A study found that the erratic movements of sunflowers help them locate sunlight, providing insights into plant behavior and potential agricultural benefits.
In a new study, physicists from the United States and Israel may have gotten to the bottom of a strange behavior in growing plants—and a mystery that intrigued Charles Darwin himself during the later decades of his life.
To many people, plants can seem stationary and even a little boring. But the green stuff actually moves a lot. For example, if you watch a time-lapse video of a sunflower sprout rising from the ground, it doesn’t just shoot straight up. Instead, as the sunflower grows, its crown spins in circles, twists into corkscrews, and generally wiggles around—albeit very slowly.
Now, researchers co-led by Orit Peleg at CU Boulder and Yasmine Meroz at Tel Aviv University have discovered a role for these chaotic movements, also known as “circles.” In greenhouse experiments and computer simulations, the group showed that sunflowers take advantage of enclosures to search the environment around them for spots of sunlight.
“Many people don’t really consider plant movement because, as humans, we usually look at plants at the wrong frame rate,” said Peleg, a co-author of the study and an associate professor at BioFrontiers. Institute and Department of Computer Science.
The team published its findings on August 15 in the journal Physical Review X.
The findings could one day help farmers devise new strategies for growing a variety of crops in more efficient arrangements.
“Our team does a lot of work on social interactions in swarms of insects and other animal groups,” said Chantal Nguyen, lead author and a postdoctoral researcher at BioFrontiers.
“But this research is particularly exciting because we are seeing similar dynamics in plants. They are rooted in the earth.”
Darwin’s cucumbers
Nguyen added that plants typically don’t move like animals, but instead move by growing in different directions over time. This phenomenon fascinated Darwin long after he returned from his voyage on the HMS Beagle, according to historical accounts.
In the 1860s, Darwin, then suffering from a series of ailments that limited his mobility, spent days observing plants in his home. He planted seeds from cucumbers and others speciesthen tracked how their crowns moved from day to day—the resulting maps look wild and random.
“I’m having a lot of fun with my needles—it’s a clumsy sort of work that suits me,” he wrote a friend in 1863.
Amused or not, Darwin could not explain why some of his threads twisted.
It’s a mystery that has puzzled even Meroz, a physicist by training. A 2017 study pointed him in the right direction. In it, scientists led by the University of Buenos Aires grew sunflower lines in cramped conditions. They found that the plants naturally and consistently arranged themselves in a zig-zag pattern, almost like the teeth of a chain. The arrangement likely helps plants maximize their access to sunlight as a group.
Meroz wondered if plant movements might be the engine that drives such patterns in plant growth.
“For climbing plants, it’s clearly a matter of looking for props to twine,” said Meroz, a professor of plant sciences and food safety. “But for other plants, it’s not clear why it’s worth it.”
Here comes the sun
To find out, she and her colleagues grew five week-old sunflowers in rows. Then, like Darwin before them, they charted how plants moved over the course of a week.
Next, Nguyen and Peleg developed a computer program to analyze the patterns behind sunflower growth. The researchers could also use their computer simulations to see what would happen if the sunflowers moved more or less—in other words, if they moved randomly or in a slow, steady pattern.
If the digital plants weren’t shaken at all, the group found, they would all wind up leaning away from each other in a straight line. If they moved too much, in contrast, they would grow in a random pattern. However, if they moved with just the right amount of randomness, the sunflowers formed that telltale zig-zag, which, in real-life plants, provides plenty of access to sunlight. Nguyen explained that plants seem to circle around to find where the best light is coming from, then grow in that direction.
“When you add some noise to the system, it allows the plant to explore its surroundings and settle into those configurations that allow each plant to find maximum light exposure,” she said. “This happens to lead to this beautiful zig-zag pattern that we see.”
In future experiments, the researchers will test how sunflowers grow in more complicated arrangements. Meroz, for her part, is happy to see plants get some credit for the movers and shakers they really are.
“If we all lived at the same time as scales with plants, you could walk down the street and see them moving,” she said. “Maybe we’d all have plants as pets.”
Reference: “Noisy surrounds facilitate self-organized shadow avoidance in sunflower” by Chantal Nguyen, Imri Dromi, Ahron Kempinski, Gabriella EC Gall, Orit Peleg, and Yasmine Meroz, 15 Aug 2024, Physical Review X.
DOI: 10.1103/PhysRevX.14.031027