Transcript Photosynthesis

Control of Growth
and Responses in
Plants
Ch. 27 - Plant Responses
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Control of Growth
and Responses in
Plants
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Ch. 27 - Plant Responses
Tropisms
Plant growth toward or away from a unidirectional
stimulus is called a tropism
- Positive is towards stimulus
- Negative is away from stimulus
- Due to differential growth - one side of organ
elongates faster than the other
Three types of tropisms:
- Phototropism - movement in response to light
- Gravitropism - movement in response to gravity
- Thigmotropism - in response to touch
Phototropism
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Control of Growth
and Responses in
Plants
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Phototropism
Positive phototropism:
Studied by Charles & Francis Darwin
 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
- Plant hormone called auxin migrates from lighted
side of stem to shady side of stem
- Cells on the shady side elongate faster than those
on the bright side, causing stem to curve toward the
light
Control of Growth
and Responses in
Plants
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Gravitropism
When a plant is placed on its side, the stem
grows upward, opposite of the pull of gravity.
This is an example of negative response called
gravitropism
Roots, in contrast, show positive gravitropism, as
they grow downwards.
Roots without root caps don’t respond to gravity
Root cap cells contain sensors called statoliths,
which are starch grains located within
amyloplasts, a type of plastid.
- Amyloplasts settle to lower part of cell & cause
bending of root.
Gravitropism
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Gravitropism
Negative gravitropism of stems
Positive gravitropism of roots
Sedimentation of statoliths
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Control of Growth
and Responses in
Plants
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Gravitropism
Auxin is responsible for:
Positive gravitropism of roots, and
Negative gravitropism of shoots
How does auxin do this:
Amyloplasts come in contact with ER which
releases stored calcium ions.
This leads to activation of auxin pumps & auxin
enters the cells
Roots & stems respond differently to auxin:
- Auxin inhibits growth of root cells, so cells on upper
surface elongate so root curves downward
- Auxin stimulates growth of stem cells, so cells on
lower surface elongate so stem curves upward
Gravitropism
Negative gravitropism of stems
Positive gravitropism of roots
Sedimentation of statoliths
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Control of Growth
and Responses in
Plants
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Thigmotropism
Unusual growth due to contact with solid objects
is called thigmotropism
Ex: Coiling of tendrils
The plant grows straight until it touches
something.
Cells in contact with object grow less while those
on the opposite side elongate.
Response can be quite rapid; within 10 minutes
Sometimes it seems to need light which might be
a need for ATP for the response.
Coiling Response
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Control of Growth
and Responses in
Plants
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Nastic Movements
Nastic movements:
Do not involve growth and
Are not dependent on the stimulus direction
Seismonastic movements result from:
Touch, shaking, or
Thermal stimulation
Due to loss of turgor pressure within a few cells
located in a thickening, called a pulvinus, at the
base of each leaflet. Touch causes K+ to flow
out of cells & then water follows.
•Ex: Mimosa leaves & Venus flytrap
Seismonastic Movement
Mimosa pudica
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Control of Growth
and Responses in
Plants
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Nastic Movements
Sleep movements:
Occur daily in response to light and dark changes
Ex: Prayer Plant
Movement due to changes in turgor pressure of
motor cells in a pulvinus located at the base of
each leaf.
Sleep Movement
Prayer plant
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Control of Growth
and Responses in
Plants
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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
- Without environmental stimuli, circadian rhythms
continue but the cycle extends to 25 or 26 hours
- Believed that the clocks are synchronized by
external stimuli such as length of daylight
compared to length of darkness. This is called the
photoperiod.
Control of Growth
and Responses in
Plants
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Plant Hormones
Almost all communication on a plant is done by
hormones
Chemical signals produced in very low
concentrations in one part of plants and then
active in another part of the plant
Hormones travel within phloem, or from cell to
cell, in response to the appropriate stimulus
Each hormone has a specific chemical structure
Control of Growth
and Responses in
Plants
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Auxins
The most common naturally occurring auxin is
indoleacetic acid (IAA).
It is produced in shoot apical meristem and is
found in young leaves and in flowers and fruits
Auxins affect many aspects of plant growth &
development
Control of Growth
and Responses in
Plants
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Effects of Auxin
Apical Dominance
Apically produced auxin prevents the growth of
axillary buds (side buds)
When a terminal bud is removed, the nearest
lateral buds begin to grow, and the plant branches
- Pruning the top of a plant generally achieves a
fuller look by removing the apical dominance
- Weak solution of auxin applied to woody cutting
causes rapid growth of adventitious roots
- Promotes fruit growth
Apical Dominance
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Control of Growth
and Responses in
Plants
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Effects of Auxin
Auxin production by seeds also promotes the
growth of fruit.
- As long as auxin is concentrated in leaves or fruits
rather than in the stem, leaves and fruits do not fall
off.
- Trees can be sprayed with auxin to keep mature
fruit from falling to ground
- Auxin is sprayed on tomatoes to induce
development of fruit without pollination creating
seedless tomatoes
Phototropism Experiments
Control of Growth
and Responses in
Plants
Darwin & Darwin (1880s)
1. Used coleoptiles (grass shoots)
2. Found that shoots bend if:
a. Tips of shoots are present &
 Normal
 Covered with clear cap
 Opaque base
3. No bending if:
a. Tip covered with cap
b. Tip was removed
4. Concluded tip senses light
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Phototropism Experiments
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Control of Growth
and Responses in
Plants
Peter Boysen-Jensen (1913)
1. Removed tips of shoots
2. Placed gelatin on stump
3. Replaced tip on top of gelatin:
a. Shoots bent towards light
4. Put piece of impermeable mica
between shoot and tip:
a. No phototropic response
5. Concluded that some mobile
chemical is responsible for the
phototropic response
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Phototropism Experiments
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A. Paal (1918)
Control of Growth
and Responses in
Plants
1. Removed tips of shoots
2. Put shoots in the dark
3. Replaced tips back on stumps
but put them off-center on stumps
4. Tip placed on right side:
a. Shoots bent towards left
5. Tip placed on left side:
a. Shoots bent towards right
6. Suggested tip produces chemical
that moves down shoot & causes
cells below it to grow
a. Light must alter its amount
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Phototropism Experiments
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Fritz Went (1926)
Control of Growth
and Responses in
Plants
1. Removed tips & placed them on
blocks of agar for an hour
2. Put blocks of agar only on cut
ends of stumps
3. If placed in center of stump:
a. Shoots grew straight upward
4. If placed off-center of stump
a. Shoots grew & bent to opposite
side
5. Blank agar blocks didn’t grow
a. Definitive evidence of a hormone
He named the hormone auxin.
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Demonstrating Phototropism Went’s Experiment
Phototropism
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How Auxins Work
Control of Growth
and Responses in
Plants
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When a stem is exposed to unidirectional light, auxin
moves to the shady side.
Auxins bind to plasma membrane receptors which leads
to a series of reactions & the generation of at least
three specific second messengers:
1. Activates a proton, H+, pump
Acidic conditions cause cell wall to loosen
Cellulose fibrils are weakened
2. Activates Golgi apparatus
Sends out vesicles laden with cell wall materials
How Auxins Work
Control of Growth
and Responses in
Plants
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3. Stimulates DNA-binding protein
Activates a particular gene
Leads to production of growth factors
Cell walls become extensible & then fill with water
by osmosis.
Turgor pressure increases due to the entry of
water & the cell elongates.
This occurs on side opposite to the light so the
stem lengthens on shady side causing a
bending toward the light.
Auxin Mode of Action
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mm
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and Responses in
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Gibberellins
Growth promoting hormones
Bring about internode elongation of stem cells
Gibberellic acid (GA3) = most common
- Stem elongation
- Can cause dwarf plants to grow huge
Sources of gibberellin:
- Young leaves, roots, embryos, seeds & fruits
Commercial uses:
- Break dormancy of buds & seeds, induce flowering,
increase size of flowers, produce larger seedless
grapes
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Effect of Gibberellins
Treated
No treatment
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and Responses in
Plants
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Gibberellins
How GA3 acts as a chemical messenger:
Embryo produces gibberellins
Amylase, enzyme that breaks down starch,
appears in cells just inside seed coat
GA3 is the first messenger
- Attaches to receptor in plasma membrane
Second messenger, calcium ions, combines with a
DNA-binding protein
- Believed to activate the gene that codes for
amylase. This acts on starch to release sugars
used as source of energy for growing embryo
Gibberellic Acid:
Structure and Mode of Action
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Control of Growth
and Responses in
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Cytokinins
Cytokinins
A class of plant hormones that promote cell
division (cytokinesis)
First isolated in 1967 from corn = zeatin
Produced in dividing tissues or roots & in seeds &
fruits
- Promotes cell division
- Prevents senescence (Aging process. Leaves die
and fall off)
- Initiates leaf growth. Lateral buds will grow when
cytokinin is applied to them.
Control of Growth
and Responses in
Plants
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Abscisic Acid
Abscisic acid (ABA): (aka stress hormone)
Initiates and maintains seed and bud dormancy
Brings about closure of stomata
Dormancy occurs when a plant readies itself for adverse
conditions by stopping growth
- ABA moves from leaves to vegetative buds in fall
- Buds are converted to winter buds which get covered by
thick, hardened scales
- In spring, reduction in level of ABA & increases in
gibberellins break seed and bud dormancy.
Produced by:
- Any “green tissue” with chloroplasts
- Monocot endosperm, and
- Roots
Abscisic Acid:
Control of Stoma Opening
ABA binding leads to influx of Ca2+ & the opening of K+
channels. Water exits guard cells & stoma closes.
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Control of Growth
and Responses in
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Ethylene
Ethylene is involved in abscission, the dropping of
leaves, fruits & flowers from a plant
Once abscission has begun:
- Ethylene stimulates certain enzymes like cellulase
- Causes leaves, fruits, or flowers to drop
Also ripens fruit by increasing activity of enzymes
that soften fruit
Uses in agriculture:
- To hasten ripening of green fruits
- To create pleasing colors before sales
Ethylene is a gas that can induce ripening of
nearby fruits
Functions of Ethylene
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Control of Growth
and Responses in
Plants
Photoperiodism
Photoperiodism is any physiological response
prompted by changes in day or night length
Photoperiod
1. The relative lengths of day and night
2. This changes with the seasons
3. Flowering, germination & dormancy all
occur at specific times of year
4. Thus, photoperiod is the major
environmental factor that needs to be
measured by plants
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and Responses in
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Photoperiodism
Three Types of Plants:
1. Short-Day Plants (Long-night)
a. Flower when days are short (fall, winter)
b. Actually controlled by night length: Night
length must be longer than a critical
value.
c. Continuity of darkness is what
matters. A flash of light will disrupt
flowering
d. Examples: Chrysanthemums,
poinsettias, rice, ragweed
Photoperiodism and Flowering
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Control of Growth
and Responses in
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Photoperiodism
2. Long-Day Plants (Short-night)
a. Flower when days are long (late spring,
summer)
b. Night must be shorter than a critical
value
c. A flash of light during the night can
induce flowering during the wrong
season
d. Examples: spinach, wheat,
lettuce, iris, petunia, mustard
Photoperiodism and Flowering
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Control of Growth
and Responses in
Plants
Photoperiodism
3. Day-Neutral Plants
a. Day length doesn’t matter
b. Flower year round
c. Examples: Roses, carnations,
dandelions, sunflowers
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Control of Growth
and Responses in
Plants
Photoperiodism
How is photoperiod detected?
1. Involves phytochrome, a light-absorbing
pigment that exists in 2 inter-changeable
forms:
a. Pr strongly absorbs red light (660-680
nm)
b. Pfr absorbs far-red light (700–730 nm)
2. When Pr absorbs red light it is converted
quickly to Pfr
3. When Pfr absorbs far-red light it is
converted slowly to Pr
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Phytochrome Conversion Cycle
happens quickly
(happens slowly)
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Control of Growth
and Responses in
Plants
Photoperiodism
4. Pfr slowly reverts back to Pr in the dark
5. At sunset, far-red light is common
a. So Pfr begins to convert to Pr
b. This marks end of day; start of night.
c. Pr accumulates slowly all night
6. At sunrise, red-light is common
a. So Pr converts to Pfr relatively quickly
b. This marks end of night; start of day.
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Phytochrome Conversion Cycle
happens quickly
(happens slowly)
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