| Litsea cubeba is an important spice and oil tree species in China.As a woody plant,Litsea cubeba can absorb,repair or adapt to heavy metals in soil,and has the advantages of barren tolerance,fast growth and large biomass.In this study,one-year old seedlings were used as experimental materials,pot control experiment was conducted.Single stress and compound stress of Cd and lead were set respectively,and the concentrations were single stress of Cd(25,50,100,200 mg/kg)and single stress of Pb(250,500,1000,2000 mg/kg).Combined stress of Cd-Pb(25-500,25-1000,100-500,100-1000 mg/kg)was used to observe the changes of plant growth,physiology and biochemistry,as well as the absorption characteristics of nutrient elements in Litsea cubeba seedlings under single or combined stress of cadmium and lead.To explore the response and enrichment characteristics of Litsea cubeba seedlings to heavy metals cadmium and lead,and to provide theoretical basis for the selection of forest land,plant cultivation and remediation of lead-cadmium polluted soil.The results show that:(1)Under the single stress of Cd and Pb,the comprehensive effect on the growth of Litsea cubeba seedlings was "promoted by low concentration and inhibited by high concentration".When Cd2+≤25 mg/kg and Pb2+≤500 mg/kg,the plant height,ground diameter and total biomass of CK in treatment group increased by 19.1%-38.36%,0.5%-25.71%and 20.04%-40.27%compared with control group.With the increase of stress concentration,plant height decreased by 74.18%,ground diameter decreased by 60%,and total biomass decreased by 54.86%.The combined CD/Pb stress inhibited the growth of Litsea cubeba seedlings,and the inhibition effect was more significant than that of single Cd/Pb stress.The plant height,ground diameter and biomass decreased by 23.28%～60.00%,17.14%～60.00%and 14.69%～44.96%,respectively,compared with the control group.According to comprehensive analysis,the inhibitory effects of three kinds of stress were as follows:Pb and Cd combined stress>single Cd stress>single Pb stress.(2)Under the single stress of Cd and Pb,the effects on chlorophyll content and photosynthetic gas exchange parameters of Litsea cubeba seedlings were also"promoted by low concentration and inhibited by high concentration".When Cd2+≤25 mg/kg and Pb2+≤500,photosynthetic rate(Pn),stomatal conductance(Gs),transpiration rate(Tr)and chlorophyll content were increased by 4.93%-25.98%,6.82-18.67%and 16.55%-43.52%,respectively,compared with control group CK.28.60%-43.52%.With the increase of stress concentration,chlorophyll content decreased and photosynthesis weakened.Under complex stress,the contents of Pn,Gs,Tr and chlorophyll showed a decreasing trend,and the contents of Pn,GS,Tr and chlorophyll in all treatment groups were lower than those in control group CK by 3.81-34.83%,7.31%-38.31%,1.07%-27.64%,10.95%-67.12%,respectively,and were lower than those under single stress with the same concentration.(3)Under single Cd/Pb stress,Litsea cubeba seedlings could respond to low concentration stress and activate the defense mechanism of Litsea cubeba seedlings,thus reducing the harm of Cd/Pb stress on them.Under single stress,SOD activity,POD activity,CAT activity and MDA activity increased by-8.37%-44.03%,-40.89%-41.47%,-30.79%-16.58%and 10.45%-293.55%in treatment groups compared with CK.SS content increased by-22.14%～39.61%,SP content increased by-34.08%～29.86%,FPro content increased by 6.95%～46.79%.Under CD/Pb combined stress,low Cd concentration(25 mg/kg)combined stress could also activate the defense mechanism of Litsea cubeba seedlings,while high Cd concentration(100 mg/kg)combined stress showed inhibitory effect.The inhibitory effects of three kinds of stress were as follows:Pb and Cd combined stress>single Cd stress>single Pb stress.(4)Under single cadmium and lead stress,heavy metal content in all organs of Litsea cubeba seedlings gradually increased with the increase of stress concentration,and the order of heavy metal content in all organs under the same concentration treatment level was root>stem>leaf.The enrichment coefficients of all organs decreased with the increase of the concentration,and the transport coefficients for cadmium and lead were less than 1.Under the combined stress,the content of Cd2+in all organs was significantly higher than that under the same concentration of cadmium single stress,while the content of Pb2+in all organs was slightly higher or lower than that under the same concentration of lead single stress.(5)Under single cadmium and lead stress,with the increase of stress concentration,the contents of N,P,K,Ca and Mg in the organs of Litsea cubeba seedlings under single cadmium stress increased first and then decreased.Under lead stress alone,the contents of N,P,K and Mg in organs also increased first and then decreased,while the contents of Ca in roots also decreased.Under the combined stress,the contents of P and K in organs increased first and then decreased,the contents of Ca and Mg decreased,while the contents of N in roots and leaves decreased,and then increased first and then decreased in leaves.The distribution pattern of nutrient elements in all organs changed obviously,mainly showed that the distribution proportion of N and Mg in the root increased obviously,the distribution proportion of K in the stem increased obviously,and the distribution proportion of P and Ca in the leaves increased obviously.In conclusion,heavy metal stress significantly affected the growth of Litsea cubeba seedlings,interfered with their photosynthetic and resistance physiological processes,and the growth of Litsea cubeba seedlings was good at low concentration,while the growth was inhibited at high concentration,indicating that Litsea cubeba seedlings had a certain tolerance to Cd and Pb.Although the transfer coefficients of Cd and Pb of Litsea cubeba seedlings were all less than 1,and a large amount of Cd was concentrated in the roots,the Cd and Pb absorbed by the stems increased with the increase of treatment concentration.Moreover,since Litsea cubeba has the characteristics of rapid growth and large biomass,it can be considered as an alternative plant for controlling soil Cd and Pb pollution. |
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