Sunflowers “dance” to avoid blocking each other’s sun
A recent study sheds light on a scientific puzzle that has puzzled researchers since Darwin
Prof. Yasmine Meroz. (Photo: Tel Aviv University)
BOULDER, Colorado – A new study from Tel Aviv University 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 collective solution that helps them find optimal growth directions. In doing so, the study sheds light on a scientific puzzle that has puzzled researchers since Darwin, namely the functional role of these inherent movements, called circumnutations.
The research was conducted under the direction 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 Examination X.
Prof. Meroz explains: “Previous studies have shown that sunflowers planted densely in a field and shading each other grow in a zigzag pattern – one forward and one backward – so as not to shade each other. In this way, they grow side by side to maximize solar radiation and therefore photosynthesis at a collective level. In fact, plants know how to distinguish between the shadow of a building and the green shadow of a leaf. When they sense the shadow of a building, they do not usually 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 shade.
In the current study, the researchers investigated how sunflowers “know” how to grow optimally (i.e., maximize the uptake of sunlight for the whole) 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 as they grew, taking a picture every few minutes. The photos were then combined into a time-lapse movie. By tracking the movement of each individual sunflower, the researchers observed that the flowers “danced” a lot.
According to the researchers, Darwin was the first to recognize that all plants exhibit a kind of cyclical movement (“circumnutation”) as they grow—both stems and roots exhibit this behavior. But until now, with the exception of a few cases, such as climbing plants that grow in large circular movements to look for something to hold on to, it was not clear whether this was an artifact or a critical feature of growth. Why would a plant invest energy in growing in random directions?
Prof. Meroz: “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 flower does not block the sunlight of its neighbor. We quantified this movement statistically and showed through computer simulations that these random movements collectively serve to minimize the amount of shadow. It was also very surprising to find that the distribution of the sunflower’s “steps” was very broad, ranging over three orders of magnitude, from a displacement close to zero to a movement of two centimeters every few minutes in one direction or the other.”
In conclusion, Prof. Meroz adds: “The sunflower takes advantage of the fact that it can take both small and slow steps and large and fast steps to find the optimal arrangement for the collective. That is, if the step distance 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 the individual dancers 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 responding to the shade of neighboring plants and random movements independent of external stimuli.”
–Tel Aviv University
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