| As a long-term and very serious environmental problem,soil Pb pollution has attracted extensive attention in China.Phyto-microbial remediation of heavy metal pollution in soil is the most suitable treatment scheme.Woody plants,because of their large biomass,and not being easily flow into the food chain,gradually began to replacement of herbs and crops as a more appropriate choice.PGPR,as a natural microbial community in soil-plant system,which is closely related to plant growth,has become one of the main applications of plant microbial joint remediation.The key of phyto-microbial remediation of Pb pollution is the effect of microorganisms on plant growth and lead tolerance under Pb stress.In this paper,two-year seedlings of Melia azedarach were planted in a pot with slag/soil at a ratio of 10%and 50%.One plant was planted in each pot.One year later,rhizosphere soil was taken to screen PGPR and test the lead tolerance.17 lead resistant PGPR strains were obtained,among which B1-B7 were the same strain,belonging to Pseudomonas sp.;D1-D10 were the same strain,belonging to Pseudomonas sp.The Pseudomonas(B1)and Pseudomonas montallii(D8),which have strong ACC deaminase activity,high IAA content and certain iron production capacity,were selected as the test strains for pot inoculation experiment.The inoculation amount was 108 CFU bacterial solution 40 ml twice in total.There are three Pb pollution concentrations:0 mg/kg,100 mg/kg and 500 mg/kg.The three treatment groups are:uninoculation,B1 bacteria inoculation,D8 bacteria inoculation,namely CK-0,CK-B,CK-D,L1-0,L1-B,L1-D,L2-0,L2-B and L2-D.The effects of rhizosphere growth promoting bacteria on Pb uptake and accumulation and growth physiology of Melia azedarach seedlings under different Pb concentration stress were studied.The main results are as follows:1.Under Pb stress,the number of culturable colonies of inoculated strains could reach 106 after 60 days,showed that the colonization of inoculated strains in rhizosphere soil was good.2.With the increase of Pb pollution level,RGR of plant height,ground diameter and biomass of leaf,stem and root decreased gradually,and seedling growth was inhibited.The activity of leaves POD enzyme first maintained and then decreased,but there was no significant change in root;the SOD enzyme activity of leaves first remained unchanged and then increased,and that in root gradually decreased;the CAT enzyme activity of leaves remained unchanged and that in root first remained unchanged and then increased;the activity of APX and GR enzyme in root and leaves first decreased and then maintained a lower level;the content of leaves soluble protein first maintained and then increased,and the content of root first decreased and then maintained;the content of MDA in leaves and root both first remained unchanged and then increased.The results showed that the activities of antioxidant enzymes in leaves and root were affected by Pb pollution.Most of the enzyme activities decreased and MDA content increased significantly under high concentration Pb treatment.3.Compared with L1-0,the content of Pb of L1-B treatment slightly decreased,the content of Pb in the aboveground part increased,the content of Pb in the root was small,but three of them all had no significant differences with L1-0,the transport coefficient(TF),the enrichment coefficient(BCF)of the aboveground part increased,the BCF of the root slightly decreased but not significant;the content of Pb in L1-D soil slightly decreased but not significant,the content of Pb and BCF in the aboveground part and the root,TF all increased significantly.Compared with L2-0,the Pb content of in L2-B treatment soil decreased by 11.42%,and the Pb content and BCF of aboveground part and root,TF all significantly increased;the Pb content of soil in L2-D treatment group slightly decreased,the Pb content of aboveground part increased,both not significantly,the Pb content of root,BCF of aboveground part and root significantly increased,and TF had no significant change.4.Compared with L1-0,there were no significant changes in relative growth rate(RGR)of plant height,ground diameter and biomass of leaf,steam and root in L1-B treatment group,the activities of peroxidase(POD),superoxide dismutase(SOD),ascorbic acid peroxidase(APX)decreased significantly,catalase(CAT)and glutathione reductase(GR)did not change significantly,the contents of soluble protein and MDA increased significantly;the activities of POD,SOD,CAT and GR in root increased significantly,APX enzyme activity,soluble protein and MDA did not change significantly.There was no significant change in RGR of plant height,ground diameter and stem biomass in L1-D treatment group,the RGR of leaf and root biomass increased significantly,and no significant changes occurred in SOD,CAT,GR activities and MDA content,the content of soluble protein increased significantly;the enzyme activies of POD,APX and GR in root had no siginificant changes,the SOD and CAT activities,the soluble protein content in root increased remarkably,the content of MDA notably decreased by 44.35%.5.Compared with L2-0,the plant height and ground diameter RGR,leaf and root biomass RGR of L2-B treatment group increased significantly,stem biomass RGR did not change significantly,leaf POD,CAT,GR enzyme activity,soluble protein and MDA contents increased significantly,and SOD enzyme activity decreased significantly,APX did not change significantly;POD,SOD,APX enzyme activity,soluble protein and MDA content in root increased significantly,but CAT and GR enzyme activities did not change significantly.There were no significant changes in plant height,stem and root biomass RGR in L2-D treatment group,but the ground diameter and leaf biomass RGR increased significantly,the activities of POD,APX,GR and MDA content in leaf increased significantly,the SOD activity decreased significantly,the activity of CAT and soluble protein content did not change significantly;the activities of SOD,CAT and MDA content in root increased significantly,but POD,APX,GR activities and soluble protein content did not change significantly.In conclusion,the growth of Melia azedarach seedlings was inhibited and the activity of antioxidant enzymes was decreased under the lead pollution treatment without inoculating.After inoculation,under the low concentration of lead stress,inoculation of strain B1 significantly promoted the transfer of Pb to the above ground,but the activity of antioxidant enzymes in leaves decreased while the content of soluble protein increased slightly,the degree of membrane lipid peroxidation in leaves deepened,and the activity of some antioxidant enzymes in roots increased,but the increase of SOD enzyme and soluble protein which played the main role was small.When inoculated with strain D8,the relative growth rate,Pb absorption and enrichment of seedlings increased significantly,and the soluble protein content and SOD enzyme activity of leaves and roots increased significantly.Under high concentration of lead stress,inoculated strain B1 had better effect than D8.Inoculated strain B1 can still maintain the ability of seedlings to transfer to the above ground,and significantly improve the antioxidant enzyme activity and Pb absorption and enrichment coefficient of leaves and roots,while inoculated strain D8 can only significantly enhance the antioxidant enzyme activity and Pb absorption and enrichment of roots.Under lead stress,the relative growth rates of leaf and root biomass of Melia azedarach seedlings were significantly increased,and the antioxidant enzyme activity of the roots was significantly enhanced when inoculated with the tested strains.There are two ways for the inoculation to improve the lead resistance of the seeds of Melia azedarach seedlings:one is to simultaneously promote the absorption and enrichment of Pb and the growth of the plant to maintain the survival of the plant,so as to increase the degree of its resistance to Pb poisoning;the other is to eliminate the ROS produced by Pb entering cells and reduce the toxicity by improving the enzyme activity of antioxidant system and promoting the synthesis of soluble protein,so as to further improve the absorption and enrichment of Pb.Both methods can improve the remediation efficiency of Pb pollution. |
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