Regulation of antioxidant enzymes by salicylic acid in cucumber leaves

Activities of the antioxidant enzymes involved in superoxide anion (O-2(radical anion)) and hydrogen peroxide (H2O2) metabolism were determined and the contents of O-2(radical anion) and H2O2 were also measured. All concentrations of salicylic acid (SA) tested (0.5, 1.0, 2.5 and 5.0 mmol/L) significantly enhanced superoxide dismutase (SOD) and peroxidase (POD) activities not only in the first treated true leaf (leaf 1) but also in the second untreated true leaf (leaf 2) of Cucumis sativus L. When the leaves were treated with 1 mmol/L SA within 6 to 72 h, the activity of POD increased by 22% to 67% in the treated leaf 1 and by 14% to 86% in the untreated leaf 2. However, no changes were observed during 3 h after treatment and at 96 h following treatment. Measurement of O-2(radical anion) and H2O2 showed that there was a significant decrease in O-2(radical anion) content and an increase in H2O2 content after SA treatment, but catalase (CAT) activity was only slightly inhibited and this suggested that the reason of the increase in H2O2 by SA treatment is not due to the inhibition of CAT but rather the increase in SOD activity. It was also found that SA at all concentrations tested could not induce new SOD isozyme but it induced 1 to 2 bands of new POD isozyme within one day after treatment. The results indicate that SA might involve in the regulation of antioxidant enzymes.