| At present,studies have proved that Macleaya cordata has high tolerance to Cd,Mo,Mn and other heavy metals in soil,showing good phytoremediation potential.Moreover,alkaloids extracted from M cordata are widely used and have high economic value.However,the tolerance mechanism of M.cordata to lead and heavy metals in lead-zinc tailings is not clear.Through the screening of modifiers in lead-zinc tailings,our team found that the compound addition of organic modifier spent mushroom compost(SMC)and inorganic modifier calcium carbonate(CaCO3)has significant effect on improving the physical and chemical properties of tailings and fixing heavy metals.Therefore,in this study,M.cordata was used as the test plant to study its tolerance,accumulation and transportation and physiological response mechanism to heavy metal lead by pot experiment in sand culture;then,the pot experiment of soil culture was used to study the stress response mechanism of M.cordata to heavy metal under the condition of improved lead-zinc tailings with SMC and CaCO3(5%+5%,mass ratio)as modifier.The purpose of this study is to reveal the tolerance mechanism of the Compound heavy metals in the lead-zinc tailings of M.cordata,and to provide a theoretical basis for the remediation of lead-zinc tailings in M.cordata.The main results are as follows:1.Tolerance,accumulation and physiological response of M.cordata to Pb(1)Low concentration of Pb stress(less than 500 mg·L-1)promoted the growth of plant height,biomass and root system,increased the content of chlorophyll,soluble protein and the activity of antioxidant enzymes,while high concentration of Pb inhibited the growth and development of M.cordata.(2)The concentration of Pb in the organs of M.cordata was in the order of root>stem>leaf.The ability of Pb accumulation and translocation decreased with the increase of Pb concentration.(3)Pb mainly existed in the root of M.cordata in the form of lead oxalate and lead phosphate.The cell wall is the main place to store and stabilize Pb.The content of Pb in the cell wall of root accounts for 53%-83%of the total.Therefore,the content of Pb in the cell wall of M.cordata was reduced by immobilizing Pb in the cell wall of roots with low activity and mobility,thus alleviating the stress on plants and ensuring the growth and development of M.cordata.When the concentration of Pb stress increased,the proportion of Pb in the tissues of M.cordata increased as inorganic salts and organic acid salts with strong migration and toxicity,indicating that high concentration of Pb stress will reduce the ability of cell wall to fix Pb,thus reducing the ability of plant root to fix Pb,which leads to Pb entering into the cell interior and damaging the cell structure.Meanwhile,it increases the form of high activity and toxicity,and inhibits the normal growth and development of M cord ata as it is transferred to the upper part of the plant.(4)Transmission electron microscopy(TEM)showed that high concentration of Pb could destroy the structure of cell wall,lead to Pb entering the cell,and affect the normal structure and function of subcellular structure.Fourier transform infrared spectroscopy(FTIR)showed that Pb stress can increase the content of-OH,-COOH and organic acids and proteins in cells,it can combine with Pb at low concentration stress to weaken its toxicity to M.cordata.2.Stress response mechanism of M.cordata to heavy metals under improved lead-zinc mine tailings(1)Heavy metals in tailings will inhibit the growth and development of M.cordata.The.improvement of SMC and CaCO3 can make the tailings loose,increase the pH and organic matter content,thus improving the permeability and fertility of tailings soil.The improved treatment reduced the activity and migration of heavy metals in the tailings,thereby promoting the growth of the roots and plants and improved the ability of the roots of M.cordata to fix heavy metals.(2)Heavy metal stress in tailings can inhibit the synthesis of chlorophyll and soluble protein.Adding SMC and CaCO3 modifier can remove excessive free radicals,increase the content of chlorophyll and soluble protein,reduce the accumulation of malondialdehyde(MDA)and reduce the degree of membrane lipid peroxidation,which helps M.cordata to grow and develop under heavy metal adversity.(3)Heavy metals are limited by cell wall fixation and vacuole partitioning.After the improvement of SMC and CaCO3,the ability of cell wall to fix heavy metals was enhanced,and the contents of Pb,Zn and Cd in organelles decreased by 52.4%,36%and 35.3%,respectively.At the same time,after the improvement treatment,the proportion of Pb,Zn,and Cd in the deionized water and ethanol extracted form content of the roots of M.cordata decreased,which further proved that the heavy metal existed in the form of weak migration activity and reduced its toxicity to M.cordata.(4)It was observed by TEM that the stress of lead-zinc tailings resulted in the damage of cell structure and the pollution of heavy metals in the cell.The cell structure integrity of M.cordata could be maintained by adding SMC and CaCO3.FTIR results showed that M.cordata could chelate with heavy metal ions to form stable compounds by increasing the number of-CH3,-COOH and-NH amide groups in the roots,so as to improve the ability of roots to fix heavy metals. |
