| Arbuscular mycorrhizal (AM) fungi have been shown to play a key role in maintaining plant health. Community diversity of AM fungi, AM fungi/pathogen ratios, their dynamic balance in the soil, and their influence on plant physiology and metabolic process during the development of soil-borne diseases have become central topics in the field of mycorrhizas. The author started working on interactions between AM fungi and Fusarium oxysporum f.sp. niveum on growth and wilt disease of watermelon (Citrullus lanatus). Then a series of experiments was carried out systematically to study the mechanisms of increasing resistance to fusarium wilt by watermelon induced by AM fungi.Firstly, the influence of AM fungi and E oxysporum f.sp niveum on growth and wilt disease of watermelon were investigated in field. Pathogen propagules in roots and the rhizosphere, the extent of root infection by F. oxysporum f.sp niveum, wilt disease incidence and disease index of watermelon seedlings inoculated with AM fungi were significantly reduced. AM fungal inoculation also increased net photosynthetic rate, transpiration rate, water use efficiency and the concentrations of N, P, B and Zn in the leaves. Consequently, AM fungal inoculation promoted the growth, yield and quality of watermelon.Watermelon seedlings inoculated with Gigaspora rosea, Glomus mosseae, and Glomus versiforme were grown in pots under greenhouse conditions. AM inoculation increased growth and dry weight of the seedlings and significantly decreased percentage root infection by F. oxysporum f.sp. niveum wilt disease incidence and disease index. The relationship between root arbuscule colonization by AM fungi and disease severity showed a negative correlation. The mycorrhizal effects of the three AM fungi were clearly different, with the degree of disease control following the sequence G. versiforme> G mosseae>Gi.rosea. In addition, the effectiveness of disease control by AM fungi were greater on susceptible cultivars than on resistant cultivars. The development of wilt disease was influenced by inoculation time and sequences of AM fungi and the pathogen. The data suggest that there competition or antagonism occurred between the AM fungi and F. oxysporum f.sp. niveum.Spore germination and colony growth of F. oxysporum f.sp. niveum were restrained by exudates from mycorrhizal watermelon seedling roots and stimulated by non-mycorrhizal roots. The exudates from mycorrhizal seedlings significantly increased growth and reduced wilt disease severity of watermelon seedlings, and the control effect increased as the degree of mycorrhizal colonization and exudates from mycorrhizal watermelon seedlings roots increased.In a low available phosphorus soil, G versiforme inoculation and phosphorus fertilization increased seedling growth, reduced wilt disease severity in watermelon seedlings, and increased the concentrations of N, P and K. However, Zn and Cu concentrations were decreased and mycorrhizal development of watermelon seedlings was partly inhibited by P fertilization. This indicates that the increase in wilt-resistance induced by inoculation with AM fungi was related to their function in improving plant P nutrition.Regarding the biochemical mechanisms of wilt-resistance induced by AM fungi, the MDA content, autoxidation rate and cell membrane permeability of the roots inoculated with both G. versiforme and F. oxysporum f.sp. niveum were lower than in roots treated only with F. oxysporum f.sp. niveum. These results suggest that the mycorrhizal status of the seedlings infected with F. oxysporum f.sp. niveum enhanced the self-regulation capability, thereby maintaining the stability of the plant cell membranes. On the other hand, double inoculation with G. versiforme and F. oxysporum f.sp. niveum, produced higher peroxidase (POD), phenylalanine ammonia lyase (PAL), β-1, 3-glucanase and chitinase enzyme activities in the seedlings compared with those inoculated with F. oxysporum f.sp. niveum alone. The peak of enzyme activities in the former treatment appeared earlier as compared with the lat... |
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