Enhancing Drought Tolerance and Fruit Characteristics in Tomato through Exogenous Melatonin Application

Drought stress not only affects the growth and development of tomato seedlings but also leads to a significant decrease in tomato fruit yield. Previous studies have shown that melatonin plays a crucial role in regulating plant tolerance to drought stress. The present study was conducted to investigate the impact of exogenous melatonin on the growth and development of tomato seedlings under drought stress, as well as its potential in improving fruit yield and quality. Our findings demonstrate that drought stress strongly suppressed growth and biomass accumulation, reduced photosynthetic pigments, and inhibited photosynthesis. Conversely, melatonin treatment led to a notable increase in plant height, stem diameter, aboveground biomass, and relative water content of tomato seedlings by 16.67%, 7.39%, 10.58%, and 13.31%, respectively, compared to the drought treatment. Moreover, the chlorophyll content increased by 40.51%, and the net photosynthetic rate increased by 1.2 times. Furthermore, the application of melatonin under drought stress resulted in a decrease in osmoregulation substances, reduced accumulation of reactive oxygen species, and enhanced activity of antioxidant enzymes in tomato seedlings. Exogenous melatonin was also found to inhibit the expression of abscisic-acid-synthesis-related genes, resulting in a reduction in the abscisic acid content in tomato seedlings. Additionally, it significantly increased the root length, root surface area, and root vitality of the plants. When compared to drought treatment, tomato plants treated with melatonin exhibited a 61.92% increase in average yield and a 37.79% increase in fruit weight per plant. Furthermore, the organic acid content decreased by 23.77%, while soluble solids and sugars increased by 15.07% and 35.49%, respectively. These findings suggest that exogenous melatonin effectively alleviates the inhibition of photosynthesis and growth in tomato seedlings under drought stress. It achieves this by regulating the content of osmotic stress substances and the activity of antioxidant enzymes, thus enhancing the resistance of tomato seedlings to drought stress. Moreover, melatonin regulates root growth by mediating the biosynthesis of endogenous ABA, thereby improving the absorption and utilization efficiency of water and nutrients in plants. Consequently, it enhances tomato fruit yield and quality under drought stress.