Sunflowers may be rooted to the ground, but that doesn't mean they can't dance.
Each day, young sunflowers trace the path of the sun across the sky, turning their faces 180 degrees from east to west.
And their slow, graceful movements continue at night. After the sun sets, the plants reorient themselves, slowly twisting their heads back to the east in anticipation of dawn.
Circadian biologist Stacey Harmer, a professor at UC Davis, became interested in studying the motion of sunflowers after watching mesmerizing time-lapse videos of this dance of the plants.
"At nighttime, you could see the whole plant rearranging itself, and it was such an amazing thing," she said. "I tell my students all the time that plants are capable of incredible things — we just don't notice because their time scale is different than ours."
The observation that juvenile sunflowers track the sun is not new — Darwin himself reported the phenomenon more than 100 years ago. But until now, no one had explained how the sunflowers move and why. In a paper published Thursday in the journal Science, Harmer and her collaborators reveal the answers to these questions.
"What they did is take a dusty old scientific curiosity, and did really great science on it," said Steven Kay, director of convergent biosciences at USC, who was not involved in the study.
The team's first step was to plant a field of sunflowers and observe what happened before they started fiddling with variables.
As the plants grew from young seedlings into mature, yellow-headed adults, the researchers found that the sun-tracking movements of the plant became less and less noticeable, until they stopped altogether.
"A really common misconception is that mature sunflowers follow the sun. Actually, they do not," Harmer said. "Mature sunflowers always face east."
The group also observed that the plants could pace their movements. For example, during the short nights of midsummer, young sunflowers took just 8 hours to swing their heads from west to east. However, during the longer nights of autumn, it took them 12 hours to accomplish the same feat.
To find out how the plants were moving, the scientists went into a field of sunflowers and marked both sides of their stems with a Sharpie pen at regular intervals.
Using a time-lapse camera, they were able to see that the east side of the stem grew longer during the day, turning the plant's head to the west. At night, the reverse was true — the west side elongated, causing the plant to face the east.
But what was controlling this growth pattern? Was it the movement of the sun or some kind of internal clock?
To answer this question the researchers moved dwarf sunflowers from an outdoor field into a controlled lighting environment in the lab. The scientists report that even when the plants were grown under constant, fixed overhead lighting, they maintained the same head-turning rhythms they displayed in the field for several days.
In another lab experiment, the researchers messed with the sunflowers' internal clocks by exposing them to a 30-hour light cycle. This thoroughly confused the plants, and they wound up turning their heads furthest to the west well before the transition to dark. During the night, the plants moved erratically.
Together these results suggest that the sunflowers' movements are regulated by something other than simple growth toward the sun. Some kind of circadian clock was also controlling the plants' twists and turns.
The next question, of course, was why. Are sunflowers served by their ability to track the sun? And is there a benefit to the mature sunflowers' decision to turn to the east?
Another series of experiments revealed the answer. Every night for 100 nights, Harmer's post-doctorate researcher Hagop Atamian went into a field of sunflowers planted in pots and rotated them so they were facing west in the morning. In multiple trials, the group found that the manipulated plants were 10% smaller compared to a control group.
"That's a really big difference," Harmer said.
The group also reported that mature sunflowers have good reason to face east. The authors found that east-facing sunflowers attract up to five times the number of pollinators compared with those that were rotated in their pots so that they were facing west.
Yet another experiment showed that this is almost certainly because east-facing sunflowers are more effectively warmed by the morning sun than sunflowers that are facing west. To come to this conclusion, another of Harmer's post-docs warmed west-facing sunflowers with a heat source until they were the same temperature as east-facing sunflowers.
Pollinators were more likely to come to the artificially warmed west-facing sunflowers than those that had not been warmed. However, the pollinators still preferred the east-facing sunflowers.
Although the scientists uncovered many of the sunflower's secrets, Harmer said there is still much to learn. In future work she plans to study what genes regulate the sunflowers' dramatic movements.
"They are really great plants, and we kept finding out fascinating things about them," she said.
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5:25 p.m. Aug. 5: This article has been updated with additional information about the study and comments from Steven Kay of USC.