Sunflowers are renowned for their ability to track the sun’s movement across the sky by turning their faces. But how do they actually “see” the sun in order to follow it? A group of plant biologists at the University of California, Davis decided to investigate. They found a new mechanism, completely different from what was previously thought.
“This was a total surprise to us,” Stacey Harmer, one of the study authors, said in a news release. “In this paper, we report that they use different molecular pathways to initiate and maintain tracking movements, and that the photoreceptors best known for causing plant bending seem to play a minor role in this remarkable process.”
Most plants display phototropism — the capacity to grow in the direction of a light source. But sunflowers take it to the next level.
Sunflowers turn their heads by growing more on the east side of the stem during the day and more on the west at night. The team at UC Davis had previously shown how sunflowers use their internal circadian clock to predict the sunrise and synchronize the opening of their florets with the appearance of pollinating insects in the morning.
Still, plant researchers had previously presumed that sunflowers’ heliotropism, their skill in tracking the sun, would rely on the same mechanisms as phototropism. This phototropism is governed by a molecule called phototropin and responds to light in the blue spectrum.
But as it turns out, sunflowers use a different mechanism entirely.
Indoor and outdoor sunflowers
Harmer’s team conducted a study to observe gene activity in sunflowers grown both indoors and outdoors. For the indoor-grown sunflowers, the genes linked to phototropin (a light receptor) were active on the side exposed to light. Conversely, genes connected to cell growth and auxins (a type of hormone) were more active on the side in the shade. However, this pattern was different in outdoor-grown sunflowers.
Upon examining the outdoor sunflowers, the team noticed no discernible difference in phototropin activity between the sunny and shaded sides of the stem. The team then conducted an experiment using shade boxes to block specific light wavelengths — blue, ultraviolet, red, or far-red — but found that it did not influence the sunflowers’ sun-tracking behavior. This suggests that multiple light-sensing mechanisms work simultaneously in these plants.
Furthermore, when sunflowers initially grown indoors were moved outside, they immediately began tracking the sun. This sun-tracking action was accompanied by a sudden surge of gene activity on the plant’s shaded side — a phenomenon that did not repeat in the subsequent days. Harmer theorizes that this indicates a form of genetic “rewiring” in the plants.
The impressive sunflowers
Sunflowers (Helianthus annuus) were first domesticated from their wild ancestor over 5,000 years ago by Native Americans. Colonialists then brought the plant to Europe during the 16th Century, where it became popular as food and decoration. By the 19th Century, the sunflower was being grown on a wide scale across Europe.
The Incas, Aztecs, and the Otomi used it as a symbol to represent their various sun gods. Now, the sunflower’s seeds are used to process sunflower oil, which can then be turned into margarine. The petals are also edible but aren’t widely eaten due to their less pleasant flavor. When planted, sunflowers can remove toxic elements from the soil, like lead.
The new study reveals previously unknown pathways for light-sensing and growth in plants, the discovery has an even broader relevance, the researchers argued. “Things that you define in a controlled environment like a growth chamber may not work out in the real world,” Harmer said in a news release. This just goes to show how much we still don’t know about these popular flowers.
The study was published in the journal PLOS Biology.