Reproductive challenges related to a complex sexual system in Euphorbia rosescens, a rare endemic herb

Abstract

When the sexual system of a rare plant species is complex, characterizing floral development, plant morphology, and subpopulation structure is essential for assessing reproductive potential. Availability of pollinator rewards and positioning of sexual morphs within inflorescences relates to outcrossing potential. We studied the complex sexual system of the recently discovered Euphorbia rosescens E.L. Bridges & Orzell, a rare species occurring in a single Florida county. Euphorbia rosescens has both monomorphic subpopulations of female plants and mixed subpopulations with male and andromonoecious plants. Most data on cyathia sex, inflorescence architecture, floral visitors, and fruit set came from one female subpopulation and one mixed subpopulation. Cyathia longevity, flowering phase, nectar secretion, pollen production, and viability were measured in female, male, and hermaphrodite cyathia. Female plants had fewer inflorescence levels and cyathia and significantly smaller cyathia than male and andromonoecious plants. Andromonoecious plants were larger and produced more cyathia relative to male plants. Within mixed populations, sex switching occurred in both directions between male and andromonoecious plants. Female plants did not switch sexes. Proximal to distal inflorescence levels transitioned from hermaphrodite to male cyathia on andromonoecious plants. Nectar secretion corresponded to initiation of staminate flowering, but in female cyathia stigmas were receptive approximately a day before nectar secretion. Pollen production and viability were similar in male and hermaphrodite cyathia. Fruit maturation was minimal, primarily observed on andromonoecious plants. Nectar availability, pollen rewards, trimorphic inequalities in inflorescence display, and low fruit set suggest high geitonogamy, thus limiting outcrossing potential for this species.