Distribution of Zn in functionally different leaf epidermal cells of the hyperaccumulator Thlaspi caerulescens

The aim of this study was to show the potential of Thlaspi caerulescens in the cleaning-up of a moderately Zn-contaminated soil and to elucidate toler ance mechanisms at the cellular and subcellular level for the detoxificatio n of the accumulated metal within the leaf. Measured Zn concentrations in s hoots were high and reached a maximum value of 83 mmol kg(-1) dry mass, whe reas total concentrations of Zn in the roots were lower (up to 13 mmol kg(- 1)). In order to visualize and quantify Zn at the subcellular level in root s and leaves, ultrathin cryosections were analysed using energy-dispersive X-ray micro-analysis. Elemental maps of ultrathin cryosections showed that T. caerulescens mainly accumulated Zn in the vacuoles of epidermal leaf cel ls and Zn was almost absent from the vacuoles of the cells from the stomata l complex, thereby protecting the guard and subsidiary cells from high Zn c oncentrations. Observed patterns of Zn distribution between the functionall y different epidermal cells were the same in both the upper and lower epide rmis, and were independent of the total Zn content of the plant. Zinc store d in vacuoles was evenly distributed and no Zn-containing crystals or depos its were observed. From the elemental maps there was no indication that P, S or Cl was associated with the high Zn concentrations in the vacuoles. In addition, Zn also accumulated in high concentrations in both the cell walls of epidermal cells and in the mesophyll cells, indicating that apoplastic compartmentation is another important mechanism involved in zinc tolerance in the leaves of T. caerulescens.