Abstract: 【Objective】 Effects of exogenous nitric oxide on the growth, root architecture, soil environment, activities of protective enzymes and oxidative damage of seedlings of Malus hupehensis Rehd. under continuous cropping were investigated. The action mechanisms of nitric oxide were explored so as to provide a theoretical basis for alleviating apple continuous cropping. 【Method】 Sodium nitropprusside (SNP, an exogenous nitric oxide donor) at concentrations of 0-1 000 μmol•L-1 were added to the containers, the plant height, leaf area, root volume, number of tips, quantity of major microorganism groups, phenolic compounds and the activities of protective enzymes (including SOD, POD, CAT and APX) in leaves, malondialdehyde (MDA) content , and   producing rate in leaves were investigated. 【Result】 The above results indicated that the best alleviating effect was 200 μmol•L-1 SNP. At this concentration, the plant height, fresh weight, chlorophyll content and leaf area were significantly increased, comparing with contrast one, increased by 59.69%, 74.25%, 45.70%, 116.58%, respectively. The parameters of root architecture, such as average diameter, total root volume and number of tips were significantly increased. The activities of SOD, POD, CAT and APX were higher significantly than untreated seedlings, while malondialdehyde (MDA) content and   producing rate were lower significantly than untreated seedlings. Exogenous nitric oxide had little effect on the quantity of major microorganism groups (bacteria, fungi and actinomycete) and phenolic compounds in rhizosphere soil. 【Conclusion】 The above results indicated that exogenous nitric oxide had an effective function on the alleviation of continuous cropping obstacle in seedlings of Malus hupehensis Rehd. Exogenous nitric oxide had some influence on the biological factors and abiotic factors, but both of them were not the main reasons for alleviating continuous cropping obstacle in seedlings of Malus hupehensis Rehd. The activities of SOD, POD, CAT were improved obviously, while malondialdehyde (MDA) content and   producing rate were decreased significantly, which were two kinds of important reasons.

Key words: Malus hupehensis Rehd., nitric oxide, continuous cropping, growth stress, oxidative damage