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Chapter 27
Control of Growth
and Responses in
Plants
Outline
Plant Responses
Tropisms
Nastic Movements
Plant Hormones
Auxins
Gibberellins
Cytokinins
Abscisic Acid
Ethylene
Photoperiodism
Control of Growth
and Responses in
Plants
2
Plant Responses
Control of Growth
and Responses in
Plants
3
Tropisms
Plant growth toward or away from a unidirectional
stimulus is called a tropism
- Positive is towards stimulus
- Negative is away from stimulus
Phototropism - Light
Gravitropism - Gravity
Thigmotropism - Touch
Phototropism
4
Phototropism
Control of Growth
and Responses in
Plants
5
Positive phototropism:
Occurs because cells on the shady side of the
stem elongate
A pigment related to riboflavin thought to act as a
photoreceptor when phototropism occurs
- Auxin migrates to shady side of stem
- Shady sides elongate faster than bright side
Gravitropism
Control of Growth
and Responses in
Plants
When a plant is placed on its side, the stem
grows upward, opposite of the pull of gravity
Stems with root caps grow downward
Root cells contain statoliths
Auxin is responsible for:
Positive gravitropism of roots, and
Negative gravitropism of shoots
6
Gravitropism
7
Thigmotropism
Control of Growth
and Responses in
Plants
8
Unusual growth due to contact with solid objects
is called thigmotropism
Coiling of tendrils
Thigmomorphogenesis occurs when the entire
plant responds to the presence of environmental
stimuli
Wind
Rain
Coiling Response
9
Nastic Movements
Control of Growth
and Responses in
Plants
Nastic movements:
Do not involve growth and
Are not dependent on the stimulus direction
Seismonastic movements result from:
Touch
Shaking, or
Thermal stimulation
Sleep movements:
Occur daily in response to light and dark changes
Prayer Plant
10
Seismonastic Movement
11
Sleep Movement
12
Circadian Rhythms
Control of Growth
and Responses in
Plants
A circadian rhythms:
Biological rhythms with a 24-hour cycle
Tend to be persistent
- Rhythm is maintained in the absence of
environmental stimuli
- Caused by a biological clock
13
Plant Hormones
Control of Growth
and Responses in
Plants
Almost all communication on a plant is done by
hormones
Synthesized in one part of the plant
Travels within phloem in response to the
appropriate stimulus
14
Auxins
Control of Growth
and Responses in
Plants
15
Auxin is produced in shoot and apical meristem
and is found in young leaves and in flowers and
fruits
Apically produced auxin prevents the growth of
axillary buds
- Apical dominance
Weak solution of auxin applied to woody cutting
causes rapid growth of adventitious roots
- Promotes fruit growth
Apical Dominance
16
Demonstrating Phototropism
17
How Auxins Work
Control of Growth
and Responses in
Plants
Auxins bind to plasma membrane receptors
Activated proton pumps H+ out of cell
Cell wall loosens
Turgor pressure increases due to the entry of
water
Cell enlarges
18
Auxin Mode of Action
19
Gibberellins
Control of Growth
and Responses in
Plants
Gibberellins:
Growth promoting hormones
Bring about elongation of the resulting cells
Gibberellic acid
- Stem elongation
- Breaking of dormancy
20
Effect of Gibberellins
21
Gibberellic Acid:
Structure and Mode of Action
22
Cytokinins
Control of Growth
and Responses in
Plants
Cytokinins
A class of plant hormones that promote cell
division
Derivatives of adenine
- Prevent senescence
- Initiate growth
23
Interaction of Hormones
24
Abscisic Acid
Control of Growth
and Responses in
Plants
25
Abscisic acid (ABA):
Initiates and maintains seed and bud dormancy,
and
Brings about closure of stomata
Produced by:
- Any “green tissue” with chloroplasts
- Monocot endosperm, and
- Roots
Abscisic Acid:
Control of Stoma Opening
26
Ethylene
Control of Growth
and Responses in
Plants
27
Ethylene is involved in abscission
Once abscission has begun:
- Ethylene stimulates certain enzymes
- Cause leaf, fruit, or flower to drop
Also ripens fruit by increasing activity of enzymes
that soften fruit
Functions of Ethylene
28
Photoperiodism
Control of Growth
and Responses in
Plants
29
Photoperiodism:
Any physiological response prompted by changes
in day or night length
- Short-day plants
- Long-day plants
- Day-neutral plants
Some plants may require a specific sequence of
day lengths in order to flower
Photoperiodism and Flowering
30
Phytochrome
Control of Growth
and Responses in
Plants
Phytochrome is a blue-green leaf pigment that
alternately exists in two forms
Phytochrome red (Pr)
Phytochrome far-red (Pfr)
Conversion of forms allows a plant to detect
photoperiod changes
Also promotes seed germination and inhibits
stem elongation
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Phytochrome Conversion Cycle
32
Phytochrome Control of Growth Pattern
33
Review
Plant Responses
Tropisms
Nastic Movements
Plant Hormones
Auxins
Gibberellins
Cytokinins
Abscisic Acid
Ethylene
Photoperiodism
Control of Growth
and Responses in
Plants
34
Ending Slide Chapter 27
Control of
Growth and
Responses in
Plants