Metals such as Zn, Cd and Cu have been reported to induce the absorption of sulfate to sustain the greater sulfur demand during the biosynthesis of GSH and phytochelatins, sulfur-containing compounds that are particularly involved in metal tolerance (Schiavon and Malagoli 2008). Zn can induce an increase in total GS

concentrations in roots and a decrease in shoots of T. caerulescens (Tolra et al. 2001). This may result from Zn-induced changes in sulfur pools and/or responses related to defence, as high Zn concentrations have been shown to inhibit sulfation of desulfo-GS in cress seedlings (Glendering and Poulton 1988). In addition, the availability of amino acid precursors (Cakmak et al. 1989; Domingo et al. 1992) for GS synthesis and the alteration of one of the substrate-specific biosynthetic steps (e.g. aldoxime biosynthesis catalysed by cytochrome P-450, flavoproteins or peroxidases) in the synthesis of GS with different side chains may also have contributed to these results (Larsen 1981; Schnug 1990). The increase in total root GS concentrations partially resulted from an increase in the concentrations of the most abundant GS in T. caerulescens, the aromatic GS sinalbin, accompanied by a decrease in indolyl GS in both roots and shoots; other changes were observed to be quantitatively less important (Tolra et al. 2001; Poschenrieder et al. 2006b). However, the extent to which Zn hyperaccumulation and/or enhanced root sinalbin concentrations contribute to defence against pathogens remains to be established. In contrast, it has already been demonstrated that organic defences (such as GS), rather than Zn, deter snails from eating the shoots of the Zn hyperaccumulator T. caerulescens (Noret et al. 2005).

Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

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