Many of us probably first got to know of heliotropism at home, kindergarten or primary school by watching the enormous yellow and black flowering heads of aptly name sunflowers, which moved as they grew. These flowers track the course of the sun spectacularly on warm and sunny, spring or summer days. Sometimes they move through an arc of almost 180 degrees from morning to evening.
The mechanics of tracking the sun
A number of flowering species display heliotropism but why do they do it? Flowers are really in the advertising game and will do anything they can to attract a suitable pollinator, as effectively and as efficiently as they can. There are several possible reasons why tracking the sun might have evolved to achieve more successful pollination.
By tracking the sun, flowers absorb more solar radiation and so remain warmer. The warmer temperature suits or even rewards insect pollinators that are more active when they have a higher body temperature.
Optimum flower warmth may also boost pollen development and germination, leading to a higher fertilisation rate and more seeds. So, the flowers are clearly moving. But how?
For many heliotropic flowering species, there’s a special layer of cells called the pulvinus just under the flower heads. These cells pump water across their cell membranes in a controlled way, so that cells can be fully pumped up like a balloon or become empty and flaccid. Changes in these cells allow the flower head to move. When potassium from neighbouring plant cells is moved into the cells of the pulvinus, water follows and the cells inflate. When they move potassium out of the cells, they become flaccid.
But sunflowers dance differently
In 2016, scientists discovered that the pin-up example of heliotropism, the sunflower, had a different way of moving. They found sunflower movement is due to significantly different growth rates on opposite sides of the flowering stem.
On the east-facing side, the cells grow and elongate quickly during the day, which slowly pushes the flower to face west as the daylight hours go by – following the sun. At night the west-side cells grow and elongate more rapidly, which pushes the flower back toward the east overnight.
Everything is then set for the whole process to begin again at dawn next day, which is repeated daily until the flower stops growing and movement ceases.
Plants are dynamic
It’s easy to think of plants as static organisms. But of course, they are forever changing, responding to their environments and growing. They are dynamic in their own way, and we tend to assume that when they do change, it will be at a very slow and steady pace. Heliotropism shows us this is not necessarily the case.