Physiological Regulation and Alleviation Effect of Melatonin on Leymus Chinensis Seedlings Under Drought Stress

Drought limits forage productivity and causes physiological dysfunction in plants. Melatonin (MT) can enhance stress tolerance, but the optimal dose and the mechanisms by which it mitigates drought-induced physiological and metabolic disturbances in Leymus chinensis remain unclear. A pot experiment under controlled soil moisture was conducted to screen the optimal MT dose for alleviating drought stress in L. chinensis seedlings and to elucidate the key physiological and metabolic mechanisms involved. Adding 100 μM MT significantly improved growth and photosynthetic performance under drought (p < 0.05). Specifically, DM100 increased plant height, root length, stem diameter, aboveground fresh weight (FW), aboveground dry weight (DW), and leaf relative water content (RWC) by 51.91%, 20.95%, 38.40%, 192.57%, 192.41% and 12.52%, respectively. Gas-exchange parameters were likewise enhanced (Gs: 183.38%, Tr: 270.37%, Pn: 114.24%), whereas intercellular CO₂ concentration (Ci) decreased by 113.34% (p < 0.05). Under drought, activities of antioxidant enzymes-superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT)-were significantly elevated, and DM100 further increased these activities; conversely, drought-induced proline (Pro) accumulation was reduced by MT treatment. Untargeted metabolomics showed that drought markedly upregulated biosynthetic pathways for tryptophan, phenylalanine, phenylpropanoids and flavonoids. DM100 selectively attenuated excessive activation of tryptophan and phenylalanine metabolism, modulated phenylpropanoid/flavonoid responses, and coordinately regulated antioxidant and osmotic-adjustment metabolism. In summary, foliar MT at 100 μmol·L^-1 appears to rebalance drought-induced metabolic perturbations by selectively modulating stress-responsive pathways rather than broadly activating metabolism, thereby improving photosynthetic performance, antioxidant capacity and growth in L. chinensis.