Antioxidative Enzymes Offer Protection from Chilling Damage in Rice Plants

Rice (Oryza sativa L.) is a tropical crop, but is also grown in temperate regions in late spring to summer. Cold temperature damage is a common problem for early-planted rice in temperate countries. Physiological responses to chilling, including antioxidative enzyme activity, were investigated in rice to identify mechanisms of chilling tolerance. Plants were exposed to 15°C (cold-acclimated) or 25°C (nonacclimated) for 3 d, under 250 μmol m ^-1 s^-1 photosynthetically active radiation (PAR). All plants were then exposed to chilling temperature at 5°C for 3 d and allowed to recover at 25°C for 5 d. Leaf fresh weight, relative water content, lipid peroxidation, chlorophyll a fluorescence, and quantum yield showed that cold-acclimated leaves were less affected by chilling compared to nonacclimated leaves. Cold-acclimated leaves also recovered faster from chilling injury than non-acclimated leaves. We analyzed the isozyme profile and activity of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR). Significant induction of expression and activity of antioxidative enzymes CAT and APX in leaves and SOD, CAT, APX, and GR in roots were observed. We deduced that CAT and APX are most important for cold acclimation and chilling tolerance. Increased activity of antioxidants in roots is more important for cold tolerance than increased activity in shoots. Chilling-sensitive rice plants can be made tolerant by cold acclimation.