By Athayde Tonhasca
Life in the British uplands can be harsh, even for species adapted to cold temperatures and scarce resources. In these habitats, a mountain avens flower (Dryas octopetala) can be a safe berth for a fly or occasional bee. Insects get more than pollen and nectar from this plant: they also get warmth: the temperature on a mountain avens flower can be up to 30C higher than the surrounding air.
Mountain avens flowers get warm because they follow the sun throughout the day, a phenomenon known as heliotropism or solar tracking. Moreover, flowers of mountain avens and some species from continental Europe such as alpine buttercup (Ranunculus adoneus) – from which we learned most of what we know about heliotropism – are usually bowl-shaped, so sunlight is reflected towards their centre. Heliotropism and flower form allow the plant to function like a satellite-tracking antenna, maximizing light interception.
But how do these flowers rotate to keep up with the sun’s position?
We don’t know for sure, but can assume auxins are behind it. This group of hormones are involved in just about every aspect of plant growth and development, including phototropism (growing towards light).
Mountain avens’ heliotropism may be similar to what happens with the common sunflower (Helianthus annuus). In the morning, the stem and upper leaves of a young sunflower plant face east. As the day progresses, auxins move from the western to the eastern side of the plant. Auxins promote water absorption and tissue elongation, so the plant slowly bends westwards. The auxin gradient is reverted at night, and the plant is reoriented eastward. However, this cyclical movement stops when the plant flowers. So contrary to what some people think, sunflowers’ flowers do not follow the sun; they are always facing east (although wind or rain can change their position).
Heliotropism is an asset for plants with short growing seasons. The temperature of an alpine buttercup’s gynoecium (the female reproductive organs) can be 5.5°C higher than the flower’s surroundings. Heat accelerates pollen germination and the growth of pollen tubes; it also leads to heavier seeds and higher germination rates. So plants have greater reproductive success.
Heliotropic flowers’ absorption of solar irradiance encourages insects to visit and stick around, basking and foraging. The extra warmth increases their metabolism, and boosts their flight capability. Frequent and long-lasting insect visits are important for many upland plants, which cannot self-fertilise and rely mostly on flies for pollination. These insects lack pollen-carrying structures and, generally speaking, are much less hairy than bees. So the longer a fly frolics on a flower, the greater the chances it will get some pollen grains stuck to it. With luck, some pollen will be carried to another flower, and pollination will happen. A warm welcome pays off for plants and insects alike.