Autointoxication In Apple Replant Disease

Apple replant disease is widespread and has been documented in all of the major fruit-growing regions of the world. The reason for this replant problem in apple orchards is not clearly understood. Studies have shown that autotoxins in root exudates or decomposition of residues may play an important role in apple replant disease. The mechanisms involved, however, have not been extensively investigated. In order to investigate the mechanisms of autotoxicity involved in apple replant disease and provide the theoretic base and technical guideline for apple tree cultivation and soil management, the following researches were done. (1) Identifying and evaluating the autotoxins in root exudates and root leachate of Malus prunifolia Boukh., (2)the autointoxication of root leachate and rhizospheric effects,(3) the effect of root leachate on the M. prunifolia seedling under drought and nutrition stress.The main results were as follows:1. The radicle growth of M. prunifolia seeds was inhibited by root exudates and root leachate. The above results clearly indicate that autotoxins were present in the root exudates and root leachate of M. prunifolia. However, the autointoxication of root leachate was higher than root exudates.2. The ethyl acetate extracts, chloroform extracts and XAD-16 resin extracts of root exudates and root leachate affected the radicle growth of M. prunifolia seeds. The inhibitory effect was strongest under XAD-16 resin extracts.3. Analysis of the root exudates and root leachate with GC-MS resulted in twenty-five and fourty-one autotoxins respectively. Root exudates of M. prunifolia mainly contained 1,2-benzenedicarboxylic acid diisooctyl ester, sitosterol, 1,2-benzenedicarboxylic acid butyl cyclohexyl ester, butylated hydroxytoluene and palmitic acid. The dominating autotoxins in M. prunifolia root leachate were phthalic acid, 4-hydroxy-Benzenepropanoic acid, butylated hydroxytoluene, dibutyl phthalate and palmitic acid. The same autotoxins were phthalic acid, butylated hydroxytoluene and palmitic acid in root exudates and root leachate. The autointoxication of these three autotoxins was identified. With the concentration increasing, the inhibitory effects get stronger.4. MDA and H2O2 content, as well as the generation rate of O2?? in M. prunifolia roots exposed to 10% root leachate showed no significant alteration with control, while their contents were strongly enhanced at higher concentrations (15% and 20%). Activities of SOD, POD, CAT, APX, GR, DHAR and MDHAR were substantially enhanced by lower concentrations of root leachate (10%). However, in the presence of higher root leachate concentrations (15% and 20%) the activities of these enzymes displayed a reduction from 20 d onward. Lower root leachate level in the medium resulted in higher reduced ascorbic acid (AsA) and glutathione (GSH) pool in roots during the whole course of the experiment. AsA and GSH content displayed an initial increase in response to higher root leachate concentrations, but did decline visibly from 15 d onward. It is concluded that the antioxidant metabolites and the activity of antioxidant enzymes in roots of M. prunifolia is up-regulated in response to root leachate stress to minimize oxidative damage, but the protective action will be exhausted as treatment time extends and treatment dose increases.5. Root leachate of M. prunifolia reduced the quantity of bacterium, actinomycetes and gross of microbes, and increased the quantity of fungi in rhizosphere soil. Simultaneously, the activities of catalase, protease, phosphatase and saccharase decreased in M. prunifolia rhizosphere soil exposed to root leachate.6. Under drought stress and different nutrition stress, root leachate treated with different density has stronger autotoxic effects on the seedlings of M. prunifolia compared with control. The root activity and chlorophyll content decreased, relative membrane permeability and MDA content increased in root leachate-treated seedlings.7. Growth of all apple rootstocks was inhibited to replant culture. After 60 days treatment,plant height, root length and plant biomass obviously declined comparing with control. Six apple rootstocks were comprehensively evaluated and revealed M. hupehensis (Pamp) Rehd. held higher tolerance to replant culture, while Malus baccata(L.)Borkh and M. robusta Rehd. were sensitive to replant culture.