The variation in degree of copper tolerance, its heritability and relationship with copper contamination of the soil and geographical location of sample, was studied on samples of seed collected from a large abandoned copper mine at Copperopolis, California. The realized heritability of variation in degree of tolerance at a high level of copper, 2 1ug cm , is shown to be about 0.40. The F1s of crosses between an upward and downward selection line and a non-tolerant line give intermediate values for tolerance. All seed samples taken from contaminated soils, and a number from populations downstream of the mine, have 100 per cent tolerant plants. Samples taken from upstream of the mine, and many populations in the vicinity, have a proportion of tolerant plants. The degree of tolerance of the samples is more variable, with rather steep dines for this character within the mine. The variation in degree of tolerance is related more to the location of the sample than to the absolute amount of copper contamination of the soil. Populations along the stream running through the centre of the mine wastes are more tolerant than populations taken from the edge of the wastes, which are more tolerant than populations collected from uncontaminated soils near the mine. The difference in steepness of the dines for the two characters, tolerance and degree of tolerance, suggests that selection against tolerance (the 'cost' of tolerance) acts more on degree of tolerance than on tolerance per se. These results are discussed in relation to the concept of geno stasis.

SUMMARYIf metal tolerant plants, by virtue of their tolerance mechanism, are less efficient at the uptake, distribution or utilization of metals then essential micronutrient deficiency may occur at the low levels of metal supply found on non-mine soils. This argument forms the basis of the metal requirement hypothesis put forward to explain the lower fitness of tolerant individuals on uncontaminated soil, the so called 'cost of tolerance'. In this paper, copper balance was investigated in Mimulus guttatus Fischer ex. DC (the yellow monkey fiower) for plants with or without the major tolerance gene which confers primary tolerance, and plants with few or many modifier genes which control degree of tolerance. No conclusive evidence to support an increased copper requirement in plants with the major tolerance gene, and/or many modifier genes was shown. Any differences in copper requirement found during vegetative growth were small, and were deemed insufficient to explain the apparent cost of tolerance.

No abstract

Circumstantial evidence suggests that plants that have evolved metal tolerance are at a disadvantage on normal soil, i.e. there is a cost of tolerance. One hypothesis for the cause of this cost is that individuals have a greater requirement for copper, and so suffer micronutrient deficiency on normal soils, as a result of a reduced uptake, distribution and\or utilization of copper. We provided highly and less copper-tolerant plants of Mimulus guttatus Fischer ex DC. (the common monkey flower) with sub-optimal copper, and demonstrated the importance of copper as an essential micronutrient during the reproductive phase, both in the production of viable pollen and in seed set. We also looked at the effect of sub-optimal copper supply on the growth of the microgametophyte, and the efficiency with which seed was set. No evidence was found that highly tolerant plants have an increased copper requirement during the reproductive phase. This is in agreement with earlier work on Mimulus guttatus, which investigated the copper requirement of highly tolerant plants during vegetative growth and found that any differences in copper requirement were small. The ' metal requirement hypothesis ' is, therefore, not the sole explanation for the cost of copper tolerance in M. guttatus.

No abstract