Types of plant growth regulators

1. Ideation of Plant growth regulators
 Plant growth regulators are biochemicals produced in plants (endogenous) or
synthetic substances applied to plants externally(exogenous) which cause
modification in plant growth & development.
 Plant groth regulators produced by plants called as phytohormones. This
substances help in increased production of roots, shoots flowers etc.
The PGRs can be broadly divided into two groups based on their function in a living
plant body.
Plant growth promoters: PGRs which are involved in growth promoting activities,
such as cell division, cell enlargement, pattern formation, flowering, fruiting and seed
formation are called plant growth promoters. e.g., auxins, gibberellins and cytokinins.
Growth retardants or Inhibitors: PGRs of the other group play an important role in
plant responses to wounds and stresses of biotic and abiotic origin and are called as
Growth retardants or Inhibitors. They are also involved in various growth inhibiting
activities such as dormancy and abscission. Eg: Abscisic acid & ethylene

2. There are five major types of Plat Growth Regulators (i) Auxins (ii) Cytokinins
(iii) Gibberellins (iv). Ethylene (v) Abscisic acid
1.Auxin:
These plant hormones are specially concerned with cell enlargement or the growth of the
shoots
Physiological function of Auxins:
Cell Elongation: 2,4-D
Root Initiation in stem cuttings: IAA, NAA, 2,4-D, naphthalene acetamide (NAd) etc. Auxin-
induced rooting helps propagation of certain plants by cuttings.
Apical Dominance:If the apical bud is intact on the plant, the growth of the lateral buds
remains suppressed. Upon removal of the apical bud, the lateral bud nearest the apical bud
establishes its dominance over the remaining buds, causing them to become in active again.
This inhibitory effect of a terminal bud upon the development of the lateral buds is called
apical dominance and produces a cone-shaped plant.
Delay (or inhibition) of abscission of leaves: The abscission of leaves can be delayed or
inhibited by the application of auxins on the surface of the lamina or on the cut surface of a
debladed petiole.

3. 2. Gibberellins:A Japanese scientist Kurosawa found that the rice seedlings
infected by the fungus Gibberella fujikuroi grow taller and turned very thin and
pale. An active substance was isolated from the infected seedlings and named as
Gibberellin. These are diterpenoids, with a gibbane ring structure. They have the
ability to elongate the stem of green seedlings especially certain dwarf and
rosette types.
Functions of Gibberellins:
• Stimulate stem growth through cell elongation and cell division.
• Involved in overcoming dormancy in seeds and buds.
• It causes internodes to stretch in relation to light intensity.
• Used commercially in stimulating seed germination &
seedling growth
• Stimulate development of flowers.
• Increasing fruit size of seedless (Parthenocarpy) in grape
• Delay Senesence: Increase photosynthesis and proteins synthesis and
thereby decrease abscission
• Break dormancy and promote leaf expansion

4. 3. Cytokinins:
Cytokinins are first isolated from Coconut milk. Miller
&Skoog and their co-workers isolated the growth factor
responsible for cellular division from a DNA preparation
calling it as cytokinins in (1950). These are synthesized in
root apex, endosperm of seeds, young fruits where cell
division takes place. eg: Zeatin.
Functions of Cytokinins:
 Cytokinins promote cell division (Cytokinesis)
 Contribute to cell elongation in leaves
 Stimulate cell differentiation in combination with
auxins.

5. 4. Ethylene:
Ethylene gas is the byproduct of gas combustion and that this gas could affect
plant growth. This same gas was naturally produced by plants and it caused faster
ripening of many fruits. Ethylene is a gaseous hormone and it produce
throughout the plant. Large amounts are released into the air by roots, the shoot
apical meristem, nodes, senescing flowers and ripening fruits.
Functions of Ethylene in Plants:
 Inhibits root and shoot initiation by blocking the transport of auxins from
apical meristems and controls stem elongation.
 Stimulates leaf curling (epinasty)
 Stimulates fruit ripening Because of this activity, ethylene is known as ripening
hormone.
 Promotes flowering in Mango, pineapple and some ornamentals.
 Hastens leaf abscission and falling down of leaves due to formation of
abscission layer.
 Determines sex expression in monoecious plants, plants that produce both
male and female flowers on the same plant. Application of ethylene causes
more female flowers in cucurbits.

6. Abscisic Acid (ABA):
The plants also do possess a few substances which inhibit growth. They retard such
processes as seed germination, root and stem elongation and bud opening. As a
matter of fact, the growth inhibitors act as chemical check upon plants preventing the
seeds from germinating and the buds from opening under unfavourable conditions.
Physiological Role in Plants:
i. Closure of Stomata: During drought, leaves synthesize large amounts of
ABA which causes stomata to close. Thus, ABA acts as a messenger and
enables plants to conserve water during drought
ii. Delays seed dormancy: Applying ABA delays seed germination. It controls
bud dormancy, counteracts the effects of other hormones. It inhibits cell
growth that is promoted by IAA. It inhibits amylase production that is induced
by Gibberellins thus delaying seed germination. It induces chlorosis and
senescence unlike cytokinins that promote rejuvenation of leaves and delay
senescence.

7. List of Plant Growth Regulators Marketed in India:
AUXINS Formul
ation
Dose Product Name Crops
2-(1-naphthyl)acetic acid SL 45 g/l Planofix Apple Pear Pineapple
4-indol-3-ylbutyric acid DP 1 g/kg Seradix B No 1 Ornamentals
4-indol-3-ylbutyric acid DP 3 g/kg Seradix B No 2 Ornamentals
4-indol-3-ylbutyric acid SL 8 g/kg Seradix B No 3 Ornamentals
GIBBERELLINS
Gibberellins SL 32 g/l ProGibb 4% Grape, Pear, Citrus,
Potato, Mango, Hops, Grape-
seedless
CYTOKININS
6-benzyl
adenine/gibberellins
SL 19/19 g/l Promalin Apple, Plum, Flowers,
Ornamentals
ETHYLENE
Ethephon SL 480 g/l Ethrel Apple, Citrus, Cotton,
Grape, Sugarcane Tobacco
GROWTH RETARDANTS
Paclobutrazol SC 250 g/l Cultar Litchi Mango Peach
Plum
Daminozide SP 850 g/kg B-Nine SP Flowers Ornamentals
Glyphosate-
isopropylamine
SL 360 g/l Glyphosate 360
Acid
Sugarcane and Grasses
Glyphosate-
isopropylamine
SL 360 g/l Mamba 360 SL Sugarcane Grasses
Glyphosate-
isopropylamine
SL 360 g/l Roundup Grasses Sugarcane
Glyphosate-
isopropylamine
SL 360 g/l Roundup Ultra Sugarcane Grasses
GROWTH INHIBITORS
Mepiquat chloride SL 50 g/l Pix Cotton
Chlormequat chloride SL 750 g/l CeCeCe 750 Pear, Wheat
Chlormequat
chloride/ethephon
SL 300/150
g/l
Uprite Wheat
DEFOLIANTS
Thidiazuron/diuron SC 120/60
g/l
Dropp Ultra Cotton