Why sunflowers dance: The code is cracked
As they grow, sunflowers “dance” to avoid blocking each other’s sunlight. A recent study sheds light on a scientific mystery that has puzzled researchers since Darwin
Sunflowers move quickly in a kind of dance that has puzzled scientists since Darwin’s time. A new study finds 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 of Tel Aviv University in collaboration with Prof. Orit Peleg of the University of Colorado Boulder in the USA.
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“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 – to avoid shading each other,” says Prof. Meroz.
“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 usually do not change their growth direction because they ‘know’ that it will not have any effect. But when they sense the shadow of a plant, they grow in a direction away from the shadow.”
The research results were published in the renowned journal Physical Review X.
In the current study, the researchers investigated how sunflowers “know” that they need to grow optimally to provide the community with as much sunlight as possible and analyzed the growth dynamics of sunflowers in the laboratory, where they exhibit a zigzag pattern.
Meroz and her team grew sunflowers in a high-density environment and photographed them as they grew, taking a photo 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.
According to the researchers (from Israel and the USA), Darwin was the first to recognise that all plants exhibit a kind of cyclic movement (‘circumnutation’) as they grow – both stems and roots exhibit this behaviour. However, 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 artefact or a critical feature of growth.
Why would a plant use energy to grow in random directions?
Prof. Meroz: “Sunflowers ‘dance’ to find the best angle so that each flower does not block the sunlight of its neighbour. We have quantified this movement statistically and shown through computer simulations that these random movements work together to minimise the amount of shadow.
“It was also very surprising that the distribution of the sunflower’s ‘steps’ was very large, 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,” she explains.
The sunflower takes advantage of the fact that it can use both small and slow steps and large and fast steps to find the optimal arrangement for the collective. This means that if the step size were smaller or larger, the arrangement would lead to more mutual shading and less photosynthesis.
This is a bit like a crowded dance party, says Meroz, where 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 one corner of the square is very crowded and the other side is empty. Sunflowers show a similar communication dynamic – a combination of responding to the shade of neighboring plants and random movements independent of external stimuli.
