Why do plants turn towards the sun? Why do plants turn towards the light? Scientists explain that the movement of the sunflower inflorescence occurs due to the uneven growth of the plant
Hagop Atamian/U.C. Davis
American scientists have found out which mechanisms are responsible for the orientation of oilseed sunflower ( Helianthus annuus) on the Sun and what evolutionary significance it has. It turned out that the ability to turn under the influence of sunlight (heliotropism) is associated with the coordinated work of light-sensitive mechanisms and circadian rhythms of the plant. The results of the work were published in the journal Science.
Young sunflowers during daylight hours turn after the Sun from east to west, and at night - back, in order to meet again in the morning sunlight. Blooming plants stop this movement and always face east. To understand why this happens, researchers from the Universities of California and Virginia conducted a series of experiments in the field and indoors.
At the first stage of their work, they artificially fixed some of the experimental sunflowers, preventing them from following the Sun. The total biomass and leaf area of such plants turned out to be on average 10 percent less than those grown without restrictions. Thus, turning behind the Sun is necessary for young plants for more intensive growth.
The reverse eastward turn during the night indicates that circadian rhythm regulation mechanisms are involved in this process. Scientists confirmed this by bringing sunflowers from the field into a room with constant lighting (the plants continued to rotate for several more days) and imposing on them an artificial 30-hour lighting cycle (the rhythm of the plants' rotations was disrupted, returning to normal with a 24-hour cycle).
Sunflowers do not have leaf pads - special motor organs that provide heliotropism in some plant species. Considering that the amplitude of sunflower movements decreases as it grows until it is completely absent in mature plants, scientists have suggested that the sunflower's rotation behind the Sun ensures uneven lengthening of the stem during the day. Experiments with plants lacking the growth hormone gibberellin, as well as studies of gene activity in the western and eastern sides of the stem, confirmed this hypothesis. Moreover, it turned out that the growth of the western side of the stem, which is more intense at night, occurs “by default,” while the growth of the eastern side, which is necessary during the day, is regulated by light-sensitive mechanisms (in particular, the redistribution of the hormone auxin under the influence of phototropins).
Changes in flower temperature during the day
Evan Brown/University of Virginia
When a sunflower stops growing and flowers, the circadian and light-sensing mechanisms lose their importance, leaving the plant oriented to the east. By turning some of the experimental sunflowers to the west, scientists became convinced that pollinating insects are practically not interested in such plants, unlike those turned to the east. 24-hour temperature recording showed that flowers oriented to the east warm up much better and faster, attracting insects. When flowers turned to the west were artificially heated, pollinators' interest in them returned.
Thus, the turns of young sunflowers behind the Sun are ensured by the joint work of circadian and light-sensitive mechanisms; they serve for an intensive increase in biomass. The orientation of adult plants to the east is necessary for their warming, which attracts pollinating insects.
Sunflowers, under the sun.
There is an established opinion that the head of a sunflower turns after the sun.
Yesterday I became convinced that this is a myth.
Under the cut there is latest photos confirming that the sun shines into the “back of the head” of the sunflower :)
And the real state of affairs
So, as we see, sunflowers do not rotate with the movement of the sun.
But there is some truth in the myth.
Namely, the sunflower looks at the sun at the moment of sunrise.
And he always looks east.
But when the sun goes down, the sunflower does not move behind it.
It's like he's waiting for the next sunrise :)
The phenomenon referred to in the myth is called Heliotropism.
Here's what Wikipedia says about it.
There is a widespread false belief that sunflowers “reach” towards the sun (heliotropism). In fact, mature sunflower flowers usually face east and do not move.
The leaves and buds of young sunflowers exhibit heliotropism. They change their orientation from east to west throughout the day.
A long time ago, people noticed that young sunflower flowers turn to follow the sun during the day, and at night they return to their original position to meet it again in the east in the morning. But until now, scientists have not been able to solve this mystery: what makes plants perform their daily ritual and why does the “worship” of the luminary stop over time?
In search of an answer, Stacey Harmer from the University of California at Davis and her colleagues conducted a series of experiments.
At the first stage, conditions were changed for sunflowers growing in their natural environment. Scientists “immobilized” one group so that the plants could not turn at all, and the other was fixed in such a way that the sunflowers at sunrise were turned to the west. When the flowers grew, it turned out that the leaves in both groups were 10% smaller than those of the “free” plants. This confirmed the guess that observing the sun is necessary for sunflowers to efficient growth.
Then the scientists decided to check whether the rhythmic “dancing” of sunflowers was due to internal clocks or environmental conditions.
They moved the plants that were growing outside into a room with constant overhead lighting and found that the sunflowers continued to turn from side to side exactly as they had before for several days.
The scientists then placed the plants in a special room with a string of lamps that turned on one at a time, imitating the movement of the sun. When the researchers programmed artificial lighting On a thirty-hour cycle of "day" and "night", the plants turned from side to side without a regular schedule. But when the light regime returned to normal, the sunflowers strictly followed the artificial "sun", showing that internal circadian rhythms were at play. important role in the movement of a flower.
But most of all, biologists were interested in the question of why, after flowering, sunflowers stop turning from side to side and freeze, “looking” towards the sunrise. Then Harmer's team turned some of the plants to the west, and then counted the number of bees and other pollinators that landed on flowers facing the west. to different parties Sveta.
It turned out that in the morning, insects visited flowers facing east five times more often than those facing in the opposite direction.
“You can see that bees go crazy for east-facing flowers and pay little attention to west-facing plants,” says Stacy Harmer.
Previous studies have shown that pollinators prefer more warm flowers, so sunflowers that receive a large dose of the first morning rays turn out to be more popular.
“I was constantly amazed at how complex plants are,” continues Harmer. “They are truly masterful at adapting to environmental conditions.”
The results of the study, published in Science, raise more difficult questions. For example, how do plants tell time and how do they find the right direction when they turn in the dark to where the sun will rise?
But, according to experts, the very fact that sunflowers have an internal clock and are guided by their own rhythms is the “Holy Grail” in the study of their complex behavior. And, as the university press release highlights, this is the first example of temporal synchronization in plants living in natural environments, which has a direct impact on growth efficiency.
Material. Those differences in material expansion due to temperature. More in the sun than in the shade. As far as I know, the base of the head at the place where the stem is attached looks like “hard cotton wool” with liquid. Maybe this liquid in the pores plays the role of muscles - are there any hydraulic operators?
[email protected] 01.08.2011
VIVAT - GOOGLE!
Name: comes from a combination of two Greek words “helios” - sun and “anthos” - flower. This name was not given to it by chance. Huge sunflower inflorescences, bordered by bright radiant petals, really resemble the sun. In addition, this plant has the unique ability to turn its head after the sun, tracing its entire path from sunrise to sunset.
Plants do not have muscles; the flower can change orientation only due to the fact that the stem that holds it grows stronger on the sunny side. Therefore, the process occurs when the sunflower grows: during the day closed flowers truly follow the course of the sun, living up to their French name tournesol. 
An even more amazing trick: during the night, the flowers manage to turn so that in the morning they again greet the sun in the east.
Thanks to this rotation, plants in the growth phase are able to capture 10-15 percent more solar energy. Grown sunflower with open flower motionlessly looking east.
The area of the stem under the flower petals contains<гормон роста>. This hormone cannot withstand direct sunlight. When exposed to the sun, this part of the stem turns to move away from it. It concentrates<гормон роста>, so it grows faster, and as a result the flower itself turns towards the sun.
So I was thinking in the right direction, I just couldn’t imagine that a plant could grow so quickly. Thanks to Google, I somehow didn’t think to Google this question. But beautiful pictures appeared in the topic. Did you know that in Germany it is customary to make bouquets of sunflower flowers? You may well be given such a bouquet for your birthday.
Alexey.n.pop***@u*****.ua Teacher 08/03/2011
No thanks to Google! Nothing is clear - the expediency of this movement is simply indicated, but what is the mechanism? And why does rotation occur at night - does it mean there is memory or celestial navigation?
It's a delusion. He does not turn FOLLOWING THE SUN. It is constantly directed in the direction whose average daily brightness is greatest... Just like cucumber leaves in a greenhouse, just like indoor flowers on a windowsill.
Take a closer look. And early in the morning, at dawn, and late in the evening at sunset at open field The heads of the sunflower flowers will be directed to the South. And in a shaded area - away from the shadow falling on it.
August 5th, 2016 , 05:59 pm
People have long noticed that young sunflower flowers turn after the Sun during the day, and at night they return to their original position in order to meet it again in the east in the morning. What makes plants perform their daily ritual and why, over time, the “worship” of the luminary stops and mature sunflower flowers do not turn behind the Sun, but remain directed only to the east.
In search of an answer, Stacey Harmer of the University of California, Davis, and her colleagues conducted a series of experiments that confirmed the hunch that observing the Sun is necessary for sunflowers to grow more efficiently. Scientists fixed the plants, preventing them from turning, or, conversely, rotated the pots, disrupting the natural course of movement. In both cases, the leaves of the plants turned out to be about 10% smaller than those of their neighbors, who calmly turned behind the Sun.
In addition, experts placed several points on the stem with a marker to study how the sunflower moves behind the Sun. Scientists monitored the points using a video camera. If the distance between them changed, it meant that the flower stem was growing where these points were drawn.
When the plants turned to follow the Sun during the day, the eastern side of the stem grew at a faster rate than the western, causing the flower itself to turn toward the Sun. And at night, the western side grew faster, and the stem turned in the other direction.
The movement of the plant is carried out with the help of special motor cells that participate in the growth mechanism and are located in the flexible base of the flower. It turned out that this movement depended on the plant's internal clock - circadian rhythms that control various life processes associated with the onset of day, night, morning and evening. The "clock" controls the rate of growth and causes one side of the stem to grow faster than the other. Thanks to this, the sunflower gradually turns to follow the Sun.
As the sunflower matures and the flower opens, overall growth slows and the plants stop moving during the day, remaining oriented to the east. The fact is that the plant reacts more strongly to sunlight early in the morning than in the afternoon, so it gradually stops moving westward during the day.
