Abstract
Heavy metals include the transition-metal elements essential to plant nutrition, iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), nickel (Ni) and molybdenum (Mo), cobalt (Co), which is required for nitrogen fixation in legumes, and the non-essential elements, chromium (Cr), cadmium (Cd), mercury (Hg) and lead (Pb). All these elements are toxic to crop plants at high tissue concentrations. In agriculture, deficiencies of essential heavy metal elements are more common than their toxicities. Nevertheless, Mn toxicity can reduce crop yields on acidic soils, and Mn and Fe toxicities occur on waterlogged or flooded soils. Toxicities can also arise in soils enriched in specific heavy metals by the weathering of the underlying rocks or anthropogenic activities. The molecular biology of heavy metal uptake and transport within plants is well understood, and the regulatory cascades enabling heavy metal homeostasis in plant cells and tissues are being elucidated. Cellular responses to excess heavy metals are also known. Many of these responses proceed through the generation of reactive oxygen species and involve the synthesis of antioxidant compounds and enzymes. Tolerance of high concentrations of heavy metals in the environment is brought about by restricting the entry of heavy metals to the root and their movement to the xylem, and by chelating heavy metals entering the cytoplasm and sequestering them in non-vital compartments, such as the apoplast and vacuole. The mechanisms by which certain plant species are able to hyperaccumulate heavy metals are also providing insight into the ability of plants to exclude and tolerate heavy metals in their tissues.