Sunflowers are fascinating little specimens of nature, and they allow us to see first-hand how plants are not the static beings we think them to be. These special flowers actually face the sun as it rises in the east and follow it across the sky until it sets in the west. Why does this happen? Are sunflowers simply desperate to get a tan?
Heliotropism – How do sunflowers face the Sun?
This property of facing the sun is mostly observed in young flowerheads and generally stops once the flower starts to bloom (mature sunflowers generally face east). The fascinating phenomenon of flowers following the sun across the sky is called heliotropism.
The question is, what makes them do this?
Plants are known to synchronize themselves with the light in their surroundings. This is known as the Circadian rhythm. They are generated by the plants themselves and are self-sustaining. There are different ways of how the plants carry it out and how they show it as well. Sunflower has its own way and that way is using a certain hormone.
The sunflower plant contains hormones called auxins. These hormones are sensitive to sunlight and do everything they can to seek shade! Therefore, they migrate from the part of the plant bathed in sunlight to the shaded region in the stem. Once there, the auxins (which are essentially growth hormones) stimulate the growth of cells. This causes the stem to become bulkier in the shaded region, so the flower ends up bending in the opposite direction – towards the Sun!
As the Sun moves across the sky, the auxins also continue to migrate to the part of the stem that is shaded, continually making the shaded part bulkier by stimulating cell growth. That’s how the sunflower continues to face the Sun – constantly bending away from the bulkier part of the stem.
Why do sunflowers face the Sun?
Well, we know how the sunflowers follow the Sun, but now it’s time to find out the reason behind this peculiar behavior.
As stated above, it is mainly young flowerheads that exhibit this property of heliotropism. This is because younger flowers have green “bracts”, which basically look like a mane; the plant also has leaves just below the flower that face the Sun. The obvious reason for the flower following the Sun at this stage would be to maximize photosynthesis.
Each sunflower plant has only one flower on its stem. Therefore, during pollination, it is essential that the plant’s only means of reproducing gets noticed by pollinators (mainly insects). Continuously, facing towards east also helps the flowers to heat up quickly. This gives them an advantage in pollination as warm flowers attract insects. Therefore, it’s in the plant’s best interest that the flower always faces the Sun, so it is always highly visible to these important pollinators.
Mature sunflowers finally stop displaying heliotropism when they start to develop seeds and therefore droop from the weight of these seeds. They end up mostly facing east from this point in their lifecycle on.
Although it would be amusing if suntanning was what these beautiful flowers were after, but the prospect of photosynthesis and getting noticed by pollinators makes a bit more sense.