Typically, plants grow in confined environments where neighboring plants shade each other and compete for light. The presence of neighbors varies over time and space, and plants have evolved the ability to recognize neighbors and grow out of their shadow. Although it is generally accepted that these responses help plants increase their individual light exposure, it is not clear how they manage to find mutually beneficial solutions. In a new study, physicists from Tel Aviv University and other universities focused on spontaneous self-organized pattern formation in sunflower groups mediated by shade avoidance. Their analysis found that circumnutations—inherent plant movements—cause random perturbations that correspond to a confined random walk.

Although circumnutations are ubiquitous in plant systems and are generally associated with exploratory movements, a quantitative understanding of their role is elusive. Nguyen et al. report for the first time on their role in promoting an optimal growth pattern for a dense group of mutually shading plants. Photo credit: Manuel H.

“Previous studies have shown that sunflowers planted densely in a field and shading each other grow in a zigzag pattern – one forward and one backward – to avoid shading each other,” said Professor Yasmine Meroz of Tel Aviv University.

“In this way, they grow side by side to maximize illumination from the sun and thus photosynthesis on a collective level.”

“In fact, plants know how to distinguish between the shadow of a building and the green shadow of a leaf.”

“When they sense the shadow of a building, they usually don’t change their growth direction because they know it won’t have any impact.”

“But when they sense the shadow of a plant, they grow in a direction away from the shade.”

In the study, researchers investigated how sunflowers “know” that they are growing optimally (i.e., maximizing sunlight for the community) and analyzed the growth dynamics of sunflowers in the laboratory, where they exhibit a zigzag pattern.

Professor Meroz and his colleagues grew sunflowers in a high-density environment and photographed them as they grew, taking a picture every few minutes. The images were then combined into a time-lapse movie.

By tracking the movement of each individual sunflower, the researchers observed that the flowers danced a lot.

According to the authors, Darwin was the first to realize that all plants exhibit a kind of cyclical movement (circumnutation) during their growth – both stems and roots exhibit this behavior.

But until now, with the exception of a few cases like climbing plants that grow in large circular movements to find something to hold on to, it was not clear whether this was an artifact or a critical growth trait. Why would a plant invest energy in growing in random directions?

“As part of our research, we conducted a physical analysis that captured the behavior of each individual sunflower within the sunflower group. We found that the sunflowers dance to find the best angle so that each neighboring flower does not block the sunlight,” said Professor Meroz.

“We have statistically quantified this movement and shown through computer simulations that these random movements are used together to minimize the amount of shadow.”

“It was also very surprising to find that the spread of the sunflower’s steps was very large, spanning three orders of magnitude, from a displacement of almost zero to a movement of two centimeters every few minutes in one direction or the other.”

“The sunflower plant takes advantage of the fact that it can use both small and slow steps and large and fast steps to find the optimal arrangement for the collective,” says Professor Meroz.

“This means that the smaller or larger the distance between the steps, the more mutual shading and lower photosynthesis will occur.”

“It’s a bit like a crowded dance party where the individual dancers dance around to get more space: if they move too much, they disturb the other dancers, but if they move too little, the crowding problem is not solved because it is very crowded in one corner of the square and empty on the other side.”

“Sunflowers show a similar communication dynamic – a combination of response to the shade of neighboring plants and random movements independent of external stimuli.”

The results were published in the journal Physical Examination X.

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Chantal Nguyen et al. 2024. Noisy circumnutations facilitate self-organized shade avoidance in sunflowers. Physical Examination X 14(3):031027; doi: 10.1103/PhysRevX.14.031027