The effect of speciation on cadmium uptake and resultant phytochelatin synthesis in plants
The pollution of agricultural land with heavy metals occurs as a result of industrial activities such-as metal mining and the application of sewage sludge to the land. Regulations concerning the maximum permissible levels of heavy metals in soils of agricultural use relate only to total concentrations, not to the chemical nature (speciation) of the metal. Bioavailability and toxicity of the metal to the plant are governed by speciation. The overall aim of the research was to investigate heavy metal plant uptake with respect to metal speciation in the growing media. Plants have been shown to possess a unique ability to adapt rapidly and evolve tolerance to toxic and potentially lethal levels of heavy metals. To alleviate the stress imposed by heavy metal exposure, plants synthesize metal-binding polypeptides termed 'Phytochelatins'. Phytochelatin synthesis is thought to act as the detoxification mechanism in plant forms, analogous to metallothionein synthesis in animals. The project involved growing Zea mays plants under carefu.lly controlled conditions - the first stage was to design and build a unit to provide a constant light and temperature source. Plant symptoms were monitored and extraction and storage procedures investigated. Various methods for the isolation and purification of phytochelatins were studied in order to determine the most effective process for their quantification. The equilibrium speciation of the aqueous growing media was computed using a sophisticated computer modelling program, after an extensive thermodynamic database was constructed for the nutrient solution components. The uptake of cadmium, and the resultant Phytochelatin synthesis, has been studied in relation to the equilibrium speciation of the growing media. Cadmium was used as the toxicant, since this has been shown to be the greatest inducer of Phytochelatins. Recently several different purification methods have been documented. Hence a major part of the study has been the development of the optimum procedures for extraction, isolation and separation of Phytochelatins from 'Zea mays' primary roots.