When the sun starts moving from east to west in the sky, the east side of the stem of a sunflower plant grows MORE rapidly than the west side. Due to this unequal growth on either side, the flower tends to bend in the direction of the sun.
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?
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.
Just like humans, plants also have internal biological clocks, which is referred to as a circadian rhythm. The circadian rhythm of a human being creates several physiological and chemical changes in the body. Similarly, the circadian rhythm of a plant allows it to respond to changes in roughly 24-hour cycles.
So how does a sunflower’s circadian rhythm play a role here?
How do sunflowers face the sun?
Before the break of dawn, a young sunflower faces East—towards the direction of the sunrise. As the sun moves from east to west, the flower turns westward as well. When the sun sets, the flower reverts to its original position, facing east, to begin the cycle again the next day.
According to an article published in the journal Science in 2016, researchers believe that sunflowers demonstrate this type of heliotropism due to their stems elongating at different rates at different times of day.
This is what researchers observed during their study: when the sun starts moving from east to west in the sky, the east side of the stem of a sunflower plant grows MORE rapidly than the west side. Due to this unequal growth on either side, the flower tends to bend in the direction of the sun.
Similarly, when the sun finally sets, the growth on the west side of the stem is greater than growth on the east. As a result of this, the stem bends east, that is, in the direction where the sun would rise again the next morning.
To make things even more interesting, researchers also tied the stem of the plant to a solid support so that they couldn’t move according to the sun’s position. In some cases, they turned the sunflower plants away from the sun. As a result, researchers observed that those plants had reduced biomass and less leaf area than the plants that were left undisturbed.
Sunflower plants’ response to light was also tested under artificial lighting conditions. According to the article published in Science, it was noted that the plants “could reliably track the movement and return at night when the artificial day was close to a 24-hour cycle, but not when it was closer to 30 hours.”
The stem of a sunflower plant experiences unequal growth at either side because of AUXINS—plant hormones that stimulate growth.
A mature sunflower DOESN’T track the sun
However, as a sunflower plant matures, it behaves differently. According to the co-author of the study published in Science. “As the overall growth of the plant slows down gradually, the circadian rhythm ensures that the plant reacts more strongly to sunlight early in the morning than the afternoon or evening. This is why a mature sunflower doesn’t move with the sun throughout the day; rather, it just faces east.”
Well, now we know how the sunflowers follow the Sun, but now it’s time to find out the reason behind this peculiar behavior.
Why do sunflowers face the Sun?
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.
When researchers compared mature flowers facing east with flowers that they had turned to face west, they made a remarkable observation. East-facing blooms attracted five times as many pollinating insects than west-facing ones.
A sunflower has a clear advantage in terms of reproduction if it faces the sun.
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.