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.

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.

Section 1 Plant Hormones

Chapter

31 Types of Plant Hormones Click below to watch the Visual Concept.

Visual Concept

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

.

Chapter

31 Auxins Section 1 Plant Hormones Click below to watch the Visual Concept.

Visual Concept

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).

Section 1 Plant Hormones

Chapter

31 Apical Dominance Click below to watch the Visual Concept.

Visual Concept

Section 1 Plant Hormones

Chapter

31 Gibberellins

•

Gibberellins

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

Section 1 Plant Hormones

Chapter

31 Ethylene

•

Ethylene

is used to ripen fruit and promote

abscission,

the detachment of leaves, flowers, or fruits.

Chapter

31 Ethylene Section 1 Plant Hormones Click below to watch the Visual Concept.

Visual Concept

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.

Chapter

31 Cytokinins Section 1 Plant Hormones Click below to watch the Visual Concept.

Visual Concept

Section 1 Plant Hormones

Chapter

31 Abscisic Acid

•

Abscisic acid

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

Section 1 Plant Hormones

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.

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.

Chapter

31 Tropism Section 2 Plant Movements Click below to watch the Visual Concept.

Visual Concept

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.

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.

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.

Chapter

31 Gravitropism in Plants Section 2 Plant Movements

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.

Section 2 Plant Movements

Chapter

31 Nastic Movements

•

Nastic movements

are responses to environmental stimuli but are independent of the direction of the stimuli.

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.

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.

Chapter

31 Types of Plants Section 2 Plant Movements

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.

Section 3 Seasonal Responses

Chapter

31 Photoperiodism

•

Photoperiodism

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

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.

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.

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.

Chapter

31 Flowering and Photoperiodism Section 3 Seasonal Responses

Chapter

31 Photoperiodism Section 3 Seasonal Responses Click below to watch the Visual Concept.

Visual Concept

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.

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.

Chapter

31 Dormancy Section 3 Seasonal Responses Click below to watch the Visual Concept.

Visual Concept