Structure and Physiological Effects of Gibberellins

Chemical Structure of Gibberellins:- Gibberellins are tetracyclic diterpenes with an ent-gibberellane ring structure.

Physiological Effects of Gibberellins:- 

i. Elongation of intact stems:- Many plants respond to application of GA by a marked increase in stem length; the effect is primarily one of internode elongation.

ii. Dwarf shoots:- Besides general increase in stem length, gibberellins specifically induce inter­nodal growth in some genetically dwarf varieties of plants like Pea and Maize. It appears that dwarf- ness of such varieties is due to internal deficiency of gibberellins.

iii. Bolting:- Gibberellins induce sub-apical meristem to develop faster. This causes elongation of reduced stem or bolting in case of rosette plants (e.g., Henbane, Cabbage) and root crops (e.g., Radish).

iv. Dormancy:- Gibberellins overcome the natural dormancy of buds, tubers, seeds etc., and allow them to grow. In this function they are antagonistic to abscisic acid (ABA).

v. Seed Germination:- During seed germination, especially of cereals, gibberellins stimulate the production of some messenger RNAs and then hydrolytic enzymes like amylases, lipases and pro­teases. The enzymes solubilise the reserve food of the seed. The same is transferred to embryo axis for its growth.

vi. Fruit Development:- Along with auxin, gibberellins control fruit growth and development. They can induce parthenocarpy or development of seedless fruits from unfertilized pistils, especially in case of pomes (e.g., Apple, Pear).

vii. Flowering:- They promote flowering in long day plants during noninductive periods.

viii. Vernalization:- Vernalization or low temperature requirement of some plants can be replaced by gibberellins.

ix. Application of gibberellins increases the number and size of several fruits, e.g., Grapes, To­mato; induce parthenocarpy in many species; and delay ripening of citrus fruits thus making storage safe.