How do sunflowers track the Sun? It turns out scientists still don't know
Sunflowers are well known for the ability to turn their flower heads to face the Sun as it moves through the sky. But how do they do this? A new study has found it remains a mystery.
Sunflowers do not use conventional botanic methods to follow the Sun across the sky and instead utilise a mechanism we are yet to discover, according to the findings of a new study.
Instantly recognisable for their classically solar appearance, the flower heads of sunflowers are famous for tracking sunlight, allowing them to make the most of the energy delivered by our star.
This behaviour — known as heliotropism — has long been assumed to be governed by the same mechanisms that control phototropism, the ability to simply grow towards a light source. The latter behaviour is common in plants, and is activated by a molecule called phototropin, which responds to light at the blue end of the spectrum.
Towards the light
Sunflowers swing their heads by growing a little more on the east side of the stem during the day (which pushes the flower west), and a little more on the west side at night (pushing the flower back east).
Previous UC Davis research showed that sunflowers use their internal clocks to anticipate the sunrise, coordinating the opening of their florets with the appearance of pollinating insects in the morning.
In the new study, the researchers built on this by looking at which genes were switched on in sunflowers grown in both laboratory conditions and outdoor sunlight.
Indoors, they observed that the sunflowers grew straight towards the light, activating genes associated with phototropin. Outdoors, on the other hand, the flowers swung their heads with the Sun and exhibited a completely different pattern of gene expression, with no apparent phototropin differences between one side of the stem and the other.
“This was a total surprise for us,” said Stacey Harmer, lead author of the study and professor of plant biology at UC Davis. “We seem to have ruled out the phototropin pathway, but we did not find a clear smoking gun.”
What the researchers did find is that blocking blue, ultraviolet, red or far-red light with shade boxes had no effect on the sun-seeking behaviour of the flowers. As the authors explain, this suggests there are probably multiple gene pathways tied to the ability, all working in tandem to angle the sunflowers towards the Sun.
Moreover, plants transferred outdoors after being grown inside acquired heliotropic abilities quite suddenly, accompanied by a burst of gene expression on the shaded side.
According to Harmer, this indicates some sort of "rewiring" is going on. But, for now at least, exactly what drives this change remains unknown and will need to be explored more deeply in future research.