Sunflowers use their internal circadian clock to follow the sun

Scientists have finally managed to unearth what it takes for the sunflower to follow the sun during the day as they grow.

Researchers from University of California, Davis have revealed through a paper published in journal Science that sunflowers use their internal circadian clock, acting on growth hormones, to follow the sun during the day as they grow. The authors of the study add that their discovery is the first example of how plant’s clock modules growth in a natural environment.

As many of you would have noticed, a sunflower effectively follows the path of the sun. Growing sunflowers begin the day with their heads facing east, swing west through the day, and turn back to the east at night. Authors, for their study, either set the plants that the sunflower can’t move or moved the potted plants around daily so that they always face the wrong way. Researchers were able to show that these actions could disrupt plant’s ability to track the sun. Following the sun provides a growth boost to the plants. Sunflowers staked so they can’t move have decreased biomass and less leaf area than those that do, the researchers found.

When plants were moved into an indoor growth chamber with immobile overhead light, they continued to swing back and forth for a few days. That is the kind of behavior you would expect from a mechanism driven by an internal clock, Harmer said.

Finally, the indoor plants did start tracking “the sun” again when the apparent source of lighting was moved across the growth chamber by turning adjacent lights on and off during the day. The plants could reliably track the movement and return at night when the artificial day was close to a 24-hour cycle, but not when it was closer to 30 hours.

When plants were tracking the sun, the east side of the stem grew more rapidly than the west side, researchers found. At night, the west side grew faster as the stem swung the other way. The team identified a number of genes that were expressed at higher levels on the sunward side of the plant during the day, or on the other side at night.

Based on this findings authors said that there appear to be two growth mechanisms at work in the sunflower stem. The first sets a basic rate of growth for the plant, based on available light. The second, controlled by the circadian clock and influenced by the direction of light, causes the stem to grow more on one side than another, and therefore sway east to west during the day.

As the sunflower matures and the flower opens up, overall growth slows down and the plants stop moving during the day and settle down facing east. This seems to be because, as overall growth slows down, the circadian clock ensures that the plant reacts more strongly to light early in the morning that in the afternoon or evening, so it gradually stops moving westwards during the day.

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