To investigate the role of exogenous selenium in regulating plant tolerance to saline-alkali stress， alfalfa （Medicago sativa L.） ‘WL298HQ’ was used as experimental material. Different concentrations of exogenous selenium were applied during seed germination and seedling stages to explore the effects on seed germination and physiological characteristics under mixed saline-alkali stress. The results showed that applying exogenous selenium at concentrations of 0.5， 1， and 2 mmol·L^-1 significantly promoted the germination of WL298HQ seeds under stress. The most pronounced promoting effect on seed germination was observed at 0.5 mmol·L^-1. Comprehensive analysis of various physiological indicators at the seedling stage revealed that 2 mmol·L^-1 exogenous selenium effectively alleviated the physiological damage caused by mixed saline-alkali stress to alfalfa seedlings. Mixed saline-alkali stress significantly inhibited the antioxidant capacity and increased cell membrane permeability of alfalfa. However， exogenous selenium application under mixed saline-alkali stress promoted plant growth， reduced the degree of membrane lipid peroxidation， and simultaneously enhanced the activities of various antioxidant enzymes in the leaves.

Alfalfa  /  Mixed saline-alkali stress  /  Exogenous selenium  /  Physiological characteristics