Enhancement of Growth and Antioxidant Enzyme Activities on Kimchi Cabbage by Melatonin Foliar Application under High Temperature and Drought Stress Conditions

Melatonin, an indoleamine molecule, regulates a wide range of physiological functions during the growth, morphogenesis, and response of plants to abiotic stresses. We examined the effect of exogenous melatonin application (5, 10, and 20 mg.L-1) on the growth and antioxidant enzyme activity of Kimchi cabbage under high temperature (air temperature 32/26 degrees C) and drought stress (soil moisture 20-24%) conditions. Application of 10 mg.L-1 melatonin alleviated stress-induced growth inhibition and increased the shoot fresh weight and the leaf surface area of seedlings by 22.7% and 13.9%, respectively, compared to the control under the stress condition. Indeed, foliar application of melatonin at 10 mg.L-1 caused an increase of net photosynthetic rate and transpiration rate under the stress condition. The lowest level of H2O2 content was also observed at 10 mg.L-1 concentration of melatonin treatment, while no significant alterations in lipid peroxidation or concentrations of malondialdehyde were detected by treatment with different concentrations of melatonin. In addition, significantly elevated activity levels of ascorbate peroxidase (APX) and superoxide dismutase (SOD) were measured at 10 mg.L-1 melatonin treatment without significant changes in ascorbic acid (AsA) and proline contents under the stress condition. These results suggest that foliar application of 10 mg.L-1 melatonin ameliorates physiological damages caused by the combination of water deficit and heat stress and enhances the activity of the antioxidant enzymes such as APX and SOD, thereby improving tolerance against the water deficit and heat stress in Kimchi cabbage.