| Ectomycorrhizal fungi, a group of important fungi in soil, can form symbioint with 70-75% of the trees for artificial reforestation. Selected mycorrhizal fungi could benefit potentially the growth, nutrient status of the host plants as well as their resistance to drought, salt and diseases caused by insects and microorganisms after the infection of the host roots. Therefore, they are widely used in forestry and artificial forestation.Some ectomycorrhizal fungus could, to some extent, improve the abilities of their hosts to resist adverse conditions by variable ways. It seems thus reasonable to suggest that they could effectively alleviate the toxicities of heavy metals and be applied in artificial forestation for the soils and ore debris contaminated by heavy metals. The mycorrhizal fungus and heavy metals studied previously in regard to toxicology are, however, too limited to summarize the fungal resistance to heavy metals. It is not thoroughly clear to understand the abilities of mycorrhizal fungi to resist heavy metal pollution, mechanisms related and potentiality in application in that case. It is thus necessary for us to use more mycorrhizal fungal species and strains intensively studied, which could be applied potentially in artificial forestation in the future, to investigate their resistance to heavy metal toxicities and mechanisms.Cd and Hg are heavy metals largely released, compared to other ones, into environment by human activities. In the present experiment, Pisolithus tinctorius 715, isolated from an eucalyptus growing red soil in Xichang, China, in 1996, and Cenococcum geophilum SIV, isolated from pine growing soil in Spain in 1970s, were adopted. Cd and Hg were added into culture solutions with different concentration gradients. The fungal growth, efflux of protons and oxalate and influx of nitrogen, phosphorus and potassium were observed. The objectives were (i) to realize their resistances to Cd and Hg and the possible mechanisms, (ii) to provide some information for the artificial forestation for soils and ore debris contaminated and (iii) to prevent the continual release of the heavy metals from polluting other territorial and aquatic ecology systems. Following are the results obtain in our experiment.Mycorrhizal fungal growth was slightly inhibited in low Hg concentrations and depressed less with the concentrations increased, indicating the resistance and adaptation to Hg toxicity. But the growth reduced suddenly or stopped as the concentrations go beyond critical ones. Fungal species varied in growth rate under Hg stress. Pt 715 was sensitive and Cg SIV was less sensitive to Hg stress. Cg SIV was quite susceptible to Cd in growth. Thebiomass lowered with Cd concentration increased. Negative correlation was found between biomass and Gd concentration gradients (y = 81.61 - 3.88X, r = -0.987). However, Pt 715 was much less susceptible to Cd compared to Cg SIV. The biomass of Cg SIV cultivated in 15mg Cd / L culture solution reduced 74.40% on the base of blank control, while it reached 73.64% of blank when Pt 715 in 140mg Cd / L solution.Mycorrhizal fungal growth was depressed variously in culture solutions with Cd or Hg added. The growth inhibition of Pt 715 by Hg was more obvious than Cd. The toxic indexes were 1.614 for Hg and 0.576 for Cd. In contrast, it behaved otherwise for Cg SIV. The toxic indexes were 0.931 for Hg and 4.59 for Cd.Efflux of organic acids and protons companied with influx of cation ions in mycorrhizal fungi in the growing period. The fungal species differentiated in proton efflux in heavy metal solutions. The proton efflux by Cg SIV changed very little if any in low Hg culture solutions compared with the blank treatment in contrast to Pt 715, which emitted less protons in this case. The efflux of protons by both Pt 715 and Cg SIV was, however, depressed in high ones. There were positive linear correlations between the proton efflux and fungal biomass, i.e. y = 0.567 + 0.027X for Cg SIV (r = 0.987) and y =0.653 + 0.087X for Pt 715 (r = 0.998). The re...
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