Absorption Capacity Evaluation And Accumulation Characteristics Of Heavy Metals Of Common Shrubs In Beijing

Heavy metal pollution in the soil is an important ecological problem in cities.Studies have shown that the concentrations of cadmium(Cd),zinc(Zn),lead(Pb)and copper(Cu)in Beijing soil were significantly higher than their background values,which should be paid attention to.Using plants to absorb heavy metals is an effective way to improve urban ecological environment.Although there were some reports on the evaluation of heavy metal absorption capacity of landscape woody plants with large biomass,strong ecological adaptability,which can be widely used in urban green space.However,the species involved were limited.Different plant species have different ability to absorb heavy metals,but the mechanism of the difference was not clear.Therefore,the absorption capacity of 16 common shrubs in Beijing to Cd,Zn,Pb and Cu was evaluated.The species with stronger and weaker absorption capacity to Cd and Zn were selected to carry out the simulation experiment of single treatment of Cd and Zn in potted soil.The characteristics and mechanisms of different absorption of Cd and Zn by plants were systematically studied.The main results are as follows1.Among the 16 shrubs in Beijing road green space,Syringa oblata,Ligustrum×vicaryi,Buxus Sinica have strong comprehensive absorption capacity of Cd,Zn,Pb,and Cu.However,Kerria japonica,Berberis thunbergii ‘Atropurpurea’ and Euonymus japonicus were weak.2.In the pot experiment,the contents of Cd and Zn in all organs of four shrubs increased with the increase of Cd and Zn concentration in soil.The concentration of roots was significantly higher than that of leaves and branches,accounting for more than 57.13%.The concentration of Cd and Zn in the whole plant of the four shrubs was as follows: Ligustrum×vicaryi > Buxus Sinica > Rosa ‘Spectra’ >Euonymus japonicus.The Cd and Zn absorption capacity of Ligustrum×vicaryi and Buxus Sinica was stronger than Rosa ‘Spectra’ and Euonymus japonicus,which was consistent with the field sampling experiment in road.3.The growth and physiological activities of four shrubs were significantly inhibited when the concentration of Cd and Zn was higher than 100 mg/kg and 1000 mg/kg,respectively.According to the comprehensive tolerance evaluation value of Cd and Zn,the tolerance of four shrubs to Cd was:Ligustrum×vicaryi > Rosa ‘Spectra’ > Buxus Sinica > Euonymus japonicus.The tolerance to Zn was as follows: Ligustrum×vicaryi > Buxus Sinica > Euonymus japonicus > Rosa ‘Spectra’.The absorption capacity of Cd and Zn was not consistent with the tolerance of the four shrubs.4.The main chemical forms of Cd in the leaves and roots of the four shrubs were ethanol-and Na Cl-extract form,while Zn were water-and Na Cl-extract form.Compared Buxus Sinica and Ligustrum×vicaryi which with stronger absorption capacity to Rosa ‘Spectra’ and Euonymus japonicus which with weaker absorption capacity,the proportion of Cd existed in ethanol-extractable form in the leaves and roots was lower under Cd treatment.The proportion of Zn existed in water-extractable form in leaves was higher and the proportion of Zn existed in residual form in roots was lower under Zn treatment.These results indicated that the difference in Cd absorption capacity of the four shrubs might be related to the proportion of Cd existed in ethanol-extractable form.And the difference in Zn absorption capacity might be related to the proportion of Zn existed in water-extractable and residual form.5.The subcellular distribution of Cd and Zn in the leaves and roots of the four shrubs was mainly accumulated in cellular debris fraction,followed by metal-rich granule and heat stable protein fractions.and less distributed in organelles and heat-sensitive fractions.Compared Buxus Sinica and Ligustrum×vicaryi which with stronger absorption capacity to Rosa ‘Spectra’ and Euonymus japonicus which with weaker absorption capacity,the proportion of Cd in heat stable protein fraction in the leaves was higher and the proportion of Cd in organelles in the roots was lower under Cd treatment.The proportion of Zn in cellular debris fraction in the leaves was lower and the proportion of Zn in heat stable protein fraction was higher under Zn treatment.These results indicated that the differences in Cd absorption capacity of the four shrubs might be related to the distribution of Cd in heat stable protein and organelle fractions,and the differences in Zn absorption capacity might be related to the distribution of Zn in cellular debris and heat stable protein fractions.6.Fourier transform infrared(FTIR)analysis showed that under Cd and Zn treatments,the hydroxyl and amino characteristic peak in root cell wall of four shrubs changed most obvious,which offset-21-15 cm.The peak of carboxyl,cellulose sugar chain and sulfate offset-4-8cm.The results showed that hydroxyl and amino groups on the cell wall of four shrubs were important functional groups for Cd and Zn binding.Carboxyl,carbohydrate and lipid also played a role in the process of Cd and Zn absorption.In this study,the absorptive capacity of Cd,Zn,Pb and Cu of 16 shrubs in Beijing was determined,and the absorption characteristics and tolerance of four shrubs to Cd and Zn were analyzed.The differences in the absorption capacity of Cd and Zn in the four shrubs were related to the chemical forms and subcellular distribution of Cd and Zn in the four shrubs.The root cell wall was the main location for the accumulation of Cd and Zn,and the hydroxyl and amino groups were the main functional groups binding Cd and Zn.