Drought-salinity interaction suppresses seed germination and seedling growth in alfalfa by altering water status, membrane stability, and antioxidant responses.

Drought and salinity stress are the primary limiting factors for the successful establishment of alfalfa (Medicago sativa) pastures. However, the integrated responses of alfalfa during early establishment to combined drought and salinity stress, particularly the physiological links among water status, membrane stability, antioxidant defense, seed germination, and seedling growth, remain insufficiently understood. This study employed a two-factor design consisting of PEG-6000-induced drought stress at three levels (5%, 15%, and 25%) and NaCl-induced salinity stress at three concentrations (50, 100, and 150 mmol·L-1). The experiment included nine drought-salinity stress combinations plus a non-stress control. The interaction between drought and salinity stress significantly affected alfalfa seed germination and seedling physiological traits. Increasing PEG and NaCl concentrations generally reduced seed germination indices, including germination percentage, germination potential, germination index, and vigor index, and inhibited seedling growth, as reflected by decreases in plant height, leaf number, fresh weight, and aboveground dry weight. Combined drought-salinity stress also disrupted water status and membrane stability, increased relative electrical conductivity, and induced stress-specific changes in antioxidant enzyme activities. Severe combined stress caused the strongest inhibition, whereas low-intensity combined stress treatments, especially 5% PEG combined with 50 or 100 mmol·L-1 NaCl, showed relatively better overall performance among the imposed stress treatments under controlled conditions. These findings provide a physiological basis for evaluating early-stage alfalfa responses to combined drought and salinity stress and may support future screening of stress-tolerant germplasm.