Abstract: To compare the physiological responses of different Medicago species to aluminum (Al) stress and identify germplasm with strong Al tolerance, four species M. polymorpha, M. lupulina, M. minima, and M. sativa were subjected to Al³⁺ concentrations of 0, 50, 100, and 150 μmol/L. Germination rate, root and shoot dry weights, Al content, and antioxidant enzyme activities were measured. Al stress significantly inhibited seed germination and seedling growth in all species (P<; 0.05), with root biomass showing the largest reduction, indicating that roots were the primary target of Al toxicity. M. polymorpha exhibited the highest Al tolerance, maintaining higher germination rate, biomass, and antioxidant enzyme activities and lower Al accumulation and malondialdehyde (MDA) content. Correlation analysis revealed significant negative relationships between Al content and growth parameters, suggesting that reduced Al accumulation and enhanced antioxidant defense contribute to Al tolerance. These findings highlighted M. polymorpha as a promising species for improving Al tolerance in Medicago breeding and for cultivation in acidic soils in southern China.

Key words: Medicago polymorpha, aluminum stress, antioxidant enzyme, aluminum tolerance, physiological response