Physiological responses of different alfalfa hybrid combinations to freezing stress

Freezing injury is the main limiting factor for alfalfa in northeast China. The screening of breeding materials that can resist freeze-thaw stress is of great significance for improving the yield and quality of alfalfa. In the present study, five different combinations of alfalfa (Medicago sativa) were selected as test materials, which were subjected to three different treatment stages: cold resistance training stage, freezing stage, and turning green stage. soluble protein(SP), soluble sugar(SS), proline(Pro), and malondialdehyde (MDA) contents, and superoxide dismutase (SOD) activity were measured. The results showed that the SP content had an overall increasing trend and was significantly higher in each test material than in the control group. The SS content fluctuated and was significantly lower in all test materials than in the control group during the cold resistant stage. The SS content was significantly higher in all test materials than the control group during the frozen stage, and the Pro content generally increased. MDA contents in the Ⅱ, Ⅲ, and Ⅴ hybrid combinations were lower than that in the control group, although this difference was not significant. SOD activities first increased and then decreased,with the contents during the other phases significantly higher than that in the control group. Membership function analysis showed that the order of cold resistance in the hybrid combinations was Ⅴ>Ⅲ>Ⅰ>Ⅱ>Ⅳ.