| Cadmium (Cd) is one of the most toxic heavy metals in the environment due to its high mobility, easy accumulation and toxicity at low concentration in organisms. Moreover, soil Cd pollution in agricultural ecosystem becomes more and more serious due to improper agriculture management and industrial wasters discharge. A number of hazardous effects on plant is evoked by Cd. Furthermore, human health may be endangered, as Cd is easily transferred to drinking water and food from Cd contaminated soils. Phytoremediation emerged as an alternative technique to remove toxic metals from soil, which offers the benefits of being in site, cost-effective and environmentally sustainable. The successful implementation of phytoremediation depends on the identification of suitable plant species that are not only capable of growing on soils containing high levels of metals, but also accumulating much more higher concentrations of metals in their shoots than normal species. These plants are termed hyperaccumulator. Up to now, more than 450 species of hyperaccumulators belonging to 45 families have been identified. But Cd-hyperaccumulator has been hardly found yet. S. alfredii Hance growing in a Pb/Zn mine area has also been identified as a Zn-hyperaccumulator native to China. It also has characteristics of large biomass, fast growth, asexual propagation and perennial. So it is an ideal plant could be applied for practice of phytoremediation. However, physiological mechanisms for Cd tolerance and uptake ability from contaminated soils by S. alfredii Hance are yet unclear. The objectives of this study were to examine the abilities of S. alfredii Hance to tolerate Cd and the effectiveness of phytoextrcation from the contaminated soils. The major results obtained were summarized as follows:1. In solution culture, the threshold for growth response to external Cd was 100 μmolL-1 for the non-mined ecotype plants (NME) and 400μmolL-1 for the mined ecotype plants of Sedum alfredii Hance (ME). For ME the dry matter yield reached maximum at 100μmolL-1 Cd. Cadmium concentrations in varied parts increased with increasing external Cd supply levels either for NME or for ME. Root Cd concentrations in NME were higher than that in ME, with maximum being 5646 mg kg-1 for NME and 2889 mg kg-1 for ME at 1000 μmol L-1 Cd. On the contrary, shoot Cd concentrations of the NME were far lower than that of ME. Maximum shoot Cdconcentrations were 533 mg kg-1 in leaves and 935 mg kg-1 in stems at 1000 μmol L-1 Cd for NME, whereas, 4933 and 3874 mg kg-1 at 400 μmol L-1 Cd for ME, respectively. The rates of Cd influx into roots (IR) and transport to shoots (TR) were greater in ME than in NME, with 5-fold for the maximum IR (Imax) and 13-fold for the maximum TR (Tmax) in NME, respectively. Meanwhile, Cadmium concentrations in the shoots of both NME and ME increased with advancing Cd treatment time. At 100μmolL-1 Cd, concentrations of Cd in leaves and stems of NME sharply increased within initial 8 and 12 days, and those in the ME increased dramatically until D20 and D16, respectively. However, leaf and stem Cd concentrations reached their maximum values on D4 for NME and D8 for ME, respectively, when the plants were exposed to 1.0 μmol L-1 Cd. Cadmium accumulation by plant shoots was obvious higher in ME than in NME at varied Cd supply levels or Cd treatment time. The maximum Cd taken up by the shoots was 1032 μg plant-1 in concentration-dependent uptake and 1699μg plant-1 in time-course uptake for ME, with 15-fold and 18-fold higher than those for NME, respectively. The ratios of shoot/root for Cd taken up were 12-39 at varied Cd supply levels and 13-24 in varied treatment time for ME, more than 10 times greater than those for NME. In addition, Cd distribution in leaves, stems and roots of ME was greatly different form those of NME. The percentage of Cd distribution in shoots was more than 79 percent at varied Cd supply levels, or 83 percent in varied treatment time for ME, either higher than tha...
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