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     

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.     

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     

Seed dormancy in Rumex species in response to environmental factors

Abstract. Many Rumex species show similar seed dormancy characteristics but there is more information concerning R. crispus and R. obtusifolius than other species. These species respond positively to red or white light. Far-red light applied for short periods may promote or inhibit germination depending on the timing of the irradiation in relation to temperature change; but long periods of far-red inhibit germination. Seeds may also be stimulated to germinate in the dark by low-temperature stratification at 15°C or less providing the temperature of the seeds is subsequently raised to a minimum of about 15°C. Seeds can, however, germinate at lower temperatures providing they have received other appropriate stimulatory treatment. Seeds also respond to alternating temperatures. In a diurnal cycle the minimum upper temperature required is about 15°C and the maximum lower temperature is about 25°C. The optimum period spent at the upper temperature is about 8 h when it is 15–25°C but the optimum period decreases as the upper temperature is increased above this range so that at 45°C, for example, it is only about 30 min. The period spent at the lower temperature in a diurnal cycle is not critical. Providing these criteria are met, the percentage germination increases with the number and amplitude of the cycles. The warming part of the cycle is necessary for the response but so far there is no convincing evidence that cooling itself is important. Secondary dormancy is induced at constant temperatures at a rate dependent on temperature, but apparently only in the presence of oxygen. This feature affects the optimum timing of a temperature change or exposure to light. Strong positive interactions are shown between stimulatory temperature treatments and white or red light. Unlike many other weed species the seeds respond only slightly to nitrate ions. The implications of these responses are discussed in relation to field behaviour.     

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     

Time, Temperature and Germination of Pearl Millet (Pennisetum typhoides S. & H.): II. ALTERNATING TEMPERATURE

Methods of analysing the response of germination to constanttemperature are extended to alternating temperatures. The analysisis illustrated for seeds of pearl millet (Pennisetum typhoidesS. & H.) germinated on a thermal gradient plate in pairsof alternating temperatures ranging from 15?C to 47?C. Alternatingtemperatures had a small but systematic effect on germinationrate such that below 42 ?C, alternations in temperature hadno effect on the maximum fraction of seeds which germinatedin the population, but increases in temperature amplitude fromzero to ?8 ?C caused a small but systematic increase in therate of germination.     

Light and temperature control of germination in Agropyron smithii seeds

In darkness, A. smithii seeds germinated poorly at constanttemperatures but well at alternating temperatures. Prolongedperiods on the high part of the temperature cycles reduced germination;the higher the temperature the shorter was the period requiredon the high part of the temperature cycles for optimum germination.Continuous, unfiltered, incandescent illumination and intermittentfar red at 15?–25?C alternation also inhibited germination;the inhibitory effects were similar to those caused by the highintensity reaction. Far red inhibited germination when appliedafter 1 and 2 complete 15?–25?C cycles in darkness butnot after 3 cycles. Less than 20% of the seeds were under phytochromecontrol at constant 20?C. When red light was applied directlyafter far red that was applied in intermittent cycles at 15?–25?C,however, 50% of the seeds caused to germinate by the alternatingtemperature were shown to be controlled by the reversible phytochromereaction. The induced high-temperature dormancy was overcome by gibberellicacid (GA₃) plus kinetin. The hormonal treatment was much moreeffective than light for breaking dormancy. Inhibition fromprolonged illumination was alleviated or eliminated by GA₃+kinetin.The failure of red light to promote good germination at 20?Cwas also overcome with GA₃+kinetin; effects of light plus thehormone treatments were more than additive. These data suggestthat optimum alternating temperatures facilitate a proper balanceand interaction of hormones, enzymes, substrates and possiblypreexistent P_fr so that the germination of A. smithii seedscan proceed without benefit of a light treatment. (Received July 7, 1976; )     

Characterization of alternating-temperature regimes that remove seed dormancy in seeds of Brachiaria humidicola (Rendle) Schweickerdt

Abstract Seeds of Brachiaria humidicola were subjected, on a thermogradienl plate, to a wide range of alternating-temperature cycles (24 h) and constant temperatures with intermittent exposure to diffuse laboratory light, in the presence and absence of 10 mol m^?3 KNO_V The optimum regime for maximum percentage germination was alternating temperatures of 35°C for 4hd^?1 and 13°C for 20 hd^?1. Almost no germination occurred at any constant temperature. Thermoperiods in which the warmer temperature was applied for the longer part of the 24 h cycle were much less stimulatory; in the presence of KNO₃, however, the germination under such regimes was much improved, although there was little effect on seeds experiencing near-optimum alternating-temperature regimes. This investigation is the first step in identifying which of 10 attributes of alternating temperatures are stimulatory, in order to predict the efficacy of different temperature regimes and to identify the stimulatory characteristics that must ultimately be explained by cellular physiology. The work shows that amplitude, thermoperiod and mean temperature must all be incorporated in a quantitative model.     

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     

Germination responses of Discopodium penninervium Hochst. to temperature and light

The germination responses of Discopodium penninervium were tested at different constant and alternating temperature regimes as well as under various light conditions both in the laboratory and glasshouse. Seeds incubated at 10, 15, 20, 25 and 30 °C failed to germinate. When the seeds were incubated at alternating temperatures of 20/12 °C and 30/12 °C under continuous light, germination was 89 and 61%, indicating that the species requires alternating temperatures as a cue for germination. However, germination declined as the amplitude of alternating temperatures increased from 8 °C and was completely inhibited at an amplitude of 23 °C, suggesting that the optimum amplitude is around 8 °C. Germination was less than 10% in light and nil in darkness at 20 °C in the laboratory. In contrast, seeds incubated at 20/12 °C germinated to 96 and 86% in light and darkness, respectively. Seeds incubated under leaf shade in the glasshouse failed to germinate whereas those incubated under direct daylight and darkness germinated to 44 and 50%, respectively, 30 days after sowing. When seeds incubated under leaf shade and in darkness were exposed afterwards to light, final percent germination was 83% from seeds incubated initially under direct daylight, 79% from those incubated under leaf shade and 86% from those incubated in darkness. The requirement for alternating temperatures and light rich in red:far red ratio to break the dormancy of seeds of D. penninervium could restrict germination to gaps in the vegetation. The results conform with the ecology of the species.     

SEED GERMINATION STUDIES IN MUSA. II. ALTERNATING TEMPERATURE REQUIREMENT FOR THE GERMINATION OF MUSA BALBISIANA

Stotzky , G., and Elsie A. Cox . (Central Research Labs., United Fruit Co., Norwood, Mass.) Seed germination studies in Musa. II. Alternating temperature requirement for the germination of Musa balbisiana. Amer. Jour. Bot. 49(7): 763–770. Illus. 1962.—Alternating temperatures were found to be required for the germination of seeds of Musa balbisiana. The temperature differentials optimal for germination in soil are dependent upon both the high and low temperatures, and range from 8–23 C. Germination is maximal when the seeds are held 6–12 hr at the high (27–35 C) and 12–18 hr at the low (12–18 C) temperatures. Some germination can be induced by short exposures to alternating temperatures followed by constant high temperatures, but continuous exposure to alternating temperatures is necessary for maximum germination. Excised embryos develop better at constant than at alternating temperatures, showing that the mechanisms affected by alternating temperatures reside elsewhere in the seed. Alternating temperatures are also required for germination of mechanically scarified seeds, although the temperature differentials are less than those necessary for intact seeds, indicating that the action of alternating temperatures is not on the permeability of the integuments.     

Characteristics of alternating temperatures which stimulate loss of dormancy in seeds of Rumex obtusifolius L. and Rumex crispus L.

Abstract. Alternating temperatures stimulate the germination of Rumex crispus L. and Rumex obtusifolius L. The optimum period spent at the lower temperature in a diurnal cycle is greater than that spent at the higher temperature. Under most conditions the optimum period at the upper temperatures is about 8 h but, as the upper temperature of a cycle is increased, the optimum period at the upper temperature becomes shorter and more critical. Thus when it is 35°C the optimum period is 2.5–4 h in the light, or about 1 h in the dark. The effect of alternating temperatures is much less in the dark than in the light and in general only extreme alternations with short periods at the higher temperature are effective in the dark. In the light any temperature alternation within the range 1–35°C is effective to at least some extent, providing the temperature difference is 5°C or more and providing the alternation includes one temperature which is above approximately 15°C and one which is below approximately 25°C. The optimum temperature difference is about 15°C. In the light, 4 to 10 cycles saturate the response, but in the dark, where the effect is much less, the response may not be saturated even by 16 cycles. KNO₃ at 10⁻³ M has little effect on the response to alternating temperatures either in the light or the dark. The response to alternating temperature regimes does not appear to vary in quality, i.e., in terms of which particular treatments are best, but it varies in magnitude with site and year of seed collection; and it increases slowly during dry storage, even when stored at a temperature as low as 1.5°C.     

Germination behaviour of Conyza bonariensis to constant and alternating temperatures across different populations

Conyza bonariensis is one of the most problematic weed species throughout the world. It is considered highly noxious due to its interference with human activities, and especially the competition it poses with economically important crops. This research investigated the temperature requirements for seed germination of four populations of C. bonariensis with distinct origin and the influence of daily alternating temperatures. For this, a set of germination tests were performed in growth chambers to explore the effect of constant and alternating temperatures. Seeds of the four populations (from Lleida, Badajoz and Seville, Spain and Bahía Blanca, Argentina) were maintained at constant temperatures ranging from 5 to 35°C. The final germination and cardinal temperatures (base, optimum and maximum) of each population were obtained. We also tested the influence of daily alternating temperatures on final germination. To do so, seeds were exposed to two temperature regimes: 5/15, 10/20, 15/25, 20/30 and 25/35°C night/day temperature (intervals increasing 5°C, with constant oscillation of 10°C) and to 18/22, 16/24, 14/26, 12/28 and 10/30°C night/day temperature (intervals with average of 20°C, but increasing the oscillation in 4°C between intervals). In general, all populations behaved similarly, with the highest germination percentages occurring in the optimum temperature range (between 21.7°C and 22.3°C) for both constant and alternating temperatures. In general, climatic origin affected germination response, where seeds obtained from the coldest origin exhibited the highest germination percentage at the lowest temperature assayed. In addition, we observed that the alternating temperatures can positively affect total germination, especially in oscillations that were further from the average optimum temperature (20°C), with high germination percentage for the oscillations of 15/25, 20/30, 18/22, 16/24, 14/26, 12/28 and 10/30°C in all populations. The cardinal temperatures obtained were significantly different across the populations. These results provide information that will facilitate a better understanding of the behaviour of Conyza and improve current field emergence models.     

新疆干旱区植物藜的种子异型性及其萌发机理

新疆干旱区分布的植物藜(Chenopodium album)的种子有黑色和褐色两种类型。对藜的异型性种子从形态结构、不同环境因素及激素或化学物质对萌发的影响以及同工酶谱等方面进行了研究,并对其萌发及适应异质环境的机理进行了讨论。结果表明:(1)藜的异型性种子在形态结构、萌发休眠特性等方面都存在明显差异:黑色种子种皮厚且硬,休眠,萌发慢,萌发率低;褐色种子种皮薄而软,不休眠,萌发快且萌发率高;(2)黑色种子的休眠可通过切除胚根外缘种皮得以完全解除;(3)赤霉素、乙烯利对黑色种子的萌发无明显促进作用;KNO3可较显著促进黑色种子的萌发;协同使用乙烯利和KNO3时,可显著提高黑种子萌发率,完全打破休眠;(4)黑色种子和褐色种子的酯酶、过氧化物酶及过氧化氢酶同工酶谱带存在差异;(5)黑色种子的萌发需要光照,而褐色种子则对光不敏感;低温贮藏对二者的萌发均无显著影响,尽管黑色种子的萌发率有波动。研究结果初步显示黑色种子的休眠是内源(胚)和外源(种皮)因素共同所致。藜的种子异型性及其萌发机理的形成是其对新疆干旱区异质化环境的高度适应。     

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.     

Analysis of the Effect of Cold Stratification on the Germination Response to Light and Alternating Temperatures using Selected Seed Populations of Ranunculus sceleratus L.

From a single population of achenes (seeds) of Ranunculus sceleratusL. sub-populations were selected on the basis of the sensitivityof individuals to an increasing number of daily light/temperatureshift cycles. Each cycle comprised a 4 h pulse of red lightfollowed by a 4 h temperature shift from 16 to 31 ?C. Selectionfor low dormancy (NND and ND) and high dormancy (D and DD) populationsresulted in a > 5-fold difference in the number of cyclesrequired for 50% germination. Despite a shift in the mean levelof dormancy the distributions of sensitivity (slopes of dose-responsecurves) were similar in all four selected populations. Differentialeffects of cold stratification on the germination response tolight and alternating temperatures were related to the depthof primary dormancy. The proportion of individuals that respondedpositively to the dormancy breaking effects of cold stratificationfollowed the trend DD < D < ND < NND. In high dormancypopulations (D and DD) the rate and uniformity of germinationof some individuals was reduced by cold stratification, indicatinga dormancy inducing effect. Over the range 2 to 11 ?C the effectivenessof dormancy release or dormancy induction was inversely relatedto temperature. The effects of cold stratification on the expressionof dormancy in R. sceleratus are discussed in relation to areproductive strategy involving winter and summer annual behaviour. Key words: Cold stratification, selection, dormancy, light, alternating temperatures, germination     

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     

Mechanisms regulating seedling emergence of orchardgrass (Dactylis glomerata L.) and western wheatgrass (Pascopyrum smithii [Rydb.] L.): Dormancy change,seed fate and seeding date

Seed germination and seedling emergence of ‘Arctic’ and ‘Lineta’ orchardgrass (Dactylis glomerata L.) and ‘Walsh’ and ‘LC9078a’ western wheatgrass (Pascopyrum smithii [Rydb.] L.) were studied both in the field and laboratory. Four seeding dates were conducted each year over 2 years and seedling emergence and seed fate in the soil were monitored. The effects of alternating temperature and light on germination were quantified and correlated with seedling emergence from soil and in the field. Orchardgrass seeds were less dormant than western wheatgrass as indicated by the disparity in germination percentage between constant and alternating temperatures. Seed germination percentage was usually higher than seedling emergence in the field for orchardgrass but lower for western wheatgrass, and temperature was not responsible for the difference. Exposing orchardgrass seeds to light during germination check helped break dormancy in orchardgrass when temperature was unfavorable (low and/or constant temperatures), while favorable temperatures (optimal, alternating temperatures) conditions overcame the inhibiting effect of light in western wheatgrass. The final seedling emergence of orchardgrass was either similar among the four seeding dates or decreased slightly from early May to early June. For western wheatgrass, however, final seedling emergence increased with seeding dates from early to late May and decreased in early June. Soil temperatures of the first 2 weeks after seeding increased from the early May to late May and then decreased. These temperatures were below or near the optimal temperatures for western wheatgrass seeds to release dormancy and germinate. Germination of the previously buried seeds indicated that orchardgrass and western wheatgrass had the potential for a high germination percentage under field conditions for all seeding dates. While soil temperatures close to the optimal temperature for dormancy breaking and germination promoted germination of orchardgrass, the same conditions could cause deterioration of seeds if they failed to germinate. For western wheatgrass, deeper dormancy reduced seed mortality.