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TRANSCRIPT
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CHAPTER 20
COORDINATION
20.4 HORMONES IN
PLANTS
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PREVIOUS LECTURE
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OBJECTIVES
a) State the roles of hormones in plants
i) Auxinii) Gibberellin
iii) Cytokinin
iv) Abscisic acid
v) Ethylene
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A group of natural and artificial plant hormones
Site of production :
Shoot apical meristem
Young leaves Seeds
Functions:
Cell elongation
Apical dominance
Fruit growth
Root growth
Abscission
AUXIN
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Cell elongation
Promote cell elongation in shoots and roots
Involve acidification of cell walls
Increases their plasticity
Water enters the cell by osmosis
Vacuole expands and the cell enlarges due to increased
turgor pressure
Involved in phototropism and geotropism
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Phototropism
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Apical dominance
Auxin produced in apical meristem inhibit lateral buds from
developing into actively growing shoots
When the apical meristem is removed, the lateral buds growto form branches
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Fruit growth
Promotes
Stimulates the ovary to produce enzymes
Develop into fruit
Root growth
At low concentration
Promotes the formation of adventitous roots At high concentration
Inhibits root formation
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Abscission
The process by which plant parts are shed
Eg: leaves, unfertilised flowers and fruits
Auxin prevents abscission
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Produced in young organs
Apical leaves
Developing buds
Root tips Germinating seeds
Functions:
Stem elongation
Seed dormancy
Flowering
Apical dominance
GIBBERELLIN
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Stem elongation
Stimulates cells to
divide and elongate Different from auxin
mechanism didnt involve cell wall
acidification
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Seed dormancy
The release of gibberellin from the embryo will trigger
germination
Break dormancy in buds and
seeds especially after winter
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Flowering
Stimulate flowering particularly in long-day plants
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Apical dominance
The function is antagonistic with auxin
Inhibit apical dominance
Promotes growth of lateral buds
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Produced in the root apical meristem
Functions:
Cell division and differentiation
Apical dominance
Senescence
CYTOKININS
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Cell division and differentiation
Promotes cell division and differentiation of youngunspeciallized cells
Requires the presence of auxin
Apical dominance
Antagonistic with auxin Promotes growth of lateral buds
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Senescene
Delaying of aging process
Anti-aging hormones
Leaves that are plucked and treated with cytokinin canmaintain their green colour for several days
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Produced by mature
leaves, fruits and
roots
Function:
Seed dormancy
Promotes dormancy
in buds and
seeds
ABSCISIC ACID
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Functions :
Fruit ripening
Abscission
Fruit ripening
As a fruit ripening, it
produces ethylene
Accelerate ripening
process
ETHYLENE
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Abscission
Promotes defoliation of leaves
Influenced by two antagonistic plant hormones
Ethylene and auxin As a leaf ages:
Auxin decreases
Ethylene is produced
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OBJECTIVES
b) Explain the roles of phytochrome in the
regulation of flowering
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Phytochrome
Phytochrome is a blue-green pigment existing in two
interconvertible forms:
Pr because it absorbs red (R: 660 nm) light
inactive form
Pfr because it absorbs far red (FR: 730 nm) light
active form
Absorption of light by one form converts it rapidly and
reversibly to the other form
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Sunlight contains more red than far-red light Pr Pfr
During the night,
Pfr Pr
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At the end of the light period
Exists in the active Pfr form
At the end of the night period
Slowly transition back to inactivePr form
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PHOTOPERIODISM PHENOMENON
Is a phenomenon in which plants respond to the
relative lengths of daylight and darkness.
Plants are classified into 3 main groups based on their
response to photoperiodism:
Short-day plants
Long-day plants
Day-neutral plants
Phytochrome and effects oflight on plant development
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Short-day plants
Plants that flower when the night length is equal to
or greater than some critical length.
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Examples:
Chrysanthemums
Poinsettias
Soybean
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Long-day plants
Plants that flower when the night length is equal
to or less than some critical length.
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Examples
Spinach
IrisMany cereals
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Day neutral plant
Plants that do not initiate flowering in response
to the day length but flower in response to some
other stimulus.
Unaffected by photoperoid
Flower when they reach a certain stage of
maturity
Does not require a specific day length to flower
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Examples:
Tomatoes
Cucumber
Rice
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Critical night length
The length of darkness that must be exceeded by
short-day plants, or not exceeded by long-day
plants, for flowering to be initiated.
R l f h h i
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Role of phytochrome in
photoperiodism and flowering
Sunlight contains more red light than far red light.
When a plant is exposed to sunlight, phytochrome Pr
would be converted to Pfr therefore increasing its
level of Pfr .
At night, Pfr would be slowly converted to Pr ,
thus reducing its level of Pfr
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For a plant to have a biological response to light, itmust contain a photoreceptor
Main photoreceptor for photoperiodism: phytochrome
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Photoperiodism in LDPs
Phytochrome Pfr stimulates flowering in LDPs
During the short nights
Only some Pfr is converted to Pr
Sufficient Pfr remain
Pfr initiates flowering
High level of Pfr stimulates the conversion of inactive
hormone precursor into a flowering hormone called
florigen
Induces flowering
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LDPs do not flower in relatively long nights
No sufficient Pfr formed
When the long night is interrupted by the exposure of
red light
Initiates flowering because
Pr converts to Pfr
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Photoperiodism in SDPs
Phytochrome Pfr inhibits flowering in SDPs
Need longer night to flower
Allows all Pfr to convert into Pr
High Prand low Pfr stimulate conversion of inactivehormone precursor to florigen
Induces flowering
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When the long night is interrupted by exposure to
red light
Prevents flowering
Pr converts to Pfr
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PR
PFR
Natural
light or red
light(660nm)
Darkness
or far-red
light
(730nm)
synthesis
Enzymically active form of phytochrome
Inhibition in short-
day plants
Promotion in
long- day plants
Precursor
of
flowering
hormone
Florigen-postulated
flowering hormoneFlorigen
formation by
enzymeconversionTranslocation
to meristems
Stem
Phytochrome and flowering, A tentative
hypotesis
Leaf
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NEXTLECTURE
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QUESTIONS
1) Give the significance of the following terms or
phrases:
a) long-day plants
b) short-day plants
2) Explain why some plants that grow near the
highway will never produce any flowering
response?
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ANSWER
1a) LDPs: Plants that usually flower when the
days become longer and the nights become
shorter
1b) SDPs: Plants that usually flower when the
light periods are shorter than the critical
night length
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ANSWER
2)
These types of plants are short-day plants
The plants require a period of darkness equal to or
longer than a critical length
Spotlight flashes from vehicles interrupt or shorten the
dark period at night and causes the conversion ofPr
into Pfr
An increased level of Pfr prevents flowering in short-day
plants