Accumulation Capacity And Tolerance Mechanism Of Four Salix Genotypes Exposed To Cadmium And Lead

With the rapid development of industry and chemical agriculture,cadmium（Cd）and lead（Pb）have become common pollutants in environmental media.Excessive Cd and Pb pollution have caused a common threat to the environment.In recent years,the application of woody species in the environmental remediation has beccome one of the hot issues in the field of phytoremediation.Salix spp.and other short-rotation tree species with fast growth rate and large biomass are more competitive.By regularly harvesting abovground biomass of plants,biomass energy production can be carried out with phytoremediation process,which is also of great significance for alleviating energy crisis.In this study,four new Salix genotypes cultivated by Shandong Academy of Forestry were studied:Salix matsudana’J172’（A7）,Salix matsudana’Shidi1’（A42）,Salix matsudana’Yankang1’（A64）and Salix psammophila’Huangpi1’（A94）were used as experimental materials to study the uptake,accumulation and translocation characteristics of Cd and Pb as well as growth and physiological biochemical response characteristics in hydroponic culture.The results will provide a theoretical basis for the selection of woody plants with strong accumulation capacity of Cd and Pb,as well as an important guideline for phytoremediation of contaminated sites.The main results are as follows:1.Under the single and combined stresses of Cd and Pb,the biomass of leaves and roots of the four Salix genotypes were significantly inhibited,and showed a trend of gradual decrease with the increase of heavy metal concentrations in the culture solution.Under the stresses of high concentration of Cd（30μM）and Cd and Pb combined stress（Cd30+Pb500）,the growth inhibition rate of the leaves for A42 and A94 exceeded 50%.Under high concentration single stresses（Cd30,Pb500）and high concentration combined stress（Cd30+Pb500）,the growth inhibition rates of roots for the four Salix genotypes all exceeded 50%.2.Photosynthetic parameters were closely related with the extraction capacity of four Salix genotypes.Under the single stress of Cd,the net photosynthetic rate and stomatal conductance of the four Salix genotypes were significantly increased,and the intercellular CO₂ concentration of the four Salix genotypes decreased under the single Pb stress.Under the combined stresses of low Pb concentration（Cd15+Pb250 and Cd30+Pb250）,the net photosynthetic rate and stomatal conductance of four Salix genotypes were significantly increased.Under the combined stresses of high Pb（Cd15+Pb500 and Cd30+Pb500）,the intercellular CO₂ concentrations and stomatal conductance of four Salix genotypes decreased.3.Under stresses of Cd and Pb,Cd was mainly accumulated in leaves and Pb was commonly accumulated in roots.The Cd uptake rates of A94 were significantly higher than those of the other three Salix genotypes.The translocation fators of Cd of the four Salix genotypes were significantly higher than that of Pb.The increased Pb concentration in culture solution reduced the accumulation of Cd in aboveground tissue under comnined stress.In addition,except for A94,increased Cd concentration would increase the accumulation of Pb in the aboveground part and reduce the accumulation of Pb in the underground part of four Salix genotypes.4.With the increase of Cd and Pb concentrations in culture solution,the activities of superoxide dismutase were enhanced,while the activities of catalase and peroxidase were inhibited,indicating that superoxide dismutase played an important role in the removal of reactive oxide species.The content of malondialdehyde in leaves and roots increased obviously,which confirmed that the level of reactive oxygen species increased.The effects of heavy metal on the osmotic pressure of cells of the four Salix genotypes depened on the concentrations of heavy metals,which was reflected in the increase of the content of free proline.At the same time,with the increase of cadmium and lead stress concentration,the soluble protein content in the leaves of the four Salix genotypes decreased,while increased in roots.5.A42 showed the best tolerance to Cd.A64 could accumulate the most Pb above ground under the combined stress of the highest concentration（Cd30+Pb500）.The tolerance of A94to two heavy metals was the lowest,but has the best uptake and translocation capacity of Cd,as well as function of fixing and isolating Pb in underground part compared with the other three Salix genotypes.In addition,under the same initial conditions and growth conditions,A94 could accumulate more biomass.