| Magnolia officinalis subsp.biloba is a Magnolia plant of the Magnoliaceae family,with ecological restoration and ornamental values.With its tall shape,well-developed root system,wide distribution area and strong adaptability,Magnolia officinalis subsp.biloba is important for soil and water conservation and ecological balance.The woody plants have the advantages of high ornamental value,rapid growth and large biomass,which are conducive to the greening and landscaping of polluted soil.In this paper,using lead and its associated metal zinc,which have a wide range of contamination and are more harmful to the living environment,as stress factors,we set 13 treatments,Pb and Zn single contamination concentrations of control(CK),200(P1,Z1),400(P2,Z2),600(P3,Z3)and 800(P4,Z4)mg·kg-1,respectively,with potted 1-year-old Magnolia officinalis subsp.biloba seedlings of woody plants as test materials and Pb-Zn combined pollution concentrations of 100+100(M1),200+200(M2),300+300(M3),400+400(M4)mg·kg-1 to investigate the phenotypic adaptation characteristics,physiological and biochemical changes and heavy metal accumulation characteristics of Magnolia officinalis subsp.biloba seedlings to different concentrations of Pb and Zn stresses.The main findings were as follows:(1)When the concentrations of Pb2+ and Zn2+were ≤400 mg·kg-1 and the combined concentrations were≤(200+200)mg·kg-1,the growth of above-ground parts and roots of Magnolia officinalis subsp.biloba seedling was promoted,while the growth of seedlings of Magnolia officinalis subsp.biloba was inhibited by high concentrations of Pb and Zn.The combined concentration of Pb2+-Zn2+was(200+200)mg·kg-1.The plant growth index was greater than that of the same concentration of P1 and Z1(200 mg·kg-1 for Pb2+ and Zn2+),indicating that Pb and Zn showed antagonistic effect with low concentration treatment,which mutually inhibited the toxic effect of each other on the plant and promoted the growth of above-ground parts and root system.(2)The chlorophyll a,chlorophyll b,chlorophyll a/b and carotenoid contents of Magnolia officinalis subsp.biloba leaves decreased with the increase of Pb,Zn single and compound stress concentrations,and the leaf photosynthetic pigment contents were higher than the CK at Pb2+ concentration≤200 mg·kg-1,Zn2+concentration≤400 mg·kg-1 and compound concentration≤(200+200)mg·kg-1.With the increase of Pb and Zn stress concentration,leaf intercellular CO2 concentration(Ci)of Magnolia officinalis subsp.biloba seedlings increased,while net photosynthetic rate(Pn)and stomatal conductance(Gs)decreased,indicating that non-stomatal factors were responsible for the decrease in photosynthesis of seedlings.Leaf quantum efficiency[Y(Ⅱ)],primary light energy conversion(Fv/Fm),PSII electron transport rate(ETR)and photochemical quenching of chlorophyll fluorescence(qP)gradually decreased with increasing stress concentration,while initial fluorescence(Fo)and non-photochemical quenching of chlorophyll fluorescence(NPQ)gradually increased,indicating that Pb and Zn stresses inhibited the PS II reaction center of Magnolia officinalis subsp.biloba seedlings.(3)With the prolongation of Pb and Zn stress time and the increase of stress concentration,it led to the intensification of cell membrane lipid peroxidation and the increase of leaf malondialdehyde(MDA)content in Magnolia officinalis subsp.biloba leaves,when the concentration of Pb2+and Zn2+ was 800 mg·kg-1 and the compound concentration was(400+400)mg·kg-1,the MDA content reached the maximum and the plants were deeply injured.Under the single and combined treatments of Pb and Zn,the leaves of Magnolia officinalis subsp.biloba.mainly reduced the accumulation of superoxide radicals and reduced cellular osmotic potential and promoted cellular uptake by increasing the superoxide dismutase(SOD)activity,soluble sugar and soluble protein contents,which enhanced the tolerance of plants to Pb and Zn to some extent.(4)The accumulation pattern of heavy metals in Pb and Zn single and compound stresses of Magnolia officinalis subsp.biloba.seedling was similar,which showed that the Pb and Zn contents in the root system were greater than the sum of the transferred heavy metal contents in the above-ground parts(stems and leaves)in general,and the accumulated Pb and Zn contents in each part of the plant showed a dose-increasing effect.Meanwhile,the transfer factor gradually decreased with the increase of Pb stress concentration,and the treatment with Pb2+ concentration of 800 mg·kg-1 decreased to the lowest,only 0.24;the transfer factor gradually decreased with the increase of Zn stress concentration,but the decrease was small,and the transfer factor decreased to 0.36 under the stress of Zn2+ concentration of 800 mg·kg-1. |
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