In a study published in the journal Science this week, researchers say they've found an answer: Sunflowers, like animals, have a circadian rhythm — an internal clock that can be set to the external world. During the day, this system sends messages to the eastern sides of their stems, telling those cells to grow slightly longer, which causes the sunflower to lean westward. At night, the message reverses, and the sunflowers tilt back toward the east.
"It's the first example of a plant's clock modulating growth in a natural environment, and having real repercussions for the plant," Stacey Harmer, professor of plant biology at University of California-Davis and senior author on the paper, said in a statement.
Even though they don't sleep, many plants have "clock genes" similar to those that direct the sleep-wake cycle in animals. For years, Harmer had been looking for a link between those genes and the release of a hormone called auxin, which controls stem growth.
Sunflowers seemed like a likely candidate to help that search. Scientists have known about the plants' sun-seeking behavior since 1898, when botanist John Schaffner described a summer he spent observing an especially "abundant and luxuriant crop of this characteristic western weed" for the journal Botanical Gazette.
So Harmer tasked a graduate student, Hagop Atamian, with putting the flowers through their paces. First Atamian staked the sunflowers so they couldn't move. The constrained flowers were smaller and frailer than their free-moving counterparts, proving that the flowers really do benefit from following the sun.
Then he moved some potted sunflowers into an indoor growth chamber, where they were subjected to continual overhead lighting. For their first few days inside, the flowers continued to swing back and forth from east to west, just as you or I would maintain a more or less 24-hour sleep cycle during the first few days of living in constant light. This suggested that the flowers' behavior was controlled by an internal rhythm, rather than a direct response to the sun.
But how was that rhythm directing the sunflowers' movement? To resolve that question, Atamian (now a postdoctoral research associate at UC Davis) went outside and put ink dots on the stems of growing sunflowers and tracked how the height of each side varied slightly over the course of the morning, afternoon and night. He snipped bits of young sunflowers' stems at different times of day to sample for the activity of growth genes, and discovered that those also varied depending on the time.
As sunflowers mature, and their broad brown and yellow faces begin to open up, they cease their dancing. Harmer believes that this is because overall growth slows in adulthood, making it hard to achieve the daily back and forth movement that younger sunflowers display. Instead, like kids who have grown up and finally settled down, they must pick one direction and stick to it. Almost all wind up facing east, where their faces warm up more quickly as the sun starts to rise.
"Bees like warm flowers," Harmer said. She and her colleagues found that east-facing flowers attracted five times as many pollinators as their western-facing counterparts. When the westerners were warmed with a portable space heater, the bumblebees started to buzz over.
“The more general point,” co-author Benjamin Blackman said in a statement, “that one of the circadian clock’s adaptive functions is to regulate the timing and strength of growth responses to environmental signals, is one that I think will apply to a broad range of traits and species.”