Effects Of Potassium On The Secretion Of Organic Acids By Ectomycorrhizal Fungi And The Concentrations Of Nitrogen, Phosphorus And Potassium In Their Hyphae

Ectomycorrhizal fungi,important microorganisms in soils with special ecological functions,are widespread in nature.While forming symbionts with roots of trees,they could promote the growth and nutrition uptake of the host plants,as well as improve their resistance to stress.Potassium,which is required only slightly lower than nitrogen by plants,is one of the essential major nutrients in forest ecosystem.The lack of potassium may cause serious negative effects on growth and development of trees.Researches showed that,when inoculated by ectomycorrhizal fungi,the secretion of low molecular organic acids by trees was promoted,the absorption of potassium by trees was enhanced remarkably.So it is of great significant to nutrition improving and growth promotion.Nine ectomycorrhizal fungal strains,Pisolithus tinctorius Pt 715(from Xichang,Sichuan), Lactarius deliciosus Ld 03(from Jinyun Mountain,Chongqing),Suillus luteus(L.Fr.)Gray(Sl 01.Sl 06,Sl 13),Seleroderma 02,Sc 02 and Boletnus sp.Bo 15(from Jinfu Mountain,Chongqing),Suillus grevillei Sg 99 and Suillus subluteus Ss 00(from Daqing Mountain,Inner Mongolia),were grown in the liquid culture for 28 days.Growth of mycelia,the secretion of proton and low molecular organic acid by Ectomycorrhizal fungi and the concentration of nitrogen,phosphorus and potassium in their hyphae were investigated.The results showed:The growth rate of ectomycorrhizal fungi were slowed in the absence of potassium,and increased after supply of potassium.The results suggested that appropriate amount of potassium nutrition could promote the growth of ectomycorrhizal fungi,and the lack of K would inhibit their growth.The increase of Ss 00 and Pt 715 was the lowest in all ectomycorrhizal fungal strains,which implied that these two ectomycorrhizal fungal strains might be tolerant to low potassium and thereby adapted in soils with low potassium content.All of the nine ectomycorrhizal fungal strains secreted oxalic acid.All of them secreted acetic acid except Pt 715.,Sl 01,Ss 00,Sl 06,Ld 03,Sg 99,Bo15 and Pt 715 secreted succinate,while Sl 01, Sl 06 and Pt 715 produced citric acid,and Sc 02 secreted formic acid.Oxalic acid accounted for 54% of the total amount of organic acids produced by ectomycorrhizal fungi,which indicated that oxalic acid is the main organic acid exuded by ectomycorrhizal fungi.For most ectomycorrhizal fungal strains,the efflux rates of H~+,oxalic acid and acetic acid by the fungi were promoted when the potassium was deficient.In general,the concentration of potassium had on significant effect on secretion of proton of ectomycorrhizal fungi while secretion of oxalic acid and acetic acid differed to some extent among different ectomycorrhizal fungi under different concentration of potassium.The efflux rate of oxalic acid was the fastest in the early phase of the culture and stabilized in the mid phase.When the concentration of oxalic acid reaches a certain degree,the efflux rate of oxalic acid remained relatively low,which revealed that excessive oxalic acid in medium played an inhibitory role.The concentration of acetic acid produced by ectomycorrhizal fungi increased in the early stage and reduced later.It showed that the efflux rate of acetic acid was greater than the decomposition rate in the early stage,whereas the decomposition rate of acetic acid was greater than the efflux rate in the latter stage.The concentrations of nitrogen,phosphorus and potassium in nine ectomycorrhizal fungi were differed significantly,which showed different uptake ability of nitrogen,phosphorus,and potassium among ectomycorrhizal fungi.Effect of potassium concentration on nitrogen contents in mycelia was different among ectomycorrhizal fungi.The concentration of phosphorus wasn't different significantly among most ectomycorrhizal fungi.The concentration of potassium in ectomycorrhizal fungi increased along with the level of potassium supply.The uptake of nitrogen,phosphorus,and potassium by ectomycorrhizal fungi increased along with the potassium supply.However,the uptake of nitrogen and phosphorus by ectomycorrhizal fungi had no significant impact by potassium concentration.The growth,the efflux rates of organic acids and oxalic acid,and the concentrations and uptake of nitrogen,phosphorus and potassium of the ectomycorrhizal fungi were diverse according to geographical origin.It showed that such aspects of the ectomycorrhizal fungal strains isolated from Chongqing Jinyun Mountain and Jinfu Mountain were better than the ectomycorrhizal fungi isolated from Inner Mongolia(Ss 00,Sg 99).Taking into account that potassium contents in the soils of the North are generally higher than that of the South,it suggested that strains such as Ss 00 and Sg 99 from high-K conditions of the north did not suitable for low-K soils of the South.It implied that the biological characteristics of ECM fungal strains were adapted to the ecological environment they lived,and that were the result of environmental selection.