| In the natural environment,lead,zinc and other heavy metal elements are easy to form complex pollution,because lead and zinc generally coexist.At present,there are many screening studies on phytoremediation of lead,zinc heavy metal pollution by hyperaccumulating or enriching plants.But these plants are limited in practical application due to their small biomass and slow growth speed,which results in unsatisfactory effects of remediation in lead,zinc heavy metal pollution.Litsea cubeba,a Litsea plant of Lauraceae,which is an important energy and spices species in southern China.It has the characteristics of fast growth,large biomass,strong adaptability and wide geographical distribution.Through the previous investigation,our team found that L.cubeba grew well in the lead-zinc tailings area,which indicated that L.cubeba had a strong tolerance to lead and zinc,and could be used as a phytoremediation basis for heavy metal pollution.In this study,pot experiments were conducted.The annual seedlings of L.cubeba were subjected to Pb and Zn stresses.The growth of plants,the changes of root morphology and structure of seedlings,and the physiological changes of plants were observed under the single and combined stresses of Pb2+,Zn2+.And the anatomical structure,accumulation characteristics of heavy metals,the absorption characteristics of main mineral elements in seedlings were analyzed.To explore the response characteristics of L.cubeba seedlings to lead and zinc stress.The main results were as follows:1.When the single concentration of Pb2+ was less than 400mg kg-1,the growth of seedlings was promoted.Under the treatment of P3(the single concentration of Pb2+was 600mg.kg-1),the above and underground biomass decreased by 12.95%and 13.86%respectively.The above-ground and root growth were significantly inhibited.But when the Zn2+ concentration was not higher than 200mg kg-1,there was no significant difference between seedling growth index and control.And when the combined concentration of Pb2+-Zn2+ was not higher than 200mg·kg-1.the growth of seedlings was slightly promoted.When the concentration of Zn2+ and Pb2+-Zn2+ was higher than 400mg kg-1,the plant growth were all significantly inhibited compared with the control.And the inhibiting effect of Zn2+ on seedling growth was greater than that under the same concentration of Pb2+.At the same time,the inhibiting effect of Pb2+-Zn2+stress on plant growth of L.cubeba was greater than that under the same concentration of single Zn2+.The inhibiting effect of high concentration of lead,zinc stress on L.cubeba was as follows:Pb-2+<Zn2+<Pb2+-Zn2+.2.The content of Chlorophyll(a)both gradually decreased under the single stress of Pb2+ and Zn2+,first increasing and then decreasing under Pb2+-Zn2+compound stress.Chlorophyll(b),(a+b)first increased,then decreased under Pb2+,Zn2+ and Pb2+-Zn2+stress,and the Chlorophyll a/b was lower than the control.With the increase of concentration under single and combined stress of lead and zinc,photosynthetic parameters of L.cubeba on Pn,Gs and Tr were increased first and then decreased.By the increase of treatment concentration,Ciincreased and Pn decreased in L.cubeba seedlings.This suggest that the decrease of photosynthesis in seedlings was caused by non-stomatal factors.With the increasing of Pb2+,Zn2+and Pb2++-Zn2+concentration,the enzyme activity of SOD,POD,CAT in L.cubeba was increased first and then decreased,the content of MDA increased gradually.Under the stress of Zn2+ and Pb2+-Zn2+,the enzyme activities of POD,CAT were stronger than Pb2+ stress,and the damage to plants was more serious.With the increasing concentration of all these three stresses,the content of soluble sugar and protein of L.cubeba increased first and then decreased,the proline contentin increased continuly.3.Under the stress of single or combined concentration of Pb2+ and Zn2+,the accumulation of Pb2+,Zn2+ in different organs of L.cubeba seedlings was different,and it was closely related to root morphological index.The correlation between the accumulation of Pb2+,Zn2+ in the above-ground or underground of seedlings and the concentration of Pb2+,Zn2+ in potted media were high.The correlation coefficients between the content of Pb2+ in the above-ground and the concentration of Pb2+ stress were extremely significant and positive,the correlation coefficients were 0.935.With a correlation coefficient of 0.738.the cumulative amount of Zn2+ in root was positively correlated with the stress concentration of Zn2+.The effects of Pb2+,Zn2+stress on root morphology of seedlings were different.We can see that the effect of Zn2+ treatment on root morphology of seedlings is greater than that of Pb2+ treatment,which indicates that the root of L.cubeba seedlings is more tolerant to Pb2+,and slightly less tolerant to Zn2+ treatment.4.The single and compound stress of Pb2+ and Zn2+ have some effects on the balance of N,P and K in seedlings.Under the single stress of Pb2+,Zn2+,the N content in the aboveground and root of i.cubeba were increased first and then decreased.Only under Zn2+800mg·kg-1 treatment,the above ground and underground P content was lower than the control,which decreased by 13.26%and 13.80%,respectively.And the N,P content of L.cubeba in the aboveground were all higher than that in the root,while the K content in the plant root was higher than in the aboveground.Under the treatment of Pb2+800mg·kg-1 concentration,K content in the above-ground was significantly lower than that in the control.Under the Pb2+ stress,the K content of root were all significantly higher than that in the control group.Under the stress of Zn2+,the minimum K content in above-ground and root were appeared at the Zn2+800mg·kg-1 treatment.Compared with the control group,they decreased by 10.73%and 5.26%,respectively.Under the combined stress of lead and zinc,the N content in above-ground and root showed a downward trend.With the increase of treatment concentration,P content in aboveground was decreased,and P content in root was increased first and then decreased.The K content in the above-ground and underground were increased first and then decreased.5.The results of leaf anatomical structure showed that three stress modes had different effects on anatomical structure of L.cubeba seedlings.Under the high concentration of lead and zinc stress,the thickness of L.cubeba leaf was decreased.The cell volume of upper and lower epidermis was smaller.The differentiation between palisade tissue and spongy tissue was not obvious,the cell structure was loose,and the ratio of palisade tissue to spongy tissue on thickness was decreased.In order to adapt to the external stress environment,the morphology of mesophyll and epidermis of L.cubeba seedlings changed in varying degrees. |
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