Abstract: Using Dongmu 70 ryegrass seedlings as experimental mate rial, we investigated its physiological response mechanism under the single artificia l simulated freezingthawing stress (10,5,0,-5,0,5,10 ℃) and combined with NaH CO₃ stress. The results show that the content of proline and malondialdehyd e (MDA) and the activity of catalase (CAT) and superoxide dismutase (SOD) in the leaves of ryegrass seedlings increased firstly and then decreased significantly under a single freezingthawing stress, the activity of peroxidase (POD) increa sed firstly and then decreased slowly, except for CAT, other physiological index es reach the maximum value at -5 ℃, which compared with other temperatures ther e are significant differences (P<0.05). Under the condition of combined stre ss treatment, physiological indexes show the rule of combined stress>single stre ss>control. The mass ratio of protein in the leaves of ryegrass seedlings raech the maximum value at 0 ℃, and combined stress and single freezingthawing gro up are higher than those in control group. Thus, the ryegrass seedling under the single freezingthawing stress and combined with NaHCO₃ stress can rapidly i mprove the activities of antioxidant enzymes and the mass ratio of osmotic adjus tment substances to alleviate damage caused by stress, and the effects of combin ed stress on seedlings are greater than the single stress.

Key words: ryegrass, NaHCO₃ stress, physiological response, freezingthawing stress