Uptake and accumulation of cadmium and zinc by Sedum alfredii Hance at different Cd/Zn supply levels

Sedum alfredii Hance has been identified to be a zinc (Zn) hyperaccumulating plant species native to China. In this study, growth responses, uptake and accumulation of cadmium (Cd) and Zn by S. alfredii were examined at Cd/Zn combining supply levels. The results showed that optimal growth for both shoots and roots was found when the plant grew at the Cd/Zn level of 1001500 mumol L-1. The concentrations of Cd and Zn in leaves and Sstems of Sedum afredii H increased with increasing Cd and Zn supply levels. The distributions of the metals in different plant parts decreased in the order: stem > leaf much greater than root for Zn and leaf > stem much greater than root for Cd. The highest concentrations of Zn (23.2 mg g(-1)) in the stems and Cd (12.1 mg g(-1)) in the leaves were noticed when the plants were grown at the Zn/Cd levels of 1000/50 and 500/400 mumol L-1, respectively. The maximum Cd and Zn accumulations in the shoots were 5.1 and 11.2 mg plant(-1) at the Cd/Zn combining levels of 400/250 and 100/500 mumol L-1, respectively. Zinc supply levels <500 mumol L-1 enhanced Cd concentrations in stems and leaves at the Cd levels <100 mumol L-1 and Cd concentration in roots at the Cd levels <50 mumol L-1. Cadmium at the supply levels greater than or equal to100 mumol L-1 decreased considerably root Zn concentrations for all the Zn levels, slightly increased leaf Zn concentrations at the Zn levels greater than or equal to250 mumol L-1, but had minimal effect on leaf Zn concentrations at the Zn levels greater than or equal to500 mumol L-1. The results indicate that S. Alfredii has an extraordinary ability to co-tolerate Cd/Zn toxicities, and to absorb and hyper-accumulate Cd and Zn under a range of Cd/Zn combining levels. Zinc addition at relatively low levels could stimulate Cd uptake and translocation, whereas Cd supply enhanced Zn translocation and partition to the shoots. Sedum alfredii is a Zn/Cd hyperaccumulator, which can be a valuable material for further study on the mechanisms of metal uptake and accumulation and for phytoremediation of the soils with Zn/Cd combined contamination.