Lead toxicity induces lipid peroxidation and alters the activities of antioxidant enzymes in growing rice plants

Abstract

When seedlings of two rice ( Oryza sativa L .) cultivars were raised in sand cultures under 500 and 1000 μM Pb(NO 3) 2 in the medium, lengths as well as weights of roots and shoots decreased with increase in Pb concentration. Pb-treated seedlings showed elevated levels of lipid peroxides with a concomitant increase in the activities of the enzymes superoxide dismutase (SOD), guaiacol peroxidase, ascorbate peroxidase and glutathione reductase compared to controls. Though Pb was readily absorbed by growing seedlings, its localization was greater in roots than shoots. The level of Pb accumulation in seedlings was far higher than the supplied one. Seedlings grown for 5–20 days in presence of 1000 μM Pb(NO 3) 2 showed about 21–177% increase in the level of thiobarbituric acid reacting substances (TBARS) in shoots indicating enhanced lipid peroxidation compared to controls. With increase in the level of Pb treatment in situ peroxidases showed more increase in activity than SOD. Under both controls as well as Pb treatments roots maintained higher activity of these enzymes than shoots. About 87–100% increase in SOD activity, 1.2–5.6 times increase in guaiacol peroxidase activity and 1.2–1.9 times increase in ascorbate peroxidase activity was observed in the roots of seedlings grown for 15 days in presence of 1000 μM Pb in the medium. Under similar treatment conditions about 128–196% increase in glutathione reductase activity was recorded in roots and 69–196% increase in shoots compared to control grown seedlings. Pb treatment resulted in a decline in catalase activity in roots whereas in shoots catalase activity increased in seedlings grown at moderately toxic Pb (500 μM) level whereas a highly toxic Pb (1000 μM) level led to a marked inhibition in enzyme activity. Two catalase isoforms were detected in roots and three in shoots of the seedlings. A highly toxic Pb (1000 μM) level led to decrease in the intensity of two pre-existing catalase isoforms in shoots. Results suggest that Pb induces oxidative stress in growing rice plants and that SOD, peroxidases and GR could serve as important components of antioxidative defense mechanism against Pb induced oxidative injury in rice.