b. Gibberellins :
It is another growth promoting hormone and is abundant in root tip and developing seeds. It shows non-polar transport through vascular tissue. Gibberellins were first isolated from the fungus Gibberella fujikuroi by Japanese scientist Kurosawa (1926). He observed that when rice plant was infected by fungus Gibberella fujikuroi, it shows extensive stem elongation called 'bakane disease'. The crystalline form of Gibberellins were isolated by Yabuta and Sumiki (1938) from the fungus culture. They named it as gibberellin. It is synthesized in young leaves, seeds, roots and stem tips. These are synthesized from mevalonic acid. More than 150 chemical types are known so far. GA3 is most common and biologically active form. Chemically it contains a gibbeane ring - a cyclic diterpene with four isoprene units.
Physiological effects and application of Gibberellins :
Dormancy of bud can be broken by gibberellin treatment. It can promote seed germination in cereals like barley and wheat by synthesizing hydrolysing enzyme amylase to produce sugar. The most striking effect of it, is the elongation of stem where internodes increase in length. It also promotes bolting i.e. elongation of internodes just prior to flowering in plants those with rosette habit e.g. beet, cabbage. It causes parthenocarpy in tomato, apple and pear, and flowering in long day plants. It is used to increase the fruit size and bunch length of grapes. When gibberellins are applied on genetically dwarf plants like maize, the stem rapidly elongates and acquires the height of normal tall varities of maize. Application of gibberellins overcomes the requirement of vernalization. Usually, it inhibits growth of root, delays senescence and prevents abscission. It also breaks dormancy of seed and hastens germination. Application of gibberellin causes production of male flowers on female plant.
