Drought stress is a major factor limiting agricultural development, and exogenous polyamines (PAs) can increase plant drought resistance by enhancing antioxidant activity, but few studies have examined whether endogenous PAs enhance the plant antioxidant system. Here, to investigate the effects of endogenous PAs on the antioxidant system of alfalfa under drought stress and the underlying mechanisms, two alfalfa cultivars, Longzhong (drought resistant) and Gannong No. 3 (drought sensitive), were used as test materials, and their seedlings were treated with polyethylene glycol (PEG-6000) for 8 days at −1.2 MPa to simulate drought stress. The levels of free PAs [putrescine (Put), spermidine (Spd) and spermine (Spm)], hydrogen peroxide (H₂O₂), malondialdehyde (MDA), key PA metabolism enzyme [arginine decarboxylase (ADC), ornithine decarboxylase (ODC), S-adenosylmethionine decarboxylase (SAMDC), polyamine oxidase (PAO), and diamine oxidase (DAO)] activities, and antioxidant enzyme [superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD)] activities were measured. These physiological indicators were used for correlation analysis to investigate the relationship between PA metabolism and the antioxidant enzyme system. The results showed that PA synthesis in alfalfa under drought stress was dominated by the ADC pathway. Spd and Spm played an important role in improving drought tolerance. The high levels of ADC and SAMDC activities were facilitated by the conversion of Put to Spd and Spm. H₂O₂ generation by oxidative decomposition of PAs was mainly dependent on the oxidative decomposition of DAO but not PAO. Low DAO activity favored low H₂O₂ production. Spd, Spm, ADC, ODC and SAMDC were positively correlated with the antioxidant enzymes SOD, CAT and POD in both cultivars under drought. Therefore, we concluded that high ADC and SAMDC activities in alfalfa promoted the conversion of Put to Spd and Spm, leading to high accumulation of Spd and Spm and low Put accumulation. Low Put levels led to low H₂O₂ production through low DAO activity, and low H₂O₂ levels induced the expression of antioxidant enzyme-encoding genes to improve antioxidant enzyme activity and reduce MDA accumulation and thereby enhanced drought resistance in alfalfa.

干旱胁迫是限制农业发展的主要因素，外源多胺（PAs）可以通过增强抗氧化活性来增强植物的抗旱能力，但很少有研究探讨内源PAs是否增强植物抗氧化系统。本研究以陇中（抗旱）和甘农3号（干旱敏感）两个苜蓿品种为试材，探讨干旱胁迫下内源PAs对苜蓿抗氧化系统的影响及其机制。用聚乙二醇（PEG-6000）在-1.2 MPa下处理幼苗8天，以模拟干旱胁迫。游离PA[腐胺(Put)、亚精胺(Spd)和精胺(Spm)]、过氧化氢(H ₂ O ₂ )、丙二醛(MDA)、关键PA代谢酶[精氨酸脱羧酶(ADC)、鸟氨酸脱羧酶(测量了ODC）、S-腺苷甲硫氨酸脱羧酶（SAMDC）、多胺氧化酶（PAO）和二胺氧化酶（DAO）]活性，以及​​抗氧化酶[超氧化物歧化酶（SOD）、过氧化氢酶（CAT）和过氧化物酶（POD）]活性。将这些生理指标进行相关性分析，探讨PA代谢与抗氧化酶系统之间的关系。结果表明，干旱胁迫下苜蓿PA合成以ADC途径为主。Spd和Spm在提高耐旱性方面发挥了重要作用。Put 转换为 Spd 和 Spm 促进了 ADC 和 SAMDC 活动的高水平。PAs氧化分解生成H ₂ O ₂主要依赖于DAO的氧化分解，而不依赖于PAO的氧化分解。低 DAO 活性有利于低 H ₂ O ₂产量。干旱条件下，两个品种的Spd、Spm、ADC、ODC和SAMDC与抗氧化酶SOD、CAT和POD呈正相关。因此，我们得出结论，苜蓿中较高的ADC和SAMDC活性促进了Put向Spd和Spm的转化，导致Spd和Spm的高积累和Put的积累较低。低Put水平通过低DAO活性导致H ₂ O _{2产量低，而低H 2} O ₂水平诱导抗氧化酶编码基因的表达，提高抗氧化酶活性，减少MDA积累，从而增强苜蓿的抗旱性。