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

Plant ResponsesChapter 31

Table of Contents

Section 1 Plant Hormones

Section 2 Plant Movements

Section 3 Seasonal Responses

Section 1 Plant HormonesChapter 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.


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

Click below to watch the Visual Concept.

Visual Concept

Types of 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.

Chapter 31


Visual Concept

Auxins



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


Visual Concept

Apical Dominance



Gibberellins

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


Ethylene

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

Chapter 31


Visual Concept

Ethylene



Cytokinins

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

Chapter 31


Visual Concept

Cytokinins



Abscisic Acid

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


Other Growth Regulators

• Growth retardants are widely used to reduce plant height.

Section 2 Plant MovementsChapter 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.


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


Visual Concept

Tropism



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.


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.


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



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.


Nastic Movements

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


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.


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 ResponsesChapter 31

Objectives

• Define photoperiodism.

• Describe the role of critical night length in flowering.

• Explain the process of vernalization.

• Explain changing fall colors in leaves.


Photoperiodism

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


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.



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.



Regulation by Phytochrome– Plants monitor changes in day length with a bluish,

light-sensitive pigment called phytochrome.

Chapter 31

Flowering and Photoperiodism


Chapter 31


Visual Concept

Photoperiodism



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.


Fall Colors

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

Chapter 31


Visual Concept

Dormancy


Documents

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