Plant Growth Regulator 

Plant Growth Regulator (PGR)

Plant Growth Regulator

 Growth & Development 

Growth:- Growth may be defined as a dynamic vital process which brings about a permanent change in any plant or its parts with respect to size, form, weight and volume.

• Growth means quantitative increase in plant body e.g. Cell division Cell enlargement
• Growth is a continuous process
• Shape of growth curve is- ‘S’-Shaped
• The rate of growth is measured in different ways by Auxanometer & Crescograph (J.C. Bose)

Development:- Irreversible change in state. It means the qualitative change in plant body. e.g.

Seed->Seedling->Vegetative->Flowering->maturation

Development is phase to phase process.

 Hormones 

Growth Hormones:- Growth hormones are the organic substances which are produced generally in meristematic tissues of the plant and trans-located towards the site of action inducing a physiological process or response and can work in extremely minute quantities.

Thimann (1948) suggested the term phytohormones for hormones of plant

Plant Growth Regulators (PGR):- PGR are such organic compounds occurring naturally in plants as well as synthetic other than nutrients which in small amounts promotes, inhibit or modify any physiological process are called PGR.

Types of PGR
1.Growth promoters	2.Growth inhibitors
Ex. Auxin, Gibberellins & Cytokinin	Ex. Abscisic acid and Ethylene

NOTE:- All plant hormone are plant growth regulators but, All plant growth regulator are not plant hormones

Synergistic effect:- When the effect is more than the sum of their individual effects it is called synergistic

Antagonistic effect:- When the action of two growth regulators is opposite it is called antagonistic

Growth substances and their functions

Harmones	Functions
Auxin	Named by F.W. Went. It has apical bud dominance, inhibit root elongation and promote cell division in tissue culture
IAA	Prevent premature fruit drop
IBA	Root formation
NAA	Fruit thinner, prevent fruit drop
2,4-D	Fruit setting hormone (< 20 ppm), and also used as herbicide (>20 ppm)
Cytokinin	Dormancy breaker, stimulate cell division
Gibberellins	Cell elongation, increase fruit size
Abscisic acid (ABA)	Induce Dormancy, stomata closer, also called as anti-gibberellin
Chlormequat (CCC)	Logging preventor, also known as cycocel
Ethylene	Ripening of fruits, Isodiametric growth of stem & roots
Maleic hydrazide	Growth retardant, prevent onion sprouting in storage, sold under trade name Sprout stop
Ethephone	T Name-Ethrel, Used as Banana fruit ripener
Glyphosate	Used as sugarcane ripner. trade name round-up

Plant Growth Regulator

1.Auxins

AUXIN:- Derived from the Greek word “auxein” means- “to grow/increase”

F.W.Went (1928) isolated the growth substance which he named Auxin. Naturally occurring auxin is-IAA (Indole acetic acid) trade name rhizophon-A

Synthetic Auxins –

a. Indole 3 butyric acid (IBA) trade name serodix

b. Indole 3 propionic acid (IPA)
c. Naphthalene acctic acid (NAA)
d. 2,4 Dichloruphenoxy acetic acid (2,4-D)
e. Malic Hydrazide (MH)

Anti-auxins:- Naphthythalamic acid (NTA), Ethylene chlorohydrins

Active sites of auxins:- Shoot tip region, coleoptiles and developing embryos etc.

• The auxin synthesis occurs rapidly in green leaves in presence of light than in the dark
• Tryptophan is the precursor of lAA and zinc is required for its synthesis
• Translocation of auxins is polar
• The site of auxin transport is located on the plasma lemma
• MH and Acid paracoumaric have the property of anti-auxins.
• 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. It was found that Auxin was responsible for curvature in Avena Coleoptilc.

Role of auxins:-

• Promotes apical bud dominance
• Auxin has inhibitory effect on root elongation due to the auxin- induce ethylene production.
• Promote the elongation of cells
• Increases cell division in cambium
• IBA promotes rooting of cutting.
• Induces uniform flowering in pineapple.
• Auxin increase in root and decrease in shoot
• Prevention of Premature Dropping of Fruits in case of Cotton and Apple- NAA & in case of Citrus fruits-2,4-D/ 2,4,5-T is used
• Foliar spray on cotton of 40 ppm NAA (20g/ 500 lit. of water / ha) at 90, 105 and 120 DAS reduces flowers, buds and squares dropping and increases yield.
• 2, 4-D is used for defoliation of Cotton plant before boll harvesting
• NAA is used for fruit thinning in Apple
• Prevention of Lodging in Cereals 30-40% Yield loss in traditional tall varieties. Alpha naphthalene acetamide is used to prevent lodging in cereals.
• Tissue Culture Auxin along with cytokinin shows successful callus formation, root-shoot differentiation etc.
• Foliar spray on potato of NAA 100 ppm at 25 DAT- to increase tuber yield

2.Gibberellins

GIBBERELLINS:- Second important growth hormone found in plants

The name ‘gibberellin’ was used by Yabuta and Sumiki (1938) for a pure crystalline chemical which was isolated from Bakanae or Foolish seedling’ diseased rice plants. Kurosawa of Japanin 1926 confirmed that the disease was caused by a fungus Gibberella fujikoroi’ (Fusarium heterosporum). Due
to this disease, rice plant grows abnormally thin and tall.

• Chemically gibberellins are known as gibberellic acid
• Precursor of Gibberellic Acid is N- Kaurene
• Most commonly available gibberellic acid is GA3

Anti-gibberellins:- phosphon D, Cycocel (CCC), Maleic hydrazide, paclobutrazol

Role of gibberellins:-

• Induction of flowering in long day plants
• Breaking of dormancy.
• Enhances seed germination
• GA has also been used to control flower sex expression in cucumbers and squash. GA application tends to promote maleness in these plants
• GA induces parthenocarpic development of fruits in tomato, apple & pear more effectively than auxin.
• The most important effect of GA is the stem elongation i.e. GA induces internode elongation or sub apical elongation
• Sugarcane- GA is used extensively to increase the sucrose yield of sugarcane.
• GA is used to increase the yield of barley malt and to decrease the time required for this process to occur.
• Foliar spray on Rice of GA, at 100 ppm during panicle initiation stage enhances the panicle exertion and increases seed weight and yield in hybrid rice.
• Foliar spray 25-50 ppm of GA, in grapes just before flowering and during fruiting enhances the fruit size, sugar content and yield.
• Spraying GA on Chrysanthemum, at 50-150 ppm increases the number of flowers and enhances the shelf life.
• Protection against plant pathogens. It reduces the number of egg fecundity of the fruit fly.

3. Cytokinin

CYTOKININS/KINETIN/KININ:- Term cytokinin proposed by Letham (1963)

Liquid endosperm of coconut called ‘Coconut Water/Milk’ contain cell division causing factors (Kinetine).

• Miller and Skoog identified kinetin
• The chemical name of kinetin is N6 furfuryl adenine or 6- furfurylamino purine.
• The first naturally occurring hormone identified -Zeatin
• Zeatin is the endogenous cytokinin of maize.
• Plays a key role in higher plants and moves through xylem
• At present it is clear that cytokinins are a part of t RNA (transfer RNA)
• Root tip is an important site of cytokinin synthesis.
• Mobility is polar and basipetal
• Precursor of cytokinin is either adenine or adenosine i.e. purine bases

Role of cytokinin:-

• Delay of senescence
• Induce flowering in short day plants
• Initiation of cell division
• Promote femaleness
• As Tissue cultures use cytokinins to induce shoot development
• Promotes stomatal opening

4. ABSCISIC ACID (ABA)

ABSCISIC ACID (ABA):-ABA first identified by P.F Weiring (1965)

Robinson and P.F.V eiring(1963-64) extracted the inhibitory substance and called ‘dormin’ because it caused dormancy.

• ABA is a naturally occurring common growth Inhibitor
• It acts as stress hormone.
• Violoxanthin serves as a precursor for biosynthesis of ABA
• Bioassays are-rice seedling growth inhibition test and inhibition of a amylase in barley endosperm
• Biosynthesis of ABA also takes place through mevalonic acid
• ABA is a terpenoids

Role of ABA:-

• Induces bud dormancy and enhances the process of abscission
• Stimulates the release of ethylene
• lt accelerates the senescence phase of growth
• It inhibits RNA and Protein synthesis.
• Brings the closure of stomata during water stress
• ABA is called Anti-Gibberellins.

5.Ethylene

ETHYLENE:- BURG (1962) established that ethylene is the only gaseous growth regulator

• It is known as ripening hormone
• Naturally occurring volatile hormone
• Production increased with increase in respiration rate
• Auxin increases ethylene level in plants and hence it has inhibitory effect on root elongation
• Maximum ethylene is formed in ripening fruits and senescing tissues
• Inhibitors of ethylene synthesis are amino-ethoxyvinyl glycine
• Precursor of ethylene is methionine, which is the sulphur centaining amino acid
• Bioassays for ethylene are triple pea test and pea stem swelling test

Role of ethylene:-

• Induces uniform flowering and ripening in pineapple
• Responsible for fruit ripening with increase in respiration
• Inhibits stem elongation and cause abscission of leaves.
• Ethrel/Ethephon-The chemical which releases ethylene.

Other Growth Regulator:-

6. Malic hydrazide (MH):- prevents premature sprouting of onion and potato trade name sprout spout

7. AMO 1618:- (A quaternary ammonium salt) is used in the cultivation of
ornamental plants

8. Paclobutrazol:- Reduces the problem of biennial bearing in mango.

9. 2,3,5-T/Tri-iodo benzoic acid (TIBA):- Increases flowering in chrysanthemum. trade name regim 8

10. Mapiquat Chloride, CCC, Chlormequat chloride (cycocel):- Used in
ornamental plants for shorter inter nodes and thicker stems. It also prevents lodging and increases tillering in cereals.

Plant Growth Regulator