Does a Sunflower Wait for the Sunrise?
Have you ever wondered whether a sunflower has some kind of tiny internal engine that helps it keep up with the Sun? Or have you been a little disappointed when you looked at a sunflower and realized it wasn't following the Sun across the sky? What happened?
Well, both are pretty good questions. Young sunflowers really do follow the Sun. When they start to grow, they face east in the morning. As the day progresses, they gradually turn westward with the moving Sun. Then, during the night, they reorient themselves, turning back toward the east in anticipation of dawn. By morning, they are ready to begin again. Isn't that incredible?
Scientists call this heliotropism, the movement of a plant in response to light. Solar tracking in developing sunflowers is described as "one of the most conspicuous diurnal rhythms observed in plants." The word heliotropism has a long history too. It was introduced by Augustin Pyramus de Candolle and later used by Charles Darwin to describe plant movement in response to light.
Mature sunflower heads in a field. Photo credit: abc.net.au
And that tiny "engine"? It isn't an engine at all, but something just as fascinating: an internal biological clock, known as a circadian rhythm, helps coordinate the movement. The stem grows differently on its east and west sides, with the pattern shifting as the Sun moves across the sky. This uneven growth causes the young plant to bend, allowing it to track the Sun during the day and return eastward at night.
As the sunflower matures and its flower opens, the back-and-forth movement stops. The mature flower settles into a stable east-facing position. Researchers found that these east-facing flowers are warmer than west-facing ones, and that this warmth attracts more pollinators. What began as movement during growth becomes a fixed orientation when the plant reaches a new stage of its life.
The sunflower's intelligence is in how its systems work together. An internal clock, environmental signals, and patterns of growth are coordinated with the predictable movement of the Sun. The young plant moves when movement is useful, then stops when a stable east-facing position better supports reproduction.
So, the next time you look at a sunflower and wonder why it isn't following the Sun, you might be looking at an older, wiser sunflower. And if you ever wondered whether a young sunflower has a tiny engine inside helping it keep up with the sky, you weren't completely off. There is no engine, of course, but there is an internal clock, a changing pattern of growth, and a remarkable ability to coordinate with the world around it. The sunflower has an amazing ability to know when to move, when to stop, and when to change the pattern.
Sunflowers facing East. Photo credit: Agota Jonas, circa 1991
Sources
J.P. Vandenbrink, Turning heads: The biology of solar tracking in sunflower, Plant Science, Volume 224, July 2014, pg 20-26
H. S. Atamian, Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits, Science, 5 Aug 2016, Vol 353, Issue 6299, pg. 587-590
