As they grow, sunflowers “dance” to avoid blocking each other’s sunlight

Flowers have long fascinated scientists and nature enthusiasts alike, not just for their beauty, but also for their subtle, almost imperceptible movements. Over a century ago, Charles Darwin was the first to observe that plants, including flowers, exhibit a kind of cyclical movement as they grow. This movement, seen in both stems and roots, puzzled researchers: Was it just a byproduct of growth, or did it serve a crucial purpose?

A new study by Tel Aviv University, in collaboration with the University of Colorado, Boulder, 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 optimal growth directions. In this way, the study sheds light on the scientific enigma that has occupied researchers since Darwin, namely the functional role of these inherent movements called circumnutations.

The research was conducted under the leadership of Prof. Yasmine Meroz from the School of Plant Sciences and Food Security at the 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.

Do flowers have a sense of direction?

Prof. Meroz explains: “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, therefore photosynthesis, on a collective level. 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”.

According to the researchers, Darwin was the first to recognize that all plants grow while exhibiting a kind of cyclical movement known as “circumnutation”, which is observed in both stems and roots. However, until today—except for a few cases, such as climbing plants that grow in large circular movements to find something to grab onto—it was unclear whether this was an artifact or a critical feature of growth. Why would a plant invest energy to grow in random directions?

In the current study, the researchers examined how sunflowers “know” to grow optimally—maximizing sunlight capture for the collective—and analyzed the growth dynamics of sunflowers in the laboratory, where they exhibit a zigzag pattern. Prof. Meroz and her team 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 tracking the movement of each sunflower, the researchers observed that the flowers were “dancing” a lot.

Shake your Tail Petal

Prof. Meroz stated, “As part of our research, we conducted a physical analysis that captured the behavior of each sunflower within the collective, revealing that the sunflowers ‘dance’ to find the optimal angle, ensuring that each flower does not block the sunlight of its neighbor. We quantified this movement statistically and demonstrated through computer simulations that these random movements are used collectively to minimize shadowing. It was also surprising to find that the distribution of the sunflowers’ ‘steps’ was very wide, ranging over three orders of magnitude, from nearly zero displacements to movements of up to two centimeters every few minutes in various directions”.

In conclusion, Prof. Meroz adds: “The sunflower plant takes advantage of its ability to use both small, slow steps and large, fast ones to find the optimal arrangement for the collective. If the range of steps were smaller or larger, the arrangement would result in more mutual shading and less photosynthesis. It’s somewhat like a crowded dance party, where individuals move around to create more space: if they move too much, they’ll interfere with the other dancers, but if they move too little, the crowding problem won’t be solved, leaving one corner of the square overcrowded and the other empty. Sunflowers exhibit a similar communication dynamic—a combination of responding to the shade of neighboring plants and making random movements regardless of external stimuli”.