That’s a result of different sides of the stem elongating depending on what time of day it is. When it turned, the west side would grow.
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To find out more about why the flowers do this, the researchers used staked plants in pots to prevent them from following the sun.
Sunflowers may look like the sun if you squint your eyes a bit, but they also do this weird thing where they turn to face the sun, hence the name. “Mature sunflowers always face east”.
The mature flower does not have the same behavior, as the clock genes stop the differential growth.
“It’s the first example of a plant’s clock modulating growth in a natural environment, and having real repercussions for the plant”, Harmer said. Previously, Harmer’s lab had discovered links between “clock” genes and the plant hormone auxin, which regulates growth. During the night also, their movement continues as then they reorient themselves and come to their original position, in the east, in the hope of dawn.
Additional co-authors of the paper are Nicky Creux of UC Davis, and Evan Brown and Austin Garner of the University of Virginia.
By staking plants so that they cannot move, or turning potted plants around daily so that they were facing the wrong way, Atamian showed that he could disrupt their ability to track the sun. So it’s clear that they’re following the sun for a specific objective and according to the paper, it’s to provide a growth boost and to attract more insects for pollination.
The science team’s first step was to plant a field of sunflowers and observe what happened before they started fiddling with variables.
Prof. Stacey Harmer, a circadian biologist from UC Davis, has explained in the journal Science as to how sunflowers move and why. That is the kind of behavior you would expect from a mechanism driven by an internal clock, Harmer said. Even when exposed to artificial light, the sunflowers would track that movement in a consistent cycle.
This revealed that something beyond the sun was causing the plants to move. To resolve that question, Atamian (now a postdoctoral research associate at UC Davis) went outside and put ink dots on the stems of growing sunflowers and tracked how the height of each side varied slightly over the course of the morning, afternoon and night. It was found that the east sides of the flowers’ stems grew more rapidly than the west sides during the day time.
There appear to be two growth mechanisms at work in the sunflower stem, Harmer said. The first one, which relied on available light, sets a basic growth rate for the plant. “At night, the higher growth rate on the west side culminates in the apex facing east at dawn”.
At dawn, whole fields of sunflowers stand at attention, all facing east, and begin their romance with the rising sun.
For the study, the researchers planted a field of sunflowers. Using an infrared camera, the researchers found that the sunflowers which faced east heated up more quickly in the morning and also attracted five times as many pollinating insects. Heating up west-facing flowers with a portable heater brought more pollinators back to the flowers.
“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 said in a statement.
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The research was funded by the National Science Foundation’s Plant Genome Research Program.
