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.