Changes in internal/external environment of organisms - stimuli
Resulting actions caused by stimuli - responses
Hormone first used by william bayliss and ernest sterling in 1902
Plant hormones = phytohormone
Auxin
- First coined by F.W.Went in 1928
- Powerful growth stimulants
- Effective at extremely low concentrations
- Main natural auxin found in plants: indole 3-acetic acid(IAA)
Functions of auxin:
- Promote growth of stem, roots and fruits through cell elongation
Hormone induces cell wall relaxation when cell wall is extensively stretched
Rate of cell elongation is proportional to auxin concentration
Stem cell elongation requires more auxin than roots
- Delay leaf senescence
- Promote growth of apical buds and inhibit growth of lateral buds
Suppression of lateral buds by apical buds: apical dominance
- Induce rooting in cutting of some plants like bougainvillaea
- Induce fruit formation without fertilisation
Development of fruits without fertilisation - parthenocarpy
Gibberellins
- Types: GA1, GA2, GA3
- Most studied form: GA3(gibberellic acid)
Functions of gibberellins:
- Promote growth of internodes by cell elongation
- Break seed dormancy; initiate germination
- Promote fruit growth; induce parthenocarpy
- Used in horticulture and food industries; enhance longitudinal growth of internodes in dwarf plants
- Used commercially to increase the length of grapes, elongate acids and improve their shape; gibberellic acid - speed up malting process in brewing industry
Cytokinins
- Discovered in 1950s by Skoog and Miller
Functions:
- Stimulate plant growth by cell division(even in non-meristematic tissues)
- Casue expansion of cotyledons in seeds
- Break seed dormancy; promote germination
- Promote chlorophyll synthesis in chloroplast, delay leaf senescence
- Inhibit apical dominance
Ethylene
- Only hormone that is a gas at ordinary temperature
- Site of synthesis and site of action are not different
- Discovered by R. Gane
- Most widely used plant growth hormone in agriculture
Functions of ethylene:
- Reduction on stem elongation
- Acceleration of senescence
- Ripening of fruits
- Initiaring germination in peanut seeds
- Sprouting of potato tuber
- Inducing flowering in mango
- Promoting root growth and root hair formation
Abscisic acid(ABA):
- Growth retarding hormone
- Found in chloroplast of leaves
- Fruits and seeds contain the highest amount of aba
Functions of aba:
- Plant growth inhibitor; slow down plant metabolism
- Inhibits seed germination and development; induces seed dormancy; help withstand desiccation
- Accelerated senescence and abscission of leaves, flowers, buds and fruits
- Stimulates closure of stomata in the epidermis and increases tolerance of plants