A cosy brewery

By Athayde Tonhasca

The stinking hellebore (Helleborus foetidus) is a favourite of many gardeners for its evergreen foliage and the abundance of bell-shaped flowers produced in late winter. The adjective ‘stinking’ is a bit libellous, and so is the plant’s alternative name, ‘dungwort’. Although not fragrant, the stinking hellebore produces a strong odour – often described as ‘meaty’ – only when its leaves are bruised.

Stinking hellebore © Uwe und Lukas, Wikipedia Creative Commons

The great British naturalist Gilbert White (1720-1793) commented on the attributed medicinal properties of the stinking hellebore: ‘The good women give the leaves powdered to children troubled with worms.’ But he added: ‘Where it killed not the patient, it would certainly kill the worms; but the worst of it is, it will sometimes kill both’. Like related buttercup or crowfoot plants (Family Ranunculaceae), the stinking hellebore is loaded with toxic glycosides.

Although a bad choice as a worm medicine, the stinking hellebore is an excellent option for a garden. It is one of earliest plants to bloom, which can happen even before the snow has melted away. This is a hard time for bees and other insects because there aren’t many other sources of pollen or nectar, so stinking hellebore flowers can be life savers for emerging bumble bee queens. 

Bees may have another good reason to visit these flowers: the warmth generated by yeast fermentation. Yeasts are single-cell fungi found in a variety of terrestrial and aquatic habitats. They are fundamental for some ecological processes such as litter decomposition, nutrient cycling and the growth of plant roots. One species, the baker’s yeast (Saccharomyces cerevisiae), is the main agent of baking, brewing and wine-making, and thus entwined with human civilisation.

Stinking hellebore nectaries are colonised by some types of yeast that ferment the nectar and warm the flower to more than 2° C above the ambient temperature. Floral warming can enhance plant reproduction by increasing pollen germination, pollen tube growth, and fertilization rates. Some of these effects have been demonstrated for plants that heat up because of their metabolism, or by absorbing solar energy by means of heliotropism, as it is the case for the mountain avens (Dryas octopetala). Higher flower temperatures may also increase the evaporation of volatile organic compounds, which help in attracting pollinators.  

Cell formations of Metschnikowia reukaufii, a yeast frequently found in flowers. Bar = 50 μm © Magyar et al. 2005. Fungal Diversity 20: 103-120

Despite the warming benefits to stinking hellebore flowers, yeasts have a negative side too. Fermentation reduces the sugar content of nectar, which makes the flower less attractive and rewarding to pollinators. We don’t know yet how these conflicting outcomes pan out for plant and pollinators. Flower yeasts however, seem to be clear winners in these interactions. They get energy from an abundant supply of nectar, and are dispersed from flower to flower by insects, just like Saccharomyces yeasts are transported by social wasps.

If you notice stinking hellebore plants flowering in your garden or local park, think about the unseen yeast cells mediating the relationship between plant and pollinator. This three-way invisible interaction is an apt representation of nature’s intricacies. 

A bumble bee, possibly an early bumble bee (Bombus pratorum), having a sip of nectar from a stinking hellebore flower © Daniel Ballmer, Wikipedia Creative Commons