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ENVIRONMENTAL STRESS AND ADAPTATION

Tissue- and Age-Dependent Differences in the Complexation of Cadmium and Zinc in the Cadmium/Zinc Hyperaccumulator Thlaspi caerulescens (Ganges Ecotype) Revealed by X-Ray Absorption Spectroscopy^1,[w]

Universität Konstanz, Mathematisch-Naturwissenschaftliche Sektion, Fachbereich Biologie, Postfach M665, D–78457 Konstanz, Germany (H.K., P.M.H.K.); University of South Bohemia, Biological Faculty, Braniovská 31, CZ–370 05 eské Budjovice, Czech Republic (H.K.); and EMBL Outstation Hamburg c/o Deutsches Elektronen-Synchrotron, Notkestrasse 85, D–22603 Hamburg, Germany (A.M., W.M.-K.)

Extended x-ray absorption fine structure measurements were performed on frozen hydrated samples of the cadmium (Cd)/zinc (Zn) hyperaccumulator Thlaspi caerulescens (Ganges ecotype) after 6 months of Zn²⁺ treatment with and without addition of Cd²⁺. Ligands depended on the metal and the function and age of the plant tissue. In mature and senescent leaves, oxygen ligands dominated. This result combined with earlier knowledge about metal compartmentation indicates that the plants prefer to detoxify hyperaccumulated metals by pumping them into vacuoles rather than to synthesize metal specific ligands. In young and mature tissues (leaves, petioles, and stems), a higher percentage of Cd was bound by sulfur (S) ligands (e.g. phytochelatins) than in senescent tissues. This may indicate that young tissues require strong ligands for metal detoxification in addition to the detoxification by sequestration in the epidermal vacuoles. Alternatively, it may reflect the known smaller proportion of epidermal metal sequestration in younger tissues, combined with a constant and high proportion of S ligands in the mesophyll. In stems, a higher proportion of Cd was coordinated by S ligands and of Zn by histidine, compared with leaves of the same age. This may suggest that metals are transported as stable complexes or that the vacuolar oxygen coordination of the metals is, like in leaves, mainly found in the epidermis. The epidermis constitutes a larger percentage of the total volume in leaves than in stems and petioles. Zn-S interaction was never observed, confirming earlier results that S ligands are not involved in Zn resistance of hyperaccumulator plants.

¹ This work was supported by the European Community "Access to Research Infrastructure Action of the Improving Human Potential Programme" to the EMBL Hamburg Outstation (contract no. HPRI–CT–1999–00017), by the Deutsche Forschungsgemeinschaft, and by the Ministry of Education of the Czech Republic (grant nos. VS96085 and ME138).

^[w] The online version of this article contains Web-only data.

^* Corresponding author; e-mail Hendrik.Kuepper{at}uni-konstanz.de; fax 607–255–2459.

Received September 4, 2003; returned for revision September 24, 2003; accepted November 11, 2003.

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