It is a well known fact that sunflowers trace the path of the sun across the sky, from east to west but the exact science behind how and why they move was explained in a recent paper published in the journal Science. In the study, the authors showed that the sunflower uses both circadian rhythms and heliotropism (bending and turning toward the light) to enhance its performance in the natural world.
“Just like people, plants rely on the daily rhythms of day and night to function,” said Anne Sylvester, the director of the National Science Foundation's Plant Genome Research Program that funded the study. “Sunflowers, like solar panel arrays, follow the sun from east to west. These researchers tap into information in the sunflower genome to understand how and why sunflowers track the sun.”
Stacey Harmer, a biologist and professor at University of California Davis, said in a press release, “At nighttime, you could see the whole plant rearranging itself, and it was such an amazing thing. 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.”
“A really common misconception is that mature sunflowers follow the sun, actually, they do not,” the Los Angeles Times reported Harmer saying. “Mature sunflowers always face east.”
The movement of sunflowers is more complex than a steady east-west motion. The plants are able to pace their movements — on short summer nights, it took them eight hours to move back to their eastward-facing morning position whereas during the longer autumn nights it took them 12 hours to cover the same.
“It’s the first example of a plant’s clock modulating growth in a natural environment, and having real repercussions for the plant,” said Harmer.
“The plant anticipates the timing and the direction of dawn, and to me that looks like a reason to have a connection between the clock and the growth pathway,” Harmer said.
To understand whether the sun’s rays had a key role in the sunflower’s movement, scientists carried out a series of experiments with sunflowers in fields, in pots outdoors and in indoor growth chambers. The young flowers continued to follow an imaginary sun’s path throughout the day even when exposed to a 30-hour light cycle.
These results suggested the role of not just the sun but also a circadian clock in influencing the sunflowers’ movements. According to Harmer, there is much more that remains to be learnt about these plants.