Chapter 6 – Chromium and the Plant: A Dangerous Affair?

Contamination of aquatic and terrestrial environments by toxic chromium (Cr) species is a worldwide increasing problem. Cr occurs in the environment mainly as Cr(III) and Cr(VI) species. Cr is an element whose toxic effects depend on its oxidation state. Cr(VI) is highly mobile and toxic to animals and plants. Conversely, Cr(III) is less mobile and less toxic. Cr does not accumulate in plants naturally, but can be absorbed easily from soils and waters. To face the challenge of adverse effects of Cr species in polluted environments, plants must use different mechanisms either to avoid the entry of toxic Cr species or to block their intracellular toxicity. Although it is always claimed that Cr(VI) is more toxic than Cr(III), when the Cr(VI) enters into cells, it is previously reduced to Cr(III) before its toxic effects occur. From this fact could arise an intriguing question on the unquestioned current dogma about the accumulation of toxic Cr(VI) by plants: that is to say, the conversion of the Cr(VI) form to the “less toxic” Cr(III) form is a key tolerance mechanism. In this chapter, we analyze relationships between Cr(VI), Cr(III), and plants which underlying as “a dangerous affair.”