Toxicity of Heavy Metals in Biological Systems

Before we can describe the toxic effects of heavy metals (given the definition provided in the first part of this chapter), it is necessary to recall two well-known facts. First, a heavy metal is not toxic per se; it is only toxic when its concentration in the plant exceeds a certain threshold ("it is the dose that makes the effect"). This is especially important to the second fact: that some elements, called micronutri-ents, have essential functions in plant cells. This has been shown for Co, Cu, Fe, Mn, Mo, Ni and Zn. Only when the internal concentration exceeds a certain threshold do they demonstrate toxic effects, and then they are commonly termed "heavy metals". As far as we know, all of these plant micronutrients are transition elements. No lead-group elements or rare earth elements have been found to be essential for higher plants. Micronutrients are essential for biosynthesis, growth, nucleic acids, growth substances, chlorophyll and secondary metabolites, carbohydrates and lipids, as well as for stress resistance. A supply of micronutrients is also essential for the integrity of membranes (Rengel 2004). The dose-response curves for essential heavy metals have been described by Berry and Wallace (1981), and show deficiency at suboptimal concentrations, tolerance at optimal concentrations (including the potential of the plant to maintain homeostasis) and toxicity at high concentrations (cf. Hagemeyer 2004). There is another not so well-known fact to be considered too. Some of the nonessential heavy metals have a stimulating or inducing effect when they are applied at very low concentrations (these are termed "low concentration stressors"). As an example, Cd produces some stimulating effects at 5 x 10-8 M in barley seedlings, as do Pb and Ti at low levels in detached barley leaves (Kovacs et al. 2009; Nyitrai et al. 2007).

However, let us now consider the toxic effects of heavy metals. Remember that this means the toxic effects of transition element ions, rare-earth (that are not only rare in the environment but are also rarely investigated) element ions and lead-group element ions on plants.

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