Factors that Influence Metal Uptake

There are different pathways associated with the entry of dissolved substances into plant cells. The cytosol is a barrier between the vacuole and the outside of the plant cell that offers high resistance to the passage of any solution that includes salts and bases (Nultsch 2001). Plants have a natural tendency to take up metals, and their passage into plant cells will probably be hampered by this barrier. The effectiveness of the metal uptake is highly dependent on the availability, which in turn depends on many factors such as pH and content of organic matter in the soil. Solubilised metal ions enter the root via either extracellular (apoplastic) or intracellular (symplastic) pathways. The apoplast is the extracellular space into which water molecules and dissolved low molecular mass substances will diffuse. On the other hand, the sym-plastic compartment consists of a continuum of cells connected via plasmodesmata.

The apoplast plays an important role in the binding, transport and distribution of ions and in cellular responses to environmental stress, contributing to the total elemental content of the roots.

This space comprises about 10-25% of the capillary space of the rhizodermis and the cortex cell walls. The ions flow with the water taken up by the apoplast into free spaces, where some of them will diffuse and some of them will bind to the carboxyl groups on the cell walls or the negatively charged groups of the proteins. The specifics of this internal dissemination depend on the metal and the plant (Greger 2004). Wierzbicka (1998) reported that most of the lead taken up by Allium cepa remains bound in the apoplast.

The ions can reach the endodermis, which is the beginning of the "internal space", by travelling along this waterway (Nultsch 2001). To get into the xylem, the ions must pass through the endodermis and the Casparian strip. The Casparian strip (Fig. 4.1) is a waterproof lipophilic surface coating in the radial cylinder of the endodermal cells of the root that consists of suberic substances and lignin. Its role is to block the passage of soluble minerals and water from the internal symplast through the cell walls (predominantly the cells in the central cylinder).

Metal uptake generally occurs in young roots without developed Casparian strips (Marschner 1995). It is not clear how metals pass through the older parts of the root.

Casparian strip

Casparian strip

Apoplastic Space
Fig. 4.1 Root uptake of solutes. 1, Free diffusion in the rhizosphere; 2, apoplastic diffusion in apparent free space and Donnan free space of the rhizodermis and parenchyma; 3, transfer to the symplast in the endodermis; 4, transpiration-stream-driven transport to the shoot

Metal uptake from the soil solution is selective and depends on specific or genetic metal ion carriers or channels located in the plasma membrane. It starts with the influx of the individual ions into the "apparent free space" (AFS) (Nultsch 2001).

Most metal ions enter the cells via an energy-dependent saturable process. Carrier systems transport cations into plant root cells. Nonessential heavy metals can compete for the same transmembrane carriers used by essential heavy metals. Heavy metals are transported acropetally to the roots.

Root uptake and transport of organic xenobiotics is determined by the so-called root concentration factor (RCF) (Schröder and Collins 2002). The RCF is heavily dependent on log Ko/w (i.e. the lipophilicity of the compound under consideration), and this seems to be governed by the absorptive properties of the root bark. Compounds with log Ko/w < 1 cannot penetrate the lipid-containing root epidermis, while compounds with log Ko/w > 2 become increasingly retained by the lipid in the root epidermis and the mucilage surrounding the root because of their enhanced hydrophobicities (Schröder and Collins 2002).

Compounds with a log Ko/w of about 2 are only transported in the transpiration stream, while those with a log Ko/w of about 1 are mobile in both phloem and xylem, although these are probably the only metabolites that enter the phloem. For compounds with log Ko/w 1.0-3.5, metabolism may occur in the leaf and stem tissue (Schröder and Collins 2002).

Once they are taken up by the roots, both organic xenobiotics and metals can be stored in underground tissues or exported to the shoot. Transport into the shoot involves loading in the xylem sap and translocation to the aerial parts.

In this case, the Casparian strip is the barrier that limits the entry of both xeno-biotics and metals into the xylem. Dissociated molecules and ions are transferred relatively easy, whereas substances with higher lipophilicities or strong binding capacities are usually retained. Inside the root stele, transfer to xylem vessels follows the laws of accelerated diffusion in the water stream moving towards the plant shoot. Xylem cell walls have a high cation exchange capacity. The metal chelate complexes reduce the interior concentration of metals in the xylem and facilitate metal transfer into the transpiration stream. Organic acids (especially citrate) as well as amino acids are the main metal chelators in the xylem.

Marschner (1995) reported that the metal uptake increases when the pH increases. The opposite happens in soils. He supposed that there is a competition between the hydrogen ions and the metal ions in the root growth area.

Experiments with aquatic plants have shown that increasing the salt content decreases Cd, Cu and Zn uptake because metal-Cl^ complexes are formed (Greger et al. 1995). These types of complexes are not appropriate for plant uptake. The opposite can be expected in sediment systems containing salt; the cadmium uptake in such a system will be significantly higher. In the case of a high salt concentration, an exchange reaction occurs between sodium ions and cadmium ions bound to colloids in the soil colloids, leading to higher cadmium concentrations in the plant (Greger et al. 1995).

Most of the metal enters the plant via the roots after metal-root contact occurs. In the case of diffuse uptake, metals migrate along their concentration gradients together with other ions to the root and across the cortex tissue. Nearby, ion mass flow can occur along the gradient in the water potential, which is held at a high level by transpiration, and this leads to enrichment in the shoot (Marschner 1995).

If the root uptake is high and the concentration of the element in the soil is low, the uptake of the element will be limited by diffusion.

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