Physiological and Biochemical Insights into Drought Tolerance of Alfalfa Cultivars with Different Sensitivities

Drought is one of the most critical abiotic stresses limiting alfalfa (Medicago sativa L.) productivity worldwide. This study investigated the physiological and biochemical responses of two contrasting alfalfa cultivars, ‘Bilensoy-80’ (sensitive) and ‘Diane’ (tolerant), exposed to drought stress induced by polyethylene glycol (PEG 6000) at different water potentials (0, −0.5, −1.0, −1.5, −2.0 MPa). Growth traits, relative water content, total chlorophyll content, osmolyte accumulation, lipid peroxidation, total phenolics, and activities of major antioxidant enzymes were evaluated. The results showed that the reductions in growth parameters, relative water content, and total chlorophyll content started to be observed at earlier drought stress levels in Bilensoy-80, whereas Diane maintained stability until more severe drought levels, compared with the control. This enhanced performance was largely attributed to increased free proline biosynthesis and the activity of key antioxidative enzymes such as catalase (CAT), and glutathione reductase (GR). Overall, the findings provide valuable insights into the mechanisms of drought tolerance in alfalfa and may inform breeding programs aimed at improving forage crop performance under water-limited conditions