Abstract: This experiment was conducted to explore the effect of low molecular weight organic acids on the biomass and physiological characteristics of Malus hupehensis Rehd. seedlings under continuous cropping. Seedlings were grown in pots were treated with oxalic acid，tartaric acid，citric acid，malic acid and the mixture of the these four organic acids. The effects of organic acids on the photosynthetic parameters，the activities of antioxidant enzymes and root system of M. hupehensis Rehd. seedlings were investigated. Compared with controls，oxalic acid could significantly promote dry and fresh weight of plant by 24.12%，35.72%，respectively. The net photosynthesis rate，stomatal conductance were significantly increased by 32.27%，76.58%，respectively. At the same time，oxalic acid could also significantly improve the activities of POD，and CAT. However tartaric acid remarkably inhibited the stomatal conductance by 40.22%. The activities of SOD and POD were also inhibited，respectively. In conclusion，the oxalic acid could promote the growth of M. hupehensis Rehd. seedlings and alleviatethe effect of replant disease on seedlings，while the tartaric acid aggravated the effect of replant obstacle on seedlings. The citric acid and malic acid showed no significant influence on replant obstacle for seedlings.

Key words: apple, replant disease, Malus hupehensis, organic acids of soil, photosynthetic indexes, protective enzyme