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Plant Hormones


Table of Content


1. First idea of plant hormones was given by Von Sachs "organ forming substance."

2. Ist Plant hormone discovered by F.W. Went was Auxin but term hormone was given by Starling & Phytohormone by Thieman.

Chemicals which are acts as natural Phytohormones are called as synthetic growth hormones. Synthetic Auxins

a-& b-NAA, 2, 4-D, 2, 4, 5-T, IPA PAA, IBA Maleic hydrazide (MH).

NAA-Napthalene acetic acid

2-4-D - 2, 4-Dichlorophenoxy acetic acid.

2, 4, 5- T - Trichloro phenoxy acetic add.

IPA - Indole propeonil pyruvic acid

PAA - Phenyl/phenoxy acetic acid·

Factors related to growth of plant


Charles Darwin & F. Darwin (1880) was the first one to study phototropism. They observed coleoptile bending in Canary grass (Phalaris). Wrote, Book "Power of movements in plants" Term­"Stimulus" By Darwin.

Boysen & Jensen 1910: Experiments on oat (Avena sativa) Plant.

In the first experiment he removed the coleoptile tip and then replaced it on stump.

On providing uniiaterallight the coleoptile tip gave positive curvature.

They observed that if gelatin inserted between the tip & cut stump, then coleoptile bends towards the unilateral light. If mice were inserted then coleoptile fail to phototropism.

Material substance term for growth hormone was given by him.

Paal: Demonstrate that when the cut tip was replaced on cut end (stump) eccentrically (asymmetrically) it cause bending even in dark.

If coca butter or platinum foil inserted there is no curvature observed.

Idea of unequal distribution of growth substance given by Paal. Large amount of substance found on shaded side.

F.W. Went (1928): Went isolated the growing tip of Avena sativa on agar plate & perform Agar-block experiment.

He give name auxin to growth substance, thus credit of Auxin discovery goes to F.W. Went.

He also found that the curvature (bending) in Avena coleoptile is proportional, with in limits to the amount of Auxin in Agar - block. This test was named as Avena curvature-test (Bioassay of auxin). Went found that 27% auxin present on illuminated side & 57% on the dark side. (About 16% auxin lost on illuminated side & rest transferred to base) Transport of natural Auxin is basipetal & polar type.

(Synthetic ® Auxin = A polar transport)

Kogl & Hagensmit (1931): Isolated an active substance from urine-of pellagra patient which was called as Auxin - A or chemically auxenotriolic acid (C18H32O5). 

Later a similar substance was isolated from corn grain oil and was named as Auxin-B or Auxenolonic acid. (C18H30O4).

Again Kogl, Erxleben & Haagen Smit 1934 - Isolated another substance from human urine and named as Heteroauxin

(IAA-C10H9O2N) by Thimann,

Auxin from Rhizopus was obtained by Thimann.

Auxin biosynthesis occurs by tryptophan amino acid in the presence of Zn++ ion.

Degradation or oxidation of Auxin

Enzymatic (by IAA oxidase)


Now IBA (Indole Butyric acid) have been also isolated from plants (natural auxin) but IAA is most widely found auxin in plants.

The compounds which can be converted into auxin are called as auxin precursor, whereas the compounds which inhibits the activity of auxin are called as antiauxins.

Indole Acetic Acid

Natural auxins: These are naturally occurring auxins in plants and therefore; regarded as phytohormones. Indole 3-acetic acid (IAA) is the best known and universal auxin. It is found in all plants and fungi.

The first naturally occurring auxin was isolated by Kogl and Haagen-Smit (1931) from human urine. It was identified as auxin-a (auxenotriolic acid, C18H32O5).s Later, in 1934 Kogl, Haagen-Smit and Erxleben obtained another, auxin, called auxin-b (auxenolonic acid, C18H30O4) from corn germ oil (extracted from germinating corn seeds), and. heteroauxin from human urine. Heteroauxin (C10H9O2N) also known as indole-3-acetic add (IAA), is the best known natural auxin, Besides IAA, indole-3-acetaldehyde, indole -3-spyruvic acid, indole ethanol, 4-chloro-idole aerie acid (4-chloro-­IAA) etc., are some other natural auxins. 

Physiological Effects and Applications of Auxin

Apical Dominance (Characteristic function of auxin):

The phenomenon in which apical bud dominates over the growth of lateral buds is called Apical Dominance. Prunning in gardens promotes densing of hedge.

Cell Division & Cell Enlargement/Callus formation

Auxin is important in Tissue culture & Grafting. It stimulates division of intrafascicular cambium. Also in healing of wounds.

Shortening of Internodes: a-NAA induces the formation of dwarf shoot or spurs in apple, pear etc., thus number of fruits increases.

Prevention of lodging: Auxin spray prevents lodging of crops, immature leaves & fruits.

Root initiation: Rooting on stem cuttings is promoted by IEA & NAA (Root growth inhibited by auxin).

Potato dormancy: MH (Maleic-Hvdrazide). a-­NAA., induces dormancy of lateral buds in potato tubers & potato can stored for long duration.

Prevention of AD scission: IAA, NAA prevents premature abscission of plant organs.

Flower initiation: Auxin is inhibitor of flowering but it promotes uniform flowering in Pine apple & Litchi plants.

Parthenocarpy: Seed less fruits can produced by spray of IAA. (By Gusteffson)

Selective weed killer: Dicot broad leave weeds can be eradicated by 2, 4-D & 2, 4, 5-T.

Agent orange is used in biowar. It was used by USA against Vietnam (1966-60).

Femaleness: Feminising effect in some plants.

Flower & Fruit thinning: Certain trees like mango form less number of fruits in alternate years. But auxins can produced normal fruit crops every year. This is known as fruit thinning.

When Antiauxin (TIBA-Tri-Iodo-Benzoic acid) are sprayed on mature cotton field then cotton balls can picked easily.


Bioassay means the testing of substance for it's activity in causing a growth response in a living plant or it's parts.

(i) A vena curvature test: Avena curvature test carried out by F.W. Went (1928), demonstrated the effect of auxins on plant growth by performing some experiments with the oat (Avena sativa) coleoptile.

(ii) Root growth inhibition test: are bioassays for examining auxin activity.


Gibberellic Acid

First of all Japanese farmers observed peculiar symptoms in rice seedlings & called the bakanae disease (Foolish seedling disease)

Rice plants become thin, tall & pale due to infection of Gibberella (Ascomycetes) or Fusarium (Duteromycetes) confirmed by Kurosawa & Swada.

Yabuta and Sumiki 1938 were first to extract a crystalline substance from the Gibberella fungus, which they named as Gibberellin.

Gibberellin is acidic & posses a Cibben ring structure that are able to overcome genetic dwarfism in plants.

100 type of Gibberellins (GA1, GA2, GA3, ……… GA100) are known GA3 [C19H26O6] is representative of all Gibberellins. First discovered Gibberellins from higher plants was GA1 (GA1 & GA20 are common GA's of higher plants)

GA found in all group of plants (algae to angiosperms) but as a flowering hormone acts only in angiosperms.

Biosynthesis of gibbereilin takes places by Mevalonic acid pathway (Kaurene ® GA)

Physiology Effects and Applications
Stem/internode elongation (characteristic function of gibberellins): GA induces internode elongation, leaf expansion & used in sugarcane cultivation.(Gibberellins induce stem elongation in Rossete plants (Cabbage). This phenomenon known as Bolting effect. (Elimination of rosset habit in some plants by gibberellins action is called bolting)

Elongation of genetic dwarf plants:

When gibberellin are applied to dwarf Maize, Pisum &. Vida faba, then they become tall. The rossett plant iii sugarbeet indicate an extreme dwarfism, this habit can be eliminate by GA.

Flowering in LDP, in short right duration: Shorting of life cycle)       

Parthenocarpy: Like Auxin, exogenous use of GA also induces the formation of seedless fruits.

Substitution of cold treatment or Vemalisation:

The biannual plants form their vegetative body in the 1st year. Then they pass through a winter season & produce flower & fruits in IInd year. GA induce flower in first year.

Breaking of dormancy: GA breaks the dormancy of seeds, buds and tubers.

Seed germination: Gibberellins induce the synthesis of hydrolysing enzymes like oamylse, Lipases & Proteases.

Sex expression: GA induce maleness in Cucumis, Cannabis.

Germination of Photoblastic seeds: Gibberellin treated light sensitive seeds can germinates in dark. Ex. Lettuce, Tobacco.

Fruit & Flower enlarger: Size of grape fruits & bunch & Geranium flowers can be increased by GA

Pomalin Þ GA (GA4 & GA7) + CK(6- Benzyladenine) - acts as apple enlarger.

In fermentation: More growth of yeast cells by CA.

Increase height of sugarcane 'plant: (More sugar contents by IAA)

Bio assay:

1. a-amylase activity test in Barley endosperm:

Endosperms are detached from embryos, sterilized and allow to remain in 1rnl of test solution far 1-2 days. There is build up of reducing sugars which is proportional to GA concentrations. Reducing sugars do not occur in endosperms kept as control.

2. Dwarf Pea & Maize test: Seeds of dwarf pea are allowed to germinate till the just emergence of plumule. GA solution is applied to some seedlings, others are kept as control. After 5 days, epicotyl length is measured. Increase in length of epicotyl over control seedlings' is proportional to GA concentration.


Cytokinin was discovered by Miller when he was working (in lab. of prof. Skoog) on tobacco pith culture. He added the contents of an old DNA­ bottle (Herring fish sperms DNA) to the culture medium & observed that the-tobacco pith callus could grow for longer period.

Miller isolated an active substance from autoclaved DNA from Herring sperm, stimulated cell division.

He named this substance as kinetin

Term cytokinin By Letham, Phytokin by Osborne and Kinin by Skoog.

The first natural cytokinin: was identified & crystallized from immature corn grains by Letham & named as Zeatin.

The most common cytokinin in plants are Zeatin and isopantanyl adenine.

Cytokinin is a derivative of Adenine base.

Root tips are major site of synthesis of CK (by Mevalonic acid pathway).

Movement of cytokinin is polar & Basipetal.

Coconut milk factor also performed activity like cytokinin, thus used in tissue culture.

Zachau obtained cytoknins from serine-t-RNA of yeast.

BAP (Benzylamino purine), Diphenylurea and Thidiazuron are synthetic cytokinins.

Physiological Effects and Applications

Cell Division (Characteristic function of cytokinin) & Cell Enlargement: One of the most important biological effect of CK (cytokinin) on plants is induction of cell division. In tissue culture also.

Formation of inter fascicular cambium and induce secondary growth.

Morphogenesis: Morphogenetic changes induces by CK in presence of IAA.

High auxin + low CK - Root formation

Counteraction of apical dominance: Promotes growth of lateral buds.

Breaking the dormancy of seeds: Like GA the dormancy of certain seeds can be broken by CK.

Seed germination: Seeds of parasite plant (Striga) can germinates in the absence of host by CK treatment.     

Delay in senescence (Richmond Lang effect): The ageing process of leaves usually accompanies with loss of chlorophyll and rapid catabolism. This is called as senescence. Senescence can be postponed by CK. (increase short life of plant parts)

Lignin biosynthesis

Parthenocarpy in some fruits

Pro-plastids modification

Phloem conduction (Nutrients Mobilisation)


Flowering in SDP (also in long days)

Induce stomatal opening


1. Tobacoo pith cell division test: Tobacco pith culture is divided into two weighted lots. One supplied with cytokinin and the other without it. After 3-5 weeks, increase of fresh weight of treated tissue over control is noted. It is a measure of stimulation of cell division and hence cytokinin activity.

2.  Chlorophyll preservation (retention) test (delay in senescance test): Leaves are cut into equal sized discs with the help of a cutter. They are divided into two lots. One lot is provided with cytokinin. After 48-72 hours, leaf discs are compared for chlorophyll contents. Cytokinin retards chlorophyll degradation.

3. Soyabean and Radish cotyledon cell division test: Excised radish cotyledons are measured and placed in test solution as well as ordinary water (as control). Enlargement of cotyledons indicates cytokinin activity.


First indication of growth inhibitors was given by Osborne.

First growth inhibitor was identified by Bennet ­Clark and Kefford (1953) from dormant potato tuber and called it b-inhibitor.

Addicott & Okhuma (1963) obtained from mature cotton fruits and named as Abscisin II. (C15H20O4)

Warieng & Robinscn: Isolated a growth inhibitor from Old Betualleaves & called as Dormin.

ABA is most wide spread growth inhibitor in the plants.

ABA synthesized by Mevalonic acid pathway & oxidation of carotenes in chloroplasts.

ABA also known as stress hormone because it protects plants from adverse conditions like water stress. ABA increases tolerance of plants to various type of stresses.

Physiological effects and applications of ABA:

Induce abscission: ABA causes ageing and abscission of leaves & fruits (Antiauxin) (Cellulase & Pectinase genes induced by ABA)

Induce Bud & Seed Dormancy: ABA regulates (anti-GA) bud & seed dormancy.

ABA plays a major role in seed maturation enabling seeds to become dormant.

Induce senescence: ABA accelerates senesce of leaves.

Inhibition of cell division" and cell elongatn Anti CK

Stomatal closing: ABA causing the stomatal des under the water stress conditions. Increases resistance to frost injury. (Anti transpirant & stress hormone)

Delaying of flowering in LDP.

Tuberisation in potato.

Inhibitor of a-amylase synthesis: Inhibition of ­seeds germination.

Geotropism in roots.

Growth inhibition in Duckweed (Lemna) Bioassay:

(1) Rice seedling growth inhibition test: Mohanty, Anjaneyulu and Sridhar (1979) used rice growth inhibition method to measure ABA like activity. The length of second leaf sheath after six days growth is measured.

(2) Inhibition of a-amylase synthesis in barley endosperm test: ABA inhibits the synthesis of amylase in the aleurone layers which is triggered by gibberellins. Goldschmidt and Monselise (1968) developed the bioassay method to estimate ABA activity by determining the extent of inhibition a-amylase synthesis induced by treating barley see endosperm with GA.  


H.H. Cousin first suggested that ripened orange are responsible for ripening of unripen bananas.

Ethylene is a gaseous pollutant hydrocarbon bi Burg reported it at a fruit ripening hormone.

Pratt Goeschl - Recognized ethylene as a nature plant growth regulator.

Biosynthesis of ethylene takes place by methionine amino acid. Ethyelene is synthesized in large quantity by ripening fruits and senescent organs.

Ethylene also formed in roots in water logged, condition.

Physiological effects and applications ethylene:

Post harvest ripening of fruits: Citrus, Oranges, Banana, Apple, Tomato, Today Ethephon/CEPA. (Chlorethyl Phosphoric acid) used at commercial level.

Stimulation of senescence & abscission of leaves: Ethylene is synthesized in large quantity by ripening fruits and senescent organs.

Flowering in pineapple.

Triple response on stem

Inhibition of stem elongation

Stimulation of radial swelling of stem

Horizontal growth of stern (Ageotropism)

Inhibits root growth: Ethylene is inhibitor of root growth but stimulates the formation of root hairs.

Epinasty of leaves.

Femaleness (Feminsing effect): Pineapple (Bromeliaceae)

Tightening of hooks of epicotyl and hypocotyls.

Inhibits the polar movement of auxin.

Bioassay: It is done on the principle of triple response which includes three characteristic effects of ehtylene on etiolated seedlings of pea-viz.

Swelling of nodes.

Inhibition of elongation of internodes of stem.

Induction of horizontal growth of stem against gravity.

Triple pea test: Pratt and Biale (1944) developed this method for bioassay of ethylene which is based on the physiological effect of ethylene to cause.

Subapical thickening of stern.

Reduction in the rate of elongation.

Horizontal nutation (transverse geotropism) of stem in etiolated pea seedlings. In presence of ethylene, epicotyls show increase growth in thickness and reduced rate of longitudinal and horizontal growth.

Pea stem swelling test: Cherry (1973) used pea, seedlings to measure ethylene concentration by marked increase of stern swelling expressed as a ratio of weight to length. In one ppm of ethylene the ratio is about 4.0.

Other Growth Regulating Substances
Wound hormone Traumatic acid:

Induce callus formation on injured parts (Healing of wounds) Chemically traumatins are auxin like substance.

Calines (Formative hormones)

Rhizocalines: Produced by leaves & induce formation of roots

Caulocalines: Produced by roots & induce formation of stem.

Phyllocalines: (Self forming hormone) produced in cotyledons & leaves, induce division of leaf mesophylls.

Morphactins or (HFCA):

These are, synthetic growth inhibitors, which are polyvalent (wide range) in action (Antiauxin property)

Inhibition of internode elongation

Reduction of apical dominance & promotion of lateral branching.

Reduces the laminar area of leaf.

Chlormequant (CCC or Cycocel: Growth inhibitor, which is used in bonsai.

Alar-85 (B-Nine): In Floriculture.

Agen organge: Mixture of 2, 4-D & 2,4, 5-T used in Bio-war. (Used by USA In Vieatnam war)

Amo-1618: In Biowar.

Phosphon-D, Cycocel, Amo-1618, Alar-85, Ancymidol (A-REST) are anti gibberellins and cause inhibition of stem growth.

Jasmonic acid (Jasmonates): According to Parthier (1991), jasmonic acid and its methyl esters are ubiquitous in plants. They have hormone properties, help regulating plant growth, development and they seem to participate in leaf senescence and in the defense mechanism against fungi.

Just like ABA jasmonates inhibit a premature germination of the oil-containing seeds of Brassica and Linum. After germination they induce the synthesis of the seed storage proteins Napin and Cruciferin as well as that of several more elaiosome­-associated proteins. 

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