Floral development, fruit set, and dispersal of the Gulf of St. Lawrence Aster (Symphyotrichum laurentianum) (Fernald) Nesom

Abstract

The Gulf of St. Lawrence aster, Symphyotrichum laurentianum (Fernald) Nesom is listed as “threatened” according to the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). This rare halophyte is found in only a few locations in New Brunswick, Prince Edward Island, and the Magdalen Islands in Quebec. Developmental evidence confirms that there are two types of florets within each flower head and that each floral type has a biseriate pappus. The centrally located “disk” florets are distinctly larger than the more numerous peripheral “pistillate” florets throughout their development. The disk florets are bisexual and consist of an ovule, a style with a bifid stigma, and four to five stamens. The peripheral florets are pistillate and consist of an ovule and a style with a bifid stigma but no stamens. The main goals of this study were to assess fruit set in both types of florets, and the wind dispersal potential of their fruit (achenes). Pistillate flowers had a lower percentage of embryo-containing (filled) achenes (15.1%) than hermaphroditic florets (27.8%) in plants grown ex situ. The majority (68.3%) of filled achenes were produced by pistillate florets. Heads grown in situ had 64.9% filled achenes. Although the achenes of this plant have structures to aid dispersal by wind, in situ observations and experimental data show that this method of dispersal may be affected by the influence of the surrounding vegetation and flooding events. The impact of other factors that may affect the reproductive biology of this rare plant are discussed.