3. The discovery and synthetic production of
PGRs have been one of the outstanding
achievements of the modern plant science,
particularly, in horticultural crops.
In 1928, Dutch Botanist, F. W. Went
remarked “Ohne Wuchstoff, Kein
wachstum” which means “ without auxins,
there is no growth”
Auxins and other allied growth regulators
like gibberellins, kinins and inhibitors play
multiple role in basic life processes.
4. Their notable effects are
Prolonging or shortening of dormancy of buds,
seeds or tubers
Enhancing rooting capacity of cuttings
Parthenocarpic production of fruits e.g. tomato,
grape, pear etc.
Increase in fruit set
Thinning of fruits
Preventing fruit drops especially pre harvest fruit
drop of apple, pear, citrus etc.
Hastening of fruit maturity
Inhibition of bud growth
Used as powerful and selective herbicides
5. After their discovery, voluminous work has
been done on evolving
Best techniques of application
Effective doses
Effect on tree and fruit
Best time of application
Dependability of the methods under variable
environmental conditions
Other considerations of their effective uses in
commercial cultivation of horticultural crops.
6. Important Definitions
Hormone:
It is a substance produced in any part of an
organism, transferred to another part where
it influences a specific physiological process.
Literally meaning is “ I arouse into activity”.
It is a Greek word and this term was first
given by Starling in 1906
7. Phytohormone:
It is a hormone or regulator specifically
produced in the plant and regulate plant
physiological processes.
Growth Hormone:
It is the phyto-hormone involved in growth
i.e. essential for cell enlargement.
Auxin:
An organic substance which promotes
growth (i.e. irreversible increase in volume)
along the longitudinal axis.
8. Auxin
Growth hormone is an itself auxin
The growth hormone may be an auxin but
not all auxins are hormones.
Synthetic auxins have been mistakenly
referred to as hormones.
According to Leopold (1960), the term
hormone should be used to refer to growth
regulators which are to be present in any
organism or plant
9. Growth Regulator
It is an organic compound, other than
nutrients, which in small quantity (may be in
1 ppm dose) is capable of modifying any
physiological process in plants.
These are the substances which stimulate,
inhibit or otherwise alter growth (Leopold,
1960).
10. These substances include:
Auxins
Antiauxins
Epinastic agents
Gibberellins
Kinins
Inhibitors
They may promote or inhibit process even if
present in minute quantities.
Their usefulness is not increased by raising
the concentration above the recommended
strength.
11. Auxin Synergists:
These are the compounds that will increase
the effectiveness of the auxins. They cannot
promote growth in auxin free medium. e.g.
ascorbic acid (Vit. C) and TIBA (2, 3, 5-tri-
iodobenzoic acid) in very small quantity.
Auxin Antagonistics:
These are the substances which are
themselves inactive such as trans-cinnamic
and DCA (2, 4-Dichloroanisole) but
counteract the activities of auxins.
12. Anti-Auxin:
These are the compounds which
competitively inhibit the action of auxins. e.g.
Caumerin, par-ascorbic acid (naturally
occuring compound) and phenyl butyric acid
(a synthetic compound).
Epinastic Agent:
These are the substances which bring about
swelling of cells causing twisting and
deformation of leaves, stem etc., instead of
bringing about normal cell elongation. e.g.
ethylene, 2, 4-D.
13. Gibberellins:
These are defined as substances possessing
the same carbon skeleton as gibberellin A3
(GA3) or very closely related to it and
biologically active in stimulating cell
division, cell elongation or both in plants.
Kinins:
It is a group of different substances of small
molecular weight possessing hormone like
properties and which stimulate cell division.
14. It should be remembered that one compound
may not show one type of growth effect.
For example:
An auxin may also have epinastic activity
An auxin synergist may also have inhibitors
effect.
16. Auxins
1st phytohormone recognized and detected in 19th
century.
Greek term “Auxein” means “to increase”
Indole acetic acid (IAA) is considered the most
physiologically active auxins in plants.
Auxins are involved in a variety of diverse plant
growth and developmental responses.
Commercial application e.g. IBA and NAA are used
for initiation of rooting especially in root cuttings.
Biosynthesis of auxins is not limited to higher plants
but microorganisms (bacteria, fungi, actinomycetes
and algae) are also involved in this process.
17. Darwin (1880) used auxins in experiments on
phototropism published in book “power of
movement in plants”. First scientist, who
gave the idea of growth regulators.
Studied the seedlings of Avena sativa (Oat)
and concluded that there is some compound,
which is transmitted from upper to lower
part of the coleoptile in response to light.
That compound was the auxin.
(Sachs, 1880) in Germany, also gave the
theory of growth controlling substances,
which move in different polar directions.
18. In 1894, Rothert, confirmed the Darwin’s findings.
In 1909-10, Hans Fitting, extracted the active
substance from the pollens of orchid and applied it
on flowers, which caused petals to drop and ovary
to swell.
Boysen Jensen, (1910-19130, a German scientist,
showed that only the decapitation of the tip
removed the phototropic sensitivity but the
replacement of the tip restored the stimulus again.
Paal, (1914-19), stated that shoot tip is the site of the
growth regulator substances, which are secreted
internally and distributed equally. He explained
that reason for bending of shoot in light only is
because, light cause its equal distribution.
19. Soding, (1925), confirmed the Paal's theory of
diffusible substance secreted by the tip.
Cholodny, (1924, 1926, 1927) (Russian) reported that
all tropism were due to a diffusible substance.
Went, (1926-28) (Dutch) was the first to isolate the
active substance from a coleoptile tip into agar and
reported that the tropistic response and the
endogenous control of growth rates were due to a
specific substance.
Nielsen, (1928), reported that the medium on which
the fungi Rhizopus suinus or Alisidia ramosa had
grown was rich in a substance active in producing
curvatures in Avena.
20. Nielson, (1930); Dolk and Thimann (1932)
extracted this substance from Rhizopus suinus.
Kogl and his associates (1931), Holland,
isolated two active materials named as auxin
‘a’ and auxin ‘b’. Auxin was first obtained
from human urine which cause strong
bending of Avena coleoptile.
In 1934, they isolated auxin ‘b’ from malt and
corn germ oil.
In 1934, they again found another substance
from urine, giving auxin activity and given
the name “hetero and yeast auxin”.
21. Thimman and Skoog (1933) found that
apical dominance is controlled by auxins and
by removing the apical portion, lateral
branches grow very rapidly.
Went (1934) also found that auxin were also
involved in root initiation.
Thimann (1935) later identified active
substance produced by R. suinus as IAA.
Gustafson (1936) said that auxins were
involved in fruit setting and development.
Haagen-smit et al (1942-46) isolated IAA
from corn and wheat, also confirmed by
Berger and Avery (1944).
22. Skoog-Tsu (1948) proclaims that nature of growth
depends upon ratio of auxins present in tissue.
Okamato et al., (1967) isolated IAA from Phaseolus
mungo. IAA has been isolated from many sources
and is widespread in higher plants. It is found to be
found in
Growing shoot tips
Young leaves
Pollen
Young ovaries of flower
Young fruit and cotyledons of germinating seed
23. Gibberellins
Kurosawa, (1926) discovered a substance in fungus
culture, that caused the elongation of rice seedlings,
during the investigation of “Foolish Seedling
Disease” of rice.
Yabuta and Sumike, (1938) isolated crystals of
substance from G. fujikuroi culture and called them
Gibberellins.
Brian and co-workers (1951) in England, found
unique ability of gibberellins to stimulate plant
growth. More than 60 gibberellins were discovered,
some from
Fungi (GA 1, 2, 3, 4, 7, 9 and 10 to 13)
Higher plants (GA 5, 6 and 8)
Anton Lang, (1957), found that gibberellin can
induce flowering.
24. Kinins
Van Overbeek et al., (1941), showed that a naturally
occuring substance found in coconut milk had the ability
to promote cellular proliferation in young Datura
embryos.
F. Skoog, C. O Miller and colleagues, in 1950s
discovered cytokinins in the studies of the growth
requirement and differentiation of cell cultures.
Miller et al., (1955), reported isolation and identification
of kinetin.
Skoog and co-workers (1956), reported another group of
low molecular weight and capable of inducing cell
division. They are Kinins (salisbury, 1957). Kinin is
derivative of adenine (building block of DNA). Later
identified as 6-furfuryl amino purine (breakdown
product of DNA). Native kinin extracted from coconut
milk, apple, fruitlets and germinating peas.
25. Abscisic Acid
Regulate seed maturation and maintenance
of embryo dormancy.
Mediates several adaptational responses
towards abiotic stress and act as negative
growth regulator.
Bennet-Clark and his colleagues, 1950s,
done initial work on this endogenous plant
hormone and found its inhibitory properties.
Liu and Carns (1961), isolated a substance
from mature cotton fruit and found that it
stimulate abscission of cotton petioles.
26. Ethylene
Affect seed germination, influence sex
determination and promote fruit ripening.
Earlier used by Egyptian and Chinese for fruit
ripening.
Neljubow (1901) determined that gaseous ethylene
is capable of altering the development of higher
plants.
Doubt (1917) discovered its abscission properties.
Denny (1924) demonstrated its ripening properties.
Gane (1934) identified it as natural plant product.
27. Brassinosteroids
Group of naturally occurring poly hydroxy steroids.
Mitchell et al., (1960) identified a new class of
lipoidal hormone, extracted from rape pollens and
termed Brassin.
Mitchell and Gregory, (1972) showed that brassin
could enhance crop yield, efficiency and seed vigor.
Milborrow and Pryce (1973) believed that brassin
were crude extract containing gibberellins and other
compounds rather than endogenous lipids.
Grove et al., (1979) identified brassinolide as the
active component in brassins.
28. Jasmonates
Promote senescence and regulate plant growth,
stimulate germination of dormant seeds.
Play role in formation of flower, fruits and seeds.
Highest conc. Found in growing tissues like shoot
apex, root tips, immature fruits and young leaves.
Demole et al. (1962) isolated (-) jasmonic acid
methyl ester from the essential oil of Jasminum
grandiflorum.
Later found that they are widespread in
angiosperm, gymnosperm and algae, indicating
their importance in plant physiology.
29. Polyamines
Accepted as plant hormone at the
international conference on plant growth
substances in 1982.
Galston and Kaur-Sawhey (1990) observed
polyamine involvement in embryogenesis.
Galston classified them hormone, on the
basis of
Widespread in all cells and regulate growth
and development at micromolar conc.
Development of plant is affected, when the
polyamine contents are genetically altered.
30. Salicylate
Johann Buchner (1828), isolated trace amounts of
salicin in willow bark.
Raffaele Piria (1838) named the active ingredient in
willow bark as Salicylic acid (SA) from the Latin
word, meaning “willow tree”.
In 1874, first commercial production of SA in
Germany.
Exogenous application of SA stimulate flowering,
inhibits biosynthesis of ethylene, stomatal closure
and ion uptake.
31. According to Zimmerman, about 500
compounds have properties of growth
regulators.
Goodwin, 1954, introduced another group,
known as unsaturated Lactones as growth
inhibitors. In general, lactones are coumarin,
scopoletin, protoanemonin and parascorbic
acid (Salisbury, 1957).
F.C Steward (1956), discovered another
group as “Coconut milk factors”. Four
substances, identified as having growth
stimulating activity, have been isolated from
cocnut and one is known as diphenyl urea.
