EFFECT OF CHILLING ON ACTIVATED OXYGEN-SCAVENGING ENZYMES IN LOW TEMPERATURE-SENSITIVE AND TEMPERATURE-TOLERANT CULTIVARS OF RICE (ORYZA-SATIVA L)

When seedlings of the rice (Oryza sativa L.) cultivar K-sen 4 were exposed, at the germination and the leaf stages, to 5 degrees C for 7 days (chilling), they withered after incubation at 25 degrees C (rewarming). In contrast, the cultivar Dunghan Shall showed chilling tolerance and successful growth after the rewarming. We tried to find whether the difference in cold sensitivity correlates with superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) (activated oxygen-scavenging system), NADP-dependent isocitrate dehydrogenase (NADP-ICDH) (TCA cycle), and glucose-6-phospahte dehydrogenase (G6P-DH) (pentose phosphate cycle) activities. CAT activities in both cultivars were drastically decreased by the chilling. In D. Shall, these activities were recovered and stimulated by the rewarming. However, in K-sen 4, the rewarming decreased the activities in the embryo and root at the leaf stage. For APX at the germination stage, chilling resistance was found with both cultivars, but the rewarming enhanced the activities effectively in D. Shall while not as effectively in K-sen 4. APX activities at the leaf stage in both cultivars were not affected very much by the chilling. However, only 10% of the initial activity was detected in K-sen 4 root after the rewarming. The other enzymes, SOD, GR, ICDH and G6P-DH, displayed no significant differences in cold sensitivities between the 2 cultivars. We concluded that the tolerance of rice cultivars to chilling injury is closely linked to the cold stability of CAT and APX.