Abstract: This study aims to assess the effect of chitosan on heat resistance of Phalaenopsis seedlings. Phalaenopsis seedlings were treated with chitosan at 0 (without chitosan addition as control), 25, 50, 100, 200, 400 mg·L^-1 respectively, at 42 ℃  for 3 days, and then the physiological changes in Phalaenopsis seedlings were determined. The results showed that with increasing the chitosan concentration from 25 to 100 mg·L^-1, the activities of superoxide dismutases (SOD), peroxidases (POD) and catalase (CAT), and the contents of proline, soluble sugar, total chlorophyll and carotenoid in Phalaenopsis seedling leaves increased gradually, while the membrane permeability and malondialdehyde (MDA) content of the leaves declined gradually, but no significant change was found in the content of soluble protein. In treatment with 100 mg·L^-1 chitosan, the activities of SOD, POD and CAT and the contents of proline, soluble sugar, total chlorophyll and carotenoid in Phalaenopsis seedling leaves reached their peak values and were significantly higher than those of the control (P<0.01), whereas the membrane permeability and MDA content of the leaves showed the lowest values and were significantly lower than those of the control (P<0.01), and the Phalaenopsis seedlings suffered a slight heat injury. With increasing chitosan concentration from 200 to 400 mg·L^-1, the activities of SOD, POD and CAT, and the contents of proline, soluble protein, soluble sugar, total chlorophyll and carotenoid of the leaves decreased significantly (P<0.01), whereas the membrane permeability and MDA content of the leaves increased significantly (P<0.01), and Phalaenopsis seedlings tended to suffer more serious heat injury. According to the results obtained, it is concluded that treatment with 100 mg·L^-1 chitosan can significantly improve heatresistance of Phalaenopsis seedlings.

Key words: water environment, stable nitrogen isotope, nitrogen cycle, measured methods, application.