30.7: Plant Sensory Systems and Responses

30.7.1: Plant Responses to Light

Learning Objective

Compare the ways plants respond to light

Key Points

• Plants grow and differentiate to optimize their space, using light in a process known as photomorphogenesis.
• Plants grow and move toward or away from light depending on their needs; this process is known as phototropism.
• Photoperiodism is illustrated by how plants flower and grow at certain times of the day or year through the use of photoreceptors that sense the wavelengths of sunlight available during the day (versus night) and throughout the seasons.
• The various wavelengths of light, red/far-red or blue regions of the visible light spectrum, trigger structural responses in plants suited for responding to those wavelengths.

Key Terms

photoreceptor

a specialized protein that is able to detect and react to light

photoperiodism

the growth, development and other responses of plants and animals according to the length of day and/or night

photomorphogenesis

the regulatory effect of light on the growth, development and differentiation of plant cells, tissues and organs

phototropism

the movement of a plant toward or away from light

Plant Responses to Light

Plants have a number of sophisticated uses for light that go far beyond their ability to perform photosynthesis. Plants can differentiate and develop in response to light (known as photomorphogenesis), which allows plants to optimize their use of light and space. Plants use light to track time, which is known as photoperiodism. They can tell the time of day and time of year by sensing and using various wavelengths of sunlight. Light can also elicit a directional response in plants that allows them to grow toward, or even away from, light; this is known as phototropism .

Phototropism of an orchid plant

This orchid plant placed next to a window grows toward the sunlight through the window. This is an example of positive phototropism.

The sensing of light in the environment is important to plants; it can be crucial for competition and survival. The response of plants to light is mediated by different photoreceptors: a protein covalently-bonded to a light-absorbing pigment called a chromophore; together, called a chromoprotein. The chromophore of the photoreceptor absorbs light of specific wavelengths, causing structural changes in the photoreceptor protein. The structural changes then elicit a cascade of signaling throughout the plant.

The red, far-red, and violet-blue regions of the visible light spectrum trigger structural development in plants. Sensory photoreceptors absorb light in these particular regions of the visible light spectrum because of the quality of light available in the daylight spectrum. In terrestrial habitats, light absorption by chlorophylls peaks in the blue and red regions of the spectrum. As light filters through the canopy and the blue and red wavelengths are absorbed, the spectrum shifts to the far-red end, shifting the plant community to those plants better adapted to respond to far-red light. Blue-light receptors allow plants to gauge the direction and abundance of sunlight, which is rich in blue–green emissions. Water absorbs red light, which makes the detection of blue light essential for algae and aquatic plants.

30.7.2: The Phytochrome System and Red Light Response

Learning Objective

Explain the response of the phytochrome system to red/far-red light

Key Points

• Exposure to red light converts the chromoprotein to the functional, active form (Pfr), while darkness or exposure to far-red light converts the chromophore to the inactive form (Pr).
• Plants grow toward sunlight because the red light from the sun converts the chromoprotein into the active form (Pfr), which triggers plant growth; plants in shade slow growth because the inactive form (Pr) is produced.
• If seeds sense light using the phytochrome system, they will germinate.
• Plants regulate photoperiodism by measuring the Pfr/Pr ratio at dawn, which then stimulates physiological processes such as flowering, setting winter buds, and vegetative growth.

Key Terms

photoperiodism

the growth, development and other responses of plants and animals according to the length of day and/or night

chromophore

the group of atoms in a molecule in which the electronic transition responsible for a given spectral band is located

phytochrome

any of a class of pigments that control most photomorphogenic responses in higher plants

The Phytochrome System and the Red/Far-Red Response

The phytochromes are a family of chromoproteins with a linear tetrapyrrole chromophore, similar to the ringed tetrapyrrole light-absorbing head group of chlorophyll. Phytochromes have two photo-interconvertible forms: Pr and Pfr. Pr absorbs red light (~667 nm) and is immediately converted to Pfr. Pfr absorbs far-red light (~730 nm) and is quickly converted back to Pr. Absorption of red or far-red light causes a massive change to the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the physiologically-active form of the protein; exposure to red light yields physiological activity in the plant. Exposure to far-red light converts the Pfr to the inactive Pr form, inhibiting phytochrome activity. Together, the two forms represent the phytochrome system .

Phytochrome system

The biologically-inactive form of phytochrome (Pr) is converted to the biologically-active form Pfr under illumination with red light. Far-red light and darkness convert the molecule back to the inactive form.

The phytochrome system acts as a biological light switch. It monitors the level, intensity, duration, and color of environmental light. The effect of red light is reversible by immediately shining far-red light on the sample, which converts the chromoprotein to the inactive Pr form. Additionally, Pfr can slowly revert to Pr in the dark or break down over time. In all instances, the physiological response induced by red light is reversed. The active form of phytochrome (Pfr) can directly activate other molecules in the cytoplasm, or it can be trafficked to the nucleus, where it directly activates or represses specific gene expression.

The Phytochrome System and Growth

Plants use the phytochrome system to grow away from shade and toward light. Unfiltered, full sunlight contains much more red light than far-red light. Any plant in the shade of another plant will be exposed to red-depleted, far-red-enriched light because the other plant has absorbed most of the other red light. The exposure to red light converts phytochrome in the shaded leaves to the Pr (inactive) form, which slows growth. The leaves in full sunlight are exposed to red light and have activated Pfr, which induces growth toward sunlit areas. Because competition for light is so fierce in a dense plant community, those plants who could grow toward light the fastest and most efficiently became the most successful.

The Phytochrome System in Seeds

In seeds, the phytochrome system is used to determine the presence or absence of light, rather than the quality. This is especially important in species with very small seeds and, therefore, food reserves. For example, if lettuce seedlings germinated a centimeter under the soil surface, the seedling would exhaust its food resources and die before reaching the surface. A seed will only germinate if exposed to light at the surface of the soil, causing Pr to be converted to Pfr, signaling the start of germination. In the dark, phytochrome is in the inactive Pr form so the seed will not germinate.

Photoperiodism

Plants also use the phytochrome system to adjust growth according to the seasons. Photoperiodism is a biological response to the timing and duration of dark and light periods. Since unfiltered sunlight is rich in red light, but deficient in far-red light, at dawn, all the phytochrome molecules in a leaf convert to the active Pfr form and remain in that form until sunset. Since Pfr reverts to Pr during darkness, there will be no Pfr remaining at sunrise if the night is long (winter) and some Pfr remaining if the night is short (summer). The amount of Pfr present stimulates flowering, setting of winter buds, and vegetative growth according to the seasons.

In addition, the phytochrome system enables plants to compare the length of dark periods over several days. Shortening nights indicate springtime to the plant; lengthening nights indicate autumn. This information, along with sensing temperature and water availability, allows plants to determine the time of the year and adjust their physiology accordingly. Short-day (long-night) plants use this information to flower in the late summer and early fall when nights exceed a critical length (often eight or fewer hours). Long-day (short-night) plants flower during the spring when darkness is less than a critical length (often 8 to 15 hours). However, day-neutral plants do not regulate flowering by day length. Not all plants use the phyotochrome system to adjust their physiological responses to the seasons.

30.7.3: Blue Light Response

Learning Objective

Differentiate among blue light responses of plants

Key Points

• In addition to phototropism, phototropins sense blue light to control leaf opening and closing, chloroplast movement, and the opening of stomata.
• When phototropins are activated by blue light, the hormone auxin accumulates on the shaded side of the plant, triggering elongation of stem cells and phototropism.
• Cryptochromes sense blue light-dependent redox reactions to control the circadian rhythm of plants.

Key Terms

cryptochrome

any of several light-sensitive flavoproteins, in the protoreceptors of plants, that regulate germination, elongation, and photoperiodism

auxin

a class of plant growth hormones that is responsible for elongation in phototropism and gravitropism and for other growth processes in the plant life cycle

phototropin

any of a class of photoreceptor flavoproteins that mediate phototropism in higher plants

skototropism

growth or movement away from light

The Blue Light Responses

Phototropism is the directional bending of a plant toward or away from a light source of blue wavelengths of light. Positive phototropism is growth toward a light source , while negative phototropism (also called skototropism) is growth away from light. Several proteins use blue light to control various physiological processes in the plant.

Blue light response of azure bluets

Azure bluets (Houstonia caerulea) display a phototropic response by bending toward the light.

Phototropins and Physiological Responses

Phototropins are protein-based receptors responsible for mediating the phototropic response in plants. Like all plant photoreceptors, phototropins consist of a protein portion and a light-absorbing portion, called the chromophore, which senses blue wavelengths of light. Phototropins belong to a class of proteins called flavoproteins because the chromophore is a covalently-bound molecule of flavin.

Phototropins control other physiological responses including leaf opening and closing, chloroplast movement, and the opening of stomata. However, of all responses controlled by phototropins, phototropism has been studied the longest and is the best understood.

Phototropism and Auxin

In 1880, Charles Darwin and his son Francis first described phototropism as the bending of seedlings toward light. Darwin observed that light was perceived by the the apical meristem (tip of the plant), but that the plant bent in response in a different part of the plant. The Darwins concluded that the signal had to travel from the apical meristem to the base of the plant, where it bent.

In 1913, Peter Boysen-Jensen conducted an experiment that demonstrated that a chemical signal produced in the plant tip was responsible for the plant's bending response at the base. He cut off the tip of a seedling, covered the cut section with a permeable layer of gelatin, and then replaced the tip. The seedling bent toward the light when illuminated even though the layer of gelatin was present. However, when impermeable mica flakes were inserted between the tip and the cut base, the seedling did not bend.

A refinement of Boysen-Jensen's experiment showed that the signal traveled on the shaded side of the seedling. When the mica plate was inserted on the illuminated side, the plant still bent toward the light. Therefore, the chemical signal from the sunlight, which is blue wavelengths of light, was a growth stimulant; the phototropic response involved faster cell elongation on the shaded side than on the illuminated side, causing the plant to bend. We now know that as light passes through a plant stem, it is diffracted and generates phototropin activation across the stem. Most activation occurs on the lit side, causing the plant hormones indole acetic acid (IAA) or auxin to accumulate on the shaded side. Stem cells elongate under the influence of IAA .

Phototropism and the distribution of auxin

Phototropism is the growth of plants in response to light. When the sun is positioned almost directly over the plant, the hormone auxin (pink dots) in the plant stem is evenly distributed. As the sun moves, the auxin is repositioned on the other side of the plant. This overload of auxin next to these cells causes them to start to grow or elongate, tipping the growth of the stem toward the light.

Cryptochromes

Cryptochromes are another class of blue-light absorbing photoreceptors. Their chromophores also contain a flavin-based chromophore. Cryptochromes set the plant's circadian rhythm (the 24-hour activity cycle) using blue light receptors. There is some evidence that cryptochromes work by sensing light-dependent redox reactions and that, together with phototropins, they mediate the phototropic response.

30.7.4: Plant Responses to Gravity

Learning Objective

Describe the role of amyloplasts in gravitropism

Key Points

• Positive gravitropism occurs when roots grow into soil because they grow in the direction of gravity while negative gravitropism occurs when shoots grow up toward sunlight in the opposite direction of gravity.
• Amyloplasts settle at the bottom of the cells of the shoots and roots in response to gravity, causing calcium signaling and the release of indole acetic acid.
• Indole acetic acid inhibits cell elongation in the lower side of roots, but stimulates cell expansion in shoots, which causes shoots to grow upward.

Key Terms

statolith

a specialized form of amyloplast involved in graviperception by plant roots and most invertebrates

amyloplast

a non-pigmented organelle found in some plant cells that is responsible for the synthesis and storage of starch granules through the polymerization of glucose

gravitropism

a plant's ability to change its growth in response to gravity

Plant Responses to Gravity

Whether or not they germinate in the light or in total darkness, shoots usually sprout up from the ground, while roots grow downward into the ground . A plant laid on its side in the dark will send shoots upward when given enough time. Gravitropism ensures that roots grow into the soil and that shoots grow toward sunlight. Growth of the shoot apical tip upward is called negative gravitropism, whereas growth of the roots downward is called positive gravitropism.

The reason plants know which way to grow in response to gravity is due to amyloplasts in the plants. Amyloplasts (also known as statoliths) are specialized plastids that contain starch granules and settle downward in response to gravity. Amyloplasts are found in shoots and in specialized cells of the root cap. When a plant is tilted, the statoliths drop to the new bottom cell wall. A few hours later, the shoot or root will show growth in the new vertical direction .

Gravitropism

This is an image of an upright tree with high curvature at the base as a result of negative gravitropism. Despite being tilted, amyloplasts will cause the shoot to grow in a vertical direction.

The mechanism that mediates gravitropism is reasonably well understood. When amyloplasts settle to the bottom of the gravity-sensing cells in the root or shoot, they physically contact the endoplasmic reticulum (ER). This causes the release of calcium ions from inside the ER. This calcium signaling in the cells causes polar transport of the plant hormone indole acetic acid (IAA) to the bottom of the cell. In roots, a high concentration of IAA inhibits cell elongation. The effect slows growth on the lower side of the root while cells develop normally on the upper side. IAA has the opposite effect in shoots, where a higher concentration at the lower side of the shoot stimulates cell expansion and causes the shoot to grow up. After the shoot or root begin to grow vertically, the amyloplasts return to their normal position. Other hypotheses, which involve the entire cell in the gravitropism effect, have been proposed to explain why some mutants that lack amyloplasts may still exhibit a weak gravitropic response.

30.7.5: Auxins, Cytokinins, and Gibberellins

Learning Objective

Differentiate among the types of plant hormones and their effects on plant growth

Key Points

• During phototropism and gravitropism, the plant hormone auxin controls cell elongation.
• The plant hormone cytokinin promotes cell division, controling many developmental processes in plants.
• Gibberellins control many aspects of plant physiology including shoot elongation, seed germination, fruit and flower maturation, seed dormancy, gender expression, seedless fruit development, and the delay of senescence in leaves and fruit.

Key Terms

cytokinin

any of a class of plant hormones involved in cell growth and division

auxin

a class of plant growth hormones that is responsible for elongation in phototropism and gravitropism and for other growth processes in the plant life cycle

gibberellin

any of a class of diterpene plant growth hormones that stimulate shoot elongation, seed germination, and fruit and flower maturation

Growth Responses

A plant's sensory response to external stimuli relies on hormones, which are simply chemical messengers. Plant hormones affect all aspects of plant life, from flowering to fruit setting and maturation, and from phototropism to leaf fall. Potentially, every cell in a plant can produce plant hormones. The hormones can act in their cell of origin or be transported to other portions of the plant body, with many plant responses involving the synergistic or antagonistic interaction of two or more hormones. In contrast, animal hormones are produced in specific glands and transported to a distant site for action, acting alone.

Plant hormones are a group of unrelated chemical substances that affect plant morphogenesis. Five major plant hormones are traditionally described: auxins, cytokinins, gibberellins, ethylene, and abscisic acid. In addition, other nutrients and environmental conditions can be characterized as growth factors. The first three plant hormones largely affect plant growth, as described below.

Auxins

The term auxin is derived from the Greek word auxein, which means "to grow. " Auxins are the main hormones responsible for cell elongation in phototropism and gravitropism. They also control the differentiation of meristem into vascular tissue and promote leaf development and arrangement. While many synthetic auxins are used as herbicides, indole acetic acid (IAA) is the only naturally-occurring auxin that shows physiological activity. Apical dominance (the inhibition of lateral bud formation) is triggered by auxins produced in the apical meristem. Flowering, fruit setting and ripening, and inhibition of abscission (leaf falling) are other plant responses under the direct or indirect control of auxins. Auxins also act as a relay for the effects of the blue light and red/far-red responses.

Commercial use of auxins is widespread in plant nurseries and for crop production. IAA is used as a rooting hormone to promote growth of adventitious roots on cuttings and detached leaves. Applying synthetic auxins to tomato plants in greenhouses promotes normal fruit development. Outdoor application of auxin promotes synchronization of fruit setting and dropping, which coordinates the harvesting season. Fruits such as seedless cucumbers can be induced to set fruit by treating unfertilized plant flowers with auxins.

Cytokinins

The effect of cytokinins was first reported when it was found that adding the liquid endosperm of coconuts to developing plant embryos in culture stimulated their growth. The stimulating growth factor was found to be cytokinin, a hormone that promotes cytokinesis (cell division). Almost 200 naturally-occurring or synthetic cytokinins are known, to date. Cytokinins are most abundant in growing tissues, such as roots, embryos, and fruits, where cell division is occurring. Cytokinins are known to delay senescence in leaf tissues, promote mitosis, and stimulate differentiation of the meristem in shoots and roots. Many effects on plant development are under the influence of cytokinins, either in conjunction with auxin or another hormone. For example, apical dominance seems to result from a balance between auxins that inhibit lateral buds and cytokinins that promote bushier growth.

Gibberellins

Gibberellins (GAs) are a group of about 125 closely-related plant hormones that stimulate shoot elongation, seed germination, and fruit and flower maturation. GAs are synthesized in the root and stem apical meristems, young leaves, and seed embryos. In urban areas, GA antagonists are sometimes applied to trees under power lines to control growth and reduce the frequency of pruning.

GAs break dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure to cold or light to germinate. Abscisic acid is a strong antagonist of GA action. Other effects of GAs include gender expression, seedless fruit development, and the delay of senescence in leaves and fruit. Seedless grapes are obtained through standard breeding methods; they contain inconspicuous seeds that fail to develop. Because GAs are produced by the seeds and because fruit development and stem elongation are under GA control, these varieties of grapes would normally produce small fruit in compact clusters. Maturing grapes are routinely treated with GA to promote larger fruit size, as well as looser bunches (longer stems), which reduces the incidence of mildew infection .

Effect of gibberellins on grapes

In grapes, application of gibberellic acid increases the size of fruit and loosens clustering.

30.7.6: Abscisic Acid, Ethylene, and Nontraditional Hormones

Learning Objective

Describe the roles played by ethylene and nontraditional hormones in plant development

Key Points

• Under stress, abscisic acid accumulates in plants, inhibiting stem elongation and inducing bud dormancy.
• The plant hormone ethylene controls fruit ripening, flower wilting, and leaf fall by stimulating the conversion of starch and acids to sugars.
• Other nontraditional hormones such as jasmonates and oligosaccharins control defense responses from herbivores and bacterial/fungal infections, respectively.

Key Terms

abscisic acid

a plant hormone that functions in many plant developmental processes, including bud dormancy, inhibition of seed germination, and plant stress tolerance.

jasmonate

any of several esters of jasmonic acid that act as plant hormones

ethylene

a plant hormone that is involved in fruit ripening, flower wilting, and leaf fall

Growth Responses

In addition to the growth hormones auxins, cytokinins, gibberellins, there are two more major types of plant hormones, abscisic acid and ethylene, as well as several other less-studied compounds that control plant physiology.

Abscisic Acid

The plant hormone abscisic acid (ABA) was first discovered as the agent that causes the abscission or dropping of cotton bolls. However, more-recent studies indicate that ABA plays only a minor role in the abscission process. ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths. Its activity counters many of the growth-promoting effects of GAs and auxins. ABA inhibits stem elongation and induces dormancy in lateral buds.

ABA induces dormancy in seeds by blocking germination and promoting the synthesis of storage proteins. Plants adapted to temperate climates require a long period of cold temperature before seeds germinate. This mechanism protects young plants from sprouting too early during unseasonably warm weather in winter. As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. Another effect of ABA is to promote the development of winter buds; it mediates the conversion of the apical meristem into a dormant bud. Low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss in winter buds.

Ethylene

Ethylene is associated with fruit ripening, flower wilting, and leaf fall. Ethylene is unusual because it is a volatile gas (C₂H₄). Hundreds of years ago, when gas street lamps were installed in city streets, trees that grew close to lamp posts developed twisted, thickened trunks, shedding their leaves earlier than expected. These effects were caused by ethylene volatilizing from the lamps.

Aging tissues (especially senescing leaves) and nodes of stems produce ethylene. The best-known effect of the hormone, however, is the promotion of fruit ripening . Ethylene stimulates the conversion of starch and acids to sugars. Some people store unripe fruit, such as avocados, in a sealed paper bag to accelerate ripening; the gas released by the first fruit to mature will speed up the maturation of the remaining fruit. Ethylene also triggers leaf and fruit abscission, flower fading and dropping, and promotes germination in some cereals and sprouting of bulbs and potatoes.

Date ripening

The plant hormone ethylene promotes ripening, as seen in the ripening of dates.

Ethylene is widely used in agriculture. Commercial fruit growers control the timing of fruit ripening with application of the gas. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses using fans and ventilation.

Nontraditional Hormones

Recent research has discovered a number of compounds that also influence plant development. Their roles are less understood than the effects of the major hormones described so far.

Jasmonates play a major role in defense responses to herbivory. Their levels increase when a plant is wounded by a predator, resulting in an increase in toxic secondary metabolites. They contribute to the production of volatile compounds that attract natural enemies of predators. For example, chewing of tomato plants by caterpillars leads to an increase in jasmonic acid levels, which in turn triggers the release of volatile compounds that attract predators of the pest.

Oligosaccharins also play a role in plant defense against bacterial and fungal infections. They act locally at the site of injury; they can also be transported to other tissues. Strigolactones promote seed germination in some species and inhibit lateral apical development in the absence of auxins. Strigolactones also play a role in the establishment of mycorrhizae, a mutualistic association of plant roots and fungi. Brassinosteroids are important to many developmental and physiological processes. Signals between these compounds and other hormones, notably auxin and GAs, amplify their physiological effect. Apical dominance, seed germination, gravitropism, and resistance to freezing are all positively influenced by hormones. Root growth and fruit dropping are inhibited by steroids.

30.7.7: Plant Responses to Wind and Touch

Learning Objective

Compare the ways plants respond to directional and non-directional stimuli

Key Points

• When subjected to constant directional pressure, such as a trellis, plants move to grow around the object providing the pressure; this process is known as thigmotropism.
• Thigmonastic responses include opening and closing leaves, petals, or other parts of the plant as a reaction to touch.
• Through thigmomorphogenesis, plants change their growth in response to repeated mechanical stress from wind, rain, or other living things.

Key Terms

thigmonastic response

a touch response independent of the direction of stimulus

thigmomorphogenesis

the response by plants to mechanical sensation (touch) by altering their growth patterns

thigmotropism

plant growth or motion in response to touch

Plant Responses to Wind and Touch

The shoot of a pea plant wraps around a trellis while a tree grows on an angle in response to strong prevailing winds. These are examples of how plants respond to touch or wind.

The movement of a plant subjected to constant directional pressure is called thigmotropism, from the Greek words thigma meaning "touch," and tropism, implying "direction." Tendrils are one example of this. A tendril is a specialized stem, leaf, or petiole with a threadlike shape that is used by climbing plants for support.The meristematic region of tendrils is very touch sensitive; light touch will evoke a quick coiling response. Cells in contact with a support surface contract, whereas cells on the opposite side of the support expand . Application of jasmonic acid is sufficient to trigger tendril coiling without a mechanical stimulus.

Thigmotropism in a redvine

Tendrils of a redvine produce auxin in response to touching a support stick and then transfer the auxin to non-touching cells. The non-touching cells elongate faster to curl around the support stick.

A thigmonastic response is a touch response independent of the direction of stimulus. In the Venus flytrap, two modified leaves are joined at a hinge and lined with thin, fork-like tines along the outer edges. Tiny hairs are located inside the trap. When an insect brushes against these trigger hairs, touching two or more of them in succession, the leaves close quickly, trapping the prey. Glands on the leaf surface secrete enzymes that slowly digest the insect. The released nutrients are absorbed by the leaves, which reopen for the next meal.

Thigmomorphogenesis is a slow developmental change in the shape of a plant subjected to continuous mechanical stress. When trees bend in the wind, for example, growth is usually stunted and the trunk thickens. Strengthening tissue, especially xylem, is produced to add stiffness to resist the wind's force. Researchers hypothesize that mechanical strain from wind, rain, or movement by other living things induces growth and differentiation to strengthen the tissues. Ethylene and jasmonate are likely involved in thigmomorphogenesis.

Attributions

• Plant Responses to Light
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "photomorphogenesis." http://en.wiktionary.org/wiki/photomorphogenesis. Wiktionary CC BY-SA 3.0.
  • "photoperiodism." http://en.wiktionary.org/wiki/photoperiodism. Wiktionary CC BY-SA 3.0.
  • "photoreceptor." http://en.wiktionary.org/wiki/photoreceptor. Wiktionary CC BY-SA 3.0.
  • "OpenStax College, Biology. October 17, 2013." http://cnx.org/content/m44711/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "phototropism." http://en.wiktionary.org/wiki/phototropism. Wiktionary CC BY-SA 3.0.
  • "Phototropism." http://en.wikipedia.org/wiki/File:Phototropism.jpg. Wikipedia CC BY.
• The Phytochrome System and Red Light Response
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "phytochrome." http://en.wiktionary.org/wiki/phytochrome. Wiktionary CC BY-SA 3.0.
  • "photoperiodism." http://en.wiktionary.org/wiki/photoperiodism. Wiktionary CC BY-SA 3.0.
  • "chromophore." http://en.wiktionary.org/wiki/chromophore. Wiktionary CC BY-SA 3.0.
  • "OpenStax College, Biology. October 17, 2013." http://cnx.org/content/m44711/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "OpenStax College, Plant Sensory Systems and Responses. October 17, 2013." http://cnx.org/content/m44711/latest/Figure_30_06_01.jpg. OpenStax CNX CC BY 3.0.
• Blue Light Response
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "cryptochrome." http://en.wiktionary.org/wiki/cryptochrome. Wiktionary CC BY-SA 3.0.
  • "skototropism." http://en.wiktionary.org/wiki/skototropism. Wiktionary CC BY-SA 3.0.
  • "phototropin." http://en.wiktionary.org/wiki/phototropin. Wiktionary CC BY-SA 3.0.
  • "OpenStax College, Biology. October 17, 2013." http://cnx.org/content/m44711/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "Phototropism Diagram." http://en.wikipedia.org/wiki/File:Phototropism_Diagram.svg. Wikipedia CC BY-SA.
  • "OpenStax College, Plant Sensory Systems and Responses. October 17, 2013." http://cnx.org/content/m44711/latest/Figure_30_06_02.jpg. OpenStax CNX CC BY 3.0.
• Plant Responses to Gravity
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "statolith." http://en.wiktionary.org/wiki/statolith. Wiktionary CC BY-SA 3.0.
  • "gravitropism." http://en.wiktionary.org/wiki/gravitropism. Wiktionary CC BY-SA 3.0.
  • "OpenStax College, Biology. October 17, 2013." http://cnx.org/content/m44711/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "amyloplast." http://en.wikipedia.org/wiki/amyloplast. Wikipedia CC BY-SA 3.0.
  • "Gravitropism tree." http://commons.wikimedia.org/wiki/File:Gravitropism_tree.jpg. Wikimedia CC BY.
• Auxins, Cytokinins, and Gibberellins
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "OpenStax College, Biology. October 17, 2013." http://cnx.org/content/m44711/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "cytokinin." http://en.wiktionary.org/wiki/cytokinin. Wiktionary CC BY-SA 3.0.
  • "OpenStax College, Plant Sensory Systems and Responses. October 17, 2013." http://cnx.org/content/m44711/latest/Figure_30_06_03.jpg. OpenStax CNX CC BY 3.0.
• Abscisic Acid, Ethylene, and Nontraditional Hormones
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "OpenStax College, Biology. October 17, 2013." http://cnx.org/content/m44711/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "abscisic acid." http://en.wikipedia.org/wiki/abscisic%20acid. Wikipedia CC BY-SA 3.0.
  • "jasmonate." http://en.wiktionary.org/wiki/jasmonate. Wiktionary CC BY-SA 3.0.
  • "Ripening." http://en.wikipedia.org/wiki/Ripening. Wikipedia CC BY.
• Plant Responses to Wind and Touch
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