Elemental distribution by cryo-micro-PIXE in the zinc and cadmium hyperaccumulator Sedum plumbizincicola grown naturally

The aim of this study was to determine Zn/Cd accumulation ability and cellular distribution in Sedum plumbizincicola from a mine area. Spatial localization of S, Cl, K, Ca, Ti, Mn, Fe, Ni, Cu, Zn and Cd in frozen-hydrated sections of S. plumbizincicola was quantitatively determined using cryo-micro-PIXE. The shoots of S. plumbizincicola hyperaccumulated up to 1,470 mg kg(-1) of Cd and 14,600 mg kg(-1) of Zn (dry weight) in field-grown plants. Micro-PIXE analyse shows that in roots Zn was concentrated in the cortex. In stems Zn was preferentially accumulated in the epidermis (5,090 mg kg(-1) wet weight) and in vascular bundles and neighbouring parenchyma cells. Interestingly, some vascular bundles of the stem were depleted in Zn. Taking tissue area into account, 22.1, 46.2 and 31.8 % of total Zn and 17.1, 71.6 and 11.3 % of total Cd in the stem were present in the epidermis, cortex and central cylinder, respectively. In the leaves, 81.3 % of total Cd and 55.2 % of total Zn were distributed in the mesophyll. The parenchyma cells, e.g. cortex in stem and mesophyll in leaf, play more important roles in Cd storage and detoxification than Zn in S. plumbizincicola.