Growth Responses and
Regulation of Growth
Chapter 11
LEARNING OBJECTIVES
1 Discuss genetic and environmental factors that affect
plant growth and development .
• 2 Describe phototropism, gravitropism, and
thigmotropism .
• 3 List several ways in which each of the following hormones affects plant growth and development: auxin, gibberellin, cytokinin, ethylene, abscisic acid.
• 4 Relate which hormone or hormones is/ are involved in
each of the following biological processes: leaf
abscission, seed germination, apical dominance.
• 5 Explain how varying amounts of light and darkness
induce flowering, and describe the role of phytochrome.
• 6 Explain how temperature affects flower induction in
certain plants.
• 7 Define circadian rhythm, and give an example.
• 8 Give an example of a turgor movement, and
distinguish between turgor movements and tropisms.
•
LEARNING OBJECTIVE 1
•
Discuss genetic and environmental
factors that affect plant growth and
development
Internal Genetic Factors
•
The location of a cell in the young plant
body affects gene expression during
development
•
Causes some genes in that cell to be
turned off and others to be turned on
External Environmental Factors
•
Factors in the physical environment
determine gene expression, affect plant
growth and development
•
•
•
changing day length
variation in precipitation
temperature
Spring Flowers
LEARNING OBJECTIVE 2
•
Describe phototropism, gravitropism,
and thigmotropism
Tropisms
•
Are directional growth responses
•
Are permanent
KEY TERMS
•
PHOTOTROPISM
•
•
GRAVITROPISM
•
•
Directional growth of a plant caused by light
Plant growth in response to direction of gravity
THIGMOTROPISM
•
Growth in response to contact with a solid object
Phototropism
Gravitropism
Fig. 11-3a, p. 223
On day 3, turned on
its side
One hour later
Fig. 11-3a, p. 223
Fig. 11-3b, p. 223
Darwins’ Experiments
Fig. 11-4a, p. 224
Fig. 11-4b, p. 224
(a)
(b)
(c)
(d)
Light rays
Fig. 11-4b, p. 224
Auxin in Coleoptiles
Coleoptile tip
(a)
Agar block
(b)
(c)
Fig. 11-5, p. 225
LEARNING OBJECTIVE 3
•
List several ways in which each of the
following hormones affects plant
growth and development: auxin,
gibberellin, cytokinin, ethylene,
abscisic acid
KEY TERMS
•
HORMONE
•
An organic chemical messenger that
regulates growth and development in plants
and other multicellular organisms
Communication Molecules
•
Five major classes of plant hormones
•
•
auxin, gibberellin, cytokinin, ethylene,
abscisic acid
A variety of signaling molecules
KEY TERMS
•
AUXIN
•
Plant hormone involved in growth and
development, including stem elongation,
apical dominance, and root formation on
cuttings
Animation: Phototropism
CLICK
TO PLAY
Auxin and Phototropism
Shaded side
of coleoptile
Light rays
Illuminated
side of
coleoptile
Fig. 11-6, p. 226
Auxin and
Root Development
Animation: Auxin’s Effects
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TO PLAY
KEY TERMS
•
GIBBERELLIN
•
Plant hormone involved in growth and
development, including stem elongation,
flowering, and seed germination
Effects of Gibberelin
KEY TERMS
•
CYTOKININ
•
Plant hormone involved in growth and
development, including cell division and delay
of senescence
Hormones and Tissue Culture
Cell division
without differentiation
(a) Initial
explant
b) Callus
Cell division
with differentiation
(c) Roots
d) Shoots
Fig. 11-10, p. 229
Cytokinin and
Senescence
KEY TERMS
•
ETHYLENE
•
A gaseous plant hormone involved in growth
and development, including leaf abscission
and fruit ripening
Ethylene and Fruit Ripening
KEY TERMS
•
ABSCISIC ACID
•
A plant hormone involved in growth and
development, including dormancy and
responses to stress
Other Signaling Molecules
Graft
Long-day
induction
Day-neutral plant grafted
to long-day plant
Both plants flower
Fig. 11-14, p. 232
Plant cell responses to infection by fungi, bacteria, or viruses. As a
result of an initial infection and a subsequent signal-transduction pathway, plants
produce a variety of antimicrobial molecules
Signal Transduction
2 Ubiquitin is
attached to proteins
that inhibit certain
genes.
Plasma membrane
Ubiquitin
Cell wall
4 Inhibited genes
are turned on.
Nucleus
Receptor
Auxin
1 Auxin
binds
to
receptor.
DNA (contains genes)
Protein
3 Proteins are
destroyed.
Cytoplasm
Nuclear envelope
Fig. 11-15, p. 233
2
Ubiquitin is
tagged to
proteins that
inhibit certain
genes.
Plasma membrane
Cell wall
Ubiquitin
4 Previously
repressed genes
Nucleus
are activated and
expressed.
Transcription
Receptor
Auxin
1 Auxin
binds to
TIR1
receptor.
Protein
3
Proteins are
targeted for
destruction.
Cytoplasm
DNA
Nuclear envelope
Stepped Art
Fig. 11-15, p. 233
LEARNING OBJECTIVE 4
•
Relate which hormone or hormones
is/are involved in each of the following
biological processes: leaf abscission,
seed germination, apical dominance
Leaf Abscission
•
Ethylene and auxin
•
As a leaf ages, auxin level in the leaf
decreases, and ethylene level increases
Seed Germination
•
Gibberellins involved in seed germination
•
•
•
in certain plants (cereals, grasses)
substitutes for low-temperature or light
requirements in some seeds (lettuce, oats,
tobacco)
Ethylene and abscisic acid
•
Ethylene promotes seed germination; abscisic
acid inhibits seed germination
Abscisic Acid and Germination
Apical Dominance 1
•
Inhibition of axillary bud growth by the
apical meristem
•
Auxin
•
•
Produced in shoot apical meristem
Inhibits axillary buds near apical meristem
from developing into actively growing shoots
Apical Dominance 2
•
Cytokinins and ethylene
•
Cytokinins promote growth of axillary buds;
ethylene inhibits axillary bud development
Auxin and
Axillary Bud Development
LEARNING OBJECTIVE 5
•
Explain how varying amounts of light
and darkness induce flowering
•
Describe the role of phytochrome
KEY TERMS
•
PHOTOPERIODISM
•
Physiological response (such as flowering) of
plants to variations in length of daylight and
darkness
Photoperiodism
•
Some plants are short-day plants, some
are long-day plants, others are
intermediate-day plants
•
•
Plant measures length of dark period
In day-neutral plants, photoperiod does
not affect flowering
Short-Day Plants
Photoperiodic Responses
Day
Night
(a) Short days
(b) Long days
(c) Short days
and long nights and short nights and long nights
(interrupted with
a brief period of
light)
(d) Long days
(interrupted with
a brief period of
dark) and short
nights
1
Chrysanthemum (short-day/ long-night plant)
2
Black-eyed Susan (long-day/short-night plant)
Fig. 11-17, p. 234
KEY TERMS
•
PHYTOCHROME
•
A blue-green proteinaceous pigment involved
in many plant responses to light, independent
of photosynthesis
Phytochrome
•
There are about five different phytochrome
proteins
•
Each exists in two forms and readily
converts from one form to the other after
absorption of light of specific wavelengths
Phytochrome Forms
•
Pr strongly absorbs red light with a
relatively short wavelength (660 nm)
•
•
Changes to the second form (Pfr)
Pfr absorbs red light with a relatively long
wavelength (730 nm)
•
The active form, triggers or inhibits responses
such as flowering
Phytochrome Conversion
Red light (660 nm)
Inactive
form
Pr
Short-lived
intermediate forms
Short-lived
intermediate forms
Far-red light
(730 nm)
Active form
Pfr
Physiological
response
(such as
flowering)
Fig. 11-18, p. 235
Animation: Phytochrome
Conversions
CLICK
TO PLAY
LEARNING OBJECTIVE 6
•
Explain how temperature affects flower
induction in certain plants
Temperature Requirements
•
Certain plants have temperature
requirements that must be met in order for
them to flower
KEY TERMS
•
VERNALIZATION
•
The low-temperature requirement for
flowering in some plant species
Temperature Requirements
LEARNING OBJECTIVE 7
•
Define circadian rhythm
•
Give an example
KEY TERMS
•
CIRCADIAN RHYTHM
•
A biological activity with an internal rhythm
that approximates the 24-hour day
Circadian Rhythms
•
Reset by the rising and setting of the sun
•
Circadian rhythms in plants affect
•
•
•
gene expression
rate of photosynthesis
opening and closing of stomata
Sleep Movements
LEARNING OBJECTIVE 8
•
Give an example of a turgor movement,
and distinguish between turgor
movements and tropisms
KEY TERMS
•
TURGOR MOVEMENT
•
•
Temporary plant movement that results from
changes in internal water pressure in a plant
part
Examples: Leaves of the sensitive plant and
Venus flytrap
Sensitive Plant
Fig. 11-21a, p. 237
Fig. 11-21b, p. 237
Fig. 11-21c, p. 237
Leaflet
open
Leaflet
Pulvinus
Vascular
tissue
Decrease of
turgor in
parenchyma
cells
Leaflet
folded
Parenchyma
cells retaining
turgor
Cross-sectional views
(c) How the folding and drooping occurs. Pulvini occur in three areas: the
base of each leaflet, the base of each cluster of leaflets, and the base of each
leaf. Only changes in the pulvini at the bases of leaflets are shown. (Top right)
A section through two leaflets, showing their pulvini when the leaf is undisturbed.
(Bottom right) A section through the two leaflets, showing how a loss
of turgor produces the folding of the leaves.
Fig. 11-21c, p. 237
Turgor and Tropisms
•
Turgor movements are temporary plant
movements
•
Tropisms are permanent growth
responses