| Ficus virens Aiton is the symbolic tree of Chongqing,which is usually used in landscape in this area.It has fast growth,long lifetime,strong resistance to pollution and high ornamental value.Pb and Cd are two kinds of heavy metals that have strong biological toxicity and are widely distributed in Chongqing.Citric acid is a natural low moclecular organic acid with less harm to ecology,which can influence the efficiency of phtoremediation by effecting the ability of plant to tolerate and accumulate heave metals.This study selected Ficus virens Aiton as experimental material,researched the ability of Ficus virens Aiton to tolerate and accumulate heavy metals in soil contaminated by Pb(250,500,700,900mg/kg)and Cd(5,10,20,30mg/kg).Three concentrations(2.5,5,10 mmol/kg)of citric acid were applied to Pb(250,500,900 mg/kg)and Cd(5,10,30 mg/kg)single-factor treatment soils to study the physiological responses of Ficus virens Aiton and the effects of citric acid on the ability to absorb and accumulate Pb and Cd in soil.The aim was analyzing the remedial ability of Ficus virens Aiton under Pb and Cd stress in soil and the influences of citric acid on the remedial ability of Ficus virens Aiton.The results provided basis for the further study of phytoremediation of Ficus virens Aiton and other trees in soils contaminated by heavy metals.There are major conclusions: 1 Physiological tolerance and characters of absorption and accumulation of Ficus virens Aiton under Pb stress1.1 Characters of morphology of Ficus virens Aiton under Pb stress: When the Pb concentration was less than 700 mg/kg,there was no obvious symptoms in the leaves and roots of Ficus virens Aiton seedlings.When the treatment concentration was greater than 700mg/kg,the plants showed obvious symptoms,mainly performed the chlorosis of the leaves,then fell off from the plants,and the symptoms of the old leaves near the plant base were more evident.Root growth was inhibited,root quantity was sparse,and the root length was short.1.2 Effects on the growth of Ficus virens Aiton under Pb stress: There was no significant differences in net growth of seedlings height and ground diameter between the treatments(250,500mg/kg)and the control.The net growth of height and ground diameter,organic and total biomass of the seedlings of treatments(700,900mg/kg)reduced significantly.After 90 days,the net growth of height and ground diameter,the total biomass in the 900mg/kg Pb-contaminated were 35.3%,64.5%,and 32.0% of the control,respectively.1.3 Effects on physiology of Ficus virens Aiton under Pb stress: After 10 days,the content of MDA and soluble sugar,the activity of SOD increased 90.21%,8.43%,27.45%,were significantly different with the control,the photosynthetic pigment content reduced 31.91%.After 40 days,MDA content increased 77.91%,the photosynthetic pigment content reduced 24.85%,compared with the control,SOD activity had no differences,which indicated the physiological toxicity of Ficus virens Aiton released with the treatment time prolonging.1.4 Characters of absorption and accumulation of Ficus virens Aiton under Pb stress: With the increasing of Pb concentration,content and accumulation of Pb in organs of Ficus virens Aiton induced and then reduced.Pb content of roots,stems and leaves reached the maximum at the concentration of 700 and 500 mg/kg,respectively,was 59.99,26.80 times,and 13.06 times the concentration of the control.In the same treatment,Pb content and accumulation in each organ of Ficus virens Aiton all showed root>stem>leaf,and the accumulation of Pb in roots accounted for 91.64%-93.38% of per plant.Under 700mg/kg Pb stress,the bioaccumulation factor reached a maximum value of 0.623,which was 1.41 times that of the control,and the transfer factor reached a minimum value of 0.118,which was 34.01% of the control.The growth of seedling was severely inhibited and the accumulation capacity was weakened when Pb concentration was 900 mg/kg(soil pollution index: 3.623,heavy contaminated),and the efficiency of remediation was low in this treatment.Under soil contaminated by 250~700mg/kg Pb(soil pollution index: 0.975~2.759,mild and middle contaminated),Ficus virens Aiton can be used to phytoremediation by fixed the Pb in soil,because of good tolerance,ability of accumulation and lowtransfer 2.Physiological tolerance and character of absorption and accumulation of Ficus virens Aiton under Cd stress 2.1 Characters of morphology of Ficus virens Aiton under Cd stress: When the concentration of Cd was 5 mg/kg(soil pollution index: 17.193,heavy contaminated),there was no significant differences in root growth with the control,and there were no obvious symptoms in the leaves.When the treatment concentrations were greater than 5mg/kg,brown grainy spots and rust spots appeared on the leaves of the seedlings,leaf tip necrosis,dehydration and dryness of the leaves.The development of the topic roots and lateral roots was restrained,and the color of roots became darker.2.2 Effects on the growth of Ficus virens Aiton under Cd stress: With inducing of the concentration of Cd,the inhibitory effects of Cd stress on the growth of Ficus virens Aiton became more.After 90 days,the net growth of height and ground diameter,root diameter,every organic biomass and the total biomass under 30 mg/kg Cd stress were 32.04%,56.83%,73.38%,34.27%,16.76%,32.29%,and 29.32% of the control,respectively.2.3 Effects on physiology of Ficus virens Aiton under Cd stress: After 10 days,soluble sugar content,SOD activity increased 10.81%,29.19%,compared with the control,the photosynthetic pigment content reduced 28.70% under 30mg/kg Cd stress,MDA content at the maximun under 20mg/kg Cd stress,increased 46.11% to the control After 40 days,SOD activity and the photosynthetic pigment content had no differences.2.4 Characters of absorption and accumulation of Ficus virens Aiton under Cd stress: The content of Cd in organs increased with the escalating of Cd concentration.The contents of Cd in different organs were in order of roots,stems and leaves.The bioaccumulation factor of Cd were all greater than 1,and the transfer factor were all less than 1.When the concentration was 30 mg/kg,the Cd content in roots,stems and leaves of seedlings and total Cd accumulation were 133.772 mg/kg,15.110 mg/kg,6.907 mg/kg,and 1025.285 ?g/plant,respectively;Cd accumulation accounted of root was 96.09% of total accumulation.The resistance and accumulation of Ficus virens Aiton under Cd stress performed that Ficus virens Aiton had potential to remedy seriously Cd-contaminated soil(soil pollution index: 17.193~94.970)because of some advantages,such as good tolerance,high accumulation and low transfer.3.Effects of citric acid on phytoremedial potential of Ficus virens Aiton under Pb stress 3.1 Effects of citric acid on physiological tolerance of Ficus virens Aiton under Pb stress: Addition of 10 mmol/kg citric acid in Pb-contaminated(250 mg/kg)soil increased the net growth of height and ground diameter,biomass.Under 500mg/kg Pb stress,applying 2.5mmol/kg citric acid reduced the net growth of height,each organic and total biomass,decreased 19.36%,55.19%,21.03%,26.19%,45.59%,25.84%,27.22%,compared with no-acid treatment.When the Pb concentration was 900mg/kg,2.5 mmol/kg citric acid resulted in seedling root length,root diameter,and total biomass reduction,which were 38.07%,52.04%,and 66.64% in the non-acid treated.3.2 Effects of citric acid on absorption and accumulation of Ficus virens Aiton under Pb stress: Addition of three concentrations citric acid in 250 mg/kg Pb-contaminated soil promoted the accumulation of Pb in each organ of Ficus virens Aiton.When the concentration of citric acid was 5 mmol/kg,Pb content in roots,stems and leaves,bioaccumulation factor,and transfer factor reached the top,was 1.62,1.68,3.34,1.63,1.29 times that of the no-acid treatment.Under 500mg/kg Pb stress,addition of 2.5 mmol/kg citric acid increased Pb content in roots and leaves of the seedlings,added 23.63% and 33.27% to the non-acid treatment.Under 900mg/kg Pbstress,Addition of citric acid increased the Pb content of roots,stems,leaves,bioaccumulation factor and transfer factor.Addition of 10mmol/kg citric acid,the Pb-accumulation of roots,stems,leaves and the total increased 69.59%,20.36%,113.32%,67.59% to non-acid treatment under 250mg/kg Pb stress;69.1%,6.44%,126.62%,67.70% to non-acid treatment under 5000mg/kg Pb stress;20.58%,172.16%,278.00%,35.17% to non-acid treatment under 900mg/kg Pb stressAdding citric acid increased Pb content of seedlings' organs,promoted Pb transfering to the aboveground,when the concentration of citric acid was high(10mmol/kg),the Pb-accumulation of organs and the total was higher than no-acid treatment.However,the effect of citric acid on physiological tolerance and uptake and accumulation of Ficus virens Aiton under different concentrations of Pb stress did not change regularly.The specific reasons needed further study.4.Effects of citric acid on phytoremedial potential of Ficus virens Aiton under Cd stress 4.1 Effects of citric acid on physiological tolerance of Ficus virens Aiton under Cd stress: Under 5mg/kg Cd stress,2.5,5mmol/kg citric acid reduced the net growth of height,root length,organic and total biomass of seedlings;10mmol/kg citric acid increased MDA content,SOD activity,and the photosynthetic pigment content in leaves;increased the content of soluble sugar,citric acid at all concentrations increased the toxicity of Cd.Under 10 mg/kg Cd stress,the net growth of height,root length,root diameter,root and stem biomass,and total biomass of seedlings decreased 25.27%,23.11%,19.40%,55.19%,44.25%,10.97%,47.77%.Under 30 mg/kg Cd stress,the net growth of height,root length and root diameter of seedlings decreased by 18.99%,35.02%,and 37.80%,respectively,compared with no-acid treatment.There were no significant differences in the groups.4.2 Effects of citric acid on absorption and accumulation of Ficus virens Aiton under Cd stress: Under 5 mg/kg Cd stress,5 mmol/kg citric acid significantly increased the Cd content of seedling roots,stems,leaves,and bioaccumulation factor,transfer factor,increased by 39.63%,210.26%,200.00%,33.01%,101.10%,compared with the no-acid treatment.;2.5,10mmol/kg citric acid significantly increased Cd content in stems and leaves of seedlings.Under 10 mg/kg Cd stress,Cd content in roots,stems and leaves of seedlings increased by 26.20%,51.43%,and 100.36% at 10 mmol/kg citric acid;Cd content in organs treated by 2.5,5 mmol/kg citric acid had no significant effect.Under 30 mg/kg Cd stress,addition of 10 mmol/kg citric acid increased the Cd content in roots,stems and leaves of seedlings by 9.19%,50.09%,and 25.16%,had no effects on bioaccumulation factor and transfer factor.Under 5mg/kg Cd stress,2.5,5,10mmol/kg citric acid significantly increased Cd-accumulation in seedling leaves,which was significantly increased by 95.58%,190.15%,and 163.41%,respectively,compared with no-acid treatmen;Cd-accumulation of roots,stems and total did not significantly promote.Under 10 mg/kg Cd stress,10 mmol/kg citric acid reduced the root,stem and total Cd accumulation by 43.69%,14.69% and 41.97%,respectively.Under the 30 mg/kg Cd stress,applying citric acid had no significant effect on the root,leaf and total Cd accumulation of seedlings.The application of citric acid increased the Cd content in various organs of seedlings,promoted the transfer of Cd to aboveground,aggravated the toxic effects of Cd,significantly reduced the biomass of seedlings,had no significant effect on the total Cd accumulation,and could not achieve the effect of improving the remedial efficiency;Under the different concentration of Cd stress,the effects of citric acid on physiological tolerance and absorption and accumulation characteristics of Ficus virens Aiton did not show regular changes.The specific reasons needed further study. |
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