Effects of exogenous selenium on physiological characteristics and cadmium accumulation in <i>Medicago sativa</i> L. under cadmium stress

L.), a forage crop, is highly sensitive to soil Cd contamination. This study investigates the role of exogenous selenium (Se) in mitigating Cd-induced stress in alfalfa, using the 'Golden Empress' cultivar in a hydroponic system. Our results show that low Se concentrations (1 mg/L) significantly improved seed germination under Cd stress. Se enhanced antioxidant defense mechanisms by increasing superoxide dismutase (SOD) and peroxidase (POD) activities. Additionally, Se reduced malondialdehyde (MDA) accumulation by 38.2% under high Cd stress (50 mg/L). Se also facilitated osmotic balance restoration, with a 7.09-fold increase in proline content compared to the control. Furthermore, Se decreased Cd accumulation in roots and leaves by 38.6% and 27.4%, respectively. These findings suggest that Se mitigates Cd toxicity through enhanced antioxidant activity, osmoregulatory substance synthesis, and modulation of Cd distribution at the subcellular level. The optimal Se concentration varies with the severity of Cd stress.