Life is not always easy for a bee: not all plants place their pollen outside. Some flowers have to be set vibrating before the pollen can get out of their tube-like stamens. Certain bee species have developed a special technique for this: they bite on the flower and use their flight muscles to generate rapid vibrations. The stamen is shaken and the pollen is released. This phenomenon is known as “buzz pollination”.

Mechanically, however, what happens is surprisingly complex. Buzz pollination is much more complicated than, say, shaking a salt shaker to release grains of salt. Until now, the role played by the mechanical properties of the flower in this process was not understood. In a collaborative FWF project with botanists from Uni Wien, a team at TU Wien has now investigated how complex-shaped stamens respond to the vibrations generated by bees. What matters is not only how fast and how strongly a flower vibrates, but also how its individual parts move relative to one another.

Different frequencies – different patterns of motion

Depending on the vibration frequency used to excite an object, it can be set into very different kinds of motion. A familiar example is a washing machine: at a certain rotational speed it may become very noisy or even start moving across the floor, while at a slightly higher or lower speed the effect suddenly disappears.

“This can happen, for example, because different parts of an object move together at certain frequencies, but move in opposite directions at others,” says Siyang Zhou from the Institute of Mechatronics and Power Electronics (MPEI) at TU Wien. “Exactly this also happens in the stamens of the tropical plant Medinilla magnifica. When we excite them at different frequencies, they respond in very different ways.”

A Tailored mechatronic setup

Previous biomechanical studies of buzz pollination have often focused on plants of the genus Solanum, which includes the tomato. Their stamens are comparatively straight and stiff and behave mechanically approximately like a rigid body moving up and down.

In Medinilla magnifica, which was investigated in the new study, the situation is considerably more complex. The stamen, which is crucial for pollen release, consists of several parts: the thin stalk, known as the filament, and the curved pollen-bearing structure, the anther, from which the pollen can escape through the small opening at the tip.

To investigate how these stamens move, the team developed an experimental test bench with a custom-built electromechanical shaker. Individual stamens were mounted on it and excited with sinusoidal motion. A laser line sensor then measured the motion at different points. “The laser point sensor conventionally used for such experiments would not be enough for Medinilla magnifica stamens,” says Siyang Zhou. “Our simulations showed that, with this approach, the stamen response above 200 hertz would become difficult to resolve reliably.”

The improved measurement makes it possible to determine not only the amplitude of the vibration, but also the vibration phase of the different parts of the stamen: Do they oscillate in synchrony? Are their motions slightly shifted relative to one another? Or do they even move in exactly opposite directions? The measurement results were then compared with different computational models: a detailed finite-element model and a simplified mechanical multibody model.

Engineering methods enable biological insights

“Our measurements show that the stamen of Medinilla magnifica has three characteristic resonances below about 1,000 hertz,” explains Siyang Zhou. The second of these resonances lies within the frequency range of 100 to 400 hertz typically used by bees to vibrate flowers. The first lies just below this range, while the third lies well above it. But finding resonances is just one part of the story.

Depending on the frequency, different parts of the stamen can perform very different kinds of motion. At certain frequencies, the filament and anther even move in opposite directions. Above the second resonance, the tip of the anther can move in the opposite direction to the rest of the stamen.

“This shows that the flower’s mechanical response is considerably more complex than a simple back-and-forth motion and that the application of mechatronic methods can lead to significant new insights in other scientific fields,” says Ernst Csencsics, project lead at the MPEI. “It suggests that pollen release depends not only on how strong the vibration is, but also on the deformation pattern it produces.”

The results therefore suggest that in buzz pollination, not only the strength and frequency of the vibration may be important, but also the shape of the stamen. Flowers with different structures could translate the bees’ vibrations into internal deformations in different ways – and thereby also influence the release of pollen.

Contact

Siyang Zhou, MSc
Institute of Mechatronics and Power Electronics 
TU Wien
+43 1 58801 369122
siyang.zhou@tuwien.ac.at

Prof. Ernst Csencsics
Institute of Mechatronics and Power Electronics 
TU Wien
+43 1 58801 369124
ernst.csencsics@tuwien.ac.at