Effects Of Arbuscular Mycorrhizal Fungus On Lead(Pb) Tolerrance Of Robinia Pseudoacacia

Robinia pseudoacacia is one of the main tree species in afforestation in Northwest China.R.pseudoacacia is prevalently employed with the characteristics of fast-growing,extensive root system and tolerance to heavy metals.In the soils suffering from heavy metal contaminations,R.pseudoacacia can establish mutualistic symbiosis with arbuscular mycorrhizal fungi?AMF?.AMF can affect the absorption,accumulation and transfer of heavy metals in host plants.Consequently,the present study investigated the effect of AMF Funneliformis mosseae on the subcellular compartmentalization and chemical forms of lead?Pb?in R.pseudoacacia.We investigated the effect of F.mosseae on root morphology and architecture of R.pseudoacacia and analyzed soil physical and chemical properties and soil enzyme activity in the rhizosphere.We also evaluated the role of glomalin-related soil protein in Pb stabilization?GRSP-Pb?and the association between GRSP-Pb and different Pb forms in soil to reveal the role of AMF in Pb stabilization in the rhizosphere.The main conclusions are as follows:1.Effects of AMF on subcellular compartmentalization and chemical forms of Pb in R.pseudoacacia under Pb stressThe results showed that Pb accumulation in roots,stems and leaves of R.pseudoacacia were increased in a range of Pb concentrations(90-3000 mg kg^-1 Pb),respectively,and the roots had the highest Pb content.Moreover,Pb content in the root of the inoculated seedlings were significantly higher than the non-inoculated seedlings.The proportion of Pb in cell wall,organelles,and solution fractions in roots,stems,and leaves of inoculated and non-inoculated R.pseudoacacia were also increased with increasing Pb concentration,respectively,with most Pb content in the cell wall.The results indicated that AMF inoculation significantly increased Pb percent in cell wall and solution and significantly reduced Pb percent in organelles in roots,stems,and leaves of R.pseudoacacia,respectively,especially the Pb proportion in the cell wall of root.Additionally,AMF inoculation decreased the active Pb content?extracted by 80%ethanol and deionized H₂O?and increased the inactive Pb content?extracted by 1 M Na Cl,2%HOAc,and 0.6 M HCl,and Pb in residues?in roots,stems,and leaves of R.pseudoacacia.The subcellular compartmentalization of Pb in different c hemical forms were highly correlated with improved growth parameters in the inoculated seedlings,indicating that AMF could improve Pb tolerance of R.pseudoacacia by altering chemical speciation in subcellular distribution in their tissues,especially the roots.2.Effects of AMF on root development of R.pseudoacacia under Pb stressThe study of effect of AMF on the root activity,root morphology and architecture of R.pseudoacacia under Pb stress showed that low Pb(90 mg kg^-1 Pb)significantly improved root activity of inoculated and non-inoculated seedlings,root activity of AMF inoculated seedlings were greater than non-inoculated seedlings;high Pb(900,3000 mg kg^-1 Pb)significantly reduced root activity of inoculated and non-inoculated seedlings.However,the decrease of root activity of inoculated seedlings were significantly lower than non-inoculated seedlings.At low Pb concentration,root length,root volume,root surface area,number of root tips and number of root forks in inoculated seedlings were increased more than the non-inoculated seedlings.Under high Pb concentration,root length,root volume,root surface area,number of root tips and number of root forks in inoculated seedlings were decreased lower than the non-inoculated seedlings.Moreover,AMF inoculation increased the root length,root surface area,root volume,and number of root tips in different root diameters.The total colonization were positively with root volume and root dry weight,but negatively with root diameter and Pb content in root.The results indicates AMF inoculation relieved Pb toxicity to root development of R.pseudoacacia by root activity,root morphology and architecture under Pb stress.3.Effects of AMF on soil physical and chemical properties and enzyme activities in the rhizosphere of R.pseudoacacia under Pb stressAMF inoculation significantly increased the activity of urease,sucrase,and alkaline phosphatase.Soil organic matter,available potassium and available phosphorus in the soil of inoculated seedlings were significantly higher than the non-inoculated treatments.The trend in nitric nitrogen in soil was different between inoculated and non-inoculated seedlings.With increasing Pb concentration,nitric nitrogen was decreased in non-inoculated seedlings but increased in inoculated seedlings.Under low Pb treatment,nitric nitrogen in the rhizosphere soil of inoculated seedlings was lower than non-inoculated seedlings,but nitric nitrogen in rhizosphere soil of inoculated seedlings was higher than non-inoculated seedlings.Ammonium nitrogen was decreased with increasing Pb concentration,with more decrement in non-inoculated seedlings.Pb treatment decreased pH value in rhizophere soil,and pH value in the inoculated treatment was significantly lower than non-inoculated treatment.The spore density was positive with the content of soil organic matter,available potassium,available phosphorus and the activity of urease,sucrose and alkaline phosphatase.This suggests that F.mosseae can influence soil properties and soil enzyme activities under Pb stress.4.Effects of AMF on Pb content in the rhizosphere soil of R.pseudoacacia under Pb stressThe effects of F.mosseae on Pb content in the rhizosphere soil of R.pseudoacacia under Pb stress were investigated.With increasing Pb concentrations,the content of diethylenetriamine pentaacetic acid extractable Pb?DTPA-Pb?,soluble Pb,oxidizable Pb,reducible Pb content,and residual Pb were gradually increased.AMF inoculation produced an increase in content of DTPA-Pb,soluble Pb and oxidizable Pb,and resulted in a reduction in content of reducible Pb and residual Pb.Glomalin,a special glycoprotein secreted by AMF,increased the content of stable Pb in soil?GRSP-Pb?,which were positively correlated with DTPA-Pb,soluble Pb,reducible Pb,oxidizable Pb and residual Pb contents.These findings suggest that AMF inoculation and GRSP-Pb influences the fractionation and bioavailability of Pb in soil,and impacts the absorption of Pb by R.pseudoacacia roots under Pb stress.