The potential for evolution of heavy metal tolerance in plants

Abstract

A simple method of screening populations for the occurrence of metal tolerance was achieved by sowing seed samples on toxic formed by mixing metalliferous waste with a certain amount of ordinary potting soil: after 3-6 months tolerant individuals can easily be distinguished by their continuing growth whereas non-tolerant individuals will have died. Using this technique it was found that: i) Normal non-tolerant populations of the grass Agrostis tenuis contain a low frequency, 1-2 per cent, of individuals which survive on slightly ameliorated copper soil. Tests showed that some of these individuals have copper tolerances approaching those of fully tolerant populations. ii) The same occurred when populations were screened on zinc soil: but no survivors were found which had full zinc tolerance. iii) When already copper-tolerant seed was sown on zinc soil it behaved rather like non-tolerant seed. The survivors were shown to be both copper and zinc tolerant. This together with experiments on soil containing two metals indicates that tolerances to different metals are independent. iv) In all experiments individuals with a wide range of different tolerances could be found, and the number of survivors varied inversely with the amount of metal in the soil. This indicates that metal tolerance must be a continuously varying character with a threshold. v) It was concluded that high selection pressure acting on large populations can pick out rare gene combinations and change the characteristics of populations in a spectacular manner. Normal non-tolerant populations of the grass Agrostis tenuis contain a low frequency, 1-2 per cent, of individuals which survive on slightly ameliorated copper soil. Tests showed that some of these individuals have copper tolerances approaching those of fully tolerant populations. The same occurred when populations were screened on zinc soil: but no survivors were found which had full zinc tolerance. When already copper-tolerant seed was sown on zinc soil it behaved rather like non-tolerant seed. The survivors were shown to be both copper and zinc tolerant. This together with experiments on soil containing two metals indicates that tolerances to different metals are independent. In all experiments individuals with a wide range of different tolerances could be found, and the number of survivors varied inversely with the amount of metal in the soil. This indicates that metal tolerance must be a continuously varying character with a threshold. It was concluded that high selection pressure acting on large populations can pick out rare gene combinations and change the characteristics of populations in a spectacular manner.