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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)  

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