Metal tolerance in plants

Nearly 60 years ago, Prat (1934) initiated the research of heavy metal resistance in plants when he was analysing the growth performance of two populations of Silene dioica (Melandrium sylvestre), one from a copper mine and one from a non-mine soil. He was able to demonstrate a heritable copper resistance in the mine population, relative to the non-mine population, which he explained as a result of evolution by natural selection. Nearly 20 years later Bradshaw (1952) and Baumeister (1954) started further research on ecological and physiological differentiation between plants from metal-enriched and noncontaminated habitats. The species chosen for study were predominantly Agrostis capillaris in the Bradshaw group (Jowett 1959; Gregory 1965; McNeilly 1965; Antonovics 1966) and Silene vulgaris in the Baumeister group (Broker 1962; Ernst 1964; Gries 1965; Riither 1966). In the late 1950s Duvigneaud (1958), while studying the vegetation on metalliferous soils in Central Africa, added to the above approaches a phytogeographic one and introduced the study of speciation processes in metallophytes. In the 1950s, the study of evolutionary and physiological aspects of metal resistance was hampered by the absence of convenient techniques for measuring metal concentrations in small plant samples. The techniques available for metal analysis were either timeconsuming, such as phase separation (Ernst 1964), or costly and only applicable for laboratory-raised plant material, i.e. radiolabelling (Turner & Gregory 1967; Peterson 1969). Only after applying atomic absorption spectrophotometry on wet-ashed plant material (Reilly 1967) did time and cost-effective metal analyses become possible.