Biomechanics and Bee Pollination

Biomechanical interactions between bees and flowers have been known to be involved in pollen transfer in plant species for more than two centuries [e.g., 1-6]. Bees are very important pollinators for many plant species. However, because bees feed their offspring nearly exclusively with pollen, plants have evolved several mechanisms to avoid overexploitation by pollen-collecting bees and to ensure pollen transfer [7-11]. Many of these mechanisms involve specific mechanical features. Four of the most important are buzz pollination and the piston, brush, and lever mechanisms.

Buzz pollination is well-known in many economically important members of the nightshade family (Solanaceae) [12]. Examples include tomato, Solanum lycopersicum; pepper, Capsicum annuum; and eggplant, S. melongena. However, buzz pollination is much more widespread and known to occur in at least 65 plant families [13]. The thecae of many of these plants dehisce only partially, and they only open small pores through which pollen can be released (poricidal anthers). To collect pollen from these plants, bees place their body near the small openings and vibrate the stamen by rapid contraction of their indirect flight muscles. Buchmann and Hurley [13] model the process of pollen release. The bee's buzzing and the consequential vibration of the anther wall transmit energy to the pollen grains inside the anther. Thus, the energy content of the anther increases while pollen grains accumulate more and more kinetic energy by repeated interactions with the walls and with each other. Release of pollen on the other hand diminishes the energy content of the anther. If buzzing continues, the number of pollen grains in the anther and the number of pollen grains that escape the anther diminish, but their average kinetic energy increases. Although this model is an extreme simplification of flower morphology, it is a cornerstone in the understanding of buzz pollination. In the case of buzz pollination, overexploitation of the pollen is prevented because only a limited amount of pollen can be "buzzed out" during a single visit. It would be interesting to test if the size of buzzing pollinators and the energy they can produce are related to morphological characters of the poricidal anthers and the energy needed to buzz the flowers of a given plant species, thereby limiting "mechanically" the pollinator spectrum.

The piston mechanism [14] often found in the pea family (Fabaceae) acts, from a functional point of view, as a pollen pump. The stamens that are hidden in the carina have coalesced filaments and form a tube that encloses the style. Flower visitors have to deform the flower actively to reach the nectar. The forces exerted by visiting insects and the concomitant deformations of the flower cause a relative forward movement of style and stigma in the staminal tube that squeezes a portion of the pollen mass out of the staminal tube. By this mechanism, pollen is transferred to the ventral part of the pollinator's body. During a single visit, only a small amount of pollen is extruded by the pollen pump, ensuring pollen dispensing.

The brush mechanism, which also occurs in the pea family, works in a similar manner [15]. In Lathyrus latifolius, a stylar brush underneath the stigma takes the pollen up during flower development. Later when the ripe flower is deformed by forces exerted by a visiting insect, the carina is lowered and the stigma and the stylar brush touch the insect consecutively, thus avoiding self-pollination. In flowers of Lathyrus latifolius, 100 millinewtons (mN) are needed to trigger this mechanism [16] (measured with a spring balance). Pollen dispensing is realized by the brush, which deposits only a dosed amount of pollen on each visitor.

Staminal levers in the broadest sense, i.e., stamina that can be tilted with or without a hinge, occur not only in the genus Salvia (see Section 6.1.2) but are also found in the Lamiaceae-Prostantheroideae [17,18] and in other families such as the Zingiberaceae. An example from the latter family is Roscoeapurpurea [19]. As the author points out, the structure of its stamen differs significantly from the Salvia lever. Flowers of R. purpurea have only one fertile stamen; the two thecae are extended; and their basal parts are sterile and block the flower entrance. The upper part of the thecae is fertile and produces pollen. Salvia, in contrast, has two fertile stamens and a different stamen morphology, as described in more detail below. In both taxa, a flower visitor releases the mechanism by pushing against the lever arm, which extends into the flower tube. Thereby, it is loaded with pollen on its head, neck, or back.

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