Abstract: The enzymes activities of nitrogen metabolism and free amino acids were studied under low temperature in Malus baccata Borkh. and M. hupehensis Rehd. The results indicated that at low temperature, glutamine synthetase(GS) activity were increased obviously by 119% and 317% respectively inM. baccata Borkh. and M. hupehensis Rehd. But low temperature decreased glutamate synthase (NADHGOGAT) and glutamate dehydrogenase (NADHGDH) activity. In M. baccata Borkh. and M. hupehensis Rehd. NADHGOGAT activity decreased to 6.11% and 48.43%, NADHGDH activity decresed to 47.07% and 33.84%. Low temperature effected GS activity and NADHGDH activity in M. hupehensis Rehd. more than in M. baccataBorkh. Analysis for GS/GDH ratio showed that GS-GDH cycle was the main pathway of ammonium assimilation irrespective of low temperature, but under low temperature stress, GS pathway was strengthened obviously, especially in M. hupehensis Rehd. At low temperature the content of free amino acids were increased by 78.34% and 89.79% respectively in M. baccata Borkh. and M. hupehensis Rehd., the total content of free amino acids increased more in M. baccata Borkh. than that of M. hupehensis Rehd. Before low temperature the main composition of free amino acids were threonine, tyrosine, aspartic acid and glutamate, but under low temperature the main composition of free amino acids were threonine, arginine, aspartic acid and glutamate. The content of arginine was obviously increased under low temperature.

Key words: Malus baccata Borkh., M. hupehensis Rehd., glutamine synthetase, glutamate synthase, glutamate dehydrogenase, free amino acids