Exogenous Melatonin Increases Root Yield and Its Medicinal Quality of Glycyrrhiza glabra Under Drought Stress

<i>Glycyrrhiza glabra</i> L. is an economically significant plant that naturally grows in arid regions and is widely used in the cosmetics, food, and pharmaceutical industries. Its roots are the economically important part. However, it has weak drought tolerance during the seedling stage, and water scarcity has become a major limiting factor for improving the yield and quality of cultivated licorice. Therefore, this study conducted a pot experiment in which melatonin was applied via root irrigation to examine its effects on alleviating drought stress in <i>G. glabra</i> seedlings and on enhancing the yield and quality of its valuable parts. The results showed that under drought conditions, applying 100 μM melatonin yielded the most significant improvements in both yield and quality. Specifically, melatonin treatment increased root biomass by 138.10% and significantly boosted the levels of key bioactive compounds, including glycyrrhizic acid, liquiritin, glabridin, liquiritigenin, and isoliquiritigenin by 60.51%, 72.08%, 182.03%, 83.86%,and 30.68%, respectively. This study uniquely combined the Mantel test and random forest modeling for a comprehensive analysis of the experimental data. The analysis indicated that these effects were attributable to exogenous melatonin, which markedly enhanced antioxidant enzyme activities in <i>G. glabra</i> seedlings and reduced membrane lipid peroxidation products, thereby strengthening their antioxidant defense capabilities. Additionally, melatonin promoted the accumulation of osmotic adjustment substances and effectively improved photosynthetic performance. Our research provides a scientific basis for increasing both the quality and yield of <i>G. glabra</i> under drought conditions.