Growth Harmones & Growth Regulators

Growth Harmones: 

Gowth Harmones are the such organic substances which are produced generally in meristematic tissues of the plant and translocated towards the site of action inducing a Physiological process or response and can work in extremely minute quantities.

Thimann (1948 ) suggested the term Phytohormone for harmones of plants.

Plant Growth Regulators (PGR) :

Such organic compounds occuring naturally in plants as well as synthetic other than nutrients which in small amounts promote , inhibit or modify any physiological processs are called PGR. 

The PGR are of two types :

1) Growth Promotor: 

e.g. auxins , gibberellins & Cytokinin

2) Growth inhibitors : 

e.g. abscisic acid and ethylene 

AUXINS:

• F.W. Went (1928) isolated the growth substance which he named Auxin .

• The plant Avena sativa (i.e. Oat) was used by went for the bioassay hence the test is known as Avena Curvature test or Avena Coleoptile test 

• Indole acetic acid (IAA) is a endogenous auxin occuring naturally in plants .

• Synthetic auxins:

a) Indole -3- Butyric acid ( IBA)

b) Indole -3 - Propionic acid (IPA) 

c) Naphthalene acetic acid (NAA)

d) Dichlorophenoxy acetic acid (2,4- D)

e) Malic Hydrazide

•Precursor of IAA is Tryptophane 

Apical dominance of Auxin :

The growth of the apical bud suppresses the growth of lower axillary buds in many plants .Such dominancy is called apical dominance 

Thimann and skoog (1934) found that dominance of the terminal bud was due to auxin .

• Physiological Effects (Practical application)

✓ Cell division:

Auxin is responsible for promoting cell division in certain tissues like cambium .

Cell division of tissue culture is entirely depsndnt on auxin.

✓ Cell elongation:

Cell elongation is activated by auxin in three ways.

a) by increasing osmotic solutes 

b) by decreasing wall pressure

c) by increasing permeability of cytoplasm to water.

Auxin has inhibitory effect on root elongation due to auxin induced production of ethylene .

3) Inhibition of lateral buds 

4) Shortening of internodes

5) Root intiation 

6) Prevention of abscission layer 

The premature drop of fruits may be stopped by spraying 2,4-D , IAA , NAA

7) Flower initiation :

Auxin generally inhibits flowering and thus is helpful in delaying the flowering in lettuce

8) Production of Parthenocarpic fruits: 

Seedless fruits are being developed by horticulturists by spraying synthetic auxins

9) Weed Control:

2,4-D is used for weed control

•Gibberellins

1) The name gibberellin was used by Yabuta and sumiki (1938) for a pure crystalline chemical was isolated from 'Bakanae or Foolish seedling ' diseased rice plants .

Kurosawa of Japan in 1926 confirmed that the disease was caused by a fungus Gibbrella fujikoroi 

2) 6 gibberellins viz. GA1 , GA2  GA3 , GA4, GA7 & GA9 were isolated from the fungus Gibbrella by cross at (1961)

3 gibberellins viz . GA5 , GA6 & GA8 were isolated from bean seeds by Mac Millan et al (1961) 

Chemically gibberellins are known as gibbrellic acid 

3) Most commonly available gibberellic acid GA3.

4) Gibberellins are synthesized through the normal isoprenoid pathway of terpene biosynthesis.

6) Gibberellin promotes shoot growth by accelerating the cell elongation and cell division in the sub apical meristem region which increases the length of internodes 

7) Gibberellin induces the synthesis of hydrolytic enzymes especially protease and Alpha amylase which triggers seed germination

8) Physiological Effects:

1) Stem elongation:

It increases the length of internodes.it speeds up the RNA synthesis

2) It converts the dwarf plant into an plant of normal height .

3) Substituting cold treatment:

Many biennials complete their life cycle within a single year by treatment with GA.

4) Parthenocarpic fruits: GA induces Parthenocarpic development of fruits in tomato , apple & pear more effectively than auxin 

5) Breaking dormamcy : 

It is effective in breaking of dormancy in potato tubers 

6) It promotes flowering in long day plants and induces maleness 

GA introduces male flowers whereas 

Ethrel / Ethephon increases femaleness

7) It increases the size of leaves and fruits

8) It prevents senescence

9) It increases the cell division and cell size.

•Cytokinin / Kinetin / Kinin

1) Cell divison inducing substance is known as Kinetin and Letham (1963)used the term Cytokinin

2) Cytokinin is a derivative of the purine base adenine

3) The chemical name of Kinetin is 

N6 Furfuryl adenine or 6 furfurylamino Purine 

4) The endosperm of coconut (Coconut milk ) also contains endogenous ( Naturally occurring) Cytokinin 

Zeatin is endogenous Cytokinin of maize

5) Physiological Effects:

1)It promotes cell division and the related DNA and RNA synthesis

2) It has morphogenesis effect, that why it is used for organ formation in a variety of tissue cultures

3) It is used in the breaking of dormancy.It also promotes the seed germination

4) It delays the phas of senescence . 

Senescence means the disappearance of Chlorophyll and the degradation of protein

Richmond and Lang (1957) 

Reported that senescence was delayed in the detached xanthium leaves for several days when they were treated with Kinetin .

Such effect of Kinetin in retarding senescence is called Richmond Lang Effect

Abscisic Acid (ABA):

1) ABA is a common growth inhibitor

2) Robinson and P.F .Weiring (1963-64)

Extracted the inhibitory substance and called it dormin because it caused dormancy

3) Okhuma et al (1963, 65) 

isolated the very active inhibitor from young cotton fruits and called it abscisin 2. Abscisin was isolated from the burns of mature cotton fruits . Later on in 1967 it was realised that the dormin and abscisin 2 were the same and was named Abscisic acid ( ABA)

4) Physiological Effects:

1) It acclerates the senescence phase of growth

2) It regulates the buds and seeds dormancy by inhibiting the growth processes

3) It inhibits gibberlin -stimulated growth hence called antigibberellin

4) It causes abscission of leaves 

5) It inhibits RNA and protein synthesis

Ethylene:

1) Ethylene (CH2= CH2) is a volatile gas which is included under Hormones in 1971

2) It is synthesized in plant from the amino acid Methionine

3) The most important effect is fruit ripening ( Climacteric rise of respiration)

4) Ethylene increase the cell permeability due of which the fruit becomes soft

5) High concentration of C02 i.e. 5-10 % inhibits the effect of ethylene.

Ag+ is also inhibitor of ethylene action 

6) Ethrel / Ethephon : The chemical which releases ethylene

Physiological effects:

1) It induces Climacteric rise and fruit ripening

2) inhibition of stem and root growth

3) Induction of flower petal discoloration.

✍️Agricos 

Vaishnavi Nimkar 🌿🌱