How Sunflowers Follow the Sun

The secret is in their stems.

Clarissa Cavalheiro / Reuters

Stacey Harmer didn't start out studying sunflowers. In fact, she did everything she could to avoid it. As far as lab plants go, they are incredibly unwieldy and inconvenient. But several years ago, at the end of her postdoc studying the circadian rhythms of a pocket-sized flowering mustard called Arabidopsis thaliana, she found herself asking a very basic question for which there seemed to be no answer. She'd seen a video of a young sunflower turning its head from east to west to follow the sun and then, in the darkness of the night, eerily ratcheting back to face east. The moment the sun rose, the sunflower was ready. “That plant,” she remembers thinking, “seems to know when and where the sun is going to rise.”

To her surprise, there did not seem to be much written on the subject. Anyone could see that a field of sunflowers will all face the same way—though mature plants don't move like the young ones—and some studies suggested that young plants' turning was the result of one side of the sunflower's stem growing faster than the other. But why did it happen? What genes were controlling the plant's movement? Were they influenced by the sun, or by some internal clock that kept track of what time it was and which way to face? And why should sunflowers face the sun, anyway?

And so, despite their size, Harmer, who is now a professor at UC Davis, found herself growing sunflowers. (She tried growing mustards and tomatoes first, to see if these more tractable plants did anything similar, but no such luck.) Later she brought on postdoc researchers who tended them in pots out in the field, sometimes spending nights out there during long experiments.

To begin with, as she and her group detail in a recent Science paper, Harmer found that plants left in the dark would continue to make their rounds for a while, even without the sun to track. That made it clear that some timing system, not just light exposure, was involved. Then, to see whether one side indeed grew faster than the other, Harmer's postdoc Hagop Atamian took time-lapse videos of sunflowers as they grew, tracking marker lines he'd made on their stems. He found that during the day, the east side of the stem rockets ahead of the west, which causes the flower's head to turn. Then, at night, the west side kicks into high gear, propelling the plant around to face east again, in preparation for sunrise.

That's remarkable because in a sunflower, east and west sides aren't fixed, like having a right and a left hand. If you turn a plant, the stem has new east and west sides. So how could the sides of the stem behave so differently? Researchers already know that plant growth happens at certain times of day under the control of the circadian clock. One explanation for the sunflower's staggered growth might have been that the plants' clocks were set to different times on either side, as though the west side was jet-lagged from the east. Every two hours for 48 hours, Atamian took samples from the stems and looked to see what genes were being expressed.

It turned out that the plant's clock was set to the same time on each side—no jet lag. Instead, genes controlled by an important growth hormone were expressed at different levels. The team is now looking closer at that hormone to see whether it builds up at different times on the opposing sides of the stem. Since the clock is tightly connected with growth, they think the hormone must be interacting with the clock somehow. “[But] we haven’t yet figured out what this mechanism might be,” Harmer says.

Why all this complicated hormonal footwork to follow the sun? To see what happened if sunflowers were prevented from their usual rounds, the researchers lashed the plants to attention on stakes, or came in every night to turn their pots around so they would be facing the wrong way when morning came. Plants treated this way had 10 percent less mass and leaf area than those left to follow the sun, suggesting that tracking helps substantially with growth.

And in another series of experiments, the group found that sunflowers facing east at dawn got many more bee visitors in the morning than those facing west (there was no difference in visitors later in the day). That seems to be because east-facing flowers are warmer: When Harmer's postdoc Nicky Creux rigged portable heaters to get west-facing flowers up to the same temperature, their bee numbers went up.

Now, Harmer's co-author Ben Blackman at UC Berkeley has been exploring whether getting early-morning pollinator visits is better for sunflowers' chances of reproduction than visits later on.

“I've actually becoming very fond of sunflower,” remarks Harmer. It's still a little frustrating, both because of its size and because there not as many tools to tinker with its genes as with the little mustard Arabidopsis. “But on the other hand, the biology is really fascinating.”