Sunflowers’ invisible colors help them attract bees and adapt to drought

The dense collection of yellow petals of a sunflower (technically an 'inflorescence', or collection of many flowers) is a familiar sight, but it's hiding something from the human eye: an ultraviolet (UV) bullseye pattern, invisible to humans but not to most insects including bees.

These bullseye patterns have long been known to improve the attractiveness of flowers to pollinators by increasing their visibility. Now, UBC researchers have found the same molecules that produce UV patterns in sunflowers are also involved in helping the plant respond to stresses such as drought or extreme temperatures, in a new paper published today in eLife, potentially providing clues to how plants can adapt to different climates.

"Unexpectedly, we noticed that sunflowers growing in drier climates had flowers with larger UV bullseyes, and found that those flowers are able to retain water more efficiently. This suggests that these larger UV bullseyes help plants adapt to these drier environments," says lead author Dr. Marco Todesco, a research associate at UBC's biodiversity research centre and department of botany.

Dr. Todesco and his colleagues grew almost 2,000 wild sunflowers of two species at the university in 2016 and 2019. They measured the sunflowers' UV patterns, and analyzed the plants' genomes, and found that wild sunflowers from different parts of North America had UV bullseyes of very different sizes: in some, the bullseye was a thin ring, while in others it covered the whole flower. Larger bullseyes were visited more frequently by bees, supporting previous research of other plant species.

The researchers found that a single gene, HaMYB111, was responsible for most of the diversity in floral UV patterns. This gene controls the production of UV-absorbing flavonol compounds, which are also known to help plants survive under different environmental stresses like drought or extreme temperatures. Larger floral UV patterns that have more of these compounds could help reduce the amount of evaporation from a sunflower in environments with lower humidity, preventing excessive water loss. In humid, hot environments, smaller UV patterns would instead promote this evaporation, keeping the plant cool and avoiding overheating, the authors say.

"Floral UV patterns appear therefore to play at least a dual role in adaptation; besides their well-known effect on enhancing pollination, they also regulate water loss from flowers," says senior author Dr. Loren Rieseberg (he/him), a professor in the department of botany and the biodiversity research centre. "That's not something you would necessarily expect a flower colour to do, and it exemplifies the complexity and efficiency of adaptation -- solving two problems with a single trait."

Sunflowers are cultivated for various purposes, including sunflower oil production, a roughly US$20 billion dollar industry in 2020. This research could help add to knowledge about how to attract pollinators, potentially increasing crop yields, says Dr. Todesco. "This work also helps us understand how sunflowers, and potentially other plants, better adapt to different areas or temperatures, which could be important in a warming climate."

The researchers also want to better understand how HaMYB111 regulates the size of UV bullseyes, and examine in more detail how those patterns affect plant physiology as well as investigate how exactly flavonol compounds affect water loss.