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Chapter 39
Plant Responses to Internal
and External Signals
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Overview: Stimuli and a Stationary Life
• Plants, being rooted to the ground
– Must respond to whatever environmental
change comes their way
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• For example, the bending of a grass seedling
toward light
– Begins with the plant sensing the direction,
quantity, and color of the light
Figure 39.1
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• Signal transduction pathways link signal
reception to response
• Plants have cellular receptors
– That they use to detect important changes in
their environment
• For a stimulus to elicit a response
– Certain cells must have an appropriate
receptor
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• A potato left growing in darkness
– Will produce shoots that do not appear healthy,
and will lack elongated roots
• These are morphological adaptations for
growing in darkness
– Collectively referred to as etiolation
(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.
Figure 39.2a
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• After the potato is exposed to light
– The plant undergoes profound changes called deetiolation, in which shoots and roots grow normally
(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 39.2b
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• The potato’s response to light
– Is an example of cell-signal processing
CYTOPLASM
CELL
WALL
1 Reception
2
Transduction
Relay molecules
Receptor
Hormone or
environmental
stimulus
Plasma membrane
Figure 39.3
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3 Response
Activation
of cellular
responses
Reception
• Internal and external signals are detected by
receptors
– Proteins that change in response to specific
stimuli
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Transduction
• Second messengers
– Transfer and amplify signals from receptors to
proteins that cause specific responses
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• An example of signal transduction in plants
2 Transduction
1 Reception
3 Response
Transcription
factor 1
NUCLEUS
CYTOPLASM
cGMP
Plasma
membrane
Second messenger
produced
Phytochrome
activated
by light
Cell
wall
2 One pathway uses cGMP as a
second messenger that activates
a specific protein kinase.The other
pathway involves an increase in
cytoplasmic Ca2+ that activates
another specific protein kinase.
Specific
protein
kinase 1
activated
P
Transcription
factor 2
P
Specific
protein
kinase 2
activated
Transcription
Light
Translation
1 The light signal is
detected by the
phytochrome receptor,
which then activates
at least two signal
transduction pathways.
Ca2+
channel
opened
Ca2+
Figure 39.4
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3 Both pathways
lead to expression
of genes for proteins
that function in the
de-etiolation
(greening) response.
De-etiolation
(greening)
response
proteins
Response
• Ultimately, a signal transduction pathway
– Leads to a regulation of one or more cellular
activities
• In most cases
– These responses to stimulation involve the
increased activity of certain enzymes
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Transcriptional Regulation
• Transcription factors bind directly to specific
regions of DNA
– And control the transcription of specific genes
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Post-Translational Modification of Proteins
• Post-translational modification
– Involves the activation of existing proteins
involved in the signal response
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De-Etioloation (“Greening”) Proteins
• Many enzymes that function in certain signal
responses are involved in photosynthesis
directly
– While others are involved in supplying the
chemical precursors necessary for chlorophyll
production
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• Plant hormones help coordinate growth,
development, and responses to stimuli
• Hormones
– Are chemical signals that coordinate the
different parts of an organism
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The Discovery of Plant Hormones
• Any growth response
– That results in curvatures of whole plant
organs toward or away from a stimulus is
called a tropism
– Is often caused by hormones
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• Charles Darwin and his son Francis
– Conducted some of the earliest experiments
on phototropism, a plant’s response to light, in
the late 19th century
http://virtualastronaut.tietronix.com/texton
ly/act25/images/phototropism.gif
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Phototropism
• Positive phototropism:
Plants grow toward light
• Negative phototropism:
Plants grow away from
light
• Results from differential
growth of cells on opposite
sides of a shoot or
coleoptile (grass seedlings)
• Cells on darker side
elongate faster than those
on the light side
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Phototropism
• Growth toward/away from light
• Differential cell elongation
results from asymmetric
distribution of auxin
• Shoot tip is the site of photoreception
– Yellow pigment in the tip is a photoreceptor
for blue light and triggers growth response
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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 39.5
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• In 1926, Frits Went
– Extracted the
chemical
messenger for
phototropism,
auxin, by
modifying earlier
experiments
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
Figure 39.6
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.
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A Survey of Plant Hormones
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• In general, hormones control plant growth and
development
– By affecting the division, elongation, and
differentiation of cells
• Plant hormones are produced in very low
concentrations
– But a minute amount can have a profound
effect on the growth and development of a
plant organ
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Auxin
• The term auxin
– Is used for any chemical substance that promotes cell
elongation in different target tissues
• Promotes elongation of young developing shoots or
coleoptiles
• Major site of production is the apical meristem
• Moves from the apex down to the zone of cell elongation
– Polar transport is unidirectional and requires metabolic
energy which is provided by chemiosmosis
– Movement aided by differences in pH between the
acidic cell wall and the neutral cytoplasm
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The Role of Auxin in Cell Elongation
• According to a model called the acid growth
hypothesis
– Proton pumps play a major role in the growth
response of cells to auxin
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• Cell elongation in response to auxin
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.
Cell wall
enzymes
Cross-linking
cell wall
polysaccharides
Microfibril
H2O
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+
Figure 39.8
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Plasma membrane
Cytoplasm
5 With the cellulose loosened,
the cell can elongate.
Other functions of Auxin
• Affects secondary growth by inducing
vascular cambium cell division and
differentiation of secondary xylem
• Promotes formation of adventitious roots
• Involved in the formation and branching of roots
• Promotes fruit growth in many plants
• Can be used as herbicides
– 2,4-D—synthetic auxin which affects dicots
selectively, allowing removal of broadleaf
weeds from a lawn
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Cytokinins
• Cytokinins
• Modified forms of adenine that stimulate
cytokinesis
• Function in several areas of plant growth
– Stimulates cell division and differentiation
– Apical dominance
– Anti-aging hormones
• Effect complemented or countered by auxin
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Control of Cell Division and Differentiation
• Cytokinins
–
Are produced in actively growing tissues such as roots, embryos,
and fruits
• Move from the roots to target tissues by moving up in the xylem sap
• Stimulate production of RNA and protein involved with cell division
• Works in conjunction with auxin
–
Stem parenchyma cells grown without cytokinins grow large but
don’t divide
–
Cytokinins alone have no affect on cells
–
Cytokinins = auxin stimulate cell growth and division, but they
remain an undifferentiated callus
–
Cytokinin > auxin causes shoot buds to develop from callus
–
Cytokinin < auxin causes roots to form
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Control of Apical Dominance
• Cytokinins, auxin, and other factors interact in the control
of apical dominance
• Antagonistic mechanism b/w auxin and cytokinin
• Auxin from the terminal bud restrains axillary bud growth,
causing the shoot to lengthen
• Cytokinins (from roots) stimulate
axillary bud growth
• Auxin can’t suppress axillary bud
growth once it has begun
Axillary buds
Figure 39.9a
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•If the terminal bud is
removed
– Plants become bushier
– Lower buds grow before
higher ones since they
are closer to the cytokinin
source than the auxin
source
•Auxin stimulates lateral root
formation while cytokinins
restrain it
“Stump” after
removal of
apical bud
Lateral branches
Figure 39.9b
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Anti-Aging Effects
• Cytokinins retard the aging of some plant
organs
– By inhibiting protein breakdown, stimulating
RNA and protein synthesis, and mobilizing
nutrients from surrounding tissues
– May slow leaf deterioration on plants since
detached leaves dipped in a cytokinin solution
stay green longer than without
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Gibberellins
•
Gibberellins have a variety of effects
–
•
Stem elongation
–
•
•
Such as stem elongation, fruit growth, and seed germination
Produced in roots and young leaves
•
Stimulate growth in leaves and stems, show little effect on roots
•
Stimulate cell division and elongation in stems
•
Cause bolting (rapid growth of floral stems, which elevates flowers)
Fruit growth
–
Controlled by gibberellins and auxin
–
Sprayed on Thompson seedless grapes; causes
grapes to grow larger and farther apart
Germination
Figure 39.10
–
Release of gibberellins signals seeds to break dormancy and germinate
–
Release stimulated by imbibed water
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Germination
• After water is imbibed, the release of gibberellins from
the embryo
– Signals the seeds to break dormancy and germinate
2 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)
Figure 39.11
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Sugar
Brassinosteroids
• Brassinosteroids
– Are similar to the sex hormones of animals
– Induce cell elongation and division
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Abscisic Acid
• Two of the many effects of abscisic acid (ABA)
are
– Seed dormancy
– Drought tolerance
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Seed Dormancy
• Ensures that the seed will germinate only when there are
optimal conditions
–
Produced in the terminal bud and helps prepare plants for winter
by suspending both primary and secondary growth
• Directs leaf primordia to develop scales that protect dormant
buds
• Inhibits cell division in vascular cambium
–
At other times, seed dormancy proves advantageous
• The ratio of ABA-gibberellins determines whether seeds
remain dormant or germinate
• In other plants, seeds germinate when ABA is washed out of
the seeds (desert plants) or degraded by some other stimulus
such as sunlight
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Drought Tolerance
• ABA is the primary internal signal
– That enables plants to withstand drought
– Acts as a stress hormone—closes stomata in times of
water-stress and reduces transpiration water loss
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Ethylene
• Plants produce ethylene
– In response to stresses such as drought,
flooding, mechanical pressure, injury, and
infection
• Gaseous hormone that diffuses through air
spaces b/w plant cells
• High levels of auxin induce its release
• Acts as a growth inhibitor
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Apoptosis: Programmed Cell Death
• A burst of ethylene
– Is associated with the programmed destruction
of cells, organs, or whole plants
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Leaf Abscission
• A change in the balance of auxin and ethylene
controls leaf abscission
– The process that occurs in autumn when a
leaf falls
• Adaptation that prevents
deciduous trees from
desiccating during winter
when roots can’t absorb water
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0.5 mm
Protective layer
Abscission layer
Stem
Petiole
Figure 39.16
Leaf Abscission
•
Prior to abscission-leaf’s essential elements are shunted to storage
tissues in the stem
•
Environmental stimuli=shortening of days and cooler temperatures.
•
Mechanics controlled by change in balance of ethylene and auxin
–
 auxin =  cell sensitivity to ethylene; cells produce more
ethylene which inhibits auxin production
–
Ethylene induces synthesis of enzymes that digest the
polysaccharides in the cell walls, further weakening the
abscission layer
–
Wind and weight cause the leaf to fall by causing a separation in
the abscission layer
–
Before the leaf falls, a layer of cork forms a protective scar in the
twig’s side of the abscission layer—prevents pathogens from
entering the plant
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Fruit Ripening
• A burst of ethylene production in the fruit
– Triggers the ripening process
• Ethylene triggers senescence and the aging
cells release more ethylene
– Breakdown of cell walls and loss of chlorophyll
– Signal to ripen spreads from fruit to fruit since
ethylene is a gas
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• Responses to light are critical for plant success
• Light cues many key events in plant growth
and development
• Effects of light on plant morphology
– Are what plant biologists call
photomorphogenesis
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• Plants not only detect the presence of light
– But also its direction, intensity, and wavelength
(color)
• A graph called an action spectrum
– Depicts the relative response of a process to
different wavelengths of light
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• Action spectra
– Are useful in the study of any process that depends
on 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.
Figure 39.17
CONCLUSION The phototropic bending toward light is caused by a photoreceptor that is
sensitive to blue and violet light, particularly blue light.
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• Research on action spectra and absorption
spectra of pigments
– Led to the identification of two major classes of
light receptors: blue-light photoreceptors and
phytochromes
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Blue-Light Photoreceptors
• Various blue-light photoreceptors
– Control hypocotyl elongation, stomatal
opening, and phototropism
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Phytochromes as Photoreceptors
• Phytochromes
– Regulate many of a plant’s responses to light
throughout its life
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Phytochromes and Seed Germination
• Studies of seed germination
– Led to the discovery of phytochromes
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• In the 1930s, scientists at the U.S. Department
of Agriculture
– Determined the action spectrum for lightinduced germination of lettuce seeds
Dark (control)
Dark
Dark
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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
Figure 39.18
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.
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• A phytochrome
– Is the photoreceptor responsible for the
opposing effects of red and far-red light
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 39.19
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• Phytochromes exist in two photoreversible
states
–
With conversion of Pr (red absorbing) to Pfr (far-red absorbing) triggering
many developmental responses
•
Pr in dark, remains Pr
•
Pr illuminated, converted to Pfr
•
Pfr triggers many plant responses to light
•
Shift in equilibrium indicates the relative amounts of red and far-red
in the sunlight
Pr
Red light
Synthesis
Far-red
light
Figure 39.20
Slow conversion
in darkness
(some plants)
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Pfr
Responses:
seed germination,
control of
flowering, etc.
Enzymatic
destruction
Phytochromes and Shade Avoidance
• The phytochrome system
– Also provides the plant with information about
the quality of light
• In the “shade avoidance” response of a tree
– The phytochrome ratio shifts in favor of Pr
when a tree is shaded
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Biological Clocks and Circadian Rhythms
• Many plant processes oscillate during the day
• Control circadian rhythms (cycle with frequency of
24h)
– Common in all eukaryotes
• Oscillator is probably endogenous and set to a 24h
period daily by environmental signals
• Most are cued to light-dark cycle resulting from the
Earth’s rotation
– May take days to reset once the cues change
• Jet lag-lack of synchronization b/w internal clock
and time zone
• Research indicates that cyclic changes in levels of a
protein form the basis for the internal clock
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• Conversion of Pr to Pfr triggers K+ fluxes in cells of the
pulvini that causes sleep movements in legumes
• Many legumes lower their leaves in the evening and raise
them in the morning
Figure 39.21
Noon
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Midnight
The Effect of Light on the Biological Clock
• Phytochrome conversion marks sunrise and
sunset
– Providing the biological clock with
environmental cues
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Photoperiodism and Responses to Seasons
• Photoperiod, the relative lengths of night and
day
– Is the environmental stimulus plants use most
often to detect the time of year
• Photoperiodism
– Is a physiological response to photoperiod
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Photoperiodism and Control of Flowering
• Some developmental processes, including flowering in
many species
– Requires a certain photoperiod
• Night length controls flowering and other responses to
photoperiod
• Some flower after a single exposure to the proper
photoperiod
• Some require several successive days of the proper
photoperiod to bloom
• Can be interrupted by red light ( 660 nm)
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Critical Night Length
• In the 1940s, researchers discovered that flowering
and other responses to photoperiod
– Are actually controlled by night length, not day length
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
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.
Figure 39.22
(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.
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• Action spectra and photoreversibility experiments
– Show that phytochrome is the pigment that receives
red light, which can interrupt the nighttime portion of
the photoperiod
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
Figure 39.23
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.
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A Flowering Hormone?
• The flowering signal, not yet chemically identified
– Is called florigen, and it may be a hormone or a
change in relative concentrations of multiple hormones
• Leaves detect photoperiod while buds produce flowers
• Florigen is produced in the leaves and moves to the buds
• Appears to be the same hormone in both short-day and
long-day plants
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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)
Figure 39.24
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.
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Meristem Transition and Flowering
• Whatever combination of environmental cues
and internal signals is necessary for flowering
to occur
– The outcome is the transition of a bud’s
meristem from a vegetative to a flowering state
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• Plants respond to a wide variety of stimuli other
than light
• Because of their immobility
– Plants must adjust to a wide range of
environmental circumstances through
developmental and physiological mechanisms
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Gravity
• Response to gravity
– Is known as gravitropism
• Roots show positive gravitropism
http://www.xtec.cat/%7Emherna23/summer03/ams
pot/picture/gravitropism.gif
• Stems show negative gravitropism
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• Plants may detect gravity by the settling of statoliths
• Mechanism:
–
Specialized plastids containing starch (statoliths) collect at the low
points in certain root cap cells
–
Triggers Ca2+ redistribution, resulting in lateral transport of
auxin in the root
–
Ca2+ and auxin accumulate on
lower side of elongation zone
–
Roots curve downward—auxin
inhibits root cell elongation,
so cells on upper side elongate
faster than those on the lower side
Statoliths
(a)
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Figure 39.25a, b
(b)
20 m
Mechanical Stimuli
• The term thigmomorphogenesis
• Occurs in vines and other climbing plants
• Directional growth in response to touch
– Contact of tendrils stimulates a coiling
response caused by differential growth of cells
on opposite sides of the tendril
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Thigmomorphogenesis
• Developmental response to mechanical
disturbance
– Results from increased ethylene production in
response to chronic mechanical stimulation
– Stem lengthening decreases while stem
thickening increases
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• Rubbing the stems of young plants a couple of
times daily
– Results in plants that are shorter than controls
Figure 39.26
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Rapid Leaf Movements
•Rapid leaf movements in response to mechanical
stimulation
Mimosa
•Reversible movements caused by
changes in turgor pressure of
specialized cells in response to
stimuli
–
–
When touched, leaf collapses and
folds together
Results from rapid loss of turgor
within pulvini (specialized motor
organs in leaf joints)
•
Motor cells lose K+, which
causes water loss by
osmosis
–
Leaf recovers in ~10 min
–
Movement travels to adjacent
leaves along the stem
•
Reduce water loss or protect
against herbivores
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(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
Figure 39.27a–c
Vein
0.5 m
http://www.ams.org/mathmedia/images/apr05flytrap-open-shut.jpg
Sleep Movements
• Lowering of leaves to a vertical position in
evening and raising of leaves to a horizontal
position in morning
Figure 39.21
• Occurs in many legumes
Noon
Midnight
• Due to daily changes in turgor pressure of motor
cells of pulvini
– Cells on one side of the pulvinus are turgid while
those on the other side are flaccid
– Migration of K+ from one side of the pulvinus to the
other is the osmotic agent leading to reversible ptake
and loss of water by motor cells
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Environmental Stresses
• Environmental stresses
– Have a potentially adverse effect on a plant’s
survival, growth, and reproduction
– Can have a devastating impact on crop yields
in agriculture
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Drought-Water Stress
• During drought
– Plants respond to water deficit by reducing
transpiration
– Guard cells lose turgor and the stomata close when a
leaf faces a water deficit
– Mesophyll cells in the leaf synthesize and release
abscisic acid which helps keep stomata closed
– Growth of young leaves is inhibited by a water deficit
since cell expansion is a turgor- dependent process
(reduces leaf surface area)
– Roots reduce growth
• Drying soil inhibits growth of shallow roots
• Deeper roots surrounded by moist soil continue to
grow
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Flooding-Oxygen deprivation
• Enzymatic destruction of cells
–
Waterlogged soil lacks air spaces that provide oxygen for cellular
respiration in roots
• Some plants form air tubes that extend from submerged roots
to the surface, thus oxygen can reach the roots
Vascular
cylinder
Air tubes
Epidermis
Figure 39.28a, b
(a) Control root (aerated)
100 m
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(b) Experimental root (nonaerated)
100 m
Salt Stress
• Plants respond to salt stress by producing
solutes tolerated at high concentrations
– Lower the water potential of the soil solution
causing a water deficit – roots lose water
– Toxic effect on plant at high concentrations
• Selective permeability to such solutes limits
water intake
– Produce compatible solutes to keep water
potential of cells more negative than the soil
solution without admitting toxic quantities of
salt
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Heat Stress
• Transpiration reduces effects of heat stress
– Evaporative cooling keeps temperature of leaf 3-10C
lower than ambient temperature
– Continues as long as stomata stay open—will close if
needed to reduce water loss
• Produce heat-shock proteins when exposed to
excessive temperatures
– Serve as temporary supports which help other proteins
fold into their functional conformations
– Help enzymes and other proteins maintain their
conformation, thus preventing denaturation
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Cold Stress
• Altering lipid composition of membranes
– Fluidity of cell membranes decreases
• Lipids become locked into crystalline structures
causing a loss of fluidity
• Solute transport and membrane protein function
are adversely affected by loss of fluidity
– Plants alter the lipid composition of their
membranes
• Proportion of unsaturated fatty acids increase—
shape reduces crystal formation and maintains
fluidity at lower temps.
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• Plants defend themselves against herbivores
and pathogens
• Plants counter external threats
– With defense systems that deter herbivory and
prevent infection or combat pathogens
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Defenses Against Herbivores
• Herbivory, animals eating plants
– Is a stress that plants face in any ecosystem
• Plants counter excessive herbivory
– With physical defenses such as thorns
– With chemical defenses such as distasteful or
toxic compounds
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings