| Zinc(Zn)is one of the mineral elements necessary for plant growth.An appropriate amount of Zn in soil can regulate plant enzyme activity and plant photosynthesis.Excessive Zn will produce toxic effects on plants.The enrichment characteristics of plants can reflect the absorption and transport of heavy metals,and the leaf treats can predict the ecological adaptation strategies of plants in the heavy metal environment.Therefore,two-years-old L.lucidum was taken as the research object in this paper,adopts pot experiment,and different treatment levels of Zn pollution was set(CK,L1,L2 and L3,respectively representing control Omg·kg-1,mild 100mg·kg-1,moderate 300mg·kg-1 and severe pollution 500mg·kg-1).By analyzing the enrichment characteristics of plants and the variation characteristics of leaf structural treats explore the tolerance of L.lucidum to heavy metal Zn,the reasons for the decrease of plant photosynthetic rate under Zn stress and the ecological adaptation strategies to provide a theoretical basis for vegetation restoration in Zn polluted areas.The main conclusions are as follows:(1)Under L1,L2 and L3 treatments,Zn mainly accumulated in plant roots,followed by leaves and the smallest in stems.The enrichment ability of Zn was L1>L2>L3.The BCF range of underground and aboveground parts of plants in Zn treatment group ranged from 0.13 to 0.46,which shows that L.lucidum has a certain remediation potential for soil Zn pollution.(2)Compared with CK,the upper epidermal thickness(TUE)and leaf thickness(LT)of L.lucidum leaves increased in Zn treatment group.The palisade tissue(Pt),lower epidermal thickness(TLE),palisade sea ratio(P/S)and structural compactness(CTR)increased under L1 and L2 treatment,and decreased under L3 treatment,and spongy tissue thickness(St)and leaf porosity(SR)are opposite.The specific leaf area(SLA)and leaf nitrogen content(TN)decreased significantly.The phosphorus content(TP)decreased significantly under L2 and L3 treatment.The N/P decreased significantly and the value of N/P was less than 14 under L1,L2 and L3 treatment.The stomatal density(SD)increased significantly and the sort of in the treatment group is L2<L1<L3.The leaf microstructure showed that under the treatment of CK,L1 and L2,the plant palisade tissue was closely arranged,and the differentiation between palisade tissue and sponge tissue was obvious.Under the treatment of L3,the cells of palisade tissue and sponge tissue were significantly smaller,loosely arranged,and the differentiation was not obvious.The above shows that the absorption of nutrient elements of plants Zn stress affects.Under L1 and L2 treatment,L.lucidum uses more substances to build guard tissue by adjusting the plasticity of leaf structure to plants can better adapt to extreme environment.Under L3 treatment,the tissue structure of L.lucidum is disordered.(3)Compared with CK,the contents of superoxide dismutase(SOD),catalase(CAT)and proline(Pro)of L.lucidum under L1,L2 and L3 treatment increased significantly,peroxidase(POD)increased significantly under L1 and L2 treatment,decreased under L3 treatment,and malondialdehyde(MDA)increased significantly under L2 and L3 treatment.Ascorbate peroxidase content(APX)increased significantly under L2 treatment.The results showed that the antioxidant enzyme system of plants was activated under Zn stress.Compared with CK,the contents of superoxide dismutase(SOD),catalase(CAT)and proline(Pro)of L.lucidum under L1,L2 and L3 treatment increased significantly.Peroxidase(POD)increased significantly under L1 and L2 treatment.Malondialdehyde(MDA)increased significantly in L2 and L3 treatment.Ascorbate peroxidase content(APX)increased significantly under L2 treatment.The above results showed that the antioxidant enzyme system of plants was activated under Zn stress.Chlorophyll a+b(Chl a+b),chlorophyll a(Chla),chlorophyll b(Chlb)and carotenoid(Car)decreased significantly.Chlorophyll a/b(Chla/b)decreased significantly under L2 and L3 treatment.The net photosynthetic rate(Pn)decreased by 16%,47%and 79%respectively and the intercellular CO2 concentration(Ci)increased by 13%,52%and 85%respectively,and the stomatal limit(Ls)decreased by 17%,25%and 46%respectively under treatment group.The results showed that Zn stress inhibited the synthesis of photosynthetic pigments,and Ls decreased and Ci increased under Zn stress,indicating that non stomatal factors were the main factor leading to the decline of photosynthesis.(4)Correlation analysis and PCA analysis showed that the leaf photosynthesis and leaf treats of L.had adaptive changes.The comprehensive scores of various indexes of L.lucidum was L1>CK>L2>L3.The comprehensive scores of L1,CK and L2 had little difference,indicating that L.lucidum could maintain the original physiological function under L1 and L2 treatment.L.lucidum could grow well under L1 treatment which the plant had a certain tolerance to Zn stress.Under L3 treatment,the leaf structure of L.lucidum was disordered and the leaf stress resistance decreased,which led to the further decline of plant photosynthetic rate.Under low and medium concentration Zn stress plants can form treats combinations with low SLA,TN,TP,Pn,St and high LT,TUE,Pt,CTR and P/S and this trait combination is located at one end of the " slow investment return”species in the leaf economic spectrum.In conclusion,L.lucidum has a certain enrichment ability for heavy metal Zn.Zn stress inhibited the photosynthesis of L.lucidum and strengthened with the increase of heavy metal Zn concentration.Under L1 and L2 treatment,plants have higher epidermal thickness,palisade tissue thickness,leaf thickness,stomatal density,superoxide dismutase,peroxiase,ascorbate peroxidase,catalase,proline and lower specific leaf area,leaf nitrogen content,phosphorus content,net photosynthetic rate and sponge tissue to improve the investment distribution of plants on leaves.Under the constraint of the limited photosynthetic products,it can reasonably allocate resources and avoid the pressure brought by the environment to ensure the normal growth of Ligustrum lucidum under the special environment of Zn stress.However,with the increase of heavy metal stress,under L3 treatment,the palisade tissue and sponge tissue cells of plant leaves became significantly smaller,arranged disorderly and did not differentiate significantly.The leaf structure was disordered,and the continuous increase of malondialdehyde caused serious damage to the membrane system,which further reduced the stress resistance of plants.Therefore,L.lucidum can be used as a restoration potential tree species in areas with light Zn pollution. |
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