| Arbuscular mycorrhiza! (AM) fungi exhibit low host specificity and form mutualistic associations with a wide range of host plant species over a wide range of environmental conditions. Some characteristics of AM fungi, including hyphal growth, distribution and metabolic activity, are closely related to mycorrhizal efficiency and to ecological function of AM fungi, and may be influenced by host plant and soil conditions. Previous studies have focused mainly on the influence of AM fungi on the physiology of the host plant, and there are few published reports on the influence of the host plant on the growth and metabolic activity of AM fungi. In the present study, a compartment cultivation system and histochemical staining were used to investigate the influence of soil available P level, plant P status and soil organic P on the growth and metabolic activity of AM fungi. Differences in metabolic activity among AM fungal isolates and the relationship between metabolic activity and mycorrhizal effectiveness were also investigated. In addition, AM fungi from a wide range of environmental conditions (originally isolated from north, central and south China) were used to study the ecological adaptability of AM fungi and the influence of edaphic conditions on AM fungal growth and metabolic activity. The objectives were to shed light on the regulation of growth and metabolic activity of AM fungi by environmental condition and select one or two AM fungus with strong ecological adaptability for use.The growth of extraradical hyphae and metabolic activity of AM fungi changed with the application rate of fertilizer P added to the soil. Fungal metabolic activity decreased with increasing addition of inorganic P to the soil and increasing plant P concentration. The growth of extraradical hyphae and hyphal density in soil were decreased significantly at high P level, however the growth of AM fungus stimulated by moderate P supplied into soil (50 mg-kg-1). Extraradical hyphal growth was inhibited at high P levels, resulting in decreased P uptake by AM fungi. It might be a contributory factor for decline in mycorrhizal efficiency. Some observations also indicated that metabolic activity and growth of extraradical hyphae were regulated by Na-phytate.Under the condition of P deficiency in host plant, AM fungi grew well and its metabolic activity was also high, while P sufficiency, AM fungi grew bad and its metabolic activity was low. These observations suggested that the growth of extraradical hyphae and metabolic activity of the AM fungi was regulated by the P status of the host plant. A hypothesis was suggested as following: Firstly, the host plants could not synthesize enough carbohydrate to meet their own growth requirements and those of the fungus when the plant in experiment conditions of severe P deficiency. As a result, extraradical hyphal growth and AM fungal metabolic activity were inhibited and therefore decline. Secondly, the structure and composition of the plasma membrane of root cortical cells lost some of their integrity, resulting in a higher permeability of root cortical cells when under conditions of moderate P deficiency. The AM fungus could obtain adequate carbon resources for itself, thus ensuring normal growth and metabolic activity owing to the efflux of carbohydrate from the plasma membrane of the host rootcortical cells to the interface of the symbiosis. Thirdly, the structure and composition of the root cortex cellular plasma membrane kept good integrity and the leakage of carbohydrate maintained at low level when the plant obtained enough P and metabolizes normally. Although enough carbohydrate was synthesized by plant, the AM fungi could not obtain enough energy, resulting in inhibition of growth of the extraradical hyphae and decreased metabolic activity of the AM fungi.Alkaline phosphatase (ALP) activity in the intraradical hyphae could reflect the functional differences among AM fungal isolates. ALP activity in intraradical hyphae at early growth stage was a useful physiological index f...
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