Exogenous Melatonin Impacts Antioxidant Levels, Photosynthesis, and Aquaporin Genes in Drought-Stressed Licorice Induced by Polyethylene Glycol (PEG) 6000

Melatonin (N-acetyl-5-methoxytryptamine) has been demonstrated to play a significant role in plant stress responses. This study aimed to evaluate the effects of exogenous melatonin (50, 100 and 200 μmol/L) on growth parameters, antioxidant enzyme activity, photosynthetic performance, and aquaporin-related gene expression in licorice (Glycyrrhiza uralensis Fisch.) seedlings under PEG-induced drought stress. The results indicated that drought significantly reduced plant height, root length, leaf mass, and antioxidant enzyme activities of leaves and roots. It also inhibited photosynthesis rate, stomatal conductance (gs), and water use efficiency (WUE), disrupted the potential activity and function of PSII, and decreased water channel protein gene expression. Melatonin treatments were found to mitigate the adverse effects of drought by restoring plant growth attributes and photosynthetic efficiency, decreasing transpiration, increasing water utilization, stomatal conductance, and water channel protein gene expression. Additionally, melatonin treatments increased fluorescence parameters such as actual photochemical efficiency of PSII (ΦPSII), electron transfer rate (ETR) and photochemical quenching (qP), as well as antioxidant enzyme activities. Water conditions are positively correlated with most photosynthetic indicators and significantly associated with the expression of the majority of aquaporin (AQPs) genes. Additionally, melatonin concentration is significantly positively correlated with stomatal limitation (Ls), ΦPSII, ETR, and the expression of the GuTIP1;3 gene in roots. In conclusion, melatonin can protect photosynthetic organs under drought stress, reduce reactive oxygen species (ROS) accumulation, and thus improve the adaptability of seedlings to drought stress.