Enhancement of Lead Uptake by Hyperaccumulator Plant Species Sedum alfredii Hance Using EDTA and IAA

Abstract

Lead (Pb) is considered as one of the most frequently encountered heavy metals of environmental concern, causing loss of vegetation, groundwater contamination, and toxicity in plants, animals, and humans (Clistenes et al. 2006). Because of its persistent nature, Pb will continue to be an environmental concern for a long time unless it is removed from the ecosystem. Conventional cleanup technologies are generally too expensive to be used for restoration of contaminated soils, and are often harmful to the normal properties of soil (Holden 1989). The emerging phytoremediation techniques, with their low cost and environmental friendly nature, have received increasing attention in the last decades (Salt et al. 1998). Over 400 hyperaccumulating plant species from all over the world can accumulate high concentrations of metals from contaminated soils (Baker et al. 2000). However, there are no reliable reports on Pb hyperaccumulating species under natural conditions, presumably since the phytoavailability of Pb is restricted by the strong complexes of Pb within solid soil fractions. To overcome this problem and increase Pb availability to plants, chelators have been used to artificially enhance Pb solubility in soil solution (Kos and Lestan 2004). Many studies have been conducted using synthetic chelators such as EDTA or HEDTA, which are added to Pb contaminated soils to promote metal translocation from roots to shoots (Huang et al. 1997; Vassil et al. 1998). Although several studies have reported elevated metal concentrations in shoots after applying chelators, most of the times it was correlated to a severe decrease in plant biomass. Therefore other amendments have also been used to increase the heavy metal uptake as well as to maintain better plant growth during chelate-assisted phytoextraction technologies. Lopez et al. (2005) studied the combined effects of EDTA and IAA on Pb uptake by Medicago sativa (alfalfa) and found that the combination of EDTA and IAA dramatically enhances the Pb uptake from root to shoot as compared to those treated with Pb alone and Pb/EDTA. Sedum alfredii Hance grows in old Pb/Zn-mined areas of southeast China and has been reported to be a Zn/Cd hyperaccumulator (Yang et al. 2002; Yang et al. 2004), and was also proved to be a Pb-accumulating species (He et al. 2002). Previous studies were mainly focused on the Pb accumulation and transportation mechanism in S. alfredii (Long et al. 2006; Yang et al. 2006), and less attention was paid to the application of chelate-assisted phytoextraction techniques. In this study, a promising method was used to investigate the uptake of Pb by two ecotypes of S. alfredii growing under hydroponic conditions. The specific objective of this study was to evaluate the effect of indole-3acetic acid (IAA), a growth-promoting auxin combined with ethylene diamine tetra acetic acid (EDTA), a synthetic chelating agent, to enhance the Pb uptake ability of S. alfredii in hydroponic culture. D. Liu T. Li X. Yang (&) E. Islam X. Jin Ministry of Education Key Lab of Environment Remediation and Ecological Health, College of Environmental and Resource Sciences, Zhejiang University, Huajiachi Campus, Hangzhou 310029, People’s Republic of China e-mail: xyang581@yahoo.com