ABSTRACT
Heavy metals comprise a small part of the earth's crust. Nevertheless, these elements play an important role in plant ecology and affect growth and performance of plant roots even in small quantities as trace elements. Trace elements can be divided into three groups. Some of them are rare and others are more abundant. The presence of both is not essential to plants. The third, smaller group includes those elements that are essential for some or for all organisms. The dose response curves of the effects of essential trace elements on organisms consist of three parts: (1) at very low concentration organisms suffer from deficiency exhibiting characteristic symptoms; (2) at a range of medium concentrations organisms grow normally; and (3) at concentrations above a critical level the elements are toxic (Berry and Wallace, 1981). High concentrations of trace elements in the rhizosphere of higher plants primarily damage the roots. Consequently, they also affect other plant parts. The threshold levels of deficiency and of toxicity differ widely for the various elements (Baker and Walker, 1989). In the nonessential elements, only normal and toxic concentration ranges (2) and (3) are found.
