Influence of sex-organ positions on pollen transfer and self-interference in plants with stylar polymorphisms: An experimental approach using three-dimensional printed flowers

Heterostylous plants are characterized by the reciprocal placement of
stigmas and anthers in the floral morphs. Darwin proposed that this
arrangement increases the proficiency of disassortative (intermorph)
pollen transfer. The floral polymorphism may also function to avoid gamete
wastage by reducing interference between sex organs. Distylous populations
consist of two morphs: the long-styled morph with stigmas above anthers
and the short-styled morph with stigmas below anthers. A related floral
polymorphism – stigma-height dimorphism – involves floral morphs that
differ in stigma-height but not anther position. We used three-dimensional
printed artificial flowers, with attached styles and anthers from
glasshouse-grown Petunia grandiflora, to investigate whether sex-organ
positioning simulating distyly and two forms of stigma-height dimorphism
influence patterns of pollen transfer. Artificial flowers were arranged in
flight cages and pollen transfer within and among flowers was mediated by
captive bumble bees (Bombus impatiens) from commercial colonies. At the
end of each trial, we quantified pollen grain number on stigmas and styles
and the amount remaining in anthers of pollen donors.  We found
partial support for Darwin’s hypothesis on the probability of pollen
reaching stigmas of the opposite morph, especially in the S-morph of
distylous arrays. The number of pollen grains deposited on stigmas was
influenced by the amount of time bumble bees spent foraging on flowers,
which was significantly longer in the long-styled morph resulting in
higher pollen loads in comparison with the short-styled morph.
Self-interference in the form of self-pollen deposition on stigmas of
donor flowers depended on the spatial separation of sexual organs. As
predicted, we found higher self-pollen deposition in stigma-height
dimorphic than distylous arrays.  Although our results do not
provide unequivocal support for Darwin’s pollen-transfer hypothesis, they
confirmed that the morphological arrangement of sexual organs within
flowers influenced pollen transfer patterns and levels of
self-interference. They also indicated that variation in sex-organ
deployment affects bumble bee foraging times with consequences for amounts
of pollen deposited on stigmas. Although preliminary, our results
highlight that three-dimensional printed flowers provide future
opportunities to investigate the functional significance of variation in
floral design in the context of plant-pollinator interactions.