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Chapter 39: Plant Responses to Internal & External Stimuli
1. How was it determined that the plant tip controlled phototropism?
Figure 39.5 What part of a coleoptile senses light, and how is the
signal transmitted?
EXPERIMENT In 1880, Charles Darwin and his son Francis designed an experiment to determine
what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to
determine how the signal for phototropism is transmitted.
RESULTS
Control
Boysen-Jensen (1913)
Darwin and Darwin (1880)
Shaded
side of
coleoptile
Light
Light
Light
Illuminated
side of
coleoptile
Tip
removed
Tip covered
by opaque
cap
Tip
covered
by transparent
cap
Base covered
by opaque
shield
Tip separated
by gelatin
block
Tip separated
by mica
CONCLUSION In the Darwins’ experiment, a phototropic response occurred only when light could
reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen
observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin)
but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested
that the signal is a light-activated mobile chemical.
Figure 39.6 Does asymmetric distribution of a growth-promoting
chemical cause a coleoptile to grow toward the light?
EXPERIMENT
EXPERIMENT
In 1926, Frits Went’s experiment identified how a growth-promoting
chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and
removed their tips, putting some tips on agar blocks that he predicted would absorb the
chemical. On a control coleoptile, he placed a block that lacked the chemical. On others,
he placed blocks containing the chemical, either centered on top of the coleoptile to
distribute the chemical evenly or offset to increase the concentration on one side.
RESULTS
The coleoptile grew straight if the chemical was distributed evenly.
If the chemical was distributed unevenly, the coleoptile curved away from the side with
the block, as if growing toward light, even though it was grown in the dark.
Excised tip placed
on agar block
Growth-promoting
chemical diffuses
into agar block
Control
Control
(agar block
lacking
chemical)
has no
effect
Agar block
with chemical
stimulates growth
Offset blocks
cause curvature
CONCLUSION
Went concluded that a coleoptile curved toward light because its dark
side had a higher concentration of the growth-promoting chemical, which he named auxin.
Chapter 39: Plant Responses to Internal & External Stimuli
1. How was it determined that the plant tip controlled phototropism?
2. What are the primary plant hormones?
Hormone
Site of Production
Effect
Auxin (IAA)
embryo of seed
germination
apical meristems
apical dominance
Cytokinins
roots
stimulates cell division
& growth, delays aging
Figure 39.9 Apical dominance
Axillary buds
“Stump” after
removal of
apical bud
Lateral branches
(a) Intact plant
(b) Plant with apical bud removed
Chapter 39: Plant Responses to Internal & External Stimuli
1. How was it determined that the plant tip controlled phototropism?
2. What are the primary plant hormones?
Hormone
Site of Production
Effect
Auxin (IAA)
embryo of seed
germination
apical meristems
apical dominance
Cytokinins
roots
stimulates cell division
& growth, delays aging
Gibberellins
apical meristems
elongation &
differentiation, flowering
fruit development
embryo
seed germination
Figure 39.10 The effect of gibberellin treatment on Thompson
seedless grapes
Figure 39.11 Gibberellins mobilize nutrients during the germination
of grain seeds
22 The aleurone responds by
synthesizing and secreting
digestive enzymes that
hydrolyze stored nutrients in
the endosperm. One example
is -amylase, which hydrolyzes
starch. (A similar enzyme in
our saliva helps in digesting
bread and other starchy foods.)
1 After a seed
imbibes water, the
embryo releases
gibberellin (GA)
as a signal to the
aleurone, the thin
outer layer of the
endosperm.
3 Sugars and other
nutrients absorbed
from the endosperm
by the scutellum
(cotyledon) are consumed
during growth of the
embryo into a seedling.
Aleurone
Endosperm
-amylase
GA
GA
Water
Radicle
Scutellum
(cotyledon)
Sugar
Chapter 39: Plant Responses to Internal & External Stimuli
1. How was it determined that the plant tip controlled phototropism?
2. What are the primary plant hormones?
Hormone
Site of Production
Effect
Auxin (IAA)
embryo of seed
germination
apical meristems
apical dominance
Cytokinins
roots
stimulates cell division
& growth, delays aging
Gibberellins
apical meristems
elongation &
differentiation, flowering
fruit development
embryo
seed germination
Abscisic acid
leaves, stems, roots,
inhibits growth
green fruit
prepares for winter
Ethylene
ripening fruit
ripens fruit
triple response
Figure 39.13 How does ethylene concentration affect the triple
response in seedlings?
EXPERIMENT
Germinating pea seedlings were placed in the
dark and exposed to varying ethylene concentrations. Their growth
was compared with a control seedling not treated with ethylene.
RESULTS
All the treated seedlings exhibited the triple
response. Response was greater with increased concentration.
0.00
0.10
0.20
0.40
0.80
Ethylene concentration (parts per million)
CONCLUSION
Ethylene induces the triple response in pea seedlings,
with increased ethylene concentration causing increased response.
Slowing elongation, stem thickening, & stem curvature
Chapter 39: Plant Responses to Internal & External Stimuli
1. How was it determined that the plant tip controlled phototropism?
2. What are the primary plant hormones?
3. How does auxin control cell elongation?
Figure 39.8 Cell elongation in response to auxin: the acid growth
hypothesis
3 Wedge-shaped expansins, activated
by low pH, separate cellulose microfibrils from
cross-linking polysaccharides. The exposed cross-linking
polysaccharides are now more accessible to cell wall enzymes.
Expansin
4 The enzymatic cleaving
of the cross-linking
CELL WALL
polysaccharides allows
the microfibrils to slide.
The extensibility of the
cell wall is increased. Turgor
causes the cell to expand.
H2O
Cell wall
enzymes
Cross-linking
cell wall
polysaccharides
Microfibril
Plasma
membrane
H+
H+
2 The cell wall
becomes more
acidic.
Cell
wall
H+
H+
H+
H+
H+
H+
1 Auxin
increases the
activity of
proton pumps.
Cytoplasm
Nucleus
Vacuole
ATP
H+
Plasma membrane
5 With the cellulose loosened,
the cell can elongate.
Chapter 39: Plant Responses to Internal & External Stimuli
1.
2.
3.
4.
How was it determined that the plant tip controlled phototropism?
What are the primary plant hormones?
How does auxin control cell elongation?
Why do leaves change colors & fall off trees?
- New red pigments made during fall + yellow & orange carotenoids
- Chlorophyll no longer produced
Figure 39.16 Abscission of a maple leaf
0.5 mm
- Aging leaves produce less auxin so
abscission layer is mores sensitive to ethylene
- Abscission layer has thin walls
- Weight of leaf causes separation
Protective layer
Abscission layer
Stem
Petiole
Chapter 39: Plant Responses to Internal & External Stimuli
1.
2.
3.
4.
5.
How was it determined that the plant tip controlled phototropism?
What are the primary plant hormones?
How does auxin control cell elongation?
Why do leaves change colors & fall off trees?
How do plants “move?”
- Tropisms – toward or away from stimuli
- Photo – light
- Gravi – gravity
- Thigmo – touch
- Turgor movements – changes in turgor pressure in specialized cells
6. How are plants able to respond to light?
- Blue-light photoreceptors
- Phytochromes
Figure 39.17 What wavelengths stimulate phototropic bending
toward light?
EXPERIMENT
Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow,
orange, and red light to test which wavelengths stimulate the phototropic bending toward light.
Phototropic effectiveness relative to 436 nm
RESULTS
The graph below shows phototropic effectiveness (curvature per photon) relative
to effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and
after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only
with wavelengths below 500 nm and was greatest with blue light.
1.0
0.8
0.6
0.4
0.2
0
400
450
500
550
600
650
700
Wavelength (nm)
Light
Time = 0 min.
Time = 90 min.
CONCLUSION
The phototropic bending toward light is caused by a photoreceptor that is sensitive
to blue and violet light, particularly blue light.
Figure 39.19 Structure of a phytochrome
A phytochrome consists of two identical proteins joined to form
one functional molecule. Each of these proteins has two domains.
Chromophore
Photoreceptor activity. One domain,
which functions as the photoreceptor,
is covalently bonded to a nonprotein
pigment, or chromophore.
Kinase activity. The other domain
has protein kinase activity. The
photoreceptor domains interact with the
kinase domains to link light reception to
cellular responses triggered by the kinase.
Figure 39.2 Light-induced de-etiolation (greening) of dark-grown
potatoes
(a) Before exposure to light. A
dark-grown potato has tall,
spindly stems and nonexpanded
leaves—morphological
adaptations that enable the
shoots to penetrate the soil. The
roots are short, but there is little
need for water absorption
because little water is lost by the
shoots.
(b) After a week’s exposure to
natural daylight. The potato
plant begins to resemble a
typical plant with broad green
leaves, short sturdy stems, and
long roots. This transformation
begins with the reception of
light by a specific pigment,
phytochrome.
Figure 39.4 An example of signal transduction in plants: the role of
phytochrome in the de-etiolation (greening) response
2 Transduction
1 Reception
3 Response
CYTOPLASM
Plasma
membrane
NUCLEUS
cGMP
Second messenger
produced
Phytochrome
activated
by light
Cell
wall
Light
Specific
protein
kinase 1
activated
Figure 39.4 An example of signal transduction in plants: the role of
phytochrome in the de-etiolation (greening) response
2 Transduction
1 Reception
3 Response
CYTOPLASM
NUCLEUS
cGMP
Plasma
membrane
Second messenger
produced
Specific
protein
kinase 1
activated
Phytochrome
activated
by light
Cell
wall
Specific
protein
kinase 2
activated
Light
Ca2+ channel
opened
Ca2+
Figure 39.4 An example of signal transduction in plants: the role of
phytochrome in the de-etiolation (greening) response
2 Transduction
1 Reception
3 Response
Transcription
factor 1
NUCLEUS
CYTOPLASM
cGMP
Plasma
membrane
Second messenger
produced
Specific
protein
kinase 1
activated
Phytochrome
activated
by light
P
Transcription
factor 2
P
Cell
wall
Specific
protein
kinase 2
activated
Transcription
Light
Translation
Ca2+
channel
opened
Ca2+
De-etiolation
(greening)
response
proteins
Red light
Pr
Phytochromes are sensitive to 2 different wavelengths
-Red light converts the phytochrome to be far-red sensitive
-Far-red converts the phytochrome to be red light sensitve
Pfr
Far-red light
Figure 39.20 Phytochrome: a molecular switching mechanism
Pr
Pfr
Red light
Responses:
seed germination,
control of
flowering, etc.
Synthesis
Far-red
light
Slow conversion
in darkness
(some plants)
Enzymatic
destruction
Figure 39.18 How does the order of red and far-red illumination
affect seed germination?
EXPERIMENT
During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red
light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in
the dark, and the results were compared with control seeds that were not exposed to light.
RESULTS
The bar below each photo indicates the sequence of red-light exposure, far-red light
exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposed
to red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right).
Dark (control)
Red
Dark
Red Far-red Red
Red Far-red
Dark
Dark
Red Far-red Red Far-red
CONCLUSION
Red light stimulated germination, and far-red light inhibited germination.
The final exposure was the determining factor. The effects of red and far-red light were reversible.
Chapter 39: Plant Responses to Internal & External Stimuli
1.
2.
3.
4.
5.
6.
7.
How was it determined that the plant tip controlled phototropism?
What are the primary plant hormones?
How does auxin control cell elongation?
Why do leaves change colors & fall off trees?
How do plants “move?”
How are plants able to respond to light?
What controls a plant’s biological clock?
- Photoperiodism – a physiological response to the duration of night & day
- Flowering
Figure 39.22 How does interrupting the dark period with a brief
exposure to light affect flowering?
EXPERIMENT
During the 1940s, researchers conducted experiments in which periods of
darkness were interrupted with brief exposure to light to test how the light and dark portions
of a photoperiod affected flowering in “short-day” and “long-day” plants.
RESULTS
Darkness
Day neutral plants are unaffected
by photoperiod….maturity important.
Flash of
light
Critical
dark
period
Light
(a) “Short-day” plants
flowered only if a period of
continuous darkness was
longer than a critical dark
period for that particular
species (13 hours in this
example). A period of
darkness can be ended by a
brief exposure to light.
(b) “Long-day” plants
flowered only if a
period of continuous
darkness was shorter
than a critical dark
period for that
particular species (13
hours in this example).
CONCLUSION
The experiments indicated that flowering of each species was determined
by a critical period of darkness (“critical night length”) for that species, not by a specific period
of light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day”
plants are really “short-night” plants.
Figure 39.23 Is phytochrome the pigment that measures the
interruption of dark periods in photoperiodic response?
EXPERIMENT
A unique characteristic of phytochrome is reversibility in response to red and far-red light.
To test whether phytochrome is the pigment measuring interruption of dark periods, researchers observed
how flashes of red light and far-red light affected flowering in “short-day” and “long-day” plants.
RESULTS
24
20
R
FR
R
R
FR
R
FR
R
FR
R
16
12
8
4
0
Short-day (long-night) plant
Long-day (short-night) plant
CONCLUSION A flash of red light shortened the dark period. A subsequent flash of far-red light
canceled the red light’s effect. If a red flash followed a far-red flash, the effect of the far-red light was
canceled. This reversibility indicated that it is phytochrome that measures the interruption of dark periods.
Figure 39.24 Is there a flowering hormone?
EXPERIMENT
To test whether there is a flowering hormone, researchers conducted an
experiment in which a plant that had been induced to flower by photoperiod was grafted to
a plant that had not been induced.
RESULTS
Plant subjected to photoperiod
that does not induce flowering
Plant subjected to photoperiod
that induces flowering
Graft
Time
(several
weeks)
YES!!!
Florigen – flowering signal
CONCLUSION Both plants flowered, indicating the transmission of a flower-inducing
substance. In some cases, the transmission worked even if one was a short-day plant
and the other was a long-day plant.
Chapter 39: Plant Responses to Internal & External Stimuli
1.
2.
3.
4.
5.
6.
7.
8.
How was it determined that the plant tip controlled phototropism?
What are the primary plant hormones?
How does auxin control cell elongation?
Why do leaves change colors & fall off trees?
How do plants “move?”
How are plants able to respond to light?
What controls a plant’s biological clock?
How does gravitropism work?
- Statoliths
Figure 39.25 Positive gravitropism in roots: the statolith hypothesis
Statoliths
(a)
(b)
20 m
Chapter 39: Plant Responses to Internal & External Stimuli
1.
2.
3.
4.
5.
6.
7.
8.
9.
How was it determined that the plant tip controlled phototropism?
What are the primary plant hormones?
How does auxin control cell elongation?
Why do leaves change colors & fall off trees?
How do plants “move?”
How are plants able to respond to light?
What controls a plant’s biological clock?
How does gravitropism work?
What’s the difference between thigmomorphogenesis & thigmotropism?
- Thigmomorpho – permanent change in shape
- Thigmo – growth in response to touch - vines
Figure 39.26 Altering gene expression by touch in Arabidopsis
Figure 39.27 Rapid turgor movements by the sensitive plant
(Mimosa pudica)
(a) Unstimulated
(b) Stimulated
Side of pulvinus with
flaccid cells
Leaflets
after
stimulation
Side of pulvinus with
turgid cells
Pulvinus
(motor
organ)
(c) Motor organs
Vein
0.5 m