Why do sunflowers always follow the Sun?

Sunflowers are known for their unique behavior of tracking the Sun across the sky, a phenomenon that has intrigued scientists for years. A recent study conducted by plant biologists at the University of California at Davis, published in PLOS Biology, has shed new light on this fascinating behavior.

Sunflowers follow the sun because of a property called phototropism. The sunflower plant contains a hormone named Auxin which is sensitive to the sun-rays. This chemical accumulates on the shaded part of stem and boosts the growth of cells on the shaded part of stem.

The stems of young sunflowers grow more during the night, but only on the west side which allows the sunflower head to bend eastward. During the day, the stem's east side grows, resulting in a swaying motion to the west with the sun. 

Sunflowers face the rising sun because increased morning warmth attracts more bees and also helps the plants reproduce more efficiently. This allows the mature plants to cross pollinate with neighboring flowers much easier. 

Sunflowers Surprise: Unique sun-tracking Mechanism

Contrary to previous assumptions, the researchers discovered that sunflowers follow the sun using a completely different mechanism. People previously believed that, like most plants, sunflowers exhibit phototropism - the ability to grow towards a light source. Typically, a molecule called phototropin, which responds to light at the blue end of the spectrum, governs this process.

However, the study revealed that sunflowers’ heliotropism, or the ability to track the Sun, does not rely on this mechanism. “This was a total surprise to us,” admitted Stacey Harmer, a professor of plant biology at UC Davis and the lead author of the paper. The exact mechanism that sunflowers use to track the Sun remains a mystery, opening up new avenues for further research in plant biology.

Sunflowers exhibit a unique movement pattern where they adjust their tops by growing slightly more on the east side of the stem during the day, causing them to tilt towards the west. At night, they grow a bit more on the west side, which makes their tops turn towards the east.

The lab of Stacey Harmer at the UC Davis School of Biological Sciences had previously discovered how sunflowers use their internal circadian clock to predict sunrise. This allows them to synchronize the blooming of their flowers with the arrival of pollinating insects in the morning.

In a recent study, graduate student Christopher Brooks, postdoctoral researcher Hagatop Atamian, and Harmer investigated the genes that were activated in sunflowers. They conducted their research on sunflowers grown indoors in laboratory growth chambers as well as those grown outdoors under natural sunlight. The findings from this study contribute to our understanding of the fascinating behavior of sunflowers.

It is not phototropism

Those planted indoors grew sunflowers directly towards the light, activating genes associated with phototropin. However, outdoor-grown plants, which moved with the sun, showed a completely different pattern of gene expression. There was no apparent difference in phototropin between one side of the stem and the other.

Researchers haven't identified the genes involved in heliotropism, the tendency to follow the sun, yet. "We seem to rule out the phototropin pathway, but we didn't find clear evidence," said Harmer.

"The scientists tested possible responses by blocking blue, ultraviolet, red or far-red light with shadow boxes, but it had no effect on the heliotropism response."

This indicates that multiple pathways likely exist, responding to different wavelengths of light, to achieve the same goal. The forthcoming works will analyze the regulation of proteins in plants.

Sunflowers quickly learn. Harmer said that the lab-grown plants began following the sun on the first day when they moved them outside. He stated that a burst of gene expression accompanied this behavior on the shaded side of the plant that did not repeat in the following days. He suggested that this shows some type of "rewiring" is occurring.

In addition to revealing previously unknown pathways for light sensing and plant growth, the discovery has great relevance, Harmer said. "Things that are defined in a controlled environment like a growth chamber may not work in the real world," he said.

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