Chapter 31
Plant Responses
Table of Contents
Section 1 Plant Hormones
Section 2 Plant Movements
Section 3 Seasonal Responses
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Chapter 31
Section 1 Plant Hormones
Objectives
• List the actions of the five major types of plant
hormones.
• Describe agricultural or gardening applications for
each of the five major types of plant hormones.
• Discuss how growth retardants are used
commercially.
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Chapter 31
Section 1 Plant Hormones
Groups of Hormones
• Plant hormones are formed in many plant parts and
regulate many aspects of growth and development.
Hormonal responses often have adaptive
advantages.
• There are five major groups of plant hormones:
auxins, gibberellins, ethylene, cytokinins, and
abscisic acid.
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Chapter 31
Section 1 Plant Hormones
Types of Plant Hormones
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Chapter 31
Section 1 Plant Hormones
Auxins
• Auxins are hormones involved in plant-cell
elongation, shoot and bud growth, and rooting.
• A well-known natural auxin is indoleacetic acid, or
IAA.
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Chapter 31
Section 1 Plant Hormones
Auxins
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Section 1 Plant Hormones
Auxins, continued
Synthetic Auxins
– Synthetic auxins are used for killing weeds,
stimulating root formation, and stimulating or
preventing fruit drop.
– Naphthalene acetic acid, or NAA, is used to
promote root formation on stem and leaf cuttings.
NAA can also be applied to a cut shoot tip of the
stem to mimic apical dominance (inhibition of
lateral bud growth due to presence of a shoot tip).
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Chapter 31
Section 1 Plant Hormones
Apical Dominance
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Chapter 31
Section 1 Plant Hormones
Gibberellins
• Gibberellins are used to increase the size of fruit, to
stimulate seed germination, and to brew beer.
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Chapter 31
Section 1 Plant Hormones
Ethylene
• Ethylene is used to ripen fruit and promote
abscission, the detachment of leaves, flowers, or
fruits.
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Chapter 31
Section 1 Plant Hormones
Ethylene
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Chapter 31
Section 1 Plant Hormones
Cytokinins
• Cytokinins are used to culture plant tissues in the
lab and to promote lateral bud growth of flower crops.
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Chapter 31
Section 1 Plant Hormones
Cytokinins
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Chapter 31
Section 1 Plant Hormones
Abscisic Acid
• Abscisic acid promotes dormancy in plant buds,
maintains dormancy in seeds, and causes stomata to
close.
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Chapter 31
Section 1 Plant Hormones
Other Growth Regulators
• Growth retardants are widely used to reduce plant
height.
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Chapter 31
Section 2 Plant Movements
Objectives
• List the environmental stimuli to which plants
respond for each type of tropism.
• Explain the current hypotheses regarding auxins and
their function in phototropism and gravitropism.
• Describe two types of nastic movements, and
explain how they help a plant survive.
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Chapter 31
Section 2 Plant Movements
Tropisms
• Tropisms and nastic movements are plant responses
to environmental stimuli.
– Tropisms occur slowly; nastic movements happen
more quickly.
• A tropism is a response in which a plant grows either
toward or away from an environmental stimulus.
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Chapter 31
Section 2 Plant Movements
Tropism
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Section 2 Plant Movements
Tropisms, continued
Phototropism
– Phototropism is thought to occur in some plants
when auxin moves to the shaded side of a plant
and causes cells there to elongate more than the
cells on the lighted side.
– Solar tracking, also called heliotropism, is the
motion of leaves or flowers as they follow the
sun’s movement across the sky.
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Chapter 31
Section 2 Plant Movements
Tropisms, continued
Thigmotropism
– Thigmotropism is a plant’s growth response to
touching a solid object.
– For example, tendrils and stems of vines, such as
morning glories, coil when they touch an object.
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Chapter 31
Section 2 Plant Movements
Tropisms, continued
Gravitropism
– Gravitropism is a plant’s response to gravity.
– It is thought to occur when auxin accumulates on
the lower sides of a horizontal root and stem.
– This accumulation causes cell elongation on the
lower side of the stem and inhibits cell elongation
on the lower side of the root.
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Chapter 31
Section 2 Plant Movements
Gravitropism in
Plants
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Section 2 Plant Movements
Tropisms, continued
Chemotropism
– Plant growth that occurs in response to a chemical
is called chemotropism.
– An example of chemotropism is the growth of a
pollen tube after a flower is pollinated.
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Chapter 31
Section 2 Plant Movements
Nastic Movements
• Nastic movements are responses to environmental
stimuli but are independent of the direction of the
stimuli.
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Chapter 31
Section 2 Plant Movements
Nastic Movements, continued
Thigmonastic movements
– Thigmonastic movements occur in response to
touch, such as the closing of the leaf trap of a
Venus’ flytrap around an insect.
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Chapter 31
Section 2 Plant Movements
Nastic Movements, continued
Nyctinastic movements
– Nyctinastic movements occur in response to the
daily cycle of light and dark, such as the cyclical
vertical and horizontal positioning of leaves in
prayer plants.
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Chapter 31
Section 2 Plant Movements
Types of Plants
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Chapter 31
Section 3 Seasonal Responses
Objectives
• Define photoperiodism.
• Describe the role of critical night length in flowering.
• Explain the process of vernalization.
• Explain changing fall colors in leaves.
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Chapter 31
Section 3 Seasonal Responses
Photoperiodism
• Photoperiodism is a plant’s response to changes in
the length of days and nights.
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Chapter 31
Section 3 Seasonal Responses
Photoperiodism, continued
Day Length and Night Length
– Plants fit in one of three photoperiodic classes for
flowering: day-neutral plants (DNPs), short-day
plants (SDPs), and long-day plants (LDPs).
– Short-day and long-day plants have a specific
requirement for darkness, called the critical night
length. Day neutral plants are not affected by day
length.
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Chapter 31
Section 3 Seasonal Responses
Photoperiodism, continued
Adjusting the Flowering Cycles of Plants
– Flower growers who want to obtain winter
flowering of LDPs simply expose them to a low
level of incandescent light in the middle of the
night.
– Summer flowering of SDPs is obtained by
covering the plants in the late afternoon with an
opaque cloth so that the SDPs receive enough
darkness.
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Chapter 31
Section 3 Seasonal Responses
Photoperiodism, continued
Regulation by Phytochrome
– Plants monitor changes in day length with a
bluish, light-sensitive pigment called
phytochrome.
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Chapter 31
Section 3 Seasonal Responses
Flowering and
Photoperiodism
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Chapter 31
Section 3 Seasonal Responses
Photoperiodism
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Chapter 31
Section 3 Seasonal Responses
Vernalization
• Vernalization is the promotion of flowering by cold
temperatures.
• Farmers often plant wheat seeds in the fall so that
the seedlings can be exposed to winter temperatures
and will flower before summer droughts begin.
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Chapter 31
Section 3 Seasonal Responses
Fall Colors
• Changing fall colors in tree leaves are due to
chlorophyll degradation, which reveals other
pigments already present.
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Chapter 31
Section 3 Seasonal Responses
Dormancy
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Chapter 31
Standardized Test Prep
Multiple Choice
1. Which of the following is the name for the process in
which flowering is stimulated by exposure of plants to
cold?
A. dormancy
B. vernalization
C. thigmotropism
D. photoperiodism
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Chapter 31
Standardized Test Prep
Multiple Choice, continued
1. Which of the following is the name for the process in
which flowering is stimulated by exposure of plants to
cold?
A. dormancy
B. vernalization
C. thigmotropism
D. photoperiodism
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Chapter 31
Standardized Test Prep
Multiple Choice, continued
2. Which of the following is a result of abscission?
F. Leaves fall off stems.
G. Cuttings are produced.
H. Plants sense night length.
J. Abscisic acid is produced.
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Chapter 31
Standardized Test Prep
Multiple Choice, continued
2. Which of the following is a result of abscission?
F. Leaves fall off stems.
G. Cuttings are produced.
H. Plants sense night length.
J. Abscisic acid is produced.
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Standardized Test Prep
Multiple Choice, continued
3. Which of the following describes the relationship
between nastic movements and a stimulus?
A. Nastic movements occur without a stimulus.
B. Nastic movements occur toward a stimulus.
C. Nastic movements occur away from a stimulus.
D. Nastic movements occur independently of the
direction of a stimulus.
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Standardized Test Prep
Multiple Choice, continued
3. Which of the following describes the relationship
between nastic movements and a stimulus?
A. Nastic movements occur without a stimulus.
B. Nastic movements occur toward a stimulus.
C. Nastic movements occur away from a stimulus.
D. Nastic movements occur independently of the
direction of a stimulus.
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Standardized Test Prep
Multiple Choice, continued
4. What is the response of a plant to the length of days
and nights called?
F. gravitropism
G. phototropism
H. photoperiodism
J. thigmotropism
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Standardized Test Prep
Multiple Choice, continued
4. What is the response of a plant to the length of days
and nights called?
F. gravitropism
G. phototropism
H. photoperiodism
J. thigmotropism
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Standardized Test Prep
Multiple Choice, continued
The illustration below shows a growing seedling. Use
the illustration to answer the question that follows.
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Standardized Test Prep
Multiple Choice, continued
5. Which of the following statements about this seedling
is true?
A. The seedling is exhibiting abscission.
B. The shoot is exhibiting positive phototropism.
C. The roots are exhibiting negative gravitropism.
D. The seedling is exhibiting negative thigmotropism.
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Standardized Test Prep
Multiple Choice, continued
5. Which of the following statements about this seedling
is true?
A. The seedling is exhibiting abscission.
B. The shoot is exhibiting positive phototropism.
C. The roots are exhibiting negative gravitropism.
D. The seedling is exhibiting negative thigmotropism.
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Multiple Choice, continued
6. cell elongation : gibberellins :: cell division :
F. auxins
G. ethylene
H. cytokinins
J. abscisic acid
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Standardized Test Prep
Multiple Choice, continued
6. cell elongation : gibberellins :: cell division :
F. auxins
G. ethylene
H. cytokinins
J. abscisic acid
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Multiple Choice, continued
The diagrams below represent three different growing
conditions. Use the diagrams to answer the question
that follows.
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Standardized Test Prep
Multiple Choice, continued
7. In which of the conditions will a long-day plant with a
critical night length of 8 hours flower?
A. 1
B. 2
C. 1 and 3
D. 2 and 3
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Multiple Choice, continued
7. In which of the conditions will a long-day plant with a
critical night length of 8 hours flower?
A. 1
B. 2
C. 1 and 3
D. 2 and 3
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Standardized Test Prep
Short Response
The Venus’ flytrap obtains nutrients by closing its leaves
around insects and then digesting the insects.
Explain why a thigmonastic movement is a more useful plant
response than a thigmotropic response would be in this
situation.
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Standardized Test Prep
Short Response, continued
The Venus’ flytrap obtains nutrients by closing its leaves
around insects and then digesting the insects.
Explain why a thigmonastic movement is a more useful plant
response than a thigmotropic response would be in this
situation.
Answer: A thigmonastic response is more appropriate in this
instance because it is very rapid. A thigmotropic response
would be slow, because it would involve plant growth. An
insect would have moved away from the plant before a
thigmotropic response could have occurred.
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Chapter 31
Standardized Test Prep
Extended Response
Base your answers to parts A & B on the information below.
Reproduction is essential for survival of flowering plant
species.
Part A Identify two plant hormones that have roles in
flowering, fruiting, and/or seed or fruit dispersal, and
describe these roles.
Part B Identify the important environmental signals for
flowering, and describe how these signals affect flowering.
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Extended Response, continued
Answer:
Part A Auxins increase the setting of fruits, stimulate fruit
maturation, and prevent fruit drop. Gibberellins increase fruit
size, release seed dormancy, and substitute for long-day
requirements for flowering. Ethylene promotes flowering in
some species, fruit ripening, and fruit drop (which aids fruit
and seed dispersal). Abscisic acid promotes seed dormancy.
Part B: The most important environmental signals for flowering
are the length of the night and temperature. Short-day plants
have a minimum length of night required for flower induction,
and long-day plants have a maximum length of night required
for flower induction. Many plants also have a requirement for
exposure to cold temperature that induces flowering.
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