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Flower orientation enhances pollen transfer in bilaterally symmetrical flowers

Abstract

Zygomorphic flowers are usually more complex than actinomorphic flowers and are more likely to be visited by specialized pollinators. Complex zygomorphic flowers tend to be oriented horizontally. It is hypothesized that a horizontal flower orientation ensures effective pollen transfer by facilitating pollinator recognition (the recognition-facilitation hypothesis) and/or pollinator landing (the landing-control hypothesis). To examine these two hypotheses, we altered the angle of Commelina communis flowers and examined the efficiency of pollen transfer, as well as the behavior of their visitors. We exposed unmanipulated (horizontal-), upward-, and downward-facing flowers to syrphid flies (mostly Episyrphus balteatus), which are natural visitors to C. communis. The frequency of pollinator approaches and landings, as well as the amount of pollen deposited by E. balteatus, decreased for the downward-facing flowers, supporting both hypotheses. The upward-facing flowers received the same numbers of approaches and landings as the unmanipulated flowers, but experienced more illegitimate landings. In addition, the visitors failed to touch the stigmas or anthers on the upward-facing flowers, leading to reduced pollen export and receipt, and supporting the landing-control hypothesis. Collectively, our data suggested that the horizontal orientation of zygomorphic flowers enhances pollen transfer by both facilitating pollinator recognition and controlling pollinator landing position. These findings suggest that zygomorphic flowers which deviate from a horizontal orientation may have lower fitness because of decreased pollen transfer.

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Acknowledgments

We thank Christina Caruso, Paul Wilson and an anonymous reviewer for giving us many valuable comments and suggestions during the review process and Daiju Kawase for his assistance in pollen counting. This research was partly supported by a grant-in-aid for young scientists (No. 17770023) from the Japan society for the promotion of science. Our experiment complies with the laws of Japan, the country in which it was performed.

Author information

Correspondence to Atushi Ushimaru.

Additional information

Communicated by Christina Caruso.

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Ushimaru, A., Dohzono, I., Takami, Y. et al. Flower orientation enhances pollen transfer in bilaterally symmetrical flowers. Oecologia 160, 667–674 (2009). https://doi.org/10.1007/s00442-009-1334-9

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Keywords

  • Commelina communis
  • Pollinator specialization
  • Horizontal
  • Flower orientation
  • Zygomorphic flower