Approximately 30,000 species of flowering plants have "poricidal anthers" from which pollen can only be released when bees apply mechanical vibrations to them. In a new study recently published in Nature Communications, a research team at the Department of Botany and Biodiversity Research at the University of Vienna found that the shape of the plant's anther pore determines how pollen is released. The scientists discovered that anthers bearing scoops around their pores expel their pollen in narrower, more targeted jets than anthers lacking such scoops, which normally scatter pollen in broad clouds. Furthermore, scoop-bearing anthers release pollen at faster speeds. The team further learned that scoop-shaped pores are sensitive in their functionality to the exact nature of the vibration applied. Findings such as these are particularly important in light of more frequent temperature extremes altering flower vibrational properties, and declines in bee abundance and diversity.

Bees are the most important pollinators of flowering plants and approximately 10% of flowering plants are functionally specialized on "buzz-pollination". Buzz-pollination is a mechanism where pollen can only be extracted from flowers through the application of mechanical vibrations. Many important crops such as tomatoes, eggplants or blueberries are buzz-pollinated because they conceal their pollen in so-called poricidal anthers – floral structures which only have a minute pore as opening, and pollen can only be released from this pore when mechanical vibrations are applied.

In a new study recently published in Nature Communications, a research team at the Department of Botany and Biodiversity Research at the University of Vienna found that how the pore is shaped determines how pollen is released. Specifically, comparing more than 500 plant species, the researchers found that pores either carry a spathulate structure termed a "scoop", or lack such a scoop. Tomatoes lack such scoops, but anthers of the large tropical plant family Melastomataceae, for example, commonly bear scoops.

Plants bearing scoops at their anthers expel their pollen in narrower, more targeted jets

PhD candidate Benjamin Lazarus from the University of Vienna went ahead to test experimentally whether the "scoop" has a specific function – and found that it does. Lazarus applied mechanical vibrations to single anthers mimicking the vibrations of bees by placing stamens into a specifically constructed artificial vibration setup. He then used high-speed videos to film the pollen clouds expelled from the different anthers and analysed the scattering angle of pollen clouds as well as the velocity with which pollen clouds moved in the air. Lazarus found that anthers bearing scoops expel their pollen in narrower, more targeted jets than anthers lacking such scoop, which normally scatter pollen in broad clouds. Furthermore, scoop-bearing anthers release pollen at faster speeds.

"Our new findings have important implications for the functionality of buzz-pollination", says study lead Agnes Dellinger from the University of Vienna. Releasing pollen in narrow jets may allow plants to target specific areas on the bee's body and place pollen in a more controlled manner. "Accuracy in pollen placement is important for plants because it can improve pollination success and at the same time enables plants to place pollen in 'safe spots' where bees can't groom and collect it off their bodies. At the same time, faster pollen release may allow plants to eject pollen across larger distances and implant it more deeply in the fur of bees, thereby again reducing pollen loss", explains Dellinger.

The functionality of the scoop-shaped pore is dependent on vibration frequency

The team further found that scoop-shaped pores are sensitive in their functionality to the exact nature of the vibration applied – while targeted pollen release showed a cyclical pattern perfectly in line with vibrations at 300 Hz and 400 Hz, pollen release became "messy" and chaotic at low frequency vibrations of around 200 Hz. Finding that the functionality of the scoop-shaped pore is dependent on vibration frequency is important because different bee species vibrate flowers at different frequencies, meaning that some bees may remove higher amounts of pollen from flowers than others.

Furthermore, recent studies from other research teams indicate that vibration frequency may be temperature-dependent, with bees vibrating flowers at higher frequencies when temperatures are high. Finding that how pollen is released from buzz-pollinated flowers is sensitive to vibrational properties is particularly important in light of more frequent temperature extremes and declines in bee abundance and diversity.

Summary:

• A research team from the University of Vienna found that the shape of a plant's anther pore determines how pollen is released.
• The team looked into the mechanism of so called "buzz-pollination". Many important crops such as tomatoes, eggplants or blueberries are buzz-pollinated.
• Plants bearing scoops around their anther pores expel their pollen in narrower, more targeted jets.
• While targeted pollen release showed a cyclical pattern perfectly in line with vibrations at 300 Hz and 400 Hz, pollen release became "messy" and chaotic at low frequency vibrations of around 200 Hz also in scoop-bearing species.
• These new findings are particularly important in light of more frequent temperature extremes potentially altering flower biomechanical properties and declines in bee abundance.

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