, Volume 92, Issue 2, pp 266-272

Indirect costs of seed production within and between seasons in a gynodioecious species

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Summary

Dimorphic species, especially those with unisexual and hermaphroditic individuals, provide an opportunity to study the dynamics of resource allocation to reproductive function. In this study, I investigated the effects of manipulating seed production on reproductive allocation within a season and between seasons in females and hermaphrodites of gynodioecious Sidalcea oregana ssp. spicata. Plants were either hand-pollinated to ensure maximum seed set (seed plants), or prohibited from pollination and seed set (no-seed plants). These treatments were effective; seed plants produced an average of 120 seeds whereas no-seed plants produced no seeds. Within a season, seed plants and no-seed plants produced similar numbers of flowers, but seed plants significantly reduced the amount of biomass allocated to each flower compared to no-seed plants. The sexual morphs differed in per-flower allocation: hermaphrodites allocated significantly more biomass to most floral structures (except to ovules) relative to females. Nonetheless, the sexual morphs exhibited similar proportional reductions in flower size in response to seed production. When allocation to floral display was characterized by petal length, the sexual morphs responded differently during the season; hermaphrodites, in both seed treatments, maintained larger petals over the season while females greatly reduced theirs. This could be interpreted as adaptive if pollen removal is positively related to petal size. In the season following the manipulations of seed production, plants which produced seeds in 1989 had reduced allocation to influorescences (and flowers) in 1990 compared to plants which did not produce seeds in 1989. A similar trend was not detected for vegetative growth between seasons. The sexual morphs did not differ significantly in their between season responses to the seed treatment.