The Effects of Heavy Metals on Plants in Industrially Contaminated Ecosystems
Many heavy metals exist naturally in soils as ions, held by interactions with soil colloids. Depending on the relative cation exchange properties of the particular soil, the concentrations of heavy metal ions available for uptake by plants will vary. If the ecosystem is not contaminated with toxic levels of heavy metals, the concentration of heavy metal ions remains at a constantly low level with optimum amounts being available to the plants within the ecosystem (Lepp, 1981). Certain heavy metals, such as copper, zinc and nickel, are essential trace metals for the majority of plants as they are contained in many of plant enzymes. Any productive soil can be damaged by an imbalance of essential mineral elements. Once this sensitive metal ion balance is disrupted, by mining or industrial waste for instance, the whole ecosystem is put in jeopardy: primary producers will experience toxic stress, resulting, ultimately, in reduced plant productivity and even death.
Certain heavy metal ions, namely lead and cadmium are particularly significant because they persist in the food chain and can accumulate amongst the higher trophic levels (Redente et al, 2002).
Obviously, the physiological effects of toxic levels of heavy metal ions varies widely depending on both the plant and the element itself. The quantity of heavy metal ion available to plants in an ecosystem may be elevated in two different ways; one being an input of ions from elsewhere to the top soil, the other being a decrease in soil pH causing adsorbed metal ions to be released from the soil colloid. Both of these mechanism may occur as a direct result of industrial activity and they often occur in conjunction. Chimney stack emissions from many industrial activities cause acid rain which acidifies the soil, thus...