A Detailed Study of Achene Composition, Germination Timing, and Competition in Heterocarpic Grindelia ciliata

Abstract

Heterocarpy, the production of multiple fruit morphs by a single plant, is a reproductive bet-hedging strategy that reduces the risks associated with environmental unpredictability by producing offspring with contrasting dispersal, dormancy, and establishment traits. Grindelia ciliata (Asteraceae), the focal species of this thesis, produces three distinct diaspore morphs: a disc morph, an intermediate morph, and a ray morph (Figure 1). The ecological function of the intermediate diaspore has been largely unexplored. This thesis investigates the differences among diaspore morphs in biomass allocation, dormancy, germination behavior, and competitive performance, while also evaluating population-level variation among three Oklahoma populations from three of Oklahoma’s 42 separate minor ecoregions (Woods, A.J., 2005). Differences in traits between populations are being compared to determine if diaspore level effects are conserved or are variable among populations. Specifically, it examines variation in diaspore and seed mass among morphs and populations, pericarp structure and scarification effects on dormancy and germination, variation in germination percentage and T50 (time to 50% germination) under different storage and environmental conditions, and differences in seedling growth and competitive performance among morphs. Disc diaspores allocated significantly more biomass to seeds and less to protective tissues than either intermediate or ray diaspores. Consequently, disc diaspores exhibited the highest germination percentages, germinated the fastest, and produced the most competitive seedlings. Intermediate and ray diaspores invested proportionally more biomass in lignified pericarps and exhibited strong physical dormancy that was substantially reduced by scarification. Although both ray and intermediate morphs germinated more slowly than disc diaspores, intermediate diaspores consistently germinated before ray diaspores, reached T50 sooner, and achieved higher germination percentages under most treatment conditions, demonstrating that they occupy a distinct germination niche. Contrary to expectations, storage treatments did not support the formation of a persistent seed bank, suggesting instead that the intermediate and ray morphs contribute to a transient seed bank. Population-level differences in biomass allocation and germination were detected but were generally weaker than differences among diaspore morphs. Greenhouse competition experiments further demonstrated that seedlings derived from disc diaspores survived longer and accumulated greater biomass than those derived from intermediate or ray diaspores, although all three morphs performed poorly under interspecific competition. Collectively, these findings demonstrate that the three diaspore morphs represent distinct functional strategies that partition reproductive risk through differences in biomass allocation, dormancy, germination timing, and seedling performance. Furthermore, the intermediate morph is neither a developmental transition nor an evolutionary relic, but rather a distinct functional morph that expands the temporal dimension of the bet-hedging strategy in Grindelia ciliata.