A research paper published the journal Science has solved a long-held mystery as to how young sunflowers follow the movement of the sun throughout the day from east to west. In night they turn back from west to east to greet the sun all over again.

As per the researchers, the sunflowers use two features- circadian rhythms and heliotropism, a phenomenon of bending and turning toward the light, for the purpose. It shall be noted that not every sunflower follow the sun, as when sunflower reaches into adulthood stage, its movement slows.

Stacey Harmer from University of California said that sunflower movement is more complex that considered. In short summer nights, sunflowers take less time to move back to their eastward-facing morning position than they do in longer autumn nights.

In the experiment, the researchers raised sunflowers indoors under lamps. Though there was no true light present, young sunflowers continued to follow an imaginary sun’s path throughout the day. The researchers exposed the flowers to a 30-hour light cycle and they continued to follow sunrise and sunset pattern.

The researchers suspected that there is much more than simply an instinct to follow the sun as the flowers were followed unnatural light as well. They consider that it could be a circadian clock. The researchers conducted an experiment in which a field of potted sunflower plants were turned to the west before the sun rose every morning.

When compared with a similar field of sunflowers that was not altered, sunflowers there were 10% smaller. Harmer said, “It’s the first example of a plant's clock modulating growth in a natural environment, and having real repercussions for the plant”.

According to a report in CS Monitor by Christina Beck, "Just like people, plants rely on the daily rhythms of day and night to function," Anne Sylvester, the director of the National Science Foundation's Plant Genome Research Program, which funded the study, said in a press release. "Sunflowers, like solar panel arrays, follow the sun from east to west. These researchers tap into information in the sunflower genome to understand how and why sunflowers track the sun."

Sunflower movement is more complex than simply following the sun across the sky by day. Researchers found that sunflowers that watch the sun’s progress return to their morning position facing east. On short summer nights, it takes sunflowers less time to move back to their eastward-facing morning position than it does on longer autumn nights.

A report published in the The Kansan said, "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."

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.

"Using a time-lapse camera, they were able to see that the east side of the stem grew longer during the day, turning the plant’s head to the west. At night, the reverse was true — the west side elongated, causing the plant to face the east. But what was controlling this growth pattern? Was it the movement of the sun or some kind of internal clock?," according to a news report published by LA Times.

To answer this question the researchers moved dwarf sunflowers from an outdoor field into a controlled lighting environment in the lab. The scientists report that even when the plants were grown under constant, fixed overhead lighting, they maintained the same head-turning rhythms they displayed in the field for several days.