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The gibberellins are metabolic products of the fungus Gibberella fujikuroi (conidial state Fusarium moniliforme). Three gibberellins are known: gibberellic acid (C19H22O6), gibberellin A1 (C19H24O6) and gibberellin A2 (C19H26O6). A structure for gibberellic acid has been proposed. Gibberellin A1 is a dihydro derivative of gibberellic acid. The structure of gibberellin A2 has not yet been established.
The biological activity of all three gibberellins is, as far as is known at present, zqualitatively similar; no truly quantitative comparisons have been reported. In describing biological results below, the abbreviation GA may refer to any one gibberellin or to mixtures.
The most characteristic effects of GA on shoot growth are increased inter-node extension, increased leaf-growth and enhanced apical dominance.
Under some circumstances, with some plant species, treatment with GA does not stimulate growth of intact roots, though some root sections do respond by increased growth. High concentrations of GA are only slightly inhibitory, results in increased dry weight. This is mainly due to increased carbon fixation and is believed to be a secondary effect of increased leaf growth.
Not all plants respond to GA by increased shoot growth and the effect on some species is greater than that on others. In species in which dwarf mutants are known, the dwarf may frequently be induced by GA to grow in a form in-distinguishable from that of the tall phenotype, genetically tall plants themselves being unaffected.
Many forms of dormancy are broken by GA. These include seed dormancy, dormancy of potato tubers and dormancy of shoot internodes and buds.
GA will induce flowering of long-short-day plants kept permanently in short-day photoperiods. I t will induce stem growth and, in long-day photoperiods but possibly not in short days, flowering in cold-requiring biennial long-day plants.
It inhibits flowering of short-day plants in inductive short-day photoperiods. I t will induce stem growth and, in long-day photoperiods but possibly not in short days, flowering in cold-requiring biennial long-day plants. It inhibits flowering of short-day plants in inductive short-day photoperiods.
In its effects on vegetative cell extension GA has certain similarities to the auxins but there are also differences. The most important differences are: (a) auxins greatly increase cell-extension in excised tissue sections, whereas GA has little effect; (6) GA induces marked cell extension in shoots of some intact plants, whereas exogenous auxins have little effect; (c) auxins inhibit root growth strongly, but GA does not.
There is evidence from several sources that GA only influences cell extension if auxin is present.
Comparison of the growth rate of excised pea internode sections with the growth rate of comparable tissues in intact plants, using untreated and GA-treated plants as sources of both types of material, has led to the conclusion that the endogenous auxins of plants are limited in their effects, and that growth is correspondingly limited, by 'an inhibitory system'GA acts by neutralizing the effects of this inhibitory system.
In its effects on leaf expansion and on some forms of dormancy, GA simulates light. In most photoperiodically sensitive plants, light, particularly in the form of long-day photoperiod, induces increased shoot growth; GA has a similar effect. The internode-inhibiting effects of light are not simulated by GA, which always promotes growth; on the other hand, GA does not physiologically reverse such inhibitions.
GA also breaks certain forms of dormancy broken in natural conditions by exposure to low temperature (vernalization).
In its effects on flowering GA also simulates light. By inducing flowering of long-day plants and long-short-day plants in short days it acts as if by extension of the light period; in inhibitory flowering in short-day plants in inductive short-day photocycles it simulates a light break in the dark period.
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