Pollen is an important food for bees, but how much remains inside a flower is not necessarily visible from outside. Some flowers store pollen inside tube-like stamens and release it through small pores. Can a bee tell by scent whether pollen remains?
In 2024, C. Douglas Moore and colleagues compared scent compounds from flowers with pollen removed by vibration and flowers retaining pollen in seven closely related Solanum taxa. They aimed to investigate whether the presence of hidden pollen is reflected in scent and could provide useful information to visiting insects.
- Flowers that release pollen through vibration
- Comparing different individuals of the same type simultaneously
- A clear change in only one of seven taxa
- Large decreases in linalool and farnesol
- Was the pollen itself emitting scent?
- Vibration effects have not yet been separated
- Is this information bees can use?
- Summary
- References
Flowers that release pollen through vibration
The Solanum flowers studied have no nectar; pollen is the reward for bees. Their pollen-containing anthers are tubular, with small pores at the tips. Bees capable of producing vibrations shake their bodies to release pollen through these openings.
In this system, losing pollen does not greatly change the anthers’ external appearance. If flowers with abundant versus depleted pollen smell different, bees might decide which to visit before vibrating them. The researchers sought to test this correspondence between scent and reward.
Comparing different individuals of the same type simultaneously
The plants belonged to the closely related Solanum group Androceras. Because two subspecies of one species were treated separately, the paper counts seven taxa. Plants were grown in a UK greenhouse under conditions excluding insect visits.
Two plants of the same taxon and origin were selected and randomly assigned to pollen removal or an untreated condition. For removal, an electric toothbrush vibrated the anthers for at least 30 seconds until no more pollen emerged. The stigma was avoided to minimize changes caused by pollination itself.
Inflorescences were then enclosed in bags, and collection of emitted volatiles began within 20 minutes of treatment and continued for six hours. Leaves were excluded, but stems and unopened buds were present. The experiment compared two conditions on different individuals simultaneously, not the same flowers before and after treatment.
Four individuals per condition were studied for each taxon, except Solanum lumholtzianum, for which insufficient flowering plants were available and three individuals per condition were used. Each plant was used once, and records from 54 plants in total were analyzed. Compound amounts were normalized to flower number and sampling time, as emissions per flower per minute.
A clear change in only one of seven taxa
The total of the five volatile compounds compared differed greatly among taxa. S. lumholtzianum had particularly high emissions, and these were significantly lower after pollen removal.
For this species, mean emissions were about 768 nanograms per flower per minute with pollen retained, versus about 67 with pollen removed. A nanogram is one-billionth of a gram. The two conditions thus showed a large difference.
No significant changes associated with pollen removal were found in the other six taxa. However, these were small comparisons, with three or four individuals per condition. Rather than proving that no change ever occurs, the findings mean that this method and sample size did not detect a clear change.
Large decreases in linalool and farnesol
Linalool and farnesol were particularly prominent in the S. lumholtzianum response. Linalool was detected at about 498 nanograms per flower per minute with pollen retained but was not detected after removal. Farnesol also decreased from about 171 to about five nanograms.
Small amounts of linalool were detected only in pollen-present conditions in two other taxa as well, but the differences there were not statistically significant. The authors suggest a possible shared mechanism among related plants while emphasizing the need for further testing.
Linalool has forms with different three-dimensional structures, and farnesol also has multiple forms. The methods did not distinguish these. The study therefore does not determine which forms affect insects in which ways.
Was the pollen itself emitting scent?
One possible explanation for reduced scent is that the removed pollen itself emitted the compounds. To examine this, the researchers pooled pollen from 40 flowers for each of three species, extracted it with solvent, and analyzed the extracts.
No volatile compounds were detected in pollen by that method. This does not establish that pollen has no scent at all. The amount collected was small, and solvent extraction differs from collecting air emitted by flowers. The authors also note differences in sampling methods and detection limits.
Another possibility is that flowers sensed pollen removal and changed emissions from other tissues, such as the anthers. Pollen is both an insect reward and essential to plant reproduction; its removal might be linked to the flower’s physiological state.
Vibration effects have not yet been separated
Because pollen was removed using vibration, the effects of pollen depletion cannot be fully separated from those of vibration or handling. There was no condition applying vibration without removing pollen.
The authors propose comparing removal by actual bees, removal by another method less likely to cause damage, and vibration without pollen release. Understanding why scent changes requires identifying both the source of compounds and the flower’s response.
Is this information bees can use?
If scent corresponds to pollen availability, it could help insects locate rewards. In ecology, honest information does not mean that a flower has an honest personality; it means that a cue corresponds to the state of a reward.
The linalool and farnesol differences in S. lumholtzianum are candidates for such cues. The study did not directly test whether bees can smell these differences, learn them, or use them to choose flowers. Connecting a chemical difference with useful information for insects is the next step.
Because most taxa studied showed no clear change, the authors discuss the possibility that scent-based information on remaining pollen is uncommon in buzz-pollinated flowers. If scent does not change, bees may find it difficult to determine pollen availability until they actually vibrate a flower.
Such a system might maintain flower visitation and increase opportunities for pollen transfer. This experiment did not, however, demonstrate increased reproductive success or intentional deception by plants. Studying other plant groups beyond these seven closely related taxa remains important.
The research received support from the UK Natural Environment Research Council’s IAPETUS doctoral training program. The authors declared no commercial or financial relationships that could represent a conflict of interest.
Summary
Following pollen removal by vibration, total volatile emissions, linalool, and farnesol clearly decreased in S. lumholtzianum, one of the seven Solanum taxa. Comparable significant changes were not found in the other six.
The study indicates that floral scent may do more than attract insects: it could also convey information about hidden rewards. Yet that correspondence need not occur in every flower. Linking the mechanisms that change scent with how insects use it is the next challenge in understanding flower–bee interactions.
References
Moore, C. D., Farman, D. I., Särkinen, T., Stevenson, P. C., & Vallejo-Marín, M. (2024). Floral scent changes in response to pollen removal are rare in buzz-pollinated Solanum. Planta, 260, 15. DOI: 10.1007/s00425-024-04403-4. Full text. © Crown 2024. The original article is published under CC BY 4.0.
