Genetic Differentiation in Morphological Responses to Simulated Foliage Shade between Populations of Impatiens capensis from Open and Woodland Sites

1. The phytochrome-mediated stem elongation response of many plant species to crowding has been hypothesized to be an example of adaptive plasticity. The photomorphogenic response is cued by the lowered red:far red ratio (R:FR) of light reflected off or transmitted through neighbouring plants. Because stem elongation could potentially be cued in low density stands by the overhead forest canopy, it has been suggested that plants of the forest understorey should be selected to be less responsive to R:FR or to be responsive only in early spring before canopy closure. 2. We tested the hypothesis that populations of a herbaceous annual from open and forest habitats differ genetically in their responses to simulated vegetation shade, which like foliage shade absorbs more red light than far red light, lowering the red:far red ratio (R:FR). For three populations of Impatiens capensis from clearing, evergreen woods and deciduous woods sites, we measured the morphogenic responses to simulated vegetation shade in a common garden experiment. 3. Impatients capensis plants grown in simulated vegetation shade were taller and had more elongated internodes than plants grown in full sun or in neutral shade. Plants from the clearing population were more responsive to simulated vegetation shade than plants from the two wood populations, as predicted. However, the populations did not differ in the timing of their sensitivity to changes in light quality. 4. The patterns of population differentiation in length and in sensitivity to light quality differed among the hypocotyl, first internode and second internode: in particular, the first internode was significantly more sensitive to light quality in the clearing population compared to the other populations. The differences amongst internodes in patterns of population differentiation suggest relatively independent evolution of internode traits.