Gibberellin involvement in dormancy-break and germination of seeds of celery (Apium graveolens L.)

The temperature-dependent, primary dormancy of cv. Florida 683 celery seeds in darkness was partially broken by a 30 min light exposure on the third day of incubation at 20–22°C, resulting in c 50 percent germination after 20 days. This light stimulation was negated by including different inhibitors of gibberellin biosynthesis in the incubation medium. Subsequent addition of a solution of the gibberellins A₄ and A₇ or of the gibberellin-active compound (1-3-chlorophthalimido)-cyclohexane carboxamide (AC94,377) overcame the inhibitory effects on germination of these GA-biosynthesis inhibitors. It is suggested that light stimulates the biosynthesis of gibberellins which are essential for dormancy-break in celery seeds and that this biosynthesis is either directly or indirectly controlled through phytochrome.Abbreviations AC94,377 1-(3-chlorophthalimido)-cyclohexane carboxamide; ancymidol, -cyclopropyl--(4-methoxyphenyl)-5-pyrimidinemethanol - AMO1618 N,N,N-2-tetramethyl-5-(1-methyl ethyl)-4-(1-piperidinylcarbonyl)oxy-benzenaminium chloride - BTS44584 S-2,5-dimethyl-4-pentamethylenecarbamoyloxyphenyl-SS-dimethyl sulphonium - P toluenesulphonate; chlormequat chloride, 2-chloroethyltrimethylammonium chloride; daminozide - N dimethylaminoscuccinamic acid; paclobutrazol, (2RS, 3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1H-1,2,4-triazol-1-yl pentan-3-ol)     

Reversing the inhibitory effect of paclobutrazol on seed germination of Amaranthus paniculatus by GA3, ethephon or ACC

The germination of Amaranthus paniculatus seeds was inhibited by applying paclobutrazol, a specific inhibitor of gibberellin biosynthesis. This inhibition was markedly counteracted by gibberellin A₃ (GA₃), suggesting that endogenous gibberellins are required for germination in this species. The inhibitory effect of paclobutrazol was also overcome by ethephon (2-chloroethylphosphonic acid) or the precursor of ethylene biosynthesis, ACC (1-aminocyclopropane-l-carboxylic acid). Thus the physiological effect of gibberellin can be mimicked by ethylene released from ethephon or synthesised from exogenous ACC. It is suggested, that endogenous gibberellins are involved in germination of Amaranthus paniculatus seeds and that action of GA₃ can be substituted by ethylene.Abbreviations ACC 1-aminocyclopropane-l-carboxylic acid - AMO-1618 (2-isopropyl-5methyl-4-trimethylammoniumchloride)-phenyl-l-piperidinium-carboxylate - ancymidol -cyclopropyl--(4-methoxyphenyl)-5-pyrimidine methanol - chloromequat chloride (2-chloroethyl)trimethylammoniumchloride - ethephon 2-chloroethylphosphonic acid - GA gibberellin A₃ - paclobutrazol (2RS, 3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-lyl)pentan-3-ol - Phosphon D 2,4,dichlorobenzyl-tributhylphosphoniumchloride - tetcyclacis 5,(4-chlorophenyl)-3,4,5,9,10-pentaaza-tetracyclo)5,4,1,0,Z,6,0^8,11 dodeca-3,9-diene     

The effect of some growth regulators on light-induced germination of Paulownia tomentosa seeds

Grubišié, D., Konjevié, R. and Neškovié, M. 1988. The effect of some growth regulators on light-induced germination of Paulownia tomentosa seeds. - Physiol. Plant. 72: 525–528.
The germination of Paulownia tomentosa Steud. (Empress tree) seeds can be induced either by red light (R) or by exogenously applied gibberellic acid (GA₃). The R induced germination is completely suppressed by far red irradiation, abscisic acid (ABA) or by growth retardants such as ancymidol, tetcyclacis and paclobutrazol, though not by AMO 1618 or chloro-choline chloride (CCC). The inhibition caused by far red light (FR) and growth retardants can be overcome by GA₃, while the ABA-induced inhibition can be reversed by fusicoccin (FC), which does not reverse the inhibition caused by FR or growth retardants. It is noteworthy that the germination of light insensitive wheat, corn, alfalfa and mung bean seeds is not inhibited by growth retardants.     

Promotion of asparagus shoot and root growth by growth retardants

Plantlets regenerated from shoot-tip culture of Asparagus officinalis L. possessed weak shoots and roots. Various combinations of auxins and cytokinins did not improve the plantlets. Incorporation of a number of growth retardants, viz. ancymidol, B-995, phosfon, Amo 1618, cycocel and paclobutrazol, promoted growth of stronger shoots and roots. The effectiveness of the growth retardants varied, with ancymidol being most effective and cycocel least effective.The response to ancymidol was prevented by exogenous GA₃ and GA_4/7. GA_1/3 and GA_4/7-like activities were detected in asparagus shoot-tip culture and these activities were reduced by the presence of the growth retardants ancymidol, Amo-1618, and cycocel.     

Studies in seed dormancy

Summary When dormant hazel seeds were subjected to six weeks chilling at 5° C their subsequent transfer to 20° C resulted in the accumulation of gibberellin (GA) followed by germination. In the presence of either phosphon D or -chlorethyltrimethylammonium chloride (CCC) at 20° C there was inhibition of both GA accumulation and germination, a finding consistent with the hypothesis that GA biosynthesis is a necessary prerequisite for the germination of chilled hazel seeds. As abscisic acid showed a strong inhibition of germination but had little effect on GA accumulation it is presumed not to have affected GA biosynthesis but to have inhibited GA action. These conclusions were supported by experiments in which the interaction of exogenous GA₃ with growth retardants and ABA was tested on the germination of chilled hazel seeds. Experiments in which the embryonic axes and cotyledons of chilled seeds were incubated separately at 20° C established that GA biosynthesis de novo occurred in the embryonic axis and indicated that in the intact seed some of the GA would have been translocated to the cotyledons. The isolated cytoledons showed no GA biosynthesis de novo but gave some release of GA from one or more bound forms.Abbreviations ABA abscisie acid - AMO 1618 2-isopropyl-4-(trimethylammonium chloride)-5-methylphenyl piperidine carboxylate - CCC -chlorethyltrimethylammonium chloride - GA gibberellin, phosphon - D tributyl-2,4-dichlorobenzylphosphonium chloride - TLC thin-layer chromatography     

The preparation in vitro of chrysanthemum for transplantation to soil

Plantlets of Dendranthema grandiflora Pennine Reel were grown from nodal sections on Sorbarods saturated with liquid medium containing 0–3 mg 1^–1 of various growth retardants. After 4 weeks they were transferred to compost and maintained at a relative humidity of 42% at 27.5°C. Wilting was assessed over a period of 3 h. Plantlets treated with paclobutrazol, flurprimidol, triapenthenol, chlorphonium chloride, uniconazol and ancymidol showed dose-related reductions in wilting up to a concentration of 3 mg 1^–1. Responses to tetcyclacis and mepiquat chloride were weaker, and no responses to chlormequat chloride, BTS 44584 or diaminozide could be detected. These observations are compatible with an hypothesis that resistance to wilting derives from inhibited synthesis of gibberellins.     

The effects of ancymidol,abscisic acid,uniconazole and paclobutrazol on somatic embryogenesis of asparagus

Summary The effects of ancymidol, abscisic acid (ABA), uniconazole, and paclobutrazol on asparagus somatic embryogenesis were evaluated. Calli induced from seedlings of genotype G447 were transferred to embryo induction medium (MS plus 3% sucrose, 0.1 mg L^–1 NAA, 0.5 mg L^–1 kinetin and 3% gelrite), with different concentrations of these compounds. After 8 weeks, the recovered bipolar or globular embryos were placed on germination medium (MS plus 6% sucrose, 0.1 mg L^–1 NAA, 0.1 mg L^–1 kinetin, 0.75 mg L^–1 ancymidol, 40 mg L^–1 adenine sulphate dihydrate, 0.17 mg L^–1 sodium phosphate monobasic and 3% gelrite) for conversion to plantlets. Inclusion of ancymidol, ABA, uniconazole and paclobutrazol in the embryo induction medium did not affect the total number of somatic embryos produced relative to the control without these compounds. However, ancymidol, ABA and uniconazole significantly improved embryo development by increasing the production of bipolar embryos 250–750% and decreasing that of globular embryos 8–35% relative to the control. The bipolar embryos produced with any of the four compounds in the embryo induction medium converted to plantlets at rates 700–1100% greater than the control. None of the globular embryos converted to plantlets. Ancymidol (0.75 mg L^–1) and ABA (0.05 mg L^–1) were the most effective treatments; 61 and 46 bipolar embryos g^–1 callus were produced, and 38% and 37% of the bipolar embryos converted to plantlets, respectively. These results indicated that ancymidol, ABA, uniconazole and paclobutrazol significantly enhanced the production of asparagus somatic embryos and their conversion to plantlets, and ancymidol and ABA were more effective than uniconazole and paclobutrazol.Abbreviations Ancymidol a-cyclopropyl-a(4-methoxyphenyi)-5-pyrimidine methanol - NAA 1-naphthaleneacetic acid - Paclobutrazol I-(4-chlorophenyl)-4,4-dimethyl-2(1H-1,2,4-triazol-1-yl)-pentan-3-ol - Uniconazole (E)-(p-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-1-pentan-3-ol - ABA abscisic acid - GA gibberellic acid     

The effect of growth retardants on anthocyanin production in carrot cell suspension cultures

The effect of growth retardants on anthocyanin production was studied in wild carrot (Daucus carota) cell suspension cultures. Paclobutrazol [(2RS,3RS) — 1 — (4-chlorophenyl) — 4,4 —dimethyl-2-(1,2,4-triazol-1-yl) pentan-3-ol], uniconazole [(E)-1-(4-chlorophenyl-4,4 —) dimethyl-2-(1,2,4-triazol-1-yl)-1-penten-3-ol], tetcyclacis [5-(4-chloro-phenyl) -3,4,5,9,10-pentaaza-tetracyclo-5, 4, 10^2,6, O^8,11 — dodeca-3, 9-diene], ancymidol [-cyclopropyl — 4 — methoxy-(pyrimidine-5-yl)benzyl alcohol] and CCC (2-chloro-ethyltrimethylammonium chloride) increased anthocyanin accumulation. AMO-1618 [(2-isopropyl-5-methyl-4-trimethyl-ammonium-chloride)-phenyl-1-piperidinium carboxylate] did not increase anthocyanin accumulation in the first passage but did increase it during the second passage on medium for improved anthocyanin accumulation. Prohexadione (3,5-dioxo-4-propionylcyclohexane carboxylic acid) decreased anthocyanin accumulation by 10%–12.5%.The inhibitory effect of gibberellin on anthocyanin accumulation was reversed by paclobutrazol. Paclobutrazol together with 10^–6M GA₃ increased anthocyanin level from 33% of control in GA₃ treated cell suspension to 76%. These results are consistent growth retardants increasing anthocyanin accumulation in carrot cell suspension cultures by inhibiting gibberellin biosynthesis.     

The effect of growth retardants on phytochrome-induced lettuce seed germination

Experiments were carried out to explore the involvement of the plant hormone gibberellin (GA) in the light-induced germination of lettuce seeds. Three growth retardants known to be inhibitors of GA biosynthesis were tested for their effect on red-light-induced germination. Chlormequat chloride (CCC) and AMO-1618 had no effect, but ancymidol was strongly inhibitory. Moreover, the inhibition caused by ancymidol was completely overcome by GA₃. CCC and AMO-1618 inhibit the formation ofent-kaurene, while ancymidol blocks the oxidation ofent-kaurene toent-kaurenoic acid. Ancymidol also was found to inhibit GA-induced dark germination of lettuce seeds, and this inhibition was partially reversed by higher levels of GA. Therefore, the results suggest two possibilities for the relationship between phytochrome and GA in this system: first, the rate-limiting step in the germination of light-sensitive lettuce seeds, that which is regulated by phytochrome, is the oxidation ofent-kaurene toent-kaurenoic acid. Alternatively, red-light treatment may result in the release of active GAlike substances which, in turn, induce germination. In either case the results presented here support the view that phytochrome exerts its effect on lettuce seed germination by means of GA rather than via an independent pathway.     

Role of endogenous gibberellins in germination of melon (Cucumis melo) seeds

The effect of the plant growth retardants ancymidol. mefluidide and uniconazole on germination of two melon accessions differing in their ability to germinate at 14°C was examined. The accessions were the cold sensitive Noy Yizre'el and the cold tolerant Persia 202. The three growth retardants were able to delay the germination of intact Noy Yizre'el seeds, but did not affect that of intact Persia 202 seeds. On the other hand germination of decoated seeds of both accessions was unaffected by these inhibitors at normal oxygen concentration, but was inhibited at 5% oxygen. When gibberellin-like activity was measured by a dwarf rice biological assay following HPLC fractionation, it was found that seeds of Persia 202 contained much more gibberellin-like activity than Noy Yizre'el seeds. Among the extracted compounds several endogenous gibberellins were identified by combined gas chromatography-mass spectrometry (GC-MS). They included GA₄, GA₂₀, GA₁ and GA₃ in Noy Yizre'el and GA₃₄, GA₂₀, GA₁ and GA₈ in Persia 202. It is suggested that the better germination of intact Persia 202 seeds, compared to Noy Yizre'el seeds at low temperature and low oxygen concentration, is due to a higher endogenous level of GA and a better seed coat permeability to oxygen.     

The use of growth retardants to improve microtuber formation by potato (Solanum tuberosum)

The growth retardant chlormequat stimulated microtuber formation by a recalcitrant cultivar of potato (Solanum tuberosum), but reduced microtuber fresh weight in a cultivar that tuberised readily in its absence. Inhibition of microtuber growth by high concentrations of chlormequat was confirmed using a different in vitro system where all cultivars tuberised in the absence of growth retardants.Alternative growth retardants were tested. Daminozide also had a detrimental effect on microtuber fresh weight, but ancymidol and paclobutrazol did not inhibit microtuber growth at the concentrations required for stimulation of tuberisation by recalcitrant cultivars. In addition, 10^-5 M ancymidol and paclobutrazol inhibited premature sprouting of microtubers in vitro.Abbreviations BA benzyladenine - DMSO dimethyl sulphoxide - FW fresh weight - PAR photosynthetically active radiation - SE standard error     

Effects of two growth retardants on tissue permeability in Pisum sativum and Beta vulgaris

The effects of growth retardants, 4-hydroxy-5-isopropyl-2-methylphenyltrimethylammonium chloride-1-piperidine carboxylate (AMO-1618 or AMO) and 2-chloroethyltrimethylammonium chloride (CCC), applied with and without gibberellic acid (GA₃), on -[³H]alanine uptake and leakage from pea (Pisum sativum L.) and betacyanin efflux from beetroot (Beta vulgaris L.) tissue were examined. Both compounds decreased the amount of -[³H]alanine taken up into pea leaf discs, and increased the quantity of radioactive label that subsequently leaked out of this tissue. Efflux of betacyanin from slices of beetroot was also found to be promoted by treatment with CCC or AMO-1618. In no case were these effects reversed by application of GA₃. It is concluded that the growth retardants may be altering tissue permeability by an interaction with the cell membranes, and this may account for some of the side effects of the retardants which cannot be explained on the basis of their inhibiting action on gibberellin synthesis.Abbreviations AMO-1618 4-hydroxy-5-isopropyl-2-methylphenyltrimethylammonium chloride-1-piperidine carboxylate - CCC 2-chloroethyltrimethylammonium chloride - GA₃ gibberellic acid     

The hormonal control of amaranthin synthesis in Amaranthus caudatus seedlings

Summary Exogenous gibberellic acid, A₃ (GA₃) inhibits phytochrome mediated betacyanin synthesis in seedlings of Amaranthus caudatus. The growth retardants, -chloroethyl-trimethylammonium chloride (CCC), 'isopropyl-4-(triethylammonium chloride)-5-methylphenyl piperidine carboxylate (AMO 1618) and tributyl-2,4,-dichlorobenzylphosphonium chloride (phosphon D) enhance pigment synthesis. Retardant stimulation of pigment synthesis is overcome by GA₃ application. Besides lowering endogenous GA levels the retardants inhibit protein synthesis by as much as 25%. Retardant inhibition of protein synthesis is not overcome by GA₃. The results suggest that amaranthin synthesis in Amaranthus caudatus can be directly controlled by endogenous GA. GA₃ has no effect on kinin induced dark pigment synthesis. Kinins, however, do not overcome GA₃ inhibition of pigment synthesis in the light.Abbreviations AMO 1618 2, 'isopropyl-4-(triethylammonium chloride)-5-methylphenyl piperidine carboxylate - CCC -chloroethyltrimethylammonium chloride - GA₃ Gibberellic acid, A3 - Phosphon D tributyl-2,4,-dichlorobenzylphosphoninm chloride     

Genetics and gibberellin production inGibberella fujikuroi

Gibberella fujikuroi (Fusarium moniliforme) is a complex group of plant pathogens. Some strains produce gibberellic acid and other gibberellins that promote growth and regulate various stages in plant development.The paper describes the research effort directed to development of genetic tools for this species. Furthermore the main features of the gibberellin biosynthetic pathway as established in Gibberella are described.Abbreviations AMO 1618 2-isopropyl-4-(trimethylammonium chloride)-5-methylphenylpiperidine-1-carboxylate - hydroxykaurenoic acid ent-kaur-16-en-7-ol-19-oic acid - kaurenal ent-kaur-16-en-19-al - kaurene ent-kaur-16-ene - kaurenoic acid ent-kaur-16-en-19-oic acid - kaurenol ent-kaur-16-en-19-ol - paclobutrazol 1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-lyl)pentan-3-ol - pefurazoate pent-4-enyl-N-furfuryl-N-imidazol-1-ylcarbonyl-DL-homoa laninate - tetcyclacis 5-(4-chlorophenyl)-3,4,5,9,10-pentaazatetracyclo-5,4,10^2.6,O^8.11-dodeca-3,9-diene - triarimol -(2,4-dichlorophenyl)--phenyl-5-pyrimidine methyl alcohol     

Growth Retardant-Induced Changes in Phototropic Reaction of Vigna radiata Seedlings

The effect of growth retardants on phototropism has been studied in mung bean (Vigna radiata) seedlings. Ancymidol, tetcyclacis, and paclobutrazol inhibited phototropism while AMO 1618 and CCC were ineffective. The fluence-response relationships for phototropism of etiolated seedlings were similar to those previously described for monocots and other dicots. Ancymidol caused a shift in the maximum phototropic response to higher fluence of light. It is suggested that ancymidol may affect phototropism through an effect on the photoreceptor system.     

Inhibitory effect of GA3 on the development of thidiazuron-induced somatic embryogenesis in geranium (Pelargonium xhortorum Bailey) hypocotyl cultures

Somatic embryogenesis in geranium (Pelargonium xhortorum Bailey cv Scarlet Orbit Improved) can be achieved by incubating hypocotyl explants on MS medium supplemented with thidiazuron (TDZ; 10 M for 3 days followed by subculture on medium devoid of any plant growth regulators. The presence of gibberellins (GAs) during both the induction and expression phases of embryogenesis was significantly detrimental to somatic embryo formation on the hypocotyl explants. The addition of the GA-synthesis inhibitors paclobutrazol, uniconazole or ancymidol during the period of growth and differentiation of somatic embryos increased the number of somatic embryos formed on each explant. However, paclobutrazol added during the period of induction had no significant influence on somatic embryo formation. Results suggest that both exogenously supplied as well as endogenous GAs play a role, albeit a negative one, on somatic embryogenesis of geranium.Abbreviations MS Murashige and Skoog (1962) medium - MSO basal medium devoid of any plant growth regulator - TDZ N-phenyl-N1,2,3-thidiazol-5-ylurea (thidiazuron)     

The control of seed germination in Trollius ledebouri A model of seed dormancy

Treatment of Trollius ledebouri seeds with gibberellins A₄+A₇ promotes germination. The efficacy of the treatment is dependent upon the duration of imbibition in distilled water prior to GA₄₊₇ application. Presoaking increases both the final percentage germination attained and also its rate of achievement. No presoaking effect is exhibited by seeds induced to germinate by testa removal in the absence of GA₄₊₇. Active washing of Trollius seeds enhances the presoaking effect and the eluent from washed seeds is inhibitory to germination. The results support the hypothesis that the presoaking effect exhibited by Trollius is the result of the leaching of a germination inhibitor from the seeds which is antagonistic to GA₄₊₇. Additionally, treatment of Trollius seeds with the gibberellin-biosynthesis inhibitor (2-chloroethyl)-trimethylammonium chloride (CCC) prior to testa removal retards germination. The inhibitory effect of CCC on germination is overcome by GA₄₊₇. Although CCC inhibits embryo growth during the presoaking of intact seeds, it does not affect the increased sensitivity of presoaked seeds to GA₄₊₇. Therefore, although endogenous gibberellins may be involved in the germination process, they do not contribute to the presoaking phenomenon. The expansion of isolated endosperm tissue is not affected by CCC. However, the chemical markedly inhibits endosperm expansion in intact seeds and implicates the embryo as both the site of production of the germination inhibitor and of gibberellin. These results are discussed in relation to previous studies and a model is presented to account for the characteristics of germination in Trollius.Abbreviations GA gibberellin - CCC (2-chloroethyl)-trimethylammonium chloride     

Effect of inhibitors of gibberellin biosynthesis on the induction of α-amylase in embryoless caryopses of Hordeum vulgare cv. Himalaya

The induction of -amylase by exogenously supplied gibberellin A₁ (GA₁) and GA₄ in embryoless caryopses of Hordeum vulgare (cv. Himalaya) was determined indirectly by measuring reducing sugars released from the endosperm. The presence of the inhibitors of GA biosynthesis, 2-isopropyl-4-dimethylamino-5-methylphenyl-1-piperidinecarboxylate methyl chloride (Amo 1618), Ancymidol, 2-chloroethyl trimethyl ammonium chloride (CCC) or (R,S)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,3-triazolyl)pentan-3-ol (PP333) did not inhibit -amylase production by either GA₁ or GA₄.Abbreviations Amo-1618 2-isopropyl-4-dimethylamino-5-methylphenyl-1-piperidinecarboxylate methyl chloride - CCC 2-chloroethyl trimethyl ammonium chloride - cv. cultivar - GA gibberellin - GC gas chromatography - GC-MS combined gas chromatography-mass spectrometry - PP333 (R,S)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,3-triazolyl) pentan-3-01     

Growth physics and water relations of red-light-induced germination in lettuce seeds

Red light (R) and gibberellins (GA) each induce a water potential decrease in the axes of lettuce (Lactuca sativa L.) embryos resulting in germination of intact seeds (achenes) or an increase in growth of the axes of isolated embryos. The fruit coat and endosperm are a substantial barrier to the penetration of exogeneous GA. Isolated embryos take up 35 times as much [³H]GA₁ as the embryos of intact seeds and respond to less than 1·10^-10 M GA₃ or GA₄₊₇. We calculated that only 1·10^-8 M of either GA₃ or GA₄₊₇ would result in 50% germination if the GA were able freely to penetrate the fruit coat. Exogenous GA₃ or GA₄₊₇, at concentrations insufficient to cause germination, result in an apparent synergistic promotion of germination when suboptimal R is applied. Yet suboptimal concentrations of exogenous GA₃ or GA₄₊₇ and suboptimal R result in only additive increases in the growth response in axes of isolated embryos. Dose-response curves demonstrate quantitative increases in the growth response of the isolated axes after R or GA treatments insufficient to induce germination in intact seeds, indicating that a threshold potential must be achieved by the embryonic axes before germination can occur.Abbreviations FR far=red light - GA gibberellin - PEG poly-ethylene glycol 4000 - P_fr far-red-absorbing phytochrome - R red light III.=Carpita et al. 1979b; IV.=Carpita et al. 1979c     

Inhibition of Gibberellin Production in the Fungi Gibberella fujikuroi and Sphaceloma manihoticola by Plant Growth Retardants

The effect of different types of plant growth retardants on fungal gibberellin (GA) formation has been studied in cultures of Gibberella fujikuroi and Sphaceloma manihoticola. Quaternary ammonium compounds (chlormequat chloride, mepiquat chloride, Amo-1618), triazoles (uniconazole and several experimental compounds), and the norbornanodiazetine tetcyclacis inhibited GA biosynthesis in both fungal species. Concentrations between 2 × 10⁻⁴ and 10⁻⁹m were required for a 50% inhibition of the production of gibberellin A₃ in Gibberella fujikuroi and of giberellin A₄ in Sphaceloma manihoticola. The formation of other prominent GAs was affected at a similar degree of intensity. Tetcyclacis was the most active compound in both fungi. Compared to the growth retardants mentioned above, the biological activity of chlorphonium chloride was low. The acylcyclohexanediones prohexadione and LAB 198 999 had virtually no activity. Most likely, this lack of activity is due to a rapid metabolism of the compounds in the cultures. For the triazole-type compounds and tetcyclacis, a relatively distinct correlation exists in their ability to inhibit GA formation in fungal cultures, to block ent-kaurene oxygenase in a cell-free system, and to reduce shoot growth of rice seedlings. Due to differences in their metabolic fate and species specificities, such conclusions cannot be made for the other compounds.