Effects of Melatonin on the Growth of Sugar Beet (Beta vulgaris L.) Seedlings Under Drought Stress

Drought stress seriously inhibits the physiological development of agricultural plants. Melatonin regulates physiological processes and improves tolerance to stress. In our study, the physiological and biochemical mechanism of exogenous melatonin for improving drought resistance of sugar beet seedlings was studied by spraying different concentrations of melatonin solution on the leaves. The results showed that under drought stress, compared with the treatment without spraying melatonin (Drought), different concentrations of melatonin increased the dry and fresh weight of sugar beet seedlings, and 100 μmol·L−1 melatonin increased the biomass by up to 47%. Leaf relative water content and water potential value increased by 61.92% and 13.67%, respectively, which was not significantly different from to control. Exogenous melatonin also significantly decreased electrolyte leakage and malondialdehyde content in sugar beets, reduced soluble sugars, soluble proteins, and free proline contents, and maintained cell stability. In addition, melatonin increased the activities of enzymatic antioxidants in the leaves by 89.35% at the highest and maintained low reactive oxygen species under drought stress. Melatonin promoted leaf gas exchange, increased chlorophyll content, and promoted photosynthesis under drought stress. These results indicate that exogenous melatonin improves the ability of sugar beet seedlings to grow under drought stress, mainly by reducing the level of membrane lipid peroxidation, increasing the activities of antioxidant enzymes, and promoting chlorophyll synthesis. Melatonin could be applied as a plant growth regulator at a large-scale field level for plants under drought stress.