Combining the observations made by Charles Darwin, experiments that home gardeners can reproduce, and sophisticated molecular biology, a team have shown how and why young sunflowers use circadian rhythms to follow the Sun across the sky. They've also demonstrated why adult flowers don't do the same thing.
The way that sunflowers track the Sun across the sky has been commented on for generations, with Darwin providing the first rigorous exploration. Explaining the mechanism took longer. Professor Stacey Harmer of the University of California, Davis, has filled in a lot more details.
"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 in a statement.
In Harmer's lab, Dr Hagop Atamian staked Helianthus annuus sunflowers so they couldn't move, and turned the pots of others to confuse them. It's probably fortunate there are no ethical guidelines for the treatment of plants. Unable to follow the Sun, the flowers grew 10 percent more slowly and produced less leaf area, demonstrating the value of maximizing their photons.
Plants that were able to move followed a different cycle in summer and autumn, demonstrating a capacity to adjust to the length of the day.
When lit by an immovable indoor lamp, the sunflowers took several days to stop turning east when the light was turned off. When confronted with moving sources of light that had cycles substantially different from 24 hours, they proved unable to adjust.
Taken together, these observations demonstrate that the movement is guided by circadian rhythms rather than photoperiodism, which is driven solely by light changes.
Atamian filmed the plants in action and demonstrated that the east side of the stem grew faster in the daytime, causing the plant to twist around, while the west side put on a spurt at night to return the plant to its original position. Sunflowers that don't produce the growth hormone gibberellin, which lengthens their stalks, were unable to track the Sun in this way, unless injected with gibberellin.
These observations were matched by the observation that genes associated with growth are expressed more strongly on one side of the plant during daylight and the other at night.
Once the flowers reach a certain point, they abandon their youthful sun-worshiping and face east throughout the day, which Harmer and Atamian suggest is driven by a stronger reaction to blue light in the mornings than afternoons.
In Science, Atamian and Harmer report that when they rotated pots containing mature sunflowers so that the flowers faced west, pollinators abandoned them. “Bees like warm flowers,” Harmer explained. When seeking flowers in the morning, bees want the warmest they can get, and naturally these are the ones facing east.
The difference was so stark that five times as many bees visited east-facing sunflowers as those looking west, making a huge difference in the chances of successful reproduction.