| The accumulation of heavy metals in soils and water poses a risk to the environmental and human health.These elements accumulate in the body tissues of living organisms and their concentrations increase as they pass from lower trophic levels to higher trophic levels.Among the techniques available for heavy metals removal from soils and water bodies,phytoremediation is very promising,cost-effective and non-invasive.The use of Salix in phytoremediation of land contaminated by heavy metals have considerable potential for its large biomass,fast-growing and strong adaptability.Cutting propagation of Salix in hydroponics has certain advantages compared with soil culture.On the basis of heavy metals accumulation by Salix and from the perspective of practical application,this paper compared the differences of three Salix species on the tolerance and accumulation of heavy metal polluted water;analyzed the effect of four equal amounts of heavy metals on physiological indexes of Salix;studied the interaction of heavy metals in the transport capacity of heavy metals;discussed the accumulation ability of Salix cutting in heavy metals and phosphorus compound polluted water and the feasibility of cutting,which used in phytoremediation of polluted water first,transplanted to mining aera in order to remediate the contaiminate soil for a long time.The results of this study are as follows:(1)The cuttings of three Salix species all took root in short period of time in water.Compared with S.babylonica and S.matsudana,S.fragilis grew best in uncontaminated environment,produced least biomass when exposed to heavy metals,and the tolerance index of shoot and root to compound heavy metals were 43 and 40.S.matsudana was not optimal under the uncontaminated condition but was the best under heavy metals treatment among the three Salix species,and the tolerance index of shoot and root to compound heavy metals were 75 and 90.S.matsudana possessed a strong metal tolerance to shield itself from HM toxicity and strong metal defensive ability which means S.matsudana could hold back more metals in the initial cutting instead of transporting metals to shoots.This suggested that the growth of different Salix species cannot reflect its accumulation of heavy metals.(2)The cutting of three Salix species even without roots had the ability of absorb and accumulate heavy metals,which was related to the diameter and position of initial cutting.The shoot and root biomass were 0.21g and 0.04g,tolerance index were 74 and 82,and phytoextraction capacity was 0.12 of cutting with large diameter(Dia>1cm)under single Cd treatment.While the shoot and root biomass of cutting with small diameter(Dia<1cm)were 0.05g and 0.02g,tolerance index were 40 and 50,and phytoextraction capacity was 0.01 under single Cd treatment.This suggests that initial cutting of S.matsudana with large diameter exhibited considerably stronger growth,presented higher tolerance,and showed greater phytoextraction and rhizofiltration efficiencies than initial cutting with small diameter.Initial cuttings of three Salix species derived from the apical position along the parent stem exhibited poorer growth performance before and after heavy metal treatment,as well as higher heavy metals accumulation ability compared with the basal cuttings.The content of Cd,Cu,Pb and Zn of cutting from the apical position along the parent stem were 5.7μg/plant,20.8 μg/plant,20.2 μg/plant and 101.8 μg/plant,while that of basal cutting were 2.9μg/plant,10.5 μg/plant,10.4 μg/plant and 53.3 μg/plant.These resultssuggest that selection of cutting from the same Salix species would present various growth condition and heavy metals accumulation ability,and there are also difference in the correlation of growth and accumulation ability.(3)The existence of the root system altered distribution and interaction of Cd and Cu in plant organs and enhanced tolerance and phytoextraction capacity of S.matsudana.The tolerance index of Cu was 38,and phytoextraction capacity was 0.15 of S.matsudana cutting without root,while that of cutting with root were 67 and 0.45.Cu inhibited plant absorption and Cd accumulated in S.matsudana and promoted Cd transportation from the initial cutting to the shoot.The translocation factor of Cd between wood and shoot increased from 13.5 to 22.5 with Cu addition.Cd inhibited Cu transport from the initial cuttings to the shoot in S.matsudana without root,while had no effect on Cu in S.matsudana with root.Cd promoted Cu absorption in S.matsudana without root and inhibited Cu absorption in S.matsudana with root.The phytoextraction capacity of Cu increased to 0.2 in cutting without root,and that in cutting with root decreased to 0.36 with Cd addition.(4)The treatment of single and compound Cd and Zn had no effect on growth of S.matsumada,while deleterious effects were found in physiological after two weeks.Content of chlorophyll a and chlorophyll bwere reduced in all treatments,while root activity was reduced only in Cd-Zn50 and Cd-Zn100 treatments,and SOD and POD activity in shoot and root were enhance only in single Cd and Cd-Zn50 treatments.The concentration of Cd and Zn in aboveground of S.matsumada were related to tissues and treatments.The concentration of Cd and Zn in shoot were 64 mg/kg and 97 mg/kg,while that in phloem were 161 mg/kg and 371 mg/kg under Cd-Zn50 treatment.The concentration of Cd in above-ground under single Cd,Cd-Zn50 and Cd-Zn200 were 93 mg/kg,84 mg/kg and 68 mg/kg,separately.The concentration varied with tissues:phloem being the major sink,while xylem the least for Cd and Zn accumulation.Besides,there was interaction between Cd and Zn in the absorption and transportion of Salix.The concentration of Cd in aboveground of S.mat.sumada was reduced by Zn,while that of Zn was enhanced by Cd.(5)Relative growth rate,fresh weight as well as biomass of shoots and roots were not affect by each metal with same concentration for the short treat time,while different response of physiological in leaves and roots to single metal treatment were inspired.The content of total chlorophyll decreased,while soluble protein content and POD activity in shoot and root were increased under each metal(Cd、Zn、Cu and Pb)treatment.Root activity decreased under Zn treatment,while was increased under Pb treatment.Cu and Pb increased free proline content in plant.The four heavy metal treatments all increased the SOD activity in different extent,and all had no effect on MDA content in shoot and root yet after two weeks treat time.S.matsudana possess different transport capacity of different heavy metals.The concentration of Zn in shoot was 98 mg/kg,which is much higher than that of Pb(0.15 mg/kg)Heavy metals transferred to aboveground of Salix were most stored in phloem.The differences of physiological variation revealed the physiological response of S.matsudana to adversity and its adaptation mechanism.(6)In order to improve the accumulation ability of heavy metals by S.matsudana,the effect of supplying phosphate to shoot was studied.The results indicate that Supplying phosphate to shoot might promote the growth of shoot and the transfer of Cd from root to shoot,resulting in strong ability of S.matsudana to accumulate Cd.The results suggest that the way of phosphorus supplied to shoot and root at the same time could not only enhance the Salix tolerance of metals,but alsoguarantee a strong accumulation capacity of Cd when use S.matsudana in phytoremediation.(7)With the increase of incubation time,the aquatic environment can’t satisfy the further growth of willow.Therefore,transplanting the Salix cutting into soil could be considered in order to study the remediation of contaminate soil.The results show that the shoot and root biomass of S.matsudana cutting grown in hydroponic for three weeks were 0.159g and 0.056g,while that of cutting grown in soil were 0.104g and 0.035g.The combined cultivate way of cutting could greatly improve the biomass of S.matsudana,and the survival rate after transplanting to soil could reach 75%or more.The shoot biomass was 0.066g when grown in water for one week,and increased by 342%after transplant to contaminate soil for three weeks(1W;1W-3S).However,shoot biomass of cutting grown in water for two weeks was 0.123,while increased by 454%after transplant to contaminate soil for three weeks(2W;2W-2S).The total chlorophyll content,POD activity and root activity of S.matsudana transplant to contaminate soil were associated with different pre-treatments and pre-treat time,which should be comprehensively considered in the practical application.The way of pre-treat cutting in water was feasible,as well as helpful to the biomass production and tolerance of plant when transplant to contaminate soil at the same period.In conclusion,the adoption of factory hydroponic cutting technology could be used in Salix cutting propagation,and the latter could be used in phytoremediation of heavy metals pollution aquatic environment at the same time.Salix species,cutting characteristics and kinds of heavy metal in water should comprehensively considered in the selection of phytoremediation plants.S.matsudana and S.babylonica had high accumulation ability of Cd,Cu and Zn.Initial cutting also possessed the ability of accumulate Cd and Zn during early Salix growth even without roots.The interaction of heavy metals would change with the emergence of root system.To reduce the damage of heavy metal on S.matsudana,enzymes activity related to resistance in shoot and root were increased,as well as soluble protein content.Chlorophyll was destroied,especially under Cu treatment.Four heavy metals(Cd,Cu,Zn and Pb)transferred to aboveground of Salix were most stored in phloem.Cutting propagation of Salix in hydroponics could be used in phytoremediation of heavy metals contaminate water,and supplying phosphate to shoot could guarantee a strong accumulation capacity of Cd.Besides,Salix cutting used in phytoremediation of heavy metal polluted water could also transplant to and used in mining soil after a period of time. |
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