Scientists find out why sunflowers dance when they grow

The study sheds light on a scientific puzzle that has occupied researchers since the days of Charles Darwin.

Researchers discovered that plants that grow in dense environments - where each plant casts a shadow on its neighbor - find a "collective solution" with the help of random movements that help them find the best growth directions.

The study by scientists at Tel Aviv University in Israel, working with colleagues at the University of Colorado, Boulder, in the United States sheds light on a puzzle that has occupied researchers since Darwin, namely the functional role of such inherent movements called circumnutations.

Study leader Professor Yasmine Meroz, from the School of Plant Sciences and Food Security at Tel Aviv University, said: "Previous studies have shown that if sunflowers are densely planted in a field where they shade each other they grow in a zigzag pattern - one forward and one back - so as not to be in each other's shadow.

"This way they grow side by side to maximize illumination from the sun, and therefore 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.

"If they sense the shadow of a building - they usually don’t change their growth direction, because they 'know' that will have no effect.

"But if they sense the shadow of a plant, they will grow in a direction away from the shadow."

For the study, published in the journal Physical Review X, researchers examined the question of how sunflowers “know” to grow in an optimal way.

The team analyzed the growth dynamics of the sunflowers in the laboratory, where they exhibit a zig-zag pattern.

Meroz and her colleagues grew sunflowers in a high-density environment and photographed them during growth, taking pictures every few minutes.

The photographs were then combined to create a time-lapse movie.

By following the movement of each individual sunflower, the team observed that the flowers were "dancing" a lot.

According to the researchers, Darwin was the first to recognize that all plants grow while exhibiting a kind of cyclical movement or “circumnutation” - both stems and roots show such behavior.

But until now - except for a few cases such as climbing plants, which grow in huge circular movements to look for something to grab onto - it was not clear whether it was a critical feature of growth.

Meroz said: "As part of our research, we conducted a physical analysis that captured the behavior of each sunflower within the sunflower collective, and we saw that the sunflowers 'dance' to find the best angle so each flower would not block the sunlight of their neighbor.

"We quantified this movement statistically and showed through computer simulations that these random movements are used collectively to minimize the amount of shadow.

"It was also very surprising to find that the distribution of the sunflower's 'steps' was very wide, ranging over three orders of magnitude, from close to zero displacement to a movement of two centimeters every few minutes in one direction or another."

Meroz added: "The sunflower plant takes advantage of the fact that it can use both small and slow steps as well as large and fast ones to find the optimum arrangement for the collective.

"That is, if the range of steps was smaller or larger the arrangement would result in more mutual shading and less photosynthesis.

"This is somewhat like a crowded dance party, where individuals dance around to get more space: if they move too much they will interfere with the other dancers, but if they move too little the crowding problem will not be solved, as it will be 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, along with random movements regardless of external stimuli."