Ecotypic divergence in Crepis tectorum (Asteraceae): inferring trait lability and correlational constraints from hormonally manipulated phenotypes

Despite long-standing interest in the evolutionary role of plant hormones, relatively few studies have used hormonally manipulated phenotypes to address questions about phenotypic evolution in plants. In the present investigation, I subjected plants of Crepis tectorum subsp. pumila to simple gibberellin (GA) treatments under both field and greenhouse conditions to assess developmental lability and correlational constraints of phenotypic traits that distinguish this dwarf ecotype from conspecific populations of the much taller weed ecotype (subsp. tectorum). The hormonally manipulated plants largely phenocopied the weed ecotype in leaf shape, plant stature and branching habit, indicative of both high lability and tight integration of traits reflecting gross morphology. Floral size traits sometimes declined after GA application, especially under field conditions. The latter result conflicts with the positive correlations between floral and vegetative size traits seen in previous comparative analyses and point to a tradeoff that could act as a constraint on ecotype divergence. The response to GA was consistent in direction for most traits, as opposed to the magnitude of response, which varied depending on the trait, the population, the growing environment and the timing and level of hormone application. Taken together, the results highlight the potential for simple hormonal changes to cause large, plastic shifts in phenotype, but also illustrate the constrained nature of such influences.