Can an increased copper requirement in copper-tolerant Mimulus guttatus explain the cost of tolerance? - II. Reproductive phase

Circumstantial evidence suggests that plants that have evolved metal tolera nce 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 grea ter requirement for copper, and so suffer micronutrient deficiency on norma l soils, as a result of a reduced uptake, distribution and/or utilization o f copper. We provided highly and less copper-tolerant plants of Mimulus gut tatus Fischer ex DC. (the common monkey flower) with sub-optimal copper, an d demonstrated the importance of copper as an essential micronutrient durin g the reproductive phase, both in the production of viable pollen and in se ed set. We also looked at the effect of sub-optimal copper supply on the gr owth of the microgametophyte, and the efficiency with which seed was set. N o evidence was found that highly tolerant plants have an increased copper r equirement during the reproductive phase. This is in agreement with earlier work on Mimulus guttatus, which investigated the copper requirement of hig hly tolerant plants during vegetative growth and found that any differences in copper requirement were small. The 'metal requirement hypothesis' is, t herefore, not the sole explanation for the cost of copper tolerance in M. g uttatus.