Study On The Mechanism Of Plant Growth-promoting Bacteria In Uranium-contaminated Soil With Associated Heavy Metals

In this study,the native plant Rumex acetosa L.in a uranium tailings area and the hyper-enriched plants of uranium and associated heavy metals,Medicago sativa,Cichorium intybus L.and Brassica juncea,were used as the research materials.A strain of plant growth-promoting bacteria were screened and isolated by enrichment culture method and average absorbance method from Rumex acetosa L.,and the growth and growth characteristics of the stain were studied.Pot experiments were conducted to find that the effects of growth-promoting bacteria on biomass and enrichment characteristics of Medicago sativa,Cichorium intybus L.and Brassica juncea under single factor experiment,and the Medicago sativa with the highest improvement in biomass and enrichment was screened.The effects of plant growth-promoting bacteria on physiological and biochemical indexes and photosynthetic performance of Medicago sativa under single pollution treatment of uranium and associated heavy metals were analyzed.This can be used as a reference for the ecological treatment of radioactive contaminated soil.The main results are as follows:?1?22 different bacterial strains were isolated from the root of Rumex acetosa L.,of which 10strains had certain growth-promoting characteristics.Finally,a strain D16 was obtained by quantitative analysis of growth-promoting characteristics.The MIC?minimum inhibitory concentration?of the strain was 350 mg/Kg?U?,1200 mg/Kg?Mn?,800 mg/Kg?Pb?,125 mg/Kg?Ni?.IAA production was 40.21 mg/L in 24 hours under normal medium.The 24-hour ACC activity in normal medium was 0.32 U/ug.The optimum IAA production conditions for the strain were 35?,pH of 7,150 r/min,yeast extract as nitrogen source and glycerol as carbon source.The optimum conditions for ACC activity of the strain were 30?,pH of 7 and 180 r/min.Combined with morphological characteristics,physiological and biochemical characteristics and 16S rDNA sequence,the strain was identified as Achromobacter xylosoxidans of Burkholderia.Fluorescence tracer experiment showed that D16 strain had the largest total amount in simulated contaminated soil on the 10th day.?2?Pot experiments were conducted to compare the effects of D16 strain on the growth of Medicago sativa,Cichorium intybus L.and Brassica juncea as well as on the enrichment characteristics of uranium and associated heavy metals.Compared with Cichorium intybus L.and Brassica juncea,the biomass of Medicago sativa in each treatment increased significantly.Under the Mn treatment of 800 mg/kg,Medicago sativa.was increased the maximum dry weight to0.970g by D16 strain.Under various U treatments,compared with the control group,D16 strain significantly increased the dry weight of Medicago sativa by 22.7%-83.3%.Under U1 treatment,the enrichment coefficient of Medicago sativa increased by 48.7%compared with the control group,which was 0.186.Under Ni1 treatment,the maximum enrichment coefficient of Medicago sativa was 0.512.Under the treatment of uranium and associated heavy metals,the transfer factor of D16strain to Medicago sativa did not increase significantly.?3?Pot experiments were conducted to study the effects of strain D16 on the antioxidant system of Medicago sativa in roots and photosynthetic system of Medicago sativa in Leaves under the treatment of uranium and associated heavy metals.The results showed that the activities of antioxidant enzymes in the roots of Medicago sativa increased compared with the control group.Under U,Mn and Ni treatments,SOD?superoxide dismutase?showed an overall increasing trend with the increase of heavy metal concentration in soil,while under Pb treatment,SOD increased first and then decreased.The maximum SOD activity was 51.17 U/g under the treatment of Pb2,which increased by 15.6%compared with the control.Under the treatment of U1,the SOD activity was increased by 25.9%compared with the control.D16 strain also promoted Medicago sativa POD,but did not reach a significant level.The maximum POD activity was 28.14 U.g-1.min-1under Mn3 treatment,which was 13.0%higher than that of the control.Under U1 treatment,the enzyme activity increased by 14.1%.CAT activity also increased in general.Under Ni1 treatment,the enzyme activity increased by 9.3%.D16 strain also reduced MDA content of Medicago sativa to a certain extent.Under Ni1 treatment,MDA had a minimum value of 1.16 umol/g,which was17.2%lower than that of the control.With the increase of the concentration of uranium and associated heavy metals,the fluorescence value gradually decreased,and increased with the addition of strain D16.Under U1 treatment,the Vj value was the highest,0.57,which increased by12.7%compared with the control.Under U2 treatment,Fv/Fm increased the most,which was11.2%more than the control.Under U1 treatment,PIabs increased by 28.2%.The change of photosynthetic value of Medicago sativa was irregular.Under U1 treatment,the photosynthetic rate increased by 86.8%.D16 strain also increased stomatal conductance by 36.1%under Mn3treatment.The intercellular CO₂ increased by 24.1%and the transpiration rate increased by 12.9%under Mn2 treatment,at the same time.