Get complete information on the Plant Hormones


Auxin :

i. Weak organic acids with the acidic group.

ii. Indole-3-acetic acid (IAA) and other natural or synthetic growth regulating substances having structures and functions similar to IAA are called auxins.


iii. First plant growth hormone to be discovered.

iv. First indication of their existence came from the work of Darwin (1880).

v. F.W. Went (1928) proved that the coleoptile tips contained a substance auxin and found that the degree of curvature of coleoptile is proportional to the amount of active substance in the agar blocks. This test is known as avena curvature test.

vi. Maximum concentration of auxins are found in the growing tips of the plant. From tip, it is transported towards the base.


vii.Same concentration of auxin exhibit different responses in different parts of the plant.


Cell elongation (stimulate the cell walls to stretch in more than one directions) ; cell division (in the vascular cambium that results in secondary growth) ; phototropism ; geotropism (high concentrations of auxins are stimulatory for stem i.e., negatively geotropic, on the other hand, higher concentrations of auxins are inhibitory for roots i.e., positively geotropic) ; apical dominance (auxins in apical bud inhibit the development of lateral buds) : prevention of abscission layer (leaves and fruits have to produce auxin continuously to prevent the formation of abscission layer) and root initiation (very low concentration of auxins promote the growth of root).

Auxin treated plants have been found to develop fruits even in the absence of pollination and seeds.


Auxins stimulate the activity of enzymes. They exhibit inhibitory effect on isoenzymes like peroxidase in tobacco. Auxin precursors : Compounds which can be converted into auxins.


Compounds which inhibit the actions of auxins.



Auxins which can easily be extracted. Active form. Bound-auxins : Auxins which are hard to extract and need the use of organic solvents. Inactive in growth.

Auxin bioassays:

The Avena curvature test and root growth inhibition test are the bioassays for examining auxin activity.

Gibberellins (GA) :


i. The discovery of gibberellin (GA) was based upon the observations made in Japan in early 1800s on the bakane or foolish seedling disease of rice.

ii. Widely distributed in nature particularly in angiosperms.

iii. Chemically, all GA are terpenes atjd weak acids.

iv. Found in abundance in young, expanding organs; being synthesised especially in embryos, young apical leaves, buds seeds and root tips.


Overcoming genetic dwarfism (help in stem elongation) ; bolting (elongation of intermodes to flowering in hosette plants) and flowering (e.g., Cabbage); breaking of dormancy and germination of seeds (after imbibitions of water, the embryo secretes gibberellin which diffuses to the aleurone layer, stimulating the synthesis of several enzymes including amylase, proteases, lipases) ; breaking of bud dormancy ; control of flowering (promote flowering in some long days plants even under short day conditions, control of fruit growth; promoting growth (in both stems and leaves) ; overcoming vernalisation; stimulate maleness (e.g., cucurbits and Cannabis) ; increases fruit yield etc.

Gibberellin bioassay is performed through dwarf maize/pea test and cereal endosperm test

Yabuta (1935) coined the term gibberellin.

About 100 different types of gibberellins have been isolated. They are denoted as GA1 GA2, GA3 and so on.

Antigibberellins like maleic hydrazide, phosphon D and chorocholins chloride (CCC) are also called antiretardants.

Cytokinins :

Skoog (1954 – 1956) found that coconut milk contained a substance which stimulated cell division in tobacco pith cultures.

Miller and his colleagues isolated and purified the substance in crystalline form from herring sperm DNA. This substance was identified as 6-furfuryl aminopurine. They named this compound kinetin because of its property to activate cell division (cytokinesis).

First naturally occurring cytokinin to be chemically identified was from young maize (Zea mays) grains in 1963, and was called zeatin.

Cytokinins are quite abundant wherever rapid cell division occurs, especially in growing tissues such as embryos, developing fruits and roots. In mature plants, these are frequently synthesized in the roots and move to the shoots through xylem.


Cell division and differentiation (in the presence of auxins, cytokinin promote cell division even in non-meristematic tissues) ; control of apical dominance (cytokinin stimulate the growth of lateral buds) ; delaying senescence (by controlling protein synthesis and mobilisation of resources) ; induce flowering ; breaking the dormancy of seeds and initiation of interfascicular cambium.

The common bioassays for cytokinins include promotion of cell division in tobacco pith culture, expansion of excised radish cotyledons, delay in senescence etc.


i. Crocker et. al. (1935) identified ethylene as natural plant hormone.

ii. Ethylene is formed from amino acid methionine.

iii. Auxin IAA is known to stimulate ethylene formation.

iv. Only gaseous natural plant growth regulator.

v. It does not generally move through the air spaces in the plants. Rather, it escapes from the plants surface.


Apical dominance (in pea seedlings and suppresses the growth of apical buds) ; stem swelling (transverse expansion of stem) ; leaf abscission (formation of abscission zone is leaves, flowers and fruits) ; epinasty (leaf bending) ; fruit growth and ripening (a rise in the rate of respiration) ; accelerates flowering ; sex expression (increases the number of female flowers and fruits in cucumber plants) and graviperception (reduces sensitivity to gravity, seedlings develop tight epicotyl hook).

Commercial compound “Ethaphone (2- chloroethyl phosphoric acid) breaks down to release ethylene in plants.

Abscisic Acid (ABA) :

i. Fredericle Addicott (1963) and P.F. Wareing (1964) discovered this hormone.

ii. Liu and Crans (1961) isolated ABA from mature cotton fruit.

iii. It is also called stress hormone or dormin because the production of hormone is stimulated by drought, water logging and other adverse environmental condition.

iv. Chemically, ABA is a Terpenoid.

v. ABA synthesized in the leaves, stems, fruits and seeds and distributed through the vascular system especially phloem.

vi. Isolated chloroplasts can synthesize it either from mevalonic acid or xanthophyll like violoxanthin.


Growth inhibitor (antagonistic to all the three growth promoters) ; bud dormancy (by inhibiting growth processes) ; senescence (of leaves) ; abscission (of leaves, flowers and fruits in plants) ; stomata closure ; hardiness (promotes cold hardiness and inhibits growth of pathogens) ; promotes flowering (in some short day plants) etc.

It is a stress hormone i.e., helps the plants to cope with adverse environmental conditions.

ABA is sprayed on tree crops to regulate fruit drop at the end of the season.

Vitamins :

i. Heterogenous group of organic compounds, needed in very small quantities for different metabolic processes.

ii. Plant stores the vitamins in different plant organs.

iii. Most of the vitamins function as coenzymes and prosthetic groups of various enzymes connected with protein, fat and carbohydrate metabolism.

iv. Some important examples of vitamins are: vitamins A, B, C, D, E and K.

v. Some vitamins are essential in maintaining cell membranes and acting as antioxidants.

Photoperiodism :

i. The response of plants to the relative lengths and alternations of light and dark periods with regard to the initiation of flowering is called photoperiodism.

ii. The effect of photoperiods on flowering was discovered by Garner and Allard (1920) in case of Maryland Mammoth variety of Tobacco (an SDP).

iii. On the basis of length of photoperoid requirements of plants, the plants have been classified into:

(A) Short-day plants (SDP):

Plants requiring light for a shorter period than their critical period e.g., Xanthium, Glycine max, Rice, Potato, Tobacco etc.

(B) Long-day plants (LDP):

Plants requiring longer exposure to light than their critical period e.g, Lettuce, Radish, Sugarbeet, Oat, Wheat etc.

(C) Day-neutral or intermediate day plants (DNP):

Plants which would flower irrespective of the photoperiod, once they are mature e.g., Cucumber, Tomato, Maize, Cotton etc.

(D) Short-long day plants (SLDP):

Plants requiring first short days and then long days e.g., Trifolium repens, Campanula medium etc.

(E) Long-short day plants (LSDP):

Long photoperiods for initiation of flowering and short photoperiods for blossoming e.g., Bryophyllum, Cestrum etc.

Critical dark period:

It is actually the length of dark period which is critical.

Critical day length:

Continuous duration of light which must always be exceeded in long day plants and must not be exceeded in short- day plants.

Short-night plants (SNP):

Long day plants do not require any continuous dark period.

Long-night plants (LNP);

Short-day plants and any interruption of critical dark period even by a flash of light prevents flowering. The reaction is called light break reaction.

Photoperiodic induction:

Phenomenon of perceiving appropriate light periods and obtaining the stimulus of flowering.

Flowering hormone:

Chailakhyan (1937) proposed the view that flower hormone namely florigen in synthesized in the leaves under favourable photoperiodic conditions. This hormone is transmitted to the growing point where the flowering takes place.

Vernalization :

Term Vernalization was coined by Lysenko (1928) to refer to the method of accelerating the flowering ability of biennials or winter annuals, by exposing their soaked seeds to low temperature for a few weeks.

The various requirements of vernalization are:

(i) Actively dividing cells

(ii) Proper nourishment

(iii) Proper hydration

(iv) Continuous low temperature for a few days

(v) Aerobic respiration.

The site of vernalization is believed to be shoot apical meristem.

Vernalisation effect is reversible and the reverse process is called devernalization.

The vernalisation stimulus can be transmitted between plants by grafting hence the stimulus is believed to be a hormone called vernalin.

A. Lang discovered that during vernalization, the levels of gibberellins increase and also that the application of gibberellins to unvernalised plants can substitute for vernalization.

Senescence :

The sum of deteriorative processes which naturally terminate that functional life of an organism.

Ageing is a sum total of changes in the total plant or its constituents while senescence represents degenerative and irreversible changes in a plant.

Senescence may be whole plant senescence (e.g., monocarpic plants) ; shoot senescence (some perrennials e.g., banana, gladiolus) ; sequential or progressive senescence (perrennials e.g., mango, Eucalyptus) and simultaneous or synchronous leaf senescence (perrennial polycarpic deciduous plants e.g., maple, elm, poplar etc.).

Senescense can be delayed by removing flowers from the plants.

Abscission :

Shedding of leaves, foliage branches, floral parts, fruits etc. without causing any injury is called abscission.

It generally occurs due to change in the hormonal balance.

Abscission zone is usually located at the base of petiole which is made up of two distinct layers i.e.,

(i) The abscission (or separation) layer where breakdown of cells takes place and

(ii) The protective layer, which develops next to abscission layer to protect the exposed surface.

Abscisic acid is known to play an important role in abscission.

Plant Movement :

Plant movement is change in position and these are not as much apparent as in case of animals.

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