As the sun rises in the morning, young sunflowers face their heads east. They gradually turn west throughout the day, tracking the sun’s trajectory across the sky. And when the sun finally settles, they swing their heads east again, waiting for sunrise.
"The plant anticipates the timing and the direction of dawn,” notes Stacey Harmer from the University of California, Davis, in a press release. Their ability to predict where the sun will rise each morning suggests that the sunflowers were relying on an internal mechanism, or clock, to track its movements.
A team of researchers led by Harmer carried out a series of experiments to find out why the sunflowers followed the sun, how they knew which direction to face, and how they were able to change their orientation.
Following the sun appeared to boost sunflower growth, as those that were staked and rendered motionless reached smaller sizes than those with freedom of movement.
The researchers dotted the sunflowers’ stems with ink and filmed them during the day to determine how they managed to move around. When plants were tracking the sun across the sky from east to west, the east side of the stem grew more rapidly than the west side, pushing the head in the right direction. Then at night, the west side grew faster as the head swung back east.
Even when plants were moved indoors with an immobile overhead light, they continued to swing back and forth for a few days. That behavior would be expected if their motion is based on an internal clock, Harmer said.
However, once the artificial light was moved to one end of the room, the flowers took to following this light much in the same way as they had tracked the sun.
The researchers concluded that two growth mechanisms were at play in the sunflower stem. First, there is the basic growth rate, which is based on how much light is available. Second, the internal clock is set to track the direction of light, leading the stem to grow unevenly, and causing the sunflower to sway east and west throughout the day.
Growth slows down as the plants age, which is why mature sunflowers are much less mobile — rather, they tend to settle facing east, and their reaction to light becomes more intense in the mornings.
But if sunlight helps the plants grow, why do mature sunflowers bother facing east? The researchers found that east-facing sunflowers had a head start on heating up in the mornings, and this helped them attract five times as many pollinators as west-facing sunflowers. A portable heater was enough to restore a large numbers of pollinators back to the west-facing ones.
"Bees like warm flowers," Harmer said.
This study was published in the journal Science.
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