Sodium carbonate and sodium bicarbonate are the two main salts that cause grassland salinization in northeast China. In spring, the plants often bear the double stress of saline and alkaline conditions. In this paper, Medicago sativa CV. Dongme-1 seedlings were used to study the effects of freeze-thaw (FT) and alkaline salt stress on the content of osmotic adjustment substances, biological membrane permeability, and antioxidant enzyme activity. The results showed that under the stress of FT+NaHCO_3 and FT+Na_2CO_3, the protein content in seedlings decreased by 1.39-7.970% and 3.93-12.290%, respectively; MDA content increased by 10.67?46.99% and 3.74-46.410%, respectively; free proline increased by 11.07-45.32% and 43.67-73.73%, respectively; and the activity of SOD and POD increased. During the FT cycle, both proline and MDA content initially increased and then decreased. Under alkaline salt and FT stress, a large amount of superoxide anion was produced in seedlings. The content of MDA increased, inhibiting seedling growth, and the contents of protein, soluble sugar and free proline in plants increased significantly. Meanwhile, the defense mechanism of seedlings induces the activity of SOD and POD to resist stress injury; the damage to seedlings under combined stress (alkaline salt and FT stress) was higher than that under a single stress (alkaline salt stress or FT stress). Moreover, Na_2CO_3 stress was more harmful to the seedlings than Na- HCO_3 stress. The results of this study prove the adaptable changes in the alfalfa seedlings, reflecting their abilities of saline-alkaline tolerance and cold resistance.
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