A young sunflower
Sunflowers are known for their unique tracking ability. As they grow, young sunflowers follow the sun’s motion from east to west from sunup to sundown. Come nightfall, the flowers pivot 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 until now, no one knew how the flowers did it.
Now comes a new study published this week in the journal Science that suggests that young sunflowers, just like people, are guided by circadian rhythms. These internal ‘body clocks’, which help regulate the plant’s growth, are set to respond to environmental cues. Moreover, they are the chief mechanism behind the plant’s unique tracking behavior.
“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 KNOW HOW TO 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 adjust their behavior accordingly.
According to Harmer, scientists uncovered this astonishing fact by using a time-lapse video to observe 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.
(Harmer notes that the research applies to young flowers only. These are the only ones that follow this circadian rhythm. Once flowers are mature, or the yellow petals have unfurled, the flowers remain in place.)
THEY TRICKED THE PLANTS
To determine what made young sunflowers follow this circadian rhythm, scientists conducted a series of experiments using plants in the field, in pots outdoors and in indoor growth chambers. In the field, they purposely turned potted plants to face west in the morning to disrupt the sunflower’s tracking ability.
Scientists discovered that 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 developing into smaller plants. Moreover, they had smaller leaves and about 10 % less biomass.
Young sunflower in the field
In a second series of experiments, the scientists moved young sunflowers (in pots) into an indoor growth chamber with a fixed overhead light. For several days, the flowers continued their daily rotation which, according to Harmer, is behavior typical of a mechanism driven by an internal clock.
The sunflowers officially started tracking “the sun” again when scientists created an artificial 24-hour day. They did this by turning adjacent lights on and off in an arc to replicate natural conditions. However, when the scientists stretched the artificial day to 30 hours, the plants could no longer reliably track the artificial sun’s movement.
THE SUNFLOWER DOESN’T ROTATE, THE STEM DOES
But what is the actual mechanism rotating the sunflower? It turns out that it isn’t the flower at all, but the stem.
By placing ink dots on the stems and filming them with a video camera, scientists found that as the plant turned to follow the sun, the east side of the stem grew more rapidly than the west side. And at night the reverse was true — the west side grew more rapidly as the plant swung the other way. The researchers concluded that this rhythmic tracking helped the sunflowers absorb as many photons from the sun as possible. This in turn helped maximize the amount of organic molecules the sunflowers used as food.
ONCE THEY GROW OLD, SUNFLOWERS FACE EAST
Finally, another series of experiments revealed that once the sunflower matures and the flower opens up, it finishes its tracking and remains in an east-facing position. This gives it a distinct advantage.
In the morning, east-facing flowers heat up more quickly. And studies show that given a choice, pollinators spend longer time on individual flowers if they are warm.
Fascinating to think about next time you see a field of sunflowers –