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     

Anoxic Seed Germination of Erythrina caffra: Ethanol Fermentation and Response to Metabolic Inhibitors

Erythrina caffra seeds were shown to be true anaerobic germinators.They exhibit a Pasteur effect, high alcohol dehydrogenase activityand produce high levels of ethanol under anoxia, in which situationgermination starts to be suppressed by as little as 0.1% externallyapplied ethanol. Toxic levels of ethanol production appear tobe prevented by a decrease in the rate of ethanol accumulation.Carbon monoxide does not inhibit germination. Cyanide, SHAM,iodoacetate, pyrazole, and 4-methylpyrazole are more inhibitoryto anoxic than aerobic germination whereas azide, arsenate,and fluoride inhibit both. Azide, pyrazole, 4-methylpyrazoleand a low concentration of cyanide and SHAM tend to stimulateethanol production in air. At 10 mol m^–3, 4-methylpyrazolestimulates anaerobic ethanol production. At higher concentrationsthis compound and all other inhibitors used suppress anaerobicethanol production initially. Inhibition of ethanol productionby 10 mol m^–3 cyanide is paralleled by an accumulationof acetaldehyde. Azide and cyanide appear to exert their inhibitoryeffect at different loci. Key words: Erythrina caffra, anoxic germination, fermentation, metabolic inhibitors     

THE EFFECTS OF TEMPERATURE ON THE GERMINATION AND RADICLE GROWTH OF PHOTOSENSITIVE LETTUCE SEED

1. The time course of germination of Grand Rapids lettuce seedshas heen followed with different combinations of temperature(3°–35°) and irradiation (red or far-red light).For each set of conditions the following three parameters weredetermined: (i) the time required for half maximum germination,(ii) the rate of germination during the actively germinatingphase, and (iii) the maximum germination attained. In general,as the temperature was lowered, with dark-imbibed seeds, (i)became longer, (ii) became lower, but (iii) became progressivelyhigher. The effect of red light at any temperature was to shorten(i) and increase (ii) and (iii) over the values dark controls.Far-red light exerted an effect opposite to that of red light.Temperatures higher than 25° inhibited (ii) and (iii) underany light conditions. The optimum temperature to the actionof red and far-red light is 25°, at which the stimulatoryeffect of red light and the inhibition of this effect by far-redlight are both maximal. 2. The growth of the radicles of de-coated seeds of Grand Rapidslettuce shows two phases at all temperatures studied. PhaseI is characterized by slow but linear growth which continuesuntil shortly after visible differentiation of the radicle intothe hypocotyl and the root. Phase II is a phase of active growthin which the total length reflects mainly the length of theroot. The optimum temperature for Phase I is 25°-35°,and that, for Phase II is 25°. In neither phase, and atnone of the temperatures studied, is there any effect of redor far-red radiation on the growth of the radicle. The firstvisible sign of radicle elongation in red light induced seeds,however, takes place at exactly the same time as that of germination. 3. Similarities and dissimilarities between the germinationand the growth are pointed out, and it is concluded that thetwo phenomena are different, but proceed at sites closely associatedin the embryo. ¹Present address: Johnson Foundation for Medical Physics, Universityof Pennsylvania, Philadelphia, Pa., U.S.A.     

Effects on Phytochrome Controlled Germination Produced by Far-Red Irradiation of Seeds Before and During Rehydration

Seeds irradiated with red light and then re-dried will respondto this light treatment on subsequent rehydration in the dark.If such high-Pfr seeds are irradiated in the dry state withfar-red light immediately before rehydration the percentagegermination is significantly reduced in the case of Plantagomajor and Sinapis arvensis but increased in Bromus steriliswhere Pfr inhibits germination. This effect of far-red lightcan be reversed by red light despite the fact that red lightalone has no effect on dry seed. This is due to the interconversionof Pfr and the red light absorbing phytochrome intermediatecomplex meta-Fa. If there is a delay between far-red irradiationand rehydration of Sinapis seeds, the inhibitory effect of thefar-red irradiation becomes progressively less the longer thedelay. This reduction in effectiveness of far-red is interpretedin terms of a dark reversal of meta-Fa to Pfr with a half-lifeof about 4–6 h. The reappearance of Pfr is either veryslow or docs not occur in dehydrated Plantago seeds, as far-redtight given 96 h prior to hydration is just as inhibitory asfar-red light given immediately before hydration. Meta-Fa doesappear to revert to Pfr in darkness in Bromus seeds, but onlyvery slowly. The rapid increase in effectiveness of red irradiationduring rehydration of high-Pfr Plantago seeds suggests that,in this species, the pre-treatment used in preparation of high-Pfrseeds may increase the receptivity or amount of the Pfr reactionpartner. Key words: Phytochrome intermediates, Seeds, Germination     

Effects of Chilling, Light and Nitrogen-containing Compounds on Germination, Rate of Germination and Seed Imbibition of Clematis vitalba L.

Effects of chilling (5 °C) period, light and applied nitrogen(N) on germination (%), rate of germination (d to 50% of totalgermination; T_50%) and seed imbibition were examined inClematisvitalba L. In the absence of chilling, light and N, germinationwas minimal (3%). When applied alone, both chilling and N increasedgermination. Chilling for 12 weeks increased germination to64%, and 2.5 mM NO^-₃or NH⁺₄increased germination to 10–12%.Light did not increase germination when applied alone, but didwhen applied in combination with chilling and/or N. Half theseed germinated when light was combined with 2.5 mM NO^-₃or NH⁺₄.The influence of chilling, light and/or N on germination wasgreater when combined, than when either factor was applied alone.Both oxidized (NO^-₃) and reduced (NH⁺₄) forms of N increasedgermination, but non-N-containing compounds did not, suggestingthe response was due to N and not ionic or osmotic effects. Without additional N, T_50%decreased from 16–20 d at zerochilling, to around 5 d at 8 and 12 weeks chilling. AlthoughT_50%was not influenced by an increase in NO^-₃or NH⁺₄from 0.5to 5.0 mM , it did increase with additional applied N thereafter.However, the magnitude of the N effect was small compared tothat of chilling. Like germination, seed imbibition increasedwith a longer chilling period, but in contrast imbibition decreasedslightly with increased applied NO^-₃or NH⁺₄. It is argued thatincreased imbibition is not directly related to an increasein total germination, but that it may be related to the rateof germination. Possible mechanisms involved in the reductionin dormancy ofC. vitalba seed are discussed. Clematis vitalba L.; germination; dormancy; imbibition; rate of germination; chilling; light; nitrate; ammonium; nitrogen; phytochrome     

Analysis of the Effect of Light and Temperature on the Fluence Response Curves for Germination of Rumex obtusifolius

Both red light (10 minutes) and 35°C treatment (60 minutes) stimulate the germination of seeds of Rumex obtusifolius otherwise maintained in darkness at 25°C. Fluence response curves were determined for the effect of red light to stimulate germination of seeds with and without 35°C treatment. The endogenous far-red absorbing form (Pfr) level in the seeds was determined using short saturating fluences of wavelengths of light which maintain different proportions of phytochrome as Pfr at equilibrium. In the seed batches investigated, the endogenous Pfr level was found to be 4% or less of the total phytochrome. High dark germination after 35°C treatment does not result from an increase in sensitivity of the whole population to Pfr. Calculated fluence response curves for germination which best fit the experimental data suggest that seeds germinate in darkness after 35°C treatment because of a nonphytochrome-related process (overriding factor).     

The Photocontrol of Spore Germination in the Fern Ceratopteris richardii

This paper describes how different wavelengths of light regulatespore germination in the fern Ceratopteris richardii. This speciesdoes not exhibit any dark germination. Maximum photosensitivityof the spores is reached 7 to 10 d after imbibition. An increasein the red light fluence above the threshold fluence of 10¹⁶quanta.m^–2 leads to a corresponding increase in germination.In sequential irradiation experiments, farred light can reversethis red light-mediated germination to the level observed withthe far-red light control. Blue light fluences above 10²⁰ quanta.m^–2can also block the germination response to red light. Moreover,this antagonistic effect of blue light is not reversed by subsequentirradiation with red light. It is therefore concluded that phytochromeand a distinct blue light photoreceptor control C. richardiispore germination. These interpretations are entirely consistentwith the published literature on other fern genera. (Received November 28, 1986; Accepted April 6, 1987)     

The Interactive Effects of Phytochrome, Nitrate and Thiourea on the Germination Response to Alternating Temperatures in Seeds of Ranunculus sceleratus L.: A Quantal Approach

Probert, R. J., Gajjar, K. H. and Haslam, I. K. 1987. The interactiveeffects of phytochrome, nitrate and thiourea on the germinationresponse to alternating temperatures in seeds of Ranunculussceleratus L.: A quantal approach.—J. exp. Bot. 38: 1012–1025. The interactive effects of phytochrome, potassium nitrate andthiourea on the germination response to alternating temperaturesin achenes (seeds) of Ranunculus sceleratus L. were studied.Using thermogradient bars, high levels of germination were recordedover a broad range of alternating temperatures providing seedsreceived daily irradiations. Reduced germination in temperaturecycles with a relatively long warm phase was related to thelevel of the active form of phytochrome (Pfr). Dose-responseexperiments to red light (R) and temperature shifts showed thatthe actions of Pfr and alternating temperatures were interdependent.Maximum germination was recorded when intermittent pulses ofR were combined with daily 4 h temperature shifts from 16°Cto 26°C. Whilst probit analysis showed that potassium nitrateand thiourea both increased population sensitivity to temperatureshifts, thiourea was a more potent stimulant. Although the effectof both chemicals was dependent on phytochrome photo-equilibriumthe threshold level of Pfr required for thiourea action wasclearly much lower than that required for nitrate action. Thioureapotentiated a response to daily temperature shifts even whenPfr was at a low, normally inhibitory level. These results indicatedifferent mechanisms of action for potassium nitrate and thioureain relation to phytochrome controlled seed germination. Key words: Phytochrome, nitrate, thiourea, alternating temperatures, germination     

Action of Respiratory Inhibitors on Seed Germination and Oxygen Uptake in Avena fatua L.

The effects of sodium azide, potassium cyanide (cytochrome oxidaseinhibitors), and salicylhydroxamic acid (SHAM; an alternativerespiration inhibitor) on germination and respiration of Avenafatua L. seeds were studied. Azide and cyanide released seeddormancy at similar concentrations and treatment durations.Cyanide, however, stimulated germination of seeds with littleafter-ripening, whereas azide had no effect under similar conditionsunless the seeds were after-ripened for several months; theduration of after-ripening required for seeds to respond toazide varied with seed batch. There was also a greater lag priorto germination in the case of azide, compared to cyanide treatedseeds. SHAM inhibited the stimulation of germination and respirationby azide, but not by cyanide. Furthermore, respiration induced by azide or cyanide could notbe inhibited by the subsequent application of SHAM. These findingssuggest that the respiration stimulated by azide and cyanideis not alternative (SHAM-sensitive) and, therefore, this respiratorypathway cannot be involved in the stimulation of germinationby cytochrome oxidase inhibitors. While embryos excised fromcontrol, azide or cyanide pretreated seeds had the capacityto perform alternative respiration, the actual contributionof this pathway was negligible. A large proportion of respirationof embryos excised from azide or cyanide pretreated seeds wasresidual, i.e. insensitive to both SHAM and cyanide. Alternative respiration, azide, cyanide, dormancy, salicylhydroxamic acid, wild oats     

Dormancy in Dioscorea: Rapid Germination of Detached Embryos from Dormant Seeds of D. tokoro

The intact dormant seeds of Dioscorea tokoro germinate slowlyif at all between 11-23°C; for full and rapid germinationthey require prior chilling treatment [Okagami and Kawai (1982)Bot. Mag. Tokyo 95: 155]. The germination abilities of zygoticembryos detached from dormant seeds of this species were studiedunder various nutritional and temperature regimes. For germinationof embryos, the minimum nutritional components in Murashigeand Skoog's (1962) medium that were required were sucrose andNO^–₃ or SO^2–₄. As the source of carbohydrate forgermination of detached embryos, sucrose, mannose and maltosewere effective; glucose and fructose were less effective; andrhamnose was entirely unable to support germination. Embryos detached from dormant seeds, incubated with the sucroseplus KNO₃, germinated more rapidly with increasing temperatureup to 35°C. However, application of sucrose and KNO₃ didnot induce germination of intact seeds above 26°C. Therefore,it is very possible that the endosperm exerts an inhibitoryfunction on germination at such high temperatures. When seeds were incubated after a cut was made over a smallpart of the edge of the endosperm in which the radicle of theembryo is encased, germination occurred rapidly but the increasein germination percentage was slight. This result suggests thatthe endosperm suppots part of the germination inhibition bymeans of a mechanical barrier or its impermeability to wateror gases. Physiological features of the endosperm alone or interactionsbetween the embryo and endosperm may contribute significantlyto the characteristics of dormancy of intact seeds of this species. (Received May 30, 1988; Accepted January 11, 1989)     

Ethylene, Nitrate and Nitrite Interactions in the Promotion of Dark Germination of Common Purslane Seeds

Ethylene (10 µ1^–1) caused about one-third of highlydark-dormant seeds of common purslane (Portulaca oleracea L.)to germinate in the dark. Attempts were made to increase germinationin the dark with nitrate and ethylene combinations. When applieddirectly to the seeds, KNO₃ did not stimulate germination andKNO₃ plus ethylene did not increase germination above that ofethylene alone. Pre-incubation of seeds in KNO₃ for 4 to 7 dbefore the ethylene applications significantly increased germination.The effects of the KNO₃ pre-incubation were additive at eachof four ethylene concentrations (0.1–100 µ11^–1).Potassium nitrate was effective only when ethylene followedthe KNO₃ pre-incubation period. Potassium nitrite stimulatedabout 25 per cent of the seeds to germinate without a pre-incubationperiod and without ethylene. Also, ethylene plus KNO₂ enhancedgermination above that achieved by either stimulus alone. Silvernitrate did not block the ethylene promotion of germination,but reversed the typical ethylene inhibition of seedling growthfollowing germination. The results support the views that nitrateexerted its effect via conversion to nitrite within the seedand that the rate of nitrate conversion may be a limiting factorin the dark germination of common purslane seeds. Ethylene mayfacilitate nitrite activity by increasing seed sensitivity tothe stimulus. Common purslane, Portulaca oleracea L., ethylene, nitrate, nitrite, germination, dormancy     

Model for variable light sensitivity in imbibed dark-dormant seeds

The level of light-induced germination of the seed of common purslane (Portulaca oleracea L.) and curly dock (Rumex crispus L.) changes with dark incubation time prior to brief, low energy, red light treatment. The rate at which phytochrome—far red-absorbing form (Pfr) acts in the light-induced population of seeds was measured by quantitating per cent reversals of the red light effect with saturating far red light exposures at successive times after the red light exposure. A linear positive correlation was found between this rate and the final germination level. These results are compatible with a model involving changing levels, during dark incubation, of a component with which Pfr interacts. In this model, germination is initiated after attainment of a certain level of interaction between Pfr and this component. These findings also support the view that the Pfr to Pr decay rate constant and total phytochrome level are stable during dark incubation.     

Promotion by gibberellin of lettuce seed germiation as a function of presoaking period

Germination of lettuce seeds (Lactuca sativa L. cv. Grand Rapids)was examined in the presence of various doses (10^–5.0–10^–3.0M) of gibberellic acid applied at various times (hour 0–8)of soaking. Germination promotion by gibberellic acid was greateras the dose of gibberellic acid was increased and attenuatedwith the length of the presoaking period. As an exception, ca.95% germination was always evoked by the largest dose (10^–3M) of gibberellic acid given at any time of soaking. The dose-responsecurve obtained for each presoaking period had a distinct sigmoidalprofile. Synergistic and photoreversible promotion by red light of thegibberellin-induced germination was also investigated. Far-redlight pulse given 6 hr after the red pulse was still effectivein removing the red light action. Application of enzyme kineticsto the gibberellin action and also to the synergism betweengibberellin and red light was suggested. ¹National Institute for Basic Biology, Myodaiji, Okazaki 444,Japan. (Received December 18, 1979; )     

Environmental Anoxia is Unnecessary for Inhibiting Chloroplast Photomorphogenesis in Rice Coleoptiles (Oryza sativa L.)

High population densities of germinating rice seedlings in initiallyair-saturated sealed aquatic environments exhibited d^–seedling growth consisting solely of coleoptile emergence inlight and dark environments. Residual oxygen tensions of 17–23%of the initially air-saturated water containing the d^–seedlings were evident after 15 d in both the light and dark.Coleoptiles of all d^– seedlings were stark white in appearance,lacked protochlorophyllide, and contained proplastids and amyloplasts,there being no evidence of normal etioplast development in thelight or dark and no chloroplast development in the light. Thus,complete environmental anoxia was observed to be unnecessaryfor inhibiting normal chloroplast photomorphogenesis in coleoptilesof light-germinated rice seedlings. Increasing the oxygen tensionsof the 15-d-old aquatic environments of light- and dark-germinatedd^– seedlings placed in the light resulted in normal chloroplastphotomorphogenesis in coleoptiles, shoots, and roots. Key words: Oryza sativa, environmental anoxia, chloroplast photomorphogenesis, rice coleoptiles     

Transition from primary dormancy to secondary dormancy in cocklebur seeds

Abstract The transition from primary dormancy to secondary dormancy was examined using upper cocklebur (Xanthium pennsylvanicum Wallr.) seeds. The non-after-ripened seeds with primary dormancy responded to chilling, anoxia, KCN, and NaN₃ with an increase in germination. However, their maximal responses to these treatments only occurred after a period of water imbibition, probably a reflection of the increasing growth potential of the axial tissue which was accompanied by the increase in the capacities of respiration and ethylene production. On the other hand, the establishment of secondary dormancy was accompanied by a decrease in respiration and ethylene production of seeds, and in the growth potential of both axial and cotyledonary tissues. The decrease in growth potential of these tissues occurred regardless of whether they were excised from after-ripened seeds or non-after-ripened seeds. It is inferred that the primary dormancy of cocklebur seeds is a state maintained in un-germinated seeds for a long time through a spontaneous transition to secondary dormancy.     

Photoinduction of Spore Germination in a Fern, Mohria caffrorum

Irradiation of spores of the fern Mohria caffrorum Sw. witheither red light (67.4 µW cm^–2) or far-red light(63.2 µW cm^–2) for a period of 24 h induced maximumlevels of germination. Brief irradiations with blue light (127.6µW cm^–2) administered before or after photoinductioncompletely nullified the effects of red or far-red light; however,with prolonged exposure to blue light, germination levels roseto near maximum. The similar effects of red and far-red lightin promoting spore germination makes the involvement of phytochromein this process questionable. Based on energy requirements,the promotive and inhibitory phases of blue light appear toinvolve independent modes of action. Mohria caffrorum, ferns, spore germination, photoinduction, phytochrome     

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 Effect of Chilling and Moisture Status on the Germination, Desiccation Tolerance and Longevity ofAesculus hippocastanumL. Seed

Effects of 2 °C chilling on the threshold moisture contentsand water potentials for various physiological processes wereestimated forAesculus hippocastanumL. seed. Seed harvested atthe time of maximum seed fall exhibited a dual response to drying:partial drying from near 50% to 32–40% moisture contentprogressively increased germination percentage (at 16 °C)up to various peak values; further desiccation was detrimental,confirming that the seeds are ‘recalcitrant’. Themoisture content for optimum germination was increased by atleast 10% as the chilling period was raised from 0 to 9 weeks.A negative linear relationship was found between log₁₀mean timeto germinate and probit final germination, regardless of pre-treatment,indicating that partial desiccation and chilling are interchangeablein promoting germination of hydrated seed. For nearly fullyhydrated seeds, increasing the chilling period from 6 to 26weeks increased the viability-loss onset point for desiccationinjury from near 40% to about 48% moisture content without alteringthe drying rates of seed tissues. Extending moist chilling invarious seed lots from 0 to 26 weeks decreased subsequent longevityat 16 °C. For 26-week-chilled seeds longevity (the periodto lose one probit of germination) differed above and belowa threshold moisture content of 48%. It remained constant inthe moisture-content range 48–38%, but increased progressivelyas moisture content was raised above 48%. This threshold moisturecontent coincided with the value above which chilled seed pre-germinatedin storage. The results indicate that post-harvest desiccationand chilling alter the water relations of various physiologicalprocesses and a schematic summary is presented which relatesthe results to an axis water sorption isotherm.Copyright 1998Annals of Botany Company Aesculus hippocastanumL., horse chestnut, chilling, moisture content, water potential, desiccation tolerance, longevity, recalcitrant seed, embryo axis, maturation, germination.     

A Comparative Study of Germination Responses to High Irradiance Light

Seeds of 19 native British herbaceous species (14 grasses andfive forbs) were exposed to white light at three photon fluencerates: high (19–2 mol m^–2 d^–1), medium (9·6mol m^–1 d^–1) and low (2·3 mol m^–2 d^–2) These photon doses have been found by previous workers to inhibitgermination in several species. High and low photon doses wereapplied only as continuous light, but the medium dose was appliedas both continuous light and as a 12 h light/12 h dark photoperiod.All four treatments, plus a dark control, were carried out at15 °C; the high and low doses were also applied with a dailyalternation of 10/20 °C. The majority of species (15) fell into one of two groups. Inseven species germination was relatively high and consistentacross all treatments, including darkness; in the other eightspp germination was inhibited only in darkness. Mostly thesedata confirmed published results for the same species In contrast in Agrostis capillaris and A. stolonifera germinationwas high only at alternating temperatures, irrespective of photondose, but was also slightly promoted by a constant temperaturecombined with light/dark alternations. Only in Bromus sterilisand B. ereclus was germination inhibited by light, in B. erectusat all photon doses and in B. sterilis only at the highest photondose These results suggest that inhibition of germination by highirradiance light is not widespread among native British species Aira caryophyllea, Arrenatherum elatius, Festuca ovina, F. rubra, Hordeum murinum, Milhim effusum, Silene dioica, Achillea millefolium, Brachypodium syhaticum, Digitalis purpurea, Holcus lanatus, Leucanlhemum vulgare, Phleum pratense, Poa trivialis, Taraxacum officinale, Agrostis capillaris, A. stolonifera, Bromus erectus, B. sterilis, seed, dormancy, germination, light, High irradiance reaction, alternating temperatures, photoperiod