| Hperaccumulators can tolerant and accumulate heavy metals, their ability to extract metals from soil and to concentrate them in shoot, has give rise to the idea of practical application such as phytoremediation and phytomining. Furthermore, these plants represent valuable models to study the mechanisms of accumulation, tolerance and detoxification of heavy metals. To understand the mechanisms of uptake and accumulation is important to discover and create a new hyperaccumulator with characteristics of tolerant to high levels of metal or multiple metals, accumulation high levels of metal(s) in harvestable parts, large biomass, potential fast growth, and easy propagation. Furthermore, the understanding is the core of engineering technique on phytoremediation and phytomining.The material in this paper is accumulation in Sedum alfredii Hance, which was found to be a superior herb plant grown on an old mining area and a new Zn-accumulating plant specie observed in Southeast China. Solution culture experiment and soil culture experiment were applied to study Cd uptake andaccumulation of Sedum alfredii Hance. And to discuss the possible mechanisms in Sedum alfredii Hance to tolerant and hyperaccumulate Cd, by means of a series of chemical ,biochemical analysis methods, such as AAS for elements determination, differential centrifugation, sequential chemical extraction method and so on. The major results were summarized as follows:1. 400 umol/L was the highest Cd concentration which mining-ecotype Sedum alfredii Hance can tolerate and grow normally under the condition of nutrition solution culture. The Cd concentration in stem and leaf reached 4511.96 and 3316.874mg/kg (DW plant) respectively when Cd concentration is 400 umol/L in nutrition solution. The maximum Cd concentration in aerial part of mining-ecotype Sedum alfredii Hance was 1423. 477 mg/kg (DW) under the condition of soil culture. These results both exceeded the standard for Cd-hyperaccumulator (1000 mg/kg). Thlaspi caeruescens was proved as a Cd-accumulator, and it can only accumulate Cd with 1140 mg/kg. And the ability of mining-ecotype Sedum alfredii Hance to accumulate Cd is 15-25 times than that of Cd accumulating vegetable crops. So, we concluded that mining-ecotype Sedum alfredii Hance is possible a new Cd-accumulator in terms of Cd content in plant.2. We studied the subcellular distribution and chemical form of Cd in root, stem and leaf of mining ecotype Sedum alfredii Hance and non-mining ecotype Sedum alfredii Hance, applying differential centrifugation technique sequential chemical extraction method. The results indicated that subcellular distribution of Cd in root, stem and leaf of mining-ecotype Sedum alfredii Hance under 400 umol/L showed the trend as follow: the subcellular distribution of Cd was uneven. The Cd content was the highest in cell wall, and then followed the soluble fraction and the least was in cell organs. And the mode was similar to other hyperaccumulators of heavy metals. However, the difference of distribution rates of Cd among cell wall, cell organ and soluble fraction in non-mining ecotype Sedum alfredii Hance was not significant. So, we think that Cell wall was the chief part for immobilizing Cd in mining-ectoypeSedum alfredii Hance, furthermore the immobilizing of Cd by means of cell wall is a effective mechanism in response to Cd stress. There existed significant difference in chemical forms of Cd in root, stem and leaf in mining ecotype Sedum alfredii Hance. De-ionized water- extractable Cd, 1mol/L sodium chloride- extractable Cd and 2%acetic acid- extractable Cd were the major chemical forms. However, there was no significant difference among all extractable Cd. In addition, salt of organic acid, heavy metals binding to protein or peptide, phosphate etc. were extracted by above three extracting agencies. These results suggested that chelation of Cd in the cytosol by proteins/ peptides and organic anions and subsequently transfer of complexes or metal to the vacuole was possible mode of Cd uptake...
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