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Young Sunflowers Really Do Follow The Sun

Photo from Pixabay

Sunflowers, like their name suggests, actually do follow and dance to the sun.

Every day, young sunflowers track the sun’s path across the sky, turning their faces 180 degrees from east to west, the Los Angeles Times reports.

These slow, graceful movements continue even when night falls, as the flowers re-orient their direction when the sun sets, turning back to the east where the sun will rise.

Stacey Harmer, a circadian biologist, and professor at the University of California Davis saw time-lapse videos of the sunflowers dancing and became interested in studying them.

She says she was amazed at seeing the plants reposition themselves at night.

I tell my students all the time that plants are capable of incredible things — we just don’t notice because their time scale is different than ours.

Harmer and colleagues decided to look deeper into how sunflowers move, and why. They planted a field of the plants and observed what happened before factoring in other variables.

As the plants matured from young seedlings to yellow adults, the researchers found that the sun-tracking dance of sunflowers slowly decreased in motion until it stopped altogether.

Harmer says there is a misconception that mature sunflowers move with the sun when they don’t. They always face east. But it’s a different story with the younger flowers.

The researchers also found that the plants were able to pace their movements, according to the season. During the shorter nights in midsummer, it took the young sunflowers only 8 hours to turn from west to east. But during the longer autumn nights, it took 12 hours to do the same.

To track the plants’ movements, the team went into the sunflower field and marked both sides of each stem at regular intervals using a Sharpie. They then used a time-lapse camera to see that each side of the stem grew longer by day as the flower turned to the west. The reverse happened at night, with the west side getting longer as the plant faced east.

The question remained as to what controlled this movement: the sun or an internal clock of some sort.

To answer this, the team transferred dwarf sunflowers from the outdoors to a lab with controlled lighting. They found that even when the plants grew under a fixed overhead light that was constantly on, they displayed the same east to west turning movement.

In a second lab experiment, the researchers decided to interrupt the sunflowers’ internal clocks, exposing them to a 30-hour light cycle. This confused the sunflowers, which began turning their heads to the west long before darkness fell. During the night, the sunflowers also exhibited erratic movements.

These results show that the sunflowers’ actions are run by more than just growing towards the sun. They had some sort of internal clock that controlled how and when they turned.

The next question was why. How would it benefit sunflowers to turn daily and is there a reason mature sunflowers look to the east?

The researchers conducted more experiments. Every night for a hundred nights, Hagop Atamian, a post-doctorate researcher on the team, went into the field of sunflowers and rotated them, so they faced west in the morning. After multiple trials, the group found that the manipulated plants were smaller in size by 10% compared to the others. Harmer said that was a significant difference.

They also found that mature sunflowers face east because they get up to five times more pollinators compared to those that were rotated to face west.

Another experiment established that the presence of more pollinators is likely because east-facing sunflowers are warmed more than those facing west. Another team member heated west-facing sunflowers with an artificial light source until they were the same temperature as those facing east to prove this.

Pollinators were more likely to flock to the artificially-warmed sunflowers compared to those that had not been warmed. But on the whole, pollinators still showed a preference for the sunflowers facing east.

Harmer says that while her team has answered many questions with regards to sunflowers’ movements, there is still much to be discovered. She intends to study what genes control the plants’ dramatic motions in the future.

The study was published in the journal Science.

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