• E. Starling coined the term
“hormone” in connection with the discovery of Secretin.
• Hormones
are organic compounds secreted in small quantities that regulate the plant’s
activities after being translocated to the organ or site of reaction where they
show their specific effect.
• Five
major kinds of plant hormones are :
Auxins
Cytokinins
Gibberellins
Ethylene
Abscisic acid
Auxins
• 1898
- proposed by Charles Darwin in “The power of movements in plants” while
working on Canary grass (Phalaris
cannariensis).
• 1928
- W. Went discovered auxins by Avena curvature test.
• 1931
- Kogl and Haagen Smit isolated Auxin-A (Auxintriolic acid)
•
1934 - Kogl and Haagen
Smit isolated Auxin-B (Auxinolonic acid)
•
1935 – isolated heteroauxins.
Natural auxin : Indole-3-acetic acid (IAA)
Synthetic auxins : Nephthalene acid (NAA), indole
butyric acid (IBA), indole propionic acid (IPA), 2,4-Dichlorophenoxyacetic acid
(2,4-D), 2,4,5-Trichlorobenzoic acid (2,4,5-T) etc.
Funtions of Auxins
• Elongation
of the cell.
• Apical
dominance
• Root
initiation (IAA, NAA, IBA, IPA )
•
Herbicides ( 2,4-D, 2,4,5-T)
•
inhibit sprouting of potato
•
induce flowering (NAA, 2,4-D)
•
abscission of fruits.
• stimulate
cell wall synthesis and parthenocarpy.
Cytokinins ( Kinetins )
• 1940s
- extracted from coconut milk.
Adenine-type cytokinis : kinetin, zeatin,
6-benzylaminopurine (BA )
Phenylurea-type cytokinis : diphenylurea, thidiazuron (TDZ)
Functions of Cytokinins
• Activates
cell division.
• Acts
in concert with Auxins : Formation of
callus , cell elongation in roots, embryos & fruit
•
Acts in reverse with Auxins : promote axillary
bud, inhibits branch roots
• Prevents
ageing and senescence in plants.
Gibberellins
•1926
- isolated from a fungus Gibberella fugikuroi in the “bakanae
disease”infected rice plant.
•
> 100 naturally occurring gibberellins
Functions of Gibberellins
• induces
seed germination
• stimulate
stem elongation and leaf expansion.
• break
dormancy in potato tubers and tree buds in winter.
• Induces
flowering in LD plants
•
causes parthenocarpy in pome fruits
• increases
the number of male flowers in moneecious
plants
Ethylene
• gaseous
hormone.
•
synthesized from methionine.
• released
from roots, shoot meristems and ripening fruits.
Functions of Ethtylene:
• induces
ripening of fruits.
• accelerates
abscission of leaves, stems, flowers and fruits.
• induces
flowering in pineapple.
• Induces
epinasty in leaves.
Abscisic acid (ABA)
• produced in mature leaves, stem, fruits and
seeds.
Functions of Abscisic acid
• accelerates
abscission.
• inhibits
seed germination
• also
accelerates senescence in leaves.
•
causes the closing of stomata
• Inhibits
fruit ripening
