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     

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     

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     

Effects of Temperature and dl-Strigol on Seed Conditioning and Germination of Witchweed (Striga asiatica)

Seed conditioning and germination in witchweed (Striga asiatica(L.) Kuntze) were temperature-dependent. With higher conditioningtemperatures, shorter conditioning time was required for germinationwith terminal dl-strigol (strigol) treatment at 30 °C. Maximumgermination (80–100%) was obtained by conditioning inwater at 20, 25, 30 and 35 °C for 14, 7, 5 and 3 d, respectively,and terminally treating with 10^–6 M strigol at 30 °C.Seeds conditioned in 10^–8 M strigol instead of water germinatedmuch less with the same terminal strigol treatment. Generally,conditioning was slower when seeds were conditioned in strigolrather than water. The reduction in germination rate by pretreatmentin strigol or pretreatment at low temperatures could be overcomeby increasing the terminal strigol concentration in the germinationtest. Conditioned seeds did not germinate at 10 and 15 °Cwith a terminal 10^–6 M strigol treatment but yielded closeto maximum germination at 25, 30 and 35 °C with the sameterminal strigol treatment. To obtain maximum germination, boththe minimum conditioning temperature and the minimum germinationtemperature for conditioned seeds were 20 °C. Factors suchas conditioning time, and strigol concentration and temperatureduring conditioning and/or germination determine whether seedsremain in the conditioning phase or shift to a germination phase. dl-Strigol, germination stimulation, parasitic plants, seed conditioning, seed germination, Striga asiatica, temperature, weed control     

Viability of onion (Allium cega) seed as influenced by temperature during seed growth

The yield and performance of seeds from crops of winter-hardy, bolting-resistant onion grown at temperatures of 15–16, 18–19 and 22–23°C in 1979, 1980 and 1982 were compared. Yields of seed from crops grown at 22–23°C were lower than those from crops grown at lower temperatures but the seeds ripened between 11 and 32 days earlier. Seeds from crops grown at mean temperatures of above 18°C gave higher percentage germination when imbibed at 30°C than 20°C and they also gave higher percentage seedling emergence than those from crops grown at lower temperatures. Seedlings from seeds produced at mean temperatures above 18°C were heavier than those from seeds of a similar weight but produced at lower temperatures. None of these differences were associated with differences in seed weight, embryo weight or seed dormancy but were positively correlated with differences in seed N-concentration. The differences were also associated with the rate of imbibition of water as high germination, high N-content seeds had a slower rate of imbibition than low germination, low N-content seeds of the same weight.     

Osmoconditioning as a Means of Counteracting the Ageing of Pepper Seeds during High-temperature Storage

Sweet pepper seeds were osmotically conditioned in 0.4 M mannitolsolution for 4 d (at 25 °C, in darkness) before or afterstorage at 35 °C for up to six months, and their germinationand viability was compared with that of untreated seeds storedunder the same conditions. Seeds that had been osmoconditionedprior to storage retained a high rate of germination and germinatedto a high final percentage (from 80 to 50 per cent) at both15 and 25 °C throughout the storage period. By contrast,both the rate and total level of germination of untreated pepperseeds declined rapidly at both germination temperatures, andby three months of storage the total level of seed viabilitywas already less than 10 per cent. Seeds that were first storedat 35 °C, and then osmoconditioned just prior to germination,showed a decline in germinability which when tested at 25 °Cwas the same as for untreated seeds, while tested at 15 °Coccurred at a slightly slower rate than for untreated seeds. It is evident that osmoconditioning prior to storage, in additionto the acceleration of germination, resulted in a dramatic delayof the ageing rate, thus increasing considerably the longevityof seeds. On the other hand, osmoconditioning after storagedid not seem to have any significant effect on seed viability,though it enhanced the germination rate. Capsicum annuum, sweet pepper, seed, germination, osmoconditioning, priming, storage, viability, ageing, longevity     

Seed Germination and Seedling Development in Cyclamen persicum

Cyclamen persicum Mill, seeds germinate in a narrow range oftemperature and germination is strongly inhibited by continuousirradiation with white light. The thermal optimum is approx.15 °C in both darkness and light. Seed germination is alsovery sensitive to oxygen deprivation and this sensitivity ismore pronounced at 20 °C than at the optimum 15 °C.Very immature seeds cannot germinate at any temperature, butgerminability increases during seed maturation Seedling development is unusual since seed reserves are usedimmediately for tuber formation. Tuberization is optimal at15–20 °C in light and in darkness. Supra-optimal temperatures(25–30 °C) or hypoxia inhibit tuber formation andlead to very elongated tubers These results allow the producers to improve the productionof homogeneous populations of cyclamen seedlings Wheat seeds, Triticum aestwum L., acetylcholinesterase, electrophoresis, germination, assay     

Winterfat (Eurotia lanata(Pursh) Moq.) Seedbed Ecology: Low Temperature Exotherms and Cold Hardiness in Hydrated Seeds as Influenced by Imbibition Temperature

Thermal analyses of freezing events in hydrated lettuce (LactucasativaL.) seeds show a correlation between low temperature exotherms(LTEs) (evidence of ice crystal formation) and seed death. Yet,weather patterns common to the Northern Great Plains of NorthAmerica regularly create conditions where non-dormant seedsof native plants hydrate with snow melt and are subsequentlyexposed to -30 °C or colder conditions. To determine ifsuch weather patterns decimate dispersed seeds, we measuredthe effects of freezing on fully hydrated winterfat (Eurotialanata(Pursh) Moq.) seeds harvested from the Northern Plainsat two USA and one Canadian location. Survival of hydrated seedsto -30 °C at a cooling rate of 2.5 °C h^-1was similarto that of seeds not subjected to cooling, even though botha high temperature exotherm (HTE) and an LTE were observed.Although the LTE was not related to winterfat seed survival,freeze-stressed seeds had reduced germination rates and reducedseedling vigour, particularly for the collection with the lightestseeds. The temperature of LTEs was similar among seed collectionswith a mean of -17.6 °C, but was warmer when the seeds wereimbibed at 0 °C compared to 5, 10 or 20 °C. We founda significant correlation between the HTE and LTE temperatures.The difference and the correlation may be due to the highermoisture content of seeds imbibed at 0 °C. After pericarpremoval, only one exotherm in the range of the LTE was observed.This was also true for the naked embryo. We conclude that anLTE indicates ice formation in the embryo, but that it doesnot signal the death of a winterfat seed.Copyright 1998 Annalsof Botany Company Eurotia lanata(Pursh) Moq.,Krascheninnikovia, Ceratoides,winterfat, exotherm, freezing tolerance, freezing avoidance, seedbed ecology, germination, D50, seedling vigour, seed collection     

Storage Behaviour of Salix alba and Salix matsudana Seeds

Effects of dehydration, storage temperature and humidificationon germination of Salix alba andS. matsudana seeds were studied.Newly released seeds showed 100% germination before and afterdehydration to 11–12% moisture content. Germination ofthe high vigour lot (100% initial normal germination) was notaffected by dehydration to 6.7% moisture content but germinationdecreased with further dehydration to 4.3%. The lower vigourlot (75% initial normal germination) was more susceptible todehydration and germination decreased following dehydrationto 6.7% moisture content. Dry seeds of both species survivedimmersion in liquid nitrogen without loss of viability. Thegermination of seeds stored with 9% moisture content decreasedto 35–40% in 5 months at -20°C or in 2 months at 5°C.However, at 25°C seeds entirely lost viability within 2weeks. Seeds showed improved performance when stored at -70°C> - 20°C > 5°C > 25°C and tolerated dehydrationto a moisture content in equilibrium with 15% relative humidity.Results suggest that they are orthodox in storage behaviouralthough they are short-lived. Humidification treatment of lowvigour seed lots resulted in a remarkable increase in germinationpercentage. Copyright 2000 Annals of Botany Company Salix alba, Salix matsudana, willow, seed storage behaviour, dehydration, humidification, cryopreservation     

Studies on the Germination of the Seeds of Striga hermonthica: III. On the Nature of Pretreatment and after-ripening

An attempt has been made to elucidate some of the changes whichtake place in seeds of Striga hermonthica (immature, ripe andoverripe), (a) during moisture-treatment at three temperatures(15°C., 22°C., and 32°C.), and (b) during after-ripeningin dry storage. Observations on the drift of respiration of immature seed throughoutpretreatment at 15°C. show that an inverse relationshipexists between germination and respirational activity. Thissuggests that germination depends on the accumulation of somemetabolite during pretreatment. It would seem that this metabolitemay also be used as a substrate for respiration during pretreatment. After-ripening of the immature seeds has been shown to changethe nature of the seeds in some way so that their peak respirationon exposure to moisture is at first progressively reduced. Thisreduction has been ascribed to the presence of some barriercapable of restricting gaseous diffusion. The data presented, togetherwith those of the previous paperof this series, suggest that the specific metabolite requiredfor germination might be proteinaceous. Certain other data appearto rule out the suggestion that germination capacity is conditionedby the sugar content of the seeds. Reasons are given which supportthe view that one of the main effects of the stimulating solutionon the seed is to increase the permeability of the seeds togaseous diffusion.     

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     

Temperature Dependence of Viability and Dormancy of Zizania palustris var. interior Seeds Stored at High Moisture Contents

Seeds (caryopses) of North American wild rice (Zizania palustrisvar. interior), a temperate aquatic grass, have been thoughtto require storage at low temperatures and high moisture contentsto preserve viability. The seeds are also deeply dormant atmaturity and require up to 6 months of stratification to breakdormancy. We report here that wild rice seeds can retain viabilityat moisture contents 30% (f. wt. basis) for up to 6 monthsat temperatures as high as 30 °C, and for at least 1 yearat temperatures below 20 °C. Dormancy is not broken at temperaturesabove 10 °C, but subsequent stratification requirementsare unaffected by prior warm storage. Cold storage is thereforenot required to maintain viability of wild rice seeds, but isnecessary to break dormancy. Hydrated wild rice seeds can befrozen to –10 °C without damage, but dormancy is notlost at subfreezing temperatures. These results provide newoptions for long-term storage of wild rice seeds. Zizania palustris var. interior (Fassett) Dore, wild rice, seed, germination, dormancy, storage, moisture content     

Effects of High Temperatures on the Permeability and Germinability of the Hard Seeds of Rhus javanica L.

The effects of temperature, 40–85 °C, on the permeabilityand germinability of the hard seeds of the pioneer tree Rhusjavanica L. with a fire syndrome were studied. The temperatureeffective for removal of the water-impermeable coat dormancyof the seeds was 55 ± 7·4 °C. With increasingtemperature, shorter exposure became sufficient to render theseeds permeable, but at temperatures above 75 °C, heat impairmentof germinability resulted in less than 60% germinability, evenwith long exposure. The most favourable regimes among thosetested were temperatures of 65–75 °C for durationsof 30–120 min, which frequently occur on denuded groundduring the midday hours of clear spring or summer days. Rhus javanica L., water-impermeable coat dormancy, seed germination, high temperature     

Seed Germination and Radicle Growth of a Halophyte, Kalidium caspicum(Chenopodiaceae)

Effects of temperature, light, NaCl and polyethylene glycol(PEG)-6000 on seed germination and radicle growth in a halophyticshrub, Kalidium caspicum(L.) Ung.-Sternb. were investigated.When seeds were incubated in deionized water at constant temperaturesbetween 10 and 30°C, the percentage germination in the darkexceeded 75%; light suppressed seed germination at alternatingtemperatures. Incubating seeds with a hypersaline solution ofNaCl for 30 d had no adverse effect on their germinability.The percentage germination of seeds incubated with a –0.8MPa NaCl solution was 73, 80 and 54% at 10, 20 and 30°C,respectively, but all radicles died before their length exceeded5 mm. In contrast, when seeds were incubated with a –0.8MPa PEG solution at 20°C, 68% of seeds germinated, and 95%of the emerging radicles survived beyond 5 mm. The high sensitivityof small radicles of this species to salinity indicated thatsalt must be removed from the soil surface for seedling establishment.Copyright2000 Annals of Botany Company Chinese desert, radicle growth, germination, halophyte, Kalidium caspicum, salinity     

Water Relations of Seed Development and Germination in Muskmelon (Cucumis melo L.): VII. INFLUENCE OF AFTER-RIPENING AND AGEING ON GERMINATION RESPONSES TO TEMPERATURE AND WATER POTENTIAL

The effects of storage conditions on the germination of developingmuskmelon (Cucumis melo L.) seeds were tested to determine whetherafter-ripening is required to obtain maximum seed vigour. Seedswere harvested at 5 d intervals from 35 (immature) to 60 (fullymature) days after anthesis (DAA), washed, dried, and storedat water contents of 3·3 to 19% (dry weight basis) at6, 20, or 30°C for up to one year. Germination was testedin water and in polyethylene glycol 8000 solutions ( –0·2to –1·2 MPa osmotic potential) at 15, 20, 25 or30°C. Germination percentages and rates (inverse of meantimes to radicle emergence) were compared to those of newlyharvested, washed and dried seeds. For 40 and 60 DAA seeds,one year of storage at 20°C and water contents <6·5%significantly increased germination percentages and rates at20°C, but had little effect on germination at 25 and 30°C.Storage reduced the estimated base temperature (T_b) and meanbase water potential (_b) for germination of both 40 and 60 DAAseeds by approximately 5°C and 0·3 MPa, respectively.Immature 35 DAA seeds showed the greatest benefit from storageat 3 to 5% water content and 30°C, as germination percentagesand rates increased at all water potentials (). Storage underthese same conditions had little effect on the germination ofmature seeds in water, but increased germination percentagesand rates at reduced 's. Accelerated ageing for one month at30°C and water contents from 15 to 19° increased germinationrates and percentages of mature seeds at reduced 's, but longerdurations resulted in sharp declines in both parameters. Immatureseeds lost viability within one month under accelerated ageingconditions. An after-ripening period is required at all stagesof muskmelon seed development to expand the temperature andwater potential ranges allowing germination and to achieve maximumgerminability and vigour. Post-harvest dormancy is deepest atthe point of maximum seed dry weight accumulation and declinesthereafter, both in situ within the ripening fruit and duringdry storage. Key words: Muskmelon, Cucumis melo L., seed, development, dormancy, germination, vigour, after-ripening     

An Intermediate Category of Seed Storage Behaviour?: I. COFFEE

Seeds of four cultivars of arabica coffee (Coffea arabica L.)were tested for germination following hermetic storage for upto 12 months at several different combinations of temperaturesbetween –20 °C and 15 °C and moisture contentsbetween 5% and 10% (wet basis). Most of the seeds from one cultivarwithstood desiccation to between 5% and 6% moisture content,a seed water potential of approximately –250 MPa, butthose of the remaining three cultivars were much more sensitiveto desiccation damage. Moreover, in all four cultivars, seedlongevity at cool and sub-zero temperatures, and at low moisturecontents did not conform with orthodox seed storage behaviour:viability was lost more rapidly under these conditions thanat either warmer temperatures or higher moisture contents. Theresults confirm that coffee seeds fail to satisfy the definitionsof either typical orthodox or recalcitrant seed storage behaviour.These results, therefore, point to the possibility of a thirdcategory of storage behaviour intermediate between those oforthodox and recalcitrant seeds. One of the main features ofthis category is that dry seeds are injured by low temperatures. Key words: coffee, Coffea arabica L., seed storage, seed longevity, desiccation, temperature     

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     

Osmoconditioning and Ageing of Pepper Seeds During Storage

The effects of osmoconditioning on the germination at 15 and25 °C of pepper (Capsicum annuum L.) seeds were studiedover a 3-year period with respect to temperature of storage.Untreated seeds stored at 5 °C showed high germinabilitythroughout the entire storage period, whereas untreated seedsstored at 25 °C showed a progressive decline in germinability,especially when assayed at 15 °C. Seeds that had been osmoconditionedprior to storage retained a high level of germinability irrespectiveof either storage or germination temperatures. When seeds thathad been stored at 25 °C were osmoconditioned after storage,there was a significantly higher germinability (assayed at 15 °C) in comparison with the corresponding untreated seeds.Seeds that were osmoconditioned twice (prior to and after storage)germinated in a similar way to those that had been osmoconditionedonce only Lactuca saliva L., lettuce, Hordeum oulgare L., barley, seed storage, moisture content, relative humidity, water potential, temperature, oxygen     

The Effect of Desiccation on the Longevity of Seeds of Araucaria hunsteinii and A. cunninghamii

Seeds of Araucaria hunsteinii K. Schum. dried quicker at 29°Cthan at 19°C and quicker with the seed-coat removed thanwhen intact; seeds enclosed in polyethylene bags increased inmoisture content. At 15°C, seeds in a flow of air driedquicker than seeds in a box with silica gel, which in turn driedquicker than seeds in a box with no desiccant. No loss of germinationability occurred on drying fresh seed from 53 to about 32 percent moisture content (fresh weight basis); during further desiccationthe percentage germination was related to percentage moisturecontent in the form of a sigmoid curve, culminating in a completefailure to germinate at approximately 14 per cent moisture content.A consistent relationship was observed for all treatments andthe mean critical moisture content for seed death (failure togerminate) was near 23 per cent. Excised embryos grew on 1 percent agar but died if previously subjected to 14 h of desiccationat 15°C. In contrast, no relationship was found between germination andmoisture content of A. cunninghamii D. Don on desiccation from21 to 7 per cent moisture content. Possible causes for the observeddifference in response to desiccation are discussed and methodsfor seed storage are considered. Araucaria hunsteinii, Klinkii pine, Araucaria cunninghamii, Hoop pine, desiccation, seed longevity, storage of seeds     

Restoration of Seed Germination at Supraoptimal Temperatures by Fluridone, an Inhibitor of Abscisic Acid Biosynthesis

Fluridone, an inhibitor of ABA biosynthesis, restored the seedgermination of lettuce (Lactuca sativa L. cv. Grand Rapids)and many other plant species at supra-optimal temperatures.ABA content in lettuce seeds after imbibition quickly decreasedat 23°C, but not at 33°C (a supraoptimal temperature).Fluridone caused a decrease in ABA content at 33°C, whichsuggests that the maintenance of high ABA content could be responsiblefor high-temperature inhibition of germination of lettuce seeds.This probably results from an increase in the rate of ABA biosynthesisat the higher temperature. The present study indicates thatABA plays a decisive role in the regulation of seed germinationat supraoptimal temperatures. ¹ Corresponding author: fax 81-22-717-8834; e-mail yoshi@bios.tohoku.ac.jp