Plant Growth Regulators – Gibberellins

Spraying of gibberellins on juvenile conifers hastens the maturity period and thus leads to early seed production.

Spraying sugarcane crop with gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre.

The 'bakanae' or 'foolish seedling' disease of rice had been known in Japan since the early 1800s.
The diseased rice plants were thin, pale green and spindle-shaped, longer by about 50 percent than the healthy plants, and were sterile.
The disease is caused by the fungal pathogen Gibberella fujikuroi.
E. Kurosawa (1926) showed that the symptoms appeared in healthy rice seedlings when they were treated with sterile filtrates of the fungus.
The active substance was later identified as gibberellic acid (GA).
ABA is a growth inhibitor, IAA is a natural auxin (discovered through experiments on coleoptile curvature) and 2,4-D is a synthetic auxin used as a herbicide.
So the foolish seedling disease of rice led to the discovery of GA (gibberellins).

Gibberellins promote elongation of the internodes. They cause bolting (sudden elongation of the internodes just before flowering) in rosette plants such as cabbage and beet.
Spraying sugarcane with gibberellins increases the length of the stem and the sugar yield; genetically dwarf plants grow to normal height when treated with gibberellins.
Cytokinins mainly promote cell division; ethylene inhibits elongation and promotes ripening and senescence; auxins (IAA) promote cell elongation mainly near the apex and in coleoptiles, but internode elongation of the whole stem is the characteristic effect of gibberellins.
So cell elongation in the internodal regions takes place due to gibberellins.

Gibberellins overcome dormancy of seeds and buds by counteracting abscisic acid.
Treatment of potato tubers with gibberellic acid makes their buds (eyes) sprout early.
ABA induces dormancy; IAA and zeatin are not used for this purpose.
So gibberellin breaks the dormancy of potato tubers.