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.     

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.     

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.     

Dormancy break of celery (Apium graveolens L.) seeds by plant derived smoke extract

Seed dormancy of a highly-dormant cultivar of celery (Apium graveolens L.) was broken by combinations of plant-derived smoke extract or N⁶-benzyladenine (BA) and gibberellins A_4/7 (GA_4/7) in the dark at temperatures between 18 and 26°C. A less dormant cultivar which responded to GA_4/7 alone showed no additional response to smoke extract or BA. Neither smoke extract nor BA affected either cultivar in the dark in the absence of GA_4/7. The partial dormancy-breaking effect of short exposures to red-light was also enhanced by smoke extracts in this highly-dormant cultivar. The results suggest that smoke extracts act in a similar way to cytokinins, by enhancing gibberellin activity in the celery seed system.Abbreviations BA N⁶-benzyladenine - GA_4/7 A₄ and A₇ gibberellin mixture     

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.     

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.     

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     

The effect of temperature on the germination-promoting activities of cytokinin and gibberellin applied to celery seeds (Apium graveolens)

Celery seeds (Apium graveolens L. cv. Lathom Blanching) made dormant by high temperature pretreatment (28–40°C) during imbibition in the dark, germinated at 22°C in the light after treatment with benzyladenine (BA). This BA-induced promotion of germination increased with increasing pre-treatment temperature from 32 to 38°C. whether BA was given before or after pretreatment. A mixture of gibberellins A₄ and A₇ (GA_4/7) given before a 4 day high temperature pretreatment at 32°C partially inhibited the germination-promoting activity of GA_4/7 given after. It is suggested that gibberellin induces the formation of a thermola-bile product which is necessary for germination, the precursor of which has a limited source.     

Possible control of gibberellin-induced release of temperature-dependent primary dormancy in seeds of celery (Apium graveolens L.) by transmembrane ion fluxes

The temperature-dependent primary dormancy of cv Florida 683 celery seeds in darkness was broken by GA_4/7 (2 × 10^-4 M) alone but other growth regulators such as BA, ethephon or daminozide were necessary to break dormancy of cv Lathom Blanching seeds in the presence of GA_4/7 at this concentration. Although AgNO₃ partially inhibited both the ethephon- and BA- induced germination of cv Lathom Blanching seeds in the presence of GA_4/7 in the dark it did not affect the promotive action of daminozide. Ethephon did not overcome the inhibitory action of high concentrations of AgNO₃ in the light. The ethylene synthesis inhibitor aminoethoxyvinylglycine (AVG) did not inhibit the germination of cv Lathom Blanching seeds induced by growth regulators in the dark or in the absence of growth regulators in the light. Fusicoccin (FC) did not break celery seed dormancy unless applied in the presence of GA_4/7. Germination of cv Lathom Blanching celery seeds treated with GA_4/7 at 16°C in the dark was inhibited by the K⁺ ionophore benzo-18-crown-C-6 (18-C-6) and in the presence of Ca²⁺ by the Ca²⁺ ionophore A23187; the 18-C-6 inhibition was reversed by BA.It is concluded that the involvement of gibberellin in celery seed dormancy is not dependent on endogenous ethylene and is directly or indirectly controlled through the action of other hormones on transmembrane ion fluxes.     

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.     

Variations in Endogenous Gibberellins in Developing Bean Seeds II. Changes Induced in Acidic and Neutral Fractions by GA(1)

Immature (8-mm), medium mature (11-mm), and mature green (16- and 17-mm) bean seeds (Phaseolus vulgaris L. cv. Kentucky Wonder and Bountiful) were incubated in gibberellin A₁ solutions for 24 hours at 20°. Extracts from the seeds were separated into nonacidic, acidic ethyl acetate, and acidic butanol fractions. These were chromatographed. The eluates of the chromatograms were tested on Progress No. 9 dwarf peas grown under red light. The level of neutral gibberellin-like substances remained unchanged in immature seed, but they increased markedly in mature green seeds. Coincident with increased levels of the neutral substances, there were significant decreases in acidic ethyl acetate-soluble gibberellin-like substances, including applied GA₁, and in 1 acidic butanol-soluble gibberellin-like substance. Seed incubation in GA₁ brought about increased activity of substance B-II in immature and medium mature seeds. The level of butanol-soluble gibberellin-like substance B-I in seeds of any size was not affected by incubation in GA₁. Considering the marked increases in activity induced in the neutral fraction and the decreases in activity of certain eluates from the chromatograms of the acidic fractions, it was concluded that the neutral fraction may serve as a reserve form of gibberellins in the dry seed. The acidic ethyl acetate substances and substance B-II may be required for normal development of the bean seed.     

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.     

The Location of Cytokinins and Gibberellins in Wheat Seeds

Wheat seeds (Triticum aestivum L. cvs. Yorkstar and Sirius) were cut transversely into embryoless and embryo-containing (embryonated) halves and the content of endogenous cytokinins and gibberellins in both halves determined before and after the seeds imbibed water for 12–15 h at 22°C in the light. Dry seeds contained little ethyl acetate-extractable gibberellin activity as measured by bioassay but n-butanol-soluble cytokinins were detected mainly in the embryoless halves. Dry, embryonated half-seeds contained water-soluble gibberellins which could be extracted into acidic ethyl acetate after treatment of the aqueous extract with either alkaline phosphatase, β-glucosidase or a crude pectolytic enzyme preparation. When half-seeds were allowed to imbibe water for 12 h and then extracted, cytokinin activity was largely lost from the embryoless halves and completely from the embryonated halves and water-soluble gibberellins were also lost from the embryonated halves. However, ethyl acetate-soluble gibberellins were present in the latter suggesting that “bound’ gibberellins were released during imbibition. The hormones present in normal and naturally embryoless dry grains of cv. Yorkstar were also determined. Both gibberellin and cytokinin activity was higher in normal grains suggesting that the presence of an embryo is essential for synthesis or accumulation of these hormones in the grain during development.     

Roles of Soluble Sugars in Protecting Phytochrome- and Gibberellin A3-Mediated Germination Control in Skotodormant Lettuce Seeds

Skotodormant seeds of Lactuca sativa Grand Rapids imbibed in darkness for 10 days (10-day DS) germinated poorly upon terminal treatment with red light (R) or gibberellin A₃ (GA₃). Soluble sugars in the imbibition solutions influenced the depth of skotodormancy. Ten-day DS seeds, imbibed in 50–500 mm sucrose or 100–500 mm glucose and given terminal GA₃ germinated completely and germinated about 80% when imbibed in 100 mm galactose, mannose, lactose, or maltose. In contrast, terminal R applied to 10-day DS seeds caused only 20–50% germination. If given R at day 0 and imbibed for 10 days in darkness in 500 mm sucrose or glucose, seeds washed free of exogenous glucose or sucrose then germinated about 50% in darkness in water. These seeds responded to terminal R or GA₃ with complete germination. When seeds were given FR at day 0, germination responses following terminal R or GA₃ were significantly lower when the duration of DS was increased from 7–10 day DS to 15 days. In 10-day DS seeds given initial FR and imbibed in either solutions of 50 or 100 mm sucrose and KNO₃, either terminal R or GA₃ treatment gave complete or near complete germination. It is concluded that seed exposure to certain soluble sugars and/or nitrate during a 10-day DS protected certain substrates and thereby extended the sensitivity of the seeds to terminal R or GA₃ treatment. The study provides substantial evidence for nonhormonal factors associated with light and GA action in the control of seed skotodormancy. Received October 30, 1996; accepted April 22, 1997     

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.     

Comparative effects of gibberellins and an N-substituted phthalimide on seed germination and extension growth of celery (Apium graveolens L.)

The N-substituted phthalimide AC 94377 (1-(3-chlorophthalimido)-cyclohexanecarboxamide) was equally effective as a mixture of the gibberellins A₄ and A₇ (GA_4/7) in breaking dormancy and stimulating germination of celery seeds when either was used in combination with ethephon or daminozide as a seed soak. Whereas seedlings emerging from GA_4/7-treated seeds became etiolated in comparison with those from untreated seeds, those from AC 94377-treated seeds showed normal development. Preharvest sprays of gibberellic acid (GA₃) increased the height of mature plants in comparison with untreated controls by about 16 per cent whereas AC 94377 was ineffective. The yield from GA₃-treated plots was about 10 per cent greater than that from AC 94377-treated plots.     

Interactions of Abscisic Acid, Cytokinin and Gibberellin in the Control of Betacyanin Synthesis in Seedlings of Amaranthus caudatus

In de-rooted seedlings of Amaranthus caudatus L., betacyanin synthesis induced by white light or cytokinin was inhibited by abscisic acid (ABA) or a mixture of gibberellins A₄ and A₇ (GA_4/7). The GA_4/7 and ABA effects were additive. Thus ABA inhibited the cytokinin action but had no effect on the gibberellin response.     

Effect of Gibberellic Acid on Dark and Light Germination at Different Temperatures of Calluna, Ledum andRhododendron Seeds

The aim of the experiments was to study the effects of gibberellic acid (GAs) on the germination of Calluna vulgaris L., Ledum palustre L. and Rhododendron lapponicum (L.) Wahlenb. seeds under different environmental conditions. Under continuous light from white fluorescense tubes (3000 lux), untreated seeds of Calluna were partly dormant at all temperatures studied (9, 15, 21, 27, 27/9, 8/16 hours). Percentage of dormant seeds increased, however, with decreasing temperature, and it varied also from seed lot to seed lot. Untreated seeds of Ledum were dormant in light at 9° and 15°C but not at higher temperatures. Untreated seeds of Rhododendron were completely dormant in light at temperatures from 13° to 24°C. Seeds of all species were completely dormant in darkness both at 15° and 27°C. GA₃ stimulated greatly the germination of all species under all studied environmental conditions. The used concentrations (0.2–3.2 mM) gave nearly 100% germination in most cases. At 9°C the dormancy in some seed lots of Calluna and Ledum was only partly broken by the used concentrations of GA₃.     

Thermomorphogenic and Photomorphogenic Control of Stem Elongation in Fuchsia Is Not Mediated by Changes in Responsiveness to Gibberellins

Stem elongation in Fuchsia × hybrida was influenced by cultivation at different day and night temperatures or in different light qualities. Internode elongation of plants grown at a day (25°C) to night (15°C) temperature difference (DIF+10) in white light was almost twofold that of plants grown at the opposite temperature regime (DIF−10). Orange light resulted in a threefold stimulation of internode elongation compared with white light DIF−10. Surprisingly, internode elongation in orange light was similar for plants grown at DIF−10 and DIF+10. Flower development was accelerated at DIF−10 compared with DIF+10 in both white and orange light. To examine whether the effects of DIF and light quality on shoot elongation were related to changes in gibberellin metabolism or plant sensitivity to gibberellins (GAs), the stem elongation responses of paclobutrazol-treated plants to applied gibberellins were determined. In the absence of applied gibberellins paclobutrazol (>0.32 μmol plant⁻¹) strongly retarded shoot elongation. This inhibition was nullified by the application of about 10–32 nmol of GA₁, GA₄, GA₉, GA₁₅, GA₁₉, GA₂₀, GA₂₄, or GA₄₄. The results are discussed in relation to possible effects of DIF and light quality on endogenous gibberellin levels and gibberellin sensitivity of fuchsia and their effects on stem elongation. Received October 4, 1997; accepted December 17, 1997     

Uptake of Gibberellic Acid in Intact and Scarified Seeds of Barbarea vulgaris

Uptake of gibberellic acid as a function of duration of exposure, external concentration, and seed lot was measured in seeds of yellow rocket (Barbarea vulgaris R. Br.) by means of lettuce hypocotyl bioassays of the acidic, basic, or neutral fractions of seed extracts and by uptake of ¹⁴C-GA₃. In intact seeds, where mM levels of GA₃ promoted only 25% germination, uptake was completed within 24 h of exposure. The maximum uptake was about 0.2% of external amount. Although germination promotion by GA₃ differed among seedlots of yellow rocket, relative uptake (percentage of the external GA₃) was nearly the same. The relative rate of uptake of GA₃ was similar for scarified and intact seeds, but germination was promoted in scarified seeds by much lower levels of GA₃ than in intact seeds. Total uptake in scarified seeds was much higher, however (about 10% of the external amount). In seeds imbibed in H₂O, practically no endogenous GA-like activity was detected in either the acidic, basic, or neutral fractions. It was also apparent that intact seeds could take up quantities of GA₃ that failed to promote germination, but were comparable to quantities that promoted germination in scarified seeds.