Alleviation Of Aluminum Toxicity By Ectomycorrhizal Fungi And Ectomycorrhizae

As soils acidify in forest ecosystems, available aluminum (Al) increases and may become phytotoxic which eventually restraints plant growth and leads to the decline of forest productivity. Some ectomycorrhizal (ECM) fungi, a group of important microorganism in soil which can form symbiont with 70~75% of the forest trees, in nature have remarkable effects on alleviating aluminum toxicity to trees grown on acidic soils. Selected ECM fungi may benefit potentially to the growth of trees for they may enhance the tolerance of roots to aluminum stress. In order to find the ways to protect trees from detrimental effects of aluminum, we need a deeper understanding of the role of ectomycorrhizal relationships in altering plant response to Al exposure and the mechanism of aluminum toxicity alleviation by ECM fungi.In the experiments, six typical ECM fungi (Pisolithus linctorius 715, Laccaria bicolor S238A, L bicolor S238N, L bicolor 270, Cenococcum geophilum SIV, and Hebeloma repandum), of which some biological aspects have been intensively studied overseas, were chose to investigate the growth, H+ and oxalate secretion, absorption of phosphorus, potassium and aluminum when exposed to Al. Then, seedlings of Pinus massoniana, which were mainly planted in South China and sensitive to Al, were inoculated with LbS238A, LbS238N and ?270, respectively, to determine the influence of mycorrhizal effect on Al toxicity.When exposed to high level of Al, the growth of Pt715 was increased by up to 12%. Cultures of Hbr and LbS238N exhibited a slight increase whereas the biomass of CgSIV was significantly promoted at low level of Al. Cultures of LbS238A and Lb210 were sensitive to Al for the growth of them decreased remarkably by up to 39% and 20% at high Al concentration. The six fungi isolates, in sequence of biomass are CgSIV>Pt715>Lb270>Hbr>LbS238A>LbS238N.Proton efflux of Hbr and CgSIV increased by up to 18% and 45%, whereas that of Pt715,LbS238A and LbS238N decreased when exposed to Al. Both the efflux of oxalate and the ratio of oxalate in total acid in all cultures increased as the concentration of aluminum in the medium increased. The ability to produce oxalate is in the sequence of Lb270>Pt715>LbS238A>LbS238N>Hbr>CgSIV, and the ratio of oxalate in total acid is in the sequence of LbS238A>LbS238N >pt715 >CgSIV >Hbr> Lb270. Accompanied with the increase of oxalate production, Al-P deposition in media dissolved significantly.Al in media significantly increased the concentrations of P, K, and Al in mycelia of five fungi, except Lb270. The fungus Hbr showed the highest increment in concentration of P (64%), followed by LbS238A (55%), LbS238A (45%), CgSIV (25%), Pt715 (21%), and Lb270 (2%). LbS238A had the highest concentration of P in mycelia, followed by LbS238N, Pt715, Lb270, Hbr, and CgSIV. Concentration of P in mycelia of LbS238A was 6.5 times than that of CgSIV. Concentration of K in mycelia is in the sequence of Lb270>Pt715>Lb238A>CgSIV>LbS238N>Hbr. Concentration of K in mycelia of Lb270 was 38 times than that of Hbr. Concentration of Al in mycelia have the sequence of LbS238N>LbS238A>Hbr>Pt715>Lb270>CgSIV. Concentration of Al in mycelia of LbS238N was 11 times than that of CgSIV.In the soil, aluminum led to decline of pH and available nutrients such as N, P, Ca, and Mg (especially P and Ca), whereas concentrations of active Al (especially KCI-soluble Al and NH4Ac-soluble Al) in rhizosphere of all seedlings and unavailable P (Al-P, Fe-P, Ca-P and O-P) in rhizosphere of Nonmycorrhizal (NM) seedlings increased. Compared to the ECM seedlings, the rhizosphere of the NM seedlings showed the highest increment on Al-P, Fe-P, Ca-P and O-P for 29%, 19%, 16%, 36% and the highest decrement on available N, P, Ca, and Mg for 13%, 27%, 17% and 14%.Compared to the rhizosphere of NM seedlings, inoculation of LbS238A decreased the proportion of KCl-soluble Al obviously, but inoculation of LbS238N and Lb270 increased the proportion of KCl-soluble Al and NH4Ac-soluble Al. The proportion of Al-P, Ca-P, and O-P decreased in rhizosphere of seedlings in...