| In order to solve the prominent problem of Cd pollution in agricultural land in Hunan Province,the Kochia scoparia with large root system,strong stem,many branches,strong adaptability and large biomass was used as the test plant.The outdoor pot experiment was conducted to study the remediation effect and mechanism of Kochia scoparia on weakly acid Cd-contaminated soil.The concentrations of Cd2+in the pot experiment were set as 0 mg·kg-1、0.3 mg·kg-1、0.9 mg·kg-1、1.5 mg·kg-1、3.0 mg·kg-1、9.0 mg·kg-1,and the p H values of the two groups were set as p H 6.1（T treatment group）and p H 5.0（TS treatment group）.Through the determination and analysis of root index,leaf photosynthetic pigments and plant enzyme activities of Kochia scoparia,the physiological and biochemical responses of Kochia scoparia to Cd-contaminated soil were investigated.Subcellular differential centrifugation and chemical reagent continuous extraction were used to explore the accumulation of Cd by Kochia scoparia,the distribution and chemical morphological characteristics of Cd in subcellular,and the accumulation characteristics and tolerance mechanism of Cd by Kochia scoparia were clarified.Macrogene sequencing was performed on the rhizosphere and non-rhizosphere soils of the Kochia scoparia enrichment group under Cd stress,and the composition and diversity of bacterial and fungi communities in rhizosphere and non-rhizosphere soils were compared and analyzed.The main results were as follows:（1）Kochia scoparia had high survival rate under different concentrations of Cd stress.During the whole experiment period,all Kochia scoparia plants survived and grew normally except one Kochia scoparia in one pot under T5 treatment and one and three in one pot under TS6 treatment.（2）Cd stress affected the physiological and biochemical indexes of Kochia scoparia.On the plant height,the average plant height of Kochia scoparia under T1treatment was the largest,which was 128.43±8.51 cm,and the average plant height of Kochia scoparia under T6 treatment was the smallest,which was 108.47±9.59 cm.The plant height of Kochia scoparia under T4 and T6 treatment was significantly lower than that of T1 treatment,and the plant height of Kochia scoparia under T6 treatment was lower than that of T1 treatment,but not significantly.The mean Kochia scoparia plant height of TS treatment group was between 122.08±10.54 cm and 130.95±11.34 cm,and there was no significant difference between each treatment and the control.The Kochia scoparia plant height of TS2,TS4 and TS6 treatments was lower than that of the control TS1.In the biomass of Kochia scoparia,the aboveground biomass of Kochia scoparia plants in all treatments was higher than that of the underground.The total biomass of Kochia scoparia under T1～T6 was 38.09±5.69～60.27±17.94 g,and the biomass of Kochia scoparia under T5 and T6 was significantly higher than that of T1.The total biomass of Kochia scoparia under TS1～TS6 treatment was 46.76±13.27～79.64±18.36 g,and there was no significant difference between each treatment and the control TS1.In the T treatment group,only the root surface area,average diameter and root tip number of Kochia scoparia in the T6 treatment were significantly decreased compared with those in the control T1.In the TS treatment group,there was no significant difference in the surface area and volume of Kochia scoparia roots compared with the control TS1.With the increase of Cd concentration,the photosynthetic pigments in Kochia scoparia leaves of T and TS treatment groups showed an overall upward-downward-upward-downward trend,and the total chlorophyll content in Kochia scoparia leaves of TS treatment group was 3.37%～22.52%higher than that of T treatment group.Under the same concentration of Cd treatment,the SOD activity of T treatment group was higher than that of TS treatment group,and the MDA content was lower than that of TS treatment group.The SOD,POD activity and MDA content of Kochia scoparia roots were the highest under T2treatment,and the lowest under T6 treatment.The antioxidant enzyme activity of Kochia scoparia roots was significantly correlated with MDA content（P<0.05）.（3）Kochia scoparia has a strong ability to accumulate Cd in soil,and Kochia scoparia accumulates 58.09%～89.35%of Cd in stems.The enrichment coefficient and transport coefficient of Kochia scoparia in T treatment group were greater than 1 under different concentrations of Cd stress,and the Cd accumulation in stems was 4.01～6.50times that in roots.The Cd accumulation in roots and stems of Kochia scoparia in T6treatment reached the highest,which were 175.15±63.09μg·pot-1 and 1009.22±219.23μg·pot-1,respectively.The removal rate of Cd by T treatment Kochia scoparia was0.84±0.20%～1.85±0.49%,the removal rate of Cd by root was 0.17±0.01%～0.31±0.02%,and the removal rate of Cd by stem was 0.67±0.19%～1.54±0.49%.The removal effect of Cd by T4 treatment Kochia scoparia was the best,reaching 1.85±0.49%.In the TS treatment group,the Cd accumulation in the roots and stems of Kochia scoparia reached the highest under TS6 treatment,which were 178.24±71.98μg·pot-1and 789.75±534.22μg·pot-1,respectively.The removal rates of Cd in the TS treatment group were 0.94±0.07%～2.38±0.22%,in which the removal rates of Cd in the roots were 0.11±0.03%～0.63±0.12%,and those in the stems were 0.72±0.06%～1.76±0.33%,The removal efficiency of Cd by Kochia scoparia under TS1 treatment reached 2.38±0.22%.（4）The roots and stems of Kochia scoparia have detoxification ability to Cd.More than 85%of Cd in roots and stems of Kochia scoparia was stored in cell walls and vacuoles,indicating that the two were important sites for the distribution of Cd in Kochia scoparia cells and detoxification.At the same time,it was found that the proportion of Cd in root organelles was lower than that in stem,which was an important reason for the accumulation of more Cd in stem.The chemical speciation of Cd in roots and stems of Kochia scoparia showed that most of Cd were extracted by acetic acid,sodium chloride and ethanol,and the content of acetic acid was the highest（T:33.20%～56.24%;TS:30.39%～52.32%）,and the content of Na Cl and ethanol was close.This Cd occurrence form was an important defense mechanism for Kochia scoparia to reduce Cd bioavailability and toxicity.（5）Under Cd stress,the biodiversity of rhizosphere soil with the best Kochia scoparia enrichment ability was higher than that of non-rhizosphere soil.The number of OTU in rhizosphere soil of Kochia scoparia was more than that in non-rhizosphere soil,and the bacterial species in rhizosphere soil were more than that in non-rhizosphere soil,but the fungi species in rhizosphere soil was lower than that in non-rhizosphere soil.At the phylum level,Chloroflexi,Proteobacteria,Bacteroidetes and Firmicutes were the dominant bacteria in rhizosphere and non-rhizosphere soils of Kochia scoparia.At the fungi phylum level,Ascomycota,Basidiomycota,Mortierellomycota and Chytridiomycota are the dominant fungi phyla in rhizosphere and non-rhizosphere soil of Kochia scoparia.Gemmatimonas,Gp1,Gp3,unclassified＿Sphingomonadaceae are the dominant bacteria in rhizosphere soil,Cheilymenia and Fusarium are the two dominant fungi in rhizosphere and non-rhizosphere soil of Kochia scoparia,and the relative abundance is more than 60%. |
