Influence of Exogenous Melatonin on the Physiological Traits of Camellia hainanica Seedlings Under Polyethylene Glycol-Induced Drought Stress

This study investigated the effects of exogenous melatonin (MT) on the physiological responses of <i>Camellia hainanica</i> seedlings under drought stress, using the drought-tolerant variety "Hai Da 1" and the drought-sensitive variety "Wan Hai 1" as test materials. Seedlings were treated with MT at concentrations of 0, 50, 100, 150, 200, and 250 μmol/L through irrigation, followed by drought stress induced by polyethylene glycol (PEG-6000). The results revealed that MT alleviated growth damage caused by PEG-simulated drought stress, with leaf relative conductivity and malondialdehyde (MDA) content showing an initial decrease followed by an increase as MT concentration rose. In contrast, relative water content, chlorophyll content, antioxidant enzyme activity, secondary metabolite levels, and carbohydrate content initially increased and then declined with increasing MT concentration. Treatment with 200 μmol/L MT notably reduced MDA content by 40-50%, enhanced antioxidant enzyme activity by 20-30%, and increased secondary metabolite levels by 11-42% in the drought-sensitive variety. These findings identified 200 μmol/L MT as the optimal concentration for mitigating drought stress in <i>C. hainanica</i> seedlings, providing a foundation for its practical application in <i>C. hainanica</i> production and further research into the drought resistance mechanisms mediated by MT.