Plants Hormones - Biology

Plants produce different kinds of chemical substances, which increase the growth and development these chemicals are called plant hormone.

    They effect the gene expression, activities or inhibit enzymes and changes the properties of membrane. These are in very less concentration.

     There are five kinds of plants hormones.

Auxins: Auxin is class of chemicals, which are both natural and synthetic. The most important natural auxin in Acetic Acid (l.A.A), which is found in higher plants. It is synthesised at tips of stem and roots (apical meristem), young leaves and in young embryo within seed. The synthetic aux ins are in dole butyric and naphthalene acetic acid.

Cell Division and Cell Enlargement: The auxin stimulate cell division and cell enlargement, increase the plant in length. It also initiate cell division in the  cambium as a result more xylem and phloem are found, which increase the trans location and thickness of plants.

Initiation of roots: Auxin initiates the development of adventitious roots when applied at the cut base of stem.

Abscission: In mature leaves and fruits less amount of auxin is present, so a abscission layer is formed at the base of petiol and stalk of fruit, this layer consists of thin walled cells.

Growth of Fruit: Auxin produces in young embryo promotes the growth of fruit.

Parthenogenesis: Auxin helps in producing parthenocarpy as seedless fruits, these fruits are formed without fertilisation. 

Apical Dominance: Growth of apical bud inhibits growth of lateral buds below the stem. This is called apical Dominance. So auxin inhibits the growth of lateral buds. If apical bud is cut then lateral buds starts to increase in size.

Weedieide: Auxin's also acts as weeds killer, they kill the broad leaves and seeds. 2-4 dichlarophenoxy acetic acid is to kill weeds in cereal crops.

Gibberellins: Gibberellins are plant hormones that increase cell division and cell enlargement Gibberellins was is covered in fungus gibberella fujikuroi fungus. This fungus cause bakanae disease in rice seeding. 
     The infected seeding elongated and falls. T-Vabuta and T-Hayashi. Isolated a chemical from the fungus and was as gibberellins. Now there are more then seven types of gibberellins.
       They are produced in roots, stem and leaves.
Role of Gibberellins: 
They promote cell division and cell elongation.
They prevent genital and physiological dwarfism.
They promote leaves and fruit growth.
They break bud and seed dormancy.
In some plants they delay senescence in leaves.
They also help to transport food from endosperm to developing embryo.

Cytokinins: These are groups of both natural and synthetic substances, which act as plant hormone. First they were obtaining from coconut milk, now other sources are herring sperm DNA and yeast extract is natural cytokinins which is obtaining from immature corn grains.

Role of Cytokinins: 
They promote rapid cell division in the presence of auxin.
They promote lateral root growth and initiated primary root growth.
They break seed dormancy.
They promote fruit develop.
They promote opening of stomata.

Abscisic acid: Abscisic acid is growth inhibitor and are produced by plant during adverse condition. e.g. Drought Condition.

1. It causes dormancy in buds and seeds.
2. It causes stomata to close.
3. They inhibit stem and root growth waste logging.
4. It promotes leaves senescence.
5. It promotes abscission.

Ethene: It is gas which act as plant hormone. It has many functions.

1. It promotes ripening of fruits.
2. It inhibits stem and roots growth.
3. It breaks dormancy of buds and seeds.
4. It promotes flowering in pineapple.
5. It contributes to leaf abscission.