Light Actions in the Germination of Cocklebur Seeds: I. DIFFERENCES IN THE LIGHT RESPONSES OF THE UPPER AND LOWER SEEDS

Germination responses to light were studied in the upper andlower seeds of cocklebur (Xanthium pennsylvanicum Wallr.). Thelower seed was dark-germinating and negatively photoblastic;the upper one had a red-light (R) requirement and was positivelyphotoblastic. Germination of the lower seeds was inhibited bya prolonged single irradiation with R, blue (B) or far-red (FR)light applied during imbibition. The maximal inhibitory effectof a single irradiation occurred 9 h and 13 h after the startof soaking at 33 °C and 23 °C, respectively. However,the inhibitory effect of R differed from that of B and FR, byonly delaying germination. A single exposure to B or FR lightcould be replaced by intermittent B or FR irradiation, and theireffects were repeatedly reversible by the following R irradiation.If the upper seeds were not exposed to R during imbibition,they failed to germinate even at 33 °C which was optimalfor germination, and the promotive effect of R increased withdelay of its application time. The photoperceptive locus incocklebur seeds was the axial tissue for all B, R and FR. Lightreceived by the cotyledonary tissue had little effect. Germinationdimorphism in response to light is discussed with respect tothe phytochrome content and the ageing of axial tissues. Key words: Blue light, Dimorphism, Far red light, Germination, Red light, Xanthium seed     

Light Actions in the Germination of Cocklebur Seeds: III. EFFECTS OF PRE-TREATMENT TEMPERATURE ON GERMINATION RESPONSES TO FAR-RED LIGHT AND ON DARK GERMINATION IN THE RED LIGHT-REQUIRING UPPER SEEDS

Esashi, Y., Oota, H., Saitoh, H. and Kodama, H. 1985. Lightactions in the germination of cocklebur seeds. III. Effectsof pre-treatment temperature on germination responses to far-redlight and on dark germination in the red light-requiring upperseeds.—J. exp. Bot. 36: 1465-1477. Red light (R) responsiveness in R-requiring upper cocklebur(Xanthium pennsylvanicum Wallr.) seeds changed in differentpatterns during a soaking period at different temperatures.At temperatures above 23°C, the responsiveness increasedand then decreased. At lower temperatures (3–18°C),however, it continued to increase throughout an experimentalperiod. The lower temperatures caused germination in the subsequentdark at 33°C, regained the R responsiveness and acquiredthe dark germinability when subsequently exposed to 8°C,to an extent proportional to the duration of the chilling. Far-red (FR) was inhibitory to germination in an earlier soakingperiod at lower temperatures, but its effect gradually decresed,and finally turned promotive. The negative FR response was repeatedlycontrolled by the following R irradiation. However, the positiveFR response was enhanced by an immediate R irradiation, andFR/R reversibility occurred after the second FR. In contrastto the R responsiveness and dark germinability, the positivegermination response to FR was not induced by soaking at 3°C,in which the growth of the axial tissue as a photoreceptivesite did not occur at all. Similarly, it was not manifestedwhen the seeds soaked at 33°C were subsequently subjectedto 8°C. Key words: Cocklebur seeds, dark germination, far-red light, low temperature, red light, seed germination, Xanthium pennsylvanicum     

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     

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)     

Possible Involvement of the Alternative Respiration System in the Ethylene-stimulated Germination of Cocklebur Seeds

Respiration of nondormant upper cocklebur (Xanthium pensylvanicum Wallr.) seeds was enhanced by exogenous C₂H₄, proportionally to the concentration of C₂H₄ and the duration of presoaking of the seeds. Benzohydroxamic acid (BHM) and salicylhydroxamic acid (SHM), inhibitors of alternative respiration, inhibited both the germination of nondormant lower cocklebur seeds and the respiration of the upper seeds presoaked for periods of 12 to 30 hours. Both the growth and respiration of axial and cotyledonary tissues were also inhibited by BHM. Moreover, BHM inhibited both the C₂H₄-induced germination of the upper seeds and their C₂H₄-stimulated respiration; the inhibition occurred only with concomitant addition of C₂H₄ and BHM. The respiration of seeds with a secondary dormancy induced by presoaking for prolonged periods was markedly stimulated by C₂H₄ but not suppressed by BHM. It was suggested that the alternative respiration system may be involved in the normal germination process of cocklebur seeds, secondary dormancy may result from its inactivation, and C₂H₄ may exert its germination-promoting action by stimulating the alternative respiration. The effects of BHM and SHM can suggest but not prove the involvement of the alternative respiration in seed germination.     

Effect of High Temperature on the Development of Cyanide-Sensitive Respiration in the Germination Process of Spinach Seeds

Germination of spinach (Spinacia oleracea L. var. grabra cv.Nobel) seeds was inhibited at a high temperature (35?C). Effectsof KCN on the respiration of seeds incubated at 20 and 35?Cwere compared in order to investigate the mechanism of inhibitionof seed germination by high temperature. Respiration of germinatingseeds incubated at 20?C was inhibited about 50% by 5 mM. KCNsolution, whereas it hardly inhibited the weak respiration ofthe seeds at 35?C. Germination of seeds was delayed by exogenousKCN. When the KCN solution was renewed daily, germination wascompletely inhibited. Pericarp removal promoted germinationat 35?C, but atypical germination (cotyledons emerging earlierthan a radicle) took up more than half of the total germination.The inhibitory action of KCN on the respiration of seeds wasnot altered by pericarp removal. A KCN addition, even at 20?C,elicited atypical germination in the pericarp-less seeds. Theseresults show that cyanide-sensitive respiration is needed toinduce typical spinach seed germination (root emergence), butis rendered inoperative by high temperatures thus bringing aboutpoor germination and atypical germination. (Received December 1, 1984; Accepted February 8, 1985)     

Light Actions in the Germination of Cocklebur Seeds: IV. DISAPPEARANCE OF RED LIGHT-REQUIREMENT FOR THE GERMINATION OF UPPER SEEDS SUBJECT TO ANOXIA, CHILLING, CYANIDE OR AZIDEPRETREATMENT

Esashi, Y., Fuwa, Nn Kojima, K. and Hase, S. 1986. Light actionsin the germination of cocklebur seeds. IV. Disappearance ofred light-requirement for the germination of upper seeds subjectto anoxia, chilling, cyanide or azide pretreatmenL—J.exp. Bot. 37: 1652–1662. The effects on the germination of positively photoblastic uppercocklebur (X anthium pennsylvanicum Wallr.) seeds by pretreatingwith anoxia, chilling, cyanide or azide, which stimulates theirdark germination, were examined in relation to light actions.Prior to experiments, seeds were pre-soaked at 23 °C inthe dark for 1 or 2 weeks to remove the pre-existing Pfr. Whenthe prctreatment conditions were suboptimal for germinationinduction, the stimulating effects of the pretreatments on germinationduring a subsequent dark period at 23 °C were manifest onlywhen seeds were irradiated with red light before or after thepretreatment Red light promotion was reversed by blue or far-redlight treatment. However, both prc-chilling for 6 d at 8 °Cand prctreatment with 1· 5 mol m ^{– 3} NaN³ for 2d could induce full germination without red light exposure.On the other hand, both pre-exposure to anoxia for 8 d and pretreatmentwith 30 mol m^–3 KCN could induce the dark germinationonly when germination occurred at 33 °C which is known toaugment the ratio of an alternative respiration flux to a cytochromeone. Moreover, the dark germination in response to these inductionswere strongly inhibited by the inhibitors of alternative respiration,propyl gallate and benzohydroxamic acid, applied during a subsequentdark period. It was thus suggested that Pfr has some relationto the operation of two respiration systems of cocklebur seeds,but it is not indispensable to germination of this positivelyphotoblastic seed. Key words: Anoxia, azide, blue light, chilling cyanide, dark germination, far-red light, red light, seed germination, X anthium pennsylvanicum     

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)     

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)     

Germination of cocklebur seed and growth of their axial and cotyledonary tissues in response to C2H4, CO2 and/or O2 under water stress

Abstract. Germination modes of lower seeds of cocklebur (Xanthium pennsylvanicum Wallr.) under different water stresses, prepared with mannitol solution, were examined in relation to gaseous factors. As the concentration of mannitol increased, germination was increasingly inhibited at a mode which was drawn by two straight lines having different slopes and meeting at an angle. One is a sharp line occurring at the lower concentrations of mannitol; the other is a gentle line occurring at higher concentrations of mannitol. The former reflected the growth response of axial tissues to mild water stress, whereas the latter reflected the growth response of cotyledonary tissues to severe water stress. The germination potential of cocklebur seeds increased with increasing temperature. Thus, the seeds were more resistant to water stress at higher than al lower temperatures. This increased germination potential under water stress resulted from the greater growth potential of axial tissues, but not cotyledonary tissues, at higher temperature. Increased O₂ levels improved both the reduced axial and cotyledonary growth under water stress. Carbon dioxide predominantly enhanced axial growth under water stress, whereas C₂H₄ exclusively enhanced cotyledonary growth. Thus, these gases were effective in potentiating germination under water stress. When combined with each other, these gases caused more pronounced growth of the axial and cotyledonary tissues, leading to germination under more severe water stresses. Maximal axial and cotyledonary growth under water stress occurred in the simultaneous presence of CO₂, C₂H₄ and O₂, which allowed the germination at higher mannitol concentrations above 0.6 kmol m^?3 From these results, it was suggested that cocklebur seeds would override water stress by depending upon both the Corresponding axial growth and the C₂H₄-responding cotyledonary growth.     

Ethylene Production in Pea and Cocklebur Seeds of Differing Vigour

Relationships between seed vigour and ethylene (C₂H₄) productionwere studied using C₂H₄-responsive fatty cocklebur seeds (Xanthiumpennsyhanicum Wallr.) and C₂H₄-insensitive starchy pea seeds(Pisum sativum L. cv. Alaska), which had been harvested in differentyears and subjected to different storage conditions. In bothspecies, the seeds with the highest vigour evolved the largestamounts of C₂H₄ during a period of water imbibition. The reductionof C₂H₄ production in cocklebur seeds occurred concomitantlywith the reduction in the growth potentials of both axial andcotyledonary tissues. Similarly, the activity of ACC-C₂H₄ conversionincreased with soaking, and was greater in seeds of high vigourcompared with those of low vigour. However, the change in ACCcontent in pea seeds differed from that in cocklebur seeds.That is, pea seeds with high vigour accumulated less ACC duringan imbibition period than those with low vigour. From theseresults it was suggested that the inferior C₂H₄ production bylow vigour pea seeds is mainly attributable to low ACC-C₂H₄conversion, whereas that by low vigour cocklebur seeds is dueto the shortage of ACC supply in addition to the reduced ACC-C₂H₄conversion. However, germination of deteriorated cocklebur seedswas not restored by exposure to ACC or C₂H₄, suggesting thatthe loss of seed vigour reduces the responsiveness of seedsto C₂H₄ as well as C₂H₄ production. Key words: Pea, cocklebur, seed vigour, ethylene production, 1-aminocyclopropane-1-carboxylic acid     

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     

Dormancy and Impotency of Cocklebur Seeds: IV. Effects of Gibberellic Acid, Benzyladenine, Thiourea, and Potassium Nitrate on the Growth of Embryonic Axis and Cotyledon Segments

Germination of nondormant but impotent small cocklebur seeds (Xanthium pennsylvanicum Wallr.) was promoted profoundly with thiourea or benzyladenine, and slightly with gibberellic acid. Gibberellic acid was ineffective in causing the germination of dormant cocklebur seeds, although thiourea and benzyladenine were effective. Experiments with excised seed pieces showed that the promotive effects of thiourea, benzyladenine, and gibberellic acid on cocklebur seed germination were associated with the enhancement of growth of seed parts; thiourea stimulated predominantly the axial growth, whereas benzyladenine stimulated predominantly the cotyledonary growth.     

Spectrophotometric characterization of phytochromes in dimorphic cocklebur seeds in relation to capacity for germination

Phytochrome was measured spectrophotometrically in different tissues of the upper (positively photoblastic) and lower (negatively photoblastic) seeds of the cocklebur (Xanthium pennsylvanicum Wallr.). Axial parts of the seeds, in particular parts of the radicle, contained high levels of phytochrome, while cotyledonary parts contained only low levels. These results were consistent with the distribution of the light-sensitive areas of the seeds that were associated with germination. Phytochrome levels in both types of dimorphic seeds increased gradually with increasing duration of dark imbibition for 4–8 h, then the rates of increase in levels of phytochrome accelerated. In both types of seed, some phytochrome was measurable even before imbibition. In the lower seeds, up to 20% of the phytochrome was occasionally observed as P_fr in samples imbibed in darkness for a short time (up to 12 h). A slight blue shift of the peak of P_T in the difference spectrum of phytochrome was observed in the case of lower seeds imbibed for 0–2 h. These results suggest that, to some extent, the lower axes contain dehydrated P_fr or intermediate(s) in the photoconversion of phytochrome. The dark reactions of P_fr were also examined in excised axes of both types of dimorphic seed after they had been pre-imbibed for 16 h in darkness. Dark destruction of P_fr was observed in both types of seed. In addition, net increases in levels of P_r were observed in the dark controls and in the samples irradiated with red light after the level of P_fr diminished. No ‘inverse’ dark reversion from P_r to P_fr was detected. Thus, after 16 h of imbibition, there were no differences in terms of properties of phytochrome between the two types of seed, and the different responses to light of upper and lower seeds might depend mainly on a difference in the physiological state of the two types of seed rather than the properties of phytochrome.     

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     

Photoinhibition of Seed Germination in the Maritime Plant Matthiola tricuspidata

The three-horned stock, Matthiola tricuspidata (L.) R. Br. isa widespread annual plant of the Mediterranean sandy shores.Its seeds are dark germinating and negatively photosensitive,in accordance with our previous findings for a number of othermaritime plants. Full germination was obtained at a wide rangeof temperatures (5-25 °C) in the dark. Inhibition of germinationunder light of various spectral qualities could be generallycorrelated, negatively and positively, respectively, with phytochromephotostationary state (ø) and relative cycling rate ofphytochrome (H). The inhibition of germination by white (fluorescent),blue and far-red light, applied either continuously or intermittently,consistently showed a linear dependence upon the logarithm ofthe flux density of the irradiation. The resulting photoinhibitioncurves had parallel slopes and, compared to those of other maritimeplants, they were shifted to higher flux densities, Continuousblue or far-red irradiations, both establishing a similar øvalue (0·26), resulted in statistically similar regressioncurves, thus favouring the hypothesis that phytochrome is thesingle photoreceptor in the photoinhibition of seed germination.Copyright1994, 1999 Academic Press Matthiola tricuspidata (L.) R. Br., three-horned stock, seed germination, light, photoinhibition, phytochrome     

EFFECTS OF LIGHT AND TEMPERATURE ON SEED GERMINATION OF FICUS HISPIDA IN XISHUANGBANNA, SOUTHWEST CHINA

 该研究应用人工气候箱设置不同的光照和温度梯度, 探讨对对叶榕(Ficus hispida)种子萌发的影响。结果表明, 荧光灯条件下(红光和远红光比例(R/FR)为4.56, 光量子密度(PPFD)约为90 μmol&;#8226;m^–2&;#8226;s^–1), 温度对种子萌发速率有显著影响, 而对最终萌发率的影响不显著。对叶榕的种子为光敏性种子, 萌发严格需光, R/FR对种子最终萌发率的影响显著, 较高的R/FR促进种子萌发, 而较低的 R/FR 抑制种子萌发。光强影响种子的萌发速率, 较弱的光照可以延缓种子萌发, 但并不能完全抑制其萌发。在30 ℃条件下, 对叶榕的种子可以在较高的 R/FR (0.42) 水平下萌发, 但温度为23/20 ℃ 时, 种子对 R/FR 的要求增高, 0.42的R/FR不能导致种子萌发。说明较低的温度和 R/FR 都可以显著抑制对叶榕的种子萌发, 而较高的温度和R/FR皆有利于种子的萌发。     

β-Glucosidase activities and HCN liberation in unimbibed and imbibed seeds, and the induction of cocklebur seed germination by cyanogenic glycosides

In many seed species, the major source of HCN evolved during water imbibition is cyanogenic glycosides. The present investigation was performed to elucidate the role of endogenous cyanogenic glycosides in the control of seed germination and to examine the involvment of β-glucosidase in this process. All seed species used here contained some activities of β-glucosidase already in the dry state before imbibition. in the decreasing order of Malus pumila, Daucus carota, Hordeum vulgare, Chenopodium album and so on. β-Gluosidase activity in upper and lower seeds of cocklebur (Xanthium pennsylvanicum Wallr.) decreased with imbibition, and in lower seeds the activity disappeared when they germinated. On the contrary, in caryopses of rice (Oryza sativa L. cv. Sasanishiki) β-glucosidase increased during imbibition, and this increase continued even after germination. β-Glucosidase in cocklebur seeds was more active in the axial than in the cotyledonary tissue. Amygdalin, prunasin and linamarin could all serve as substrattes for the β-glucosidase(s) from both cocklebur and rice. Amygdalin, prunasin and linamarin as well as KCN, were effective in stimulating the germination of upper cocklebur seeds. The seeds evolved much more free HCN gas when they were exposed to the cyanogenic glycosides than when the glycosides were absent. Moreover, the application of the cyanogenic glycosides or of KCN caused accumulation of bound HCN in the seeds. Carbon monoxide, which stimulated cocklebur seed germination only slightly, did not cause accumulation of bound HCN. We suggest that a balance between the cytochrome and the alternative respiration pathways, which is adequate for germination (Esashi et al. 1987. Plant Cell Physiol. 28: 141–150), may be brought about by the action of endogenous HCN; a large portion of which is liberated from cyanogenic glycosides via the action of β-glucosidase. In addition to the partial suppression of the cytochrome path and unlike carbon monoxide, the HCN thus produced may act to supply cyanide group(s) to unknown compounds necessary for germination.     

Germination Inhibitors in Fruit Extracts of Red Beet (Beta vulgaris cv. rubra

Germination inhibitors in methanol and water extracts of redbeet fruits (Beta vulgaris cv. rubra L.) have been studied usinglettuce and red beet seed germination as bioassays. The methanolextracts contained substances which inhibited lettuce seed germination,but had no effect on the germination of red beet seeds. Germinationof both lettuce seeds and of water-leached or sulphuric acid-treatedred beet seed balls were inhibited by the water extracts. Theconcentrations of ammonia, ferulic acid, and oxalate in thewater extracts were much lower than required for inhibitionof red beet seed germination. The water extracts contained,however, large amounts of inorganic ions, and the results clearlydemonstrated that the inhibitory effect of the water extractson red beet seed germination was mainly due to the content ofsuch inorganic ions.     

Effects of Repetitive Acid Immersion, Red Light, and Gibberellin A3 Treatments on Phytochrome-mediated Germination Control in Skotodormant Lettuce Seeds

Dry lettuce seeds (Lactuca sativa L. cv. Grand Rapids), whichreceived 5 min far-red light (FR) 0.5 h after the onset of waterimbibition, showed 17% and 50% germination without and withacid immersion treatment (pH 0.1) for 1 h and rinsing with water,respectively. The acid treatment caused only 6% germinationor less in FR-treated seeds held for 10 to 30 d in dark storage.The 10 to 30 d skotodormant seeds did not respond to red light(R) or gibberellin A₃ (GA₃) singly, but showed 84% or higherpercentage germination if 1 h acid immersion was given beforeR or GA₃. The 20 d skotodormant seeds, which received R treatmentat day 10 but remained dormant showed 89% germination with onlyacid treatment. Similar values were obtained with 30 d skotodormantseeds which received one or two R treatments at day 10 or 20,i.e. the only requirement for these R-treated dormant seedswas an acid immersion. This releases the skotodormancy and rendersthe seeds more sensitive to R or GA₃, but the skotodormancywas initiated again if no light or hormone treatments were givenimmediately. The repetitive R or GA₃ treatments, which did notcause skotodormant seeds to germinate, lessened the degree ofskotodormancy. The germination of these skotodormant seeds canonly be induced by the synergistic action of R and GA₃. In thisstudy, GA₃ caused higher germination percentages in R-treatedskotodormant seeds than R stimulated in GA3-treated seeds. Itis suggested that (i) repetitive R or Ga₃ treatments maintaina high endogenous level of the far-red-absorbing form of phytochrome(P_fr) and GA activity, respectively, (ii) the accumulated stableintermediates of phytochrome persist in fully-imbibed skotodormantseeds for up to 20 d, without phytochrome expressing its functionuntil the seeds are acidified and (iii) a model is formulatedto interpret the results of acidification, growth promotersand R effects on germination of light-sensitive lettuce seeds. Key words: Phytochrome, Latuca saliva, seed germination, dark reversion of phytochrome, gibberellin A₃, acidification, skotodormancy