Germination of seeds in Hyptis suaveolens Poit

Seeds of Hyptis suaveolens require long illumination periodsto promote full germination, both light and dark germinationbeing controlled by the phytochrome system. Germination in thisspecies is inhibited both by relatively low (up to 20°C)and high temperatures (45°C). Experiments in which seedswere transferred after different periods from sub-optimal orsupra-optimal temperatures to a favourable one, suggest thatboth high and low temperature inhibition may be explained onthe basis of phytochrome action. Thus, a temperature of 20°Cinduces dark dormancy due, probably, to dark reversion of phytochrometo the inactive form; whereas, at 45°C the dominant processseems to be phytochrome decay. No phytochrome loss has beenobserved at 10°C. If, however, phytochrome levels are reducedby a particular treatment, no irreversible alteration is produced;seeds simply require longer illumination periods under diesecircumstances. A low initial concentration of phytochrome couldaccount for the requirement of long illumination periods. Other factors affecting germination in this species, such asalternating temperatures, gibberellic acid and time of storage,are discussed. ¹ Present address: Instituto Venezolano de Investigaciones Cientificas,Apartado 1827, Caracas, Venezuela. (Received January 21, 1971; )     

Phytochrome Action during Prechilling Induced Germination of Betula papyrifera Marsh

Seeds of paper birch (Betula papyrifera Marsh.) were induced to germinate by prechilling at 3 C or by red light. The light requirement was mediated by phytochrome and the action of phytochrome during prechilling was investigated. Red irradiation (R) prior to prechilling markedly enhanced the effectiveness of the prechilling treatment in inducing subsequent germination at 18 C. Reversal of this enhancement by far-red irradiation (FR) was more effective when FR was supplied after a 1-week prechill treatment than after a 2-week treatment. The R enhancement effect exhibited a sharp drop as prechilling temperature was increased from 5 to 7 C. This decline is consistent with a membrane phase transition at about 7 C where Pfr action is diminished by a loss in sensitivity of its receptor sites. Although phytochrome action was observed during prechilling treatments, the seeds failed to germinate at prechilling temperatures. Therefore, it was concluded that while potentiation of germination by Pfr occurred during prechilling, some other reaction(s) leading to radicle protrusion requires higher temperatures. In one seed source loss of germination potential was observed with protracted storage at 3 C. This was prevented by R supplied during the prechilling treatment. Taken collectively the data suggest that action of phytochrome during prechilling is accentuated in these seeds by two factors: (a) an increase in the sensitivity (or number) of Pfr receptor sites; and (b) preservation of Pfr by deferment of thermal reversion.     

Alternating Temperatures and Rate of Seed Germination in Lentil

The effect of alternating temperatures on the times taken byseeds of lentil (Lens culinaris Medikus) to germinate was investigatedusing a two-way temperature-gradient plate. Between 5 and 25°C,warmer temperatures increased the rate of germination. Variationamong the individual seeds in the times required for germinationat different constant temperatures within this range were describedwell by a log-normal distribution of thermal times, accumulatedabove a base temperature of 1·5°C. Even with amplitudesas great as 20°C, no effect of alternation per se on thethermal time required for germination was detected—whetherthe cool temperature was applied for 8 or 16 h d^-1. Similarly,in alternating temperature regimes where the minimum temperatureof the diurnal cycle was between 0°C and the base temperature,the thermal times required for germination (where no thermaltime accrued during the periods when temperature was below T_b)were in close agreement with those values provided by the modeldetermined at warmer constant temperatures. However, where theminimum temperature applied was < 0°C the germinationof all but the earliest germinators was delayed beyond modelpredictions, and more so where the sub-zero minimum temperaturewas applied for 16 rather than 8 h d^-1. The results, therefore,contradict the view that alternation in temperature per se reducesthe thermal time required for seed germination. Rather, rateof germination responds instantaneously to current temperature,but prolonged exposure to sub-zero temperatures can result indamage sufficient to delay germination when seeds are returnedto regimes warmer than the base temperature.Copyright 1994,1999 Academic Press Lens culinaris Medikus, lentil, seed germination, alternating temperatures, thermal time, temperature-gradient plate     

A Model of the Effects of a Wide Range of Constant and Alternating Temperatures on Seed Germination of FourOrobanche Species

Seeds of the obligate parasitic plants, Orobanche spp., wereconditioned in water or GA₃for 2 or 12 weeks and then stimulatedto germinate by the synthetic stimulant GR24. Temperature treatmentsduring the germination tests comprised 169 different constantand alternating temperature regimes on a two-dimensional gradientplate. Optimum temperatures for germination of seeds of O. aegyptiacaand O. crenata were 18–21 °C and 18 °C, respectively.However, longer conditioning periods slightly lowered the optimain both species, and the maximum germination percentage wasalso reduced due to an induction of secondary dormancy. At agiven mean temperature, more seeds germinated at constant thanat alternating temperatures. Results were analysed in termsof characteristics of alternating temperatures that appearedto control germination, i.e. mean temperature, maximum temperature,amplitude (difference between daily maximum and minimum temperatures)and thermoperiod (the time spent at the maximum temperatureeach day). Final germination was modelled on the basis of therebeing two prerequisites for germination: a minimum mean temperaturewhich must be exceeded and a maximum temperature above whichthe seed will not germinate. These two requirements were assumedto be independent and to be normally distributed in the seedpopulation so that final germination could be described by amultiplicative probability model. Because of the response tomaximum temperature, inhibitory effects were more evident atalternating temperatures. Amplitude and thermoperiod influencedthis effect of maximum temperature. The implications of thedetrimental effect of alternating temperatures for germinationofOrobanche spp. in the field are discussed. Copyright 1999Annals of Botany Company Orobanche aegyptiaca, O. crenata, O. cernua, O. minor, broomrape, seed germination, temperature, germination model, secondary dormancy.     

Phytochrome-induced Germination, Endosperm Softening and Embryo Growth Potential in Datura ferox Seeds: Sensitivity to Low Water Potential and Time to Escape to FR Reversal

In Datura ferox seeds, the far-red absorbing form of phytochrome(Pfr) induces endosperm softening, larger embryo growthpotential,and germination. We investigated the effect of exposing theseeds to a range of water potentials in the presence of Pfronits induction of these responses. In addition, the escape timeto far-red-light (FR) reversal of the three responses wasdetermined. Low water potential inhibited Pfr action on endosperm softeningand germination in a similar way. In both cases, a 50% reductionin the response to a saturating red-light (R) irradiation wasobserved at a water potential of c. —0·5 MPa andtherewas very good correlation between the percentage numberof seeds with softened endosperm at 45 h after R and germinationat 72 h after R (R2=0·95). In contrast, the effect ofdecreasing the external water potential on Pfr induction ofa larger embryogrowth potential was more complex. Moderate decreasesin water potential (—0·3 to —0·5 MPa)enhanced Pfr action and thegrowth potential of the embryos waslarger (20—25%) than the water controls; water potentialsbelow —0·7 MPa inhibited the Pfr stimulus. The escape time to FR reversal of the R effect was shorter forthe increase in embryo growth potential than for endospermsoftening.Twenty-four h after R, the embryo response had escaped in morethan 80% of the population whereas endospermsoftening and germinationwere susceptible to FR inhibition in 100% of the seeds. These results indicate that in D. ferox seeds the increase inembryo growth potential is not sufficient for germination andthatendosperm softening is a necessary condition. Key words: Germination, dormancy, phytochrome, endosperm softening, water potential     

A Comparison of Seed and Seedling Phytochrome in Avena sativa L. using Monoclonal Antibodies

Hilton, J.R. and Thomas, B. 1985. A comparison of seed and seedlingphytochrome in Avena saliva L.using monoclonal antibodies.—J.exp. Bot. 36: 1937–1946.The kinetics of phytochrome accumulationduring imbibition and germination of seeds of Avena saliva L.cv. Saladin has been determined in vivo by spectrophotometryand in extracts by using Enzyme–Linked Immunosorbent Assay(EL1SA). In vivo measurements show two phases of phytochromeincrease. The first occurs during the initial 2 h of imbibitionand is associated with the hydration of the seed proteins; thesecond larger increase begins after 16 h, due probably to denovo synthesis. An ELISA using monoclonal antibodies purifiedfrom dark grown Avena seedlings detected only the second increasein phytochrome content. Mixing experiments indicate that theinability to detect phytochrome by ELISA during the first 16h is not due to the presence of inhibitors in the extracts.It is concluded that pre–existent seed phytochrome isantigenically dissimilar to seedling phytochrome. These twopools of phytochrome are stable and unstable respectively withregard to Pfr destruction. Key words: Immunology, phytochrome, seed     

Germination of Talinum triangulare L. Seeds as Affected by Various Chemical and Physical Treatments

Three-month ‘old’ and ‘fresh’ seedsof Talinum triangulare were subjected to various treatmentsto induce early and rapid germination. Scarification and activated carbon were the most effective treatmentsin improving total germination in fresh seeds, while the 3 and5 per cent thiourea treatments were most effective in improvingtotal germination in old seeds. Activated carbon, scarificationand 5 per cent thiourea treatments enhanced early germinationin both old and fresh seeds. Cumulative percentage germinationwas very high in fresh seeds after scarification or after treatmentwith activated carbon and 5 per cent thiourea, and lowest inseeds treated with 3 per cent thiourea and hot water. In oldseeds, highest cumulative percentage germination was obtainedwith 3 and 5 per cent thiourea treatments and scarification.Generally, higher germination was obtained with fresh seedsthan with old seeds. Partial seed-coat removal and treatment with 5 per cent thiouriaresulted in a higher rate of and cumulative percentage germinationcompared with seeds with the coat partially removed but nottreated with thiourea. Constantly high temperature (34 °C) increased both rateand total germination compared with seeds planted at room temperature(20–23 °C). Treatments that did not induce germinationwere 1 per cent thiourea, H₂SO₄, cold water soaking at roomtemperature, 6 per cent hydrogen peroxide and soil planting.These treatments effected less than 3 per cent germination. Talinum triangulare L, seed scarification, activated charcoal, thiourea, germination     

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     

An Investigation of the Influence of Constant and Alternating Temperature on the Germination of Cassava Seed using a Two-dimensional Temperature Gradient Plate

The germination of cassava seed in response to various constantand alternating temperature regimes within the range 19–40°C was investigated using a two-dimensional temperaturegradient plate. It was found that almost all seeds were incapableof germination unless the temperature for part of the day exceeded30 °C and the mean temperature was at least 24 °C. However,dormant seeds required environments where the temperature forpart of the day exceeded 36 °C, the mean temperature wasat least 33 °C, and the amplitude of the diurnal temperaturealteration was within the range 3–18 °C. Providingthese conditions were met, the times spent at the upper andlower temperatures within a diurnal cycle were not critical.Hermetic storage of the seed for 77 days at 40 °C with 7.9per cent moisture content did not influence the pattern of germinationin response to constant and alternating temperatures. It issuggested that an alternating temperature regime of 30 °Cfor 8 h/38 °C for 16 h applied for a minimum of 21 daysis appropriate for cassava seed viability tests. Manihot esculenta Crantz, cassava, germination, dormancy, temperature     

Effect of light and temperature on the germination of lettuce seeds

A short period (15–30 min) at 30° C promotes germination of seeds of Lactuca sativa L. cv. Repolhuda in darkness. Far-red light reverses this stimulation, and the escape curves for phytochrome and high-temperature action are quite similar, indicating that the two factors act at a common point in the chain of events leading to germination. It is suggested that high temperature acts by decreasing the threshold of the active, far-red absorbing, form of phytochrome (Pfr) needed to promote germination.Abbreviations FR far-red light - Pfr far-red-absorbing form of phytochrome - R red light     

An Analysis of the Effect of Alternating Temperatures on Germination of Lycopus europaeus L.

The germination of Lycopus europaeus seeds depends absolutelyon exposure to light and fluctuating temperatures. Studies oftemperature responses were made to establish the minimum fluctuationrequired for a response, the interaction of temperature andexposure time in different parts of the alternating temperaturecycle, and the effects of successive transfers between cyclingtemperature conditions. There was a complex interaction betweenthese three. The minimum fluctuation never fell below 6.5 °Cbut varied up to c. 15 °C according to other test conditions.High temperatures favoured rapid responses, and exposure totemperatures above 20 °C in one or other phase of the temperaturecycle was essential for a full response. No response occurredeither at any temperature under constant conditions, or if onlyone temperature change was given. Under some conditions a singlecycle of alternating temperatures, including two changes oftemperature, promoted high germination rates.     

A Model for Germination Responses to Alternating Temperatures

When seeds of Chenopodium album are imbibed in 0–01 Mpotassium nitrate solution in the light at constant temperature,percentage germination increases to an optimum at 24 °C,above which it decreases. These relationships are linear ifpercentage germination values are transformed to normal deviates.At supra-optimal temperatures, alternating temperatures havelittle or no effect on this basic relationship. However, atsub-optimal temperatures normal deviate germination increaseslinearly with amplitude at constant mean temperature and theincreases are relatively greater at lower mean temperatures.The effect of amplitude is also greater when more time is spentat the wanner temperature in the diurnal cycle. Seeds of Panicummaximum show very similar responses except that the effect ofamplitude is greater when a shorter time is spent at the warmertemperature in the diurnal cycle. These observations form thebasis of a quantitative model which, at sub- and supra-optimaltemperatures, respectively, accounts for 90 and 75% of the variationin germination of C. album seeds subject to a very wide rangeof thermal environments on a two-dimensional temperature gradientplate. The data presented for P. maximum are less comprehensive,but again the model accounts for 80% of the variation. The relevanceof the same model to two unrelated species from different climatesand of different ecological behaviour suggests that it may begenerally useful in determining optimum temperatures for seedviability testing regimes and ultimately for predicting fieldbehaviour These results allow the producers to improve the productionof homogeneous populations of cyclamen seedlings Chenopodium album L., Panicum maximum Jacq., seed dormancy, germination, alternating temperatures, temperature gradient plate     

Interaction of light and temperature on the germination of Rumex obtusifolius L.

Seeds (nutlets) of Rumex obtusifolius L. fail to germinate in darkness at 25° C, but are stimulated by short exposure to red light (R) the effectiveness of which can be negated by a subsequent short exposure to far red light (F) indicating phytochrome control. Short periods of elevated temperature treatment (e.g. 5 min at 35° C) can induce complete germination in darkness. Although short F cannot revert the effect of 35° C treatment, cycling the phytochrome pool by exposure to short R before short F results in reversion of at least 50% of the population. Prolonged or intermittent F can also revert the germination induced by 35° C treatment. The effect of elevated temperature treatment is interpreted on the basis of two possible models; (i) that it increases the sensitivity of the seeds to a low level of pre-existing active form of phytochrome (Pfr) (ii) that it induces the appearance of Pfr in the dark. In both cases it is envisaged that elevated temperature treatment and Pfr control germination at a common point in the series of reactions that lead to germination.Abbreviations D Dark - F far red light - P phytochrome - Pr red absorbing form of P - Pfr far red absorbing form of P - R red light     

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     

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     

Germination of Tagetes minuta L. I. Temperature Effects

Initial studies have indicated that Tagetes minuta achenes haveboth a temperature and a light requirement for germination.Temperatures tested were 10, 20, 25, 30 and 35 °C. Germinationwas optimal at 25 °C under white light conditions. Underthese conditions 100 per cent of achenes germinated within 7days of imbibition. There was no germination at 10 or 35 °Ceither in the light or in the dark. Achenes imbibed and incubatedat 35 °C for 4 days showed no visible signs of germinationbut on transfer to 25 °C, 100 per cent of these achenesgerminated within 24 h. Furthermore, achenes given this hightemperature (35 °C) treatment could be dried at 25 °C,re-imbibed at 25 °C and again 100 per cent of achenes germinatedwithin 24 h of re-imbibition. This rapid germination responsefollowing removal from the high temperature regime could alsobe induced by transfer to temperatures of 20 °C or 20 °C(16 h) alternating with 10 °C (8 h). Tagetes minuta L., weed seeds, germination, temperature, light     

The Influence of Temperature on Seed Germination Rate in Grain Legumes: III. A COMPARISON OF FIVE FABA BEAN GENOTYPES AT CONSTANT TEMPERATURES USING A NEW SCREENING METHOD

Ellis, R. H., Simon, G. and Covell, S. 1987. The influence oftemperature on seed germination rate in grain legumes. III.A comparison of five faba bean genotypes at constant temperaturesusing a new screening method.—J. exp. Bot. 38: 1033–1043. A screening procedure which requires information on the progressof germination at only four temperatures was able to definethe response of the rate of seed germination to sub- and supra-optimaltemperatures for whole seed populations of each of five fababean (Vicia faba L.) genotypes. In one population of the cultivarSutton the models for sub- and supra-optimal temperatures derivedfrom the screen satisfactorily explained observations from anearlier separate investigation at a wider range of temperatures.Two discrete groups of genotypes were identified. Within eachgroup the base temperature T_b did not differ significantly:for the landraces Lebanese Local Large and Syrian Local Largethe value was estimated to be –7·5°C and forthe landrace Lebanese Local Small and the cultivars Sutton andAquadulce it was –4·0°C. The optimum temperaturefor the 50th percentile [T_o(50), at which temperature the rateof germination is maximal] also varied between these two groupsof genotypes, being 20·5–21·5°C forthe first group and 24·5–26·0°C forthe second. In several temperature regimes some of the viableseeds within a seed population failed to germinate. Nevertheless,even at temperatures where a substantial proportion of the seedsfailed to germinate the models defined by the screening methodpredicted the germination times of those seeds which did germinate. Key words: Faba bean, seed gemination rate, temperature     

Photoregulation of Germination in Freshly-harvested and Dried Seeds of Bromus sterilis L.

Freshly-harvested and dried seeds of Bromus sterilis L. withrespectively 52-54% and less than 10% moisture contents, differedmarkedly in their rates of germination and in their responsesto light. Dried seeds attained full germination in darknessat 15 ?C by 4 d from sowing and were inhibited by the formationof Pfr. In contrast, freshly-harvested seeds required 3-4 weeksto attain maximum germination and were promoted by Pfr. Themodified photoresponse of the seeds was induced only when theseeds were dried in an alternating temperature regime. The resultsare discussed in relation to the importance of desiccation asthe trigger for the transition from the developmental to thegerminative and growth phases of seed maturation of Bromus steriliswhich is well-documented in seeds of other species. Key words: Bromus sterilis, desiccation, light     

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     

Changes in Trifolium arvense Seed Quality Following Alternating Temperature Treatment using Liquid Nitrogen

The effects of alternating temperatures, using liquid nitrogen(–196°C) as the low temperature phase, on Trifolium arvense L.seed quality was investigated. Treatment reduced hardseedednesswith the seeds exhibiting a linear increase in the probit ofgermination with the logarithm of the number of alternatingtemperature cycles. Germination levels equal to ‘chipped’controls were achieved after four cycles or more, dependingon the initial depth of hardseededness, and the response wasunaffected by either the rate of cooling or the rate of warming. Imbibition time was reduced with increasing alternating temperaturecycles. Resealing experiments identified the main area of wateruptake as the lens, and SEM showed that deepening of alreadypresent cracks may be important in removing hardseededness.Pre-treatment of seeds with alternating temperatures predisposesthe seeds to subsequent damage by sulphuric acid treatment,but not by impaction. Stress effects were not restricted to the seed coat surfaceas between 10 and 20 % of seeds treated for 3–15 cyclesdeveloped into abnormal seedlings with detached cotyledons.Seed vigour is also reduced as treatment leads to contractionof the temperature response curve for germination. The resultshave implications for the cryopreservation of papilionoid legumeseeds as a means of genetic conservation. Trifolium arvense, Hare's foot, hardseededness, quantal response, cryopreservation, liquid nitrogen