ALTERED ZN COMPARTMENTATION IN THE ROOT SYMPLASM AND STIMULATED ZN ABSORPTION INTO THE LEAF AS MECHANISMS INVOLVED IN ZN HYPERACCUMULATION IN THLASPI-CAERULESCENS

We investigated Zn compartmentation in the root, Zn transport into the xylem, and Zn absorption into leaf cells in Thlaspi caerulescens,, a Zn-hyperaccumulator species, and compared them with those of a related nonaccumulator species, Thlaspi arvense. Zn-65-compartmental analysis conducted with roots of the two species indicated that a significant fraction of symplasmic Zn was stored in the root vacuole of T. arvense , and presumably became unavailable for loading into the xylem and sub sequent translocation to the shoot. In T. caerulescens, however, a sma ller fraction of the absorbed Zn was stored in the root vacuole and wa s readily transported back into the cytoplasm. We conclude that in T. caerulescens, Zn absorbed by roots is readily available for loading in to the xylem. This is supported by analysis of xylem exudate collected from detopped Thlaspi species seedlings. When seedlings of the two sp ecies were grown on either low (1 mu M) Or high (50 mu M) Zn, xylem sa p of T. caerulescens contained approximately 5-fold more Zn than that of T. arvense. This increase was not correlated with a stimulated prod uction of any particular organic or amino acid. The capacity of Thlasp i species cells to absorb Zn-65 was studied in leaf sections and leaf protoplasts. At low external Zn levels (10 and 100 CIM), there was no difference in leaf Zn uptake between the two Thlaspi species. However, at 1 mM Zn2+, 2.2-fold more Zn accumulated in leaf sections of T. cae rulescens. These findings indicate that altered tonoplast Zn transport in root cells and stimulated Zn uptake in leaf cells play a role in t he dramatic Zn hyperaccumulation expressed in T. caerulescens.