WEINIG, CYNTHIA* and JOHANNA SCHMITT. Dept. of Ecology and Evolutionary Biology Box G-W, Brown University, Providence RI 02912. - Antagonistic selection in complex environments: adaptive plasticity to UV vs. competition.
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