Plant Growth Regulators used in agriculture to manipulate plant vegetative and reproductive growth. Major plant growth regulators are Auxin, gibberellin, Cytokinin, ethylene and ABA etc.

1. DR. SASMITA PRIYADARSHINI DASH

2. GROWTH REGULATOR:-
 An organic compound
 Can be natural or synthetic
 If the compound is produced within the plant it is called a
plant hormone.
 Both internal plant hormones and lab created hormones are
called plant growth regulators.

3. There are five major types of plant
growth regulators:-
Auxin
Gibberellin
Cytokinin
Abscisic acid
Ethylene

4. Auxin :-
 Auxin is derived from Greek word ‘auxem’ means to grow.
 First time discovered by F.W Went in 1928 in coleoptiles of
Avena sativa .
 The most important natural auxin in plants is indole acetic acid(IAA)
Some of the Synthetic auxin like 2,4-Dand NAA.
 Auxin is synthesized in shoot tip region & meristematic region.
Physiological role:-
Apical dominance
Stem &root growth
Cell division & cell elongation
Root initiation
Flower & fruit development
 Parthenocarpy
 Root growth

5. Gibberellins :-
 Gibberellins were discovered by Eiichi Kurosawa, a japanese plant pathologist in
1926 from Gibberella fujikuroi ( Fusarium moniliforme ).
 The most commonly occurring gibberellin is GA3 which is also called as
gibberellic acid.
High in immature seeds.
Physiological role :-
Seed germination
Prevention of genetically dwarfism
Breaking dormancy
Bolting & flowering
Parthenocarpy
Sex expression
Stem elongation
Flowering

6. Cytokinins :-
 The term cytokinin was derived from cytokinesis meaning cell division.
 The first cytokinin was isolated from herring sperm in 1955 by Miller and
his associates.
 This compound was named kinetin because of its ability to promote
cytokinesis.
 Letham discovered the first naturally occuring cytokin called zeatin in corn
kernal in 1963.
Produced in meristematic regions of the plant.
 Kinein and 6-benzyladenine (BA)& DPU ( N,N- diphenyl urea) are the
synthetic cytokinins.
Physiological role:-
Cell division
Delay senescence

7. Ethylene:-
A russian graduate Nelijubow in 1901 from coal gas used to illuminate
lamps,discovered ethylene.
Cousins in 1910 first time suggested that plant might produce
ethylene.
Denny established the role of ethylene as a ripening agent.
Ethylene is the only gaseous hormone.
Physiological role:-
Ripening of fruits
Inhibition of root growth
abscission

8. Abscisic acid:-
Abscisic acid wasdiscovered by P.F.Wareing and F.R.Addicott in 1961
as a growth inhibiting hormone from the leaves of Acer pseudoplatanus
and fruit of cotton plant.
In sixth International conference on plant growth substances held in
1967 at Canada,Its name abscisic acid and abbrevation as ABA was
proposed,because it causes abscission in plant part.
Physiological role:-
Dormancy
Abscission
Senescence

9. APPLICATION OF PLANT GROWTH SUBSTANCRS IN VEGETABLES :-
Seed germination:-
 Pre sowing treatment of seed with growth regulators has been reported to enhance seed
emergence.
In tomato higher germination with GA3 at 0.5 mg/l&2,4-Dat 0.5 mg/l is reported.
Soaking of seeds in ethephon at 480mg/l for 24hrs improved germination in muskmelon,
bottle gourd, squash melon & water melon at low temperature.
 Gibberellins mobilize the stored carbohydrates in seed by activating α-amylase enzyme
resulting germination.
Seed dormancy:-
Main problem has been potato where freshly harvested tubers fail to sprout
Before the termination of rest period.
Chemicals which have been reported to break the rest period are GA, ethylene,
chlorhydrin & thiourea .
The treatment which has been used for breaking of dormancy in potato comprise the
vapour treatment with ethylene chlorohydrin (1lt/20q) follwed by dipping in thiourea
(1%sol)for 1hr finally in GA(1mg/l) for 2 seconds.

10. Flowering:-
Induction of flowering in plants which otherwise fail to flower has also been
reported with the use of various plant growth regulators.
Application of GA at 50mg/l to young leaves of non flowering varieties of potato,
when floral buds had just formed , resulted in flower induction in all varieties.
MH delayed flowering in okra.GA has been reported to induce early flowering in
lettuce.
Sex expression:-
Sex expression in cucurbits is influnced by various natural and induced factors.
The most important such environmental factors are day length, light intensity,
temperature, mineral nutrition and ambient atmosphere.
The treatment with growth regulators has been found to change sex expression in
cucurbits, okra & pepper.
Leaves and roots play an important role in expression of sex , roots are
responsible to enhance female sex expression where as leaves seem to promote
male sex expression.

11. Gametocides :-
Some plants growth regulators possess gametocidal actions to produce male
sterility which can be used for F1 hybrid seed production.
The chemicals which has been reported to show good performance are MH
at 100 to 500 mg/l in egg plant,okra,peppers&tomato,GA3 in onion,2,3-
dichloroisobutyrate(0.2 to 0.8%) in egg plants, muskmelon, okra, onion, root
crops, spinach , tomato &TIBA in cucumber, egg plants, onion & tomato.
Gaat 100mg/l can also be used for inducing male sterility in pepper.
Hybrid seed production:-
Growth regulators can also be used as an aid in hybrid seed production.
Use of ethephon has been used for producing temporary female lines in some
cucurbits.
In muskmelon foliar spray of silver thiosulphate at 400mg/l was found best
for induction of male flower on gynocious lines.

12. Fruit set :-
Poor fruit set is a major problem in tomato, brinjal & chillies which is frequently
caused by adverse weather conditions during flowering.
Plant growth regulators have been reported to enhance fruit set under both
normal &adverse weather conditions.
Foliar spray of MH(20ppm) and NAA or PCPA (50ppm)is very effective for
increasing fruit set in tomato under both normal and adverse conditions.
Parthenocarpy :-
Studies on use of growth regulators in many vegetables have shown an increase
in the fruit size with their applications.
The role of plant growth regulators in fruit development can also be seen from
the fact that with their help it is possible to stimulate fruit development without
fertilization (parthenocarpy). In brinjal, application of 2,4-D at 0.00025% in lanolin
paste to cut end of styles or as foliar sprays to freshly opened flower cluster has
been reported to induce parthenocarpy.

13. Fruit ripening:-
Uniform ripening and colour development in tomato and chilli are important
factors for marketing point of view.
Ethephon, an ethylene releasing compound has been reported to induce ripening
in tomato & pepper.
Field application of ethephon at 1000mg/l at turning stage of earliest fruits
induced early ripening of fruits thus increasing the early fruit yield by
30-35% post harvest dip treatment with ethephon at 500-2000mg/l has also been
reported to induce ripening in mature green tomatoes.
Fruit yield:-
Growth regulators have been reported to increase yield in several vegetables.
In brinjal soaking of seedlings roots in NAA at 0.2mg/l and ascorbic acid at
250mg/l has been reported to produce higher fruit yield.
In onion foliar spray of NAA(300ppm) or GA3(60PPM) at bulb formation has
been reported to improve bulb yield.

14. Hardening and establishment of seedlings:-
Some times vegetable seedlings show poor establishment due to transplanting
shock.
Spray of 500-1000ppm CCC is very effective in inducing hardening of tomato
seedlings and also reduces leaf curl infestation.
Seedling root dip in NAA at 0.1 to 0.2 ppm for 24 hrs has been found most
effective treatment in tomato, brinjal, cauliflower & cabbage.
Induction of exerted flowering in tomato ( heterostyly ):-
Exerted stigma in tomato prevents self pollination so that tedious work of
emasculation can be avoided in tomato.
Application of GA3 at the time of flowering elongates the stigmatic position of
flower and avoid selfing . Such Lines can also be used as female line in hybrid
seed production programme.
In tomato maintenance of anther less mutant is a problem.
GA3 has been successfully utilized for this purpose.

15. Effect of plant growth substances on quality of vegetables:-
Growth
regulator
Concentration
ppm
crop Effect on quality
GA3 15 muskmelon Improves rind
thickness
GA3 5-15 Cabbage, cauliflower Increases head or curd
size
Ethephon 250 Tomato Increases TSS
MH 2500 Potato Reduces sprouting
&rotting of tubers in
storage

16. Growth regulators also have role in:-
Induction of biotic resistance:-
Growth substances can be used to reduce the incidence of diseases
in vegetable crops .
Auxins like 2,4-D,NAA,TIBAand IAA reduce the seviarity of
Fusarium wilt, TLCV and TMV.
 Allevation of abiotic stresses
1) Frost resistance :-
Spray of ethephon on tomato nursery 12 days prior to transplanting
increases the frost tolerance in plant.
In cucumber ABA is reported to be involved in development of
frost tolerance in plant.

17. 2) Water stress:-
Plant growth regulators play an important role in plant water relation.
Under water stress, the synthesis of auxins in shoot and its transport to
the root is inhibited.
Under such conditions, ABA signal moves from roots to the shoots to
regulate growth and development.An increased level of ABA leads to
stomatal and transpiration control and modifies water balance by
restricting leaf growth.
3) Salinity stress:-
Certain plant growth substances are known to alleviate the harmful
effect of soluble salts on plant.
 The physiological changes in plants under salt stress are caused by
hormanal imbalance.
Plant growth, photosynthetic activity and translocation of assimilates
are inhibited under salt stress condition.
In bean and tomato , application of cycocel at 5-12mg/plant as soil
application or as foliar spray at 0.1-0.3%improves considerable tolerance
to salinity however no reduction in uptake of salt could be noticed.

18. 4) Low temperature stress :-
Chilling temperatures frequently damages many vegetable crops.
 Absisic acid application has been found to increase cold hardiness in
several vegetable crops.
 Spraying of potato haulms with 0.74kg/ha of CCC causes a short term
retardation of growth, stimulates the root growth and increases resistance
of plants to frost.
5) High temperature stress :-
plants can grow only at certain temperature regimes .
In general high temperature accelarates the most of the plant metabolic
processes.
The harmful effect of high temperature is usually aggravated by lack of
available moisture .
Hot dry winds damage the crop by increasing evapotranspiration causing
desiccation of leaves .
Application of certain growth regulators can alleviate the harmful effect
of high temperature in vegetable crops improved growth in tomato under
high temperature conditions can be achieved by spraying of sodium salt of
PCPA(50ppm)or NAA(10ppm)at flowering.

19. Improvement of shelf life:-
Use of growth regulators and other chemicals has been found helpful in
enhancing shelf life of several vegetables.
 Auxin :-
foliar spray of 50-500ppm CIPA, four days before harvest is effective in
reducing weight loss and retention of vitamin C content in snap beans.
 Gibberellin:-
 Gibberellins reduce the respiration rate and delay climacteric peak and
chlorophyll degradation in leaves, thereby enhance shelf life in vegetable crops.
Tomato fruit dipping in solution of GA3 (100-500ppm) retards the
Ripening and extends the storage life.
 Cytokinin :-
Post harvest dip of broccoli heads in 6-benzyl amino purine(BA) at 10ppm
concentration ,increases shelf life by reducing weight loss,co2 evolution and o2
uptake.
 Cycocel and Maleic Hydrazide are also used to enhance shelf life of
vegetables.
 Enhancement of shelf life through restricting ethylene production in storage.