Any of a group of plant growth substances first discovered through their ability to cause greatly increased stem elongation in intact plants. Gibberelhns have subsequently been shown to affect numerous aspects of plant growth and development. Most are chemically related to *gibberellic acid
. A few compounds with gibberellin-like activity, e.g. helminthosporol from the fungus Helminthosporium sativum, have a quite different chemical structure. Gib-berellin can overcome certain forms of genetic dwarfism and dwarf varieties are often used in bioassays for gibberellin. Gibberellin-induced stem extension is due to the effect of gibberellin on cell expansion, which it increases by influencing cell-wall expansibility. This however does not occur in the absence of auxin, as illustrated by the fact that gibberellin does not cause extension of excised internodes, cut off from their auxin supply.
Gibberellins have also been found to break dormancy in buds and seeds that normally have a light or chilling requirement. They can also partly or completely replace the photoperiod or cold requirement necessary to some species for flowering. Gibberellin levels are high in young leaves and, if applied to ageing leaves, can delay senescence. Levels are also high in developing seeds and fruits. Gibberellin applications can induce parthenocarpy and this has been put to commercial use in the production of seedless varieties of fruit. In barley seeds gibberellin has been found to stimulate the synthesis of the enzyme a-amylase. It does this by making available the messenger RNA responsible for a-amylase synthesis. This effect has proved of use in the brewing industry where a-amylase activity is essential for the production of malt. Gibberellin can even promote the enzyme's activity in inviable seed.
Gibberellins interact with other growth substances in various ways. There is evidence that *abscisic acid
reduces gibberellin levels, hence the antagonistic effects of these substances. Gibberellin is believed to interact with auxin in the control of sex expression in dioecious plants. Gynoecious plants usually have low levels of gibberellin and high levels of auxin. Application of gibberellin can induce the formation of male flowers. This effect has been put to use in the cucumber industry where certain hybrid varieties are naturally gynoecious. Since pollen production is necessary for fruit set some of the plants are sprayed with gibberellin to produce the necessary male flowers.