Light Actions in the Germination of Cocklebur Seeds: V. EFFECTS OF ETHYLENE, CARBON DIOXIDE AND OXYGEN ON GERMINATION IN RELATION TO LIGHT

Esashi, Y., Hase, S. and Kojima, K. 1987. Light actions in thegermination of cocklebur seeds. V. Effects of ethylene, carbondioxide and oxygen on germination in relation to light.–J.exp. Bot. 38: 702–710. Effects of ethylene, CO² and O² on the germination of after-ripenedupper cocklebur (Xanthium pennsylvanicum Wallr.) seeds wereexamined in relation to pre-irradiation by red (R) or far-red(FR) light In order to remove the pre-existing Pfr, seeds weresoaked in the dark for various periods prior to light irradiationand gas treatments. Regardless of light, 0.3 Pa C₂H₄ promotedgermination at 23 ?C, but it strongly inhibited germinationwhen applied at 33 ?C, the optimal temperature for the germinationof this seed. However, delayed application of C₂H₄ during 33?C incubation stimulated germination independently of lightin a similar manner to that seen at 23 ?C. It is, therefore,suggested that the germination-regulating action of C2H4 iscompletely independent of phytochrome. In contrast, the germination-promoting effect of 3–0 kPaCO₂ was pronounced only when the seeds were previously irradiatedby R, regardless of temperature, suggesting that CO₂ actionto promote germination depends upon Pfr. A synergism betweenCO₂ and C₂H₄ at 23 ?C was observed only in the germination ofseeds pre-irradiated by R, while at 33 ?C an antagonism occurredindependently of light. The stimulation of C₂H₄ production byCO₂ was most striking in the cotyledonary tissue pre-irradiatedby R. However, the R-dependent enhancement of CO₂-stimulatedC₂H₄ production was negated by the subsequent FR and it wasnot found in the presence of 1-aminocyclopropane-1-carboxylicacid (ACC). Moreover, the R dependency of the germination-promotingCO₂ effect disappeared in the presence of C₂H₄. The R-dependentC₂H₄ production enhanced by CO₂ may thus be involved, at leastpartially, in some step of conversion from methionine to ACC. The germination-promoting effect of C₂H₄, but not CO₂, was enhancedby O₂ enrichment regardless of light. However, the germination-promotingeffect of pure O₂ itself appeared to depend upon pre-irradiationwith R Key words: Carbon dioxide, cocklebur seed, ethylene, far-red light, germination, oxygen, red light, Xanthium pennsyloanicum     

{alpha}-Aminoisobutyric acid : A probable competitive inhibitor of conversion of 1-aminocyclopropane-1-carboxylic acid to ethylene

Of 16 compounds related to 1-aminocyclopropane-1-carboxylicacid (ACC), aminoisobutyric acid (AIB) inhibited the productionof endogenous ethylene in the cotyledonary segments of cocklebur(Xanthium pennsylvanicum Wallr.) seeds most strongly. AIB at4 mM inhibited the formation of ethylene by about 50%, althoughthe O₂ uptake of the segments was not affected even at 20 mM.AIB also inhibited ethylene formation in the stem segments ofetiolated pea (Pisum sativum L. cv. Alaska) seedlings. Kineticanalysis with cell free extracts from etiolated pea shoots revealedthat AIB competitively inhibits the conversion of ACC into ethylene. (Received May 26, 1980; )     

Possible involvement of ethylene-activated {beta}-cyanoalanine synthase in the regulation of cocklebur seed germination

A possible involvement of ß-cyanoalanine synthase(CAS: EC 4.4.1.9 [EC] ) in germination processes of seeds was demonstratedusing pre-soaked upper seeds of cocklebur (Xanthium pennsylvanicumWallr.). Pretreatment in anoxia not only with KCN but also cysteine,as the substrates for CAS, stimulated the subsequent germinationof cocklebur seeds in air. However, the effect of cysteine wasmanifested even in air when applied together with C₂H₄, andits effect was further enhanced in combination with KCN. Thegermination-stimulating effect of KCN was intensified by C₂H₄only when 0₂ was present. In contrast, serine, another substrateof CAS, was effective in air only when combined with C₂H₄ and/orKCN. The addition of cysteine greatly reduced the cyanogenicglycoside content of seeds, but increased HCN evolution. Onthe other hand, glutathione did not have any effect on cockleburseed germination, HCN evolution or bound cyanogen content, suggestingthat cysteine is not acting as a reducing reagent. It is suggestedthat CAS regulates the process of cocklebur seed germinationby the dual action of enlarging the pool of amino acids andsupplying sulphydryl bases, the latter being more determinatelyimportant. Serine is effective only via the former action, whilecysteine would act via both. Key words: Cyanide, cyanogenic glycoside, ß-cyanoalanine synthase, seed germination, Xanthium pennsylvanicum     

The Role of Anaerobic Respiration in Germinating Muskmelon Seeds: I. IN RELATION TO SEED LOT QUALITY

Five muskmelon (Cucumis melo L.) cultivar seedlots from a commercialsource and freshly produced seeds of two cultivars, when artificiallyaged, were found to differ in their viability and vigour asdetermined by germination tests. Furthermore, the various commercialseedlots without ageing also exhibit a range of deteriorationlevels. Low vigour seeds had higher respiratory quotient valuesthan the high vigour seeds as a result of a higher level ofCO₂ production. This high level of CO₂ evolution in low vigourseeds may have been due to anaerobic respiration. Levels of acetaldehyde and ethanol produced by imbibing seedswere negatively associated with seed viability and vigour. After6 h of imbibition low vigour seeds produced significantly moreethanol and acetaldehyde than high vigour seeds. After 24 hof imbibition, ethanol continued to accumulate in the commercialseedlots up to 10-fold the amount produced after 6 h of imbibition,whereas, acetaldehyde levels increased less. However, in thefreshly produced, artificially aged seeds (except the most extremeageing), ethanol levels were reduced and no acetaldehyde productioncould be detected, indicating re-utilization of ethanol. It is suggested that ethanol production in the first hours ofimbibition can be used as a reliable index to predict germinationin muskmelon seedlots. Key words: Germination, Cucumis melo L., Seeds, Anaerobic respiration     

Control of the germination of secondary dormant cocklebur seeds by various germination stimulants

The responsiveness of non-dormant, upper cocklebur (Xanthiumpennsylvanicum Wallr.) seeds to various germination stimulants,such as CO₂ C₂H₄ CS(NH₂)₂, BA and enriched O₂, decreased withincreasing periods of water imbibition and was completely lostin the state of secondary dormancy. Unlike CO₂ BA and CS(NH₂)₂however, C₂H₂ and enriched O₂ effectively prevented the developmentof secondary dormancy, and their combination was the most effectivefor stimulating the germination of seeds which had undergoneimbibition for a long time. CS(NH₂)₂ and BA were effective,not by themselves but either under anaerobiosis or elevatedO₂ tension. Growth of the axial and cotyledonary segments excisedfrom aged seeds remained responsive to these germination stimulantsand could be further stimulated by exogenous C₂H₂. With imbibitionat a lower temperature, the seeds maintained high germinationin response to various stimulants and a high rate of C₂H₂ andCO₂ production during a long period of water imbibition. Theseresults are discussed in terms of the two possible causes forthe loss of responsiveness or induction of the secondary dormancy. (Received June 27, 1978; )     

The Role of Anaerobic Respiration in Germinating Muskmelon Seeds: II. EFFECT OF ANOXIA TREATMENT AND ALCOHOL DEHYDROGENASE ACTIVITY

Exposing muskmelon (Cucumis melo L.) cultivar seedlots to N₂atmosphere created totally anaerobic conditions which stimulatedethanol production and accumulation in both high and low vigourseeds. However, accumulation of ethanol was consistently higherin the low vigour seeds than in the high vigour ones. In addition,CO₂ production under N₂ and in air suggests the presence ofan apparent ‘Pasteur effect’ in the low vigour seedsbut not in the high vigour seeds. Acetaldehyde production underN₂ was very low and did not seem to be associated with seedvigour, probably because of its nature as an intermediate inethanol production. The fast shift toward ethanol may be dueto the fact that alcohol dehydrogenase, the enzyme which catalyzesthe conversion of acetaldehyde to ethanol, exists in sufficientamounts in the imbibing seeds so that it is not a limiting factorin the conversion to ethanol. Alcohol dehydrogenase activitydid not appear to be related to seed vigour. Key words: Cucumis melo L., Anaerobic respiration, Germination, Seeds.     

Ethylene Production, 1-Aminocyclopropane-l-Carboxylic Acid Content and Its Conversion to Ethylene in Axial Segments of Dormant and Nondormant Cocklebur Seeds

During the imbibition of water, the change in the ethylene productionof axial segments of nondormant (ND) cocklebur (Xanthium pennsylvanicumWallr.) seeds paralleled the change in the content of free 1-aminocyclopropane-l-carboxylicacid (ACC), but not the change in conjugated hydrolysable ACCin the axes. Aminoethoxyvinylglycine, anoxia and a-aminoisobutyricacid inhibited ethylene production, the lattertwo compoundscausing the accumulation of free ACC. Administration of ACCgreatly enhanced ethylene production in the axes. Thus, freeACC seems to be a direct precursor of ethylene production inthe axial tissue of cocklebur seeds. Imbibed dormant (D) axes characterized by inferior ethyleneproduction had less ability to convert exogenously applied ACCto ethylene as compared to ND axes. But, there was little differencebetween the D and ND axes in the endogenous contents of freeand conjugated ACC. This suggests that the inferior ethyleneproduction found in detached D axes is associated with the lowactivity of an ACC-ethylene converting system. (Received December 17, 1982; Accepted April 30, 1983)     

Dormancy and impotency of cocklebur seeds III. CO2- and C2H4-dependent growth of the embryonic axis and cotyledon segments

Growth of segments of embryonic axes and cotyledons excisedfrom dormant or nondormant cocklebur (Xanthium pennsylvanicumWallr.) seeds and CO₂ and C₂H₄ production in these segmentswere examined in relation to the effects of temperature, CO₂and C₂H₄. Both the nondormant axes and cotyledons grew evenat low temperatures below 23°C, but the dormant ones failedto grow. There was only little difference in the CO₂ evolutionbetween the nondormant and dormant ones, but both the axis andcotyledon segments from the dormant seeds exhibited little orno C₂H₄ productivity, unlike the nondormant ones, at low temperatures.However, a high temperature of 33°C caused rapid extensiongrowth and C₂2H₄ production even in dormant axes and cotyledons. The inability of dormant axes and cotyledons to grow disappearedcompletely in the presence of C₂H₄ at fairly low concentrations.Removal of endogenous CO₂ and C₂H₄ reduced the growth in bothaxes and cotyledons, while exogenous CO₂ mainly enhaced axialgrowth although exogenous C₂H₄ strongly stimulated the growthof both organs. Regardless of the dormant status, however, maximumgrowth of these organs occurred when C₂H₄ was given togetherwith CO₂. We suggest that dormancy in cocklebur seeds is dueto the lack of growing ability in both organs, caused by thelack of C₂H₄ productivity in both dormant axes and cotyledons,particularly in the former. (Received December 2, 1974; )     

Promotion of Cocklebur Seed Germination by Allyl, Sulfur and Cyanogenic Compounds

The effects of allyl, sulfur and cyanogenic compounds on thegermination of upper cocklebur (Xanthium pennsylvanicum Wallr.)seeds were examined. Mercaptoethanol and methylmercaptan aswell as KCN, substrates for rßcyanoalanine synthase(CAS), and H₂S and thiocyanate, the products of the CAS catalyzingreaction, were effective in promoting germination, suggestingthe involvement of CAS in germination. Most of allyl compounds, especially allylthiourea, as well asethylene which activated CAS [Hasegawa et al. (1994) Physiol.Plant. 91: 141], promoted the germination in an abnormal typewhich occurred by the predominant growth of cotyledons as didC₂H₄ [Katoh and Esashi (1975) Plant Cell Physiol. 16: 687].However, they failed to activate CAS unlike ethylene, and toliberate free ethylene during an incubation period. It was thuspossible that an C₂H₄-like double bond within allyl compoundscan act to promote seed germination. (Received June 10, 1996; Accepted August 21, 1996)     

Changes in Ethylene Production and in 1-Aminocyclopropane-1-carboxylic Acid (ACC) and Malonyl-ACC Contents of Cocklebur Seeds during Their Development

Ethylene production in developing cocklebur (Xanthium pennsyluanicumWallr.) seeds peaked when the dry weight of the seeds beganto increase in the early period of development. The productionthen began to decrease and stopped when the dry weight increasewas completed. The upsurge of ethylene production in the earlydevelopmental period paralleled increases in ACC synthase activityand the 1-aminocyclopropane-1-carboxylic acid (ACC) contentof the seeds, both of which rapidly decreased later. Malonyl-ACC (MACC) accumulated in developing cocklebur seedsduring the early period of development, before the ACC contentand ethylene production increased. Although the ACC synthaseactivity, ACC content and ethylene production showed markeddecreases, the MACC content remained almost unchanged duringthe middle period of seed development, with a pronounced decreaseoccurring in the late period. Exogenous application of MACCdid not promote ethylene production of seeds collected at thelate developmental stage. Aminoethoxyvinylglycine, an inhibitorof ACC synthase, strongly inhibited the ethylene productionof the same lot of seeds. Therefore, the decrease in the MACCcontent in developing cocklebur seeds was not due to reuse ofMACC for ethylene production. (Received May 24, 1984; Accepted August 15, 1984)     

Dormancy and impotency of cocklebur seeds I. CO2, C2H4 O2 and high temperature

High O₂ tensions, CO₄, C₂H₄ and high temperatures were effectivenot only in breaking the dormancy of cocklebur (Xanthium pennsylvanicumWallr.) seeds but also in increasing the germination potentialof the nondormant but small seeds. There were few qualitativedifferences in response to these factors between the dormantand impotent seeds. Unlike CO₂, however, enriched O₂ and C₂H₄were stimulative even at the low temperature of 13°C. Germination induced by CO₂, C₂H₄ and high temperature treatmentswas lowered when endogenously evolved C₂H₄ or CO₂ was removed,whereas the effect of O₂ enrichment was not affected by theirremoval. CO₂ and high temperatures remarkably stimulated C₂H₄production, whereas O₂ enrichment had no such effect. C₂H₄ productivity was lower in the dormant than non-dormantseeds, suggesting that the after-ripening is characterized byincreasing C₂H₄ production. (Received August 20, 1974; )     

Dormancy and impotency of cocklebur seeds II. Phase sequence in germination process

Interactions of C₂H₄, CO₂, O₂ and high temperature in stimulatinggermination of cocklebur (Xanthium pennsylvanicum Wallr.) seedswere studied and the phase sensitive to each factor during germinationwas determined. The combination of CO₂ and enriched O₂, andparticularly that of C₂H₄ and enriched O₂, much more effectivelystimulated germination than CO₂ and C₂H₄. At low temperature,however, the cooperation of CO₂ and enriched O₂ was lost andonly the effect of C₂H₄ in combination with CO₂ or enrichedO₂ remained. Carbon dioxide stimulated C₂H₄ production and induced germinationwhen it was applied in the first period of water imbibition,corresponding to the passive thrust forming phase. C₂H₄ becameeffective after the CO₂-responsive phase. In contrast, bothO₂ enrichment and high temperature became increasingly effectivewith the imbibition times. Anaerobiosis applied during the firstperiod of the germination process showed no inhibitory effect,whereas CO₂ and C₂H₄ were stimulative even under such a condition. (Received August 26, 1974; )     

Stimulation of ethylene production by hydroxamic acid, in particular, by benzohydroxamic acid

The effects on the ethylene production of known inhibitors ofa cyanide-insensitive, alternative respiration in plants wereinvestigated using cotyledonary segments of cocklebur (Xanthiumpennsylvanicum Wallr.) seeds. Benzohydroxamic acid (BHAM) at3 mM stimulated ethylene production 4- to 8-fold over the control,but respiration of the segments was hardly affected at thatconcentration. The stimulatory effects of 3-chlorobenzohydroxamicacid (CLAM) and salicylhydroxamic acid were far smaller thanthat of BHAM. BHAM at 3 mM also markedly stimulated the ethyleneformation in the epicotyl or hypocotyl sections of etiolatedpea (Pisum sativum L.) and mung bean (Vigna radiata [L.] Wilczek)seedlings. Moreover, 3 mM BHAM further promoted the increasedethylene formation which was caused by administration of 1-aminocyclopro-pane-1-carboxylicacid (ACC) to the cotyledonary segments. The promoting effectsby BHAM and CLAM were also found in the conversion of ACC intoethylene in pea stem homogenates. (Received July 26, 1980; )     

Inhibitory Effects of Methyl Jasmonate on the Germination and Ethylene Production in Cocklebur Seeds

Methyl jasmonate (JA-Me) inhibited the germination of cocklebur (Xanthium pennsylvanicum Wallr.) seeds. The inhibition of the germination of cocklebur seeds treated with JA-Me at concentrations less than 300 μm was nullified by ethylene applied exogenously, although the inhibitory effect of 1,000 μm JA-Me was not recovered completely even by high concentrations of ethylene (10,000 μL/liter). JA-Me inhibited ethylene production before seed germination. The level of 1-aminocyclopropane-1-carboxylic acid (ACC) in the cotyledonary tissues treated with JA-Me decreased but not the level of 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC). JA-Me inhibited the conversion of ACC to ethylene in the tissues. These results suggested that JA-Me inhibits ethylene production by prevention of ACC oxidation in addition to ACC synthesis. We believe that the inhibition of ethylene production by JA-Me results in the retardation of the germination of cocklebur seeds. Received June 4, 1997; accepted October 23, 1997     

Stimulation of ethylene production by hydroxamic acid, in particular, by benzohydroxamic acid

The effects on the ethylene production of known inhibitors ofa cyanide-insensitive, alternative respiration in plants wereinvestigated using cotyledonary segments of cocklebur (Xanthiumpennsylvanicum Wallr.) seeds. Benzohydroxamic acid (BHAM) at3 mM stimulated ethylene production 4- to 8-fold over the control,but respiration of the segments was hardly affected at thatconcentration. The stimulatory effects of 3-chlorobenzohydroxamicacid (CLAM) and salicylhydroxamic acid were far smaller thanthat of BHAM. BHAM at 3 mM also markedly stimulated the ethyleneformation in the epicotyl or hypocotyl sections of etiolatedpea (Pisum sativum L.) and mung bean (Vigna radiata [L.] Wilczek)seedlings. Moreover, 3 mM BHAM further promoted the increasedethylene formation which was caused by administration of 1-aminocyclopro-pane-1-carboxylicacid (ACC) to the cotyledonary segments. The promoting effectsby BHAM and CLAM were also found in the conversion of ACC intoethylene in pea stem homogenates. (Received July 26, 1980; )     

The Role of C2H4 in anaerobic induction of cocklebur seed germination

Non-dormant small cocklebur seeds (Xanthium pennsylvanicum Wallr.)are potentiated to germinate, if they are subjected to anaerobiccondition for certain time periods after being sufficientlypre-soaked under aerobic conditions. This is termed "anaerobicinduction" of seed germination. Such induction was slightlyinhibited by CO₂ applied during anaerobiosis, but markedly promotedby C²H⁴ Thus, C²H⁴ can exert its action even in anaerobiosis,but does not enhance the fermentative CO₂ evolution. No actualanaerobic induction occurred when over 1? O₂ was present, evenif C²H⁴ had been applied. Therefore, anaerobic induction seemsto be due to a concerted action of some anaerobically proceedingevents and the anaerobically produced C²H⁴. (Received May 31, 1976; )     

Further Study on the Involvement of Cyanide-Resistant Respiration in the Germination of Cocklebur Seeds

The effect of propyl-gallate (PG) and benzohydroxamic acid (BHAM),inhibitors of cyanide-resistant, alternative respiration path(AP), on germination were examined using after-ripened upperand lower cocklebur (Xanthium pennsylvanicum Wallr.) seeds pre-soakedat 23?C for various periods. Germination was strongly suppressedby PG or BHAM at concentration above 2 mM. However, germinationwas enhanced by low concentrations of PG or BHAM (0.25 or 0.5mM) which reduced some portions of AP operation. Similarly,the high temperature-induced germination of pre-soaked upperseeds was promoted by the same low concentration range of PGor BHAM, in which PG and BHAM were effective only when appliedat the start of high temperature incubation. The inhibitionof germination by C₂H₄ at high temperature occurred only whenseeds were exposed to C₂H₄ during the earlier period of hightemperature incubation, and delayed application tended to promotetheir germination, although most of germinated seeds did notexhibit the normal germination behaviour of predominant radicleprotrusion. If the upper seeds had been subjected to a short-timepre-soaking, the inhibition of high temperature-induced germinationby C₂H₄ was prevented by the low concentrations of PG or BHAM,although the germination restored was mostly an abnormal, predominantlycotyldonary growth, suggesting that the germination inhibitionby C₂H₄ may be involved in some step of axial growth or in thegrowth of some specific axial zone. The lower concentrationsof PG or BHAM were promotive to the axial growth even at 33?C.Based on these results, the involvement of AP in cocklebur seedgerminaton is discussed in relation to the differential growthof axial and cotyledonary tissues. (Received May 2, 1986; Accepted October 27, 1986)     

Interrelation between Ethylene and Carbon Dioxide in Relation to Respiration and Adenylate Content in the Pre-Germination Period of Cocklebur Seeds

Effects of C₂H₄ and CO₂ on respiration of pre-soaked upper cocklebur(Xanthium pennsylvanicum Wallr.) seeds during a pre-germinationperiod were examined in relation to effects of the two gaseson germination. At 33?C, cocklebur seed germination was greatlystimulated. This high temperature-stimulated germination wasseverely inhibited by C₂H₄, but not by CO₂, although both gasesstimulated germination at 23?C. C₂H₄ promoted seed respirationat 23?C, but its promotive effect decreases with increasingtemperature and disappeared at about 35?C, while CO₂ stimulatedrespiration regardless of temperature. CO₂ augmented the operationof the CN-sensitive, cytochrome path (CP) regardless of temperature,resulting in an increase in the ratio of the CP flux to a CN-resistant,alternative path (AP) flux. On the other hand, C₂H₄ augmentedthe operation of both paths, particularly of the AP, at 23?C,where it promoted germination. However, at 33?C where germinationis suppressed by C₂H₄, C₂H₄ preferentially stimulated respirationvia the AP, thus leading to an extremely high ratio of AP toCP. The inhibitory effect of C₂H₄ on germination at 33?C disappearedcompletely in enriched O₂, under which conditions CP is knownto be augmented. At 23?C, CO₂ and C₂H₄ acted independently incontrolling seed respiration, but they were antagonistic at33?C. The independent action appeared when the AP flux was verylow relative to the CP flux, while the antagonism appeared whenthe AP flux had risen. This differential action of the two gasesat different temperatures was also observed in the ATP level,adenylate pool size and energy charge of the axial tissues.These results suggest that the germination-controlling actionsof both CO₂ and C₂H₄ are fundamentally manifested through themodification of respiratory systems. However, the germination-inhibitingeffect of C₂H₄ at 33 ?C was not removed by inhibitors of AP,and there was little difference in the adenylate compounds betweenthe C₂H₄-treated and non-treated seeds at 33?C. Therefore, thephysiological action of C₂H₄ can not be explained only in termsof regulation of the respiratory system. (Received January 24, 1986; Accepted November 17, 1986)     

Differential Responsiveness in Zonal Growth of Cocklebur Seed Axes to CO2, C2H4, O2 and Respiration Inhibitors

The axial growth of de-coated cocklebur (Xanthium pennsylvanicumWallr.) seeds, whose axes were divided into 4 zones, was examinedin relation to the temperature-dependent shift of the effectof C₂H₄ on germination. At 23?C, where both C₂H₄ and CO₂ stimulatedgermination, CO₂ promoted the axial growth at the radicle tipzone, whereas C₂H₄ promoted growth in the proximal portion ofthe axis. At 33?C, C₂H₄ inhibited germination, and stronglysuppressed the growth at the radicle tip, whereas the effectof CO₂ did not change. The inhibition of growth at the radicletip zone was alleviated by O₂ enrichment, which also reversedthe inhibition of germination. It is thus apparent that thetemperature-dependent shift of the action of C₂H₄ is associatedwith a temperature-dependent responsiveness of the radicle tipzone to C₂H₄. Growth of the radicle tip zone was sensitive toNaN₃, whereas the proximal portion was sensitive to benzohydroxamicacid, an inhibitor of alternative respiration, suggesting thatthere may be an increase in the operation of the alternativerespiration path along a gradient of axial tissue from the tiptowards the cotyledonary side. The effects of CO₂ and C₂H₄ arediscussed in relation to the different respiratory activitiesin each axial zone of cocklebur seeds. (Received May 9, 1986; Accepted November 6, 1986)     

Dormancy and Impotency of Cocklebur Seeds. IX. Changes in ACC-Ethylene Conversion Activity and ACC Content of Dormant and Nondormant Seeds during Soaking

Differences in ethylene production between dormant (D) and nondormant(ND) lower seeds of cocklebur (Xanthium pennsylvanicum Wallr.)were studied with respect to changes in the activity of conversionof 1-aminocyclopropane-l-carboxylic acid (ACC) to ethylene andin the contents of ACC and malonyl-ACC in their axial-tissuesduring soaking. Superior ethylene production in ND seeds ascompared to D seeds became evident during a soaking period rangingfrom 12–24 h, when the radicle protrusion in ND seedshad not yet occurred. Ethylene production in ND seeds increasedabruptly after the radicle protrusion. The inhibitors of ethyleneproduction, aminoethoxyvinyglycine, cobaltous ion and -aminoisobutyricacid, inhibited the germination of ND seeds, whereas ACC enabledD seeds to germinate. Activity of ACC-ethylene conversion was absent in dry axialtissues and developed with soaking. After 24 h, this activityin ND axes was superior to that in D axes. Under hypoxia, however,the difference in the ACC-conversion activity appeared before24 h. On the other hand, the contents of ACC in both D and NDaxes remained almost unchanged until 24 h of soaking. It isthus suggested that the inferior ethylene production in D seedsis associated mainly with their low activity of ACC-ethyleneconversion, though partly with their low activity of ACC supply. Activity of ACC-ethylene conversion in the axes of ND seedsincreased sharply after radicle protrusion which occurred after24 h of soaking. Correspondingly, the contents of both ACC andmalonyl-ACC increased in the axes of germinated ND seeds. Theseimply that the high ethylene production in the ND seeds in thepost-germination period comes from the increasing activitiesof ACC supply as well as ACC-ethylene conversion in their axes. Key words: Cocklebur seeds, Dormancy, Ethylene production, 1-aminocyclopropane-1-carboxylic acid, Germination, Xanthium