How do sunflowers follow the sun?

31 May 2009

Question

I grow sunflowers in my garden for several years now and it always follow sunrise from east to west each day, but what I would like to know is, do they slowly unwind during the night so that at sunrise they’re pointing east again ready and waiting for the sun-up? Or do they remain facing where the sun sets in the west until the sun starts to rise, and then suddenly whiz around to face it?

Answer

We asked David Henke, Senior Lecturer in Plant Sciences, at Cambridge University...

It is actually very simple: there is a kind of driver which is growth. If you look at a sunflower there is a narrow neck which is growing, and it is in this narrow neck where most of the cell expansion, and therefore most of the expansion of the stem takes place. And this takes place at different rates on different sides of the stem. So, in the morning, most of this growth is on the West side of the stem so the flower tilts to the East; later on in the day you get stronger and faster growth on the north side, so the flower becomes tilted and so on until the evening when it ends up facing West. At night the growth is corrected and you have a great deal of growth on the West side so at the beginning of the day it is facing East again. This pattern is probably driven by some kind of internal clock, which is set by the transition at the end of the day from light to dark, which then starts the whole process of West side growth in the flower.

We know that the sensitivity of plants to light in terms of the sensors capable of picking up light are quite remarkable, and you can show that the light of a full moon on a completely clear night is just about perceptible to a plant, and the problem is that most of the time the moon isn't full.

Sunflowers do unwind at night using the same alternating growth mechanism as in the day. But what is also interesting is that no one really knows why the flowers themselves follow the Sun. The best guess is that they need more heat to grow more seeds...

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