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 examined 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 tested 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 orientations of donor and recipient flowers affects the receipt of outcross pollen by S. rostratum. Results: For the three Solanum species studied, there was no effect of floral orientation on total pollen deposition (both self and outcross) when flowers shared 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. Whether other bilaterally symmetrical, buzz-pollinated flowers also benefit from increased cross-pollination when presented horizontally remains to be established.

Flower angle favors pollen export efficiency in the snowdrop Galanthus nivalis (Linnaeus, 1753) but not in the lesser celandine Ficaria verna (Huds, 1762)

“…We recorded enhanced pollen loads on bee’s bodies in upward and downward flowers by counting pollen grains deposited on pollinator bodies, although previous studies examined pollen grains remaining in visited flowers. 9 , 10 , 17–19 We consider these methods complementary, showing that downward flowers are more efficient in transferring pollen grains on pollinator bodies than upward flowers, but further research is necessary to determine whether stigmatic pollen loads 9 , 20 in the snowdrop are influenced by flower angle.…”

Effects of floral symmetry and orientation on the consistency of pollinator entry angle

“…It is likely that horizontal presentation of flowers promoted the evolution of symmetric sexual organs in flowers through the increased accuracy of pollen placement. Studies have shown that both zygomorphy and horizontal/semi-pendant positioning increase the accuracy of pollen placement (Stewart et al 2022 ), and that floral orientation and bilateral symmetry combined promote outcross-pollen transfer (Nevard and Vallejo-Marin 2022 ). While they showed little difference in the effect of zygomorphy and orientation, zygomorphic flowers are capable of reorienting themselves when they are damaged, whereas actinomorphic flowers are not (Armbruster and Muchhala 2020 ).…”