Other Growth Regulators
Plant Growth Regulators
- Certain substances affect the growth quite miraculously.
These were referred to Hormones. Hormone means to urge or to stimulate
- Hormones of plants are referred as phyto Hormones.
Phyto Hormones are organic substances which are naturally produced
in plants, control the growth or other physiological functions,
at a sight remote from its place of production and active in extreme
- Growth Hormones can also be defined as substances
which are naturally produced in plants, control the growth are
other physiological functions, at a sight remote form its place
of production and active in extreme minute quantities.
- Growth Hormones can also be defined as substances
synthised in particular cells and are transferred to other cells
where in extremely small quantities influence development process.
- Florigen Etc.,
- Now different categories of substance affecting
plant growth are known which can be broadly classified as.
Naturally occurring growth
Synthetic Growth Substances
- To include both the terms they can be called
as Growth Regulating Substances.
Acid (ABA), Phaseic Acid
Phytochrome Tranmatic Substances
1618, Phosphin - D, Morphacting, Malformis.
Auxins, Synthetic Cytokinins.
- Characteristics Features Polar translocation
- Apical dominance - Variable Behaviour or root and shoot growth
- Root Initiation - Delay in abscision and differentiation of
Role of Auxins
- Apical Dominance:Removal
of apical bud stimulates lateral buds. Auxins inhibit lateral
bud formation since they are synthesised in apex. This phenomena
is called apical dominence. Eg: Potato tubers for apical buds
- Cell Division And Elongation:
Shoot and Root growth.
- Xylem Differentiation:
Auxins helps in establishing contact between vascular tissues
of the callus and that of the bud and makes it possible for the
bud to grow properly in callus. By adding auxin and sugar continued
growth of callus may be obtained and new shoots and even new plant
can be produced.
- Nucleic Acid Activities
of IAA increases total RNA - synthesises specific enzymes lead
to cell enlargement.
- Manifold Activities Play specific role in seed
germination, growth, rooting, flowering (Reproductive phase),
abscission, parthenorcarpy and tissue culture.
- Germination: IAA, IBA, NBA, 2,4-D are mostly
used in soaking seed for germination- at low concentrations promotes
germination but these effects are subjected to variation depending
on form and species of plants.
- Root: NAA, 10% induces 100% rooting in mango:
Dashri, langra IBA+SUGAR application leads to greater number of
roots-structure of roots also changed (Vascular bundles).
- Flowering: Play floragenic role in day neutral
plants IAA promotes formation of female flowers. Increased spikelet
number, leaf number and weight and number of grains in wheat.
NAA & IAA increases boll-set (G.hirsutum) induced more pine-apple.
Fruit weight increases.
- Parthenocarpy: IBA, NAA produces seed less/fruits
- smaller sized fruits, but more in number, hence yield not affected.
- Fruit setting: By using 2,4,5 T fruit setting
and yield of ber/fruit increased. IAA, IBA, and NAA induce high
percentage fruit set.
- Prevention of pre-mature drop of fruits: 2,4,D,IAA,IBA,
2,4,5-T, are used to prevent pre-harvest drop of sweet oranges(
100 to 500 ppm)
- Tissue and Organ culture: IAA & Kinetin
- Auxins as inhibitors: High concentration of auxins
inhibit the growth and exert toxic effect on plants. In normal
case, self produced auxins inhibit the growth and development
of lateral buds, and as a result apical buds, remains dormant.
Auxins use in Agriculture
- Propagation of plants by hormone treatment of
- Prevention of pre harvest drops of plants.
- Increasing parthenocarpy.
- Increasing fruit set.
- Prevention of sprouting by inhibiting buds.
- Inhibition of prolonged dormancy.
- Control of flowering.
- Defoliation of plants
- Prevention of leaf fall or abscission.
- Thinning of compact fruits.
- Selective weed killer.
- Second important growth
Hormone. More than 60 types of Gibberellins are known. They are
named as GA1, GA2------ upto GA60. About 51 types are found in
- Mechanism of Gibberellins:
GA exerts its physiological effect on altering the Auxin status
of tissue. It acts at the gene level to cause depressions of specific
- The activated genes by producing new enzymes,
bring about observed morphologic changes. Alerts the RNA. GA appears
to involve in alteration of nucleic acid directed protein synthesis
in some long term regulatory action and some other types of activation
phenomena in short term regulatory action.
Role of Endogenous Gibberellins
- Apical bud dormancy
- Role in sub apical meristem
- Cell elongation
- Fruit growth
- Metabolisation of food in seed storage cells.
- Germination: Increases length of hypocotil and
cotyledenary leaf area.
- Root Growth: Inhibits root growth
- Leaf Expansion: Leaves become broader and enlarged
(Cabbage, Sweet corn).
- Hyponesty of leaves: GA treated leaves of chrysanthemum
plants holds their leaves more erect.
- Flowering: Induces flowering in long day plants
and in plants requiring cold induction. Also promotes formation
of male flowers.
- Parthenocorpy: Brinjal, Guava(Alahabad round).
- Fruit setting: Increased fruit setting (Phalsa,
Sweet lime, Grapes).
- Fruit Drop: Not much effective.
- Stem elongation: Chorchorus capsularis: extention
of stem and increased number of internodes. However leaf area,
basal diameter of stem and fibre quality are reduced.
- Pollen Germination: Sugar cane 15 out of 34 germinated
against normal conditions.
- Breaking Dormancy: In temperate plants buds become
dormant in later summer and do not sprout even when exposed to
sufficient moisture, temperature and oxygen. They require low
temperatures or long days or red light. GA overcomes this dormancy.
Enhanced cell elongation push through the endosperm (seed coat)
Potato tubers can be made to sprout in winter by GA.
- Other uses: Sprayed on Fruits to prevent rind
disorder. Thomson seedless grape bunches if sprayed with GA, causes
elongation of bunch, so they are less tightly packed and less
succesptible to fungi.
- Initiation of cell division
- Delay of senescence
- Use in tissue culture
- Counteract apical dominence.
Action and application
- Cell division
- Cell enlargement
- Apical dominance
- Mobility: Immobile obstructs the movement of
amino acid, phosphate and various other substances
- Nucleic acid metobolism: Quick increase in the
amount of RNA and decreases DNA
- Protein synthesis: Increases DNA
- Protein synthesis: Increased rate
- Florigens: Induction of flowering in short day
Absicissic Acid (ABA)
Role of ABA
- Induces bud dormancy
- Promotes senescence
- Accelerates leaf abscission in cotton plant
- Induces flowering during long days in certain
short day plants - ineffective in short day plants.
- Counteracts GA
- Highly useful in inducing
fruit ripening. Ethylene is a natural product of ripening fruit.
Ethylene is a gas at temperatures under which a plant can live.
- Auxins increase ethylene level in plants and
auxin actions are attributed through ethylene such as increased
percentage of female flowers, apical bud dominance and leaf epinasty.
Role of Ethylene
- Abscission: Principle
accelerator of abscission - Capable of promoting changes associated
with pre - abscission and aging of leaves, petioles, flowers and
fruits. Ethylene degreening is a commercial practice (5-10 ppm).
- Degreening occurs after
ethylene treated are exposed to air - accelerates maturity and
induces uniform ripening (Pine apple).
- It is responsible for flower initiation in plants
- synthesised in older leaves and then transferred to growing
region, where it innitiates floral bud.
- They are synthetic growth regulators, acts in
variety of ways on the natural regulation of mechanisms of plants.
The important are:
acid (synthetic auxin)
- Substituted benzoic
- Maleic acid hydrazide
- Flurene - carboxylic
acid and their derevatives
- Methyle benzilate
- Dichlorflurenol etc.
- The action of these
substances are systemic and after their uptake they are transported
and distributed not polarly, but basipetally and acropetally.
Role of Morphactins
- Seed Germination - inhibition
- Growth of seedlings
- inhibits growth of both root and shoot this property is similar
- Stem elongation: dwarfing
- Apical dominance and
branching : treatment to grasses and cereals increased tillering
and also increased no of laterals. Stimulates extension of lateral
- Prolonged bud dormancy
: Root growth and root branching: Lateral roots are inhibited
and primary roots are promoted. The action of morphactin on the
longitudinal growth of root system may be considered as a reverse
of their action on the shoot system.
- Flowering: Prevents flowering in short day plants,
sequence of flowering, position and no. of flowers and parthenocarpy
- Hormone activity increases in response to wound
or injury. A wound hormone called traumatic acid has been extracted,
which stimulated meristamatic of cork, cambium to cover wounded
or injured portions.
Some Other Growth Regulators
- Triacontanol: Saturated primary alchohol isolated
from shoots of alfa-alfa. Response is very rapid inincreasing
growth. 20% enhanced growth in rice and maize.
- Brassins: Steroid growth promoters isolated from
polen grain of rape causes distninct growth promoting activity.
- Xanthoxin: Potent growth inhibitor can be converted
metabolically to ABA
- Batasins:Isolated from yam plants -causes dormancy
- Vitamins: Organic compounds - occur in several
plants, substances inhibit growth of certain parts of plants.
- Vitamins:Organic compounds - Occur in natural
food stuffs either as such are as utilisable precursors which
are required in minute amounts for normal growth, maintenance
and reproduction. They have cetalytic and regulatory functions
in cell metabolism.