Thermoperiodic response and effect of photoperiod on thermoperiodic induction of diapause in Colaphellus bowringi

The effects of thermoperiods on diapause induction under continuous darkness (DD), continuous light (LL), and an L12:D12 photoperiod were investigated in the cabbage beetle, Colaphellus bowringi Baly (Coleoptera: Chrysomelidae), a short‐day species. Diapause could be induced by thermoperiod under both LL and DD; however, in the range of 24–30 °C, lower incidences of diapause were observed under LL than under DD. The critical cryophase was found to be dependent on the mean temperature of the thermoperiod applied. Although the thermoperiodic response pattern was similar under LL and DD, the incidence of diapause was higher under LL when the duration of the cryophase did not exceed 12 h. In contrast, when the duration of the cryophase was longer than 12 h, the incidence of diapause under LL was lower or equal to that under DD. When a thermoperiod of 24 °C (cryophase) and 28 °C (thermophase) was applied, the incidence of diapause was higher under LL than under DD, regardless of the duration of the cryophase. Thermoperiodic responses under a photoperiod of L12:D12 and under DD further revealed that induction of diapause was strongly influenced by the photophase temperature. Moreover, the incidence of diapause was lower when the thermophase coincided with the photophase than when the cryophase coincided with the photophase.     

Effects of thermoperiods on diapause induction in the cabbage beetle, Colaphellus bowringi (Coleoptera: Chrysomelidae)

Abstract. The effects of thermoperiods on diapause induction in continuous darkness or under a 12 : 12 h LD photoperiod were investigated in the cabbage beetle, Colaphellus bowringi Baly, a typical short‐day species. The diapause response curves both at different constant temperatures and at the thermocycle of format CT x: (24 ? x) h (16 : 28 °C) under continuously dark rearing conditions showed that the incidence of diapause depended mainly on whether or not the mean temperature was ≤20 °C or >20 °C. If the mean temperature was ≤20 °C, all individuals entered diapause; if >20 °C, the incidence of diapause declined gradually with increasing mean temperatures. The thermocycle (CT 12 : 12 h) with a series of different cryophases (8–22 °C) and thermophases (24–32 °C) under continuous darkness demonstrated a cryophase response threshold temperature of approximately 19 °C and a thermophase response threshold temperature of approximately 31 °C. Thermoperiodic amplitude (temperature difference between cryophase and thermophase) was shown to have a significant influence on diapause induction at the mean temperatures of 22, 23 and 24 °C, but not at ≥25 °C. Thermoperiodic responses under LD 12 : 12 h clearly showed that the incidence of diapause was influenced strongly by the photophase temperature. The thermoperiod under LD 12 : 12 h induced a much lower incidence of diapause than the thermoperiod with the same temperature in continuous darkness. The ecological significance of thermoperiodic induction of diapause in this species is discussed.     

Thermoperiodic induction of larval diapause in the European corn borer, Ostrinia nubilalis

Thermoperiodic induction of larval diapause was shown to occur in the European corn borer, Ostrinia nubilalis (Hübner). Using continuous darkness and thermoperiods of the format XC:24-XT(15:30°C), incidence of diapause was shown to be dependent on phase durations, with a critical cryophase of about 9.5 hr. A thermoperiodic response threshold was demonstrated; it was found to be very close to 17.5°C. Thermoperiodic amplitude (temperature difference between cryophase and thermophase) was shown to have no influence on the induction of diapause, within a relatively broad range of physiological temperatures. Thermoperiodic induction of diapause was shown to be at least partially independent from the effects of temperature on larval developmental rates.     

Thermal and thermoperiodic effects on larval development and diapause in the European corn borer, Ostrinia nubilalis

Effects of temperature and thermoperiod on larval development and the induction of diapause were investigated in the European corn borer, Ostrinia nubilalis. Developmental threshold temperatures characteristic of the first four larval instars were estimated under both thermoperiods and constant temperatures. Threshold values were similar under the two conditions, but the 4th-larval instar was shown to display a significantly lower developmental threshold temperature than was characteristic of the earlier instars. Although developmental times (days per instar) were not greatly affected by fluctuating temperature regimes, the quantity of growth (weight, head width) during the 4th and 5th (last) stadia was found to be much greater under thermoperiodic regimes than under comparable constant temperature conditions. By means of thermoperiodic regimes having identical mean temperatures but different cryophase durations, it was demonstrated that the induction of diapause is dependent on the duration of the cryophase rather than on the mean temperature of the thermoperiod. To be effective, the cryophase must be colder than the insect's thermoperiodic response threshold. It was also demonstrated that thermoperiodic responses that are readily produced under continuously dark rearing conditions are not manifested under continuous light.     

Diapause induction in Eotetranychus smithi (Acari: Tetranychidae): effect of average temperature,but not of thermoperiod

Facultative diapause of Eotetranychus smithi appears to occur at the egg stage and is induced by temperatures ≤17.5 °C, independent of photoperiod. However, the effect of thermoperiod on the induction of diapause remains unclear. To answer this question, we exposed female E. smithi to various thermoperiods under constant light conditions. First, we found that the deposition order of eggs affected the incidence of diapause: the first eggs (exclusively males) tended to avert diapause compared with the second and third eggs (most of them are females), possibly because of the sex of the eggs. Next, the incidence of diapause of the second eggs decreased with shortening of the cryophase, which was associated with an increase of the average temperature, and it showed clear long‐day‐type thermoperiodic response curves. However, this species does not sense the ratio of day (thermophase) to night (cryophase) of a given thermoperiod. Short thermoperiods did not increase the incidence of diapause, but rather precluded the entry into diapause. We detected no sign of the involvement of the circadian system in diapause induction in the thermoperiodic Nanda–Hamner protocol. We conclude that diapause induction of E. smithi does not involve the circadian system, and thus does not show thermoperiodism. Diapause induction under the various thermoperiodic conditions tested in the present study appears to be derived from the temperature itself. E. smithi is an exceptional species that relies on temperature alone to induce diapause.     

Photoperiodism and thermoperiodism in the predatory mite Amblyseius potentillae are probably based on the same mechanism

Summary A comparative study was made of the photoperiodic and thermoperiodic induction of diapause in the phytoseiid mite Amblyseius potentillae. Sensitivity to thermoperiod was found to be highest during the protonymphal and deutonymphal stages, with some sensitivity still being present in the young adult. Summation of both photoperiodic and thermoperiodic cycles was shown to take place, which demonstrated the presence of a photoperiodic counter as well as a thermoperiodic counter in these mites. Vitamin A appeared to be necessary for some early step in the physiological mechanism of diapause induction and not just for the expression of the diapause response. The light sensitivity threshold for photoperiodic induction of diapause was found to be extremely low, viz. less than 0.02 W/cm². Moreover, the light sensitivity threshold appeared to be strongly temperature dependent in A. potentillae. Experiments in which the mites experienced various sequences of short-day photoperiods and short-day thermoperiods, applied either concurrently or in succession, showed that the information collected by the photoperiodic counter and the thermoperiodic counter is integrated into one induction sum. These results strongly suggest that photoperiodic and thermoperiodic induction of diapause in these mites is based on the same physiological mechanism.Abbreviations DD continuous darkness - LL continuous light - LD light-dark cycle (e.g. LD 16:8 is a cycle of 16 h of light and 8 h of darkness) - TC thermoperiodic cycle (e.g. TC 16:8 (27°: 15°) is a thermoperiod with a 16 h thermophase of 27 °C and an 18 h cryophase of 15°C)     

Effect of constant and fluctuating temperature on daily melatonin production by eyecups from Rana perezi

We analysed the effect of daily temperature cycles in relation to constant temperature on day/night melatonin synthesis in frog eyecups in culture. Eyecups were cultured for 24 h under 12L:12D photoperiod and two thermal regimes, constant temperature (25, 15 and 5 °C) and thermoperiod (WL/CD, thermophase coinciding with photophase and cryophase coinciding with scotophase; and CL/WD, cryophase coinciding with photophase and thermophase coinciding with scotophase). A negative correlation between ocular serotonin N-acetyltransferase activity and culture temperature for both diurnal and nocturnal activities has been observed. This effect of increased ocular activity at low temperature is more pronounced than the well-known stimulatory effect of darkness, and it does not depend on the photoperiod phase. The lack of interactions between the phase of photoperiod and culture temperature indicates that the effects of both factors are independent. Nighttime temperature is the key factor in determining the amplitude of the melatonin rhythm in the Rana perezi retina. However, daytime temperature can not counteract the inhibitory effect of light on ocular melatonin synthesis. Accepted: 22 June 1995     

Thermoperiodic acclimations enhance cold hardiness of the eggs of the migratory locust

Treatment of thermoperiods that simulate the patterns of natural occurrence is most efficient in enhancing cold hardiness. To examine the effects of different thermoperiods on cold hardiness of eggs in the migratory locust, Locusta migratoria (L.), the survival rates, cryoprotectant levels and three hsps expressions in mid-stage eggs (7-day-old) were measured after the eggs were subjected to three different thermoperiod regimes, : short (2 day), long (10 day), and nature-mimicking thermoperiodic acclimation. The thermoperiodic acclimations resulted in the highest egg survival rates in both the short and the long period acclimation groups in comparison with the groups treated with constant temperatures. The egg survival of nature-mimicking thermoperiod groups was significantly higher than those of constant temperature groups for the same acclimation duration. The survival rate of eggs under single daily thermoperiod was higher than that of multiple daily thermoperiods. The concentration of cryoprotectants (myo-inositol, trehalose, mannitol and sorbitol) and the expression levels of hsp20.5, hsp70, and hsp90 all increased in thermoperiodic acclimation eggs.     

Comparison of the circadian eclosion rhythm between non-diapause and diapause pupae in the onion fly,Delia antiqua: the effect of thermoperiod

When non-diapause and diapause pupae of Deliaantiqua were exposed to various thermoperiods where thermophase (T) was 25 °C and the cryophase (C) was 15 or 20 °C (TC₁₅ or TC₂₀) in constant darkness (DD), the majority of both types of flies emerged before the rise in temperature. Eclosion time was delayed at the lower cryophase temperature. Moreover, there was a significant difference in the time of adult eclosion between non-diapause and diapause pupae; diapause pupae eclosed earlier than non-diapause pupae. When the two types of pupae were transferred to a constant low temperature (15 or 20 °C) after having experienced TC₁₅ or TC₂₀ 12:12 h, they showed circadian rhythmicity in eclosion. The free-running period (τ) of the eclosion rhythm changed after transfer to constant low temperatures in both non-diapause and diapause pupae, suggesting that this change represents a transient cycle until the temperature-sensitive oscillator is coupled again to the temperature-insensitive pacemaker. However, diapause pupae tended to show a shorter τ than non-diapause pupae. This observation suggests that the difference in adult eclosion time under thermoperiodic conditions between non-diapause and diapause pupae is related to their different τ s.     

Influence of the photocycle and thermocycle on rhythms of plasma thyroxine and plasma and ocular melatonin in late metamorphic stages of the bullfrog tadpole, Rana catesbeiana

The diel fluctuations in plasma thyroxine (T(4)) and plasma and ocular melatonin entrain to the light/dark (LD) cycle in the bullfrog tadpole, although the phase of the rhythms changes during development. Previous studies on the rhythmicity of these hormones were conducted under various LD cycles, but with a constant temperature, raising the question of the role of the natural thermocycle in determining the phase of the rhythms, and the changes that occur in the hormone levels and rhythms during late metamorphosis. To study this question, tadpoles were acclimated to simulated natural conditions of 14.5L:9.5D with a corresponding thermocycle in which the thermophase was 28 degrees C and the cryophase was 18 degrees C, or to the same thermocycle under constant light (24L). On both photoregimens, the diel fluctuations changed between prometamorphosis and metamorphic climax. However, more statistically significant rhythms, as indicated by the cosinor, occurred on 14.5L:9.5D than on 24L. At climax on the LD cycle, all hormones peaked around the same time in the late scotocryophase, whereas on 24L, plasma T(4) peaked in the thermophase and plasma and ocular melatonin peaks occurred some distance from each other early in the cryophase. The earlier peaks of plasma and ocular melatonin on 24L were due to a transient rise in these hormones at the onset of the cryophase, which was not sustained in the absence of an LD cycle. On 14.5L:9.5D with a corresponding thermocycle, the hormone rhythms had nearly the same phases as was found in previous work on 12L:12D at a constant temperature of 22 degrees C, allowing for minor phase shifting due to the photocycle differences, indicating that in this species laboratory studies on constant temperature give valid results even in the absence of a thermocycle. The findings show that the phases of the hormone rhythms are determined by the LD cycle although the onset of the cryophase, in the absence of a photocycle, may exert some influence on the nighttime rise in melatonin. The developmental rise in plasma T(4), and drop in plasma melatonin, occurred on both 14.5L:9.5D and 24L, indicating, taken together with previous work, that these climactic changes were independent of temperature and light cycling.     

Adult eclosion rhythm of the Indian meal moth Plodia interpunctella: response to various thermocycles with different means and amplitudes

In addition to photoperiod, thermoperiod (or thermocycle) might be an important Zeitgeber for entraining the circadian oscillator controlling adult eclosion rhythm in the Indian meal moth Plodia interpunctella Hübner (Lepidoptera: Pyralidae). This is confirmed by exposing larvae receiving diapause‐preventing treatments to various thermocycles with different means and amplitudes of temperature. The thermocycles investigated in the present study are TC 8 : 16 h, TC 12 : 12 h, TC 16 : 8 h and TC 20 : 4 h, where T and C represent thermophase (30 °C) and cryophase (20 °C), respectively. For all thermocycles, the peak of adult eclosion rhythm occurs at around the mid‐thermophase. This indicates that the larvae use both ‘temperature‐rise’ and ‘temperature‐fall’ signals to adjust the eclosion phase in each thermocycle. The absence (DD) or presence (LL) of light affects this time‐keeping system slightly under the given thermocycle. The rhythmic adult eclosion noted after exposure of larvae to 30 °C DD for 14 days is recorded in the thermocycles (TC 12 : 12 h, DD; mean temperature = 25 °C) with different amplitudes of 27.5/22.5 °C, 26.5/23.5 °C and 25.5/24.5 °C. The peak in adult eclosion advances in time as the amplitude of the temperature cycle decreases. In the temperature cycle of 25.5/24.5 °C, a peak occurs at the end of the cryophase, 2 h before the temperature‐rise. The adult eclosion rhythm is also observed under various thermocycles (TC 12 : 12 h, DD) consisting of different temperature levels (30 to 20 °C) with different amplitudes. It is found that the temporal position of the peak advances significantly when the amplitude of the thermocycle becomes lower.     

Photoperiodic and thermoperiodic interactions in the regulation of the larval diapause of Diatraea grandiosella

The effect of various combinations of photoperiod and temperature on the induction and termination of the mature larval diapause of a Missouri strain of the southwestern corn borer. Diatraea grandiosella, was examined. Larval exposure to regimes in which the low phase of a 30°:23°C thermoperiod coincided with a scotophase of 10 to 14 hr duration led to high incidence of diapause. Larval exposure to 30°:24°C, 33°:21°C, and 36°:18°C thermoperiods with half cycles of 12 hr in continuous darkness yielded a diapause incidence of 16%, 22%, and 59%, respectively, whereas exposure to a 30°:24°C thermoperiod in continuous illumination yielded a completely nondiapause generation. Larval exposure to one of a series of 36°:18°C thermoperiods in which the duration of the high phase was increased in 2 hr increments from 0 to 24 hr in continuous darkness showed that “short-day” thermoperiods yielded a high incidence of diapause. However, no clearly defined critical thermoperiod was observed. An examination of photoperiodic and thermoperiodic effects on diapause development showed that, in general, those combinations of temperature and light cycles which were diapause inductive also retarded diapause development. The relationship between seasonal photoperiods and thermoperiods in southeastern Missouri was examined.     

Insect photoperiodism: diversity of results in night-break experiments, including nonresponsiveness to light

Three night-break experiment protocols were utilized in an attempt to help clarify the role of the circadian system in photoperiodic time measurement in the European corn borer, Ostrinia nubilalis. Larvae raised in a light-dark (LD) cycle consisting of 12 hr of light alternating with 12 hr of darkness (LD 12:12), at a constant temperature of 30 degrees C, enter a state of arrested growth and development known as diapause (Takeda and Skopik, 1985). In the present research (Experiment 1), the induction of diapause was prevented by 1-hr light pulses that systematically scanned the dark phase of LD 12:12. Thus, the importance of 12 hr of uninterrupted darkness for maximal induction of diapause is stressed. The same experimental protocol applied to larvae already in diapause (Experiment 2), however, resulted in a bimodal curve of diapause termination. Although this result is consistent with the proposition that a nonperiodic hourglass timer underlies this event (Skopik and Takeda, 1986), it does not rule out the circadian system. Like LD 12:12, a thermoperiod in constant darkness (12 hr at 4 degrees C alternating with 12 hr at 25 degrees C) also induces diapause. Scanning such a thermoperiod with 1-hr light pulses, however, resulted in only a small effect (reduction of diapause) when light fell in the early to middle part of the warm phase (Experiment 3). Thus, the time-measuring system, under these experimental conditions, showed only a weak response to light. This unexpected result is discussed with respect to Experiment 1 and two general models that have been proposed to account for photoperiodic time measurement in insects.     

Ultradian, circadian and seasonal variations of plasma progesterone and LH concentrations during the luteal phase

The circadian variations in plasma progesterone (P) and LH concentrations were investigated in six women, aged 23-40 years. All were studied in the mid-luteal phase (7 +/- 2 days after LH mid-cycle surge). Experiments were conducted in autumn and in spring. Blood samples were obtained every 15 min for 24 hr. Plasma P and LH concentrations were measured by RIA. Each subject's time-series was analysed using three methods; visual inspection (chronogram), spectral analysis to estimate component periods of rhythms (tau) and cosinor analysis to quantify the rhythms parameters. Marked temporal variations in plasma P concentration were observed in each subject. The maximal variations over a 24-hr period, ranged between 13-58.5 mmol/l. Differences related to sampling time were statistically validated by ANOVA (p less than 0.00001). Significant harmonic periods were detected by spectral analysis but differed among subjects. In all subjects but one, a circadian rhythm was detected. The acrophase location was similar (about 0700 hr) in the four subjects studied in autumn, but ranged from 1940 to 0320 hr in those studied in spring. An ultradian rhythm with tau = 8 hr was also validated in six time-series with similar acrophases (about 0200, 1000, and 1800 hr). Cosinor analysis of pooled data revealed that the 24-hr, 12-hr, and 8-hr rhythms were statistically significant (p = 0.001) in autumn. algebraic sum of these three cosine functions yielded a circadian waveform with peak-times occurring near 0300 and 1130 hr and a trough-time about 2200 hr. In spring, the circadian pattern appeared quite different, and peak-times were found near 0700 and 2000 hr, and trough-times near 0300 and 1500 hr. Furthermore, the 24-hr mean of P was higher in autumn (28.9 +/- 0.4 nmol/l) than in spring (17.2 +/- 0.4 nmol/l), p from ANOVA less than 0.00001. The evidence for a similar circadian LH pattern is not as strong. Seasonal, circadian and ultradian rhythms characterize the physiologic time structure of plasma P concentration in mid-luteal phase.     

Light and temperature cooperate to regulate the circadian locomotor rhythm of wild type and period mutants of Drosophila melanogaster

In the wild type (Canton-S) and period mutant flies of Drosophila melanogaster, we examined the effects of light and temperature on the circadian locomotor rhythm. Under light dark cycles, the wild type and per(S) flies were diurnal at 25 degrees C. However, at 30 degrees C, the daytime activity commonly decreased to form a rather nocturnal pattern, and ultradian rhythms of a 2 approximately 4h period were observed more frequently than at 25 degrees C. The change in activity pattern was more clearly observed in per(0) flies, suggesting that these temperature dependent changes in activity pattern are mainly attributable to the system other than the circadian clock. In a 12h 30 degrees C:12h 25 degrees C temperature cycle (HTLT12:12), per(0) flies were active during the thermophase in constant darkness (DD) but during the cryophase in constant light (LL). The results of experiments with per(0);eya flies suggest that the compound eye is the main source of the photic information for this reversal. Wild type and per(0) flies were synchronized to HTLT12:12 both under LL and DD, while per(S) and per(L) flies were synchronized only in LL. This suggests that the circadian clock is entrainable to the temperature cycle, but the entrainability is reduced in the per(S) and per(L) flies to this particular thermoperiod length, and that temperature cycle forces the clock to move in LL, where the rhythm is believed to be stopped at constant temperature.     

Effect of thermoperiod amplitude on the adult eclosion rhythm of Indian meal moth,Plodia interpunctella (Lepidoptera: Pyralidae)

The thermoperiodic time cue is known to be one of several zeitgebers (Zt) that entrain an organism's biological rhythms. This study investigated the adult eclosion rhythm of the Indian meal moth, Plodia interpunctella, (Lepidoptera: Pyralidae) under various thermocycles in constant darkness. The insects were exposed to different thermoperiods (e.g. 25.5°C/24.5°C). Thermophase : cryophase ratios (TC) ranged from 4 h : 20 h to 20 h : 4 h. The peak in adult eclosion rhythm occurred at Zt 18.8 ± 4.4 (mean ± SD) on a given day and at Zt 0.7 ± 3.8 on the subsequent day under TC 4 h : 20 h and TC 20 h : 4 h conditions, respectively (start of temperature increase = Zt 0). Thus, longer thermophase periods delayed the peak in eclosion. The regression line obtained was then compared with that obtained under the temperature cycle of 30°C/20°C, and the two lines were found to lie parallel to each other. The peak in adult eclosion rhythm occurred earlier under the 25.5°C/24.5°C conditions than under the 30°C/20°C conditions. The results strongly suggest that the amplitude of the temperature cycle is an important factor determining the temporal position of the peak in adult eclosion. This is a significant finding among insect species.     

Effect of temperature on thermoperiodic determination of diapause

Under continuous darkness, thermoperiodic determination of larval diapause in the European corn borer, Ostrinia nubilalis, is strongly influenced by the temperatures of the thermoperiodic phases. Cryophase temperatures are most inductive if lower than the thermoperiodic response threshold (17.5°) but not below about 10°, the larvae's developmental threshold. Thermophases are most effective if their temperatures are only slightly above the thermoperiodic response threshold, especially in regimes in which the cryophase temperatures approach the freezing point. The implications of these findings are discussed from the standpoint of biological periodism and biological clock function.     

Interacting effect of thermoperiod and photoperiod on the eclosion rhythm in the onion fly, Delia antiqua supports the two-oscillator model

Daily light and temperature cycles entrain adult eclosion rhythms in many insect species, but little is known about their interaction. We studied this problem in the onion fly, Delia antiqua. Pupae were subjected to various combinations of a photoperiod of 12L:12D and thermoperiods. The thermoperiods consisted of 12 h warm phase (W) and 12 h cool phase (C), giving a mean temperature of 25 °C with different temperature steps of 8, 4 and 1 °C. As the phase relation of the two Zeitgebers was varied, the phase of eclosion rhythm was shifted, depending on the phase angle with the light cycle and the amplitude of the temperature cycle. When the temperature step in the thermoperiod was 8 °C (WC 29:21 °C), the eclosion rhythm was entrained mainly to thermoperiod rather than photoperiod. In the regime with a 4 °C temperature step (WC 27:23 °C), both thermoperiod and photoperiod affected eclosion rhythm, and a phase jump of the eclosion rhythm occurred when the warm phase of thermoperiod was delayed 15-18 h from light-on. In regimes with a 1 °C temperature step (WC 25.5:24.5 °C), the eclosion rhythm was completely entrained to photoperiod. The observed interacting effect of light and temperature cycle on the eclosion rhythm in D. antiqua can be explained by the two-oscillator model proposed by Pittendrigh and Bruce (1959).     

Photoperiodic induction in quail as a function of the period of the light-dark cycle: implications for models of time measurement.

The earliest detectable event in the photoperiodic response of quail is a rise in luteinizing hormone (LH) secretion beginning at about hour 20 on the first long day. The timing of this rise was measured in castrated quail after entrainment to short daylengths which cause significant phase angle differences in the circadian system: (1) LD 2:22 and LD 10:14, and (2) LD 3:21 (T = 24 hr) and LD 3:24 (T = 27 hr). The quail were then exposed to 24 hr of light (by delaying lights-off), and the time of the first LH rise was measured; it was similar in all schedules. Quail were also entrained to LD 3:21 or LD 3:24 and then given a single 6-hr nightbreak 6-12, 7-13, or 13-19 hr after dawn. The earlier pulse was marginally more inductive in the 27-hr cycle. Thus the entrainment characteristics of the photoinducible rhythm (phi i) in quail appear very different from those of the locomotor circadian rhythm, and raise doubts as to whether phi i is a primary circadian oscillator.     

The temperature dimension of the seed germination niche in fen wetlands

The seed germination niche partly determines adaptation, ecological breadth and geographic range in plant species. In temperate wetlands, environmental temperature is the chief regulator of germination timing, but the ecological significance of high and low temperatures during dormancy break and germination is still poorly understood. Our aim was to characterize the temperature dimension of the germination niche in mountain base-rich fens, determining (1) the effect of different temperatures on dormancy break and germination, and (2) whether different germination strategies may be identified at the species level. We conducted laboratory germination experiments with seeds of 15 species from these habitats, collected in 18 fen sites in the Cantabrian Mountains (Spain) for two consecutive years. In all the species, the seeds were totally or conditionally dormant at dispersal and stratification produced a significant increase of germination. In most cases, there was not an obligatory requirement for cold temperatures during dormancy break, since warm stratification promoted germination as well. Although the optimal germination thermoperiod was generally high (30/20 °C), most species could also germinate at lower temperatures after cold-stratification. We also identified a group of species associated to cold-water springs that germinated only at low temperatures. Our results demonstrate that dormancy break in mountain base-rich fens does not obligatorily depend on cold temperatures during overwintering. Furthermore, germination at cool temperatures may be more widespread in wetland habitats than previously thought. The existence of two distinctive germination strategies, ‘warm’ and ‘cool’, can potentially give rise to divergent species responses to climate change.