PLANT HORMONES

Certain substance affect growth, they are called plant hormones or phytohormones and also termed as growth regulators . Growth hormones may be defined as . "They are synthesized in young embryonic tissue as there is no specific organ for their production in plant. they are transferred to other cells where they are required and influence the process of development . There are five kinds of plant hormones.

1.Auxins: 

                 Auxins are compound ,which induce elongation in shoot cells and other growing parts. Auxin is a class of chemicals both naturally and synthesized ones . Indole acetic acid (1AA) is the naturally occurring auxin of higher plants. It is synthesized at the apices of stem and root (apical meristem), young leaves and young embryo within seed . Synthetic chemicals having effect like auxin are indole butyric acid (IBA) and naphthalene acetic acid (NAA).

            Auxins were first collected from oat plant.Fritwent  (1926) was first to use term auxin. He reported his activity in oat plant. Went (1928) was other scientist who performed oat-curvature test.

Role of Auxins: Some important roles of auxin are as follows:
(a) Cell division and cell elongation : Auxin take part in the cell division and cell elongation at the apices of root and stem.so their length is increased. It also helps in the  cell division of cambium to produce vascular tissues and to increase the width of plant body.
(b) Initiation of  roots: Auxin helps in root formation It also produces adventitious roots from the base of stem.
(c) Abscission of leaves and fruits: When leaves and fruits are mature, auxin production is stopped and at their lower region a layer of cells is formed called abscission layer, due to the jerk the leaves and fruits fall on the ground.
(d) Growth of fruit:  Auxin produced in young embryo promotes the growth of fruit.
(e) Parthenocapy : Use of auxin also helps in producing fruits and seeds without fertilization.
(f) Apical dominance :Auxin  helps in the rapid growth in the apical region of stem and root, it is called apical dominance.
 (g) Weedicide: Auxin  are selective weed killer. 2-4-dichlorophenoxy acetic acid (2-4-D) is used to kill weeds in lawns and central crops.

 2. Gibberellins:
                       It is group if chemicals that promote cell division and cell elongation . It is an important growth hormone of plant. First noticed in Gibberella fujikurel fungus which infected rice seeding and produced a disease called bakanae (Foolish seeding ). They infected seeding elongated and ultimately fell over without  producing grains. Two Japanese scientists T-Yabuta and T .Hayashi succeeded in isolating the active substance from the fungus and was named Gibberellin, after the name of genus . Soon after its 70 different types of Gibberellin have isolated from the higher plants. They are named as A1, A2, A3 to A70.

Role of Gibberellins:
                     Giberellins prevent the genetic and physiological dwarfism in plants. Gibberellins are found to be effective in braking the dominancy. They induce flowering in long day plants.In some cases these hormones help in fruit formation by 
  Parthenocarpy. They promote cell division and cell elongation of stem and leaves. They slimutale gemination of pollengrain and formation of pollen-tube in many plants.

3.Cytokinins:
           These are a group of substance both natural and synthetic , which react with auxin to induce cell division Miller (1954) isolated it from herring sperm DNA and named  it as Kinetin. Cytokinin have been extracted from coconut milk, tomato juice ,fruit of pear , plum ,yeast, maize and other plants. The cytokinin of maize grains is called zeatin.
Role of Cytokinin: It takes part in cell division in the presence of auxins. It is involved in the elongation of cells. It initiates the growth and root formation. It breaks dormancy. It indices flowering in short day plants. It delays senescence that is old age and detachment of leaves from plant.

4. Abscisic acid:
      It does not take part in growth of plant body, but during unfavourable conditions it reduces or stops the growth. It is called stress hormone. In dry season and cold weather it stops the formation of buds and seeds, causes stomata to close, modifies leaves into scales which protects the buds and increases the process of leaf fall.

5. Ethene: 
It is gas but it work as hormone. It is called ripening hormone. It takes part in the ripening of fruits. It effects permeability of cell membrane, which allows enzymes responsible for destroying chloroplast with the result that red and yellow colours are unmarked and fruit assumes ripened colour. It takes part in the abscission of leaves. It induce flowering in some parts, such as pineapple.