Plant Hormone :
The term hormone was coined by Starling. Plant hormone is a complex organic compound. The growth and development of plants are controlled by a special class of chemical substances called plant hormones. They are needed in small quantities at very low concentrations as compared to enzymes. They are rarely effective at the site of their synthesis.
There are different types of plant hormones:-
1 Auxin:-
- It is the first growth regulator plant hormone.
- Charles Darwinpered experiments on Canary grass and described them in his book.
- Paal said that it is a growth substance synthesized at the tip of the plant which is translocated downwards.
- The quantity of auxin is maximum in the dark region and minimum in the light region.
- Auxin is synthesized at the apex and soluble in water.
There are two types of Auxin:-
Natural Auxin:-
- Auxin-A:- Isolated from human urine.
- Auxin-B:- Isolated from corn germ oil.
- Auxin- IAA :- It is the universal natural auxin.
Synthetic Auxin:-
- I.B.A (Indole butyric acid )
- N.A.A ( Naphthalene acetic acid )
- 2,4-D (2,4-dichloro phenoxy acetic acid )
- 2,4,5-T (2,4,5-trichloro phenoxy acetic acid )
Function of Auxin:-
Apical dominance:-
The apical bud grows and the lower axillary buds are suppressed. Removal of the apical bud results in the growth of lower buds. The auxin (IAA) of the terminal bud inhibits the growth of lateral buds.
Root Growth:-
Auxin initiates the growth of roots and many new plants are usually propagated by stem cutting Rose, Bougainvillea. If we dip the lower cut end of a cutting in a dilute solution of auxins very soon large number of roots develop on the cut ends due to which these cuttings develop into successful plants.
Parthenocarpy:-
Parthenocarpy can be induced by spraying the ovary of flowers with a dilute solution of IAA then without fertilization seedless fruits are formed. Ex- Lemon, orange, and papaya.
Control of abscission:-
Leaf, fruits, and flowers are attached to the plant by petiole. The abscission layer is formed in the initial part of the petiole due to this premature leaf, fruits and flowers are dropped. Premature drops of fruits such as apples, pears, and citrus can be prevented by spraying the trees with a dilute solution of NAA or 2,4-D.
Cell elongation:-
Auxins promote the elongation and growth, of stems and roots and the enlargement of many fruits by stimulating the elongation of cells in all directions.
2 Gibberellins:-
- Gibberellin was first discovered by Kurosawa in fungus.
- The fungus causes Bakanae disease or foolish seedling disease of rice plants in Japan.
- More than 136 different types of gibberellins are known in which GA1, GA2, and GA3 are used in agriculture.
- Gibberellins are synthesized from acetylenes, co-A by primordial leaf and cell of seed and translocated in all directions.
Functions of gibberellins:-
- The gibberellins induce the elongation of plants and it elongates even the genetically dwarf varieties of plants.
- It promotes flowering in long-day plants under unfavorable short-day conditions.
- Gibberellins overcome the natural dormancy of buds, tubers, seeds, etc., and allow them to grow.
- Biennial plants are converted into annual plants by gibberellins.
- It induces parthenocarpy.
- Leaves of plants become large and broad by spraying gibberellins hence it is used in cauliflower, tomato, and pea.
3 Cytokinin:-
- The first cytokinin was discovered by Skoog and Miller in yeast and coined the term kinetin.
- Letham coined the term cytokinin. Letham discovered the first natural, cytokinin in unripe maize grain. It was named Zeatin.
- Cytokinin is formed from the dissociation of nucleic acid.
- Coconut milk also contains many kinetin-like substance which helps in cell division.
- In plants synthesis of cytokinin takes place in cells of the root and is translocated upwards.
Functions of cytokinin:-
- Its main function is to stimulate cell division in the presence of auxin.
- Cytokinin delays the senescence in plants which is known as the Richmond Lang Effect. Degradation of chlorophyll and protein are the characteristics of senescence.
- Cytokinin promotes the growth of lateral buds and suppresses the formation of apical buds.
- Cytokinin induces flowering in short-day plants.
4 Ethylene (CH₂CH₂):
- It is the only gaseous plant hormone which is an unsaturated hydrocarbon.
- It is synthesized from methionine amino acid.
Functions of ethylene:-
- Its main function is the ripening of fruits. At the time of ripening of respiration, it increases that are known as climacteric respiration
- Ethaphone or CEPA (2-chloroethyl phosphoric acid) is used in industries for ripening of fruits.
- It causes “sleeping Sickness “ disease in plants and the petals of flowers are closed.
- It increases the no. of female flowers.
- It stimulates the abscission of leaves and fruits
5 Abscisic acid on ABA :-
- It was discovered by Cornforth and Addicott from cotton balls and coined the term abscisin.
- It is also called the stress hormone.
- It is synthesized from Xanthophyll pigment in mature leaves.
- Ethylene & ABA are growth inhibitor hormones.
Functions of abscisic acid:-
- Its main function is to cause senescence and abscision.
- It promotes tubers in potatoes.
- It checks the germination of seed.
- It causes the closing of stomata in leaves.
- It is antagonistic to growth promoter Gibberellin hence it is called as anti Gibbrellic plant hormone.