Effects of floral morphology on plant-pollinator interactions in buzz-pollinated flowers

Abstract

The remarkable diversity of floral forms across angiosperms is largely shaped by interactions between plants and pollinators. Buzz-pollinated flowers have a tight relationship with bees, their primary pollinators, as they require externally produced vibrations for pollen release. These flowers have evolved various morphologies during this relationship, many of them variations on the same theme, yet the functional significance of these forms remains understudied. In this thesis, I examine the effects of different buzz-pollinated morphologies on plant-pollinator interactions. The species I studied vary in stamen arrangement and stamen dimorphism. First, I compared vibration transmission between flowers with contrasting stamen arrangements using artificial vibrations applied to a focal anther. Second, I used cantilever theory and displacement experiments to calculate the natural frequency of two different stamen types in six Solanum taxa, which differ in size and mating system. Third, I assessed bumblebee buzz characteristics on two of these species, to find out whether they match their natural frequency to each flower type. Finally, I asked whether the role of floral orientation differs between radial and bilateral buzz-pollinated flowers, by manipulating their orientations and measuring bumblebee-mediated pollen deposition on the stigmas. I found that the transmission of vibrations varies with stamen arrangement: more uniformly arranged flowers transmit vibrations more faithfully than non-uniform flowers and some arrangements promote the amplification of vibrations. I also found that stamen natural frequency differs between plants with differing mating systems. Bees do not appear to adjust their buzzing frequencies to match these natural frequencies, although they may respond to differences in pollen release. Last, I found that the horizontal orientation of bilateral flowers may function to increase pollen transfer in outcrossing flowers. These findings shed light on key biomechanical and behavioural aspects of buzz pollination and point to potential functions of different floral morphologies.