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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