Science Questions

How do sunflowers follow the sun?

Sun, 31st May 2009

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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 A sunflowermost 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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At night, the amount of water on each side is equal and the flower rests between east and west.


Don't sunflower plants permanently face east at some stage in their life cycle? Chemistry4me, Thu, 28th May 2009

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.

Now I'm a bit confused (than usual). Chemistry4me, Thu, 28th May 2009

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?

In the same way that a seed sends a root downwards (positive geotropism and negative phototropism) and a shoot upwards (positive phototropism and negative geotropism), the stems of green plants actively seek out light in order to maximise the solar return for the energy invested in growth. However, they have no muscles to make them move so the only way to do this is by adding cells asymmetrically to make the plant grow in the direction of light.

I think this is what happens with the sunflower - it grows more on the side OPPOSITE the light, pushing the stem towards the light source. As the sun crosses the sky this growth pattern alters in step.

Whether plants can respond to moonlight, however, I have no idea. Therefore, how the flower gets itself back to pointing the right way again for the next sunrise, or if it even does anticipate sunrise, I am not sure...

Chris chris, Sun, 31st May 2009

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.

The flower doesn't photosynthesize, so is it a result of the rest of the plant moving for photosynthesis? Or is it related to the hypothesis/theory that warm flowers attract pollinators? _Stefan_, Sun, 31st May 2009

Is that true? Petals are merely morphed leaves after all. Trees photosynthesis through their bark, so it would surprise me very much if flowers did not photosynthesis at all. dentstudent, Sun, 31st May 2009;col1

Morning light arrives early on the high slopes of the Colorado Rocky Mountains. At 5:00 A.M., a climber on a westward route to the summit of a fourteen-thousand-foot peak can count on a cheerful welcome from thousands of snow buttercups (Ranunculus adoneus), their dazzling yellow flowers facing east in the glow of the sun's early light. It's as if, overnight, a blast of wind blew down the slope, all the little yellow umbrellas in its path. Then, as the sun moves across the sky from east to west, the buttercups turn to follow it, and their bright faces greet the climber once again as she makes her descent.
Solar-tracking, or heliotropic, flowers are most common in arctic and alpine environments, where the air is often cool and the growing season is short. The satellite dish-shaped flowers of the snow buttercup, the arctic poppy, and other heliotropic flowers collect the sun's rays so efficiently that they heat up, becoming as much as fourteen degrees Fahrenheit warmer than the air around them. These miniature saunas are enticing to insects, which are by and large unable to generate their own heat and must wait for the sun to warm them up before starting the day's activities. Anyone who has observed bees on a flower early in the morning, when they are so still as to seem drugged or nearly dead, is familiar with insects' dependence on external heat sources. Solar-tracking flowers provide their insect visitors with a warm retreat for basking, foraging, even mating. In return, the visitors pollinate their hosts. The considerable heat absorbed by heliotropic flowers also jump-starts the development of newly fertilized ovules, helping the plants complete seed maturation in as few as eight weeks.

Harsh arctic and alpine conditions provide a "motive" for solar tracking, but what of the means? The mechanisms that leaves use to follow the sun are much better known than those flowers use. Movement in solar-tracking leaves, first written about by Charles Darwin in his 1880 book The Power of Movement in Plants, can occur rapidly and is reversible--two defining features of heliotropism. In nasturtiums, for example, a specialized organ at the base of the leaf--the pulvinus--continually orients the leaf surface at a right angle to the sun's rays, maximizing light interception for photosynthesis. The plant equivalent of a muscle, the pulvinus consists of specialized extensor and flexor cells that swell or shrink with changes in turgor pressure (determined by the amount of water in the cell). As extensor cells swell and flexor cells shrink, the leaf blade is reoriented to track the changing position of the sun.

Experiments with the snow buttercup have begun to reveal the sensory and developmental processes that lead to heliotropism in flowers. My colleague Maureen Stanton, of the University of California, Davis, and I started with a simple yet vital question: How do flowers sense the position of the sun? We knew that light provides plants with information as well as energy. Photomorphogenesis (plants' developmental responses to light) begins with photosensitive molecules in the cells of certain plant organs. Phototropism (one kind of photomorphogenesis) orients growing organs toward a light source. The sunflower, which is the plant kingdom's version of a morning person, shows phototropic stem growth, with the flower at the tip of the main stem always facing east. While phototropism does not exhibit the reversibility seen in heliotropism, we reasoned that similar sensory mechanisms might be involved in true solar tracking.

Anecdote has it that in the nineteenth century, a Frenchman noticed that some plants situated behind bottles of red wine failed to grow toward the sunlight. This early observation suggested that short-wavelength blue light, known to be blocked by red pigment, was necessary for phototropism. Blue light was later shown to play a crucial role in changing turgor in the pulvinus of heliotropic leaves. To determine whether blue light also cues solar-tracking in flowers, we performed our "buttercup in a Wine bottle" experiment. But we had no intention of carrying cases of cabernet up to twelve-thousand-foot elevations in the Rockies. Instead, we constructed lightweight cubes out of red acrylic filters. In the evening, we placed the cubes over snow buttercup plants. The next morning, we discovered that the filters had completely disabled the plants' ability to locate the sun. The confused flowers faced every which way, on average more than thirty degrees away from the sun's rays. Flowers in the two groups of control plants--those surrounded by blue-light-transmitting filters and those simply left in the open--tracked the sun much more successfully, facing, on average, within fifteen degrees of it.

Our experiment with acrylic cubes confirmed that blue light guides the movement of heliotropic flowers. But which organs actually perceive the light signal and translate it into the biochemical language of plant growth? Molecular biologists approach such questions by searching for gene products involved in photomorphogenesis. Rebecca Sherry, formerly a colleague at the University of Missouri-Columbia, and I took a less sophisticated tack, simply removing portions of the buttercup plant and discerning whether what remained could accomplish the task of solar tracking. Inspired by the Queen of Hearts in Alice's Adventures in Wonderland, we began with the edict "Off with its head!" and decapitated a number of innocent snow buttercups, removing the solitary flowers from their supporting stems. Neighboring buttercups were spared, as controls. Surprisingly, we found that the stems of decapitated buttercups continued to move over the course of the day, along with those of the control plants, aligning the ghosts of flowers past with the rays of the sun. Barring a paranormal phenomenon, this result means that the guidance system for flower heliotropism is housed in the stem rather than in the flower.
The flower stem of a snow buttercup is divided into three regions by two sets of bracts (modified leaves) that form sheaths around the stem. To study it, we again took our cue from Darwin, who performed an experiment since repeated by countless college botany students. When he illuminated young canary grass seedlings with light from one side, Darwin found that their shoots responded by bending toward the light source--unless he placed a foil hat over the tip of the shoot. Darwin concluded that the upper part of the shoot senses light and somehow reports the light's whereabouts to the lower region of the stem, which does the bending. We now know that this involves the growth-promoting hormone auxin. This hormone moves away from the light source and collects on the shaded side of the stem, where it causes cells to elongate more rapidly than on the sunlit side, thus producing a bend.
Realizing that foil caps were a technique better suited to the English garden than to the breezy alpine provinces of snow buttercups, we used Liquid Paper (opaque correction fluid) as a sunscreen to block different portions of the flower stem from sunlight. We found that if we blocked only the tops of the buttercups' stems, the stems lost their tracking ability. In contrast, when we painted middle or bottom portions of the stems with Liquid Paper, the plants continued to track accurately. These findings place the guidance system for heliotropism in the upper portion of the stem, just below the flower. Yet the bend needed for solar tracking occurs in the middle of the stem, well below the site of light reception. What messenger coordinates these activities?

The obvious suspect was auxin. We collected bent buttercup stems to inspect under the microscope back at the laboratory. In nearly every instance, the cells on the side of the stem that had been in the shade were longer than those on the sunlit side, as expected with auxin-induced growth. In fact, during solar tracking, the sunlit side of a flower stem grows only about half as fast as the shaded side.

Our results have revealed that, mechanistically, heliotropic flowers have more in common with seedlings reaching for the sun than with solar-tracking leaves. Leaves, for the most part, must be capable of orienting independently of the central stem of a plant. Ivy stems, for example, grow toward shade ("looking" for something to climb on), while ivy leaves seek the sun. In contrast, solar-tracking flowers, as extensions of the tip of the central stem, may utilize the same machinery that, earlier in a plant's life, served to orient the seedling shoot. Going further with our exploration of flower heliotropism may require that we trade our polar fleece for lab coats. Applying the tools of molecular genetics may determine whether the same genes control both seedling phototropism and flower heliotropism. But how glad I am to have begun the search in the crisp mountain air, inspired by the spectacular views that greet snow buttercups every day.

dentstudent, Sun, 31st May 2009

Nice article, dentstudent.

Do flowers contain much chlorophyll? Wouldn't the green show through, especially in light coloured flowers? _Stefan_, Mon, 1st Jun 2009

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.

Chris chris, Mon, 1st Jun 2009

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.

Chris chris, Tue, 23rd Jun 2009

It's good to be back, I missed y'all! WylieE, Wed, 24th Jun 2009

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

Why did some plant close there leaves in the evening abeilekpor christian, Sun, 9th Feb 2014

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