The most interesting thing about sunflowers is arguably their movement that appears to follow the sun.
They face east in the morning toward the rising sun following the star over the course of the day as it moves across the sky, eventually turning their big yellow heads westward by sundown. At night, the flowers turn their face back east in the dark in an apparent readiness to catch the first rays of sunlight at dawn.
How the sunflower does this, however, has been a mystery. In a new study published in the journal Science on Friday, Aug. 5, researchers revealed that the sunflower's ability to detect light and its internal clock work together to turn on growth genes allowing its stems to bend with the arc of the sun.
Plant biologist Hagop Atamian, from the University of California, Davis, and colleagues placed sunflowers in a room with lights that mimic the path taken by the sun on different light and dark cycles. Although the plants behaved as expected on a 24-hour cycle, the researchers observed that the plants appeared to be confused during a 30-hour day.
The plants that were used to a 24-hour cycle outdoors, on the other hand, continued to bend from east to west for a few days after they were placed under a fixed light indoors, which means that a 24-hour circadian rhythm guides the movement of the sunflowers.
The researchers also found that the plants' stems have something to do with their movement. They identified the different growth genes that were expressed at higher levels on the sunward side during the day or on the other side at night time.
They said that two growth mechanisms appear to be at work in the sunflower stem. The first one, which relied on available light, sets a basic growth rate for the plant.
The second that is controlled by the circadian clock and influenced by the light's direction causes the plant's stem to grow more on one side than the other causing the plant to move east to west during the day.
"Solar tracking movements are driven by antiphasic patterns of elongation on the east and west sides of the stem," the researchers wrote in their study. "Genes implicated in control of phototropic growth, but not clock genes, are differentially expressed on the opposite sides of solar tracking stems."
As the plant matures, its overall growth slows down and it stops moving during the day, finally settling down to facing east.