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 a scientific puzzle 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, 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 (Noisy Circumnutations Facilitate Self-Organized Shade Avoidance in Sunflowers).
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, and 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.
In the current study, the researchers examined the question of how sunflowers “know” to grow in an optimal way (i.e. maximize capture of sunlight for the collective) and analyzed the growth dynamics of the sunflowers in the laboratory, where they exhibit a zig-zag 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 following the movement of each individual sunflower, the researchers 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 (“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?
Prof. Meroz: “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.”
In conclusion, Prof. Meroz adds: “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.”
