Plant growth regulators are organic compounds, either natural, or synthetic, that modify or control one or more specific physiological processes with a plant. Natural plant growth regulators are produced by plants and to differentiate these from hormones in animals, the term plant hormones or phytohormones is used for such substances. Plant hormones are naturally occurring compounds produced by the plant to accelerate or retard the rate of growth or maturation.

1. Sem. III, Paper VI- Plant Metabolism
Unit 3- Plant Growth Regulators
Dr. Seema A. Gaikwad
Dept. of Botany
Vidnyan Mahavidyalaya, Sangola

2. PLANT GROWTH REGULATORS
As the plants require oxygen, water, sunlight, and nutrition to grow and develop, they do
require certain chemical substances to manage their growth and development. These
chemicals substances are called Plant Growth Regulators and are produced naturally by
the plants itself.
These are simple organic molecules having several chemical compositions. They are also
described as phytohormones, plant growth substances, or plant hormones. Plant hormones
are organic compounds which are either produced naturally within the plants or are
synthesized in laboratories.
Plant growth regulators can be defined as chemicals which signal to regulate and control
the growth of plants. They can accelerate as well as retard the rate of growth in plants.

3. Discovery of Plant Growth Regulators
The discovery of major Plant growth regulators began with observations of plant movements by Charles
Darwin (1884) and his son, Francis Darwin. They observed the growth of coleoptiles of canary grass
(Phalares canariensis) towards the light source-phototropism. Followed by a series of experiments, they
concluded the presence of a transmittable substance that influences the growth of canary grass towards the
light. That transmittable substance was what we know as auxin which was isolated later by F.W. Went
(1928). Effect of Auxin on Plant Growth
Later many scientists discovered and isolated different
plant growth regulators. Gibberellins or gibberellic acid
was formerly found in uninfected rice seedlings and was
reported by E. Kurosawa. Skoog and Miller discovered
another growth promoting substance named kinetin, which is now known as cytokinin.

4. Types of Plant Growth Regulators
Generally, there are five types of plant hormones namely, auxin, gibberellins
(GAs), cytokinins, abscisic acid (ABA) and ethylene. In addition to these,
there are more derivative compounds, both natural and synthetic like CCC,
which also act as plant growth regulators. Auxins, Gibberellins, and
Cytokinins are grouped into Plant growth promoters while Abscisic acid,
Ethylene and CCC are grouped into Plant growth inhibitors. Ethylene can
be grouped either into the promoters or into the plant inhibitors.
 Plant Growth Promoters- Auxins, Gibberellins, and Cytokinins.
 Plant Growth Inhibitors- Abscisic acid, Ethylene and CCC.

5. Plant Growth Promoters
a. Auxins: The first phytohormones to be discovered is the Auxin and it was discovered by the biologist Charles Darwin.
Auxins are one of the most important plant hormones. The chief naturally occurring auxin is indole-3 acetic acid – IAA and
other related compounds. The term Auxin is derived from the Greek word meaning to grow. Auxins are a group of
phytohormones produced in the shoot and root apices and they migrate from the apex to the zone of elongation. The term,
auxin was introduced by Kogl and Haagen- Smit (1931). Went (1928) isolated auxin from the Avena coleoptile tips by a
method called Avena coleoptile or curvature test and concluded that no growth can occur without auxin. Auxins are widely
distributed throughout the plant however, abundant in the growing tips such as coleoptile tip, buds, root tips and leaves.
Indole Acetic Acid (IAA) is the only naturally occurring auxin in plants.
The synthetic auxins include - IBA: Indole Butyric Acid, NAA: Naphthalene Acetic acid, MENA: Methyl ester of
Naphthalene acetic acid, MCPA: 2 Methyl 4 chloro phenoxy acetic acid, TIBA: 2, 3, 5 Tri iodo benzoic acid, 2, 4-D: 2, 4
dichloro phenoxy acetic acid, 2, 4, 5-T: 2, 4, 5 – Trichloro phenoxy acetic acid.
Chemical Structure: Auxin molecules are normally derived from the amino acid tryptophan.
This amino acid has a six-sided carbon ring, attached to a 5-sided ring containing carbon.
This 5-sided ring has a group attached. All auxins are compounds with aromatic ring and
a carboxylic acid group.
IAA (Indole-3-acetic acid) – C10H9O2N

6. Physiological Roles and Practical Applications of Auxins
1. Cell division and elongation: The primary physiological effects of auxin are cell division and cell elongation in
the shoots. It is responsible for initiation and promotion of cell division. It is important in the secondary growth of
stem and differentiation of xylem and phloem tissues.
Practical Application: In grafting and tissue culture techniques, callus formation is important and can be achieved
by IAA application. In tissue culture, stimulates the cell division and callus formation in combination with cytokinins.
Besides cell elongation, auxin may also be active in cell division. In many tissue cultures, where the callus growth is
quite normal, the continued growth of such callus takes place only after the addition of auxin.
2. Root Formation: In contrast to stem, the higher concentration of auxin inhibits the elongation of roots but the
number of lateral roots is considerably increased i.e., higher concentration of auxin induces more lateral branch roots.
Application of IAA in lanolin paste (lanolin is a soft fat prepared from wool and is good solvent for auxin) to the cut
end of a young stem results in an early and extensive rooting.
Practical Application: Initiation of adventitious roots on stem cuttings. Lateral root development in tissue culture.
This fact is of great practical importance and has been widely utilized to promote root formation in economically
useful plants which are propagated by cuttings.

7. Physiological Roles and Practical Applications of Auxins
3. Apical dominance: In many plants, if the terminal bud is intact and growing, the growth of lateral buds just
below it remains suppressed. Removal of the apical bud results in the rapid growth of lateral buds. This
phenomenon in which the apical bud dominates over the lateral buds and does not allow the lateral buds to
grow is known as apical dominance. The apical dominance is due to presence of auxins present at that site,
which suppresses lateral buds through nutritional status.
Practical Application: This property of auxin is used in agriculture for checking the sprouting of lateral buds
on tubers during storage. By spraying any one solutions of auxin like IBA, NAA, potato and other
commercially important tubers can be stored for longer duration. Similarly, in case of some horticultural plants,
by removing apical buds, branches are allowed to develop, for production of more flowers.
4. Prevention of abscission: Natural auxins prevent the formation of abscission layer which may otherwise
result in the fall of leaves, flowers and fruits. The development of abscission layers begins with decrease in
auxin concentration below certain minimum level. But if the auxin concentration in the cells is maintained, it
prevents the formation of abscission layer prematurely. Prevent dropping of fruits and leaves at early stages.
Practical Application: Promote natural detachment (abscission) of older leaves and fruits. Premature fruit fall
in commercially important plants like Apple, pear, Citrus is responsible for heavy loss to the cultivators. This
loss can be prevented by spraying dilute solution of IAA, NAA, 2-4-D like auxins. By spraying plants with low
concentrations of 2-4-D defoliation in cabbage and Cauliflower can be prevented.

8. Physiological Roles and Practical Applications of Auxins
5. Parthenocarpy: Auxin can induce the formation of parthenocarpic fruits (fruit formation without pollination
and fertilization). In parthenocarpic fruits, the concentration of auxin in the ovaries is higher than in the ovaries
of plants which produce fruits only after fertilization. These seedless fruits are known as parthenocarpic fruits.
• Practical Application: In many plants like orange, Banana and grapes natural parthenocarpy can be observed
due to internal auxins. Now parthenocarpic seedless fruits are developed in many plants which helps to
improve agriculture yield.
6. Eradication of weeds: The unwanted, harmful plants which compete with crops and reduce crop yield, are
termed as weeds. The roots of many weeds are very sensitive to auxins. Some synthetic auxins especially 2, 4-
D and 2, 4, 5-T are useful in eradication of weeds at higher concentrations.
• Practical Application: The above property of auxins can be used in agriculture by farmers to destroy various
harmful weed plants and increase the crop yield. Some synthetic auxins especially 2, 4- D and 2, 4, 5-T are
useful in eradication of weeds at higher concentrations. Used by gardeners to keep lawns free from weeds.

9. Physiological Roles and Practical Applications of Auxins
7. Spur Formation: Auxins at higher concentration prevent excessive elongation of stem and induce development of
more buds. Flowers are produced on the dwarf branches and their production is controlled by maintaining higher
auxin concentration.
• Practical Application: In apple and Pear plants fruit develops on dwarf branches or spurs. If these terminal buds
are treated with NAA it prevents the formation of elongated branches and increases number of dwarf branches. This
increases development of more flower buds and fruits which gives more profit to cultivator.
8. Effect on Flowering and sex expression: Auxins take indirect part in the physiology of flowering by controlling
the vegetative growth. In some plants through effect of auxin, synchronization of completion of vegetative growth
and simultaneous beginning of reproductive growth in different individuals in a field.
• Practical Application: Auxins generally inhibit flowering but in pine apple and lettuce it promotes uniform
flowering. This property of auxin has been commercially utilized in case of pine apple, sugarcane and lettuce. By
spraying synthetic auxins like NAA, in appropriate

10. Physiological Roles and Practical Applications of Auxins
9. Prevention of Lodging: Prevention of plants from lodging and reduction in crop
yield is checked by auxins.
• Practical Application: Many crop plants belonging to Poaceae and Leguminosae
tend to lodge (fall down) due to excessive elongation and softening of cells in the
basal internodes of the stem. This reduces crop yield. But if auxins like NAA is
treated to basal part of stem, to grow stiff, woody and erect.
10. Movements in plants: Auxins are involved in phototrophic and geostrophic
movements in plants due to their polar transport and effect on cellular growth.

11. b. Gibberellins: Gibberellins are synthesized in young tissues of the shoots, uncertainly in roots and also in the
developing seeds. There are also some evidences that leaves may be the source of some biosynthesis.
• Discovery of Gibberellins- A Japanese scientist E. Kurosawa in 1926 found that some of the rice seedlings grow
taller than others. He discovered the seedlings to be infected by the pathogenic fungus Gibberella fujikuroi. In his
work he found that the rice plant disease- bakanae (foolish seedling) caused by this fungus is having some
metabolites that might be responsible for the stimulated growth of seedlings. In 1935, an agricultural chemist T.
Yabuta isolated a non-crystalline material from the similar type of culture that he named as “gibberellins”. The
gibberellins are named GA1, GA2, GA3 ... GAn in their order of discovery. Gibberellic acid -GA3, was first
structurally characterized gibberellin. There are at present 136 GAs that have been identified from plants, fungi and
bacteria.
• Chemical Structure: Basically, gibberellins are terpenoids. They have either
19 or 20 carbon atoms into either four or five ring systems. This skeleton is called
“gibberellane” or “gibben skeleton”. Different forms of gibberellins differ from
one another in carbon atom number and positions of substituent hydroxyl groups.

12. Physiological Roles and Practical Applications of Gibberellins
1. Seed germination
Certain light sensitive seeds e.g., Barley, Lettuce and tobacco show poor germination in dark. Germination
starts vigorously if these seeds are exposed to low temperature, light or red light. This requirement of light is
overcome if the seeds are treated with gibberellic acid in dark.
• Practical Application: Dormancy in economically important seeds can be broken and germination
percentage can be increased. Breaks seed dormancy in some plants which requires light for germination.
2. Dormancy of buds
In temperature regions the buds formed in autumn remain dormant until next spring due to severe cold. This
dormancy of buds can be broken by gibberellin treatments. In potato also, there is a dormant period after
harvest, but the application of gibberellin breaks dormancy in potato tubers.
• Practical Application: Break bud dormancy, in tubers and trees of economically importance and process of
growth can be induced using gibberellin solution at proper concentration.

13. Physiological Roles and Practical Applications of Gibberellins
3. Elongation of internodes or Stem Elongation
The most pronounced effect of gibberellins on the plant growth is the elongation of the internodes. Therefore, in many
plants such as dwarf pea, dwarf maize etc. gibberellins overcome the genetic dwarfism. Seedling of Tomato, Radish etc.
showed marked hypocotyle elongation by application of GA.
• Practical Application: GA can be applied to sugarcane sets for stem elongation which will increase yield and sugar
percentage. Thus farmer can obtain more money. Helps in increasing the crop yield by increasing the height in plants such
as sugarcane and increase the axis length in plants such as grape stalks.
4. Bolting and flowering
In many herbaceous plants, the early period of growth shows rosette habit with short stem and small leaves. Under short
days, the rosette habit is retained while under long days the stem elongates rapidly and is converted into polar axis bearing
flower primordia. This phenomenon is known as ‘bolting’, which is induced due to winter cold. This bolting can also be
induced in such plants by the application of gibberellins even under non-inductive short days.
In Hyoscyamus niger (a long day plant) gibberellin treatment causes bolting and flowering occurs under non-inductive short
days. Promote bolting in cabbages and beet.
• Practical Application: By treating biennial plants of economic importance with GA, bolting can be induced and seeds
can be obtained in one season or year instead of two.

14. Physiological Roles and Practical Applications of Gibberellins
5. Parthenocarpy: Germination of the pollen grains is stimulated by gibberellins; likewise, the growth of the fruit and the
formation of seedless fruits without fertilization can be induced by gibberellin treatment. When GA is applied to stigmatic
surface at anthesis to emasculated flowers parthenocarpic fruit develops.
• Practical Application: In many cases, e.g., Pome and stone fruits where auxins have failed to induce parthenocarpy, the
gibberellins have proven to be successful. Seedless and fleshy tomatoes and large sized seedless grapes are produced by
gibberellin treatments on commercial scale by farmers. The grape varieties like “Thompson- seedless”, “Sonaka”, and
“Sharad- seedless”, are treated with GA3 for better size and yield. For improving size, color, quality of fruits like apple,
Tomatoes are treated with GA3 in the form of sprays. GA can induce parthenocarpy in cherry fruits.
6. Synthesis of the enzyme α – amylase: One important function of gibberellins is to cause the synthesis of the enzyme α-
amylase in the aleurone layer of the endosperm of cereal grains during germination. This enzyme brings about hydrolysis of
starch to form simple sugars which are then translocated to growing embryo to provide energy source.
• Practical Application: The α – amylase activity increased in the germinating barley seeds due to GA treatment is widely
used for malt production in beer industry. Used by the brewing industry for increasing the speed on the malting process.

15. Physiological Roles and Practical Applications of Gibberellins
7. Reversal of Genetic Dwarfism: The most striking effect of gibberellins is conversion of genetically dwarf plants to normal height.
Rosette sugar beet, dwarf pea and corn varieties show marked longitudinal growth and attain normal height due to GA treatment. Original
dwarf plants have low GA concentration and external application will stimulate cell growth due to activation of sub-apical meristems,
resulting into increased growth.
• Practical Application: To increase the height of genetically dwarf plants are useful for plant breeding programmes and also in many
dwarf plants of horticultural importance GA is used to increase height.
8. Substituting Cold Treatment and Flowering Induction: Winter variety plants require low temperature for induction of flowering. In
such plants flowering can be induced by GA application without cold treatment.
• Practical Application: Winter variety plants of economic importance can be made to flower during any season by application of GA
e.g. Radish, Xanthium spp. etc.
9. Reversion of sex expression: GA application can change sex expression in some plants. The plants with some unisexual flowers, when
treated with GA shows increase in number of opposite sex flowers. e.g. In Cucumis sativus, Cannibis spp. male flowers are converted to
female flowers and their number is increased. However, in Maize female flowers are converted to male.
• Practical application: By using GA, in case of economically important fruit plants with unisexual flowers, number of female flowers
can be increased, due to which increase in number of fruits per plant can be increased. This helps the cultivator.

16. c. Cytokinins: In Cytokinin, cyto = cell + kinin = division, i.e., meaning is cell division.
Cytokinin is also called as cytokine. These are the compounds resembling with adenine
(aminopurine) which stimulates cell division.
• Discovery of Cytokinins- Kinetin was discovered by Skoog and Miller (1950)
from the tobacco pith callus and the chemical substance was identified as 6-
furfuryl aminopurine. Because of its specific effect on cytokinesis (cell division), it
was called as cytokinins or kinetin. In 1963, zeatin was isolated from immature
sweet corn seeds (Zea mays) by D. S. Letham; it was the first natural cytokinin
derived from plant that has kinetin-like activity. The term, cytokinin was
proposed by Letham (1963). Some of the very important and commonly known
naturally occurring cytokinins are Coconut milk factor and Zeatin.

17. Chemical Structure: The chemical substance was identified as 6-furfuryl amino
purine. Kinetin has been found to be a derivative of purine base, adenine which
bears furfuryl substituent at 6th position. All cytokinins have purine ring
with a side chain at N6 position (amino substituted adenine).

18. Physiological Roles and Practical Applications of Cytokinin
1. Cell division – Cytokinin stimulates cell division.
• Practical Application: Used in tissue culture to induce cell division in mature tissues.
The most important biological effect of kinetin on plants is to induce cell division
especially in tobacco pith callus, carrot root tissue, soybean cotyledon, pea callus etc.
2. Cell enlargement- Like auxins and gibberellins, the kinetin may also induce cell
enlargement. Significant cell enlargement has been observed in the leaves of Phaseolus
vulgaris, pumpkin cotyledons, tobacco pith culture, cortical cells of tobacco roots etc.
• Practical Application: Enlargement in cells increases the size and weight of the organ.
Therefore in case of some fruit plants of commercial importance to increase the size and
weight of fruits cytokinins are used.

19. Physiological Roles and Practical Applications of Cytokinin
3. Counteraction of apical dominance - Promote lateral shoot growth and adventitious shoot formation. Stimulates
the growth of lateral buds and leaf expansion by cell division & enlargement and hence counteracts the effect of
apical dominance.
• Practical Application: External application of cytokinin promotes the growth of lateral buds.
4. Dormancy of seeds- Break bud and seed dormancy. Cytokinin application is found effective in breaking of seed
dormancy and promotion of seed germination.
• Practical Application: Like gibberellins, the dormancy of certain light sensitive seeds such as lettuce and tobacco
can also be broken by kinetin treatment.
5. Delay of senescence (Richmond - Lang effect)- The senescence of leaves usually accompanies with loss of
chlorophyll and rapid breakdown of proteins. Senescence can be postponed to several days by kinetin treatment by
improving RNA synthesis followed by protein synthesis. Promote nutrient mobilization which in turn helps delay leaf
senescence
• Practical Application: Richmand and Lang (1957) while working on detached leaves of Xanthium found that
kinetin was able to postpone the senescence for a number of days. Helps in delaying the process of ageing
(senescence) in fresh leaf crops like cabbage and lettuce.

20. Physiological Roles and Practical Applications of Cytokinin
6. Flower induction: Cytokinins can be employed successfully to induce flowering in short day plants.
They are used to keep flowers fresh for a longer time.
7. Morphogenesis- Stimulates morphogenesis in tissue culture. Morphogenesis in plants by cytokinins is
associated with auxin level. It has been shown that high auxin and low kinetin produced only roots whereas
high kinetin and low auxin could promote formation of shoot buds.
• Practical Application: During tissue culture experiments, in the beginning only callus is formed. To
induce root and shoot apices, in the callus, cytokinins are essential. So cytokinins are useful in tissue
culture to multiply economically important plants rapidly.
8. Accumulation and translocation of solutes- Plants accumulate solutes very actively with the help of
Cytokinin and also help in solute translocation in phloem.
9. Protein synthesis- Osborne (1962) demonstrated the increased rate of protein synthesis due to
translocation by kinetin treatment.

21. Physiological Roles and Practical Applications of Cytokinin
10. Promotion of chloroplast development- Involved in the formation of new leaves and
chloroplast organelles within the plant cell. Stimulation of chlorophyll synthesis that causes the
conversion of etioplasts into chloroplasts.
11. Other effects- Cytokinins provide resistance to high temperature, cold and diseases in some
plants. They also help in flowering by substituting the photoperiodic requirements. In some cases,
they stimulate synthesis of several enzymes involved in photosynthesis.
12. Commercial applications- Cytokinins have been used for increasing shelf life of fruits,
quickening of root induction and producing efficient root system, increasing yield and oil contents
of oil seeds like ground nut.

22. Plant Growth Inhibitors- Abscisic acid, Ethylene and CCC
a. Abscisic acid: Abscisic acid is a naturally occurring growth inhibitor. This
growth inhibitor is synthesized within the stem, leaves, fruits, and seeds of the
plant. Abscisic acid mostly acts as an antagonist to Gibberellic acid. It is also
known as the stress hormone as it helps by increasing the tolerance of plants to
different kinds of stress.

23. •Discovery of Abscisic acid: In 1963, F. Addicott and his associates first identified
and characterized the abscisic acid from young cotton fruits that is responsible for
the abscission of fruits. They isolated two compounds and called abscisin I &
abscisin II. Abscisin II is presently known as abscisic acid (ABA). This substance
also induces dormancy of buds therefore it also named as Dormin.
Chemical Structure: ABA is a sesquiterpenoid
consisting of 15 carbon atoms, having asymmetric
carbon. It has a six-carbon ring structure to which a side
chain is attached. The naturally occurring S- abscisic acid
is dextro-rotatory and has got inhibitor action.

24. Physiological Roles and Practical Applications of Abscisic acid (ABA)
The two main physiological effects are- 1. Geotropism in roots 2. Stomatal closing
1. Geotropism in roots-Geotropic curvature of root is mainly due to translocation of ABA in basipetal
direction towards the root tip.
2. Stomatal closing- Stimulates closing of stomata in the epidermis. ABA is synthesized and stored in
mesophyll chloroplast. In respond to water stress, the permeability of chloroplast membrane is lost which
resulted is diffusion of ABA out of chloroplast into the cytoplasm of the mesophyll cells. From mesophyll
cells it diffuses into guard cells where it causes closing of stomata.
3. Water stress and ABA- Stress responses especially to water deficiency. ABA seems to play an
important role during water stress and drought conditions. This occurs due to ABA promoted stomatal
closure which checks water loss in transpiration.
• Practical application: ABA application will be helpful in drought resistance development in plants.
More ABA is accumulated during drought cultivator plants than the sensitive plants.

25. Physiological Roles and Practical Applications of Abscisic acid (ABA)
4. Inhibition of seed germination and seedling growth - External supply of ABA inhibits seed
germination because of inhibition of activity of enzyme amylase and proteases. The seedling growth in
Glycine max and growth in buds in many woody angiosperms is inhibited by ABA even at very low
concentrations.
5. Abscission and Senescence: ABA induces senescence of leaves, abscission of leaves, flowers and fruits.
It causes ageing and promotes and promotes development of abscission layer in stalks of flowers, leaves,
fruits thus affecting their shading or fall off from the plant body. Leaf senescence and yellowing of leaves
are found to be stimulated by ABA.
6. Anti-gibberellin Effect: ABA generally inhibits the action of gibberellin in many plants called anti-
gibberellin effect. It also inhibits GA promoted ER synthesis and β-amylase.
7. Fruit Growth and Ripening: ABA plays significant role in fruit growth and ripening. ABA seems to
help fruit ripening, coincided with stoppage of fruit growth and initiation of colour. This is because
induction of ethylene by ABA.
• Practical application: In horticultural practices, for early ripening of fruits, ABA sprays will be helpful
for premature harvest of fruits to yield more. Unripened Citrus and Tomato fruits due ABA treatment ripen
early.

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