Study On Cadmium-Resistant Endophytes Promoting Trifolium Repens L. To Rehabilitate Soil Cadmium Pollution

In recent years,it is difficult to repair the soil in Qinling Lead-Zinc tailing area and its surrounding soils,because of the pollution of heavy metals such as Cadmium.The existing physical and chemical remediation technology had trouble in repairing soils polluted by heavy metals thoroughly.However,due to the presence of high concentration heavy metals such as Cadmium,surrounding plants grew slowly.Therefore,it is a safe,environmental friendly,simple and effective way to construct a plant-microbial combined remediation system for Cadmium-contaminated soil in tailing and its surrounding areas using screened Cadmium-resistant strains with growth-promoting ability and hyper-accumulating plants.In this study,the endophyte of Periploca sepium Bunge and Melilotus suaveolens Ledeb,the dominant plants in the Qinling lead-zinc tailing area,were isolated and identified by morphological and molecular biology methods.At the same time,the removal rate of Cd²⁺and the growth-promoting potential of the strains,such as the activity of indole acetic acid?IAA?and Siderophore,were studied.The strains GR1 and GR44 were screened by pot experiment,which could effectively improve the cadmium tolerance of Trifolium repens seedlings.By adding cadmium-resistant endophyte?GR1 and GR44?to Trifolium repens under different cadmium concentration stress,and the biomass,physiological index,cadmium content of Trifolium repens as well as cadmium content,enzymatic activity,physical and chemical properties and microbial quantity in rhizosphere soil,were determined to study the effects of cadmium-resistant endophyte?GR1 and GR44?on the remediation of cadmium contaminated soil by Trifolium repens.It can provide technical support for plant-microbial combined remediation technology to repair Cadmium-contaminated soil.The main conclusions are as follows:1.The results showed that 11 Cadmium-resistant endophytic bacteria were isolated and screened with the condition of 800.0 mg/L concentration.All strains were identified as Bacillus sp..Biological characteristics showed that the removal rate of Cd²⁺for all strains were range of 14.56%⁷9.45%at 800.0 mg/L Cd²⁺concentration level and the strain GR44 reached 79.45%was the highest.There were7 strains which had activity of IAA and strain GR44,GR49,CG7 were higher.Siderophore activities of all strains were more than 50%and each one had activity of ACC deaminase.Among of them,Siderophore activity of GR44 was the highest,and ACC deaminase activity of GR54 was the highest.The results of pot experiment of Trifolium repens showed that compared with the control group,the physiological indexes of Trifolium repens seedlings treated with strain GR44 increased significantly,plant height increased 94.79%,root length increased 99.81%,fresh weight increased63.93%,dry weight increased 106.58%,chlorophyll content increased 52.10%,soluble sugar content increased 26.87%,and soluble protein content increased20.72%.Therefore,the overall promotion effect was significant.?P<0.05?.The Cadmium-resistant endophytic bacteria which was GR44 could be used as dominant strains for constructing the plant-microbial combined remediation technology,which had positive significance for reclamation of tailing and its surrounding areas.2.The results showed that 5 strains of endophytic fungi which could tolerate 30000.0 mg/LCd²⁺were screened and identified.GR1 was belonging to Talaromyces sp.,GR4 was belonging to Colletotrichum sp.,GR9 was belonging to Purpureocillium sp.,both GR7 and GR10 were belonging to Fusarium sp..When the concentration of Cd²⁺was 1 000.0 mg/L,GR7 had the highest removal rate of Cd²⁺?84.45%?.4 strains had the IAA secretion ability,GR4 was the highest one?99.417±0.59 mg/L?.GR7 had the highest siderophore activity?84.64%?.GR9 had the highest extracellular CAT activity?10.01 U/mL?.Among them,strain GR1 had the highest available phosphorus content?201.67 mg/L?and strain GR4 had the highest available potassium content?105.11 mg/L?.The results showed that all five endophytic fungi with cadmium resistance could significantly improve the cadmium tolerance of Trifolium repens seedlings.Compared with the control group,the plant height,root length,fresh weight,dry weight,chlorophyll content,soluble sugar content and soluble protein content increased by 65.28%,108.92%,63.58%,106.42%,72.16%,13.43%and 12.05%respectively.Therefore,the overall promotion effect was significant?P<0.05?.Experiment results showed that the Cd-resistant endophytic fungi GR1 could be used as a dominant strain for plant-microbial combined remediation technology to repair soil.3.Cd-resistant endophytes GR1 and GR44 were cultured and were added to Trifolium repens under different concentrations of Cd²⁺stress.Compared with the control group,when Trifolium repens was under Cadmium Stress of 30.0-50.0 mg/kg,the biomass and physiological indexes of the treated group were significantly higher than those of the control group by adding Cadmium-resistant endophyte GR1 and GR44.The cadmium content,enzyme activity,physical and chemical properties and microbial quantity in rhizosphere soil had significant restoration effects.The combination system which associated Cadmium-resistant endophyty?GR1 and GR44?with Trifolium repens can enhance the remediation effect of soil Cadmium pollution.Through the experiment of combined remediation of screened Cd-resistant endophytes and Trifolium repens,we provided technical support for plant-microbial combined remediation of cadmium contamination in tailing and its surrounding areas in order to improve the remediation efficiency of Cadmium contamination soil.