Element Toxicity

For macroelements the adequate range is usually very broad. This is different for some micronutrients. Their reactivity is the reason why they can easily become toxic (Fig. 7.2). Thus, the concentration range between deficiency and toxicity is comparatively narrow. Mn availability can become very high in acidic soils, and the resulting Mn toxicity is a secondary problem of low pH besides the (far more important) Al toxicity. A negative redox potential of flooded soil is another reason for high Mn availability and toxicity (Box 7.1). Plants in many arid regions such as southern Australia suffer from boron toxicity.

Similarly, beneficial elements are quite often available in the soil at a concentration that exceeds the toxicity threshold. Unlike silicon, aluminium and sodium are required in only very small quantities for growth stimulation. Many soils and soil conditions exist, however, that are associated with high availability of these elements and their respective toxicities. Survival of plants in salt marshes entails the expression of particular salt tolerance mechanisms. Growth in acidic soils where Al³⁺ becomes available for uptake by plant roots requires specific adaptations to cope with the associated toxicity.

Another relevant aspect to consider is the potential availability of elements that have no biological function and are potentially highly toxic (Clemens 2006). Some are taken up into cells because of their chemical similarity to essential elements. The most important examples are arsenic (As) and Cd. The arsenate anion [(AsO4)^3-] closely resembles the phosphate anion and cannot be discriminated against accurately enough by phosphate uptake systems. Cd is below Zn in the periodic table and therefore shares many chemical characteristics with this essential element. Again, transporters for the essential Zn²⁺ ion are not perfectly selective and transport Cd²⁺ ions as well.

Some plants have evolved the capacity to survive in soils with very high levels of toxic elements that far exceed the concentrations tolerable for most terrestrial plants. In the case of Na they are referred to as halophytes. Vegetation on metal-rich sites is dominated by metallophytes. The adaptations to such extreme habitats represent very instructive models for rapid evolution in action and will therefore be described from this angle.