A few months ago I was sharing my car for a road trip. After some minutes of small talk with my fellow passenger it was unavoidable for me to admit that I was a botanist. I am always scared of that moment. It gives your counterpart the unique opportunity to ask this one question, he or she always wanted to ask about plants. And that’s exactly what happened:

“I always wondered: How does the sunflower turn its head towards the sun? Do they have some kind of motor that moves their heads? How come you are not able to explain this to me in every last detail? Did you not say, you were a botanist?”

Dear person, who made this trip the seemingly longest one of my live: I am very sorry I could not give you a satisfying answer as specific, immediate, scientifically correct and ‘easy-to-understand-also-for-non-scientists’ as you expected me to.  You might not believe it, but: the movement of the sunflower head is not as trivial as us turning our heads towards a ringing cell phone or the doorbell. Still, I feel like I owe you (and of course all other readers who are intersted in fascinating natural mechanisms) an answer. I am therefore proudly presenting today:

How sunflowers actually track the sun – ‘FAQ’

(based on Atamian et al. 2016 – a beautiful and highly up-to-date publication from Science as well as Kutscher & Briggs 2016, who published an amazing paper about sunflowers considering the history of its research and the bigger picture)

How do sunflowers turn their heads?

Sunflowers do not possess something resembling a motor or a joint to move their head. Sorry to disappoint. Some plants possess so-called ‘pulvini’, joint-like structures which can cause movement of plant organs via changes in turgor (the pressure of the cell plasma against the cell wall). This can cause swelling of tissues and result in their reorientation. The sunflower does not! The tracking of solar light with the flower head is caused by the elongation of the plant stem. During the day, the side of the sunflower stem facing east grows more than the side of the stem facing west. That causes the tip of the plant (also called the apex) and therefore the flower to move gradually from the east side (the rising sun) in the morning towards the west side (sunset) in the evening.

And how does the sunflower stem know when that it faces the east side (the bright side) in the morning and consequently has to grow more?

Trying to make it simple, this has two reasons: 1) The circadian clock of the sunflower (an intrinsic 24-hours-rhythm) and 2) the regulation of auxin (a plant hormone) by photoreceptors, inducing differences in gene expression on the opposite sides of the stem. This causes unequal growth on the stem sides.

Do all sunflowers turn their heads?

No. A sunflower needs to grow to be able to turn its head (as you can read above). Therefore only young sunflowers turn their heads towards the west side during the day. With progressing maturation of the sunflower, orientation towards the west side ceases until the flower finally stops its movement and keeps facing east until the end of its life.

And why do sunflowers turn their heads?

Most probably, this has an ecological reason. Flowers facing the sun get warm. It could be shown that warm flowers are more often visited by pollinators, such as bees, than flowers with a lower temperature. Still – this is a question which is still to be researched more intensively in the future. Maybe the flower temperature has positive effects on the floral physiology?

Becoming a sunflower expert and spreading fun facts about the sunflower, which scientific terms do I have to know?

• Helianthus annuus: the scientific name for the common sunflower
• Heliotropism: solar tracking/ portions of a plant following the sun
• Circadian clock: intrinsic rhythm of approximately 24h

And what does all of that have to do with Biomimetics?

Well… what do you think? Sunflowers take advantage from exposing their flowers to the sun and developed a sophisticated mechanism to achieve this. Is this something we need to apply in technology? And if yes, are we even able mimick the sunflowers’s principle? Maybe we could improve our solar panels efficiency via constructing them in a way they turn towards the sun. But if we install a photosensor and a motor – is this still Biomimetics, as it funtions completely different then the sunflower?

I hope I could give you some insights in the solar tracking movement of sunflowers. If there is anything else you always wanted to know about sunflowers, please do not hesitate to ask via comment or e-mail and I’ll do my very best to answer!

… or is there may be anything else you always wanted to know about plants and you never had the chance to ask …? 🙂

The common sunflower

Reference:

1. Atamian, H. S., N. M. Creux, E. A. Brown, A. G. Garner, B. K. Blackman and S. L. Harmer (2016) Circadian regulation of sunflower heliotropism, floral orientations, and pollinator visits. Science 353:6299, 587-590.
2. Kutschera, U. and W. R. Briggs (2016). Phototropic solar tracking in sunflower plants: an integrative perspective. Annals of Botany 117:1, 1-8.