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Plant growth regulators

Thiamann – suggested the use of term “Phytohormone” in plantsPhytohormone are organic substances which are naturally produced in plants

AUXINS (weakly acidic growth hormone):Plant growth regulators
Auxin was named by KOGL. It is a Greek word derived from ‘Auxein’ which means
to grow.

  • Naturally occurring Auxin – IAA
  • Synthetically produced auxins are – NAA, IBA, 2, 4 – D, MCPA
  • Anti-auxins – Naphthythalamic acid (NTA) , Ethylene chlorohydrins
  • Active sites of auxins – shoot tip region, coleoptiles and developing embryos etc
  • The Auxin synthesis occurs rapidly in green leaves in presence of light than the in the
  • Tryptophan is the precursor of IAA and zinc is required for its synthesis
  • Translocation of auxins is polar
  • The site of Auxin transport is located on the plasma lemma
  • Avena curvature test and split pea stem curvature test are the bioassays that are
    generally used for auxins

Role of auxins:
1. Promotes apical dominance
2. Increases cell division in cambium
3. Promote the elongation of cells
4. Auxin increase in shoot and decrease in root
5. Induces uniform flowering in pineapple
6. IBA promotes rooting of cutting

Plant growth regulators

 Second important growth hormone found in plants
 Discovered by KUROSAWA (1926)
 First isolated from Gibberella fujikuroi, the causal organism of “foolish seedling of
rice” or commonly called Bakanae disease of rice.
 Gibberellins are CYCLIC DITERPENES with gibbane skeleton
 Gibberellins moves in both xylem and phloem
 Chemically gibberellins are related to terpenoids and its precursor is N- Kaurene
 Anti- gibberellins: phosphon D, Cycocel (CCC), Maleic hydrazide, paclobutrazol

 Role of gibberellins:

1. Breaking of dormancy
2. Induction of flowering in long day plants
3. Promotes male flowers production
4. Enhances seed germination
5. The most important effect of GA is the stem elongation i.e. GA induces internode
elongation or sub apical elongation

Plant growth regulators


 Plays a key role in higher plants and moves through xylem
 Miller and Skoog – identified kinetin
 Term cytokinin proposed by Letham (1963)
 The first naturally occurring hormone identified – Zeatin
 Root tip is an important site of cytokinin synthesis
 Precursor of cytokinin is either adenine or adenosine i.e. purine bases
 Mobility is polar and basipetal

 Role of cytokinin:

1. Initiation of cell division
2. Delay of senescence
3. Induce flowering in short day plants
4. Promotes stomatal opening
5. Promote femaleness in male flowers

Plant growth regulators


 Naturally occurring growth regulator
 It acts as stress hormone
 ABA first identified by WAREING (1965)
 Lunalaric acid found in algae and liverworts acts similar to abscissic acid
 Violoxanthin serves as a precursor for biosynthesis of ABA
 Biosynthesis of ABA also takes place through mevalonic acid
 It is a terpenoids
 Bioassays are – rice seedling growth inhibition test and inhibition of α amylase in
barley endosperm

 Role of ABA:

1. Induces bud dormancy and enhances the process of abscission
2. Senescence of leaf is promoted by ABA
3. Stimulates the release of ethylene
4. Brings the closure of stomata during water stress

Plant growth regulators

 It is known as RIPENING HORMONE
 Production increased with increase in respiration rate
 Auxin increases ethylene level in plants
 Naturally occurring volatile hormone
 BURG (1962) established that ethylene is the only gaseous growth regulator
 Maximum ethylene is formed in ripening fruits and senescing tissues
 Biosynthesis of ethylene occurs from methionine which is a sulphur containing amino
 Inhibitors of ethylene synthesis are amino-ethoxyvinylglcine
 Bioassays for ethylene are triple pea test and pea stem swelling test

Role of ethylene:

1. Responsible for fruit ripening with increase in respiration
2. Induces uniform flowering and ripening in pineapple
3. Inhibits stem elongation and cause abscission of leaves
4. Induces fruiting in ornamental plants
5. ETHEPHON- increase latex flow in rubber

Plant growth regulators

Glysophosine – used to ripen sugarcane
Florigen (flowering hormone) – initiation of flowering in plants
Traumatic acid (wound hormone) – found in injured portions of a plant
Xanthoxin – destruction product of Violoxanthin and forms ABA
Brassins – steroid, isolated from pollen grains of Brassica
Jasmonic acid – Methyl ester in jasmine, inhibits growth and promote senescence

Important points:

Potassium ions (K+) play an important role in the opening and closing of stomata
Plant transpirants – colourless plastics, silicone ols, phenyl mercuric acetate, Absiccic
acid, Co2 etc.
Porometer is used for measuring transpiration
 The growth is maximum during exponential phase
Transpiration takes place through stomata, lenticels or cuticle
Guttation refers to exudation of water from plants in the form of liquids

Short day plants – soybean, potato, sugarcane, cosmos, chrysanthemum, tobacco,
rice, onion, upland cotton, strawberry, datura etc
Long day plants – spinach, lettuce, radish, alfalfa, sugar beet, opium, poppy, oats,
wheat etc.
Day neutral plants –tomato, cucumber, cotton, pea, sunflower, maize etc.
Vernalisation – refers to method on inducing early flowering in plants by pretreatment
of their seeds at very low temperature
 Hormone responsible for vernalisation is vernalin
 Water use efficiency is highest in CAM plants followed by C4 and C3 plants
 Photosynthetic efficiency is highest in C4 plants
 To make one molecule of glucose, 6 turns of Calvin cycle are required
 The efficiency of photosynthesis is 40 %
 The ratio of photosynthesis to respiration during day time is 10:1
 In most succulent plants, Co2 is fixed by the activity of PEP carboxylase
 The ratio of Co2 reduced and oxygen released during photosynthesis is 1:1
DCMU is an example of photosynthetic inhibitor
 The products of light reaction are ATP and NADPH2
 Major form of carbon transfer in plants is by sucrose
 For photosynthesis, the visible range of spectrum between 250 to 750 nm is essential.

Plant growth regulators

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