Plants and Heavy Metals

Abstract

Heavy metals are chemical elements with a specific gravity greater than 5.0. Among the 90 naturally occurring elements, 21 are non-metals, 16 are light metals, and the remaining 53 are (with As included) heavy metals. The definition thus includes the majority of naturally occurring elements, which from a biological perspective is not very helpful. However, only a limited number of heavy metals are soluble under physiological conditions and thus bioavailable to living organ- isms. Some of these are considered essential for life, including Fe, Co, Mo, Mn, Zn, V, Ni, Cr, Cu, and W. They are required as micronutrients or trace elements because they often act as cofactors in biochemical reactions, but they are toxic when present in excess. Other heavy metals such as Cd, Hg, Ag, Pb, and U have no known biological function and are toxic even at very low concentrations. Heavy metals are often present as natural components of ultramafic or calamine soils, but the prevalence of heavy metals in the environment has increased more recently as a result of human activity. Metal processing facilities, mines, refuse dumps, sewage sludge, and traffic are all sources of heavy metals. In addition, the intense use of phosphate fertilizers and municipal sewage sludge in agriculture contributes to the accumulation of heavy metals in soils. The increasing concen- trations are potentially toxic to both animal and plant life. Metal mobility in the soil is strongly influenced by root exudates and microbes in the rhizosphere. Mobilized metals bind to root cell walls and are then taken across the plasma membrane by transport systems. Diverse families of metal transporters are induced under metal deficiency conditions, indicating their involvement in the regulation of metal uptake, transport, and distribution in the aerial parts of the plant. For most plants, both essential and non-essential heavy metals cause toxicity symptoms and growth inhibition when present in excess. Heavy metals induce oxidative stress by generating free radicals and reactive oxygen species; they displace essential ions from proteins and other molecules; and then bind strongly to oxygen, nitrogen and sulfur groups, and hence inactivate enzymes by binding to cysteine residues.