To investigate the effects of exogenous Melatonin （MT） on the growth and physiological characteristics of alfalfa （Medicago sativa） under low phosphorus stress， germplasms ‘7890’ and ‘15578’ were used as materials. The normal phosphorus （NP， 1.0 mmol·L^-1 KH₂PO₄） and low phosphorus （LP， 0.01 mmol·L^-1 KH₂PO₄） stresses were imposed by nutrient solution sand culture method. Five different concentrations of MT were applied to the roots of alfalfa seedlings under LP stress to determine their morphology and physiological indicators. The results show that： 90 μmol·L^-1 MT has the best effect in alleviating low phosphorus stress in alfalfa. The plant height， leaf area， aboveground and underground biomass， relative water content， root vitality， transpiration rate， stomatal conductance， net photosynthetic rate， contents of chlorophyll a， chlorophyll b， carotenoids， chlorophyll， proline， soluble sugar and soluble protein， as well as the activities of superoxide dismutase， peroxidase and catalase were all significantly increased. The concentrations of malondialdehyde and intercellular CO₂ were significantly reduced. Principal component analysis indicated that chlorophyll content， soluble sugar content， and leaf stomatal conductance could be used as key indicators for the growth and development of alfalfa under low phosphorus stress by MT. In conclusion， exogenous MT can enhance the activity of antioxidant enzymes and the content of osmotic regulatory substances in alfalfa seedlings under LP stress， and improve their tolerance to low phosphorus.

Alfalfa  /  Melatonin  /  Low phosphorus stress  /  Peroxidase