Phytohormones Gibberellins and Physiological effects of gibberellin

Phytohormones Gibberellins and Physiological effects of gibberellin are illustrated with notes.

Gibberellins

Gibberellin was first discovered in Japan by Kurusowa.

He observed from his field that some of the rice seedlings had grown much taller than the others.

On further observation, he found that such taller rice plants had shown unusual internodal elongation.

This internodal elongation is known as the ‘bakanae’ or ‘foolish seedling’ disease of rice.

Later, it was discovered that the elongation was due to the action of a substance produced by a fungus, Gibberella fujikuroi.

This substance was successfully isolated from the fungus and it was named as gibberellic acid.

There are over 90 different gibberellins isolated from fungi and from
higher plants.

Gibberellins occur in various plant organs.

They are named as GA1, GA2, GA3, etc. These phytohormones occur in all groups of plants.

Gibberellins produce extraordinary elongation of stem. The elongation of stem is caused by the cell division and cell elongation induced by gibberellic acid.
One of the most striking effects of the gibberellins is the reversal of
dwarfism in many genetically dwarf plants. For e.g. ‘Rosette’ plant
of sugar beet, when treated with GA undergoes marked longitudinal
growth of axis attaining the normal size.
Rosette plants usually show reduced internodal growth. These plants
exhibit excessive internodal growth when they are treated with
gibberellin. This sudden elongation of stem followed by flowering is
called bolting.
Many biennials usually flower during the second year of their growth.
For flowering to take place, these plants should be exposed to cold
season. Such plants could be made to flower without exposure to
cold season in the first year itself, when they are treated with
gibberellins.
Formation of seedless fruits without fertilization can also be induced
by gibberellin treatment in many plants. eg. Tomatoes, apples,
cucumbers, etc.,
Some of the light sensitive seeds can germinate by the treatment of
gibberellic acid even in complete darkness. eg. barley,
m Gibberellin breaks dormancy in potato tubers.