| Soils contaminated with metals pose a major environmental and human health problem, which is still in need of an effective and affordable technological solution. The phytoremediation is a cost-effective -green- technology based on the use of metal-accumulating plants to remove toxic metals. Plant growth and lead accumulation of accumulating and non-accumulating ecotypes of Sedum alfredii Hance were studied with hydroponic, pot and field experiments. To elucidate the characteristics of Pb tolerance and accumulation in the accumulating ecotype of Sedum alfredii Hance, a series of chemical, biochemical analytic techniques were used to study the absorption, distribution of Pb at cellular and subcellullar in plant, especially in the cell wall fractions, as well its application to phytoremediation. The major results were summarized as follows:1. In the hydroponic experiment, shoot growth of the mining ecotype was not inhibited by Pb levels up to 320mg L-1, whereas the growth of the non-mining ecotype was inhibited in all Pb levels treated. Lead concentrations in the roots and shoots of the mining ecotype increased with increasing of Pb level in the nutrition solution. The maximum Pb concentrations in the shoots and roots of mining ecotype were 514 mg kg-1 and 13922 mg kg-1, and were 2.27 times and 2.62 times as that of the non-mining ecotype, respectively. The highest rate of lead accumulation for mining ecotype was 8.62 jag plant-1 d-1, being 7.16 time as that of the non-mining ecotype. Due to its fast growth rate and high Pb-accumulating ability, it was suggested that 5-. alfredii Hance is a Pb-tolerant and Pb-accumulating ecotype.2. Lead uptake and accumulation in roots, stems and leaves of the accumulating ecotype were affected by the Pb levels supplied and treatment time. During a short-term treatment (36h), the Pb concentrations in roots, stems and leaves of the accumulating ecotype supplied with lOmg L-1 Pb had not significant differences. With 160 mg kg-1 Pb treatment, the increase of the root Pb was observed firstly at 2h, followed by that of stem at 8h, and the Pb concentration in leaves was2h, followed by that of stem at 8h, and the Pb concentration in leaves was increased at 24h. During the long-term treatment (40d), lead concentrations in roots, stems and leaves of accumulating ecotype with 10 mg L -' level increased with time, peaked at 22, 22 and 28d, respectively. At 160 mg L-1 Pb treatment, lead concentrations in roots, stems and leaves increased with time, peaked at 16, 22 and 28d, respectively. After the plants were grown at various Pb levels for 28d, the highest Pb concentration in leaves occurred at 40mg Pb L-1, and that in stems and roots occurred at 160 mg Pb L-1. From the results, it was suggested that lead uptake and accumulation by the accumulating ecotype are time and concentration dependent.The subcellular distributions of Pb in root, stem and leaf tissues were studied using sucrose differential centrifugation. Lead was distributed mainly in cell walls of the roots, stems and leaves for both of the accumulating and non-accumulating ecotypes. For the accumulating ecotype, the proportions of Pb in cell walls of roots, stems and leaves at different Pb supply levels were averaged 89 %, 61 %, 86 % respectively, whereas in the non-accumulating ecotype those averaged 59 %, 59 %, 77 %, respectively. About 50 % of Pb was found to be associated with the cell wall fraction in the stems of accumulating ecotype and the percentage increased to about 80 % both in roots and leaves. The results indicated that the distribution of Pb in cell walls may mainly account for high tolerance to Pb for the accumulating ecotype. There were significant differences in chemical forms of Pb in roots, leaves and sterns between the two ecotypes. In roots, the most Pb of both ecotypes could be extracted with 0.6 mol L-1 HC1. The percentage of NaCl-extractable Pb in the roots of accumulating ecotype was higher than that of non-accumulating ecotype. In stems, NFLtOAc-extractable Pb...
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