scavenging enzymes in Mg- and K-deficient leaves indicate a higher production of H2O2 and related toxic O-2 species. It is suggested that in Mg- and K-deficient leaves, utilization of photoreductants in CO2 fixation is restricted because of impaired export and thus accumulation of photosynthates. This disturbance might lead to enhanced photoreduction of molecular O-2 to toxic O-2 species causing chlorophyll destruction (chlorosis), a process which is not important in P-deficient leaves where export of sucrose is not affected.