Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, Japan.
Plant Biol (Stuttg). 2018 May;20(3):537-545. doi: 10.1111/plb.12699. Epub 2018 Mar 1.
Explosive pollen release is a mechanism used by some angiosperms that serves to attach pollen to a pollinator's body. It is usually adopted by species with zygomorphic tubular flowers and pollinated by birds and bees. The tree genus Meliosma (Sabiaceae, Proteales) has unique disc-like flowers that are externally actinomorphic, but internally zygomorphic, and release pollen explosively. To elucidate the adaptive significance of explosive pollen release, we observed flowering behaviour, the breeding system and pollinator visits to flowers of the Japanese species Meliosma tenuis in a temperate forest. Flowers bloomed in June and were nectariferous and protandrous. Explosive pollen release was triggered by slight tactile stimuli to anther filaments or staminodes in male-stage flowers. Because pollen cannot come into contact with the pistils enclosed by staminodes, M. tenuis is functionally protandrous. Artificial pollination treatments revealed that M. tenuis is allogamous. The dominant flower visitors were nectar-seeking drones of the bumblebee species Bombus ardens (Apidae). The drones' behaviour, pollen attachment on their bodies and fruit set of visit-restricted flowers suggest that they are the only agent triggering the explosive pollen release mechanism, and are the main pollinator of M. tenuis. The finding that bumblebee workers rarely visit these flowers suggests that the explosive pollen release has another function, namely to discourage pollen-harvesting bumblebee workers.
花粉爆炸释放是一些被子植物用来将花粉附着在传粉者身上的一种机制。它通常被具有左右对称管状花的物种采用，并由鸟类和蜜蜂授粉。树属杨梅（杨梅科，核心花目）具有独特的盘状花，外部辐射对称，但内部左右对称，并具有花粉爆炸释放的特点。为了阐明花粉爆炸释放的适应意义，我们观察了日本杨梅（Meliosma tenuis）在温带森林中的开花行为、繁殖系统和传粉者对花的访问。花朵在 6 月开花，具有花蜜和雄蕊先熟。在雄性阶段的花朵中，轻微的触感触发了花粉的爆炸释放。由于花粉无法与由雄蕊包围的雌蕊接触，因此 M. tenuis 在功能上是雄蕊先熟的。人工授粉处理表明，M. tenuis 是异交的。主要的花朵访客是熊蜂属（Bombus ardens）的花蜜寻求雄蜂（熊蜂科）。雄蜂的行为、它们身上附着的花粉以及访问受限花朵的结实情况表明，它们是唯一触发花粉爆炸释放机制的媒介，也是 M. tenuis 的主要传粉者。工蜂很少访问这些花朵的发现表明，花粉爆炸释放还有另一个功能，即阻止采集花粉的熊蜂工蜂。
