Abstract: To investigate the effects of exogenous melatonin (MT) on the growth of alfalfa (Medicago sativa) under drought stress, this study used the cultivar ‘Zhongmu 3’ alfalfa as experimental material. Under drought stress treatments, potted alfalfa seedlings were root-irrigated with different concentrations of melatonin. Phenotypic and physiological-biochemical indicators of seedlings were measured to further explore the optimal concentration of MT for alleviating drought stress. The results showed that treatment with 90 μmol·L^-1 melatonin significantly increased the plant height, stem thickness, relative water content, and relative chlorophyll content of the alfalfa seedlings. In addition, 90 μmol·L^-1 MT treatment significantly increased the antioxidant enzyme activities and the gene expression levels of the enzymes in the leaves of alfalfa seedlings under drought stress. The treatment with 90 μmol·L^-1 MT also reduced the contents of proline, soluble sugar, malondialdehyde, electrolyte leakage, and hydrogen peroxide content. Therefore, root irrigation of 90 μmol·L^-1 MT is helpful to improve drought resistance of alfalfa seedlings. This study provides certain theoretical support for planting efficient forage alfalfa in arid areas.

Key words: Alfalfa, Drought, Melatonin, Osmotic adjustment, Antioxidase

摘要： 为探究褪黑素对干旱胁迫下紫花苜蓿（Medicago sativa L.）生长的影响，本研究以‘中苜3号’紫花苜蓿为试验材料，设置正常供水和干旱2个水分梯度，采用盆栽试验根灌不同浓度褪黑素，分析不同条件下幼苗表型、生理生化指标的变化。结果表明：90 μmol·L^-1褪黑素处理能够显著提高紫花苜蓿幼苗株高、茎粗、相对含水量、相对叶绿素含量，此外，90 μmol·L^-1褪黑素处理显著增加干旱胁迫下紫花苜蓿幼苗叶片抗氧化酶活性及酶合成相关基因表达水平，并降低脯氨酸、可溶性糖含量及丙二醛、相对电导率、过氧化氢含量。综上所述，褪黑素提高干旱胁迫下紫花苜蓿幼苗抗氧化酶生物合成基因表达水平，增强抗氧化酶活性，清除过量活性氧，缓解膜损伤，提高幼苗抗旱性。

关键词: 紫花苜蓿, 干旱, 褪黑素, 渗透调节, 抗氧化酶