| Open-pit mine tailings have the characteristics of high heavy metal content,extreme pH,low organic matter content,and sandy soil,which are not conducive to plant colonization,and there are serious ecological and environmental safety issues.Choosing woody plants as pioneer plants to restore vegetation in tailings ponds is a more appropriate method of restoration.Ectomycorrhizal fungi are common in nature and can form ectomycorrhizal fungi with most woody plants,which help the host plant absorb water and nutrients,promote plant growth,and improve host plant tolerance to stress.Based on the field investigation of mine tailings,our study investigates the resources of ectomycorrhizal fungi in the tailings dam and its surroundings,screens out highly tolerant ectomycorrhizal fungi,initially explores their tolerance mechanism to heavy metals,and clarifies the ectomycorrhizal fungi's absorption of tailings by host plants Influence of heavy metals in reservoir soils and selection of suitable ectomycorrhizal fungi-woody plant symbiota for tailing reservoir repair.Field surveys of 5 Pb/Zn/Cd tailings ponds in Guizhou found that 3 tailings ponds only have a small amount of Pinus plants,and the other 2 tailings ponds have a small amount of woody and herb.Identification of ectomycorrhizal fungi on pine plants growing in tailings ponds,found that most of the native ectomycorrhizal fungi in tailings ponds are Suillus,and a small amount of Amphinema sp.and Trichophaea sp.A variety of ectomycorrhizal fungi fruiting bodies were collected from the surroundings of the above-mentioned five tailings ponds,and 7 species of Suillus mycorrhizal fungi were isolated and purified.The evolutionary analysis of the native ectomycorrhizal fungi and the collected fruiting bodies of the tailings pond revealed that one of the fungi had high homology with the native ectomycorrhizal fungi of the tailing pond.The analysis identified these fungi as S.luteus(S1).The Cd high-tolerancefungi S.cavipes(Sc)was screened by conducting heavy metal(Cd)tolerance tests on the seven Suillus in vitro.Both fungi can grow under higher Cd stress.Among them,Sl can grow at a maximum of 60 mg/L.and Sc can grow at a maximum of 80 mg/L Cd.Further tests on the heavy metal resistance of the two ectomycorrhizal fungi S.luteus(Sl)and S.cavipes(Sc)were carried out.It was found that the two fungi have high resistance to a variety of types(Pb,Cd,and Zn).It can accumulate a large amount of three heavy metals.Under the treatment of 150 mg/L Zn2+,Sc can accumulate 2005.2 mg/kg Zn.The growth of Sl under 5mg/L Cd2+stress was better than that of Sc,but it was the opposite under 20mg/L Cd2+stress.Sc can still grow under the stress of 80mg/L Cd2+or 300mg/L Pb2+or 1000mg/L Zn2+.The research on the Cd tolerance mechanism of two fungi found that the physiological mechanisms of Cd2+tolerance of the two exophytes mainly include cell wall adsorption,intracellular compartmentalization,improvement of ion balance,and organic acid adsorption.With the increase of Cd2+concentration,the Cd bound to the cell wall of Sl was transformed to the organelle,and the cell wall adsorption decreased.Sc cell wall-bound Cd transitioned to the soluble part,and cell wall adsorption increased.Under low Cd2+treatment,the high-toxicity ratio of ethanol and ultrapure water in Sl bacteria was lower than that in Sc.The results were opposite under 20mg/L Cd2+treatment.The ratio of weaker hydrochloric acid was higher than that in Sl.The nutritional status(Pi,Mg,Fe,and Ca absorption)of Sl was better than that of Sc under low concentration Cd2+treatment,but the result was opposite under high concentration Cd2+ stress.Under Cd2+stress,the formic,malic,and succinic acid secretions of the two fungi increased,which helped reduce Cd2+toxicity.In order to investigate the effects of two fungi S.luteus(Sl)and S.cavipes(Sc)on Pinus sylvestris to absorb heavy metals in tailings pond soil,a root bag experiment was designed,and non-mine fungi Cenococcum geophilum(Cg)and non-mycorrhizal(NM)P.sylvestris as controls.The experiment designed two kinds of root bags:open bag(allow fungal hyphae to pass)and closed bag(no fungal hyphae to pass).Both kinds of root bags are filled with the same non-contaminated soil.Five months later,the mycelia of the three ECM P.sylvestris in the open bag passed through the open bag filter membrane into the tailings pond soil.while the closed bag did not pass through the plastic film.As the mycelium penetrated the root sack and entered the soil of the tailings pond around the root sack.the soil heavy metal content in the open root sack of ECM P.sylvestris increased.The heavy metals of plant roots are absorbed and transported into the soil of rhizosphere.The inoculation of ECM fungi can significantly increase the biomass of the host plant.This growth-promoting effect results in no significant difference in the concentrations of heavy metals in ECM and NM plants(Pb concentration in the root of ECM P.sylvestris is even lower than NM).Compared with NM plants,ECM plants in the open root bag significantly increased the total heavy metal accumulation.In addition,compared with the closed root bag,the nutrient status of ECM in P.sylvestris in the open root bag is better than that in NM P.sylvestris,indicating that the extended hyphae can also promote the host plant to obtain the essential mineral elements in the soil outside the root bag.The three types of ECM P.sylvestris exhibited different heavy metal accumulation,which was similar to the fungal Cd tolerance.The high heavy metal-resistant bacteria Sc mycorrhizal P.sylvestris showed the highest level of heavy metal accumulation,followed by Sl,and Cg had the lowest heavy metal enrichment level.In order to explore the repair effect of two Suillus auxiliary plants on the tailings pond,we selected four additional ectomycorrhizal fungi Cenococcum geophilum(Cg),Pisolithus sp1.(P1),Pisolithus sp2.(P2),and Laccaria sp(Ll).As control comparisons.And non-mycorrhizal P.sylvestris,a total of 6 species of ectomycorrhizal fungi,were planted in the tailing pond soil.After 4 months,the plant growth,photo synthetic parameters,nutrients and heavy metal content,as well as changes in soil physical and chemical properties,enzyme activity,and available state of heavy metals were measured.The results show that inoculation with ECM fungi can increase biomass,photosynthesis(photosynthesis rate(Pn),stomatal conductance(Gs),transpiration rate(Tr),intercellular CO2 concentration(Ci)),and mineral absorption(phosphorus(Pi),Magnesium(Mg),iron(Fe),calcium(Ca))to improve the survival rate of the host plant,while reducing the transfer coefficients of Cd,Pb and Zn,and increasing the bioaccumulation coefficient(above-ground heavy metal concentration/soil heavy metal concentration).In addition,compared with NM plants,ECM P.sylvestris accumulates more Cd,Pb and Zn,while reducing soil pH and available heavy metal states(DTPA-Cd,DTPA-Pb,DTPA-Zn)and increased soil enzyme(acid phosphatase,alkaline phosphatase,urease)activity.Compared with non-mineral ECM fungi,Sl and Sc infected P.sylvestris had the highest accumulation of heavy metals(Pb,Zn,and Cd),photosynthetic parameters,biomass,soil enzyme activity,and BCF.Therefore,the selection of indigenous ectomycorrhizal fungi with high heavy metal tolerance can more effectively assist pioneer plants in repairing the tailings pond soil and improve the plant colonization of abandoned bare tailings ponds. |
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