Thiamann – suggested the use of term “Phytohormone” in plantsPhytohormone are organic substances which are naturally produced in plants
AUXINS (weakly acidic growth hormone):
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
dark. - 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
GIBBERELLINS:
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
CYTOKININS:
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
ABSCISSIC ACID (ABA):
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
5. ABA is called ANTI- GIBBERELLIN.
ETHYLENE:
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
acid
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
OTHERS:
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.
