| Polygonum capitatum Buch. -Ham. ex D.Don attributes to Polygonum, Polygonacea,it is a kind of perennial wild medicinal plant with high economic and ornamental value. It is attributes include being able to survive in shade, arid as well as cold environments. By taking wild groundcover plant—P. capitatum as the research material and the pot-culture method, this experiment studied the effect of growth, changes in physiological and biological index of the plant and the absorption, accumulation, as well as transference properties of Cd and Pb in the plant under different concentrations of Cadmium and Lead and the compound of Cadmium and Lead stress. In addition, this experiment investigated how P. capitatum survived when introduced to Cadmium and Lead. The results are as follows:1. Low concentration Cd2+, Pb2+ had little effect on the germination of P. capitatum.1.0mg·L-Cd2+ promoted germination rate(GR)and germination energy,and was able to influence the seedling root length. With the increasing concentration of Cd2+(≥1.0 mg·kg-1) and Pb2+ (≥300 mg·kg-1), germination rate (GR), germination energy (GE), germination index (GI), vigor index (VI), root length, shoot height, root biomass and shoot biomass decreased gradually and the seedling growth was seriously inhibited. Browning, and even death of seedling could also be observed, as the Pb2+ concentration in soil was increased by 1000 mg·kg-1, The inhibition on root of Cd2+ and Pb2 stress was stronger than on the seedling. Toxicity of root was heavier than seedling.2.The single pollution of Cd2+(≤10 mg·kg-1) was able to influence the seedling height, root length, leaf areas, aboveground biomass, root biomass at lower concentration, but the difference was not significant. With the increase in the concentration of Cd2+,the more toxical effect of the growth.. At low concentration of Pb2+(300 mg·L-1), the seedling height, leaf areas, aboveground biomass were increased by 0.07%,0.56% and 2.09%, while the root length, root biomass had some decrease, both had less difference with the control. While the concentration of Pb2+ was up to more than 300 mg·L-1, the seedling height, root length, leaf areas, aboveground biomass, root biomass all had some decrease. Under the introduction of combined pollution, the seedling height, root length, leaf areas, aboveground biomass, root biomass decreased with increasing stressing concentration.3.Under the introduction of Cd2+,Pb2- and Cd2+and Pb2+ compounds pollution, palisade mesophyll, spongy mesophyll, the thickness ratio of palisade and sponge tissue diminished, with increased concentration of Cd2+,Pb2+ and Cd2++Pb2+,the greater the inhibition on the palisade mesophyll, spongy mesophyll and the thickness ratio of palisade and sponge tissue.4.Under the introduction of Cd2+ pollution,the membrane permeability of the mesophyll of P. capitatum increased with increasing stressing time.The values of the membrane permeability reached maximum in the 40th day under Cd2+ 1.0 mg·kg-1 concentration. Under 0.5 mg·kg-1,1.0 mg·kg-1,5.0mg·kg-1 Cd2+ concentration, the MDA content, the SOD activities of P. capitatum increased with increasing stressing time,while under 10 mg·kg-1,50.0mg·kg-1concentration the two values decreased with increasing stressing time. Soluble sugar content, soluble protein content of P. capitatum increased at lower Cd2+ (≤5 mg·kg-1) concentration, but decreased as expected at higher Cd2+(>5 mg·kg-1) concentration and with increasing stressing time.5. Under the introduction of Pb2+ pollution, the membrane permeability of the mesophyll and the MDA content of P. capitatum increased with increasing stressing concentration. At low concentration of Pb2+(≤300 mg·kg-1) for the first 10th day,the two valus had less difference with the control,while other treatment group had most difference with the control. The SOD activities of P. capitatum increased at lower Pb2+(≤1000 mg·kg-1) concentration, but decreased as expected at higher Pb2+ (>1000 mg·kg-1) concentration and with increasing stressing time. Soluble sugar content, soluble protein content of P. capitatum had less difference with the control under the concentration of 300mg·kg-1,while under 800mg-kg-1,1000 mg·kg-1,1500 mg·kg-1,2000 mg·kg-1 the two values increased with increasing stressing concentration, had most difference with the control.6. Under the condition of combined pollution, the permeability of membrane and the SOD activities of P. capitatum increased at lower Cd2+and Pb2+ combined concentration (T1), but significantly decreased at higher Cd2+and Pb2+ combined concentration (T2,T3), reached a peak on the 20th day at the T1 treatment, increased by 61.54% and 89.73%; The content of MDA ascended with the increase of the Cd2+and Pb2+ combined concentration and the days of introduction, reached a peak on the 40th day at the treatment of T3,while the soluble sugar content and soluble protein content of P. capitatum showed a decreasing trend during 40 days,be reduced to the minimum on the 40th day at the T3 treatment. The effects of Cd2+ and Pb2+ combined compounds on the physiological characters of P.capitatum were stronger than single Cd2+ and Pb2+ stress.7. The results also indicated that the photosynthetic characteristics of P. capitatum seedlings were influenced severely by Cd2+, Pb2+and their combined concentrations. With the increase of Cd2+, Pb2+and their combined concentrations in cultivation medium, the content of chlorophyll, net photosynthetic rate, transpiration rate and stomatal conductance of P. capitatum decreased significantly, while the intercellular CO2 concentration increased, the relative low concentrations of Cd (<5.0 mg/kg) improved content of chlorophyll,net photosynthetic rate, transpiration rate and stomatal conductance, reduced the intercellular CO2 concentration.8. Accumulation of Cd2+and Pb2+in vegetative organs of P. capitatum would increase as the increase of treatment concentrations of Cd2+and Pb2+ in soils, and reached a peak at the Cd2+concentration of 50 mg·kg-1, the maximum quantity being 182.69 mg·kg-1 in root,31.49 mg·kg-1 in leaf and 10.34 mg·kg-1 in stem. When the Pb2+ concentration in soil was increased by 1500 mg·kg-1, the contents of Pb in root was accumulated by 246.74 mg·kg-1, was 36.72 times of the control. As the Pb2+ concentration in soil was increased by2000 mg·kg-1, the contents of Pb in steam and leaf were accumulated by 128.79 mg·kg-1 and 36.21 mg·kg-1, both being up the peak. Pb and Cd in the plant distributed noticeable difference accumulation of Cd2+ in different vegetative organs of P. capitatum was in the order of root> leaf>stem, while the accumulation amounts of Pb was in the order of root> stem>leaf. The bioaccumulation coefficient of Cd and Pb were 0.459～1.546 and 0.023～0.067, and the translocation coefficient was 0.099～0.138 for Cd and 0.142～0.668 for Pb, and the largest total transference amount in shoot was up to 100.09 ug-plant-1 for Cd and 222.536 ug-plant-1 for Pb.According to the research on the Cd2+, Pb2+ and their combined concentrations tolerance for P. capitatum, it was found out that P. capitatum has strong adaptability to the Cd2+,Pb2+ and their combined introduction environment. It could grow normally at the Cd2+ 50 mg·kg-1, Pb2+ 2000 mg·kg-1 and Cd2+, Pb2+ combined concentrations of (50+2000) mg·kg-1. Therefore, it is a preferable germplasm resources of landscape groundcover plant for soil introduced with Cd2+, and Pb2+...
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