Untitled Document

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PLANT HORMONES AND GROWTH SUBSTANCES

DEFINITIONS, TYPES AND PROPERTIES

Hormone - an endogenous or naturally-occurring compound that is produced or synthesized in one
part of the plant and causes a change in physiology, growth or development in another
part of the plant; usually present in very small quantities.

Growth Substance - all naturally-occurring or synthetically produced substances that affect the
physiology, growth and development of plants.

Naturally-Occurring
Example
Synthetic
Example
Structure
Site of Production

Auxin

indoleacetic acid (IAA)

indolebutyric acid (IBA)
naphthaleneacetic acid (NAA)
2,4-dichlorophenoxyacetic acid (2,4-D)

shoot tips
embryos

Cytokinin

zeatin
kinetin (not in plants)

benzyladenine (BA)
pyranylbenzyladenine (PBA)

root tips
embryos

Gibberellic Acid (GA)

over 90

none

shoot tips
root tips
embryos

Ethylene

ethylene

ethephon or ethrel
(both release ethylene inside plant)

ripening fruits
aging flowers
germinating seeds
wounded tissue

Abscisic Acid (ABA)

abscisic acid

none

plastids, especially
chloroplast

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EFFECTS AND PRACTICAL APPLICATIONS OF HORMONES/GROWTH SUBSTANCES

AUXIN
1) Tropism - response of plants to environmental or physical stimuli.
    a) phototropism - response to light
    b) geotropism - response to gravity
    c) thigmotropism - response to touch
2) Apical dominance - determined by apical bud, partly due to auxin produced
3) Fruit set - low concentrations stimulate
4) Fruit or flower thinning - high concentrations cause
5) Herbicides - 2,4-D at high concentrations
6) Adventitious root formation
    a) stem and leaf cuttings
    b) tissue culture

CYTOKININ
1) Leaf aging or abscission - may delay
2) Seed germination - may overcome dormancy or stimulate germination
3) Adventitious shoot formation
    a) leaf and root cuttings
    b) tissue culture

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EFFECTS AND PRACTICAL APPLICATIONS OF HORMONES/GROWTH SUBSTANCES - cont.

GIBBERELLIC ACID (GA)
1) Rosette or dwarf plants - lack of endogenous GA often causes

growth retardants - chemicals that block GA synthesis; are used in greenhouse and
bedding plant production to produce compact plants.

2) Flowering - may cause bolting in biennials
3) Fruit size - increases size of seedless grapes
4) Bud dormancy - may overcome and substitute for cold treatment
5) Seed germination - may increase or speed up
6) Sex expression - favors staminate flower formation on monoecious plants

ETHYLENE
1) Fruit ripening - stimulates in many fruits, ex. banana
2) Flowering - triggers flowering in some bromeliads, ex. pineapple.
3) Flower longevity - causes senescence (death) of cut flowers
4) Leaf abscission (leaf drop) - causes in some plants
5) Leaf epinasty (curling and contortion of leaves) - causes in some plants
6) Sex expression - favors pistillate flower formation on monoecious plants

ABSCISIC ACID (ABA)
1) Dormancy - causes bud or seed dormancy
2) Leaf abscission (leaf drop) - may cause in some plants
3) Stomata - causes stomata to close (a response to drought stress)