Differential effect of sorbitol and polyethylene glycol on antioxidant enzymes in rice leaves

Polyethylene glycol (PEG) and sorbitol (ST) have each been used in osmotically induced water stress studies in plants, however, these osmotica may not have equivalent effects in plants. The present study was designed to examine whether antioxidant enzyme responses in rice leaves are different for PEG and ST of osmotic potential -1.5 MPa. As judged by relative water content, PEG treatment resulted in a higher degree of water stress in rice leaves than ST treatment. PEG treatment markedly increased lipid peroxidation, judged by malondialdehyde content, in rice leaves. However, ST treatment had no effect on lipid peroxidation. An increase in peroxidase (POX), ascorbate peroxidase (APX) and glutathione reductase (GR) activities was observed in rice leaves treated with ST. PEG treatment had no effect on POX and APX activities and decreased GR activity in rice leaves. The decrease in superoxide dismutase activity induced by PEG was more pronounced than by ST. Cycloheximide blocked the enhanced activities of POX, APX and GR by ST, indicating de novo synthesis of the enzymes. Results suggest that ST but not PEG treatment can up-regulate antioxidant system in rice leaves.