Abstract: Chitins were added to the replant soils in pots to the final dosages at 0，0.5，1.0，2.5 g ? kg-1，respectively. The seedlings of Malus hupehensis Rehd. were planted in the pots. The effects of chitin on the photosynthesis，the contents of reactive oxygen species（ROS）and the activities of antioxidative enzymes in leaves of Malus hupehensis Rehd. seedlings under the replant conditions were studied. The results showed that the addition of chitin at 1.0 g ? kg-1 obviously enhanced the plant height，ground diameter，up and under ground dry weight，root shoot ratio and photosynthetic parameters. The root shoot ratio of plants treated with 1.0 g ? kg-1 chitins was 1.51 higher than that of the control. Treatment with 1.0 g ? kg-1 chitins also increased the content of photosynthetic pigments，net photosynthetic rate（Pn） and transpiration rate（Tr）in leaves of seedlings. The Pn of plants treated with 1.0 g ? kg-1 chitins was 1.30 higher than that of the control. The seedlings treated with 1.0 g ? kg-1 chitin exhibited higher activities of antioxidant enzymes including superoxide dismutase（SOD），peroxidase（POD），catalase（CAT），and ascorbate peroxidase（APX），which were 1.10，1.85，1.77 and 1.43 times as high as that of the control，respectively. Treatment with 1.0 g ? kg-1 chitin not only decreased the contents of MDA，H2O2，and  to 73%，62% and 34% of the control，but also reduced the contents of proline（Pro）and soluble sugars. Addition of chitin at 2.5 g ? kg-1 significantly decreased the biomass，photosynthetic rate and the activities of antioxidant enzymes，promoted the accumulation of the  and proline. Those results suggested that appropriate dosages of chitin could alleviate the replant disease of apple.

Key words: apple, Malus hupehensis, chitin, photosynthetic rate, reactive oxygen species, antioxidant enzymes, continuous cropping