The Role Of Root Exudates In Scirpus Triqueter On Phytoremediation Of Pyrene-lead Co-contaminated Soil

For clarifying the mechanisms of Scirpus triqueter root exudates on remediation of pyrene-lead co-contaminated soil, the different compositions of Scirpus triqueter root exudates under different growth conditions; the effects of Scirpus triqueter root exudates on pyrene and lead desorption from soil; the effects of Scirpus triqueter root exudates on reparation of pyrene-lead co-contaminated soil; the influence of Scirpus triqueter root exudates on form and the bioavailability of pyrene and lead; the influence of root exudates on Scirpus triqueter response to pyrene-lead stress were analysed. Get the following conclusions:(1) The components of Scirpus triqueter root exudates in pyrene and lead cocontaminated soil is different from uncontaminated soil. Roots exudates of Scirpus triqueter mainly contained 27 kinds of compounds, organic acids were the main compounds, 17 kinds of organic acids including palmitic acid, pentadecane acid, vanillic acid, stearic acid, citric acid et al. Under the stress of pyrene and lead, the amount of Scirpus triqueter root exudates of water soluble organic acid increased significantly, such as citric acid, succinic acid and glutaric acid, so these three kinds of organic acids were considered to be the special root exudates under pyrene and lead stress.(2) Organic acid treatments enhanced desorption amount and rate of pyrene and lead. Roots exudates of organic acids concentrations increased from 0 to 50 g/L, the desorption amount and rate of pyrene and lead increased quickly at beginning, however increased slowly after the concentrations of 10 g/L, and were constantly after the concentration of 40 g/L. Organic acid treatments enhanced the desorption amount of pyrene and lead from soil following the order: citric acid>succinic acid> artificial root exudates>glutaric acid. The desorption rate of pyrene and lead in pyrene-lead cocontaminated in soil was higher than pyrene and lead single pollution soil.(3) Scirpus triqueter root exudates treatments enhanced the phytoremediation of pyrene-lead co-contaminated soil. The biomass of Scirpus triqueter with addition of C(citric acid) treatment nearly more than 1 times higher than the control group, addition of R(raw root exudates) was about 60% higher than control group, addition of S(succinic acid) about 35% higher than the control group. The effects of Scirpus triqueter root exudates on promotion of Scirpus triqueter shoots and roots growth was different, the order from high to low was C(citric acid)>R(raw exudates)>S(succinic acid). The effects of removing contaminations with different kinds of Scirpus triqueter root exudates treatments were also different, following the order by C(citric acid)>R(raw exudates)>S(succinic acid). To a certain extent, the addition of Scirpus triqueter root exudates also increased the soil enzyme activities, the enzyme activities in different soil types following the order by C(citric acid)>R(raw exudates)>S(succinic acid), and the enzyme activities with the same root exudates treatment following the order by lead single pollution soil<pyrene single pollution soil<pyrene-lead co-contaminated soil. Enzyme activities and pyrene removal rate has a significant positive correlation, suggests that plant roots exudates promote the activities of the enzyme, improve the biodegradation of polycyclic aromatic hydrocarbons.(4) Scirpus triqueter root exudates of organic acids effectively improved the bioavailable content of pyrene and lead and greatly influenced other chemical form of pyrene and lead. After the addition of organic acids, compared with the control, the highest bioavailable content of pyrene increased 41.0%, and the highest bioavailable content of lead increased 872.7% in soil, in the citric acid group, compared with the group. The addition of organic acids improved the bioavailability of pyrene and lead in soil following the order: citric acid>artificial root exudates>succinic acid>glutaric acid. The addition of three organic acids improved the water-soluble and acid-extractable form of lead and n-butanol extractable form of pyrene, reduced the residual form of pyrene and lead.(5) Under different concentrations of pyrene and lead stress, the biochemical indicators of Scirpus triqueter changed. Under low and high concentrations of pyrene and lead stress, the content of MDA increased in shoot of Scirpus triqueter, and decreased after the recovery of organic acids(Scirpus triqueter root exudates) treatment. Under the stress of pyrene and lead, plasm H+-ATPase activity and tonoplast H+-ATPase activity and H+-PPase activity increased, and after the organic acids recovery processing it falled. Under the stress of pyrene and lead, the shoot of the catalase(CAT), superoxide dismutase(SOD) and peroxidase(POD) activity increased, and decreased after the recovery of organic acid treatment. Similar trends were discovered in shoots and roots of hydrogen peroxide(H2O2) and superoxide free radical(O2-) content of Scirpus triqueter. Pollution stress induced plant cytoplasmic membrane peroxidation and oxidative damage, but after organic acid recovery processing, this phenomenon was alleviated and citric acid has the best effect.(6) Scirpus triqueter root exudates in rhizosphere was the main repair mechanisms of phytoremediation, the root exudates in the rhizosphere environment promote the desorption of pollutants, enhance the ability of plant to remove and uptake contaminants, exudates of related antioxidant enzymes or release the specific root exudates, activation of pollutants, promote the bioavailability of contaminants, relieve the stress effects of pollutants on Scirpus triqueter and promote restoration of the compound pollution of soil, all these results and discussion could clarify the mechanisms of Scirpus triqueter root exudates on remediation of pyrene and lead co-contaminated soil.