Effects of light quality on the circadian rhythm of leaf movement of a short-day-plant

Studies were made of the effects of blue, green, red and far-red (FR) light on the circadian rhythm of leaf movement of Coleus blumei × C. frederici, a short day plant. Under continuous illumination with blue light, there was a significant lengthening of the period of the rhythm to about 24.0 hr, as compared to 22.5 hr in continuous darkness. Under continuous red light, the period length was significantly shortened to 20.5 hr. Under continuous green or FR, the period length was not significantly different from the dark control. It was observed that under continuous FR illumination, the leaves tended to oscillate in a more downward position. Eight-hr red light signals were effective in advancing the phase of the rhythm as compared to a control under continuous green light. Blue light signals were effective in delaying the phase of the rhythm. FR light signals were ineffective in producing either delay or advance phase shifts. Far-red light did not reverse the effects of either red or blue light signals. On the basis of these results it is suggested, that pigments which absorb blue or red light, rather than phytochrome, mediate the effect of light on the circadian rhythm of leaf movement.     

Night length measurements by the circadian clock controlling diapause induction in the mosquito Aedes atropalpus

Night interruption experiments were used to investigate the behavior of the clock controlling diapause induction in the mosquito, Aedes atropalpus. The data from these experiments indicated that the clock included a circadian oscillator which was phase set at dusk. Following this event the oscillator proceeded to drive a nightly rhythm of sensitivity to light. This rhythm included a photoinducible phase where light interruption inhibited diapause. The photoinducible phase was fixed, occurring 7 to 9 hr after dusk in all photoperiod regimens tested. The photoinducible phase was followed by a refractory phase, which continued until dawn. During the refractory period light did not inhibit diapause. These observations indicated that the circadian clock behaved like an interval timer which was set at dusk. The rhythm of sensitivity to light, an inherited time scale, limited the induction of diapause to seasonal periods when nights were longer than 9 hr. As a result, diapause was induced only when the daylength dropped below the critical photoperiod of L15:D9 (hours of light:hours of dark).A ‘T’ experimental design was used to confirm the importance of the length of the night in clock controlled induction of diapause in this mosquito.     

Aspects of the induction of diapause in a laboratory strain of the mite Tetranychus urticae

Some diapause characteristics were studied in a strain of the spider mite. Tetranychus urticae. which had been reared on bean plants in the laboratory for over 15 yr. The diapause induction response curve was of the long-day type, showing a sharply defined critical daylength of 13 hr 50 min. In constant darkness no diapause induction occurred, but with a photoperiod of 1L:23D diapause incidence was already complete. A thermoperiod with a 5°C amplitude induced diapause in combination with a short-day photoperiod only when the low phase of the thermoperiod coincided with the scotophase. The same thermoperiod did not induce any diapause in constant darkness. The photoperiodic reaction of the laboratory strain used in these experiments appeared to remain constant over a very long period of time and to be independent of the diapause history of previous generations of mites.Although photoperiodic sensitivity was demonstrated during the whole postembryonic development, sensitivity was maximal at the end of the protonymphal instar and declined rapidly during the deutonymphal instar. Only 2 inductive cycles of 10L:14D were required to induce up to 62% diapause if the mites were kept in continuous darkness during the remainder of their development. Long days or continuous light could reverse the inductive effect of a sequence of short-day cycles previously applied to the mites.Light breaks of 1 hr duration applied at different times during the dark period of a 10L:14D photoperiod generated a sharp bimodal response curve with two discrete points of sensitivity to the light breaks at 10 hr after ‘dusk’ and 10 hr before ‘dawn’, thus showing a remarkable similarity with the results obtained in light break experiments with some species of insects.     

Regulation by Photoperiod of Seasonal Changes in Body Mass and Reproductive Function in Gray Mouse Lemurs (Microcebus murinus): Differential Responses by Sex

Microcebus murinus exhibits highly seasonal biological rhythms to cope with extreme seasonality in availability of resources. To study the role of daylength on seasonal changes in body mass and reproductive function, we exposed male and female gray mouse lemurs to natural, constant, or alternating light cycles for 2 years under constant environmental conditions. When exposed to either constant short (SD: 10 h light/day), long (LD: 14 h light/day), or intermediate (ID: 12 h light/day) daylength, males and females maintained a constant body mass with no spontaneous cyclic variation. We only observed typical seasonal body mass changes in subjects exposed to alternating periods of SD and LD, the weight gain being triggered by SD, whereas weight loss occurred under LD. Reproductive activity in females proceeded from an endogenous rhythm that was expressed under constant daylengths. In contrast, changes in reproductive activity in males depended on daylength variation. In both sexes, SD and LD have direct inhibitory or stimulatory effects on reproductive activity. In females, daylength regulates breeding season by synchronizing an endogenous sexual rhythm with the season, whereas in males, the perception of a critical photoperiod is used to determine the subsequent onset or arrest of their breeding season. These sexual differences in the effect of daylength could be related to sex-specific differences in reproductive constraints.     

Influence of Photoperiod, Daylength, and Feeding Schedule on Tadpole Growth and Development

The metamorphic rate of Rana pipiens tadpoles was studied under different photoperiods, daylengths, and feeding schedules. Tail resorption and hindlimb growth and development induced by immersion in 30 μg/l thyroxine (T₄) were accelerated under longer photoperiods and continuous light when 6L: 18D, 12L: 12D, 18L: 6D, and 24L regimes were compared. Constant light exposure did not produce faster development than an 18 hr photoperiod, and initially was less effective. The rate of spontaneous and T₄-induced metamorphosis was greater the shorter the day on 9L:9D, 12L: 12D, or 15L: 15D cycles, although all groups received the same overall amount of light, but in different dosages. When feeding schedule but not the LD cycle was varied, groups of tadpoles fed on 18, 24, or 30 hr regimes showed no differences in growth and development rate on 19L: 5D, and only random variations under continuous light. Differences in metamorphic rate on 18, 24, or 30 hr days are not due to the feeding schedules, but to the LD cycles. From these experiments we conclude that illumination, particularly the length and frequency of the photoperiod, affects the utilization of T₄. Development rates independent of the total amount of illumination, but related to daylength and light schedule, suggest interaction of light with an endogenous timing mechanism.     

The influence of artificial photoperiod on the growth,appetite and reproductive status of male red deer and sheep

Young male red deer and Suffolk X (Finn X Dorset) sheep were kept on an artificial photoperiod such that two cycles of daylength occurred during one calendar year. They were penned separately, fed to appetite, weighed weekly and measured tri-weekly.Both species showed two cycles of intake, growth and gonadal activity in response to the daylength when only one would have been shown on natural photoperiod, although in the sheep these cycles were of lower amplitude than in the deer. The deer grew two sets of antlers during the study. A lag of some 3–4 months occurred between an event such as peak food intake and the time it would have been expected to occur relative to the daylength cycle. It is considered that although daylength controls these cycles, there is an endogenous rhythm which photoperiod cannot completely suppress.     

Calling behaviour in M.convecta females under different temperature and photoperiodic conditions

Abstract. . The effects of temperature and photoperiod on calling behaviour in females of the Australian common armyworm, Mythimna corwecta (Walker) (Lepidoptera: Noctuidae), were studied in the laboratory. Age at first calling was greater at 15 and 20^oC compared with 25^oC, but there were no significant differences between 25 and 30^oC. Females kept at 10^oC did not call, but if transferred at 10 days to 20^oC they called after a period similar to the pre-calling period at constant 20^oC. Onset time of calling was earlier at 15 and 20^oC compared with 25 and 30^oC, but there were no significant differences in calling duration. At 20^oC, age at first calling was increased by shorter day length (12h and 14h, compared with 16h), and there was an interaction between daylength and moth age affecting the duration of calling. Onset times advanced with longer daylength, but peak calling was later in relation to the lengtii of the scotophase. These results are discussed in connection with migration in M.convecta. Evidence for a circadian rhythm of calling was found in females entrained for 3 and 8 days after emergence under reverse-cycle conditions and then transferred to constant darkness. However, after 56 h and 80 h respectively of darkness, calling became de-synchronized. Subsequently, it appeared to re-synchronize to a different clock, which approximately correlated with the normal photoperiods the moths had experienced during larval development.     

Sex pheromone-releasing behaviour in females of the dermestid beetle,Trogoderma glabrum

Adult Trogoderma glabrum females assume stationary postures characterized by abdominal elevation and partial exposure of the ovipositor segments. The postural activity was studied in relation to sex pheromone production and was designated calling, or sex pheromone-releasing behaviour because it was accompanied by an increase in the rate of release of sex pheromone(s). Calling was largely restricted to an 8 hr interval centered toward the middle of a 16 hr photophase. Mating frequency and female pheromone content increased during the same interval. Because females extracted during the calling period yielded at least 10 times more pheromone than extracts prepared at other times, and because pheromone activity released during one calling period was estimated to be at least 10-fold higher than that which could be extracted from whole females several hours prior to the start of calling, heightened production of active pheromone(s) was postulated to occur during calling. Functional significance of the calling posture is discussed in relation to sex pheromone gland location.     

Photoperiodic Control of Flowering in Dark-Grown Seedlings of Pharbitis nil Choisy : The Effect of Skeleton and Continuous Light Photoperiods

The control of night-break timing was studied in dark-grown seedlings of Pharbitis nil (Choisy cv. Violet) following a single continuous or skeleton photoperiod. There was a rhythmic response to a red (R) interruption of an inductive dark period, and the phasing of the rhythm was influenced by the preceding light treatment.

Following a continuous white light photoperiod of 6 hours or less, the points of maximum inhibition of flowering were constant in real time. Following a continuous photoperiod of more than 6 hours, maximum inhibition occurred at 9 and 32.5 hours after the end of the light period. The amplitude of the rhythm during the second circadian cycle was much reduced following prolonged photoperiods.

Following a skeleton photoperiod, the time of maximum sensitivity to a R interruption was always related to the second pulse of the skeleton, R₂, with the first point of maximum inhibition of flowering occurring after 12 to 18 hours and the second after 39 hours. Without a second R pulse, the time of maximum sensitivity to a R interruption was related to the initial R₁ pulse. A `light-off' or dusk signal was not mimicked by a R pulse ending a skeleton photoperiod; such a pulse only generated a `light-on' signal and initiated a new rhythm.

It is concluded that the timing of sensitivity to a R interruption of an inductive dark period in Pharbitis nil is controlled by a single circadian rhythm initiated by a light-on signal. After 6 hours in continuous white light, the phase of this rhythm is determined by the transition to darkness. Following an extended photoperiod, the timing characteristics were those of an hourglass; this seemed to be due to an effect on the coupling or expression of a single circadian timer during the second and subsequent cycles, rather than to the operation of a different timing mechanism.

In addition to the effects on timing, the photoperiod affected the magnitude of the flowering response.

     

Effects of temperature and light on calling in the tiger moth Holomelina lamae (Freeman) (Lepidoptera: Arctiidae)

ABSTRACT. In Holomelina lamae Freeman daily eclosion of adults is gated, with males emerging before females. By advancing the onset of photophase and by delaying the onset of scotophase, it was demonstrated that lights-on acts as the main phase-setting cue for calling. Few females call on the day they eclose. Calling is initiated c. 9 h after the onset of photophase in 2-day-old females, and shifts to earlier times in older females. The duration of calling also increases with age. That calling is controlled by an endogenous circadian clock is indicated by its persistence in continuous light (LL) and dark (DD). In LL calling is dampened rapidly, but a single scotophase re-entrains the rhythm. Decreases in temperature advance the onset of calling and the mean hour of calling, while increases in temperature delay both. However, the magnitudes of such phase-shifts depended upon hour of the photoperiod. Moreover, cooling and heating appears to exert both transient and long-term effects on the calling rhythm. An 8 h period at a reduced temperature in LL induces calling in females whose calling is dampened, and entrains the calling rhythm. Females maintained in DD from second instar larvae to the adult stage exhibit a circadian calling rhythm set by eclosion.     

Entrainment of the melatonin rhythms in early postnatal lambs and their mothers

Although the developing sheep can produce an appropriately timed melatonin rhythm as early as 1 week after birth, it is not known whether the lamb is able to adjust its melatonin rhythm to a change in daylength. The ability of the young lamb to entrain its pattern of melatonin secretion to a new photoperiod was determined in the present study. Eight female lambs and their mothers were raised in long days (LD 16:8) beginning 2 weeks postpartum. At 7 weeks of age, the time of lights-off was advanced 8 hr, the short-day photoperiod then being LD 8:16; the time of lights-on remained unchanged. Concentrations of melatonin were measured in blood samples collected hourly on days - 1, 0, 2, 4, 6, and 13 relative to the light change. On day 0, all mothers and daughters had advanced the onset of melatonin secretion by at least 1 hr, and by day 13, 12 of 16 had completely entrained to the new photoperiod. The rate of entrainment among individuals varied; the mean rate for lambs and mothers did not differ. This study provides evidence that the melatonin-rhythm-generating system matures shortly after birth.     

Circadian Rhythms of Enzymes' Activity in Mice Tissues During Exposure to Continuous Illumination

Activity rhythms of enzymes were determined in various tissues of C57BL/6J male mice. The determinations were carried out on mice which were kept in 14 hr light: 10 hr dark regimen, and on day 2, day 5 and day 21 during exposure to continuous illumination. Locomotor activity rhythms were followed in light: dark and up to the seventh day in constant light. All the activities exhibited a significant circadian rhythm in the light: dark regimen. During the exposure to continuous illumination, the locomotor activity exhibit a free running circadian rhythm with a consistent 24 hr and 40 min, major period component. At the same time recording the rhythms of enzyme activity; enzymes exhibited various formats of response which differed from those of the locomotor activity. The results suggest that rhythms of enzyme activity, as well as the desynchronization of the rhythms, are not enzyme specific.     

Latitudinal clines in the properties of a circadian pacemaker

The circadian rhythm of eclosion activity and its pacemaker were analyzed in a series of latitudinal races of Drosophila auraria ranging from 34.2 degrees to 42.9 degrees N in Japan. The phase of the rhythm (psi EL) to the daily photoperiod (PP) changes as daylength is increased, and the slope of psi EL (PP) changes with latitude. Is is sufficiently greater in the north to cause a phase reversal of northern and southern races on long versus short photoperiods. This reversal is found in assays of the pacemaker's phase (psi PL) as well as that of the rhythm (psi EL). Assay of the pacemaker shows that its period (tau) is longer in northern than in southern races, and that the amplitude of its phase response curve (PRC) is lower in the north. The period of the rhythm in all latitudinal races is longer than 24 hr in short photoperiods (LD 1:23), but is probably less than 24 hr (as an aftereffect of photoperiod) in longer days such as LD 14:10. The observed north-south differences in the phase relation of both pacemaker and rhythm to the light cycle are explained by the latitudinal clines in pacemaker properties and a postulated aftereffect of photoperiod on tau. It is suggested that the latitudinal cline in PRC amplitude has functional significance in conserving the amplitude of the pacemaker's signal to the rest of the system it times. Computer simulation shows that without such a reduction in the perceived light intensity, pacemaker amplitude will be lowered by the increase in duration of the daily light at higher latitudes.     

Reproductive activity in Finnish Landrace x Rambouillet ewes exposed to various daylength cycles applied over periods of six months

Twenty-six Finnish Landrace x Rambouillet ewes were exposed to a series of artificial daylength cycles over periods of 6 mo. The cycles in general consisted of 3 mo. of long or increasing daylength, followed by 3 mo. of short or decreasing daylength. Increasing daylength to 22 hr.; 17.5 hr. or 15.75 hr. but not 14 hr. resulted in the onset of anestrus in all ewes. Only four of eight ewes in the 14 hr. cycle became anestrus. Abrupt decreases in daylength resulted in a more rapid return to reproductive cyclicity than a gradual decline. In addition, there appeared to be an inverse relationship between the length of the longest day of a treatment cycle and the time taken for ewes to return to reproductive cyclicity, following a subsequent abrupt decrease in daylength. Reducing daylength form 22 hr. to 17.5 hr. did not induce reproductive cyclicity. Reducing daylength from 17.5 hr. to 15 hr.; 12.75 hr. or 7.5 hr. did result in the onset of reproductive cyclicity. Finally, extending the period of long day exposure in a daylength cycle decreased the response time to a subsequent reduction in daylength. It was concluded that reproductive seasonality may be initiated by the interaction of darkness with a daily photosensitive phase in the ewe.     

The Circadian Rhythm of Leaf Movement of Coleus blumei x C. frederici, a Short Day Plant. II. The Effects of Light and Temperature Signals

The phase response curve for the circadian rhythm of leaf movement of Coleus blumei x C. frederici, a short day plant, is generally similar to those reported for other organisms. An increase in the duration of the light signal caused an increase in the extreme values of the phase response curve and shortened the time for transition from maximum delays to maximum advances. Experiments with 2 light signals showed that the overt rhythm of leaf movement represents the rhythm of the light sensitive oscillator even during the transient period that followed the first light signal. A temperature decrease of 7° for 8 hr caused only a transient phase shift in the following 2 cycles but not in the steady state. The combination of such a temperature decrease and a light signal showed that only the overt rhythm of leaf movement was disturbed by the temperature decrease whereas the light sensitive oscillator was free running. A temperature decrease of 11° for 10 hr caused a steady state phase shift and affected the light sensitive oscillator as well.     

An aphid circadian rhythm: Factors affecting the release of sex pheromone by oviparae of the greenbug, Schizaphis graminum

Adult oviparae of Schizaphis graminum emit a sex pheromone from scent plaques on the metathoracic tibiae, as do oviparae of other aphid species. By allowing males to walk along a wire walkway, a turning response and an increased rate of antennation were observed when the aphids were within 1–2 cm of a pheromone source. Adult males responded but 4th instar larval males did not.The onset of the release of the pheromone by the oviparae is triggered by the initiation of the light phase in a LD-cycle, and is governed by a circadian rhythm with a free-running period of 25.6 hr under continuous illumination of 15 lx. Daily pheromone release peaks 4–7 hr after lights-on and reaches a maximum on days 6–8 of adulthood.     

Rhythms as photoperiodic timers in the control of flowring in Chenopodium rubrum L.

Summary In C. rubrum, the amount of flowering that is induced by a single dark period interrupting continuous light depends upon the duration of darkness. A rhythmic oscillation in sensitivity to the time that light terminates darkness regulates the level of flowering. The period length of this oscillation is close to 30 hours, peaks of the rhythm occurring at about 13, 43 and 73 h of darkness.Phasing of the rhythm by 6-, 12- and 18-h photoperiods was studied by exposing plants to a given photoperiod at different phases of the free-running oscillation in darkness. The shift in phase of the rhythm was then determined by varying the length of the dark period following the photoperiod; this dark period was terminated by continuous light.With a 6-h photoperiod the timing of both the light-on and light-off signals is shown to control rhythm phasing. However, when the photoperiod is increased to 12 or 18 h, only the light-off signal determines phasing of the rhythm. In prolonged periods of irradiation-12 to 62 h light—a durational response to light overrides any interaction between the timing of the light period and the position of the oscillation at which light is administered. Such prolonged periods of irradiation apparently suspend or otherwise interact with the rhythm so that, in a following dark period, it is reinitiated at a fixed phase relative to the time of the light-off signal to give a peak of the rhythm 13 h after the dusk signal.In daily photoperiodic cycles rhythm phasing by a 6-h photocycle was also estimated by progressively increasing the number of cycles given prior to a single dark period of varied duration.In confirmation of Bünning's (1936) hypothesis, calculated and observed phasing of the rhythm controlling flowering in c. rubrum accounts for the photoperiodic response of this species. Evidence is also discussed which indicates that the timing of disappearance of phytochrome P_fr may limit flowering over the early hours of darkness.     

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.     

Endogenous rhythmic activity of photosynthesis, transpiration, dark respiration, and carbon dioxide compensation point of peanut leaves

At 14-hour day length, 25 C leaf temperature, 9 mm Hg vapor-pressure deficit, and 1.17 joules cm⁻² min⁻¹ irradiance, the diurnal change in daily photosynthesis of the cultivated peanut (Arachis hypogaea L.) is a result of an endogenously controlled circadian rhythm in net photosynthesis which peaks near noon and troughs near midnight. By resetting the day-night light regime, the rhythm rephased in continuous light. The free-running rhythm approximates 26 hours. Both transpiration and dark respiration show similar rhythmicity, with transpiration closely in phase with the rhythm in photosynthesis. The rhythm in carbon dioxide compensation point is approximately 12 hours out of phase, peaking at midnight and troughing at midday. Endogenous changes in stomatal aperture seemed to be the major control of the rhythm in photosynthesis. The activity of ribulose-1,5-diphosphate carboxylase increased during the normal photoperiod, leveling off after 12 hours; however, the activity was not correlated with the rhythmic change in photosynthesis.     

Circadian rhythms in the mating behavior of the cockroach, Leucophaea maderae

Mating behavior of small populations of virgin males and females of the cockroach Leucophaea maderae were continuously monitored via time-lapse video recording in controlled laboratory conditions. The time of onset of copulation was found to be rhythmic in a light cycle of 12 h light alternated with 12 h of darkness, with the peak of mating behavior occurring near the light to dark transition. This rhythm persisted in constant dim red illumination and constant temperature. In constant conditions, the period of the rhythm was slightly less than 24 h, with a peak of copulation during the late subjective day. These data demonstrated that mating behavior is gated by a circadian clock. When males and females were taken from light cycles that were 12 h out of phase, a bimodal rhythm was observed with one peak in the males' late subjective day and a second peak of equal amplitude in the late subjective day of females. The results indicated that circadian systems in both males and females contribute to the circadian rhythm in copulation. Bilateral section of the optic tracts (OTX) of both males and females abolished the rhythm, but the rhythm persisted when OTX females were paired with intact males or when OTX males were paired with intact females. Furthermore, when OTX males or OTX females were paired with intact animals that were 12 h out of phase, a bimodal rhythm was still observed. These results suggested that the circadian pacemaker in the optic lobes of both male and female cockroaches participates in the control of mating, but that a pacemaker outside the optic lobes is also likely involved. Finally, it was shown that the female's olfactory response (measured by electroantennogram) to components of the male sex pheromone exhibited a circadian rhythm, but the data suggested the peripheral olfactory rhythm is not likely to be involved in the rhythm of mating behavior.