The majority of plant species have a weak ability to selectively extract essential TEs from the rhizosphere (Marschner 1995); therefore, they take up non-essential (Si, Co) and even highly toxic ones (Hg, Pb, Cd). Some of potentially toxic metals (Cr, Ag, Sn) due to lower solubility in the most soil circumstances are practically phyto-unavailable. In contrast, Cd is one of the most soluble TEs with relatively high bioavailability (Adriano et al. 2004; Clemens 2006). The preferred metal forms that are taken up by roots are soluble free cations (Cd2 +, Zn2+, Cu2+), but also complexes with organic ligands (e.g. Zn, Broadley et al. 2007) . Free metal forms create (bioavail-able?) complexes with in/organic natural and/or synthetic ligands dissolved in the rhizosphere. However, confirmation of metallo-complex acquisition by roots is relatively scarce and based mostly on assumptions.
Increased uptake and phytoaccumulation of Cd, Zn and/or Cu in the presence of excessive inorganic ligands (e.g. Cl-) in the soil/rhizosphere have been showed in a wide range of experimental conditions (Smolders and McLaughlin 1996; Weggler et al. 2004; Ondrasek et al. 2009a, b). Also, Chiang et al. (2011) confirmed that LMW-OAs concentration in root exudates was positively correlated with the amount of Cd accumulated in millet shoots/roots, whereas Hoffland et al. (2006) found that Zn uptake efficiency in rice correlates with exudation rates of LMW organic anions. Similar results were obtained with giant alga Chara whereby Al3+ complexation with sulphate and citrate anions increased Al uptake by up to greater than twofold (Taylor et al. 2000) . Likewise to LMW, positive influence of HMW organic ligands (e.g. HAs) on phytoextraction of TEs (Cd, Cu, Zn, etc.) was confirmed in tobacco (Evangelou et al. 2004), forage (Neunhauserer et al. 2001) and recently in fodder radish (Bandiera et al. 2009) (see Sect. 6).
Undoubtedly, metal chelation and/or compl-exation with in/organic ligands enhance desorption from soil solids, and having mobilised TEs into a dissolved phase, improve their root uptake/ phytoaccumulation, although underlying mechanisms are unclear. Uptake of metals from the rhizosphere is mediated by a specific (Cu, Fe, Zn) and/or nonspecific (e.g. Cd) transporter proteins embedded into the plasma membrane of root cells (e.g. see review by Clemens 2006). Due to similar physical (ionic radius) and chemical (redox-activity, Lewis acidity) properties among TEs, non-essential and highly toxic ones (e.g. Cd) most probably enter roots via highly specific routes for essential nutrients (e.g. Zn), especially under stressful conditions (e.g. Ondrasek et al. 2009a) .
The TEs may be adsorbed/sequestered by numerous reactive groups and constituents in the apoplast (cell wall) and finally after crossing the plasma membrane in the symplast. In most cultured species, the largest portion of taken TEs is retained in the below-ground tissues (e.g. >70% Cd, Chiang et al. 2011; 65% Mn, Pearson and Rengel 1995) and the remaining portion is translocated to aerial parts in the following decreasing order: stem > old leaves > young leaves > fruits > seed. Information on the speciation of metals
Table 22.1 Possible intake of TEs via the foodstuffs consumption [adapted from ANZFA (1997) and Ondrasek (2008a)]
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