Using the noninvasive technique of X-ray absorption spectroscopy (XAS), we have been able to determine the ligand environment of Zn in different tissues of the Zn-hyperaccumulator Thlaspi caerulescens (J. & C. Presl.). The majority of intracellular Zn in roots of T. caerulescens was found to be coordinated with histidine. In the xylem sap. Zn was found to be transported mainly as the free hydrated Zn2+ cation with a smaller proportion coordinated with organic acids. In the shoots, Zn coordination occurred mainly via organic acids, with a smaller proportion present as the hydrated cation and coordinated with histidine and the cell wall. Our data suggest that histidine plays an important role in Zn homeostasis in the roots, whereas organic acids are involved in xylem transport and Zn storage in shoots.

Zinc Ligands in the Metal Hyperaccumulator Thlaspi caerulescens As Determined Using X-ray Absorption Spectroscopy