Tomato Seed Germination. Osmotic Pretreatment and Far Red Inhibition

At 25 °C germination of tomato (Lycopersicon lycopersicum)seeds is inhibited by continuous and intermittent far red illumination.It is also inhibited by a single 30 min far red irradiationgiven 8 h from the start of imbibition. The incubation of seedsin a mannitol solution inhibitory for germination has no effecton the final germination percentage after seeds are subsequentlytransferred to water. A 30 min far red irradiation at the timeof transfer results in partial inhibition of germination. Thisinhibition can be released by the continuation of osmotic incubationfor several days before the transfer to water. At the end ofa 7 d dark period of osmotic incubation, inhibition of subsequentgermination in water can be realized only by continuous farred illumination. Seeds osmotically pretreated for 7 d and afterwardsdried-back show a mean time to 50% germination significantlylower than that of untreated seeds. Moreover, besides singleand intermittent, even continuous far red light has no inhibitoryeffect on the germination of these seeds. It is concluded that,in addition to the already known germination advantages, osmoticpresowing treatment also induces the ability of seeds to germinateunder unfavourable light conditi.     

Effects of Photoperiod on Germination of Seeds of Talinum triangulare (Jacq.) Willd

Seed germination in Talinum triangulare as affected by photoperiod,with or without previous incubation in the dark in water at25 or 4 °C, was studied. The time course and quantity ofseed germination in photoperiods of 1 h and above were similarwith or without dark pretreatment, but the time to half maximumgermination was reduced from 12 days in non-dark pretreatedseeds to 4 days in seeds given 20 days in the dark at 25°C.A photoperiod of 0·25 h gave a lower rate and total germinationthan photoperiods of 1 h and above. Un-pretreated seeds required17 cycles of 24 h photoperiod for maximum germination as comparedwith 7 or less cycles if the seeds received more than 10 daysdark pretreatment at 25 °C. Both the rate and total germinationin light increased as the length of dark pretreatment at 25°C was increased from zero to 30 days. Incubation of theseeds in water in the dark at 4 °C for 5 to 30 days priorto illumination at 21 °C, reduced both the rate and quantityof seed germination in light as compared with those similarlyincubated in the dark at 25 °C. However, previous incubationin the dark for 30 days at 4 °C partially substituted forthe light requirement. The possible mechanism of breakage ofseed dormancy in Talinumis discussed in relation to these andother findings. Talinum triangulare (Jacq.), Willd, light, photoperiod, seed germination     

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     

Photoinduced Seed Germination of Oenothera biennis L: I. General Characteristics

General characteristics of light-induced germination of Oenothera biennis L. seeds were investigated at 24°C. During dark imbibition, seeds reached maximal respiration in 7 hours and maximal water content and photosensitivity in 24 hours. After dark imbibition of 24 hours, seeds required a long exposure (>36 hours) to red or white light for maximal germination. Two photoperiods (12 and 2 hours) separated by a period of darkness of 10 to 16 hours gave near maximal germination. For the two photoperiod regime, the first light potentiates a reversible phytochrome response by the second light. A 35°C treatment for 2 to 3 hours in the dark immediately prior or subsequent to 8 hours of light caused a higher percentage of germination. A 2 hour treatment at 35°C also potentiates a reversible phytochrome response. Halved seeds germinated at 100% in light or darkness indicating that the light requirement of the seeds is lost in the halving procedure. After-ripened seeds required less light and germinated more rapidly and at higher percentages than seeds tested shortly after maturation.     

SUCCESSIVE PROCESSES INVOLVED IN THE GERMINATION RESPONSE OF NASTURTIUM SEEDS

1. The seeds ofNasturtium palustreDC. do not germinate, eitherin the light or darkness, at various constant temperatures,but require for their full germination a certain period of alow temperature (5°) applied immediately after light irradiation.These results indicate the existance of at least two processes,a light-dependent process and a low temperature-requiring process,in the initiation of germination ofNasturtiumseeds. Experimentalevidence indicated further that the light exposure causes twodifferent processes in the seed germination. 2. When a dark period at 23° was inserted between the lightirradiation and the low temperature treatment the germinationwas suppressed. The inhibitory effect of the inserted dark periodat 23° was eliminated by a short irradiation during thedarkness (light-break). 3. Prolonged exposure ofNasturtium seeds to any concentrationof gibberellin brought about no germination when exposure wasgiven in complete darkness. The germination was promoted onlywhen light irradiation was applied to the seeds. A short applicationof gibberellin at a fairly high concentration was, however,remarkably effective for the germination even in the darkness,and the germination was inhibited as the gibberellin applicationwas lengthened. It was considered that gibberellin could substitutefor the combined effect of light irradiation and low temperaturetreatment to induce the germination of Nasturtium seeds, andthat gibberellin was inhibitive toward the reactions followingthe above treatments which induced the germination (Received October 31, 1996; )     

Factors affecting the induction and release of secondary dormancy in Kalanchoë seeds

Abstract. Several short daily R irradiations are required from the first day of incubation on water to induce germination of Kalanchoë seeds. When the same light treatment is given after a prolonged dark incubation period at 20°C, secondary dormancy prevents germination. Factors controlling the induction and breaking of secondary dormancy have been investigated. The induction of secondary dormancy is very temperature dependent. Locally puncturing the seed coat strongly delays it. Secondary dormancy is not induced in the presence of GA₃ during the first 10 d of dark incubation, although this growth substance cannot induce dark germination. Prolonged or cyclic daily R irradiations can relieve secondary dormancy of seeds kept on water, even after a dark period of 20 d. A 24 h treatment at 4°C restores responsiveness to short R exposures of slightly secondarily dormant seeds. The synergism between GA₃ and Pfr in non-dormant Kalanchoë seeds, leading to high effectiveness of even one short FR irradiation, still occurs in seeds made secondarily dormant before transfer to GA₃, but more R or FR irradiations, in combination with GA₃, are required for the release of secondary dormancy. A combination of red light and 6-benzyl-aminopurine is ineffective in removing dormancy.     

The prevention of dehydration injury in celery seeds by polyethylene glycol, abscisic acid, dark and high temperature

Celery seeds ( Apium graveolens L.) given a germination induction period (3 days imbibition at 17°C in the light) could be prevented from germinating by up to 14 days subsequent exposure to high temperature (32°C), polyethylene glycol (PEG), abscisic acid (ABA) or dark (22°C). When the seeds were returned to 17°C in the light, germination occurred and, except for the high temperature treatment, was more rapid compared to seeds given a germination induction period only.
Celery seeds incubated for 3 days at 17°C in the light and then air-dried at 20°C germinated slowly when re-sown at 17°C in the light, and achieved only 19% germination after 21 days. Exposing the seeds to high temperature, PEG, ABA or dark for up to 14 days before drying maintained seed viability and subsequent germination was faster. The longer treatment periods gave increased benefit, and PEG was the most effective treatment. It is suggested that the effectiveness of the treatments in inducing dehydration tolerance relates to their ability to inhibit germination possibly via their prevention of cell expansion.     

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     

Effects of Fluridone and Norflurazon on Conditioning and Germination of Striga asiatica Seeds

Inhibitors of carotenoid biosynthesis are known to prevent abscisic acid (ABA) biosynthesis and to affect germination and dormancy of seeds in many plants. In this study, the effects of three carotenoid biosynthesis inhibitors, fluridone, norflurazon and diflufenican, on the conditioning and germination of Striga asiatica seeds were examined. Fluridone and norflurazon shortened the conditioning period required before S. asiatica seeds would germinate after exposure to the germination stimulant strigol, and prevented the inhibitory effects of both light and supraoptimal temperature (40 °C) on seed germination. In addition, treatment with fluridone or norflurazon after conditioning in water induced seed germination in a manner similar to the effect of natural germination stimulants. Moreover, the seedlings developing after conditioned with fluridone formed haustorium-like structures without the involvement of haustorium inducing factors. In contrast, diflufenican had no effect on the conditioning and germination of S. asiatica seeds. These results indicate that fluridone and norflurazon have various effects on the germination of S. asiatica seeds and might be available for control of root parasites.     

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.     

Germination and Dormancy in Immature and Fresh-mature Lettuce Seeds

The effects of red light, far-red light, Gibberellin A₃, andethephon were studied on the germination of lettuce seeds cv.Grand Rapids harvested at different stages of development. Seeds did not become capable of germination until 8 days afteranthesis. Red light promoted seed germination from the age of8–9 days following anthesis up to the newly mature stage.Ten or 11 days following anthesis, a large percentage of seedsbecame capable of germination in the dark and therefore couldbe considered not dormant. They were affected by far-red light,but less so than the mature seeds. The effect of light on the germination of developing seeds appearedto be similar to the known light effect on mature lettuce seedgermination. Gibberellin A₃ and ethephon had no effect on immatureand fresh seed germination. Lactuca sativa L., Lettuce, germination, dormancy, red light, far-red light, gibberellin A₃, ethephon     

Induction of Secondary Dormancy in Chenopodium bonus-henricus L. Seeds by Osmotic and High Temperature Treatments and Its Prevention by Light and Growth Regulators

Factors controlling the establishment and removal of secondary dormancy in Chenopodium bonus-henricus L. seeds were investigated. Unchilled seeds required light for germination. A moist-chilling treatment at 4 C for 28 to 30 days removed this primary dormancy. Chilled seeds now germinated in the dark. When chilled seeds were held in the dark in −8.6 bars polyethylene glycol 6000 solution at 15 C or in water at 29 C a secondary dormancy was induced which increased progressively with time as determined by subsequent germination. These seeds now failed to germinate under the condition (darkness) which previously allowed their germination. Continuous light or daily brief red light irradiations during prolonged imbibition in polyethylene glycol solution at 15 C or in water at 29 C prevented the establishment of the secondary dormancy and caused an advancement of subsequent germination. Far red irradiations immediately following red irradiation reestablished the secondary dormancy indicating phytochrome participation in “pregerminative” processes. The growth regulator combination, kinetin + ethephon + gibberellin A₄+A₇ (GA₄₊₇), and to a relatively lesser extent GA₄₊₇, was effective in preventing the establishment of the secondary dormancy and in advancing the germination or emergence time. Following the establishment of the secondary dormancy by osmotic or high temperature treatments the regulator combination was relatively more active than light or GA₄₊₇ in removing the dormancy. Prolonged dark treatment at 29 C seemed to induce changes that were partially independent of light or GA₄₊₇ control. The data presented here indicate that changes during germination preventing dark treatment determine whether the seed will germinate, show an advancement effect, or will become secondarily dormant. These changes appear to be modulated by light and hormones.     

Nitrate Reductase Independent Stimulation of Seed Germination in Sisymbrium officinale L. (Hedge Mustard) by Light and Nitrate

After a 72 h preincubation in darkness at 15 °C seed germinationof Sisymbrium officinale L. (hedge mustard) at 24 °C wasstimulated by a combination of red light and nitrate. In thepresence of nitrate the seeds escaped from the inhibiting effectof far-red irradiation with an escape time of approx. 8 h. Afterred light, the exposure of seeds to nitrate could be delayedfor 3 h without affecting maximal germination. Prolonged delayresulted in a decrease of the germination response. The possibilitythat nitrate reduction was involved in the stimulation of germinationwas studied by pre-incubating seeds for 72 h in nitrate andsubsequently transferring them to water and irradiating withred light. During the first 8 h period after the red irradiationin which induction of germination occurred, total nitrate levels(endogenous + leachate) remained constant, indicating an absenceof nitrate reductase activity. During the next 8 h visible germinationstarted and total nitrate levels declined, suggesting inductionof nitrate reduction. It is concluded that nitrate reductiondocs not play a role in the induction of germination. The conclusionwas supported by the lack of inhibition of seed germinationby sodium chlorate and sodium tungstate in spite of an inhibitionof nitrate reduction of 80 and 100%, respectively. The contrastsbetween our results and hypotheses concerning the mechanismof action of nitrate in seed germination are discussed Sisymbrium officinale L., hedge mustard, germination, light, nitrate, nitrate reductase     

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     

PHOTOCONTROL AND TEMPERATURE DEPENDENCE OF GERMINATION OF RUMEX SEEDS

1. Light is not obligatory for the germination of the seed ofRumex obtusifolius L. subsp.agrestis DANSER, which has beenregarded as being a typical light sensitive seed. Even in continuousdarkness, a short period of high (30°) or low temperature(5°) treatment evokes germination very readily. 2. Germination is markedly promoted by 1 min exposure to a redlight and this red light effect is completely removed by 1-hrexposure to a far-red light. Alternations of the red and far-redradiation bring about an alternate promotion and inhihibitionof germination. 3. When a dark interval is inserted between the red and thefar-red treatments, inhibition of germination becomes less distinctas the duration of darkness increases. When the seeds are irradiatedwith far-red prior to red, with an inserted darkness, germinationpromotion due to the red light also decreases with the durationof inserted darkness. 4. Complicated interdependence between the light and temperatureeffects are demonstrated. This suggests a participation of somereactants besides pigments in the photoreaction. 5. The observed interdependence between the light and temperatureeffects on the germination of Rumex seeds implies that, if,as BORTHWICK has assumed, two forms of pigment, viz., a far-red-absorbingform and a red-absorbing one, are participating in the photoreaction,they should be presumed to coexist from the start of imbibition. (Received September 27, 1960; )     

After-ripening and phytochrome action in seeds of dormant lines of wild oat (Avena fatua)

The effects of a short exposure to red, far-red or alternate red/far-red light on the germination of seeds after-ripened for different periods of time were studied in dormant lines of wild oat ( Avena fatua L.). Three stages were distinguishable in the after-ripening period in the response of germination to light. Seeds stayed dormant and showed no response to light during stage I. Phytochrome-mediated germination was observed in seeds during stage II. The phytochrome action disappeared during stage III, i.e. seeds fully germinated following treatments of all light qualities. When the seeds were imbibed in polyethylene glycol solutions, dark germination was reduced and phytochrome again had an effect, which suggested the involvement of phytochrome in water uptake of the seed.     

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.     

Environmental Factors Influencing the Expression of Dormancy Patterns in Weed Seeds

Buried seeds often show seasonal periodicity of dormancy. Dormancypatterns of Chenopodium album, Polygonum persicaria, Sisymbriumofficinale and Spergula arvensis were studied by burying seedsunder field conditions in sandy loam in December, 1986. Seedswere exhumed at regular intervals and germination was subsequentlytested in the laboratory. It was shown that the conditions ofthe germination test influenced the expression of the dormancypattern. Germination of C. album and 5. arvensis always dependedon the presence of light, whereas seeds of S. officinale completelylost their light dependency during the first winter. Applicationof nitrate during the germination test in light improved germinationof all species. Dark germination was not stimulated by nitratealone. Desiccation of the exhumed seeds at a r.h. of approx.15% enhanced germination under all conditions. A combinationof several stimulating factors revealed breaking of dormancymuch earlier in the season. During induction of secondary dormancythe effect of the test conditions was even more pronounced.Dormancy induction could be overlooked for several months whenseeds were desiccated and/or given nitrate during the germinationtest in light. It is hypothesized that in the field both desiccation- due to cultivation and dry spells - and nitrate enrichmentof the soil will influence the expression of the seasonal patternof dormancy and therefore enlarge the period of possible seedlingemergence Desiccation, dormancy pattern, germination, light, nitrate, seeds, weeds, Chenopodium album, Polygonum persicaria, Sisymbrium officinale, Spergula arvensis     

Seasonal Periodicity in Germination of Seeds of Chenopodium album L.

Seeds of Chenopodium album L. were buried under field and controlledconditions. The germination capacity of these seeds was testedover a range of conditions at regular intervals. Seed buriedin the field only showed small seasonal changes in germinationcapacity when tested at constant temperatures in incubators.However, when germination was tested at field temperatures,seasonal changes in germination were more obvious. Nitrate andlight always promoted germination. There was a strong positiveinteraction between the effects of the two factors. When nitrateand light were combined, exhumed seeds germinated over a muchlonger period of the year than in water with or without light.Desiccation only stimulated under particular conditions, forexample, when germination was tested in nitrate in darkness.A regression model was developed with the data from the germinationtests in incubators. The model describes the changes in dormancyand germination and estimates germination at field temperaturesaccurately throughout the year. Despite the absence of clearseasonal changes in the temperatures suitable for germination(computed with the model), germination in the field showed seasonalperiodicity, because the field temperature and the germination-temperaturerange only overlapped from spring to late summer.Copyright 1993,1999 Academic Press Chenopodium album, lamb's quarters, dormancy pattern, germination, regression model, temperature, light, nitrate, desiccation     

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