Effects Of External Phosphorus Supplies On The Biomass, Efflux Of Oxalate And Protons As Well As Phosphatase Activities Of Ectomycorrhizal Fungi

Ectomycorrhizal fungi play a vital role in the nutrient cycling of soil-plant systems. They could, in general, improve plant nutrition under nutrient stress status. Previous research shows that ectomycorrhizal fungi can promote phosphorus absorption and growth of the trees. Ectomycorrhizal trees absorb phosphorus efficiently by extensive external hyphae which distribute widely in the soil since they have large surface contacted with soils which benefits contact absorption of phosphorus. Also, ectomycorrhizal fungi can secrete organic acids and protons to mobilize insoluble phosphorous. Km and Cmin for phosphorus (H2PO₄^1-/HPO₄^2-) absorption by ectomycorrhizal fungi are quite low. In addition, the content of organic matter, an important source of phosphorus, in forest soil is usually higher. Phosphatase catalyizes the decomposition of organic phosphorus such as phosphohpids into inorganic one for plant use. In our test, external phosphorus was supplied to study the biomass, efflux of oxalate and protons as well as phosphatase activities of ectomycorrhizal fungi in order to explore the reasons and factors for the mobilization of phosphorus by the fungi.Three strains of Lactarius delicious and Laccaria bicolor (strain: S238N) were used in the test. Variable phosphorous was added into culture mediums to culture the fungi for 21 days at 25℃in the dark and then fungal biomass, phosphatase activities and efflus of oxalates and protons by ectomycorrhizal fungi were detected. Following are the results obtained in our experiment:Concentrations of phosphorus in the mediums significantly influenced greatly fungal growth. In general, less growth rate was observed than those with suitable amount of phosphorous supplied and high phohporus in the culture mediums inhebited the growth of ectomycorrhizal fungi. The mean growth rate of the four fungi varied greatly in different phosphorous supplied: Ld-01≈Ld-02 > Lb S238N≈Ld-03. It is worth pointing out Ld-1 and Ld-3 grew faster than Lb S238N, indicating high sensibility of Lb S238N to external phosphorous.Concentrations of phosphorus in culture mediums influenced remarkably efflux of oxalate and the amount effluxed changed in following sequence: without P>low P>optimal P>high P, ranged from 2.32 to 8.66 times of variations. Fungal strains varied in the efflux of oxalate: Lb S238N > Ld-02≥Ld-03≈Ld-01. It seems reasonable to suggest that Lb S238N had stronger abilities to mobilize Al-P, Fe-P and Ca-P in soils than other three fungi.Concentrations of phosphorus in culture mediums influenced obviously efflux of protons and the amount effluxed changed in following sequence: without P>low P>optimal P>high P, valued from 1.18 to 5.86 times of differences. Fungal strains varied in the efflux ofprotons: Ld-03 > Ld-02 > Ld-01 > Lb S238N.Concentrations of phosphorus in culture mediums influenced significantly efflux of acid phosphatase by three strains of Lactarius delicious. The activity of acid phosphatase in the mediums is the highest without phosphorus, middle with low phosphorus and low with high phosphorus supplied. Without phosphorus added in the culture mediums, the activities of acid phosphatase varied in the sequence: Ld-03 > Ld-01 > Ld-02 > Lb S238N, ranged from 1.36 to 16.88 times of differences.