Seed vernalization and gibberellic acid interact to affect life cycle type in facultative winter annual Canadian horseweed ( Erigeron canadensis )

Background and Aims: Plants display enormous variation in the phenological traits that make up their life cycle both within and between populations. Facultative winter annual species are particularly interesting because they can adopt either a fall-emerging/spring-flowering or spring-emerging/summer-flowering life cycle at the population level via evolution or at the individual level via within-generation and transgenerational plasticity. We hypothesized that the growth hormone gibberellic acid (GA) may be a mechanism by which these plants adjust their life cycle in response to cold temperatures. Methods: We conducted growth chamber and greenhouse experiments using the facultative winter annual agricultural weed Erigeron canadensis (horseweed) to investigate the interactive effects of genetic variation; transgenerational responses to parent plant life cycle; and plastic responses to temperature, light, and GA treatments. Key Results: We found that contrary to a prior report, exposing imbibed seeds to 3-4 weeks of cold (i.e., seed vernalization) does not always result in summer annual type growth, with considerable variation found among field-collected seeds from 10 populations. Further, we found that seed vernalization and exogenous application of GA both tended to increase summer annual characteristics, interacting in ways that were largely consistent with the hypothesis that GA is a mechanism for cold-induced life cycle differentiation. Light treatment did not significantly affect life cycle traits, while parent life cycle type had marginal effects on offspring life cycle type. Finally, genetic variation among and within sites explained far less of the variation in life cycle traits than the plastic responses to seed vernalization and GA treatments. Conclusions: Our study proposes that the seasonality of this harmful agricultural weed is influenced by a GA-mediated response to vernalization of seeds during winter, yet highlights the need for further study, given the variability in this response.