Following the process of root absorption and transportation, the shoot plays a role as the final destination of Cd.
Cadmium is transferred from the substrate to the above ground parts through xylem transportation, and then reaches the final storage location via xylem unloading. Corresponding to the xylem loading of Cd translocation, there are also two main pathways for Cd xylem unloading: apoplastic bypass and symplastic bypass (Clemens et al. 2002; Karley et al. 2000). In the leaves of T. caerulescens, Cd was found in cells lying on the way of water migration from the vascular cylinder to epidermal cells, which is in line with passive Cd transport by the transpiration stream (Wojcik et al. 2005). Leaf cells accumulate solutes differently depending on their cell types (Karley et al. 2000). Several families of transporters may be involved in the xylem unloading of Cd in shoots, such as HMA and CDF (please see Sect. 2.2 for details, Fig. 1d).
The distribution of heavy metals at plant tissue, cellular, and sub-cellular levels is considered as one of the important physiological mechanisms for metal hypertolerance and hyperaccumulation (Cosio et al. 2005; Kupper et al. 1999, 2000; Ma et al. 2005). On the whole, Cd is stored principally in the less metaboli-cally active parts of leaf cells. In T. caerulescens, Cd is mainly localized at the edges of the leaves, and also in points of higher concentration spread over the whole limb surface (Cosio et al. 2005). Cadmium is found in leaf epidermal cells in T. caerulescens, with concentrations two times higher than that in mesophyll cells (Ma et al. 2005). Besides, Cd is also found in epidermal trichomes of A. halleri (Kupper et al. 2000) and Brassica juncea (Salt et al. 1995). Most of the Cd in the leaf has been suggested to be localized at the base of trichomes (Kupper et al. 2000; Ma et al. 2005). This preferred storage in leaf epidermal cells may be associated with avoidance of heavy metal damage to photosynthesis. Entrance of Cd into the mesophyll cells may cause phytotoxicity in plants (Hu et al. 2009). Although metals are accumulated to much higher levels in the epidermis, a significant fraction of the total leaf metal accumulation still occurs in the mesophyll, mainly due to low biomass of the epidermis (Ma et al. 2005).
On the sub-cellular level, Cd is found both inside the cells and on the cell walls (Cosio et al. 2005). Similarly, Zhang et al. (2010) have suggested that Cd is mainly distributed in the vascular tissue and localized on the cell wall. In leaves of T. caerulescens, Cd and Zn are sequestered predominantly in the epidermal vacuoles (Cosio et al. 2005; Kupper et al. 1999; Ma et al. 2005). Hu et al. (2009) have shown that the major storage sites for Zn and Cd in P. griffithii leaves are vacuoles of epidermis and bundle sheath, and to a lesser extent in cell wall or cytosol, and vacuolar storage may play a major role in strong tolerance and hyperaccumulation of Zn and Cd.
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