One of the earliest questions in botany was how plants knew where the sun was, and how they grew towards the light. For a sessile organism, these feats of movement are astonishing, and can lead someone to question whether plants are more alive than they seem.

While there are intriguing implications for new discoveries in plant senses and what a plant knows about its environment, there is one phenomenon that started this curiosity. Something that has fascinated many botanists, and the general public for years is why – and how – young sunflowers seemed to follow the sun throughout the day.

With more attention on the plant world, it was found that all plants reached for the sun, even if it was not as obvious as young sunflowers made it.

In a room where the light comes from overhead, if you place a potted plant on its side, it will begin modifying its growth to reach up, towards the sun, even though the plant is technically growing sideways. You may have even experienced this phenomenon yourself, if you have ever had a plant on a windowsill that wasn’t receiving enough sunlight. You will notice that the plant will extend itself or grow “leggy” and tall to reach for the sun.

This is the same reason why you find yourself having to rotate your houseplants every now and then, because they will start growing towards the window, rather than straight up; unless you have skylights that light can penetrate. Greenhouses have translucent tops not just for their famous greenhouse effect, helping warm the interior, but also to allow the most light in so plants can benefit from the natural light cycle.

This phenomenon is called phototropism, and in this HerbSpeak article, you will learn why plants experience this effect, how it works, the effects of different types of light, and much more.

Phototropism – Definition

Phototropism is a form of tropism, which is a growth response in all or part of an organism due to an external stimulus. In this case, the organism, plants, are responding to light. Photo-, meaning light, and -tropism combined means, essentially, growth in response to light.

In most examples of this event, the plant is compelled to move towards light, a case of positive movement. In other instances, plants are compelled to move away from light, an example of negative movement.

Phototropic responses are modulated by a phytohormone called Auxin which reacts to the presence of light, which we discuss in this article.

Why Do Plants Experience a Phototropic Response?

Perhaps one of the most endearing parts about plants is their desire to reach for the sun. But why, on a physiological level, do plants desire more sunlight? We know that plants require sunlight to grow, but they are considered to be sessile organisms, entities that are quite literally rooted in place. So, what compels them to alter their growth patterns and strive for the sun?

At the most basic functionality of all plant life, each organism needs three basic things: water, oxygen, and sunlight. Most plants also need some additional nutrients in the soil.

Green plants photosynthesize, which is a process that allows the plant to capture sunlight and, using oxygen and water, synthesize phytonutrients and energy for further development and strength.

The plant’s stem, leaves, some flower buds, and all other plant cells that contain chlorophyll (the green pigment) photosynthesize.

Shaded areas of a plant are unable to photosynthesize as much since they are unable to reach as much sunlight, whether the area is shaded by its own stems or by neighboring plants. For many plants, an extended period of being unable to reach as much sunlight as it needs will result in the plant becoming sickly, or even dying.

That said, most plants have a limit to how much sunlight they can process, so underbrush plants, also known as shade plants, may prefer this level of light. In any case, the plants need to reach adequate levels of light, because even low-light plants will die if they are kept in a dark area without any sunlight.

Phototropism is suggested to have evolved in higher plants as a way to help them maximize the surface area that sunlight reaches, allowing more photosynthesis throughout the plant.

With adequate levels of photosynthesis, the more nutrients and energy the plant creates. With more energy and nutrients, the stronger the plant becomes.

The stronger the plant, the more leaves develop, and the more it is able to photosynthesize. The cycle goes on and on.

It wasn’t until the late 1870s until phototropism was truly studied and given a name. Charles Darwin and his son, Francis Darwin, were two of the earliest scientists to study this biological event and how it worked in plants. This is something that will be discussed more in-depth later in this article.

Darwin was a lover of science and many of his experiments and expeditions across the globe were in study of plant life and how it worked. Many of his experiments discovered reactions and actions in plant senses that modern botanical scientists are still struggling to understand completely. Phototropism, fortunately, has been studied well enough that we now know what compels the plant to react the way it does.

Negative Phototropism VS. Positive Phototropism

There are two types of phototropism. The first and most common instance is positive phototropism, where a plant will move towards the sunlight. Then, there is negative phototropism where the plant will move away from sunlight.

Plants use positive phototropism to orient their leaves, stems, and any buds towards the sun so that it may weave its way through competition and maximize the surface area able to photosynthesize.

One example of negative phototropism is in a plant’s roots; pull a plant out of the soil so that its roots sit on top, and it will begin to grow downwards. Now, this is in addition to another tropism called gravitropism, which is the plant’s response to gravity. All plants are influenced by gravitropism unless they are in zero gravity, which makes experiments solely on phototropism difficult as a plant is a complex organism.  

How Does Phototropism Help a Plant?

Phototropism helps plants by giving it the ability to capture the most sunlight across photosynthetic tissue. Additionally, phototropism helps plants compete for survival in populated forests or cities, seeking areas of light that might be just out of reach.

Without this biologically driven movement, plants in windowsills or those over-shaded by neighboring plants would grow sickly and weak, rather than growing tall to reach for more energy and nutrients.

Plants rely on tropism responses greatly to interact with their environment, with many other external stimuli affecting plant growth, such as geotropism (response to gravity), hydrotropism (response to water), and thigmotropism (response to mechanical stimulation or touch) being some of the most influential.

How Phototropism Works: Components of a Phototropic Plant

In this section, we will learn how phototropism works on a cellular level from inside the plant. There are dozens of tangential topics to get lost under to get a complete view of how plants react to light and how their growth patterns might be altered, but the mechanisms and methods involved in phototropism are fairly straightforward.

Phototropism is a response generated and influenced by several phytohormones, or plant hormones. There are many plant hormones that affect plant growth and are influential in phototropism on some level, but the main phytohormone responsible for the plant’s response to light is called auxin.

What Is Auxin?

Auxin is a phytohormone that is essential to plant growth and structural development. This substance is significantly influenced by environmental factors, affecting the plant and helping regulate development. (1) Surprisingly, this substance was only isolated in the 1930s by chemists studying maize, though many scientists hypothesized that some sort of mobile signal was responsible for the elongation of plant cells and specific growth patterns towards light.

“(Auxin) is extremely potent in controlling many aspects of plant growth and development, despite its relatively simple chemical structure. It controls cell division, cell expansion, and cell differentiation. It has a ubiquitous and context-dependent function, making it difficult to assign a single function to auxin.”

Sebastien Paque

This phytohormone promotes stem elongation and inhibits the growth of axillary buds, keeping plants straight and sturdy. Many plant cells are able to produce auxin, but the concentration can vary depending on what part of the plant it is present in.

The meristem of the plant, the tip of the stem with active cell division and plant growth, is especially high in auxin content. It is in this area of the plant that reacts the most to phototropism.

What Is A Phototropin?

Phototropins play an important role in the phototropic response. All plants are filled with proteins and flavoproteins that act like photoreceptors. (2) These photoreceptors allow the plant to sense the presence of light, as well as differentiate what wavelength the light is and exhibit different results depending on the plant cell signaling. 

Most plants contain a variety of photoreceptors, such as phytochromes, cryptochromes, and phototropins. While all photoreceptors play some role in the phototropic response, the phototropin is directly linked with phototropism.

These receptors are activated by UV and blue light wavelengths, which is important for a phototropic response as plants use blue light as a cue to exhibit a growth pattern, bending towards the light.

“Plants perceive and respond to blue light by means of specific blue-light absorbing photoreceptors, which control and influence many aspects of plant growth and development. Two major classes of blue-light receptors have been identified to date: the cryptochromes and phototropins, both of them are flavoproteins and are active principally in the range of 360 to 500 nm of the visible spectrum.”

Bin Kang

These phototropin proteins also help stimulate chloroplast movement inside plant cells and cause the stomata in leaves to open for transpiration and photosynthesis. Additionally, the phototropin proteins are responsible for mediating the first changes in stem elongation before the cryptochrome receptors activate, and many other important metabolic and growth processes in plants.

What Are Cryptochromes?

Cryptochromes are known to regulate plant development and photomorphogenesis, which is how the plant will exhibit its overall development, form, and structure affected by light. These are another class of photoreceptors present in the plant tissues much like the phototropin.

Like a phototropin, cryptochromes help regulate chloroplast movement and are responsible for a variety of metabolic and growth processes in plants, such as regulation of flowering time and the circadian rhythm, as well as stem elongation.

These photoreceptors, cryptochromes, also play a significant role in phototropism. Though they are structurally unrelated to a phototropin protein, they work together to produce the phototropic response.

What is the Plant Measuring, and How Does it Activate Phototropism?

For the phototropic response to occur, the plant measures blue light wavelengths. Blue light is essential for healthy chlorophyll production and strong, healthy stems and leaves, while red light is most beneficial for producing beautiful flower blooms and plump fruits.

Blue light wavelengths are also responsible for healthy photosynthetic activity, but it also suppresses the growth response. [3] Plants grown under higher percentages of blue light will have stouter stems and darker leaves than plants grown under lights without a blue wavelength.

While blue light is a primary trigger for photomorphogenesis, it also suppresses the growth response in plants. Red light appears to have the opposite effect on plants, with plants grown only under red light growing tall and spindly, searching for more a more substantial source of sunlight. However, when even a small portion of blue light is added to the red light source, the plants’ growth is regulated.

“[…] only a low intensity of blue is needed in a light spectrum for fully functional photosynthesis. […] Generally, blue light suppresses extension growth; plants grown with blue light are usually shorter and have smaller, thicker and darker green leaves compared to plants grown without blue light”

Erik Runkle

Specifically, blue light at around 450 nanometers – a wavelength that is a mix of blue and violet light – triggers a response in the plant’s photoreceptors and signals auxin to move. In the case of positive phototropism, it forces auxin to move to the shaded side of the stem.

The concentrated levels of auxin in the shaded side of the plant stimulates the release of hydrogen ions. These hydrogen ions case the pH of these shaded cells to decrease, which activates certain enzymes called expansins. These enzymes cause the shaded cells to swell, causing the stem to bend towards the light.

Because the shaded cells are now elongated compared to the side that is receiving light, the plant is changed in a very permanent, structural way.

Now, if the light source were to stay in this spot, this process would continue until the plant climbs ever higher, reaching for the light source in an attempt to capture as much light as possible.

After all, on a forest floor, tiny saplings and sprouts must find their way through towering competitors, wavering and weaving through other plants to find the light and give them the best chance of success. This causes the plant to exhibit an elongated or “leggy” appearance.

If a plant is subject to an ever-changing light direction, such as unshaded plants experiencing natural light cycles, the plant tends to sit upright unless another environmental factor affects its growth (such as geotropism.) This is because the light is, in most cases, above the plant for most of the day. Likewise, the ever-changing light will cause auxin to move to all sides of the plant throughout the day, resulting in an even growth pattern; however, plants almost never grow straight up.

In the video shared above, we see that plants are naturally inclined to exhibit a pattern of growth in a spiral motion, winding their way up, rather than growing what appears to be straight up at all times. Almost every plant in nature grows with this winding motion, allowing the plant to sense its environment and assess its best opportunities for growth, even when grown in unchanging laboratory conditions.

Charles Darwin’s Experiment on Phototropism in the Coleoptile

Charles Darwin and his son, Francis Darwin, were the first scientists to begin pinpointing why plants reached for the sun, as well as what part of the plant signaled the changes.

This experiment was performed in the late 1870s on coleoptiles of plants and published in 1880. Coleoptiles are not a type of plant, but rather, they are a part of plant anatomy; the protective sheath which forms around the embryonic stem in grass seeds, particularly canary grass, and oats.

Image: The stem of a plant naturally bends towards light.

Darwin and his son discovered that when they allowed a plant to grow in a dark room with a light source above it, it grew as normal. When they turned the artificial light source to the side of the plant, however, it began to bend towards the light as it if were reaching towards the light source.

In their case, since they did not have access to the wide assortment of light wavelengths in lightbulbs today, they used a small candle that was dim enough so that he “could not even see the clock on the wall.”

This began a series of experiments to determine what part of the plant signaled these changes. Darwin’s theory was that this was that the tip of the plant sensed the light, that it was the “eyes” of the plant which could see light.

He experimented on several plant varieties to get to this point in his trials, but he made the most obvious progress with a species of canary grass.

Image: By cutting off the tip of the plant, Darwin would discover if his hypothesis was correct.

In the first set of experiments, he decided to start strong by cutting the tip of the plant completely off the plant. As he suspected, the plant did not bend towards the light; but this was such an imprecise method that he decided to continue his experiments, as the damage to the plant could have just as easily been a factor in preventing the plant’s bending and further growth.

Image: The stem did not bend when covered completely with foil.

Next, he had to determine whether it was the tip that sensed light, as he suspected, or another part of the plant. Darwin covered all parts of the plant with a metal foil, blocking any incoming light. The plant did not bend towards the light while it was covered. Once he removed the foil, however, the plant began to grow towards the light again. So, he concluded, the plant was indeed sensing the presence of light through some part of its anatomy.

Image: The stem bent towards the light when the plant was covered with foil except for the tips.

The next experiment was to cover the growing region of the coleoptiles, rather than the tips of the plant. The growing region is just below the tip, and for good measure, the rest of the plant was covered as well.

While covered with the tip exposed, the plant bent towards the light. Darwin was on track to proving his hypothesis that the tip of the plant were the “eyes” that sensed light.

Image: The stem did not bend when covered with an opaque cap.

Darwin’s next experiment brought him even closer to a conclusion. Next, he took an opaque cap that light could not shine through, and he covered the tip of the plant with it. As he suspected in this experiment, the plant did not grow towards the light in this scenario either. If he removed the cap, however, the plant would grow towards the light – so it did not have anything to do with a lack of ability to grow, but the cap.

Unfortunately, even the lightest cap is heavy to a small plant. Darwin could not conclude that it was the tip of the plant that sensed light yet because he could not guarantee that it was not another factor keeping the plant from growing, such as the weight from the cap. He continued to another set of experiments on his specimen of canary grass.

Image: The stem bent towards the light when covered with the translucent glass cap.

Next, Darwin placed a glass cap on the tip of the plant and waited. This glass cap was not any heavier than the opaque cap he had used in his earlier experiment, but it did allow the light to pass through to the plant. This experiment could rule out the possibility of the previous cap being too heavy for the plant, if only it worked.

Likely to his surprise and excitement, the experiment proved what he had hoped; the canary grass bent towards the light, even with the weight of the glass cap on its tip.

Darwin concluded that there is something in the tip of the plant that senses the light and sends the signal for movement towards the bottom of the plant, where the bending occurs. He suspected it had some internal influence to signal this change which moved from the tip of the plant into the lower growing regions. This discovery helped scientists begin learning about the existence and influence of phytohormones, which, at that point, were unknown.

Further Discoveries in Phototropism

Darwin performed a number of other experiments as well in an attempt to discover what sort of signal was sent to the lower growing region of the plant, but he could not identify any one source. Later in 1913, a scientist named Peter Boysen-Jensen developed Darwin’s experiments further.

Image: Plant response with objects between decapitated stem and tip.
From left to right: decapitation and replacement: no bend. / Gelatin: bend. / Mica flake: no bend. / Foil: no bend.

These experiments revisited the cutting of the plant’s tip, which had previously stopped the plant from growing – but what if Boysen-Jensen replaced the tip?

In his first experiment, he reproduced the experiment of cutting off the plant’s tip. To no one’s surprise at that point, the plant would not bend towards light, just as it had not bent towards the light in Darwin’s experiments years earlier.

He then took another specimen and cut its tip off, placed a layer of gelatin, which is water-soluable, on the stump, and then replaced the tip. The plant bent towards the source of light, even though it had been decapitated and replaced.

When a thin flake of mica, which is impervious to chemical or electrical signals, was placed between another specimen’s decapitated tip and the growing region, however, the plant was unable to bend towards the light.

The plant was also unable to bend towards the light if a thin layer of foil was placed between it and its decapitated tip, ruling out the possibility of an electrical signal.

Image: The stem bends if the tip is placed back on one side of the stem, even when in complete darkness.

In 1918, a scientist named Arpad Paal continued Boysen-Jensen’s experiments by performing an experiment on coleoptiles grown in the dark. The experiment was simple; decapitate a specimen and place the tip back on, but not wholly on the stem. Instead, Paal placed the tip on only one side of the stem, and the plant, despite the darkness, bent as if it were reaching for a light source.

This allowed scientists to pinpoint that, indeed, the chemical causing the phototropic response gathered on the shaded side of the plant, not the illuminated side.

Image: When in the dark, the stem bent towards the light when agar was placed only on one side of the stem. This did not occur when agar was placed in the center of the stem.

Later, a scientist named Frits Went finalized these experiments in 1926 with the discovery of the phytohormone that was responsible for several decades of befuddlement and confusion. He performed these experiments in a different way, however, taking inspiration from all the experiments done before him. (4)

“Among the historical references to plant phototropism, Darwin’s The Power of Movement of Plants (1880) is arguably the most well-known. Therein, Darwin describes a mysterious substance that is transduced from the tip of the seedling, where the light signal is perceived, to lower portions of the seedling, where the signal response can be observed in the form of directional growth changes (Darwin, 1880). It was not until the 1920s that a significant breakthrough occurred when Frits Went (Went, 1926), working on phototropism in the oat (Avena sativa) coleoptile, isolated and identified Darwin’s mysterious substance as the plant hormone auxin.”

Emmanuel Liscum

He prepared blocks of agar, which would allow whatever hormonal or chemical influence to travel through, much like Boysen-Jenson’s experiment with gelatin. He prepared his specimens of grass, and one key difference with inspiration taken from Arpad Paal: he performed these experiments with coleoptiles grown in the dark.

Frits Went first cut the tips of the plant off and did nothing with them, as the others had done before him, and the plant did not grow.

Next, he placed agar blocks between the plant stem and its decapitated tip, much like the gelatin experiment as before. The plant grew straight up, confirming that there was indeed some signal transmitted from the tips to the growing region of the plant.

Image: Auxin moves to the shaded side of the plant. The concentration causes cell elongation and the plant begins to bend towards light.

Lastly, he placed the agar with the cut tips on one side of the plant’s stem. The side of the plant that had the agar elongated, causing the plant to bend much like it would have towards a light source.

The chemical was named auxin, which comes from a Greek word meaning “to grow.”

Influences of Other Plant Hormones

We have discussed auxin and its ability to elongate cells on the shaded side of a stem to cause the plant to bend towards the source of light. Auxin is also present in the roots, but it has the opposite effect. (5) This is an excellent example of negative phototropism.

Auxins have the opposite effect on root cells. In a root, the shaded side contains more auxin and grows less – causing the root to bend away from the light.”

BBC.co.uk

Plants, like humans and animals, are complex organisms that require a lot of molecules and cells working together to keep everything functioning correctly. To this end, there are plenty of phytohormones that affect photomorphogenesis, which is the overall development and structure of plants in relation to light.

For example, phytochromes are responsible for detecting light, but they also play a crucial role in plant development, photosynthesis, and other physiological responses, not just phototropism. Vice versa, several phytohormones can affect phototropism but not play a primary role with that process.

Many of the phytohormones that affect the plant’s ability to sense or react to light do play some role in phototropism, even if it is miniscule. Unfortunately, scientists have only begun to scratch the surface of this subject, so even if a phytohormones has been identified in a role, it might not be fully understood yet.

Ethylene is one phytohormone which affects the phototropic response as well as leaf abscission, ageing, and fruit ripening. (6) Ethylene may be responsible for the inhibition of auxin transport, at least in mung beans according to one study.

“We have performed experiments under hypobaric conditions which indicate that light-induced ethylene might participate in causing such transport inhibition in mung bean.”

Thomas Brennan

Gibberellins are another phytohormone which, among other physiological processes, help promote cell elongation and division in plants. Gibberrelic acid also helps regulate the transcription of other phototropic phytohormones, enabling plants to better respond to light.  

Cytokinins are a class of phytohormones which aid in exponential cell division in plants and, when combined with auxins, help promote callus cell differentiation. Callus cells are a layer of cells which cover wounds on plants; essentially, these phytohormones help promote creating these cells and signaling which function those cells should serve. Cells that must cover wounds in bark are physiologically different than cells that must cover wounds on a leaf or root.

Since cytokinins are essential to cell division and differentiation, they aid the plant in developing cells that then bend towards the light with the help of auxin’s shade signaling.

Effects of Red and Far Red Light

The sun emits a full range of colored light wavelengths, some of which are not even visible to our eyes, giving plants the full spectrum of light to regulate their growth and photosynthetic activity. Plants are capable of sensing all different wavelengths of light. These wavelengths are emitted by the sun and travel down through the atmosphere and onto the earth’s floor where there are trees, shrubs, and herbs, down to the smallest sprouts of grass. (7)

The wavelengths that plants can sense vary based on the position of the sun. For example, when the sun is setting, there is much more far red light hitting the forest floor than red light. Plants use this signal as a way to tell where the sun is and what time of day it is through photoperiodism. 

“Light has numerous effects on seedling development: inhibition of hypocotyl elongation, promotion of cotyledon expansion, primary leaf development, development of the chloroplasts, and regulation of gene expression.”

C. Frankhauser

Plant tissues contain something called a chromoprotein, which have an active form (Pfr) and an inactive form (Pr). These proteins affect a group of molecules within the plant, called chromophores. These chromophores help detect and capture light wavelengths – much like the other photoreceptors discussed earlier – and they are responsible for helping facilitate biological changes within the plant when they detect light.

When the sun is shining in the sky, it is producing all these wavelengths, however, it is predominately emitting blue light and, to a lesser degree, red light. When the sun is low in the horizon, it produces more red light than blue light.

Blue light strongly triggers phototropic responses, inhibiting growth, while the red light will trigger growth. With the natural light cycle for open-air plants, this constant change in light wavelength keeps the plant’s physiology in equilibrium – though other environmental factors such as gravity or weather may still alter the plant’s growth.

Image: From left to right – the sun produces more blue light wavelengths during the day than red, and more red than blue when setting.

Focusing solely on red light wavelengths, we can see that the sun predominately emits red light when it is high in the sky, and far red light when it is low in the atmosphere, such as when it is about to set. This means the plant senses the red light coming from sunlight and activates the active form of the chromoprotein (Pfr) triggering plant growth.

Plants will actively grow under Pfr light, however, when the sun reaches the horizon, the levels of far red light spike and the inactive form is produced, slowing plant growth. In a way, this prepares the plant for the nighttime where it will “rest”. For many flowering plants, a process called photoperiodism – discussed in the next section – is how the plant tells what time of year it is and controls flowering actions.

Plants constantly measure the ratio of Pfr/Pr which activates many different physiological processes depending on the plant. The ratio at dawn in especially important to regulating flowering, winter bud development, and vegetative growth.

Prolonged periods of far red light will cause the plant to extend, seeking a more substantial source of photosynthetic light wavelengths. Even if the sun is above emitting red light, the direct light will filter down to the forest floor and, in the shade of other plants, the light will become far red light. To the plant, a prolonged emission of far red light signals that something is shading it from above so it must move out of the way or risk becoming overshaded.

In laboratory studies, plants grown under exclusively far red light were spindly and weak, over-extending themselves in blind directions to seek a better source of light as there is no blue light to regulate their growth. Additionally, this is why shade plants typically take longer to grow than those that require full indirect or direct sunlight.

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Measuring Day Length: Photoperiodism

Before we go any further, there is a question that has yet to be answered: how does the plant sense light, and how does it know how long the day is?

Plants, being sessile organisms that must still react to their environment. Part of that reaction and adaptation is controlled by the changing of seasons; when the days get shorter, the plant can begin to brace themselves for colder weather. When colder weather hits and the days get shorter, plants prepare for winter.

Likewise, in the spring and summer, plants must sense when it is safe for them to bloom again. Warmer temperatures and longer day lengths signals the changing season to plants.

This phenomenon is seen most notably in deciduous trees, which shed their leaves to prepare for cold weather. Leaf abscission and color changes are triggered by a combination of colder weather and shorter days. Bud development and flowering is triggered by warmer weather and longer day lengths.

It is worth noting that not all plants respond the same; some plants bloom at night, others prefer colder rather than warmer weather. How they sense these changes in season is, however, the same.

Most plants fall under two categories: short-day plants and long-day plants. Plants that flower only when the day length is shorter than a certain threshold are called short-day plants, whereas plants that flower only when the day length is longer than a certain threshold are long-day plants.

Some plants are day-neutral and their flowering is not dependent on the day length.

Day length is a strong trigger for flower development, and flower growers are known to control the timing of their blooms by controlling the timing of their lights in an indoor setup.

An odd quirk of how the terminology is defined versus the truth of the matter: plants do not necessarily sense the day length, but the length of the night.

Short-day plants are truly long-night plants, while long-day plants are truly short-night plants. If a short-day (long night) plant in the lab is expecting 16 hours of night, then a researcher comes in in the middle of the night to turn a light on for just a few minutes – then turns it back off before they leave – the plant will not flower because it has been interrupted.

The plant, in a way, will remember the last time it saw red light from the lightbulb, and it will send the signal along that the night is not long enough to bloom yet. This is compared to a short-day plant that was in the same conditions at the same age but allowed to rest the full night without interruption – that plant would then begin to flower.

Phototropism Examples

There are many examples of phototropism in nature and the best part is, if you own plants, you probably don’t even need to leave your home to see this tropism in action.

Before Charles Darwin’s series of experiments which showed there was some sort of mobile signal inside the plant communicating to the growing region where the light was, botanists did not even believe that light was a significant factor in plant growth. Little did they know, less than a hundred years later, we would understand far more about light and its effects on plants than had previously been made in history.

Two popular examples of phototropism are the sunflower, Helianthus annus, and Pilobolus fungus, Pilobolus crystallinus.

Sunflowers

Sunflowers are highly susceptible to phototropic reactions. So much so that they can be visibly seen throughout the day tracking the sun’s movements. This motion was part of the reason they earned their name, the sunflower.

This phenomenon is most visible in young sunflowers, before they are at peak maturity. In the morning, the east-facing plants greet the sun and slowly turn west, tracking the sun across the sky to maximize sunlight exposure to the broad petals. This is because the levels of auxin change throughout the day depending on the sun’s position. When one side becomes shaded, the auxin levels shift to the shaded side and begin elongating the plant cells on that side, causing the stem – and thus the flower head – to turn towards the sunlight.

In the evening, once the sun has set, the now-west-facing flowers slowly turn their way back around at night as the auxin level settle in the stems, ready to greet the rising sun in the east again the next morning.

Image credit: britannica.com

Pilobolus Fungi

The Pilobolus fungi is also known as the hat-thrower fungus because the semi-translucent fungi grow dark sporangia that look like hats. They grow on grazing animal feces and must get their spores onto the food of foragers so that it may pass through the digestive tract and continue to grow a new generation of feces-loving fungus.

Unfortunately, not many animals will forage near feces, so Pilobolus uses a squirt-gun method – meaning high water pressure propulsion – within the fungus to “throw” the spores onto fresh food sources for grazing animals. These sporangia can be thrown up to 10 feet (3 m) away from the original fungus, reaching an incredible speed of 56 mph (90 km/h) if needed to propel the spores.

This fungus utilizes phototropism as a way to ensure their spores have the best chance at dispersal. Using these phototropic responses the same way plants do, these fungi orient themselves towards light, giving the best chance at propelling their spores through gaps in the grass and onto vegetation far enough away from feces that foraging herbivores are still likely to consume the spores.

References
  1. Sebastien Paque & Dolf Weijers, Q&A: Auxin: The Plant Molecule That Influences Almost Anything, https://bmcbiol.biomedcentral.com/articles/10.1186/s12915-016-0291-0
  2. Bin Kang, Nicolas Grancher, Vladimir Koyffmann, Danielle Lardemer, Sarah Burney, Margaret Ahmad, Multiple Interactions Between Cryptochrome and Phototropin Blue-Light Signalling Pathways in Arabidopsis Thaliana, https://pubmed.ncbi.nlm.nih.gov/18183416/
  3. Erik Runkle, Effects of Blue Light on Plants, https://www.canr.msu.edu/floriculture/uploads/files/blue-light.pdf
  4. Emmanuel Liscum, Scott K. Askinosie, Daniel L. Leuchtman, Johanna Morrow, Kyle T. Willenburg, Diana Roberts Coats, Phototropism: Growing Towards an Understanding of Plant Movement, http://www.plantcell.org/content/26/1/38
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