Sunflower ‘Dancing’ Mystery Solved: New Israeli Study Explains Darwin’s Puzzle

Prof. Yasmine Meroz with Sunflowers (Tel Aviv University)

A joint study by Tel Aviv University and the University of Colorado, Boulder, has revealed that plants, such as sunflowers, that grow in crowded conditions, where each casts a shadow on its neighbor, employ a collaborative strategy to optimize their growth. By engaging in random movements, these plants collectively find optimal growth directions. This research offers valuable insights into a long-standing scientific mystery, the functional role of circumnutations, which are inherent plant movements that have intrigued scientists since Darwin’s time.

The research was conducted under the leadership of Prof. Yasmine Meroz from the School of Plant Sciences and Food Security, Wise Faculty of Life Sciences at Tel Aviv University, in collaboration with Prof. Orit Peleg from the University of Colorado Boulder in the USA. The research team included Dr. Chantal Nguyen (Boulder), Roni Kempinski and Imri Dromi (TAU). The research was published in the prestigious journal Physical Review X.

“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,” explained Prof. Meroz.

In this way, they grow side by side to maximize illumination from the sun, and therefore engage in photosynthes, on a collective level. In fact, plants actually know how to distinguish between the shadows cast by different kinds of objects such as those 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 can sense that doin so will have no effect. But if they sense the shadow of a plant, they will grow in a direction away from the shadow.

In their study, the researchers investigated how sunflowers are able to instinctively optimize their growth for maximum sunlight absorption, benefiting the collective. They analyzed the growth patterns of sunflowers in a laboratory setting, where they exhibit a distinctive zig-zag pattern. Professor Meroz and her team cultivated sunflowers in a high-density environment and captured their growth process through time-lapse photography, taking pictures every few minutes. By tracking the movement of each individual sunflower, the researchers discovered that the flowers were engaged in a dynamic and coordinated dance.

According to the researchers, Darwin was the first to recognize that all plants grow while exhibiting a kind of cyclical movement (“circumnutation”) – both stems and roots show this behavior. But until today, – 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 an artifact or a critical feature of growth. Why would a plant invest energy to grow in random directions?

The researchers said they conducted a physical analysis that “captured” the behavior of each sunflower within the sunflower collective, and 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,” said Prof. Meroz. 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.”

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,” Prof. Meroz explains. “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.”