Abstract: 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.

Key words: Alfalfa, Melatonin, Low phosphorus stress, Peroxidase

摘要： 为研究外源褪黑素（Melatonin，MT）对低磷胁迫下紫花苜蓿（Medicago sativa）生长及生理特性的影响，以种质‘7890’和‘15578’为材料，采用营养液砂培法进行常磷（Normal phosphorus，NP 1.0 mmol·L^-1 KH₂PO₄）和低磷（Low phosphorus，LP 0.01 mmol·L^-1 KH₂PO₄）胁迫，对LP胁迫下苜蓿幼苗根灌5种不同浓度MT，测定其形态及生理指标。结果表明：90 μmol·L^-1 MT缓解苜蓿低磷胁迫效果最佳，其株高、叶面积、地上及地下生物量、相对含水量、根系活力、蒸腾速率、气孔导度、净光合速率、叶绿素a、叶绿素b、类胡萝卜素、叶绿素、脯氨酸、可溶性糖和可溶性蛋白含量及超氧化物歧化酶、过氧化物酶、过氧化氢酶活性均显著升高，丙二醛和胞间CO₂浓度显著降低。主成分分析表明，叶绿素含量、可溶性糖含量、叶片气孔导度可作为MT对低磷胁迫下苜蓿生长发育的关键指标。综上，外源MT能增强LP胁迫下苜蓿幼苗抗氧化酶活性和渗透调节物质含量，提高其耐低磷性。

关键词: 苜蓿, 褪黑素, 低磷胁迫, 过氧化物酶