by Dieterich Mabin et al.
Bumble bee on alfalfa with some tripped flowers
Insect pollinators move pollen between flowers and plants, and therefore affect pollen transfer, pollen movement, and pollen dispersal. Two important components of pollen transfer dynamics include the accumulation of pollen on a bee’s body as a bee visits flowers in succession, and the number of pollen grains deposited on the stigma as a bee revisits a flower. These characteristics have typically been described in plants without a tripping mechanism. However, the flowers of many plant species in the families Labiatae and Fabaceae have closed flowers that require a pollinator or other force to exert pressure on the keel of a flower in order to release the anthers and stigmas. In this study, we investigate the two fundamental aspects of pollen transfer dynamics mentioned above for a bumble bee species visiting flowers with a tripping mechanism. The number of pollen grains on a bee’s body increased with the number of flowers tripped by a bee. In fact, for each flower tripped in a foraging bout, the number of pollen grains on a bee’s body increased by an average of 954 grains. However, revisiting tripped flowers did not increase the number of pollen grains deposited on a stigma. This contrasts with flowers that do not have a tripping mechanism where the number of pollen grains deposited on a stigma increases with the number of pollinator visits to a flower. Moreover, plants with a tripping mechanism can be categorized into two groups, one where the flowers remain open after tripping such as the plant species used in this study, Medicago sativa, commonly known as alfalfa or lucerne, and the other where the flowers close after tripping as occurs in clover or Trifolium species. We hypothesize that staggered anther dehiscence, where pollen is released over time from a flower, would be beneficial in plant species whose flowers close after tripping. Moreover, revisits to a flower could increase pollen deposition on stigmas in flowers that close after tripping especially if staggered anther dehiscence is present. Tripping of a flower affects pollen transfer dynamics, and the effect may vary with the mode of tripping.