1. Chapter 31 Table of Contents Section 1 Plant Hormones
Plant Responses
Chapter 31
Table of Contents
Section 1 Plant Hormones
Section 2 Plant Movements
Section 3 Seasonal Responses

2. 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.

3. Chapter 31 Groups of Hormones
Section 1 Plant Hormones
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.

4. Types of Plant Hormones
Section 1 Plant Hormones
Chapter 31
Types of Plant Hormones
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5. Section 1 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.

6. Chapter 31 Auxins Section 1 Plant Hormones
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7. Chapter 31 Auxins, continued Synthetic Auxins
Section 1 Plant Hormones
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).

8. Chapter 31 Apical Dominance Section 1 Plant Hormones
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9. Section 1 Plant Hormones
Chapter 31
Gibberellins
Gibberellins are used to increase the size of fruit, to stimulate seed germination, and to brew beer.

10. Section 1 Plant Hormones
Chapter 31
Ethylene
Ethylene is used to ripen fruit and promote abscission, the detachment of leaves, flowers, or fruits.

11. Chapter 31 Ethylene Section 1 Plant Hormones
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12. Section 1 Plant Hormones
Chapter 31
Cytokinins
Cytokinins are used to culture plant tissues in the lab and to promote lateral bud growth of flower crops.

13. Chapter 31 Cytokinins Section 1 Plant Hormones
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14. Section 1 Plant Hormones
Chapter 31
Abscisic Acid
Abscisic acid promotes dormancy in plant buds, maintains dormancy in seeds, and causes stomata to close.

15. Other Growth Regulators
Section 1 Plant Hormones
Chapter 31
Other Growth Regulators
Growth retardants are widely used to reduce plant height.

16. 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.

17. Section 2 Plant Movements
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.

18. Chapter 31 Tropism Section 2 Plant Movements
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19. Chapter 31 Tropisms, continued Phototropism
Section 2 Plant Movements
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.

20. Chapter 31 Tropisms, continued Thigmotropism
Section 2 Plant Movements
Chapter 31
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.

21. Chapter 31 Tropisms, continued Gravitropism
Section 2 Plant Movements
Chapter 31
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.

22. Gravitropism in Plants
Section 2 Plant Movements
Chapter 31
Gravitropism in Plants

23. Chapter 31 Tropisms, continued Chemotropism
Section 2 Plant Movements
Chapter 31
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.

24. Chapter 31 Nastic Movements
Section 2 Plant Movements
Chapter 31
Nastic Movements
Nastic movements are responses to environmental stimuli but are independent of the direction of the stimuli.

25. Nastic Movements, continued
Section 2 Plant Movements
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.

26. Nastic Movements, continued
Section 2 Plant Movements
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.

27. Section 2 Plant Movements
Chapter 31
Types of Plants

28. Chapter 31 Objectives Define photoperiodism.
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.

29. Chapter 31 Photoperiodism
Section 3 Seasonal Responses
Chapter 31
Photoperiodism
Photoperiodism is a plant’s response to changes in the length of days and nights.

30. Photoperiodism, continued
Section 3 Seasonal Responses
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.

31. Photoperiodism, continued
Section 3 Seasonal Responses
Chapter 31
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.

32. Photoperiodism, continued
Section 3 Seasonal Responses
Chapter 31
Photoperiodism, continued
Regulation by Phytochrome
Plants monitor changes in day length with a bluish, light-sensitive pigment called phytochrome.

33. Flowering and Photoperiodism
Section 3 Seasonal Responses
Chapter 31
Flowering and Photoperiodism

34. Chapter 31 Photoperiodism Section 3 Seasonal Responses
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35. Chapter 31 Vernalization
Section 3 Seasonal Responses
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.

36. Section 3 Seasonal Responses
Chapter 31
Fall Colors
Changing fall colors in tree leaves are due to chlorophyll degradation, which reveals other pigments already present.

37. Chapter 31 Dormancy Section 3 Seasonal Responses
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