The non-photic induction of spermatogenic development in European starlings

Detailed studies of the photosexual biology of male European starlings (Sturnus vulgaris) document a non-obligatory involvement of photoperiod in the induction of testicular metamorphosis. Although post-winter solstice increases in daily photophase duration are responsible for the ecologically correct chronology of the annual reproductive cycle, starlings maintained in the absence of daily photostimulation under go testicular metamorphosis with complete spermatogenic development. Present experiments reveal that the rate of testicular growth in starlings held in constant darkness (DD) is affected by previous photoperiodic experience. Birds held under a natural northtemperate zone photoperiod and transferred to DD on 13 September require significantly fewer days to achieve spermatogenic testes than birds pretreated under 12-and 14-h photoperiods or in constant light (LL). Complete spermatogenesis in the 14-h group is achieved only after a greater duration of DD exposure than in all other birds. Variations in the extent of the 12-h pretreatment period do not alter the testis growth rate in starlings subsequently transferred to DD. It is suggested that photoperiodic conditions applied prior to the initiation of DD treatment may affect the characteristics of circadian oscillations that occur in the absence of a photoperiodic zeitgeber, and thus change the reproductive response rate through alterations of hormonal secretions from the hypothalamo-hypophyseal axis.Presented at the Eighth International Congress of Biometeorology, 9–14 September 1979, Shefayim, Israel.     

Stable daily light regimens as inductive factors of endogenous testicular cycles in the European starling,Sturnus vulgaris

European starlings (Sturnus vulgaris ) maintained under chronic 12L:12D exhibit testicular cycles with a periodicity of 9–10 months. These circannual testicular cycles incorporate all of the physiologically distinct phases observed during gonadal cycles in starlings under temperate-zone photoperiods. Starlings maintained under chronic 6L-18D also undergo testicular cycles but these cycles: (a) have a relatively short periodicity (about 6 months); (b) include periods of testicular involution, though not to the minimal quiescent level for this species; and (c) do not include the physiologically distinct photorefractory phase separating testicular cycles in starlings under chronic 12L:12D and under temperate-zone photoperiods. While it is possible that testicular cycles in starlings under certain daily light regimens of fixed duration are a function of an endogenous circannual reproductive rhythm, we believe that the testicular cycles generated under both 12L:12D and 6L:18D are the product of gonadotropin secretion rates controlled by circadian (not circannual) oscillations periodically entrained by these chronic photoperiods.     

Seasonality of Mobbing Intensity in the Pinyon Jay

In the wild, pinyon jays (Gymnorhinus cyanocephalus) mob owls at all times of year, but our observations of wild birds suggested that the mobbing response was strongest during the breeding season. To test this, we placed four groups of five wild caught adult pinyon jays in environmental chambers during late autumn when their testes were regressed. These birds received photoperiods of 12, 12, 13 and 14 h for 30 days prior to being exposed singly to a live great horned owl (Bubo virginianus). Three groups of five birds were maintained under natural photoperiods in outdoor aviaries and were tested with the owl in Aug., Sep. and Nov. Birds on long artificial photoperiods mobbed more strongly than birds exposed to shorter photoperiods. The jays under natural photoperiods showed the opposite response: those tested in November mobbed significantly more strongly than birds tested in August. Jays with the weakest mobbing responses were molting, and those with the strongest responses seemed to be closest to reproductive readiness. There was no correlation between gonad size and mobbing intensity, but our results suggest a strong relationship between reproductive readiness and mobbing intensity in pinyon jays.     

Melatonin Does Not Modulate Testicular Responsiveness to Altered Photoperiods. A Study During Different Phases of the Annual Gonadal Cycle in Roseringed Parakeets (Psittacula krameri)

The roseringed parakeet has been shown to exhibit a variable testicular responsiveness to both altered photoperiodic regimens and to treatment with melatonin during different phases of the annual gonadal cycle. Adult male roseringed parakeets were held under either natural photoperiods (NP), or long photoperiods (LP; 16L 8D), or short photoperiods (SP; 8L 16D) for a total period of 90 days. From day 46 onward, half of the total birds in each group were administered with the vehicle of melatonin, and the other birds were injected daily in the afternoon with melatonin (25 µg/ 100 g body wt.) till the end of the experiment. An identical experimental schedule was followed during the four different (preparatory, progressive, pre-breeding, and breeding) phases of the annual testicular cycle. The testicular activities in various bird groups were evaluated by volumetric, gravimetric, histometric and karyometric measurements, and by quantitative histological studies. The findings revealed that exogenous melatonin may exert either a suppressive influence or none at all on the testicular functions in relation to the photoperiodic schedule as well as to the reproductive phase of the concerned bird, but in no case modulates gonadal responsiveness to artificially altered photoperiods.     

Melatonin Does Not Modulate Testicular Responsiveness to Altered Photoperiods. A Study During Different Phases of the Annual Gonadal Cycle in Roseringed Parakeets (Psittacula krameri)

The roseringed parakeet has been shown to exhibit a variable testicular responsiveness to both altered photoperiodic regimens and to treatment with melatonin during different phases of the annual gonadal cycle. Adult male roseringed parakeets were held under either natural photoperiods (NP), or long photoperiods (LP; 16L 8D), or short photoperiods (SP; 8L 16D) for a total period of 90 days. From day 46 onward, half of the total birds in each group were administered with the vehicle of melatonin, and the other birds were injected daily in the afternoon with melatonin (25 µg/ 100 g body wt.) till the end of the experiment. An identical experimental schedule was followed during the four different (preparatory, progressive, pre-breeding, and breeding) phases of the annual testicular cycle. The testicular activities in various bird groups were evaluated by volumetric, gravimetric, histometric and karyometric measurements, and by quantitative histological studies. The findings revealed that exogenous melatonin may exert either a suppressive influence or none at all on the testicular functions in relation to the photoperiodic schedule as well as to the reproductive phase of the concerned bird, but in no case modulates gonadal responsiveness to artificially altered photoperiods.     

Hypothalamic expression of thyroid hormone-activating and -inactivating enzyme genes in relation to photorefractoriness in birds and mammals

Photorefractoriness is the insensitivity of gonadal development to the stimulatory effects of long photoperiods in birds and to the inhibitory effects of short photoperiods in small mammals. Its molecular mechanism remains unknown. Recently, it has been shown that reciprocal expression of thyroid hormone-activating enzyme [type 2 deiodinase (Dio2)] and -inactivating enzyme [type 3 deiodinase (Dio3)] genes in the mediobasal hypothalamus is critical for photoperiodically induced gonadal growth. Since thyroid hormones are required not only for photoinduction, but also for the induction of photorefractoriness, we examined the expression of these genes in relation to photorefractoriness in birds and mammals. Transfer of birds to long photoperiods induced strong expression of Dio2. This was maintained in tree sparrow when they later became photorefractory, but decreased somewhat in quail. In hamsters, transfer to long photoperiods also induced strong expression of Dio2. High values were not maintained under long photoperiods, and, indeed, expression decreased at the same rate as in animals transferred to short photoperiods. There was no renewed expression of Dio2 associated with testicular growth as animals became refractory to short photoperiods. Expression of Dio3 was high under short photoperiods and low under long photoperiods in all the animals examined, except for the short photoperiod-refractory hamsters. Our present study revealed complex regulation of deiodinase genes in the photoinduction and photorefractory processes in birds and mammals. These gene changes may be involved in the regulation of photorefractoriness, as well as photoinduction.     

Circannual rhythms in European starlings: Why do they stop under long photoperiods?

Summary In European starlings (Sturnus vulgaris) exposed in winter to photoperiods of 12 1/2 h or more, testes go through a cycle of growth and regression but then stay inactive for many months. Under a photoperiod of 12 h, in contrast, testes usually go through repeated circannual cycles. We have tested the hypothesis that the failure of starlings held under long photoperiods to initiate a second testicular cycle is a consequence of the fact that photorefractoriness is not broken under photoperiods longer than 12 h. The results of 2 experiments are consistent with this hypothesis: whereas the testes of starlings held continuously in a 13-h photoperiod or in continuous light, remained inactive after an initial testicular cycle, testicular growth was re-initiated after birds were exposed for 4 to 8 weeks to a short 8-h photoperiod.     

BREEDING CONDITION OF MALE RED BISHOPS UNDER ARTIFICIAL PHOTOPERIODS

Craig, A. J. F. K. 1985. Breeding condition of male Red Bishops under artificial photoperiods. Ostrich 56: 74–78.

Testis development, as indicated by bill colour changes, and moult were followed over two years in nine birds kept under constant conditions of 14 h light:10 h dark. All birds maintained at least partial nuptial dumage throughout this period, but most continued normal cycles of wing moult. The testes continued to show cycles of enlargement and regression.     

Biological clocks help reduce the physiological conflicts in avian migrants

We present evidence from experiments on overwintering populations of two Palearctic-Indian latitudinal migratory birds, the black-headed bunting (Emberiza melanocephala) and the red-headed bunting (E. bruniceps), that the bird’s clock in interaction with day length regulates seasonal rhythms of migration and reproduction such that physiological conflict between them is reduced. Initiation and termination of the body mass and testicular cycles are separately regulated photoperiodic events. For example, under stimulatory photoperiods the response curve of body mass does not overlap with that of the testicular growth. A response-specific photoperiodism is adaptive, since gain in body mass, critical to spring migration, precedes gonadal recrudescence. Finally, migration as indicated by the night-time migratory restlessness under experimental situations (e.g., intense locomotion under caged condition-called Zugunruhe) appears to be regulated by a separate circadian oscillator.     

The seasonal development of photoperiodic responsiveness in an equatorial migrant, the garden warblerSylvia borin

Summary Garden warblers (Sylvia borin) are migrating European passerines that spend the winter in tropical Africa. To investigate how photoperiod controls the annual cycles of reproduction, molt and migration, garden warblers were exposed to photoperiodic changes simulating those experienced by conspecifics with an equatorial wintering area. At three different times groups of birds were moved from a constant equatorial photoperiod of 12.8 h to a 15-h photoperiod: group 1 on November 19, group 2 on February 24, and group 3 on April 12. In all birds the changes in body weight, nocturnal migratory restlessness, testicular width or diameter of the largest oocyte, and plasma LH concentrations were determined. In addition, the birds were examined for molt. In all birds of groups 2 and 3 the transfer to long photoperiods induced a gonadal cycle associated with spring migratory restlessness and, in the group 3 birds, an increase in LH. In group 1 only few birds showed marginal gonadal responses and the gonadal cycles of the group 2 birds were less pronounced than those of group 3. The results suggest that the hypothalamo-pituitary-gonadal axis was refractory to stimulation by long photoperiods in autumn but, later in the season, became responsive to the same long photoperiods. Unlike the situation in other temperate zone birds, the transition from the photorefractory to the photosensitive state seems to be a gradual process which develops spontaneously and with the appropriate temporal characteristics in the absence of photoperiodic change.Abbreviation LH luteinizing hormone     

Photoperiodic regulation of annual testicular events in the Indian major carp Catla catla

The importance of photoperiods in the regulation of annual testicular events in the carp Catla catla was evaluated by subjecting them to either long (16 h light : 8 h dark) or short (8 h light : 16 h dark) photoperiods for 30 days during the preparatory, prespawning, spawning and postspawning phases of an annual gonadal cycle. In each reproductive phase, testicular responsiveness to subjected photoperiods was determined by comparing the gonadal status in corresponding groups of control or natural photoperiodic fish. The values of testicular weight, gametogenic index, as well as testicular activity of two steroidogenic enzymes (Δ⁵3β‐, and 17β‐hydroxysteroid dehydrogenase), and the serum titre of testosterone were considered as the indices of functional status of the testis in the fish concerned. During the prespawning phase, exposure of fish to a daily long photoperiod schedule resulted in precocious maturation of testis, while retardation of testicular growth was noted under the influences of short photoperiod. However, none of the employed photo‐schedules could influence the gametogenic and steroidogenic functions of the testis in the remaining part of the gonadal cycle. Collectively, the present study provides evidence for the first time that in the case of a commercially important carp, Catla catla, artificial, long photoperiods may be used for advanced testicular maturation, while reductions in maturation‐associated growth and deterioration in flesh quality may be avoided by submitting the fish to shorter day lengths during the prespawning phase of the reproductive cycle.     

Environmental regulation of the reproductive system in a flexibly breeding Sonoran Desert bird, the Rufous-winged Sparrow, Aimophila carpalis

We investigated reproductive regulation in male Rufous-winged Sparrows, Aimophila carpalis, a Sonoran Desert passerine that breeds after irregular summer rains. Field and captive data demonstrate that increased photoperiod stimulates testicular development in March and maintains it until early September. Free-living birds caught in July and placed on captive long days (16L: 8D) maintained developed testes for up to 7 months, and free-living birds caught in September, during testicular regression, redeveloped testes when placed on captive long days, indicating that these birds were still photosensitive. Captive birds on long days maintained testicular development when exposed to temperatures mimicking those occurring during regression in free-living birds. In free-living birds, testicular development was observed during spring and summer, but unless this was associated with rainfall, breeding (indicated by juveniles) did not occur. Large increases in plasma luteinizing hormone (LH) in free-living males were correlated with heavy rainfall in July/August, when the birds bred, and in November, when they did not breed. In captive birds, plasma LH concentrations were unresponsive to photoperiodic changes, but may have responded to social cues. Plasma prolactin concentrations were directly correlated with photoperiod in free-living birds, but an effect of photoperiod on prolactin secretion was not seen in captive birds. It is concluded that male Rufous-winged Sparrows use long photoperiods to stimulate and maintain testicular development, but exposure to long photoperiods does not terminate breeding by inducing absolute photorefractoriness. The specific timing of reproductive behaviors is apparently determined by elevated plasma LH coinciding with long day stimulated gonad development.     

Starling circannual systems: are they arrested in long photoperiods?

Summary In the European starling,Sturnus vulgaris, circannual rhythms in gonadal size, molt and other related functions persist only in photoperiods close to 12 h, but are absent in longer or shorter daylengths. To find out whether the arrhythmia seen in long photoperiods results from an arrest of the underlying clock system, three groups of male starlings were held for 10, 14, or 20 months in a 13 h photoperiod and then transferred to a 12 h photoperiod. A control group was held in the 13 h photoperiod throughout the experiment for 28 months. During the initial exposure to the 13 h photoperiod, all birds went through a gonadal cycle, followed by a complete molt. Subsequently, the control birds retained small testes to the end of the experiment and there was no further molt. In contrast, most of the experimental birds re-initiated a testicular cycle, following transfer to the 12 h photoperiod and molted after its completion. The latency between the transfer to the 12 h photoperiod and the onset of testicular growth was not significantly different among the three groups, indicating that the underlying circannual clock had been arrested in the 13 h photoperiod and restarted in the 12 h photoperiod. The pattern of the second testicular cycle did, however, differ among groups. Particularly its amplitude decreased from group 1 to group 3, suggesting that the capacity of the birds to respond to a 12 h photoperiod decreased with increasing duration of exposure to the 13 h photoperiod.Dedicated to Prof. Dr. C.S. Pittendrigh on the occasion of his seventieth birthday.     

Photoperiodic regulation of feeding and reproduction in a brooding sea star from central California

Summary

Individuals of the sea star genus Leptasterias from the Monterey Peninsula, California, were maintained for 34 months under two contrasting regimes of seasonally changing photoperiod. In one regime the lights turned on and off in phase with local sunrise and sunset; in the other the daily photoperiod was kept 6 months out of phase with ambient photoperiods. Annual cycles of feeding, gametogenesis, and embryo brooding of the out-of-phase animals were all shifted out of phase with respect to those of the in-phase animals. These small, relatively hardy sea stars appear well suited for more detailed analyses of photoperiodism in asteroids.     

Culture of the marine phytoplankter,Dunaliella tertiolecta,with light-dark cycles

Summary Dunaliella tertiolecta was grown in continuous culture, maintained by a single daily dilution to a constant cell concentration, with photoperiods of duration 3 to 18 hours. Illumination was provided with filtered tungsten light having a maximum intensity at 580–590 m. Average intensity at the culture surface was maintained at 0.05 cal/cm²xmin. Temperature was regulated at 20° C. Daily cell production and the fraction of cells dividing daily are reported for each of the photoperiods. Cyclic diurnal variations were noted in cell pigment content, cell volume and photosynthetic rate. Corresponding variations in cell carbon/chlorophyll a ratios were small and may be unimportant for the estimation of biomass from chlorophyll a in field work. An equation for calculating daily growth rate in continuous light, from light absorption by cell chlorophyll a, derived previously, was modified to include the influence of providing light in discrete photoperiods.Supported by U. S. Atomic Energy Commission, Contract AT(11-1)-34 Project 108.     

Thyroxine can mimic photoperiodically induced gonadal growth in Japanese quail

Summary Pharmacological doses of thyroxine are able to mimic the effects of long photoperiods in Japanese quail. In birds maintained on short daylengths thrice-weekly injections of 100 g thyroxine cause full testicular maturation at rates not greatly different from those seen if quail are exposed to long days. Thyroxine stimulates increases in the secretion of FSH and LH, in pituitary prolactin content and in the hypothalamic content of Gn-RH. The effects are dose-dependent. If female quail kept on short daylengths are given thyroxine their ovaries develop and they lay eggs. In castrated male quail on short days thyroxine causes a ten-fold increase in circulating LH within a week. Thyroxine injections are also capable of maintaining quail in a photorefractory state even when they are transferred to short daylengths. The results suggest that thyroxine mimics long days by acting high in the photoneuroendocrine system and does not simply act to facilitate hormone secretion per se. This is in line with growing evidence in mammals and birds that parts of the photoperiodic machinery are sensitive to thyroid hormones.Abbreviations Gn-RH gonadotropin releasing hormone - FSH follicle stimulating hormone - LH luteinizing hormone - T ₄ thyroxine     

Support for a rare pattern of temperature-dependent sex determination in archaic reptiles: evidence from two species of tuatara (Sphenodon)

Background

In many birds, day length (=photoperiod) regulates reproductive cycle. The photoperiodic environment varies between different seasons and latitudes. As a consequence, species at different latitudes may have evolved separate photoperiodic strategies or modified them as per their adaptive need. We studied this using house sparrow as a model since it is found worldwide and is widely investigated. In particular, we examined whether photoperiodism in house sparrows (Passer domesticus) at 27°N, 81°E shared features with those exhibited by its conspecifics at high latitudes.

Results

Initial experiment described in the wild and captive conditions the gonad development and molt (only in captives) cycles over a 12-month period. Both male and female sparrows had similar seasonal cycles, linked with annual variations in day length; this suggested that seasonal reproduction in house sparrows was under the photoperiodic control. However, a slower testis and attenuated follicular growth among captives indicated that other (supplementary) factors are also involved in controlling the reproductive cycle. Next experiment examined if sparrows underwent seasonal variations in their response to stimulatory effects of long day lengths. When birds were transferred every month over a period of 1 year to 16 hours light:8 hours darkness (16L:8D) for 17–26 weeks, there was indeed a time-of-year effect on the growth-regression cycle of gonads. The final experiment investigated response of house sparrows to a variety of light-dark (LD) cycles. In the first set, sparrows were exposed for 31 weeks to photoperiods that were close to what they receive in between the period from sunrise to sunset at this latitude: 9L:15D (close to shortest day length in December), 12L:12D (equinox, in March and September) 15L:9D (close to longest day length in June). They underwent testicular growth and regression and molt in 12L and 15L photoperiods, but not in 9L photoperiod. In the second set, sparrows were exposed for 17 weeks to photoperiods with light periods extending to different duration of the daily photosensitivity rhythm (e.g. 2L:22D, 6L:18D, 10L:14D, 14L:10D, 18L:6D and 22L:2D). Interestingly, a slow and small testicular response occurred under 2L and 10L photoperiods; 6L:18D was non-inductive. On the other hand, 14L, 18L and 22L photoperiods produced testicular growth and subsequent regression response as is typical of a long day photostimulation.

Conclusion

Subtropical house sparrows exhibit photoperiodic responses similar to that is reported for its population living at high latitudes. This may suggest the conservation of the photoperiodic control mechanisms in birds evolved over a long period of time, as a physiological strategy in a temporally changing environment ensuring reproduction at the best suited time of the year.     

Hypothalamic neurosecretion and photoinduced testicular growth in the tree sparrow,Spizella arborea

Summary To assess the role of the hypothalamic neurosecretory system in the mechanism of photoperiodic control of testicular growth in Spizella arborea, we have examined the effect of (1) photostimulation on the density of paraldehyde-fuchsin-stainable (PAF+) material in the neurovascular zone of the median eminence, and (2) interruption of the hypothalamo-hypophysial (neurosecretory) tract on the photoperiodic testicular response. Birds were captured during midwinter and retained on 8-hr daily photoperiods several months prior to the experiment. Interruption of the neurosecretory tract was effected by a small knife fashioned from stainless steel wire; radiography was employed to facilitate stereotaxic placement of the knife assembly.Extending the daily photoperiod from 8 to 20 hrs induced rapid testicular growth in photosensitive Spizella arborea, but had no consistent effect on the density of PAF+ material in the palisade layer of the anterior median eminence. Whereas some terminal controls showed moderate depletion after 21 days' exposure to 20-hr daily photoperiods, others retained as much or more stainable material than initial controls in which testicular growth was not induced. Moreover, testicular growth was induced at essentially identical rates whether the PAF+ innervation of the anterior median eminence was intact or eliminated to varying degrees. The finding that the rate of photoperiodically induced testicular growth is independent of the density or pattern of accumulation of PAF+material in the neurovascular zone of the median eminence, coupled with the inconstant effect of photostimulation on the density of stainable material therein, strongly suggests that the eminential component of the hypothalamic neurosecretory system is not a part of the mechanism that controls photoperiodic testicular growth in S. arborea.Contribution No. 980, Division of Biology, Kansas Agricultural Experiment Station, Manhattan 66502. This investigation was supported in part by the Undergraduate Research Participation Program (GY-771) of the National Science Foundation. We are grateful to French's Pet Bird Laboratory, Rochester, N.Y., for the parakeet foods, to Richard S. Donham, Michael D. Ruff, Thomas G. Shane, and Philip G. Watt for assisting with field operations, to Mr. Gary R. Dierking for preparing the drawing, and to Mr. Don Pihlaja for assisting with the photomicrography.     

Melatonin Binding in the House Sparrow Song Control System: Sexual Dimorphism and the Effect of Photoperiod

Avian song is a sexually dimorphic behavior which is regulated seasonally. This regulation involves the construction and growth of song control structures: the high vocal center (HVC),nucleus robustus archistrialis(RA),nucleus magnocellularis anterior(MAN), and Area X. Song behavior and its neural correlates are controlled by steroid-dependent and independent processes. The avian circadian system is known to be involved in both daily processes and seasonal reproduction. A major part of this system is the circadian secretion of melatonin by the pineal gland. To determine possible interactions of the circadian and song control systems, the distribution and density of 2-[¹²⁵I]iodomelatonin (IMEL) binding, an indicator of melatonin sensitivity, were determined in male and female house sparrow brains. Specific binding was found in visual system centers of both genders, but binding in HVC, RA, and Area X was present only in males. Binding in MAN was present in both sexes. Although the effects of short and long photoperiods on male house sparrow IMEL binding in song structures revealed no systematic changes, there were significant differences in binding under different photoperiods in HVC and RA. IMEL binding in the tectofugalnucleus rotundus,however, was consistently highest under short day conditions. IMEL binding in song control nuclei was independent of testicular influence, since castration did not affect it significantly. The data point to a role for the circadian system of house sparrows in song control, but a specific role for melatonin in the daily or seasonal regulation of the song control system in birds, could not be determined.     

Response to experimental photoperiods by a migratory bunting, Emberiza melanocephala

Little is known about the effects of photoperiod on avian migrants that visit southeast Asia. In this paper, we report experiments performed on an emberizid finch, the Black-headed Bunting Emberiza melanocephala , to investigate its photoperiodic responses under artificial photoperiods, and continuous light and darkness.
Two series of experiments were performed with the photosensitive male birds. In the first series, different groups were exposed to seven different artificial photoperiods: 3L/21D, 6L/18D, 8L./16D, 11L/13D, 12L/12D, 15L/9D and 20L/4D, for 30 days. They were weighed and laparotomized at the beginning and end of the experiments. The birds responded to 12L/12D, 15L/9D and 20L/4D, but not to 3L/21D, 6L/18D, 8L/16D and 11L/13D. In the second series, photosensitive birds were placed under continuous light (LL) and dark (DD) conditions for 130 and 90 days. Periodic observations indicated that testicular growth and fattening followed by involution and fat-depletion had resulted in birds under LL, indicating the onset of photorefractoriness, while DD had no effect either on gonads or fattening in the buntings.
Our results demonstrate that light stimulation is a prerequisite to reproductive and metabolic activities (pre-migratory and migratory changes, fattening and weight gain) in the Black-headed Bunting, which has a photoperiodic threshold to these events at between 11 and 12 h daily photoperiods.