Research Reveals How and Why Sunflowers Turn Their Golden Heads

A new study shows sunflowers have an internal clock and face east to keep bees and other insects happy

Sunflowers 2
Ben Blackman/UC Berkeley

Sunflowers may be beautiful but there's something that's also a little bit creepy about them. During the growing season, the young sunflowers rotate their bright yellow heads during the day to track the sun's movement across the sky. They reset overnight, swinging their face back to the east. Now, a new study published in the journal Science, suggests how and why the big bloomers do it. 

To figure out why the sunflowers rotate, a team of researchers from the University of California, Davis looked at whether the flowers were following the sun or following an internal cue from a circadian rhythm. JoAnna Klein at The New York Times reports that the researchers tested this by placing sunflowers in an indoor room with lights designed to mimic the sun’s daily path. During a 24-hour cycle, they behaved normally. But once they were put on a 30-hour cycle, their rhythm was off. This means the plants likely follow an internal circadian rhythm.

“It’s the first example of a plant’s clock modulating growth in a natural environment, and having real repercussions for the plant,” UC Davis plant biologist and senior author of the study Stacey Harmer says in a press release.

But how do they turn their heads? The researchers found that during the day, genes click on causing the east-facing half of the stem to grow. The lengthening stem causes the flower head to slowly bend to the west during the day. At night, genes causing the west side of the stem to grow activate, causing the head to flip back to the east, explains Klein.

To tackle the question of why, the team studied if there were any benefits to facing the sun all the time—a trait common among older sunflowers. reports Emily Benson reports for New Scientist that the researchers studied sunflowers in pots, restraining some so they could not move their heads and rotating others so they could not follow the sun. What they found is the leaves of those sunflowers were 10 percent smaller than their freewheeling kin. Moving their head, they conclude gives the plants a boost in efficiency.

When the plant fully matures, it faces permanently east, which benefits the aging bloom as well. The researchers found that east-facing flowers heat up more quickly in the morning, attracting five times as many pollinators as west-facing blooms. This find supports previous studies, which suggest that bees and other pollinators prefer warmer flowers in the morning.

“I’m continually astonished at how sophisticated plants are,” Harmer tells Benson. “They’re really masters of coping with the environment.”

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