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Annals of Botany 2007 99(5):869-875; doi:10.1093/aob/mcm038
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© The Author 2007. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Sorghum Roots are Inefficient in Uptake of EDTA-chelated Lead

Yong Xu1, Naoki Yamaji1, Renfang Shen2 and Jian Feng Ma1,*

1 Research Institute for Bioresources, Okayama University, Chuo 2-20-1, Kurashiki 710-0046, Japan
2 State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China

* For correspondence. E-mail: maj{at}rib.okayama-u.ac.jp

Received: 24 October 2006    Returned for revision: 9 January 2007    Accepted: 30 January 2007   

Background and Aims: Ethylene diamine tetraacetic acid (EDTA)-assisted phytoremediation has been developed to clean up lead (Pb)-contaminated soil; however, the mechanism responsible for the uptake of EDTA–Pb complex is not well understood. In this study, the accumulation process of Pb from EDTA–Pb is characterized in comparison to ionic Pb [Pb(NO3)2] in sorghum (Sorghum bicolor).

Methods: Sorghum seedlings were exposed to a 0·5 mM CaCl2 (pH 5·0) solution containing 0, 1 mM Pb(NO3)2 or EDTA–Pb complexes at a molar ratio of 1:0·5, 1:1, 1:2 and 1:4 (Pb:EDTA). The root elongation of sorghum at different ratios of Pb:EDTA was measured. Xylem sap was collected after the stem was severed at different times. The concentration of Pb in the shoots and roots were determined by an atomic absorption spectrometer. In addition, the roots were stained with Fluostain I for observation of the root structure.

Key Results: Lead accumulation in the shoots of the plants exposed to EDTA–Pb at 1:1 ratio was only one-fifth of that exposed to ionic Pb at the same concentration. Lead accumulation decreased when transpiration was suppressed. The concentration of Pb in the xylem sap from the EDTA–Pb-treated plants was about 1/25 000 of that in the external solution. Root elongation was severely inhibited by ionic Pb, but not by EDTA–Pb at a 1:1 ratio. Root staining showed that a physiological barrier was damaged in the roots exposed to ionic Pb, but not in the roots exposed to EDTA–Pb.

Conclusions: All these results suggest that sorghum roots are inefficient in uptake of EDTA-chelated Pb and that enhanced Pb accumulation from ionic Pb was attributed to the damaged structure of the roots.

Key words: Complex, EDTA, form, Pb, phytoremediation, uptake system, sorghum (Sorghum bicolor)


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