Induction of Thermotolerance and Activation of Antioxidant Enzymes in H2O2 Pre-applied Leaves of Cucumber and Tomato Seedlings

The thermotolerance and activation of antioxidant enzymes in the leaves of cucumber and tomato seedlings were induced by foliar application of H2O2. The induction of thermotolerance by foliar application of H2O2 was detected with decreased electrolyte leakage and malondialdehyde (MDA) content and increased water-soluble protein and chlorophyll content compared with heat-stressed seedlings. The levels of H2O2 and superoxide anions in the leaves of heat-stressed seedlings were also reduced by H2O2 pre-application before exposure to heat stress. The specific activities of ascorbic peroxidase (APX) and glucose-6-phosphate dehydrogenase (G6PDH) in the leaves of cucumber and tomato seedlings were significantly increased by foliar application of H2O2. The gel activities of superoxide dismutase (SOD), hydrogen peroxidase (HPX), and polyphenol oxidase (PPO) isozymes were abruptly increased by foliar application of H2O2 and these were maintained for 10 d afterward. The gel activities of guaiacol peroxidase (GPX) in the leaves of cucumber seedlings were significantly increased by foliar application of H2O2, but this did not occur in tomato leaves. Although one or two isoforms of SOD, HPX, and PPO isozymes in tomato leaves were suppressed by foliar application of H2O2, the total gel activities of the antioxidant enzymes were increased. These results suggest that foliar application of H2O2 before a crop is subjected to heat stress may be an effective and method to alleviate heat stress.