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Plant Responses

Plant Responses

Have you ever tried growing plants indoors? Did you notice how they tend to grow towards the window or whatever light source you have? This happens for the same reason that sunflowers face east during dawn and swing west for the duration of the day: plants tend to follow the direction of light.

So, let's talk about plant responses!

  • First, we will look at how plants respond to light.
  • Then, we will look at plant responses to gravity.
  • After, we will explore plant responses to water limitations.

Plant Responses to Stimuli

Unlike animals, plants cannot just uproot themselves and move to another location when responding to environmental stimuli. Instead, plants have hormones (chemical messengers) and other sophisticated mechanisms that detect stimuli such as light, gravity, and water and send signals to initiate physiological changes in response to these stimuli.

Plant Responses to Light

The ability to detect light in the environment is essential for a plant's competitiveness and survival.

Plants have photoreceptors that can detect and respond to at least three wavelengths of light:

  1. Blue light

  2. Red light

  3. Far-red light

Photoreceptors consist of chromoproteins. A chromoprotein is comprised of a protein attached to a light-absorbing pigment via a covalent bond.

Plant response to blue light: phototropism

Some plants respond to environmental changes by developing their stems, roots, or leaves toward or away from the stimulus; such responses are called tropisms.

Phototropism refers to a plant’s movement toward or away from a light source.

Plants tend to grow toward a light source because they need light energy to produce sugars.

The chromoproteins responsible for regulating phototropism are called phototropins. In addition to phototropism, phototropins also regulate other plant responses including the opening and closing of leaves, the movement of chloroplasts within cells, and the opening of stomata for gas exchange during photosynthesis.

The process by which phototropins cause plants to bend toward a light source are summarized as follows:

  • Phototropins called phot1 and phot2 in the apical meristem detect blue light triggering the accumulation of a plant hormone called auxin (also known as indole acetic acid) on the shaded side of the plant.

  • Auxin stimulates cell elongation by pumping protons from the cells to the space between the plasma membrane and the cell wall, causing the cells to expand.

  • Because cell expansion takes place only on the shaded side of the stem, the plant bends toward the light source.

Figure 1 below shows how auxin regulates plant response to light.

Apical meristem in plants refers to the tip of roots and shoots that have the capacity to undergo cell division hence growth.

Plant response to red light / far-red light: stem elongation, germination, photoperiodism

While blue light promotes bending, red light promotes stem elongation. Red light–as opposed to far-red light–promotes stem elongation because to a plant, red light means full sun, while far-red light means it is being shadowed out by another plant. This is due to the fact that unfiltered, full sunshine includes significantly more red light than far-red light.

Chlorophyll absorbs more strongly in the red part of the visible spectrum than in the far-red region, so a plant's ability to distinguish between red light and far-red light enables it to grow away from shaded areas toward light.

The chromoproteins that detect red and far-red light are called phytochromes. Phytochromes have two forms:

  1. Pr (phytochrome red) which is capable of absorbing red light, and
  2. Pfr (phytochrome far-red) which is capable of absorbing far-red light.

When Pr absorbs red light, it changes into Pfr, and when Pfr absorbs far-red light, it quickly changes back to Pr. The absorption of red or far-red light alters the structure of the chromophore, affecting the conformation and activity of the phytochrome protein to which it is attached.

In short, phytochrome activity is initiated by red light and inhibited by far-red light. The two forms of phytochrome–collectively called the phytochrome system–act as a biological switch.

Phytochrome promotes plant growth toward red light via cytokinin (a hormone that promotes cell division) and gibberellin (a hormone that stimulates stem elongation). Cytokinin is triggered by the Pfr form of phytochrome, promoting cell division in apical meristems exposed to red light.

In many plant species, the phytochrome system also controls seed germination.

Seed germination refers to the active metabolic processes that lead to the emergence of a new seedling.

Seeds may become dormant after fertilization so that they germinate at a time and place where the seedling has a better chance of survival.

For some plant species, exposure to red light indicates that the seed is in a suitable place for access to sunlight following germination. Some seeds may not germinate in the dark, where the phytochrome is in the Pr. The conversion of Pr into Pfr promotes the transcription of amylase (an enzyme that changes starch reserves in the seed into simple sugars), initiating seed germination.

It is important to note that not all plant species need light to germinate. Some seeds germinate through a light-independent process that is regulated by a plant hormone called gibberellin.

The phytochrome system also enables plants to detect seasonal changes. Photoperiodism refers to a plant's response to the duration and timing of day and night. It regulates processes such as flowering and the formation of winter buds. Due to the stimuli brought about by seasonal changes, a plant's ability to detect seasonal changes is crucial to its survival. While temperature and light intensity affect plant development, they are not accurate indicators of seasonal changes because they fluctuate from year to year. In contrast, the duration of the day (which is stable) is a better predictor of the season.

Plant response to gravity

Shoots typically sprout up from the earth, while roots grow down into the ground whether exposed to light or in complete darkness. When given enough time, a plant lying on its side in the dark will eventually develop upward shoots. This is because of gravitropism.

Gravitropism is the tendency of roots to grow down into the soil and the tendency of branches to grow upwards toward the sun due to the force of gravity. Gravitropism can be negative or positive:

  • Negative gravitropism refers to the upward growth of the shoot apical tip.

  • Positive gravitropism refers to the downward growth of the roots.

Gravitropism is regulated by auxins and amyloplasts. As mentioned before, auxins are plant hormones that promote cell elongation. Amyloplasts, on the other hand, are cell organelles containing heavy starch granules that fall to the bottom of the cell in response to gravity.

When amyloplasts fall to the bottom of the cell, they come into contact with the endoplasmic reticulum (ER) which releases calcium ions. In turn, the calcium ions signal the cells to transport auxin to the bottom of the cell.

This means that when the plant is tilted, the amyloplasts move, causing auxin to accumulate in what the plant perceives to be the new bottom of the root, that is, in the direction of gravity. Amyloplasts can be found in shoots and in the root cap.

Auxin affects the growth of roots and shoots differently:

  • In roots, a high concentration of auxin suppresses cell elongation, so cells grow slowly on the lower side while cells grow normally on the upper side, causing the root to bend toward the high concentration of auxin which is downward.

  • In shoots, a higher concentration of auxin promotes cell elongation causing the shoot to bend away from the region with greater auxin concentration. As such, gravitropism causes shoots to grow upward.

Plant responses to water limitations

Water consumption causes the hormone gibberellin to signal the transcription of the gene encoding amylase, an enzyme that changes starch reserves in the seed into simple sugars, initiating seed germination.

When the plant lacks water, germination is inhibited by abscisic acid (also known as ABA), a hormone that suppresses the function of gibberellins. Thus, gibberellins and abscisic acid have contradictory functions that work hand-in-hand to regulate germination in response to stimuli like water.

Additionally, in the absence of water, abscisic acid also causes stomata (pores in leaves) to close, preventing gas exchange and inhibiting photosynthesis. If the stomata of a plant remain closed for too long, the plant begins to die in localized regions (in leaves and stems, for instance). This process is regulated by the hormone ethylene, which has the ability to induce localized cell death.

Other plant responses related to growth

There are other responses to stimuli that affect the growth and development of plants. Here we will discuss two: apical dominance and leaf abscission.

Apical dominance

Many plants grow at a single apex which is dominant over other stems. Apical dominance means that the growth of a single apex exceeds those of other stems. Apical dominance is controlled by the presence of auxin at the apical meristem.

Other growth-regulating hormones, like cytokinins, require auxin to function. Cytokinins increase cell division only in the presence of auxin. Together, auxin and cytokinins promote cell growth. Because auxin is only present in the apical bud and not the lateral buds, plant growth takes place only in the apical bud.

Leaf abscission

Some plants shed their leaves in response to seasonal changes (based on temperatures, light, water, or other environmental stimuli). This process is known as leaf abscission, and it is controlled by interactions between auxin and ethylene.

During the growth season, the leaf generates a lot of auxins, which inhibits ethylene activity; however, when the seasons change, the leaf produces less auxin. Lower auxin levels allow ethylene to commence senescence (maturing) and, eventually, programmed cell death at the point of leaf attachment to the stem, enabling the leaf to fall off in a regulated way without causing harm to the remainder of the plant.

Plant Responses - Key takeaways

  • Plants have hormones (chemical messengers) and other sophisticated mechanisms that detect stimuli and send signals to initiate physiological changes in response to these stimuli.
  • Plants have photoreceptors that can detect and respond to at least three wavelengths of light: blue light, red light, and far-red light.
  • Gravitropism is the tendency of roots to grow down into the soil and the tendency of branches to grow upwards toward the sun due to the force of gravity.
  • The absence of water can inhibit germination and cause localized cell death.
  • Other plant responses related to growth include apical dominance and leaf abscission.

References

  1. Georgia Tech Biological Sciences. (n.d.). Plant Hormones and Sensory Systems. Organismal Biology. Retrieved June 16, 2022, from https://organismalbio.biosci.gatech.edu/chemical-and-electrical-signals/plant-hormones-and-sensory-systems/
  2. Plant Sensory Systems and Responses. OpenStaxCollege. (2012, August 22). Retrieved June 16, 2022, from http://pressbooks-dev.oer.hawaii.edu/biology/chapter/plant-sensory-systems-and-responses/

Frequently Asked Questions about Plant Responses

Examples of plant responses to stimuli include phototropism (movement towards light) and gravitropism (movement in the direction of gravity).

5 ways in which plants respond to their environment include phototropism, germination, photoperiodism, gravitropism, and leaf abscission.

Plant growth responses in which they develop toward or away from certain stimuli are called tropisms.

Abscisic acid is produced in response to water deficiency.

The different responses of plants to their environment include phototropism, germination, photoperiodism, gravitropism, and leaf abscission.

Final Plant Responses Quiz

Question

Which of the following is not a major plant hormone?

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Answer

Testosterone

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Question

What do plant hormones help regulate in plants?

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Answer

Plant hormones help to regulate growth and development and response to stimuli in a plant's environment. 

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Question

Hormones are ________ _________ that help trigger a response to an environmental stimulus. 

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Answer

Signaling substances

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Question

Which of the following are true statements about signal transduction pathways?

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Answer

They amplify the effects of small amounts of hormones. 

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Question

_________ is the tendency of plants to grow towards light. 

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Answer

Phototropism

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Question

Auxin concentration on the ______ side of the stem results in a plant growing toward light (phototropism).

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Answer

The shady side. The auxin causes stem elongation on the shady side, causing the plant to bend toward the light source. 

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Question

__________ are the plant hormone involved in the dominance of the apical meristem (apical dominance).


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Answer

Auxins

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Question

________ _________ is when the tip of the stem (apical meristem) prevents the development of axillary buds (lateral growth) so that the plant can continue growing upward toward the sunlight.  

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Answer

Apical dominance 

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Question

Which of the following do gibberellins NOT play a major role in?

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Answer

Stomatal response to water stress

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Question

Which of the following is NOT a function of cytokinins?

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Answer

Apical dominance

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Question

Hormones are said to be __________ if they have competing or opposite functions.

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Answer

Antagonistic

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Question

Which hormone is responsible for initiating the closing of stomata when faced with a lack of water?

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Answer

Abscisic acid

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Question

Which of the following is a function of abscisic acid?

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Answer

Keeps seeds dormant if the environmental conditions are not ideal

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Question

If you buy bananas and notice that overnight, they seemed to have ripened on your counter, you would most likely be observing the effects of what plant hormone?

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Answer

Ethylene gas

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Question

Which of the following is true about plant and animal hormones?

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Answer

Plant hormones are often smaller, less-complex organic molecules, whereas animal hormones are typically larger and more complex. 

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Question

True or False: Plant hormones are typically more specialized than animal hormones.

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Answer

False; animal hormones have more specific functions; plant hormones have a diversity of functions. 

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Question

What is gravitropism?

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Answer

Gravitropism in plants is the development or growth of plant organs toward a certain direction in response to gravity.

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Question

This type of gravitropism is where the growth of plant organs is parallel to the pull of gravity.

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Answer

Orthogravitropism

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This type of gravitropism is where the growth of plant organs is in the same direction as the pull of gravity.

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Positive gravitropism

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This type of gravitropism is where the growth of plant organs is in the direction that is opposite of the pull of gravity.

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Negative gravitropism

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Question

This type of gravitropism can be observed in stolons and rhizomes due to the horizontal growth of their roots or stems.

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Diagravitropism

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Question

Explain how positive gravitropism helps in plant growth.

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By growing vertically downwards, positive gravitropism enables primary roots to efficiently take up water and nutrients as well as to secure anchorage in the soil.

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Explain how negative gravitropism helps in plant growth.

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Negative gravitropism manifests in the upward growth of shoots. By growing upward, leaves are positioned properly for efficient photosynthesis and respiration. 

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Question

Summarize the process of gravitropism in plants.

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Answer

The plant perceives the direction of gravity, which leads to an asymmetry in cell structure or biochemistry. A signal is sent to the site in the plant organ causing cells to elongate at a differential rate.

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Question

Explain the role of amyloplasts in gravity perception.

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Answer

Amyloplasts are heavy starch-filled organelles that can weigh down the cell due to the pull of gravity. Its weight can cause asymmetry in the cell structure or biochemistry.

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Question

What happens during the reaction phase?

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Answer

In roots, a high concentration of auxin slows cell elongation, causing the root to bend down toward the region where auxin is highly concentrated.

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Question

Why is gravitropism important in plants?

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Answer

Gravitropism is important in the growth and development of plants. It is a strategy that enables plants to compete for resources in their immediate surroundings and to resume vertical growth after being toppled down by wind, rain, or other environmental factors. It also enables plants to protect their seeds from moisture and pathogens that may be present in the soil.

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Question

A corn seedling was flipped to its side. In about 20 to 30 minutes, cells along the higher side of the root close to the root tip will begin to elongate quicker than cells along the lower side, causing the root to bend downward. What type of gravitropism does this illustrate?

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Answer

Positive gravitropism

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Question

A Coleus plant is turned on its side. After an hour, gravitropic curvature begins to form at the tip of the shoot. The leaves around the plant's base realign and settle into a more horizontal posture, just as they were before the plant was flipped on its side. What type of gravitropism does this example demonstrate?

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Answer

Negative gravitropism

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Question

Explain how lateral roots exhibit gravitropism.

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Answer

Gravitropic response can also be observed in lateral roots which tend to grow at a progressively oblique angle. After emergence, lateral roots would curve toward an initial preferred angle in relation to the pull of gravity (which is usually quite shallow), then straighten up and grow at this angle for a while. They may begin to steeply curve until they reach an angle that is nearly vertically downward which could result in an axial root system, or they may maintain a shallow angle for a long time which could result in a radially expanded root system. 

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Question

Which statement best describes the translocation phase?

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Answer

One or more signals move to the site of the reaction phase on both sides of the organ

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Question

What is the plant hormone that helps cause a phototropic response?

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Answer

Auxin

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Question

 When a plant responds by directional growth to a stimulus, we call that a ______.

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Answer

Tropism

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Question

Phototropism happens when a plant grows __________, either toward or away from a light source.

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Answer

Directionally

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A negative phototropism is when a plant grows ______ a light source.

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Away from

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Positive phototropism occurs when a plant grows _____ a light stimulus.

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Toward

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Question

The light absorbing part of a photoreceptor in a plant is known as a ____________.

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Answer

Chromophore

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Question

True or False: A photoreceptor is a protein that can capture and respond to a light stimulus.

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Answer

True

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Question

A photoreceptor is made up of a ______ and a ________.

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Answer

Protein and chromophore

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Question

What is the name of the photoreceptor associated with the phototropic response?

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Answer

Phototropin

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Question

Phototropin absorbs _________ wavelengths, and must absorb this wavelength to initiate a phototropic response.

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Answer

Blue-light

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Question

When a plant stem is going to bend toward the light source, auxin concentrates on the ______ side of the stem, causing growth and curvature.

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Answer

Shady

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Question

Negative phototropism is typically observed in the plant's _________.

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Answer

Roots

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Positive phototropism is typically observed in the _______ of plants.

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Stems

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Question

Charles Darwin and his son found that by blocking the shoot tip, the coleoptile…

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Answer

Did not bend or exhibit phototropism

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Boysen-Jensen's experiments showed that there was a signal being sent from the shoot tip of the plant further down the stem. Scientists later discovered this signal was...

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Answer

The plant hormone auxin

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Question

What is photoperiodism?

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Answer

Photoperiodism refers to the ability of plants to determine the time of day and year by detecting wavelengths of sunlight

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Question

What is the environmental cue that tells a plant what time of the year it is?

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Answer

A change in the length of a day

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Question

The length of day or the number of hours of exposure to daylight is referred to as ___.

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Answer

Photoperiod

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Question

_______ are chromoproteins that detect red and far-red light. 

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Answer

Phytochromes

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