It's summertime, and the fields of Yolo County are filled with ranks of sunflowers, dutifully watching the rising sun. At the nearby University of California, Davis, plant biologists have now discovered how sunflowers use their internal circadian clock, acting on growth hormones, to follow the sun during the day as they grow. "It's the first example of a plant's clock modulating growth in a natural environment, and having real repercussions for the plant," said Stacey Harmer, professor of plant biology at UC Davis and senior author on the paper to be published Aug. 5 in the journal Science.

Harmer is a molecular biologist who studies circadian clocks, usually in the lab plant Arabidopsis, which is distinctly smaller than a sunflower. Previously, Harmer's lab had discovered links between "clock" genes and the plant hormone auxin, which regulates growth. But Harmer and her collaborator Benjamin Blackman at UC Berkeley and the University of Virginia needed an example to work on, and they found it in the sunflower. 

East to West, and back

Growing sunflowers begin the day with their heads facing east, swing west through the day, and turn back to the east at night.

"The plant anticipates the timing and the direction of dawn, and to me that looks like a reason to have a connection between the clock and the growth pathway," Harmer said. This behavior of sunflowers had been described by scientists as far back as 1898, but no one had previously thought to associate it with circadian rhythms.

Hagop Atamian, a postdoctoral researcher in Harmer's lab, in collaboration with Blackman's lab at the University of Virginia (now at UC Berkeley), carried out a series of experiments with sunflowers in the field, in pots outdoors, and in indoor growth chambers.

By staking plants so that they cannot move, or turning potted plants around daily so that they were facing the wrong way, Atamian showed that he could disrupt their ability to track the sun. Following the sun provides a growth boost to the plants. Sunflowers staked so they can't move have decreased biomass and less leaf area than those that do, the researchers found.

When plants were moved into an indoor growth chamber with immobile overhead light, they continued to swing back and forth for a few days. That is the kind of behavior you would expect from a mechanism driven by an internal clock, Harmer said.

Finally, the indoor plants did start tracking "the sun" again when the apparent source of lighting was moved across the growth chamber by turning adjacent lights on and off during the day. The plants could reliably track the movement and return at night when the artificial day was close to a 24-hour cycle, but not when it was closer to 30 hours.