The three-dimensional arrangement of flowers and sexual function within plant inflorescences is known as inflorescence architecture. Some plants possess an arrangement of male and female flowers on vertical inflorescences, such as racemes, where female(-phase) flowers are arranged below male(-phase) flowers. As pollinators often move upwards while foraging in racemes, Darwin’s syndrome has been suggested to reduce inter-flower self-pollen transport. Reducing inter-flower pollen transport can improve plant fitness by reducing inbreeding and increasing pollen export. Despite these observations, the influences of the inflorescence and directional movement of pollinators have been, until recently, overlooked. In this thesis I investigate the effects of inflorescence architecture on directional foraging and pollen transfer.
Pollen transfer simulation models are used to assess the impacts of differing pollinator movement within plants. Plants where all flowers function bisexually and when flowers function as either males or females are investigated. These simulations reveal that consistent movement should increase outcrossing and pollen export for both inflorescence types, not just those where sexual function is separated over the inflorescence. These advantages were dependent on the consistency of pollinator foraging behaviour. Therefore, selection should favour traits that encourage directional foraging in both inflorescence types. However these pollen transfer advantages were much greater in plants showing Darwin’s syndrome, suggesting selection for arrangements which encourage male flowers to be visited after female flowers.
Observation of wild pollinators revealed five bee species foraged differently on C. angustifolium, in manners that simulations suggested should alter geitonogamy and therefore plant fitness. Furthermore, upwards flight in the two most common visitors to C. angustifolium was observed to be less time consuming then downward movements, providing an energetic explanation for the largely unanswered question of why many pollinators show upward movement in vertical inflorescences. Together, these findings demonstrate the significant role of inflorescence architecture in modulating plant-pollinator interactions
