The effect of inflorescence display size and flower position on pollination success in two deceptive and one rewarding orchids.

Inflorescence display size and flower position along inflorescence play an important role in plant reproduction, in the formation of fruits primarily linked to pollinator behavior. In our study, we aimed at determining how visitation rate and choices of pollinators depend on the number and position of flowers along the inflorescence using orchids as target species. We measured reproductive success in (1) natural condition, (2) in hand-pollination experiments and (3) in an experimental design, by modifying the composition of the inflorescences, in populations of two deceptive orchids, Orchis anthropophora and O. italica, and one rewarding orchid, Anacamptis coriophora ssp. fragrans. No differences of natural fruit production among the flower position along the inflorescence (i.e. upper vs lower part) have been observed, suggesting no preference by pollinators for different parts of the inflorescence. Hand pollination experiments highlighted low value of pollen limitation in A. coriophora ssp. fragrans and high values in Orchis italica and O. anthropophora. Reproductive success of deceptive orchids in experimental plots in which flowers on upper half of the inflorescence were removed decreased significantly due to low and small floral display, while reproductive success of the nectariferous species did not showed significant variations. Our data highlighted that the examined orchids showed no clear relationship between fruit formation and flower position along the inflorescences. Thus we can affirm that for orchids the entire inflorescence has a dominant role in the attractiveness of insects and the choice of the insect is not conditioned by the position of the flower. This implies that all flowers have the same possibility of receiving visits from pollinators, and therefore each flower has the same opportunity to ripen into fruit.