TEL AVIV, Israel (Press Release) — A new study from Tel Aviv University (TAU) in collaboration with the University of Colorado, Boulder, has found that plants growing in dense environments where each plant casts shadows on its neighbors use random movements to find a common solution that helps them find optimal growth directions. The research sheds light on a scientific puzzle that has puzzled researchers since Darwin.

The research was led by Professor Yasmine Meroz of the School of Plant Sciences and Food Security at TAU’s George S. Wise Faculty of Life Sciences in collaboration with Professor Orit Peleg of the University of Colorado, Boulder. The research team included Dr. Chantal Nguyen of Boulder and Roni Kempinski and Imri Dromi of TAU. The research was published in the journal on August 15, 2024 Physical Examination X.

“Previous studies have shown that sunflowers planted closely together in a field and shading each other grow in a zigzag pattern – one forward and one backward – so that they do not shade each other,” explains Professor Meroz. “In this way, they grow side by side to maximize solar radiation and therefore photosynthesis on a collective level.

“In fact, plants can distinguish between the shadow of a building and the green shadow of a leaf. When they sense the shadow of a building, they usually do not change their growth direction because they ‘know’ it will have no effect. But when they sense the shadow of a plant, they grow in a direction away from the shadow.”

The researchers investigated how sunflowers know how to grow optimally to obtain as much sunlight as possible for the community. They also analyzed the growth dynamics of sunflowers in the laboratory, where they exhibit a zigzag pattern.

Professor Meroz and her team grew sunflowers in a high-density environment and photographed them as they grew, taking a picture every few minutes. The photos were then stitched together to create a time-lapse movie. By tracking the movement of each individual sunflower, the researchers observed that the flowers “danced” a lot.

Charles Darwin was the first to recognize that all plants exhibit some sort of cyclical movement as they grow, the researchers say. Both stems and roots exhibit this behavior. But before this study, it was not clear whether this was a mere artifact or a crucial feature of growth. 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 collective. We found that the sunflowers ‘dance’ to find the best angle so that each individual flower does not block the sunlight of its neighbor,” says Professor Meroz. “We quantified this movement statistically and showed through computer simulations that these random movements are used together to minimize the amount of shadow.”

“The sunflower plant takes advantage of the fact that it can make both small and slow steps and large and fast ones to find the optimal arrangement for the collective. That is, if the step area were smaller or larger, the arrangement would result in more mutual shading and less photosynthesis. This is a bit like a crowded dance party where individuals 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 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 and random movements independent of external stimuli,” concludes Professor Meroz.

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The foregoing provided by Tel Aviv University.