Sunflowers dance together to balance sunlight and shade, Israeli researchers say
Tel Aviv University team explains how the plants grow close together and move in zigzag patterns for optimal growth in a collective ‘dance party’
Reporter at The Times of Israel
Researchers at Tel Aviv University say they have discovered that sunflowers growing together in dense environments move in a zigzag pattern as if they’re dancing, not only to get more sunlight but also to avoid blocking the sunlight of their neighbor.
The discovery sheds light on a scientific idea that Charles Darwin first explored 200 years ago when he observed that plants moved in circular movements, called circumnutations.
The study, led by Prof. Yasmine Meroz from Tel Aviv’s School of Plant Sciences and Food Security and Prof. Orit Peleg from the University of Colorado Boulder, along with other researchers, was published last week in the peer-reviewed journal Physical Review X.
Sunflowers at a ‘dance party’
In describing how sunflowers moved in their experiment, Meroz and Peleg — friends who met while doing a postdoctorate at Harvard University a few years ago — told The Times of Israel by telephone to “imagine a party where it’s very, very crowded.”
If nobody moves, said Meroz, “then you’re stuck in some places which are kind of empty, and other places, like next to the buffet, where it’s very, very crowded.”
But if “you dance just the right amount,” she said, “then you will have enough space.”
The experiment showed that sunflowers “seem to wiggle.”
One flower will “go to the right, another to the left, in a zigzag pattern,” the researchers said, moving in circumnutations that seem to be coordinated so that a crowded group of plants can move about “collectively to find more light.”
Why do plants move?
The researchers said their work was inspired by research done on sunflowers grown closely together in a field. The flowers perceive light signals from their immediate neighbors and change their positions for optimal growth of the crowded group.
Most people notice that climbing plants grow in circular movements around a beam, for example, as a way for them to find new support to grow higher.
But it wasn’t clear why other plants made this circular movement, first noticed by Darwin.
Meroz and Peleg decided to conduct a controlled experiment on sunflowers in a lab.
They grew five sunflowers close together in a row and followed their movements by taking a time-lapse photo every couple of minutes over a week to 10 days to make a movie where the sunflowers seemed to be wiggling in a zigzag pattern like in a dance.
“We saw that the plants moved a lot — a lot more than we thought they would,” Meroz said.
“The idea came to us that these circumnutations might have a function in helping these plants find “an optimal configuration, where all of them have enough sunlight,” Meroz said.
Statistical physics
Peleg, who merges tools from physics, biology, engineering and computer science to understand the behavior of what seems like “disordered living systems,” wanted to know whether the sunflowers were moving randomly or whether their movements were synchronized.
To get the answer, she used models from what is known as statistical physics.
In simple terms, she said, random motion refers to a movement that appears erratic, “like dust particles floating aimlessly in a beam of sunlight.”
She found it fascinating that this phenomenon was first observed by a botanist, Robert Brown, who noticed how “pollen moves randomly in water.”
“Although such motion seems unpredictable, physicists like Albert Einstein developed mathematical formulas to predict the average behavior of large groups of such particles,” Peleg said. “This is why it’s called ‘statistical physics.’”
In the experiment, the researchers built a model to determine the optimal amount the plants would need to move to give one another enough sunlight.
“We quantified this movement statistically and showed through computer simulations that these random movements are used collectively to minimize the amount of shadow,” Meroz said.
She said it was also “very surprising” to find that the distribution of the sunflower’s “steps” was very wide in one direction or another.
Peleg said the plants found a “sweet spot” where they moved just the right amount so that the group balanced sunlight and shade.
She said that plants have memory, and, like other organisms, use seemingly random movements to navigate and send signals.
“If a dog is sniffing a trail, it will usually move its head right and left, and compare what it smells here and there, and in this way, it will find the trail,” Peleg said. “So one question we can ask is, do plants do the same thing? They move their heads, so to speak, to the right and the left.”
She also said that the sunflowers’ behavior is similar to the collective behavior of birds or fish.
“It’s mathematically very similar,” she said. “The question people like to ask is, how is information moved across the system?”
Meroz said that she hoped the experiment — which they call “a feat of friends” — would get people to look at plants differently.
“If we lived in the time scale of a plant and moved very, very slowly, then we can imagine walking in the street and seeing the plants moving around you,” she said. “You probably would think about them in a very different way. I think they would look different to us as humans.”
