Physiology and morphology of Mimulus guttatus vary with water level. Under flooded conditions, root to shoot ratio is significantly higher and photosynthetic rate and conductance are significantly lower than at field capacity. Is this phenotypic plasticity adaptive? Plants were grown from seed in the greenhouse under two water levels. At 5, 7 and 10 weeks from emergence, a subset of the plants was switched into the other environment (high water into low water and vice versa). The third switch resulted in a crossover in fitness between the switched and control plants: within each environment, the plants whose early environmental cues corresponded to their ultimate environment had a higher fitness than those responding to “incorrect” early cues. This provides evidence that plasticity in response to water level is adaptive in this species. The time courses of root to shoot ratio, leaf area ratio, specific leaf area, reproductive allocation, maximum photosynthetic rate and instantaneous water use efficiency were followed, and fitness responses are interpreted in terms of changes in these traits over time.

Key words: adaptive phenotypic plasticity, flood tolerance, Mimulus guttatus