Differentiation for flowering time and phenotypic integration in Arabidopsis thaliana in response to season length and vernalization.

The response of plants or animals to different environmental regimes may take the form of specialization of their life history patterns to match the prevailing conditions in a geographical area. In turn, the evolution of different life histories implies that there are trade-offs between distinct components of the life cycle. We investigate some of the possible explanations for the existence of distinct types of populations in the weed Arabidopsis thaliana (Brassicaceae), differentiated by flowering schedule. The so-called early flowering and late flowering "ecotypes" are hypothesized to result from adaptation to harsh winters or short seasons as opposed to mild winters or long seasons, respectively. We carried out two experiments in which we studied the reaction of natural populations to an increase in season length and to conditions simulating mild winter or spring. Unfortunately, only one of our accessions turned out to be a late flowering population; however, it did have a fitness disadvantage when the season was too short, although it had a higher reproductive output at the end of longer growing seasons. Most populations reacted to the simulation of a mild winter by extending their vegetative phase and increasing their reproductive output; however, this could be offset by increased winter mortality under harsh conditions. Character correlations (phenotypic integration) showed contrasting patterns of change in response to the two environmental factors: at the shortest season's length many correlations were negative, displaying a trade-off between vegetative and reproductive traits; during longer seasons, all correlations were positive and there was no evidence of vegetative-reproductive trade-offs. Exposure to cold did not trigger any major change in the pattern of character correlations.