Factors influencing seed: Ovule ratios and reproductive success in four cleistogamous species: A comparison between two flower types

Ovule and seed numbers, and the ratio between the two, were determined in chasmogamous (CH) and cleistogamous (CL) flowers/fruits in four perennial herbs: Viola hirta, V mirabilis, V. riviniana, and Oxalis acetosella. The results were applied to data from earlier studies on fruit set and progeny performance in these species, to find implications for factors influencing seed: ovule (S:O) ratio, to relate S:O ratio to other fitness components, and to compare final numbers of seedlings per initiated ovule between the two reproductive modes. Mechanisms considered to have a potential impact on S:O ratios were pollen limitation, resource limitation, and "genetic load". In V. mirabilis and O. acetosella, mean S:O ratio was significantly higher in the CL phase than in the CH phase; in V. hirta and V riviniana, there was no difference. Pollen limitation seemed improbable as a factor important for seed set in all species, and no indication was found of resource limitation. There was a negative correlation between S:O ratio and seed germination rate in O. acetosella, indicating that seed abortion, particularly in CH fruits, may be a way to purge inferior zygotes. The high CH S:O ratios in all species suggest that substantial selfing also occurs in this phase. The final number of seedlings per ovule did not differ significantly between modes in any of the species, partly due to poorer germination in CL seeds. This indicates that, despite the often lower pre-emergent reproductive success (fruit and seed set) in the CH phase, total reproductive success will not differ between the two modes because post-emergent selection is stronger on CL progeny. The strongest limitation on pre-emergent reproductive success in both phases is apparently related to flower/fruit abortion.