Sarah Miskimmin, West Sussex asked:
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?
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...
At night, the amount of water on each side is equal and the flower rests between east and west.
during their early bud stages face the East every morning waiting for the sun to rise and actually turn and move to track the sun throughout the day before returning to face the East again at night.
The fact that they "follow" the Sun i.e. have a photo-tropism is more to do with growth than active pursuit isn't it?
It occurred to me that I've never learned why the flowers follow the sun at all, and I can't find the answer online.
Nice article, dentstudent.
There are various accessory pigments that utilize the light wave frequencies that the standard chlorophyll does not, which is why some leaves are more red than green (since they are reflecting the red wavelengths). But the pigmentation of the petal colour is probably much stronger (to us at least) than the accessory photosynthesis pigment which is why we see it. Of course, the pollinators are likely to see something else entirely. Just because we see a pretty yellow flower, doesn't mean that this is the functional "colour". dentstudent, Mon, 1st Jun 2009
Thanks _Stefan_, Mon, 1st Jun 2009
I pointed out on the show last night (it will be published tomorrow on as a podcast) that one reason the flowers track the sun is that the light will warm the flower, thereby making it more attractive to pollinators. This was shown in 2006 by Beverley Glover and her colleagues at Cambridge University when they demonstrated in a Nature paper that bees prefer warm nectar. Hence flowers are equipped with conical cells that act as miniature solar collectors to warm the petals.
As to if they would follow the moonlight, I would suspect not. The circadian clock in plants works to "gate" most light responses. That is, if you shine light on a plant when it expects it to be night (as determined by its internal clock), it won't be fooled into turning on its photosynthetic machinery. So it is probably likely that this response is also gated. There is some evidence that hormones, such as auxin, are also regulated by the circadian clock. WylieE, Tue, 23rd Jun 2009
Thanks Colleen - and good to see you back.
They follow the sun because of growth hormones the plants produce called auxins. Auxins make the plant grow faster and sunlight destroys auxin. So the side in the sun grows slower than the shaded side so it "bends" toward the sun. It's the same thing that happens when you put a plant in a window and it bends toward the window. kurt120, Wed, 5th Aug 2009
Do sunflowers really follow the sun? I have my doubts. All the sunflowers in my front- and backgarden are facing east but I am yet see them move with the sun as people say. I have even turned one of my pots to the west and I have observed day after day that they are turning to the east again. RAPTEE, Tue, 22nd Sep 2009
Sorry folks, sunflower flower heads DO NOT follow the sun. The buds do, but once open, they face East and don't move. Grow some sunflowers and find out yourself... or check Wikipedia! John Cossham, Mon, 11th Jul 2011
My son planted a sunflower a few months back. Prior to the head flowering, it would follow the sun during the day but now it's flowered, it resolutely points east. Matt B, Fri, 12th Aug 2011
I am as far from a scientist as you get, but I have observed lots of sunflowers. I have Never seen them face any direction except east, this includes fields of them and the ones we plant in our garden. Therefore, I am very confused with the answers, that say they Do follow the sun. Jill, Mon, 9th Sep 2013