Cellular compartmentation of cadmium and zinc in relation to other elements in the hyperaccumulator Arabidopsis halleri

The cellular compartmentation of elements was analysed in the Zn hyperaccum ulator Arabidopsis halleri (L.) O'Kane & Al-Shehbaz (=Cardaminopsis halleri ) using energy-dispersive X-ray microanalysis of frozen-hydrated tissues. Q uantitative data were obtained using oxygen as an internal standard in the analyses of vacuoles, whereas a peak/background ratio method was used for q uantification of elements in pollen and dehydrated trichomes. Arabidopsis h alleri was found to hyperaccumulate not only Zn but also Cd in the shoot bi omass. While large concentrations of Zn and Cd were found in the leaves and roots, flowers contained very little. In roots grown hydroponically, Zn an d Cd accumulated in the cell wall of the rhizodermis (root epidermis), main ly due to precipitation of Zn/Cd phosphates. In leaves, the trichomes had b y far the largest concentrations of Zn and Cd. Inside the trichomes there w as a striking sub-cellular compartmentation, with almost all the Zn and Cd being accumulated in a narrow ring in the trichome base. This distribution pattern was very different from that for Ca and P. The epidermal cells othe r than trichomes were very small and contained lower concentrations of Zn a nd Cd than mesophyll cells. In particular, the concentrations of Cd and Zn in the mesophyll cells increased markedly in response to increasing Zn and Cd concentrations in the nutrient solution. This indicates that the mesophy ll cells in the leaves of A. halleri are the major storage site for Zn and Cd, and play an important role in their hyperaccumulation.