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Standardized Test Prep



Plant Responses

Chapter 31

Table of Contents

Section 1 Plant Hormones

Section 2 Plant Movements

Section 3 Seasonal Responses



Section 1 Plant Hormones

Chapter 31

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.




Chapter 31

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.




Chapter 31

Types of Plant Hormones




Chapter 31

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 .



Chapter 31

Auxins





Chapter 31

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).




Chapter 31

Apical Dominance




Chapter 31

Gibberellins

• Gibberellins are used to increase the size of fruit, to stimulate seed germination, and to brew beer.




Chapter 31

Ethylene

• Ethylene is used to ripen fruit and promote abscission, the detachment of leaves, flowers, or fruits.



Chapter 31

Ethylene





Chapter 31

Cytokinins

• Cytokinins are used to culture plant tissues in the lab and to promote lateral bud growth of flower crops.



Chapter 31

Cytokinins





Chapter 31

Abscisic Acid

• Abscisic acid promotes dormancy in plant buds, maintains dormancy in seeds, and causes stomata to close.




Chapter 31

Other Growth Regulators

• Growth retardants are widely used to reduce plant height.



Section 2 Plant Movements

Chapter 31

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.




Chapter 31

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.



Chapter 31

Tropism





Chapter 31

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.




Chapter 31


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.




Chapter 31


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.



Chapter 31

Gravitropism in

Plants





Chapter 31


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.




Chapter 31

Nastic Movements

• Nastic movements are responses to environmental stimuli but are independent of the direction of the stimuli.




Chapter 31

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.




Chapter 31

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.



Chapter 31

Types of Plants




Section 3 Seasonal Responses

Chapter 31

Objectives

• Define photoperiodism.

• Describe the role of critical night length in flowering.

• Explain the process of vernalization.

• Explain changing fall colors in leaves.




Chapter 31

Photoperiodism

• Photoperiodism is a plant’s response to changes in the length of days and nights.




Chapter 31

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.




Chapter 31


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.




Chapter 31


Regulation by Phytochrome

– Plants monitor changes in day length with a bluish, light-sensitive pigment called phytochrome.



Chapter 31

Flowering and

Photoperiodism




Chapter 31

Photoperiodism





Chapter 31

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.




Chapter 31

Fall Colors

• Changing fall colors in tree leaves are due to chlorophyll degradation, which reveals other pigments already present.



Chapter 31

Dormancy




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




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





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.





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.





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.





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.





4. What is the response of a plant to the length of days and nights called?

F. gravitropism

G. phototropism

H. photoperiodism

J. thigmotropism





4. What is the response of a plant to the length of days and nights called?

F. gravitropism

G. phototropism

H. photoperiodism

J. thigmotropism





The illustration below shows a growing seedling. Use the illustration to answer the question that follows.





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.





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.





6. cell elongation : gibberellins :: cell division :

F. auxins

G. ethylene

H. cytokinins

J. abscisic acid





6. cell elongation : gibberellins :: cell division :

F. auxins

G. ethylene

H. cytokinins

J. abscisic acid





The diagrams below represent three different growing conditions. Use the diagrams to answer the question that follows.





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





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




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.




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.




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.




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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