LIGHT AND EXCESS MANGANESE - IMPLICATIONS FOR OXIDATIVE STRESS IN COMMON BEAN

The effect of light intensity on antioxidants, antioxidant enzymes, an d chlorophyll content was studied in common bean (Phaseolus vulgaris L .) exposed to excess Mn. Leaves of bean genotypes contrasting in Mn to lerance were exposed to two different light intensities and to excess Mn; light was controlled by shading a leaflet with filter paper. After 5 d of Mn treatment ascorbate was depleted by 45% in leaves of the Mn -sensitive genotype ZPV-292 and by 20% in the Mn-tolerant genotype CAL IMA. Nonprotein sulfhydryl groups and glutathione reductase were not a ffected by Mn or light treatment. Ten days of Mn-toxicity stress incre ased leaf ascorbate peroxidase activity of cv ZPV-292 by 78% in low li ght and by 235% in high light, and superoxide dismutase activity follo wed a similar trend. Increases of ascorbate peroxidase and superoxide dismutase activity observed in cv CALIMA were lower than those observe d in the susceptible cv ZPV-292. The cv CALIMA had less ascorbate oxid ation under excess Mn-toxicity stress. Depletion of ascorbate occurred before the onset of chlorosis in Mn-stressed plants, especially in cv ZPV-292. Lipid peroxidation was not detected in floating leaf discs o f mature leaves exposed to excess Mn. Our results suggest that Mn toxi city may be mediated by oxidative stress, and that the tolerant genoty pe may maintain higher ascorbate levels under stress than the sensitiv e genotype.