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M. Lasat1, L. Kochian1, M. Toaspern2, K.W. Smolenski2, K. Finkelstein2, E. Fontes2, D. Bilderback2
1USDA-ARS Plant, Soil and Nutrition Laboratory, Cornell University
2CHESS, Cornell University

The existence of plant species that can accumulate very high levels of heavy metals suggests that the genetic potential exists to use terrestrial plants to clean surface soils contaminated with toxic metals. One of the best known heavy metal hyperaccumulators is Thlaspi caerulescens, a member of the cabbage family which has been shown to accumulate up to 3% Zn on a dry weight basis in without showing toxicity symptoms. X-ray fluorescence and absorbance spectroscopy (XANES) measurements at CHESS have been used investigate Zn hyperaccumulation in T. caerulescens.

Flouresent X-ray          Flouresent X-ray
Zn fluorescence images of a T. caerulescens leaf from (left): a plant grown for forty days on 1 : M Zn2+ and then grown for the last 10 days in a nutrient solution without Zn and (right): a plant grown for forty days on 1 M Zn and then grown for the last 10 days in a nutrient solution supplemented with 150 M Zn2+. The scale of pseudo-color images relating relative Zn concentration to color is:
yellow (highest) > red > green > brown (least).

Shown in the x-ray fluorescence images above, the pattern of Zn accumulation in leaves was different in leaves of T. caerulescens compared with a related nonaccumulator species. For plants grown on a range of Zn concentrations, very little Zn fluorescence was emitted from leaves of the nonaccumulator species and this signal was associated exclusively with the leaf vascular tissue. In the Zn-hyperaccumulator T. caerulescens, the pattern of Zn accumulation was dependent upon the amount of Zn stored in the leaves. After growth on low Zn levels, most of the Zn was localized in the vascular tissue. As Zn accumulation in leaves increased, Zn fluorescence increased from leaf tissue between the veins, indicating that the excess Zn is a stored inside living leaf cells.

These results are important because they suggest that Zn hyperaccumulation in T. caerulescens involves alterations in Zn transport systems and chemical speciation in leaves. Studies have been initiated to characterize the chemical environment associated with Zn accumulation in leaf cells and thereby determine what organic ligands might be complexed with Zn.