Plant hormones

Hormones

Hormones are non nutrient chemical which act as a intercellular messengers and are produce in trace amounts.

Plant hormones are:

• Auxin
• Gibberellins
• Cytokinins
• Ethylene
• Abscisic acid

Auxin

Auxin term is derived from Greek word 'auxein' which means to grow.

F.w.went

• Discovery of auxin 
•  coined the term auxin.
• Isolated auxin from tips of coleoptiles of oat seedlings.
• Bioassay of auxin(Avena curvature test)

Kogl and Haagensmit

• First time isolated auxin from human urine.
• Precursor of auxin is tryptophan.
• Zn is also necessary for biosynthesis of auxin.

Natural auxin

• Indole-3-acetic acid (IAA)
• Indole 3butyric acid (IBA)

Synthetic auxin

Artificially synthesise

Example:- naphthalene acetic acid (NAA)

2,4-D(2,4-Dichlorophenoxy acetic acid)

IPA(indole propionic acid)

Natural and synthetic auxin have been used extensively in agriculture and horticulture practice.

Physiological effect and application of auxin.

1. Apical dominance:- Higher plant growing apical bud inhibit  growth of lateral buds called apical dominance
2. Root initiation
3. Flowering
4. Abcission
5. Parthenocarpy
6. Herbicide
7. Xylem differentiation
8. Cell division
9. Potato dormancy.

Bioassay of auxin

Bioassay is a biological testing of a substance (PGR)  on a plant

1. Avena curvature test
2. Root growth inhibition

Gibberellins

Physiological  effect and application of gibberellins

• Bolting

Gibberellins promote bolting in Beet, and Cabbage etc.

• Overcome genetic dwarfism

Gibberellins promote internode elongation in genetically dwarf  plant like maize,pisum, and vicia faba etc

• Improve yield in sugarcane.

Sugarcane store carbohydrates as sugar in their stems.

Spraying sugarcane crop with gibberellins increase the length of stems

• Improve shape in Apple

Gibberellins cause fruit like apple to elongate and improve its shape.

• Improve yield in grapes.

Ability of gibberellins to cause increase in length of axis.

• Senescence

Gibberellins delay senescence .

• Early maturity in conifers

Spraying juvenile conifers with gibberellins hastens the maturity.

• Brewing industry

Gibberellins is used to speed up malting process in brewing  industry.

• Dormancy

Gibberellin induced  the synthesis of hydrolyzing enzymes  alpha-amylase ,lipases and proteases

Bioassay of gibberellins

1. Barley endosperm test
2. Dwarf pea and maize test

Cytokinins

Discovered by F.skoog.and his co workers

Miller:-isolate and active some substance from the autoclaved DNA of sperm cells of herring fish.

Natural cytokinin  are synthesized in the region where rapid cell division occur. Example Root apices and shoot apices.

Physiological effect and application

1. Cell division
2. Overcome apical dominance: - cytokinin promote the growth of lateral buds.
3. Role of cytokines in is antagonist to auxin.
4. Senescence:-cytokinin promote nutrient mobilization towards younger parts which help in the delay of leaf  senescence.
5. Help to produce new leaves
6. Promote lateral shoot growth
7. Induced opening of stomata
8. Tissue culture

Bioassay of cytokines

1. Tobacco pith cell division test
2. Chlorophyll preservation test

Ethylene

• Ethylene is volatile substance.
• Gaseous PGR
• Methionine is a precursor of ethylene.
• Ethylene is one of the most widely used PGR in agriculture.

Physiological effect and application of ethylene

1. Ethylene promote fruit ripening
2.  Ethylene promote senescence  and abscission
3. Triple response

• Horizontal growth of seedling
• Swelling of the axis
• Apical hook formation in dicot seedlings

1. Dormancy :- ethylene breaks seed and bud dormancy
2. Increase absorption surface
3. Initiate flowering
4. Promote female flower in cucumbers
5. Ethylene promote rapid internode.

Abscisic acid(ABA)

Synthesis:- precursor of abscisic acid is carotenoids (violoxanthin)

1. Abscisic acid act as a general plant growth inhibitor
2. ABA inhibit seed germination.
3. Abscisic acid stimulate closing of stomata.
4. Abscisic acid is known as stress hormones.
5. Abscisic acid important role in seed development, maturation and dormancy.
6. Abscisic act as antagonist to gibberellins.