Comprehensive evaluation of physio-morphological traits of alfalfa (Medicago sativa L.) varieties under salt stress

Salt is a major abiotic factor significantly affecting plant growth and development. Alfalfa (Medicago sativa L.), a crucial perennial crop for livestock feed, shows significant differences in salt tolerance among different varieties. This study aimed to comprehensively evaluate the salt tolerance of 30 varieties of alfalfa under salt stress (0, 150, and 300 mmol L^-1 NaCl). It showed that shoot height (SH), root length (RL), shoot fresh weight (SFW), and root fresh weight (RFW) were decreased by 37.68%, 35.83%, 43.79% and 48.86%, respectively, under high salt stress. Photosynthesis-related parameters, including chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (TChl), and carotenoids (Car) contents of all varieties were decreased by 50.13%, 43.73%, 48.17% and 60.86%, respectively, and minimum inhibition of photosynthetic pigment contents was observed in the variety Salsa. The changes of antioxidant enzymes in different alfalfa varieties were also found under salt stress. For example, the APX of Sardi 7 decreased by 641.84%, while the APX of Phabulous decreased by 88.33% compared to the non-treated controls. Principal component analysis (PCA) identified five major variables: Car, TChl, APX, Chl a, and POD. Finally, each variety's comprehensive tolerance membership function values were calculated by the membership function method, and the 30 varieties were classified into four categories by cluster analysis. Our findings indicate that Sardi 7, Salsa, Gannong No.8, Daye, and Instinct are alfalfa salt-tolerant varieties. Our study provided baseline information on the response of alfalfa varieties to different salinity levels, which will help select or breed salt-tolerant varieties.