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1Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Ch. 27 - Plant Responses
2Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Ch. 27 - Plant Responses
TropismsPlant growth toward or away from a unidirectional stimulus is called a tropism Positive is towards stimulusPositive is towards stimulusNegative is away from stimulusNegative is away from stimulusDue to differential growth - one side of organ Due to differential growth - one side of organ elongates faster than the otherelongates 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
4Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
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 Plant hormone called auxinauxin migrates from lighted migrates from lighted side of stem to shady side of stemside of stem to shady side of stemCells on the shady side elongate faster than those Cells on the shady side elongate faster than those on the bright side, causing stem to curve toward the on the bright side, causing stem to curve toward the lightlight
5Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
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 gravityRoot 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.
7Gravitropism
Negative gravitropism of stems
Positive gravitropism of roots
Sedimentation of statoliths
8Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Gravitropism
Auxin is responsible for:Positive gravitropism of roots, andNegative 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
9Gravitropism
Negative gravitropism of stems
Positive gravitropism of roots
Sedimentation of statoliths
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Thigmotropism
Unusual growth due to contact with solid objects is called thigmotropismEx: 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 minutesSometimes it seems to need light which might be a need for ATP for the response.
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Nastic Movements
Nastic movements:Do not involve growth andAre not dependent on the stimulus direction
Seismonastic movements result from:Touch, shaking, orThermal 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
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Nastic Movements
Sleep movements:Occur daily in response to light and dark changesEx: Prayer PlantMovement due to changes in turgor pressure of motor cells in a pulvinus located at the base of each leaf.
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Circadian Rhythms
Biological rhythms with a 24-hour cycleTend to be persistentRhythm is maintained in the absence of Rhythm is maintained in the absence of environmental stimuli environmental stimuli Caused by a Caused by a biological clockbiological clockWithout environmental stimuli, circadian rhythms Without environmental stimuli, circadian rhythms continue but the cycle extends to 25 or 26 hourscontinue but the cycle extends to 25 or 26 hours Believed that the clocks are synchronized by Believed that the clocks are synchronized by external stimuli such as length of daylight external stimuli such as length of daylight compared to length of darkness. This is called the compared to length of darkness. This is called the photoperiod.photoperiod.
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Plant Hormones
Almost all communication on a plant is done by hormonesChemical 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
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
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
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Effects of Auxin
Apical DominanceApically 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 Pruning the top of a plant generally achieves a fuller look by removing the apical dominancefuller look by removing the apical dominanceWeak solution of auxin applied to woody cutting Weak solution of auxin applied to woody cutting causes rapid growth of adventitious rootscauses rapid growth of adventitious roots Promotes fruit growthPromotes fruit growth
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Effects of Auxin
Auxin production by seeds also promotes the growth of fruit. As long as auxin is concentrated in leaves or fruits As long as auxin is concentrated in leaves or fruits rather than in the stem, leaves and fruits do not fall rather than in the stem, leaves and fruits do not fall off.off. Trees can be sprayed with auxin to keep mature Trees can be sprayed with auxin to keep mature fruit from falling to groundfruit from falling to ground Auxin is sprayed on tomatoes to induce Auxin is sprayed on tomatoes to induce development of fruit without pollination creating development of fruit without pollination creating seedless tomatoes seedless tomatoes
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants Phototropism Experiments
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 capb. Tip was removed
4. Concluded tip senses light
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Control of GroControl of Growth and Respowth and Responses in Plantsnses 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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Control of GroControl of Growth and Respowth and Responses in Plantsnses in PlantsA. Paal (1918)
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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Control of GroControl of Growth and Respowth and Responses in Plantsnses in PlantsFritz Went (1926)
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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Control of GroControl of Growth and Respowth and Responses in Plantsnses in PlantsHow Auxins Work
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 loosenCellulose fibrils are weakened
2. Activates Golgi apparatusSends out vesicles laden with cell wall materials
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in PlantsHow Auxins Work
3. Stimulates DNA-binding protein Activates a particular geneLeads 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.
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Gibberellins
Growth promoting hormonesBring about internode elongation of stem cellsGibberellic acid (GA3) = most common Stem elongationStem elongationCan cause dwarf plants to grow hugeCan cause dwarf plants to grow hugeSources of gibberellin:Sources of gibberellin: Young leaves, roots, embryos, seeds & fruitsYoung leaves, roots, embryos, seeds & fruitsCommercial uses:Commercial uses: Break dormancy of buds & seeds, induce flowering, Break dormancy of buds & seeds, induce flowering, increase size of flowers, produce larger seedless increase size of flowers, produce larger seedless grapesgrapes
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Gibberellins
How GA3 acts as a chemical messenger:Embryo produces gibberellinsAmylase, enzyme that breaks down starch, appears in cells just inside seed coat
GA3 is the first messenger Attaches to receptor in plasma membraneAttaches to receptor in plasma membraneSecond messenger, calcium ions, combines with a Second messenger, calcium ions, combines with a DNA-binding proteinDNA-binding protein Believed to activate the gene that codes for Believed to activate the gene that codes for amylase. This acts on starch to release sugars amylase. This acts on starch to release sugars used as source of energy for growing embryoused as source of energy for growing embryo
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Cytokinins
CytokininsA class of plant hormones that promote cell division (cytokinesis)
First isolated in 1967 from corn = zeatinProduced in dividing tissues or roots & in seeds & fruits Promotes cell divisionPromotes cell division Prevents Prevents senescence senescence (Aging process. Leaves die (Aging process. Leaves die and fall off)and fall off) Initiates leaf growth. Lateral buds will grow when Initiates leaf growth. Lateral buds will grow when cytokinin is applied to them. cytokinin is applied to them.
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Abscisic AcidAbscisic 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 fallABA moves from leaves to vegetative buds in fall Buds are converted to winter buds which get covered by Buds are converted to winter buds which get covered by
thick, hardened scalesthick, hardened scales In spring, reduction in level of ABA & increases in In spring, reduction in level of ABA & increases in
gibberellins break seed and bud dormancy.gibberellins break seed and bud dormancy. Produced by: Any “green tissue” with chloroplastsAny “green tissue” with chloroplasts Monocot endosperm, andMonocot endosperm, and RootsRoots
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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 GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Ethylene
Ethylene is involved in abscission, the dropping of leaves, fruits & flowers from a plantOnce abscission has begun: Ethylene stimulates certain enzymes like cellulaseEthylene stimulates certain enzymes like cellulaseCauses leaves, fruits, or flowers to dropCauses 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 fruitsTo hasten ripening of green fruits To create pleasing colors before salesTo create pleasing colors before sales
Ethylene is a gas that can induce ripening of nearby fruits
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
Photoperiodism
Photoperiodism is any physiological response prompted by changes in day or night length
PhotoperiodPhotoperiod1. The relative lengths of day and night2. This changes with the seasons3. Flowering, germination & dormancy all occur at specific times of year4. Thus, photoperiod is the major environmental factor that needs to be measured by plants
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
PhotoperiodismThree 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
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Control of GroControl of Growth and Respowth and Responses in Plantsnses in Plants
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
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Control of GroControl of Growth and Respowth and Responses in Plantsnses 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 GroControl of Growth and Respowth and Responses in Plantsnses 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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Control of GroControl of Growth and Respowth and Responses in Plantsnses 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.