Flowering stinking hellebore in the snow. Photo: Carlos Herrera
The stinging hellebore has a natural heating system: yeast cultures in their nectar raise the temperature of the flowering plant in winter. In this way, the plant attracts pollinators despite the cold and can therefore multiply more easily than other plants. This Spanish researchers discovered when they examined the influence of yeast fungi on the heat balance of the buttercup. The heater not only brings an advantage to the hellebore, but also to the pollinating insects: a warm nectar meal allows the cold-sensitive animal to stay active longer. It has been known for some time that certain plants have heating mechanisms. However, the scientists have previously assumed that the flora knows only two ways to get to more heat: either by giving off heat, whereby the plant regulates its temperature from the inside, or by passive absorption of sunlight. Now the researchers around Herrera of the Estación Biológica de Doñana in Seville have discovered a completely new heating mechanism: The nectar in the flowers of the Stinking hellebore (Helleborus foetidus) can be up to six degrees warmer than the closer environment of the plant. Yeasts that live in the nectar are responsible for this elevated temperature. These degrade the sugar in the sweet sap, releasing large amounts of heat.

For their study, the scientists removed the yeast fungi from the flowers of some hellebore plants. The temperature of the nectar immediately dropped off sharply. In a second experiment, they mixed young, yeast-free flowers with the fungus and measured the temperature of the nectar, the air temperature inside the flowers and the ambient air temperature. The researchers discovered that the nectar warmed up as a result of the yeast activity and the temperature difference between the flower interior and the environment also increased.

The yeast is actually considered a parasite, the scientists write: It reduces the sugar content in the nectar that the plant produces for possible pollinators. Thus, the sweet plant juice loses its appeal for the food-seeking insects. However, according to the researchers, this interpretation implies that neither plants nor pollinators benefit from the presence of yeast. However, this is not the case: the insects visit the Stinking hellebore not primarily because of the nectar, but because of the heat in their calyx. This creates a "win-win situation" for the plants and the pollinators thanks to the parasites.

Carlos Herrera (Estación Biológica de Doñana, Seville) Proceedings of The Royal Society B, online pre-publication, doi: 10.1098 / rspb.2009.2252 ddp / wissenschaft.de? Regula Brassel advertisement

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