Surely you have already heard that sunflowers always follow the sun; this feature gives its name to these beautiful flowers. The question you may be asking yourself now is: why do sunflowers always follow the sun? From the outset, we are going to dismantle this belief. The idea that sunflowers always follow the sun is a bit exaggerated: only the young heads do it before flowering at all and once the sunflower has matured, it remains fixed in the same position and looks east permanently instead of following the sun.
The fact that mature sunflowers do not look west at any time is another mystery. Why do sunflowers stop following the sun once they have matured? Let’s see what happens in more detail:
Why do sunflowers follow the sun until they ripen?
The behavior of young sunflowers is actually very common among plants (and people). The flowers follow a circadian rhythm (or a 24-hour rhythm), that is, they are used to feeling the changes in light and temperature as the day progresses. Since our days are based on the light and movement of the sun, living things conserve their energy because they are active by day and inactive by night.
In the case of sunflowers, the plant stretches its stem during the day, which allows it to adapt to a behavior called heliotropism. The plant is genetically programmed so that its growth hormones react to heat and light. The reason why sunflowers react like this is simple. The brown face of the plant contains several tiny flowers (it is not a single large plant) and to make sure that they can all develop properly, they need plenty of sunlight. As Hagop S. Atamian argues, when this rhythm is interrupted, sunflowers can lose up to 10% of their biomass and the size of their leaves is reduced. That is, without this behavior, the plants would not grow properly or would not mature at all.
Sunflower growth begins the day with the head facing east, turning west through the day, and the plant returns again to face east at night.
“The plant anticipates timing and steering in the early morning, and it seems to me that’s a reason to have a connection between the internal clock and the growth pathway,” Harmer said. This behavior of sunflowers had been described by scientists and dates back to 1898, but no one had previously thought of associating it with circadian rhythms.
Hagop Atamian, a postdoctoral researcher in Harmer’s lab, in collaboration with Blackman’s lab at the University of Virginia (now at UC Berkeley), conducts a series of experiments with sunflowers in the field, in outdoor pots, and with indoor growth chambers.
By fixing the plants so they can’t move or spinning the potted plants daily to face the wrong path, Atamian showed that it could disturb their ability to track the sun. The sun provides a growth boost to plants. Sunflowers, planted so they can’t move, had decreased their biomass and have less leaf area than they do, the researchers found.
When the plants were moved to an indoor growth chamber with immobile overhead light, they continued their round trip for a few days. That’s the kind of behavior you’d expect from a mechanism powered by an internal clock, Harmer said.
Finally, houseplants started tracking the “sun” again when the apparent source of illumination moved through the growth chambers lit during the day and turned them off at night. Plants could reliably track 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.
Why do mature sunflowers only look east?
So how do plants move their stems during the day? Atamian put ink dots on the stems and filmed them with a video camera. In a time-lapse video, he was able to measure the distance between the changing points.
Harmer says there appear to be two growth mechanisms in the sunflower stem. The first establishes a basic type of plant growth, based on the available light. The second, controlled by the circadian clock and the influence of the direction of light, causes the stem to grow more on one side than the other, and thus influences from east to west during the day.
As the sunflower matures and the flower opens, global growth slows down and the plants stop moving during the day and settle facing east. This seems to be due to global growth slowing down, and the circadian clock ensures that the plant reacts more strongly to early morning light than in the afternoon or evening, so it gradually stops moving westward during the day. The researchers grew sunflowers in pots in the field, and some of them were spinning westward.
By measuring the flowers with an infrared camera, they found that east-facing sunflowers warmed more quickly in the morning – and also attracted five times the number of pollinating insects. Heating the west-facing flowers with a portable heater, attracted more pollinators back to the flowers.
“Bees like warm flowers,” Harmer said.
“Like people, plants depend on the daily rhythms of day and night to function,” said program director Anne Sylvester. “Sunflowers, like the set of solar panels, follow the sun from east to west. These researchers leveraged information in the sunflower genome to understand how and why sunflowers follow the sun.”