Plasticity is commonly viewed as an adaptive mechanism enabling organisms to express a phenotype suited to local conditions. However, the physical environment is complex, and adaptive plasticity to one selective agent may counter responses to a second selective agent. In a recent study with Impatiens capensis, we tested whether plasticity of stem elongation to ultra-violet radiation was adaptive and whether this response affected plasticity to competitive cues. Plants possess photoreceptors that enable them to detect many aspects of the ambient light environment, such as levels of ultra-violet, red, and blue light. In some cases, shifts in light quality are reliably associated with changes in the physical environment. For instance, the ratio of red:far-red light is reliably correlated with neighbor proximity, because chlorophyll selectively absorbs red light while transmitting far-red. The phytochrome photoreceptors, which switch photoreversibly between red- and far-red-absorbing forms, enable plants to detect R:FR cues of neighbor proximity. Perception of low R:FR stimulates increases in stem elongation, which enables plants to overtop their neighbors and achieve higher fitness. This response also increases exposure to ultra-violet radiation, which is known to decrease elongation in other species. We found that selection favored decreased elongation in high UV environments, but increased elongation in non-UV environments, demonstrating that plasticity of stem elongation is explicitly adaptive. The physiological basis for this selection is unclear, however, elongation responses in the natural environment may reflect a compromise between selection imposed by UV and competition.

Key words: Impatiens capensis , antagonistic selection, photomorphogenesis, shade-avoidance responses, stem-elongation responses, ultra-violet radiation