Enhanced peroxidase activity in rice leaves in response to excess iron, copper and zinc.

The effect of excess Fe(2+) on the peroxidase (POD) activity in detached rice leaves was investigated. FeSO(4) was effective in stimulating POD activity in detached rice leaves under both light and dark conditions. FeSO(4) but not K(2)SO(4) induced POD activity, indicating that POD activity is induced by Fe(2+). FeSO(4)-induced POD activity is not specific for the rice cultivar used in this study. CuSO(4) and ZnSO(4) were also observed to induce POD activity in detached rice leaves. Cycloheximide blocked the enhanced activity of POD by Fe(2+), Cu(2+) or Zn(2+), indicating de novo biosynthesis of the enzyme. Paraquat treatment resulted in a decrease in POD activity. H(2)O(2) had no effect on POD activity in detached rice leaves. It seems that Fe(2+)-, Cu(2+)- or Zn(2+)-induced POD may not be mediated by free radicals. Using isoelectric focusing to separate POD, it was found that excess Fe(2+), Cu(2+) or Zn(2+) induced both quantitative and qualitative metal-specific changes in POD isozyme pattern in detached rice leaves. A new POD isozyme with a pI of 4.81 can be induced by Fe(2+), Cu(2+) and Zn(2+) in detached rice leaves.