Mobilization And Utilization Of Phosphorus And Potassium From Soils By Ectomycorrhizal Fungi Under Manganese Stress

Acid soils account for about 60% of total forestry soils on the earth.Manganese(Mn)toxicity is the second harmful factor to influence tree growth just after aluminum and the deficiencies of phosphorus(P)and potassium(K)occur together with Mn toxicity in acid soils.Ectomycorrhizal fungi(ECMF)can resist Mn toxicity,and utilize P and K nutrition in soils,which is of important implications for trees adapting to acid soil environments,increasing forest productivity,afforestation and Mn pollution management.Taking ECMF as the test strains,including Russula delica Fr.(Rd),Pisolithus tinctorius 715(Pt 715),Cenococcum geophilum Fr.(Cg),and Suillus grevillei(Kl.)Sing(Sg),a liquid culture experiment was conducted to explore the influence of different Mn concentrations on ECMF growth,nutrients uptake,organic acid and proton efflux,and Mn distribution in fungal hyphae.In addition,the fungal growth,organic acid and proton efflux,and P and K utilization were also studied under Mn stress with soil as sole P and K source.Following are the main results:1.ECMF varied significantly in response to Mn and the abilities resistant to Mn change in the sequence: Rd>Sg>Pt 715>Cg.Rd and Pt 715 also showed higher abilities to mobilize P from soils than Cg and to mobilize K than Cg and Sg.Therefore,the ECMF from the southern forests were more adaptable to the soil environments with high Mn but low P and K than those in boreal forests.2.Most of Mn absorbed by fungal hyphae located in apoplasts and only few in protoplasm.Mn in apoplast was between 5.23 and 9.21 folds of protoplasm.Therefore,plasma membrane could be considered as an important barrier of ECM fungi against Mn entrance into the cells.3.Fungal P content and uptake increase under Mn stress and these increments varied greatly among fungal species(strains).Mn fixation by phosphate or polyphosphates in hyphae could be one of mechanisms of ECMF resistant to Mn.4.Under Mn stress,Rd and Sg,the ECMF with higher abilities resistant to Mn,showed lower rate of proton efflux and Mn content in hyphae,particularly in protoplasm,than those of Cg and Pt 715 which were sensitive to Mn.The decreased availability and absorption of Mn could thus be considered as an important mechanism of ECMF to alleviate Mn toxicity.5.Under Mn stress,the efflux of organic acids by ECMF showed diverse changes among fungi grown in kind and concentration.All tested ECMF could secrete oxalate.Moreover,efflux of oxalate of stronger resistance strains was significantly higher than that of sensitive strains,which benefited to chelate Mn2+,Fe2/3+ and Al2/3+ whereby to eliminate Mn toxicity and to release inorganic P and K from phosphates and soil minerals.6.Under Mn stress,ECMF could effectively absorb extraneous P source in a quite low concentration(0.007~0.027 mg/L)with soil as P source.All tested fungi reduced AlP,Fe-P and O-P in soil.Total soil inorganic P(Al-P,Fe-P,O-P and Ca-P)was decreased similarly by Rd and Pt 715 grown in the mediums with and without Mn,although there was less decrease in total inorganic P in soil by Cg as Mn concentration increased in culture solution.It could be concluded that Mn might resistant sensitive strain to mobilize and utilize inorganic P in soil.7.K concentration and uptake by Rd,Pt 715 and Cg with Mn were higher than those without Mn.ECMF could still uptake K effectively with water soluble K in soil lower than 1.48 mg/kg.After fungal culturing for 28 days,the total amount of water soluble K,exchangeable K,and slow released K reduced in the soil was much lower than fungal K uptake.The results suggest that ECMF absorbed K from soil with extremely low K concentrations and some of K accumulated in ECMF could come from those present in mineral structures.In addition,adequate Mn promoted Rd,Pt 715 and Cg to activate and utilize soil mineral structure K Nevertheless,Mn in the culture medium inhibited the activation and utilization of soil mineral structure K by Sg.8.In liquid culture solution with Mn and soil as the sole K source,the H~+ secretion and K absorption of ECMF as well as decrease of soil mineral structure K increased with the increase of Mn concentration.It was speculated that ECMF might be stimulated to secret more H~+ by adequate Mn to decompose mineral structure K.In conclusion,different ECMF had different ability to resist(or endure)Mn.Plasma membrane could be considered as an important barrier of ECM fungi against Mn entrance into the cells.Under Mn stress,ECMF could adjust the phosphorus absorption and secretion of organic acids(especially oxalate)and H~+ to resist(or endure)Mn toxicity.Mn could affect ECMF in activation and utilization of insoluble P and soil mineral structure K.In a word,ECMF might play a potential role in the adaptation of manganese toxicity,P and K deficiency in acid soil for trees.