Plants In Motion: Why Sunflowers Follow the Sun

Sunflowers are known for their unique tracking ability. As they grow, young sunflowers follow the sun from east to west across the sky. Come nightfall, the flowers pivot back from west to east, only to begin the cycle all over again at dawn.

Scientists have observed this behavior as far back as 1898. But it took a team of plant biologists to finally figure out how (and why) the flowers did it. Their research revealed an interesting phenomenon. It turns out that young sunflowers, like people, are guided by circadian rhythms.  These internal ‘body clocks’ not only help regulate the plant’s growth, but they are also the chief mechanism behind the plant’s unique tracking behavior.

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“It’s the first example of a plant’s clock modulating growth in a natural environment, and having real repercussions for the plant,” said Stacey Harmer, professor of plant biology at University of California-Davis, and senior author of the paper reporting the discovery.

SUNFLOWERS CAN CONTROL THEIR BEHAVIOR

And that’s only half the story. It turns out that young sunflowers are not only attuned to the positional changes of the sun, they can even anticipate the seasonal shift between long and short days. As a result, they can adjust their behavior accordingly.

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Scientists uncovered this astonishing fact by using a time-lapse video. By tracking how the plants moved during different times of the year, they discovered that during long summer days the sunflowers rotated more slowly. But come nightfall, they quickly repositioned themselves to face east before dawn.

However, in September when the days grew shorter, the flowers took longer to reorient themselves. This indicated that they knew when the sun was coming up.

(Note- this phenomenon applies to growing flowers only. Once sunflowers mature, their stems stiffen and they remain facing towards the east – see why below.) 
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THEY TRICKED THE PLANTS

To determine what made young sunflowers follow this circadian rhythm, researchers conducted a series of experiments. They set up potted plants in the field and in indoor growth chambers. In the field, they purposely turned the potted plants to face west in the morning to disrupt the sunflower’s tracking ability.

Amazingly, those plants that were deliberately faced westward still rotated to follow the sun (in reverse). However, they grew more slowly than the other sunflowers. Eventually, they developed into smaller plants with smaller leaves.

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Young sunflower in the field

Meanwhile, in the indoor growth chamber, the potted sunflowers that were faced westward continued their daily rotation for a few days before petering out. They didn’t officially start tracking “the sun” again until scientists created an artificial 24-hour day by turning adjacent lights on and off in an arc to replicate natural conditions.

THE SUNFLOWER DOESN’T ROTATE, THE STEM DOES

So what is the actual mechanism rotating the sunflower? It turns out that it isn’t the flower at all, but the stem.

Researchers have discovered that the direction of light causes one side of the stem to grow more rapidly than the other. During the day, the flower swings east to west to absorb as many photons from the sun as possible. This in turn helps maximize energy absorption for the growing flower.

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MATURE SUNFLOWERS STAY FACING EAST

As the sunflower matures, however, its overall growth slows down and the plant stops moving. It finishes its tracking and remains in an east-facing position. This gives it a distinct advantage.

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In the morning, eastward-oriented flowers heat up more quickly. And studies show that given a choice, pollinators are more attracted to warmer flowers.

 

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About carole funger

I'm a landscape designer and Maryland Master Gardener living in the Washington, DC area. I blog about new trends in horticulture, inspiring gardens to visit and the latest tips and ideas for how to nurture your own beautiful garden. Every garden tells a story. What's yours?

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