It’s well-known that the sunflower (Helianthus annuus) follows the sun: its flower buds face east in the morning, south at noon and west in the afternoon, then return to their original position overnight. When the inflorescence finally opens, however, all movement ceases and the flower remains in an eastward orientation. But why?
Almost all living things, from germs to plants to mammals, follow a timetable of about 24 hours. This is called circadian rhythm and is a sort of biological clock. In its simplest expression, the clock tells the cells when to wake up and when to sleep, but its effect is much more extensive than that. In the case of the sunflower, in particular, it tells the stem how to grow.
A Stem That Grows at Different Speeds
During the day, beginning in the morning, the sunflower stem elongate more rapidly on the shady side than on the sunny side, which makes the stem tilt in the opposite orientation. As the day progresses, shade moves eastward and so does the part of the stem that elongates fastest, pushing the bud westward. As a result, the flower bud follows the sun.
When the sun sets, the stem changes tactics and elongates mostly on the west side and this continues all night, thus pushing the flower bud back to its original position facing east.
So that’s how it’s done… but what advantage does the plant find in this movement?
More Sun, More Energy
Before it opens, the inflorescence is covered with green leaf-like sepals that, like all the green parts of the plant, carry out photosynthesis. By always facing the sun, they help the plant gain more energy. When a sunflower stem is attached to a stake to prevent it from moving, this reduces the plant’s biomass, a proof that photosynthesis was not as effective in providing storable energy as in a free-moving sunflower.
When sunflowers are subjected to 30-hour days under laboratory conditions, this totally upsets their circadian rhythm: they lose the ability to redirect their flower buds eastward overnight.
Flowers are For Bees
But why then does the stem stop moving and remain facing east once the flower opens?
As the inflorescence expands and yellow ray flowers open, the green sepals, once out front, are pushed back behind the flower so they are largely hidden from the sun and therefore become less useful as a source of energy.
However, a flower that faces the morning sun warms up faster than a flower facing any other direction. And bees and other pollinating insects, being cold-blooded, need extra warmth in the morning to overcome the chilliness still in the air. That’s why, in the morning, when the air is still cool, they tend to pollinate flowers oriented to the east over those facing any other orientation and to stay on them for a longer time.
One study showed that, in total, about five times more pollinators visited east-facing sunflowers than sunflowers that were staked to face other directions.
Later in the day, when the air is warmer, insects become indifferent to the position of flowers, but its eastward-facing habit has already given the the sunflower a clear advantage over the other flowers in its surroundings in the battle to attract pollinators.
And that is why sunflowers follow the sun!