Within-carpel and among-carpel competition during seed development, and selection on carpel number, in the apocarpous perennial herb Helleborus foetidus L. (Ranunculaceae)

Apocarpous flowers share opportunities for post-fertilization ovule selection among more functional levels than syncarpous flowers, because the occurrence of a variable number of unfused carpels adds a new source of variation to the likelihood of successful female reproduction. The extent to which post-fertilization events might differ among these unfused carpels may promote variations in the reproductive strategies of plants. We report a study of the variation, within and among carpels and flowers, in seed production and mass in the apocarpous Helleborus foetidus (Ranunculaceae), in relation to the number of carpels per flower. Differences within and among carpels in female reproductive success were affected by carpel number and pollination environment. When analysing whole flowers as functional units we also found that the magnitude of the differences related to carpel number and pollination treatment actually depended on the “distribution” of pollen types within flowers. Thus, variable within-flower pollination environments, more likely to occur in apocarpous than in syncarpous flowers, may affect the strategies of resource allocation for fruit development at different stages of the reproductive process. Regarding seed production, we found that producing more flowers with four carpels was under directional; however, when mean diaspore mass was considered as a measure of fitness, directional selection was found on producing flowers with two and three carpels (the modal carpel number found in wild populations). We discuss ecological and developmental reasons which could explain the observed pattern, and conclude that selection on an optimum carpel number may be very variable across the species range, as the discussed reasons may impose constraints on eventual evolutionary response, thus contributing to the maintenance of the intra-individual variability in carpel number.