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Phototropism

Phototropism

Phototropism (pronounced foe-TA-tro-piz-em) is the growth of a plant in the direction of its light source. Plants are very sensitive to their environment and have evolved many forms of "tropisms" in order to ensure their survival. A tropism is the growth of a plant as a response to a stimulus, and phototropism occurs when a plant responds to light by bending in the direction of the light. Although plant physiologists (scientists who study how the processes of a plant actually work) know that this growth is caused by a plant hormone, they still do not fully understand exactly how it works.

Bending toward the light

Most of us at some time have noticed a houseplant on a windowsill that seems to have all of its thin stems leaning in the same direction, as if it were trying to press itself against the glass. Picking it up and turning the entire pot in the opposite direction so that the plant is pointing away from the window will only result, about eight hours later, in the plant having reversed itself and going about its business of pointing its leaves toward the window again. This is not because plants especially like

windows but rather because light is essential to their survival, and they have developed ways of making sure they get all they need.

We know then that it is the light coming through the window that the plants are striving to get closer to, but how is a plant, which is rooted in soil, able to "move" toward the light? Actually, the plant does not so much move toward the light source as it grows in that direction. As already noted, this growth of a plant that occurs as a response to a stimulus is called a tropism. There are several forms of tropisms, such as gravitropism or geotropism, in which a plant reacts to the force of gravity; hydrotropism, in which the presence of water causes a response; galvanotropism, in which a plant reacts to a direct electrical current; thigmotropism, in which a plant responds to being touched or some form of contact; and chemotropism, in which a plant reacts to a chemical stimulus. Since the prefix "photo" refers to light, phototropism involves a plant responding to light. In all of these tropisms, the plant's response involves some form of growth. Finally, all tropisms are either positive or negative, although these words are not always used. So when a plant's leaves grow toward the light (stimulus), it is technically called positive phototropism. When its roots normally grow away from the light, it is called negative phototropism.

Words to Know

Auxin: Any of various hormones or similar synthetic substances that regulate the growth and development of plants.

Photosynthesis: Chemical process by which plants containing chlorophyll use sunlight to manufacture their own food by converting carbon dioxide and water to carbohydrates, releasing oxygen as a by-product.

Tropism: The growth or movement of a plant toward or away from a stimulus.

How phototropism works

It is known that as long ago as 1809, Swiss botanist Augustin Pyrame de Candolle (17781841) observed the growth of a plant toward the light and stated that it was caused by an unequal growth on only one part of the plant. However, he could not understand how this was happening. Some seventy years later, English naturalist Charles Darwin (18091882) began to grow canary grass in order to feed the birds he kept, and he eventually discovered that it was the tips of the sprouting seedlings that were influenced by the direction of their light source. He and his son Francis learned this when they covered the tips of some seedlings and found that they did not move toward the light. When only the seedlings' stems were covered, however, they still moved toward the light.

It was not until the 1920s that Dutch botanist Frits W. Went (19031990) proved the connection between phototropism and a plant hormone called auxin. Went discovered that plants manufacture a growth stimulant (which he named auxin) in their tips, which they then send to other cells in the plant. In phototropism, however, this growth hormone is distributed unevenly when the light source comes from only one direction. Specifically, more auxin flows down the dark side, meaning that it grows faster than the exposed side of the plant. This unequal or one-sided growth (also called differential growth) brings about the curving or bending of the plant toward the light source. Went named this growth hormone after the Greek word auxein, which means "to increase." Although it was isolated and named, auxin was not understood chemically until twenty years later when it was finally identified chemically as indole-3-acetic acid.

Plants can react and adjust

Understanding what plant tropism is and, specifically, what happens during phototropism makes us realize that plants, as living things, necessarily demonstrate the several characteristics of life. Specifically, this includes growth, response to stimuli, and adaptation. It is because of its hormones that a plant's stem always grows upwards and its roots always grown downward. Since plants must make their own food to survive (by changing light energy into chemical energya process called photosynthesis), the ability to capture as much of this light energy as possible is crucial to its survival. Thus, plants have developed a chemical response to light or the lack of it that causes their stems to bend toward the stronger light.

Today, we know that a certain minimal amount of light (whether natural or artificial) has to be present for the plant to react chemically. This is called its threshold value. Despite our understanding of the basic stages and phases of phototropism, we are only now beginning to obtain the most basic knowledge of what goes on at the genetic and molecular level. We do realize however that plants are living, sensitive things that can adjust to their environment and actually seek out the light they need if they are not getting enough.

[See also Plant ]

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phototropism

phototropism (heliotropism) The growth of plant organs in response to light. Aerial shoots usually grow towards light, while some aerial roots grow away from light. The phototropic response is thought to be controlled by the differential distribution of the plant growth substance auxin in the illuminated part, which in turn causes differential growth of the shoot or root. See tropism.

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phototropism

phototropism Growth of a plant in response to the stimulus of light. Cell growth increases on the shaded side of the plant, resulting in curvature towards the source of light. Auxin hormones are involved in this process.

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phototropism

phototropism (heliotropism) A tropic response of a plant or plant organ to the stimulus of light.

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phototropism

phototropism See heliotropism.

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Phototropism

Phototropism

History of phototropism research

Cholodny-Went theory

The photoreceptor pigment

Phototropism in other organisms

Phototropism in nature

Resources

Phototropism is the orientation of an organism in response to asymmetric illumination. Phototropism is commonly observed in the stems of higher plants, which bend toward a light source as they grow. Phototropism can be positive (bending toward a light source) or negative (bending away from a light source), depending on the organism and nature of the illumination. Phototropism and other tropisms are different from nastic movements, which are also common in plants. A tropism is the orientation of an organism in response to an external stimulus in which the stimulus determines the orientation of the movement. A nastic movement is a growth movement in which the stimulus does not determine the orientation of the movement.

History of phototropism research

Plant physiologists have investigated phototropism for over 100 years. The best-known early research on phototropism was by Charles Darwin, who reported his experiments in a book published in 1880, The Power of Movement in Plants. Although Darwin was better known for his earlier books on evolution (The Origin of Species and The Descent of Man ), this book was an important contribution to plant physiology.

Darwin studied phototropism in canary grass and oat coleoptiles. The coleoptile is a hollow sheath of tissue which surrounds the apical axis (stem) of these and other grasses. Darwin demonstrated that coleop-tiles are phototropic in that they bend toward a light source. When he covered the tips of the coleoptiles, they were not phototropic but when he covered the lower portions of the coleoptiles, they were photo-tropic. Darwin concluded from these and other experiments that (a) the tip of the coleoptile is the most photosensitive region; (b) the middle of the coleoptile is responsible for most of the bending; and (c) an influence which causes bending is transmitted from the top to the middle of the coleoptile.

The Dutch-American botanist Frits Went built upon Darwins studies and began his own research on phototropism as a student in the 1920s. In particular, Went attempted to isolate the chemical influence that Darwin described. He took tips of oat coleoptiles and placed them on small blocks of agar, a special type of gel. Then, he placed these agar blocks on the sides of other coleoptiles whose tops he cut off. Each coleoptile bent away from the side that had the agar block. Went also performed important control experiments. He

observed that plain agar blocks, which were placed beneath the lower portions of coleoptiles had no effect on coleoptile bending. Went concluded that the coleoptile tips contained a chemical substance which diffused into the agar blocks and he named this substance auxin. The auxin that Went studied was subsequently identified by chemists as indole-3-acetic acid (IAA). IAA is one of many plant hormones which control a number of aspects of plant growth and development.

Cholodny-Went theory

These and other experiments by Went led to what has become known as the Cholodny-Went theory of tropic curvature. In terms of phototropism, the Cholodny-Went theory proposes that (a) auxin is synthesized in the coleoptile tip; (b) the coleoptile tip perceives the asymmetric illumination and this causes auxin to move into the un-irradiated side; (c) auxin moves down the coleoptile so that lower regions develop an auxin asymmetry; and (d) the higher auxin concentration on the un-irradiated side causes the coleoptile to bend toward the light source.

There is currently vigorous debate among plant physiologists about the Cholodny-Went theory. Critics have noted that Went and other early researchers never actually measured the auxin concentrations but only relied on bioassays performed with agar blocks. Furthermore, the early studies relied on small sample sizes which were statistically unreliable, and the researchers may have wounded the coleoptiles during tip removal.

In addition, numerous recent experiments indicate that the coleoptile tip is not always necessary for tropic responses and that auxin gradients form in the tissue more slowly than the development of curvature.

Despite these criticisms, many plant physiologists maintain that the basic features of the Cholodny-Went theory have been upheld. The debate about the Cholodny-Went theory has stimulated much new research in phototropism and gravitropism. Many researchers currently are investigating tropic curvature using modern time-lapse photography. Others are examining the role of additional plant hormones in regulating phototropism and gravitropism.

The photoreceptor pigment

There has also been an active search for the identity of the photoreceptor pigment, an aspect of photo-tropism not covered by the Cholodny-Went theory. In the 1930s, many researchers believed the photoreceptor was a carotenoid, a class of mostly orange plant pigments. They argued that carotenoids strongly absorb blue light and phototropism is most effectively elicited by blue light. Furthermore, retinal, a carotenoid derivative, was identified as the photoreceptive pigment controlling vision in humans and other animals.

However, more recent experiments appear to rule out a carotenoid as the photoreceptor. In particular, when seedlings are treated with norflurazon, a chemical inhibitor of carotenoid synthesis, they still exhibit phototropism. In addition, mutants of plants and fungi which have greatly reduced amounts of carotenoids are unaffected in their phototropic responses.

A great variety of different experiments now indicate that a flavin (vitamin B-2) is the photoreceptor pigment. Like carotenoids, flavins strongly absorb blue light. However, unlike most carotenoids, they also strongly absorb radiation in the near-ultraviolet (370 nm) region. Radiation in the near-ultraviolet region of the spectrum is also highly effective in phototropism.

Phototropism in other organisms

While phototropism has been most intensively studied in vascular plants, many other organisms also exhibit phototropism. Phototropism occurs in the filaments and rhizoids of algae, germ tubes and protonemas of mosses, rhizoids and protonemas of ferns, spore-bearing stalks of certain fungi, and numerous other organisms.

Many phototropism experiments have been performed on Phycomyces blakesleeanus, a zygomycete fungus. Phycomyces has slender spore-bearing stalks, referred to as sporangiophores, which bend in response to light and other external stimuli. Incredibly, the sporangiophore of Phycomyces is about as photosensitive as the eyes of humans and about one thousand times more photosensitive than a grass coleoptile. Furthermore, the sporangiophore has the ability to adapt to a one hundred million-fold change in ambient light intensity. These and other interesting characteristics of Phycomyces have made it an excellent model organism for investigation of phototropism.

Phototropism in nature

Laboratory studies of phototropism have a bearing upon the life of plants in nature. It is advantageous for a young seedling, such as a coleoptile, to bend toward the light so that its leaves can intercept more sunlight for photosynthesis and grow faster. Phototropism is also related to solar tracking, the orientation of a plants leaves in response to the sun.

KEY TERMS

Agar Carbohydrate derived from a red alga which biologists use in a gel form for culture media or other purposes.

Bioassay Estimation of the amount of a substance, such as a hormone, based upon its effect on some easily measured response of an organism.

Coleoptile Hollow sheath of tissue which surrounds the stem of young grass plants.

Gravitropism Orientation of an organism in response to gravity.

Nastic movement Growth movement controlled by external or endogenous factors in which the orientation of the movement is not determined by an external stimulus.

Tropism Orientation of an organism in response to an external stimulus such as light, gravity, wind, or other stimuli, in which the stimulus determines the orientation of the movement.

Unlike the response in coleoptiles, which is caused by differential stem growth, solar tracking responses in most species are caused by pressure changes in special cells at the leaf base. Depending on the species and other factors, the blades of a mature leaf may be oriented perpendicular to the suns rays to maximize photosynthesis or parallel to the suns rays to avoid over-heating and desiccation.

See also Geotropism.

Resources

BOOKS

Osborne, Daphne J. and Michael T. McManus. Hormones, Signals and Target Cells in Plant Development. New York: Cambridge University Press, 2005.

Peter A. Ensminger

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Phototropism

Phototropism

Phototropism is the orientation of an organism in response to asymmetric illumination. Phototropism is commonly observed in the stems of higher plants, which grow bent toward a light source. Phototropism can be positive (bending toward a light source) or negative (bending away from a light source), depending on the organism and nature of the illumination. Phototropism and other tropisms are different from nastic movements, which are also common in plants. A tropism is the orientation of an organism in response to an external stimulus in which the stimulus determines the orientation of the movement. A nastic movement is a growth movement in which the stimulus does not determine the orientation of the movement.


History of phototropism research

Plant physiologists have investigated phototropism for over 100 years. The best known early research on phototropism was by Charles Darwin, who reported his experiments in a book published in 1880, The Power of Movement in Plants. Although Darwin was better known for his earlier books on evolution (The Origin of Species and The Descent of Man), this book was an important contribution to plant physiology .

Darwin studied phototropism in canary grass and oat coleoptiles. The coleoptile is a hollow sheath of tissue which surrounds the apical axis (stem) of these and other grasses . Darwin demonstrated that these coleoptiles are phototropic in that they bend toward a light source. When he covered the tips of the coleoptiles, they were not phototropic but when he covered the lower portions of the coleoptiles, they were phototropic. Darwin concluded from these and other experiments that (a) the tip of the coleoptile is the most photosensitive region; (b) the middle of the coleoptile is responsible for most of the bending; and (c) an influence which causes bending is transmitted from the top to the middle of the coleoptile.

The Dutch-American botanist Frits Went built upon Darwin's studies and began his own research on phototropism as a student in the 1920s. In particular, Went attempted to isolate the chemical influence which Darwin described. He took tips of oat coleoptiles and placed them on small blocks of agar, a special type of gel. Then, he placed these agar blocks on the sides of other coleoptiles whose tops he cut off. Each coleoptile bent away from the side which had the agar block. Went also performed important control experiments. He observed that plain agar blocks which were placed beneath the lower portions of coleoptiles had no effect on coleoptile bending. Went concluded that the coleoptile tips contained a chemical substance which diffused into the agar blocks and he named this substance auxin. The auxin which Went studied was subsequently identified by chemists as indole-3-acetic acid (IAA). IAA is one of many plant hormones which control a number of aspects of plant growth and development.


Cholodny-Went theory

These and other experiments by Went led to what has become known as the Cholodny-Went theory of tropic curvature. In terms of phototropism, the Cholodny-Went theory proposes that (a) auxin is synthesized in the coleoptile tip; (b) the coleoptile tip perceives the asymmetric illumination and this causes auxin to move into the un-irradiated side; (c) auxin moves down the coleoptile so that lower regions develop an auxin asymmetry; and (d) the higher auxin concentration on the un-irradiated side causes the coleoptile to bend toward the light source.

There is currently vigorous debate among plant physiologists about the Cholodny-Went theory. Critics have noted that Went and other early researchers never actually measured the auxin concentrations but only relied on bioassays performed with agar blocks. Furthermore, the early studies relied on small sample sizes which were statistically unreliable, and the researchers may have wounded the coleoptiles during tip removal.

In addition, numerous recent experiments indicate that the coleoptile tip is not always necessary for tropic responses and that auxin gradients form in the tissue more slowly than the development of curvature.

Despite these criticisms, many plant physiologists maintain that the basic features of the Cholodny-Went theory have been upheld. The debate about the Cholodny-Went theory has stimulated much new research in phototropism and gravitropism. Many researchers currently are investigating tropic curvature using modern time-lapse photography . Others are examining the role of additional plant hormones in regulating phototropism and gravitropism.


The photoreceptor pigment

There has also been an active search for the identity of the photoreceptor pigment, an aspect of phototropism not covered by the Cholodny-Went theory. In the 1930s, many researchers believed the photoreceptor was a carotenoid, a class of mostly orange plant pigments. They argued that carotenoids strongly absorb blue light and phototropism is most effectively elicited by blue light. Furthermore, retinal, a carotenoid derivative, was identified as the photoreceptive pigment controlling vision in humans and other animals.

However, more recent experiments appear to rule out a carotenoid as the photoreceptor. In particular, when seedlings are treated with norflurazon, a chemical inhibitor of carotenoid synthesis, they still exhibit phototropism. In addition, mutants of plants and fungi which have greatly reduced amounts of carotenoids are unaffected in their phototropic responses.

A great variety of different experiments now indicate that a flavin (vitamin B-2) is the photoreceptor pigment. Like carotenoids, flavins strongly absorb blue light. However, unlike most carotenoids, they also strongly absorb radiation in the near-ultraviolet (370 nm) region. Radiation in the near-ultraviolet region of the spectrum is also highly effective in phototropism.


Phototropism in other organisms

While phototropism has been most intensively studied in higher plants, many other organisms also exhibit phototropism. Phototropism occurs in the filaments and rhizoids of algae , germ tubes and protonemas of mosses, rhizoids and protonemas of ferns , spore-bearing stalks of certain fungi, and numerous other organisms.

Many phototropism experiments have been performed on Phycomyces blakesleeanus, a zygomycete fungus. Phycomyces has slender spore-bearing stalks, referred to as sporangiophores, which bend in response to light and other external stimuli. Incredibly, the sporangiophore of Phycomyces is about as photosensitive as the eyes of humans and about one thousand times more photosensitive than a grass coleoptile. Furthermore, the sporangiophore has the ability to adapt to a one hundred million fold change in ambient light intensity. These and other interesting characteristics of Phycomyces have made it an excellent model organism for investigation of phototropism.

Phototropism in nature

Laboratory studies of phototropism have a bearing upon the life of plants in nature. It is advantageous for a young seedling, such as a coleoptile, to bend toward the light so that its leaves can intercept more sunlight for photosynthesis and grow faster. Phototropism is also related to solar tracking, the orientation of a plant's leaves in response to the Sun . Unlike the response in coleoptiles, which is caused by differential stem growth, solar tracking responses in most species are caused by pressure changes in special cells at the leaf base. Depending on the species and other factors, the blades of a mature leaf may be oriented perpendicular to the Sun's rays to maximize photosynthesis or parallel to the Sun's rays to avoid over-heating and desiccation.

See also Geotropism.


Resources

books

Hart, J.W. Plant Tropisms and Other Growth Movements. London: Routledge, Chapman & Hall, 1990.


Peter A. Ensminger

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Agar

—Carbohydrate derived from a red alga which biologists use in a gel form for culture media or other purposes.

Bioassay

—Estimation of the amount of a substance, such as a hormone, based upon its effect on some easily measured response of an organism.

Coleoptile

—Hollow sheath of tissue which surrounds the stem of young grass plants.

Gravitropism

—Orientation of an organism in response to gravity.

Nastic movement

—Growth movement controlled by external or endogenous factors in which the orientation of the movement is not determined by an external stimulus.

Tropism

—Orientation of an organism in response to an external stimulus such as light, gravity, wind, or other stimuli, in which the stimulus determines the orientation of the movement.

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Phototropism

Phototropism


Phototropism is the term used to describe a plant's response to light. When we notice that a potted plant on a windowsill has turned its leaves toward the light, we are witnessing phototropism. This is but one form of tropism or plant "behavior." A tropism is a phenomenon in which a plant grows in response to some outside stimulus.

Although plants cannot move in the manner of other organisms, plants are living things and, therefore, are sensitive to external stimuli. Their reactions to the many different outside forces they meet sometimes give the impression that they have indeed moved. Although plants usually appear motionless unless they are moved by the wind, they are in fact growing

much of the time and responding to a variety of environmental stimuli. When a plant's reactions to a stimulus result directly in any type of plant growth, botanists call this phenomenon a tropism. Tropisms can be positive or negative. A positive tropism means that the plant begins to grow toward the outside stimulus. Negative tropism means that it grows away from the source.

When a plant responds to a light source by growing in the direction of that source, it is called positive phototropism. (Photo means light in Latin.) There are many other forms of tropisms, but all must involve plant growth as a response. Chemotropism is a plant's response to chemicals; thigmotropism is its response to being touched; geotropism is its response to the force of gravity. Since all tropisms can be positive or negative, the growth of a seed's roots downward into the soil is positive geotropism (in the direction of the source) but also negative phototropism (away from the light). The upward growth of the new shoot is the reverse (positive phototropism and negative geotropism).

Plants have evolved tropism in order to maximize a particular function and therefore be better able to compete, survive, and reproduce. When a plant grows toward the light (positive phototropism), it can grow more rapidly and if necessary, out-compete its neighbor for scarce resources. An obvious example of tropism is hydrotropism in which a plant during a drought will make its root system grow away from its natural, gravity-pulled downward course and off in a direction containing life-sustaining water. Tropisms are one means that plants have to battle for their survival.

The actual mechanism by which a tropism (stimulus/growth) occurs could be described as uneven growth. Specifically, when a stem or root moves toward or away from an outside stimulus, it must grow in a curve. It achieves this "curved" growth by having the outside of the curve grow faster than the inside. This is caused by the plant hormone called auxin. According to what type of specialized tissue is receiving the stimulus (such as a root or a stem), a larger amount of auxin moves from the growing tip and down one side than moves down the other. Since auxins come from the tip, when a plant wants to move toward the light it sends an unequal amount of auxin down its sides. More auxin goes to the shaded side and less to the sunny side, meaning that the shaded side grows more than the sunny side and the plant therefore grows in a curve toward the light source.

[See alsoLight; Plants ]

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