Abstract
The role of superoxide dismutase (SOD) in defense against SO2 toxicity was investigated using leaves of poplar and spinach. Young poplar leaves having five times the SOD of the old leaves were more resistant to the toxicity of SO2. Spraying spinach leaves with diethyldithiocarbamate caused a marked loss of SOD activity which resulted in a decrease in their resistance to the toxic effects of SO2. The SOD activity in poplar leaves was increased by fumigation with 0.1 ppm SO2, and this was more evident in young leaves than in old ones. The increased SOD activity was inhibited by cyanide. The poplar leaves having high SOD activity induced with SO2 fumigation were more resistant to 2.0 ppm SO2 than the control leaves. These findings suggest that SO2 toxicity is in part due to the superoxide radical and that SOD participates in the defense mechanism against SO2 toxicity.

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