12th botany neet school

Phytohormones Gibberellins and Physiological effects of gibberellin

Phytohormones Gibberellins and Physiological effects of gibberellin

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


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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.

Physiological effects of gibberellin

  1. Gibberellins produce extraordinary elongation of stem. The elongation of stem is caused by the cell division and cell elongation induced by gibberellic acid.
  2. 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.
  3. 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.
  4. 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
  5. Formation of seedless fruits without fertilization can also be induced
    by gibberellin treatment in many plants. eg. Tomatoes, apples,
    cucumbers, etc.,
  6. 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.

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Other links 

Plant tissue culture – origin and techniques

Plant physiology – photosynthesis and its significance

Site of photosynthesis and Mechanism of photosynthesis

Electron transport system and photophosphorylation types

Dark reaction

C3 and C4 pathways

Photorespiration or C2 cycle

Factors affecting photosynthesis

Test tube and funnel experiment, Ganong’s light screen experiment

Mode of nutrition – Autotrophic, Heterotrophic


Mechanism of Respiration – Glycolysis

Mechanism of Respiration – Oxidative decarboxylation , Krebs cycle

Mechanism of Respiration – Electron Transport Chain, Energy Yield

Ganong’s respiroscope, Pentose phosphate pathway

Anaerobic respiration, Respiratory quotient, Compensation point, Kuhne’s fermentation tube experiment

Plant growth and Measurement of plant growth

Phytohormones Auxins

Phytohormones Cytokinin, Ethylene, Abscisic Acid, Growth Inhibitors – Physiological Effects

Photoperiodism and vernalization, Phytochromes and flowering


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