“No, sunflowers do not face each other on cloudy days because they follow the circadian rhythm of heliotropism.”
There is a misconception among a lot of people that on cloudy day’s sunflowers face each other to harness the mutual energy. But according to a scientific study conducted by the Department of Plant Biology, University of California, sunflowers follow the circadian rhythm of heliotropism even in the absence of light.
Let’s try to understand how sunflower tracks sun during day and reset its position during night time.
Heliotropism in sunflowers
We all know that sunflower turns their face towards the sun and track it during the day time. This form of the diurnal motion of sunflower in response to the direction of the sun is known as Heliotropism. Flowers that track sun are also known as heliotropic or phototropic flowers.
In this dynamic form of phototropism, photoreceptor cells present in the shoot apices of the sunflower stem help the plant to detect the source of light, and then the motor cells present just below the flower (called pulvinus) helps to bend towards the light. During the dawn, the plant faces the east side and then slowly shift towards the west side till dusk as they track the sun. During the night time, they slowly reorient themselves towards to east side.
Along with flowers, heliotropism can be found in leaves as well. The plant leaves orient themselves perpendicular to the sunrays and tracks the sun during the day time. This phenomenon is known as leaf heliotropism.
Benefits of heliotropism
Here are some of the benefits of heliotropism in plants
- According to the study based on the heliotropism of sunflower, it is found that the warm flowers due to heliotropism received more pollinator’s visit as compared to flowers not facing the sun.
- Heliotropism also promotes plant growth by aligning their photosynthetic organs with the direction of the sunlight.
Therefore, heliotropism promotes plant growth and reproductive success by increasing pollination, fertilization, and seed development.
Effect of cloudy weather on heliotropism
It is found that sunflowers follow the circadian rhythm even if there is not lack of sunlight or cloudy weather. If the sunflower plant is kept under such conditions for many days, then these rhythmic movements dampened over time.
It is interesting to note that the sunflower does not track the sunlight when it gets matured. Heliotropism is found only in immature and young sunflower plants. Fully matured sunflower plants generally stay in one direction mainly east direction. It is also found that in the morning hours, the flowers facing the east side face five times more pollinators than the flowers facing the west side.
Therefore, it is just a myth that during cloudy days or unclear sky, sunflowers face each other. In these conditions also, sunflowers track the sun during the day time and reorient during the night towards the east side.
Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits, Hagop S. Atamian, Nicky M. Creux, Evan A. Brown, Austin G. Garner, Benjamin K. Blackman, Stacey L. Harmer