The levels of GA3 and GA20 may be associated with dormancy release in Onopordum nervosum seeds

Endogenous levels of two gibberellins, GA₃ andGA₂₀, were quantified in unimbibed Onopordumnervosum seeds collected from two different populations, whichshoweddifferences in their germination capacity. After purifying the seed extracts,gibberellin levels were evaluated by gas chromatography mass spectrometry byusing selected ion monitoring (GC-MS-SIM) adding deuterated gibberellins asinternal standards. The intraspecific differences in germination capacity wereassociated with differences in the endogenous levels of both gibberellins. Thecontents of GA₃ and GA₂₀ in seeds with high germinationrate were twice and five times higher, respectively, than those from seeds witha low germination rate, indicating a possible role of gibberellins in dormancyrelease in this plant species.     

Dormancy of <Emphasis Type="Italic">Nicotiana benthamiana</Emphasis> seeds can be broken by different compounds

The influence of after-ripening, sodium nitroprusside, potassium ferricyanide, cyanide, paclobutrazol and nitrite on germination of seeds of Nicotiana benthamiana was investigated as well as the influence of plant hormones such as gibberellins and abscisic acid. Dormancy of N. benthamiana seeds was broken by all treatments except treatments with abscisic acid, paclobutrazol and gibberellic acid (GA₃). Gibberellins had an interesting effect on dormancy breakage of studied seeds which was dependent on use of particular gibberellin: GA₃ or GA₄₊₇. Unlike GA₃, GA₄₊₇ had broken seed dormancy.     

The effect of light on the endogenous levels of cytokinins and gibberellins in seeds of sitka spruce (Picea sitchensis Carriere)

Abstract Maximum germination of Sitka spruce seed was obtained at 22°C under continuous fluorescent light or following a single 10 min exposure to red light (RL) given after 48 h of dark imbibition. The levels of endogenous cytokinins and gibberellins were examined in seeds which were exposed to different light regimes. Cytokinin levels in Sitka spruce seeds increased approximately 1.7–fold during exposure to continuous white light, during dark stratification, or within 1 h or a 30 min exposure to RL. A single peak of activity could be eluted from the LH^?20 Sephadex column which appeared in the same volume as authentic zeatin riboside. Gibber-ellin activity in Sitka spruce seeds increased rapidly during exposure to 30 min of RL and then declined during subsequent dark incubation. Similarly, under continuous white light, gibberellin activity was at least twice as high as in the dark. Two gibberellm-like substances were found in Sitka spruce seeds which appeared to be similar to GA₇ and GA₉ on the basis of their chromato-graphic properties. The zone of gibberellin activity directly associated with photo-excitation of Sitka spruce seeds contains conjugate gibberellins, possibly glucosyl esters, which when hydrolysed with β-glucosidase form two products which are similar to the free gibberellins which were detected. The light response in Sitka spruce seed is compared to a similar response in Scots pine (Pinus sylvestris L.) seeds.     

Germination of Tagetes minuta L. II. Role of gibberellins

The effect of the application of gibberellins to Tagetes minuta L. achenes (seeds) was determined at both 25°C, the optimal germination temperature, and 35°C, at which temperature the achenes are thermoinhibited. Both GA₃ and GA₄₊₇ accelerated germination at 25°C. Seed germination at 25°C was inhibited by paclobutrazol, but on subsequent application of GA₄₊₇ rapid germination was induced. Following application of GA₃ or GA₄₊₇ to thermoinhibited seeds, a significantly higher final germination percentage was observed than in the distilled water control. However, endogenous gibberellin levels in germinating and thermoinhibited seeds did not differ significantly.     

Thermodormancy in Celery Seeds and its Removal by Cytokinins and Gibberellins

Imbibition of celery (Apium graveolens L.) seeds at 32°C for up to 96 h lowered the upper temperature limit for germination. If this high temperature treatment was given in the light, these seeds germinated slightly earlier than those treated in the dark although the final percentage germination was similar for both treatments. The inhibitory effect of the high temperature treatment was completely removed by allowing the seeds to imbibe in a mixture of the gibberellins A₄ and A₇ (GA_4/7) and partially removed by the cytokinin N⁶-benzylaminopurine (BA). GA_4/7 was less effective when added before rather than after the high temperature treatment, whereas the opposite was true of BA. At constant temperatures more GA_4/7 was required to promote germination as the temperature was raised but addition of BA reduced the concentration of GA_4/7 required. A model is proposed for the control of celery seed germination by light and temperature through the action of endogenous cytokinins and gibberellins.     

Gibberellins in developing fruits of Pisum sativum cv. Alaska: Studies on their role in pod growth and seed development

Gibberellins A₁, A₈, A₂₀ and A₂₉ were identified by capillary gas chromatography-mass spectrometry in the pods and seeds from 5-d-old pollinated ovaries of pea (Pisum sativum cv. Alaska). These gibberellins were also identified in 4-d-old non-developing, parthenocarpic and pollinated ovaries. The level of gibberellin A₁ within these ovary types was correlated with pod size. Gibberellin A₁, applied to emasculated ovaries cultured in vitro, was three to five times more active than gibberellin A₂₀. Using pollinated ovary explants cultured in vitro, the effects of inhibitors of gibberellin biosynthesis on pod growth and seed development were examined. The inhibitors retarded pod growth during the first 7 d after anthesis, and this inhibition was reversed by simultaneous application of gibberellin A₃. In contrast, the inhibitors, when supplied to 4-d-old pollinated ovaries for 16 d, had little effect on seed fresh weight although they reduced the levels of endogenous gibberellins A₂₀ and A₂₉ in the enlarging seeds to almost zero. Paclobutrazol, which was one of the inhibitors used, is xylem-mobile and it efficiently reduced the level of seed gibberellins without being taken up into the seed. In intact fruits the pod may therefore be a source of precursors for gibberellin biosynthesis in the seed. Overall, the results indicate that gibberellin A₁, present in parthenocarpic and pollinated fruits early in development, regulates pod growth. In contrast the high levels of gibberellins A₂₀ and A₂₉, which accumulate during seed enlargement, appear to be unnecessary for normal seed development or for subsequent germination.Abbreviations GA_(a) gibberellin A_n - GC-MS combined gas chromatography-mass spectrometry - HPLC high-performance liquid chromatography - PFK perfluorokerosene - PVP polyvinylpyrrolidone     

Evidence for the accumulation of a germination inhibitor during progressive thermoinhibition of seeds of celery (Apium graveolens L.)

The germination of seeds of celery (Apium graveolens L.) becomes progressively thermoinhibited on incubation in the dark at high temperatures, the inhibitory temperature being dependent on the cultivar used. In two high-dormancy cultivars of celery, the production of germination inhibitors in seeds incubated in the dark at 26°C gradually increased over a 7-day period. Inhibitor production was measured by incubating seeds of the low-dormancy cultivar Florida 683 in homogenates of the thermoinhibited seeds of the high-dormancy cultivars and recording germination either in the light or with the gibberellins A₄ and A₇ (GA_4/7) in the dark. Most Florida 683 seeds which failed to germinate in the homogenates after 15 days were induced to germinate by addition of N⁶-benzyladenine (BA). The presence of BA in addition to GA_4/7 throughout incubation in the dark completely overcame the inhibitory effects of homogenates. This indicates that thermoinhibition of celery seeds is associated with the accumulation of a germination inhibitor which interacts with cytokinins. This does not appear to be abscisic acid (ABA) since ABA levels in thermoinhibited seeds were lower than in untreated seeds and did not increase with duration of high temperature treatment.Abbreviations ABA Abscisic acid - BA N⁶-benzyladenine - GA_4/7 a mixture of the gibberellins A₄ and A₇ - HTP high-temperature pretreatment     

Growth Regulators and the Phytochrome-Mediated Dormancy of Celery Seeds

Seeds of five celery (Apium graveolens L.) cultivars germinated at 15°C in the light or dark but at 22°C only in the light. This light requirement was overcome by treatment with a mixture of the gibberellins GA₄ and GA₇ (GA_4/7) but interactions of cytokinins, daminozide, ethephon, EDTA and N-phenyl-N′-4-pyridylurea (NC5392) with GA_4/7 were observed. Varietal differences in response to GA_4/7 concentration and the requirement for cytokinins were related to the upper temperature limits for germination of the different cultivars. Seeds of cultivars responding to low concentrations of GA_4/7 appeared to contain less natural inhibitor than those requiring either high concentrations of GA_4/7 or cytokinin in addition to low GA_4/7. The cytokinin requirement for germination was partially removed by leaching the seeds with water. Interaction studies with applied hormones indicated that in seeds incubated in the light inhibition by abscisic acid was partially alleviated by N⁶-benzyladenine but not by GA_4/7 application. The implications of these results are discussed in relation to the involvement of natural plant hormones in the dormancy mechanism of celery seeds.     

Relationships between position on the parent plant and germination characteristics of seeds of parsley (Petroselinium crispum Nym.)

The percentage germination of seeds of parsley cv. Imperial Curled was higher in the light than in the dark, the high temperature limits for germination being 30 and 28°C for light and dark respectively. At the higher temperatures, the germination rate was slower in the dark. At 30°C, treatment with a gibberellin A_4/7 mixture at 2 × 10^–4 M partially alleviated the inhibiting effect of darkness on the germination percentage. Pre-incubation of parsley seeds at 35°C in the dark for 30 h increased the rate, but decreased the percentage, of germination of seeds incubated at 15°C in the light. Germination and seedling emergence studies were made on seed harvested from four different umbel positions. Although heavier seeds were produced from primary umbels than from other umbel orders, they were less viable as measured by seedling emergence in the glasshouse. The rate of emergence was decreased with increasing umbel order i.e. with later seed development: this was reflected in subsequent seedling weights, with seedlings from quarternary umbel seeds being about half the weight of those from primary umbel seeds. The upper temperature limit for dark germination was only slightly affected by umbel order, with quarternary umbel seeds being the most thermo-inhibited.Abbreviations BA N⁶-benzyladenine - GA_4/7 a mixture of gibberellins A₄ and A₇ - SD8339 6-benzyl-amino-9-(tetrahydropyran-2-yl)-9H-purine     

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     

Enhancement by Low pH of Gibberellin Effects on Dormant Celery Seeds and Embryoless Half-Seeds of Barley

The action of gibberellins on celery (Apium graveolens L.) seed germination and the release of reducing sugars from barley (Hordeum vulgare L.) seed endosperm were enhanced by decreasing the pH of the incubation solution to below the pKa point. In most cases, low pH was obtained by mixing the solution with weak acids such as succinic acid 2.2 dimethylhydrazide (SADH), 2-chloroethylphosphonic acid (CEPA), or citric acid. However, lowering the pH of the gibberellin solution with strong acid (HCl) also increased markedly the activity of low concentrations of GA_4/7. The synergistic action of ethylenediaminetetraacetic acid (EDTA) with gibberellin was not dependent on the pH level of the incubation solution. The response of celery seeds to gibberellins was increased when their distal ends were removed: solution pH and EDTA had no effect on this response. The possible explanations of the synergism between low pH compounds and gibberellins are discussed.     

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     

Biosynthesis and Contents of Gibberellins in Seeded and Seedless Sweet Orange (<Emphasis Type="Italic">Citrus sinensis</Emphasis> L. Osbeck) Cultivars

In this work, we study the capacity to biosynthesize gibberellins (GA) of ovules (either fertilised or unfertilised), developing seeds and pericarp from fruitlets and their relation with fruit set capacity. Experiments were performed in adult, 12-year-old trees of seeded (Pineapple) and seedless parthenocarpic (Washington navel) sweet orange [Citrus sinensis L. Osbeck] cultivars. The activity of GA20-, GA3- and GA2-oxidases and gibberellin levels were measured in the ovules and pericarp of fruitlets in different development states. The results indicate that ovules are the main sites of gibberellin synthesis in fruitlets during the post-anthesis period. The most intense GA₁ synthesis—coincident with the highest expression of GA20ox2, GA3ox1 and GA2ox1—was detected in the ovules of the seeded cultivar, probably induced by fecundation and associated with low early fruitlet abscission rates. By contrast, the low activity detected in the sterile cultivar appears to be rather developmentally or constitutively regulated. As a fruitlet develops, the GA₁ concentration is augmented in the pericarp in comparison to ovules or developing seeds, and levels therein did not exhibit noticeable differences between varieties. Furthermore, developing seeds from pineapple had higher GA₁ content than the unfertilised abortive ovules from Washington navel. Taken together, data suggest a main role for this hormone in the control of fruitlet abscission, and also demonstrate a function in seed development.     

Radioactive gibberellin a(5) and its metabolism in dwarf peas

Radioactive gibberellin A₅ (³H-GA₅) was synthesized from gibberellic acid. When it was applied to dwarf peas grown in the dark, an average of 3% was converted to another acid gibberellin within 48 hours. The biological activity of the metabolite did not account for the response to applied GA₅. GA₅ is therefore assumed to be biologically active per se.³H-GA₅ did not appear to form a stable complex with a macromolecule in pea shoots. When injected into dwarf pea pods, ³H-GA₅ was readily metabolized by maturing seed to more water-soluble substances and to two other acidic compounds. This metabolism continued even throughout germination of the seed without reconversion of the metabolites to GA₅. It is concluded that “bound” GA₅ plays no part in the germination of dwarf pea seeds.     

Enhancement of Gibberellin Activity by Ethylenediaminetetraacetic Acid in Celery Seeds and Embryoless Barley Seeds

The phytochrome-controlled dormancy of celery seeds (Apium graveolens L.) can be broken by exogenously applied growth regulators. Gibberellins, especially the mixture of GA₄ and GA₇ appeared to be the primary stimuli for germination of darkincubated seeds. However, concentrations less than 1 mM were ineffective in replacing the light requirement in one celery cultivar (cv.Utah), and in another cultivar (cv.Lathom Blanching), concentrations above 1 mM were insufficient. The addition of ethylenediaminetetraacetic acid (EDTA) to gibberellins, increased markedly their activity and reduced by several orders of magnitude the gibberellin concentrations needed to stimulate a light treatment. This synergistic activity is similar to the effect of benzyladenine (BA) or Succinic acid-2,2-dimethylhydrazide (SADH) when added to gibberellins. In a diverse plant system, the mixture of gibberellin with EDTA increased significantly the amount of reducing sugars released by embryoless barley seeds.     

Interactions between Abscisic Acid, Gibberellins and Cytokinins in Grand Rapids Lettuce Seeds Germination

Experiments with Grand Rapids lettuce seeds (Lactuca sativa L.) maintained in darkness or irradiated with red light have shown that the inhibition of germination induced by low concentrations of ABA (2, 4, 6 μM) could be overcome by gibberellins (GA₃ or GA₄). The same results were obtained, although to a lesser extent, under the influence of two out of the four cytokinins tested (K and BAP) for seeds maintained in darkness. To suppress the block induced by higher concentrations of ABA (for example 8 μM), it was necessary to apply a cytokinin (K, BAP, Z or 2iP) and a gibberellin (GA₄ or GA₃) simultaneously, or a cytokinin following a red light treatment. Experiments conducted in darkness in which ABA (8 μM) was applied together with a cytokinin (BAP) and a gibberellin (GA₄) showed that the gibberellin and the cytokinin played similar roles towards each other and towards ABA.     

Dual Effect of Light on the Gibberellin- and Nitrate-Stimulated Seed Germination of Sisymbrium officinale and Arabidopsis thaliana

Red light (R) has a dual effect on the seed germination of the two related species Arabidopsis thaliana and Sisymbrium officinale. The two species provide different means to separate the light-effects. In S. officinale, stimulation of germination by R depends on the stimultaneous presence of nitrate (light-effect I). The effect of both factors is completely blocked by tetcyclacis, an inhibitor of gibberellin (GA)-biosynthesis. Addition of a mixture of gibberellins A₄ and A₇ (GA₄₊₇) antagonizes the inhibition. In the absence of nitrate, R shifts germination to lower GA-requirement (light-effect II). In A. thaliana a similar second light-effect is seen on the GA-requirement of GA-deficient ga-1 mutant seeds. R stimulates germination of wild type seeds in water (light-effect I). For both species, light-effect I shows a fluence threshold value of approximately 10⁻⁵ moles per square meter, which is independent of the nitrate concentration. Increasing nitrate concentrations narrow the fluence-range required for maximal germination whereby the product of nitrate concentration and fluence value determines the germination level, indicating a multiplicative interaction between R and nitrate. Fluence-response curves for light-effect II are similar for both species. Germination occurs in the range of 10⁻⁶ to 10⁻² moles per square meter fluence. The maximal level of germination is determined by the level of dark-germination and light-effect II. Increasing GA₄₊₇ concentrations induce a shift to lower fluence values. It is shown that in the second effect the co-action of R and exogenous GA₄₊₇ is clearly additive. It is concluded that light-effect I induces a chain of events leading to GA biosynthesis. Light-effect II seems to enhance the sensitivity of the seeds to GAs.     

Relationship between Position on the Parent Plant and Dormancy Characteristics of Seeds of Three Cultivars of Celery (Apium graveolens)

Germination and seedling emergence studies were made on seeds harvested from four different umbel positions of three cultivars of celery (Apium graveolens L.). Although heavier seeds were produced from primary umbels than from other umbels, these were less viable as measured by the germination percentage at I8°C in the light. However, germination of viable seeds from quaternary umbels was lower than that of seeds from primary umbels at 18°C in the dark when incubated with GA₄, (2 × 10 ^?4M) and seed from secondary and tertiary umbels tended to be intermediate in response. All viable seeds germinated when N⁶-benzyladenine (10^?2M) was used in combination with GA₄. Seeds from quaternary umbels of two of the cultivars had a lower high-temperature limit for germination in the dark than did seeds from other umbels. In glasshouse experiments the emergence of viable ‘quaternary’ seeds of these cultivars was higher than that of ‘primary’ seeds. Under these conditions the time to 50% of the final emergence as determined after 42 days was similar for seeds from all umbel positions within each cultivar. In two varieties seedling weights were greater from seeds of primary as compared to quaternary umbels, and in general, the largest seedlings arose from the heaviest seeds and the smallest from the lightest seeds in all three cultivars.     

Isolation and Characterization of Gibberellins in Mature Seeds of Phaseolus vulgaris

The change of endogenous gibberellin in the germinated seeds of Phaseolus vuigaris cv. Kentucky Wonder was investigated. Based on this result, endogenous gibberellins in the mature seeds were extensively investigated to result in the isolation of GA₁, GA₈, GA₈ glucoside, AB-II (unknown gibberellin-like substance) and glucosyl esters of GA₁, GA₄, GA₃₇ and GA₃₈.