37. Plant Sensation and Response

Phototropism

PLANT HORMONES - Auxin Gibberellin Cytokinin Ethylene Abscisic Acid

Afferent 2- Sensory receptors

A barley mutant lacking a gibberellic acid receptor would
(A) fail to make GA.
(B) catalyze starch more quickly.
(C) fail to make a-amylase.
(D) fail to take up water.

In general, small seeds that have few food reserves must be exposed to red light before they will germinate. (Lettuce is an example.) In contrast, large seeds that have substantial food reserves typically do not depend on red light as a stimulus to trigger germination. State a hypothesis to explain these observations.

 The phytochrome receptor moves from the plasma membrane into the cytoplasm. 

 No. Unlike most receptors, phytochrome receptors are found in the cytoplasm. 

 No. This does happen in response to phytochrome activation but not immediately following activation. The cGMP second messenger must first be produced. 

 No. This is the ultimate response to light activation of the phytochrome, but other things must occur first. 

 Genes for proteins that function in de-etiolation are transcribed. 

 No. This does occur after light activates a phytochrome but not immediately following activation; other things must occur first. 

Ca2+ channels are opened, and guanylyl cyclase is activated.

Review the signal transduction mechanism in the de-etiolation response.

What happens immediately after a phytochrome is activated by light?

Plant Hormones - Tropisms & Auxins  #52

Plant Responses to the Environment: Tropisms and Defenses

 the ability to perceive shortwave ultraviolet light 

 No. Photoperiodism is not necessarily related to an ability to perceive shortwave ultraviolet light. 

 No. An attraction to light would be positive phototropism. 

 No. Animals also exhibit photoperiodism. 

 No. This would be negative phototropism. 

 a physiological response of an organism to alternating light and dark cycles 

Consider what is meant by the prefix "photo-" and what is meant by periodicity.

Photoperiodism is __________.

Photomorphogenesis

Principle of Cellular Communication | Overview of Cell Signalling

Auxin

Auxin enhances cell elongation in all of these ways except
a. increased uptake of solutes.
b. gene activation.
c. acid-induced denaturation of cell wall proteins.
d. cell wall loosening.

Phytochromes can be considered “shade detectors,” while phototropins such as PHOT1 can be considered “sunlight detectors.” Explain why these characterizations are valid.

Which of the following statements about phytochrome is not correct?
a. It is photoreversible.
b. Its function was understood long before the protein itself was isolated.
c. The Pfr form activates the responses to red light.
d. It is involved in guard-cell opening.

Phototropism Explained

Major plant hormones (& how to remember) | Control & Coordination | Biology | Khan Academy

Evaluate the following assertions about phototropism. Select True or False for each statement.
T/F Cells on the illuminated side of a stem elongate more than cells on the shaded side.
T/F Phototropism is triggered by blue light.
T/F Phototropins play a significant role in phototropism.
T/F The bending of the plant is due to cell elongation in response to auxin.

Sensory Receptors (Mnemonic)

Mechanisms of Plant Growth

A certain short-day plant flowers only when days are less than 12 hours long. Which of the following would cause it to flower?
a. a 9-hour night and 15-hour day with 1 minute of darkness after 7 hours
b. an 8-hour day and 16-hour night with a flash of white light after 8 hours
c. a 13-hour night and 11-hour day with 1 minute of darkness after 6 hours
d. a 12-hour day and 12-hour night with a flash of red light after 6 hours

Photoperiodism II Role of Phytochromes in Flowering II Red Light and Far-red light effect

Photoperiodic Flowering - Part 1

9.4 The Phytochrome System

Sensory Receptors

If a long-day plant has a critical night length of 9 hours, which 24-hour cycle would prevent flowering?
a. 16 hours light/8 hours dark
b. 14 hours light/10 hours dark
c. 4 hours light/8 hours dark/4 hours light/8 hours dark
d. 8 hours light/8 hours dark/light flash/8 hours dark

Phototropism  | Plant Biology | Khan Academy

Short Video: Photropism

Signal Transduction and Response

Signal transduction in a "touch" neuron

Photoperiodism  | Plant Biology | Khan Academy

Tropism (Types, positive & negative) | Control & Coordination | Biology | Khan Academy

Photoderiodism

Charles and Francis Darwin discovered that
a. auxin is responsible for phototropic curvature.
b. red light is most effective in shoot phototropism.
c. light destroys auxin.
d. light is perceived by the tips of coleoptiles.

Auxin causes a shoot to bend toward light by
a. causing cells to shrink on the dark side of the shoot.
b. stimulating growth on the dark side of the shoot.
c. causing cells to shrink on the lighted side of the shoot.
d. stimulating growth on the lighted side of the shoot.

 remove the apical meristem and apply auxin before beginning light treatments 

 No. By applying auxin, you are replacing the auxin synthesized by the apical meristem. 

 shine light from one side only. Then measure the auxin diffusing down the stem on the light and the shaded sides 

 No. Light will still affect both sides of the stem. 

 spray auxin on just one part of the plant at a time to determine which one stimulates flowering 

 No. This proposed experiment does not involve light. 

 try light treatments of different colors (first blue, then yellow, then green) to see which promotes flowering the fastest 

 No. Flowering and phototropism are two different photomorphogenetic processes. 

 cover one part (for instance, the tip or base) with an opaque barrier before beginning light treatments 

Consider the experiments of Charles and Francis Darwin.

You are interested in determining what part of a plant senses the light, which leads to phototropism. A good first experiment would be to __________.

How plants tell time - Dasha Savage

A plant nursery manager tells the new night security guard to stay out of a room where chrysanthemums (which are short-day plants) are about to flower. Around midnight, the guard accidentally opens the door to the chrysanthemum room and turns on the lights for a moment. How might this affect the chrysanthemums? How could the guard correct the mistake?

Understanding Photoperiodism: How Plants Respond To Periods of Light and Darkness

SCIENTIFIC THINKING The discovery of auxin is a good example of how scientists build upon each other’s work. For each of the three sets of researchers discussed in the chapter (the Darwins, Boyle-Jensen, and Went), write a one-sentence summary of what they discovered. If you were going to credit one person as being the “discoverer” of auxin, who would it be?

How may a plant respond to severe heat stress?
a. by reorienting leaves to increase evaporative cooling
b. by creating air tubes for ventilation
c. by producing heat-shock proteins, which may protect the plant’s proteins from denaturing
d. by increasing the proportion of unsaturated fatty acids in cell membranes, reducing their fluidity

 No. The length of the dark period is insufficient to cause flowering. 

 An 8-hour day and a 16-hour night with 1 minute of white light after 8 hours 

 No. The flash of light interrupts the dark period, resulting in a dark period that is insufficient to cause flowering. 

 A 12-hour day and a 12-hour night with 1 minute of white light after 6 hours 

 No. The flash of light interrupts the dark period, resulting in a length of uninterrupted darkness insufficient to cause flowering. 

 Alternating 4-hour periods of light and darkness 

 No. The length of uninterrupted darkness is insufficient to cause flowering. 

 An 11-hour day with 1 minute of darkness after 6 hours, followed by a 13-hour night 

Consider that short-day plants are actually long-night plants.

A certain short-day plant flowers when days are less than 12 hours long. Which of the following treatments will result in flowering?

The signaling molecule for flowering might be released earlier than usual in a long-day plant exposed to flashes of
a. far-red light during the night.
b. red light during the night.
c. red light followed by far-red light during the night.
d. far-red light during the day.

 Responses to the number of hours of daylight 

 No. Circadian rhythms are not directly tied to any known environmental variable. 

 Monthly cycles of behavior or physiological change 

 No. Circadian rhythms are not monthly cycles. 

 Metabolic processes related to the light/dark cycling of the environment 

 None of the listed responses is correct. 

 No. One of the other choices is correct. 

 Innate 24-hour cycles of behavior or physiological change 

Consider the meanings of the root circa- and the root di- (as in diem).

What are circadian rhythms?

No. The exposure to red light would maintain Pfr, the phytochrome isomer that promotes germination.

No. The exposure to red light would maintain Pfr, the phytochrome isomer that promotes germination. The effect of the initial far-red light exposure would be "canceled."

No. The exposure to red light would maintain Pfr, the phytochrome isomer that stimulates germination. The green light does not affect phytochrome.

 Red light and far-red light together, followed by red light only 

No. The final exposure to red light would convert Pr to Pfr, the phytochrome isomer that promotes germination.

 Exposure of seeds to red light followed by far-red light 

Consider the effect of different wavelengths of light on Pfr and Pr. .

Some seeds require light for germination. This process is controlled by the phytochrome system. Which of the following germination treatments would inhibit germination of such seeds?

 No. Phototropism is the tendency of plants to grow toward light. 

 No. Leaving the lights on would eliminate periods of darkness. Long-night (short-day) plants would not flower. 

 The lights must have emitted far-red light. 

 No. Red light affects plant flowering when it interrupts a period of darkness. 

 No. Leaving the lights on would eliminate periods of darkness, preventing short-day (long-night) plants from flowering. 

Consider how this light regimen would affect phytochrome.

An Alaskan trapper worried about being attacked by grizzly bears left the lights on in his cabin all the time. Plants just outside the cabin flowered a month early. Which of the following best explains this?

 No. Bending toward light is phototropism, which is unrelated to phytochromes. 

 No. Increased levels of ethylene and decreased levels of auxin trigger fruit drop. 

 No. Bending toward gravity is gravitropism, which is unrelated to phytochromes. 

 causes changes in a plant in response to stresses such as water shortage 

 No. Phytochrome function is unrelated to stress responses. 

Consider the role of phytochrome in activities such as flowering.

A conformational change in a substance called phytochrome __________.

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