Studies On Effects Of AMF On Growth And Resistance Of Cucumber Seedlings To Wilt Disease Under Protected Conditions

Cucumber (Cucumis sativus L.) wilt is currently the main problem occurring in cucumber cultivation under greenhouse conditions. Studies on the potential of arbuscular mycorrhizas (AM) in nursing stronger seedlings that are resistant to wilt and mechanisms involved in resistance were made to provide scientific evidence to make good use of AM fungi for disease control in agricultural practice. Three AM fungi, Glomus mosseae, Glomus intraradices, and Glomus versiforme were studied in addition to Fusarium oxysporum f. sp. cucumerinumAM fungi were introduced into the nursing process and 30 days later, compared with corresponding controls, the shoot and root biomass of plants inoculated with G. mosseae were increased by 21.42 and 12.50 %. Concentrations of root nitrogen (N) and phosphorus (P) in were increased by 11.69 and 114.28%.When mycorrhizal seedlings inoculated with G. versiforme were transplanted to continuous cropping in sterilized soil, growth-promoting effects of the AM fungus were significant until flowering. As for disease incidence index, compared with plants infected with F. oxysporum f. sp. cucumerinum alone, those colonized by G. versiforme and infected with F. osysporum f. sp. cucumerinum had a 37.5 % lower index.Irrigating seedling roots with a spore suspension of F. oxysporum f. sp. cucumerinum (spore density 106 cfu ml-1), G. versiforme was effective in resisting wilt if the P concentration was 31mg kg-1 in the nutrient solution. The change in the Polyphenol Oxidase was related to the disease resistance capability while the change in the Peroxidase was only a index of the symptoms.The mechanisms involved in the biological control are as follows.(1) Mycorrhizal plants are sensitive to infection by F. oxysporum f. sp. cucumerinum. The loss can be compensated by accumulating much more biomass. Seedlings grown for 4 weeks were infected with F. oxysporum f. sp. cucumerinum (spore density 106 cfu ml-1) and 2 weeks later, in the case of plants pre-inoculated with G. versiforme, the relative reductions in shoot and root biomass caused by F. oxysporum f. sp. cucumerinum were 1.54 and 1.64 times compared with plants infected with Fusarium alone, but the total biomass was 2 times than that of the control.(2) In the case of mycorrhizal plants, the harm caused by F. oxysporum f. sp. cucumerinum can be reduced. Seedlings grown for 4 weeks were infected with F. oxysporum f. sp. cucumerinum (spore density 106 cfu ml-1) and 2 weeks later the concentrations of Malondiadehyde in roots pre-inoculated with G. mosseae and G. versiforme were reduced by 32 and 35 % respectively. Compared with non-mycorrhizal plants infected with F. oxysporum f. sp. cucumerinum alone, the damage to cell membranes was evidently lower than in the controls. In the leaves of plants pre-inoculated with G. versiforme, concentrations of N and P were significantly lower than in the controls but concentrations of potassium (K) were much higher than in the controls.(3) Development of F. oxysporum f. sp. cucumerinum in the roots of mycorrhizal plants can be inhibited by much higher concentrations of secondary metabolites. Seedlings grown for 4 weeks wereinfected with F. oxysporum f. sp. cucumerinum (spore density 104 cfu ml-1), and 9 days later the disease index had reached 15 % in non-mycorrhizal plants, but mycorrhizal plants continued to show healthy growth. The concentrations of flavone and total phenol in mycorrhizal seedling roots were higher than those in non-mycorrhizal seedlings from the infection period to the incubation period.(4) Prophylactic enzyme can be induced much earlier in mycorrhizal plants, and this protects plants from further attack by F. oxysporum f. sp. cucumerinum. Seedlings grown for 4 weeks were infected with F. oxysporum f. sp. cucumerinum(spore density 104 cfu ml-1). Chitinase, β-1,3-glucanase and phenylalanine ammonium-lyase in the roots of plants pre-inoculated with G. versiforme were induced earlier and to a much extent than in the controls. Sodium dodecyl sulfate polyacrylamide gel electrophoresis separations were ca...