Floral orientation affects outcross pollen deposition in buzz-pollinated flowers with bilateral symmetry

Abstract

Premise: Floral orientation is central to plant-pollinator interactions and is commonly associated with floral symmetry. Bilaterally symmetrical flowers are often oriented horizontally for optimal pollinator positioning and pollen transfer efficiency, while the orientation of radially symmetrical flowers is variable. Buzz-pollinated species (pollinated by vibration-producing bees) include bilateral, horizontally oriented flowers, and radial, pendant flowers. The effect of floral orientation on pollen transfer has never been tested in buzz-pollinated species. Methods: Here, we examine the effect of floral orientation on bumblebee-mediated pollen deposition in three buzz-pollinated Solanum species with different floral symmetry and natural orientations: S. lycopersicum and S. seaforthianum (radial, pendant), and S. rostratum (bilateral, horizontal). We test whether orientation affects total stigmatic pollen deposition (both self and outcross pollen) when all flowers have the same orientation (either pendant or horizontal). In a second experiment, we evaluated whether different orientation in donor and recipient flowers affects outcross pollen receipt in S. rostratum. Results: For the three Solanum species studied, there is no effect of floral orientation on total pollen deposition (both self and outcross) when flowers share the same orientation. In contrast, in our experiment with S. rostratum, we found that pendant flowers received fewer outcross pollen grains when paired with pendant donors. Conclusions: We suggest that floral orientation influences the quality of pollen transferred, with more outcross pollen transferred to horizontally oriented recipients in the bilaterally symmetrical S. rostratum. It remains to be established whether other bilaterally symmetrical, buzz-pollinated flowers also benefit from increased cross-pollination when presented horizontally.