The closure of flower heads is driven not only by falling light or temperature, as previously thought, but also by pollinators. In a set of field experiments, Jochen Fründ at Georg August University in Göttingen, Germany, and his team show that flowers in the Asteraceae family, including smooth hawksbeard (Crepis capillaris), close within three hours of pollination. By contrast, the heads of unpollinated flowers stayed open for hours longer — until late afternoon.
Flower-head closure earlier in the day affects interactions between plants and the pollinators of the surrounding area, and can change the times at which some insects are active. The authors suggest that their results be considered in plant and pollinator field surveys to ensure accuracy.
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Pollination drives floral clock. Nature 476, 9 (2011). https://doi.org/10.1038/476009d