Effects of Gibberellins on Plant Growth and Development
Authors: BRIAN P. W. 
Published: January 2008

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.

The work of Galston (1957) suggests that this inhibitory system might be an auxin‐destroying enzyme. Not all experimental observations are completely compatible with this view.

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. The action spectrum of he light‐induced effects mentioned in this paragraph and in paragraph 13 is similar, red light (650 mp) being most active. Characteristically an inducing exposure to red light may be reversed by an exposure to far‐red (735 mμ).

In cold‐requiring biennials GA simulates vernalization.

GA is not florigen, the postulated flowering hormone common both to short‐day plants and long‐day plants.

Hormones with physiological properties similar to those of GA have been detected in several plant tissues. The active material has been isolated from Phaseolus seeds and shown to be gibberellin A,. It is suggested that growth regulation in plants is based on a balance of auxins, GA‐like hormones and a growth‐inhibitory system.

Effects of photoperiod on plants are not confined to flowering. In general, short‐day photoperiods tend to induce retarded shoot extension and dormancy; shoot extension is accelerated and dormancy broken by exposure to long‐day photoperiods, to low temperature, or to exogenous GA.

There is thus a fundamental unity in the effects of GA on plant development, in which GA closely simulates effects usually induced in nature either by exposure to light or by vernalization. Accordingly, the explanation already offered (paragraphs 11 and 18 of this Summary) of the effects of GA on vegetative growth of day‐neutral plants, may be extended to cover phenomena of flowering and dormancy.

A hypothetical scheme is presented to explain regulation of growth and flowering, based primarily on the activity of GA‐like hormones; this scheme is a modification of those proposed by Borthwick et al. (1952) and Liverman & Bonner (1953). It may be summarized as follows: In response to light, GA‐like hormones are formed in leaves, a physiologically inactive, or less active, precursor (P) being an intermediary. The hormone is converted slowly back to P in darkness, more rapidly if the leaves are exposed to far‐red light. If the leaf is then exposed to red light once more, the hormone is again formed from P. Thus in long‐day conditions increasing concentrations of the hormone will be built up but in short‐days concentrations will be much lower. If it is supposed that high levels of GA‐like hormone induce flowering in long‐day plants but that flowering of SDP only takes place when levels of the hormone are low, the flowering and vegetative responses of both types of plant to light and to exogenous GA can be accounted for. This scheme can be adapted to explain other light‐induced responses, and also the effects of vernalization if it is assumed that low‐temperature treatments also are concerned with synthesis of GA‐like hormones.