COMPARTMENTATION OF ZINC IN ROOTS AND LEAVES OF THE ZINC HYPERACCUMULATOR THLASPI-CAERULESCENS J AND C PRESL

Thlaspi caerulescens is a metallophyte that is able to hyperaccumulate Zn. In the present study the subcellular compartmentation of Zn was investigated in roots and leaves of this species by means of X-ray microanalysis. Leaves accumulated higher average Zn concentrations than roots. In roots of plants exposed to 10 mu M Zn, Zn concentrations in the apoplast were similar to those in vacuoles, while in plants treated with 100 mu M Zn considerably higher Zn concentrations were detected in vacuoles than in the apoplast. In epidermal and sub-epidermal cells of leaves of plants from both treatments, Zn mainly accumulated in vacuoles and, to a lesser extent, in the apoplast. In vacuoles from plants exposed to 100 mu M Zn, high Zn concentrations were associated with variable amounts of P, Ca and K. In leaves, the highest Zn concentrations (13,600 mu g g(-1) d.m.) were found in globular crystals present in many vacuoles of epidermal and subepidermal cells. Smaller deposits with a variable Zn concentration between 1,000 and 18,300 mu g g(-1) d.m. were observed in the epidermal and subepidermal cells of roots. Both the high Zn/P element ratios found in the crystals and the absence of Mg indicate that, in contrast to other plant species, myo-inositol hexaphosphate (phytate) is not the main storage form for Zn in Thlaspi caerulescens.