TOXIC EFFECTS OF HEAVY METALS ON CROP PLANTS

Accumulation of heavy metals in agricultural soils in high concentration can be toxic to crops. Their effects and bioavailability depend on soil p H , organic acids in the soil, presence of other elements and plant species. Seed germination, number of roots, root length and shoot height of rice (Oryza sativa), barley (Hordeum vulgare) and wheat (Triticum aestivum) are affected by various concentrations of copper (Cu) and zinc (Zn). Cu has a higher effect than Zn and the inhibiting effect of Cu on seed germination is more pronounced on rice than on wheat and barley. Heavy metals reduce grain yields and dry weight of wheat plants, with Cadmium (Cd) having the highest effect and Chromium (Cr) the lowest. Plants resist heavy metal effects by producing metal binding proteins, the storage of heavy metals in trichomes of epidermis and increase in anti-oxidation enzyme activities. There is need to reduce human activities generating heavy metals to prevent heavy metal pollution. Heavy metal is a term used to describe more than a dozen elements that are metals or metalloids. Many different definitions have been proposed based on density, atomic number, atomic weight, chemical properties and toxicity. Generally, heavy metals have densities above 5g/cm 3 . Examples of heavy metals include lead (Pb), copper (Cu), nickel (Ni), cadmium (Cd), chromium (Cr) and mercury (Hg). (Hawkes, 1997: Duffus, 2002) Heavy Metal Pollution