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

Abstract

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.