Metal Transport in Shoots

The transport of heavy metals in phloem can be complicated because ions can easily be coupled to the phloem of living cells. Cadmium for example can be found in the stipule and in the leaf stalk of pea after the leaves have been treated, but it is not transported further (Greger et al. 1993). Stephan and Scholz (1993) suppose that nicotinamide, which is a metal chelator, influences the content of heavy metals in the phloem. Aquatic plants transport heavy metals in both vessel types. If the osmotic potential around the roots increases, the basipetal transport of zinc and cadmium will also increase; in contrast, the acropetal transport was overbalanced when the leaves were treated with osmotica (Greger 2004).

The plasma membrane acts like a barrier to toxic elements and inhibits uncontrolled uptake into the cell lumen. Metals are taken up in the form of cations and cation transport systems may be used by designated elements. Zinc is transported by specific zinc transporters (Lasat et al. 2000), and copper passes through the membranes via the ATP-dependent copper outflow (Knauer et al. 1997). Light and temperature affect the uptake of cadmium and lead (Hu et al. 1996; Chawla et al. 1991; Hooda and Alloway 1993). An increase in biomass production promotes the uptake of such elements. Accumulation in plants is reduced with dilution, which is affected by growth (Ekvall and Greger 2003).

Costa and Morel (1994) hypothesise that 30% of cadmium is taken up passively by plants; the rest passes through the membrane actively via specific H+-ATPases. It is not yet known whether these ATPases are identical to the MDR-like tonoplast carriers responsible for the sequestration of xenobiotic glutathione conjugates. In the cytoplasm, the metal binds to negatively charged macromolecules, or to parts of bigger cell structures. Biomolecules, for example phytochelatins, can spontaneously form complexes with the cadmium; using specific transporters, these complexes can pass through the tonoplast and reach the vacuole, where the cadmium separates and complexes with organic acids. The free phytochelatin molecules exit the vacuole and again become available to act as binding partners for metals in the cytosol. The cleavage of the phytochelatin-cadmium complexes happens spontaneously under the low-pH regime of the vacuole (Steffens 1990).

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