A CIRCADIAN RHYTHM IN THE LONG-DAY PHOTOPERIODIC INDUCTION OF ERECT AXIS DEVELOPMENT IN THE MARINE RED ALGA NEMALION HELMINTHOIDES1,2

Two isolates of Nemalion helminthoides from the west coast of Ireland showed a heteromorphic life history in which tetraspores formed under short-day conditions on uniaxial, prostrate tetrasporophytes gave rise to uniaxial, prostrate growths similar in morphology to the tetrasporophytes. The induction of multiaxial, erect axes on tetraspore-derived plants was a long-day photoperiodic response. At 11°C, 16:8 h LD, nitrate concentration in the enriched seawater medium had little effect on the numbers of erect exes formed. Induction of erect axes occurred only in daylengths ≥ 12 h, mainly between 7–13° C, and the critical daylength, which generally lies between 14 and 16 h, changed with temperature. Night-breaks (NB) of 1 h light in the middle of 16 h night were ineffective in the promotion of erect axis formation, and day-breaks (DB) of 1 h darkness in the middle of a 16 h day did not inhibit axis formation. Neither continuous light nor NB of 1–1.5 h given at various times during the dark period of an 8:16 h LD cycle promoted the formation of erect axes. At 9° C, equivalent photon exposures (0.69 and 0.34 mol·m^?2) at two different instantaneous photon flux densities resulted in erect axis formation only in the 14 h, 16 h and DB regimes. Photoregimes of 16 h light in combination with dark cycles of various lengths resulted in the maximal promotion of the formation of erect axes in diurnal (22–24 h) and bi-diurnal (44–48 h) cycles, a diminishing response in non-diurnal cycles greater than 24 and 48 h, and a minimal response at 32 h. These data show that the formation of erect axes is a long-day photoperiodic response and provide further evidence for a connection between endogenous circadian rhythms and long-day photoperiodic responses.     

Circadian Timekeeping for the Photoperiodic Control of Budset in Picea abies (Norway Spruce) Seedlings

Romanian populations of Norway spruce are induced to set terminal buds by four inductive cycles of 8 h light/16 h darkness. To distinguish between circadian and hourglass timekeeping for the photoperiodic control of budset, seedlings were raised in continuous light at 300 µmol m^-2s^-1 at 20°C for 10 weeks. They were then exposed to an extended night regime consisting of three cycles of 8 h light/40 h dark with 4-h or 1-h nightbreaks (120 µmol m^-2s^-1) applied to groups of plants at intervals during the extended night. Following a final cycle of 8 h light/16 h dark to maximize budset, the plants were transferred to continuous light. Budset was delayed when the night-break was applied close to the critical nightlength (CNL) of 6–7 h or about 22–23 h later in the extended night, consistent with circadian rather than hourglass timekeeping. Confidence intervals were calculated for the times to maximum effect of the night-breaks.     

Evidence that photoperiodic, dark time measurement in Pharbitis nil involves a circadian rather than a semidian rhythm

Experiments were carried out to determine whether a semidian (12 h) rhythm in flowering response operates in Pharbitis nil as the basis for photoperiodic time measurement. The effect of 5 min far-red light followed by 85 min dark (FRD) given 4, 8,14 and 22 h before the end of a 48 h photoperiod on night-break timing and critical night length was determined. When given 4 h before the end of a 48 h photoperiod, an interruption with FRD advanced the phase of the circadian rhythm in the night-break inhibition of flowering. In contrast, earlier interruptions of the photoperiod had no effect on the phase of the rhythm. The critical night length was modified by FRD given 4 h (shortened) or 8 h (lengthened) before the end of the photo-period; when given at other times FRD did not alter the critical night length. The results are discussed in relation to the basis for photoperiodic timekeeping, with particular reference to suggestions for the involvement of a semidian rhythm. A circadian model based on the concept of limit cycles is described.     

The photoperiodic control of wing development in the black bean aphid,Aphis fabae

Newly born presumptive gynoparae of Aphis fabae were transferred from their prenatal short-day rearing conditions (light-dark 12:12, 15°C) to a variety of postnatal photoperiodic regimes. Long days prevented wing formation and the majority of aphids developed into apterous or alate-apterous intermediate adults. Continued short days resulted, almost exclusively, in winged adults. The photoperiodic-response curve (T = 24) revealed a critical photoperiod of light-dark 13.5:10.5 and further investigations showed that morphogenesis depended more upon night- than day-length. Maximal apterization occurred with 8 or 9 h dark regardless of the length of the photophase but at photophases of 12 h or less the critical night length was reduced to 8.5 h. Night-interruption experiments revealed two peaks of photosensitivity when the scotophase was 12–14 h long but only a single peak was seen with longer dark periods. A series of experiments using early night interruptions followed by extended dark revealed apparent critical night lengths which decreased as the interruption was placed later in the scotophase. Resonance experiments involving 12 or 16 h photophases and extended scotophases at 15°C revealed long-day effects with scotophases shorter than critical and short-day effects with longer dark periods. However, similar regimes at 20°C produced three peaks of apterization 24 and 20 h apart indicating the possibility of a circadian element involved in the photoperiodic response. The results are compared with the photoperiodic responses of other insects.     

MORPHOLOGY AND REPRODUCTION OF THE RED ALGA ACROCHAETIUM PECTINATUM IN CULTURE12

The life history of the marine red alga Acrochaetium pectinatum (Kylin) Hamel was studied in unialgal culture using supplemented natural seawater media. The tetrasporophytes are larger than the gametophytes, have a compact filamentous basal system, and produce monosporangia and tetrasporangia. Mono-spores give rise to tetrasporophytes. Tetraspores develop into small gametophytes with unicellular bases. The gametophytes are heterothallic when small (usually less than 500 μ) but as some females become larger (2-3 mm) they produce spermatangia as well as carpogonia. Gametophytes may bear mono-sporangia in addition to carpogonia or spermatangia. These monospores give rise to gametophytes. Fertilization of the carpogonia has not been observed. The tetrasporophytes produce only monosporangia in day-lengths of 12-16 hr, but both tetrasporangia and monosporangia are formed in daylengths of 6–10 hr. Tetrasporangial production is reduced at 15 C compared with 10 C. Light intensity in the range of 5-200 ft-c (cool white fluorescent lighting) has no apparent influence on induction of tetrasporangia. Induction of tetrasporangia is not a photoperiodic response because their development is not inhibited by a brief light break in the middle of the dark period in short daylengths. Plastid morphology, origin, and frequency of sporangia and vegetative branching are variable during the ontogeny, and consequently are somewhat unreliable as taxonomic criteria. Differences in basal systems between gametophytes and tetrasporophytes also indicate that this feature, which is used to distinguish major subgeneric groupings in Acrochaetium, may not be as useful as previously thought.     

Circadian Timekeeping for the Photoperiodic Control of Budset in Picea abies (Norway Spruce) Seedlings

Romanian populations of Norway spruce are induced to set terminal buds by four inductive cycles of 8 h light/16 h darkness. To distinguish between circadian and hourglass timekeeping for the photoperiodic control of budset, seedlings were raised in continuous light at 300 µmol m^-2s^-1 at 20°C for 10 weeks. They were then exposed to an extended night regime consisting of three cycles of 8 h light/40 h dark with 4-h or 1-h nightbreaks (120 µmol m^-2s^-1) applied to groups of plants at intervals during the extended night. Following a final cycle of 8 h light/16 h dark to maximize budset, the plants were transferred to continuous light. Budset was delayed when the night-break was applied close to the critical nightlength (CNL) of 6-7 h or about 22-23 h later in the extended night, consistent with circadian rather than hourglass timekeeping. Confidence intervals were calculated for the times to maximum effect of the night-breaks.     

Differential effects of photoperiodic history on the responses of gonadotrophins and prolactin to intermediate daylengths in the male Syrian hamster

The effect of photoperiodic history on the neuroendocrine response to intermediate daylengths (11-13.5 hr of light) was investigated in the male Syrian hamster. The duration of the nocturnal peak of pineal melatonin content was inversely proportional to photoperiod and independent of photoperiodic history. Serum levels of prolactin were lower in animals exposed to shorter photoperiods. Photoperiodic history had little effect on the response of serum prolactin to intermediate daylengths. Serum luteinizing hormone (LH) concentrations were also lower in shorter photoperiods, but in addition were sensitive to the direction of photoperiodic change, so that a single photoperiod could be interpreted as either stimulatory or inhibitory to LH secretion. This effect of photoperiodic history was expressed at intermediate photoperiods with 12-13.5 hr of light. The sensitivity of serum follicle-stimulating hormone (FSH) levels to photoperiodic history was masked by an early onset of photorefractoriness. Testicular size and serum testosterone levels revealed weaker effects of photoperiodic history; these were attributed to the dissociation between gonadotrophin and prolactin secretion induced by intermediate daylengths. The contrasting effects of photoperiodic history on the secretion of LH and prolactin may represent the expression of multiple photoperiodic time-measuring systems.     

Photoperiodic long-day control of sporophyll and hair formation in the brown alga Undaria pinnatifida

Two photoperiodic responses, the development of sporophylls and hairs, havebeen quantified in sporophytes of the brown alga Undaria pinnatifida. In a finalexperiment, the algae were cultivated in outdoor, 2000-L seawater tanks in agreenhouse for up to 12 weeks, and daylength was regulated by automatic blindsmounted on top of the tanks. Vegetative young sporophytes were treated undershort-day (SD; 8 h light per day) or long-day conditions (LD; 16 h light perday), at 12 h light per day or in a night-break regime (NB; 8 h light per day,7.5 h dark, 1 h light, 7.5 h dark). The earliest sporophyll development wasobserved 6, 7 or 9 weeks under LD, NB or SD conditions, respectively. After 12 weeksthe sporophylls were significantly longer and wider under LD or NB conditions than inthe SD regime, and only half of the experimental algae had formed sporophyllsunder SD conditions, but all algae under LD or NB conditions. In a foregoing 7-weekculture experiment performed in 300-lL indoor tanks, enhanced sporophyll formationhad also been observed under LD and not under SD conditions (NB omitted). In bothexperiments, blade elongation rates remained high until the end of theexperiments in SD, but declined during sporophyll initiation in LD, NB or at 12 hlight per day. Another difference caused by photoperiod was observed in regard to thedevelopment of surface hair spots which occurred in both experiments on the bladesin LD, NB or at 12 h light per day with identical densities, but were completelylacking under SD conditions. It is concluded that U. pinnatifida is afacultatative long-day plant in regard to reproduction forming vigorously sporophyllsin long days, and an obligate long-day plant in regard to hair formation.     

Morphology,temperature and photoperiodic responses in Audouinella botryocarpa (Harvey) Woelkerling (Acrochaetiaceae,Rhodophyta) from Ireland

Abstract

Audouinella botryocarpa is reported for the first time from the British Isles. On the west and south coasts of Ireland and in the Isle of Man it grows in the intertidal on wave-splashed limpet shells and rock surfaces. Populations examined monthly in Galway Bay formed monosporangia throughout the year and tetrasporangia from December to March and May but no gametangial plants were found. Plants isolated into culture from monospores formed monosporangia at daylengths of 8, 10, 12, 14 and 16 h at 6.5, 8, 10, 11 and 15°C; the same plants formed tetrasporangia at daylengths ≤10 h at 8, 10 and 11°C but not at 6.5 and 15°C. Spores isolated from these plants gave rise to further monosporangial plants that also formed tetrasporangia under inductive conditions. Night-breaks of 1 h in a 16 h night prevented tetrasporangial reproduction and a critical daylength of ～10 h was found at 10°C. British Isles Audouinella botryocarpa typically has 6 rounded or irregularly-shaped chloroplasts per cell, each of which has a single, centrally-placed pyrenoid. The chloroplasts grow together in mature cells, typically giving the appearance of a single plate-like chloroplast with 6 pyrenoids. The implications of these observations for the generic classification of acrochaetioid algae is discussed and it is concluded that chloroplast morphology, number and the presence or absence of pyrenoids may provide the basis for a future generic realignment of acrochaetioid algae.     

Photoperiod and temperature as triggers in the seasonality ofDelesseria sanguinea

The red algaDelesseria sanguinea is strongly seasonal, producing gametangia in early tetrasporangia in mid- and new blades in late winter. A lamp was installed in the shallow subtidal off the Isle of Man, illuminating about 40 plants ofDelesseria for one hour in the night or day. Single blades from separate plants were held in laboratory tanks at different temperatures and in short days, long days and with a night-break. In the sea, the night-break prevented fertility in tetrasporophytes but some gametophytes became fertile. New blades were stimulated, arising 6 weeks early. Their lengths indicated a saturation level of about 10 μmol m⁻² s⁻¹ for one hour in 24. Growth rate calculations suggested a delay in stimulation until the ambient sea temperature dropped to 13°C. Tetrasporangia were formed after the night-break ceased in December but not January. In day-addition of light there was slight, if any, stimulation of blade production. In the laboratory, gametogenesis occurred readily in short days but not in long days or with a night-break. There was little or no effect of temperature between 8 and 14°C. Tetraspores were rarely formed in the laboratory. The timing of gametogenesis suggested a critical daylength of about 14 h. New blades were clearly stimulated by lower temperatures in the laboratory, few forming at 14°C and many at 7–10°C. They appeared mainly in long days or with a night-break but formed in short days after gametangia production. It is concluded that both gamete and tetraspore production are under photoperiodic control but require different conditions, possibly gametogenesis needing fewer cycles. There is some evidence for antagonism between new blade and reproductive structure initiation. The critical daylength could involve a timing differential of a month over the species geographical range. On the other hand it is suggested that its southern limit could be determined by the winter isotherm of 13°C, warmer than which might not allow blade initiation.     

Photoperiodic control of larval development in the semivoltine cockroachPeriplaneta japonica (Blattidae: Dictyoptera)

Periplaneta japonica is semivoltine, entering early diapause in any (except the first) larval instar before the last, and late diapause in the last instar. Early diapause was induced under a short day of 13 h or less at 28°C, and under both short and long daylength (12–16 h) at 20°C. The shorter the daylength and the lower the temperature, the younger the instar was entering early diapause. Early diapause was terminated by a long day (16 h) or a high temperature (28°C), after which larvae grew faster in short days than in long days until the last instar, when they again entered diapause, always in short days and frequently in long days. This late diapause was terminated also by an increase in daylength and was always followed by adult emergence. In this case, 13 and 14 h daylengths after exposure to 12 h daylength were as effective as 16 h daylength. Ourdoor samples collected in late autumn, winter and early spring at Hirosaki (40.5°N) comprised two distinct size groups, corresponding with the early and late diapause instars. Based on these results, the seasonal development strategy and intriguing aspects of the photoperiodic response in this cockroach are discussed.     

Responses to stepwise photoperiodic changes for the larval diapause of the Indian meal moth Plodia interpunctella

Abstract The Indian meal moth Plodia interpunctella Hübner (Lepidoptera: Pyralidae) diapauses as a last‐instar (fifth) larva. At 30 °C, no larvae enter diapause under any photoperiodic conditions; at 25 °C, the photoperiodic response curve is a long‐day type with a critical length of approximately 13 h light; at 20 °C, diapause is induced moderately even under long days (> 13 h). Cumulative effects of short days or long days on diapause induction are determined by alternate, stepwise and gradually changing regimes of photoperiod at 25 °C. When the larvae are repeatedly exposed to LD 16 : 8 h and LD 12 : 12 h photoperiods every other day, the incidence of diapause is 37%. When the larvae are placed under an LD 16 : 8 h photoperiod for 2 days and then under an LD 12 : 12 h photoperiod for 1 day, it is 38 %. Exposure to an LD 16 : 8 h photoperiod for 1 day and then to an LD 12 : 12 h photoperiod for 2 days induces only 15% diapause. This may indicate that the photoperiodic information is not accumulated in a simple fashion despite the generally accepted hypothesis (i.e. photoperiodic counter). Larvae exposed to an LD 16 : 8 h photoperiod for 5 days after oviposition express a very high incidence of diapause even under short days between an LD 2 : 22 h and LD 12 : 12 h photoperiod. After 10 days exposure to an LD 16 : 8 h photoperiod, however, the short day does not induce diapause strongly. On the other hand, an LD 12 : 12 h photoperiod in the early larval life is highly effective in the induction of diapause. A gradual increase or decrease of photoperiod (2 min day^?1) shows that the direction of photoperiodic change does not affect the diapause determination.     

Maternal and direct effects of natural-like changes of photoperiod and food condition manipulation on life history parameters in <Emphasis Type="Italic">Daphnia</Emphasis>

This study analyzed changes of life history parameters of a Daphnia pulicaria clone in response to increasing (spring conditions) versus decreasing (fall conditions) photoperiod and low and high food concentrations in maternal and offspring environments. One treatment increased the photoperiod from 12 to 16 h at a rate of 3 min per day, while the other treatment decreased the photoperiod from 16 to 12 h at the same rate. We also used two food concentrations: low (0.2 mg C/l) and high (0.5 mg C/l). Maturation time, first clutch size, survivorship, and somatic growth in offspring depended on the direction of photoperiodic changes for Daphnia grown under high food concentration. Natural-like changes of photoperiod on life history parameters and abrupt changes resulted in similar effects on life history parameters of Daphnia (see Alekseev and Lampert 2004 Hydrobilogia 526:225–230).     

Independence of circadian entrainment state and responses to melatonin in male Siberian hamsters

Background

Seasonal fluctuations in physiology and behavior depend on the duration of nocturnal melatonin secretion programmed by the circadian system. A melatonin signal of a given duration, however, can elicit different responses depending on whether an animal was previously exposed to longer or shorter photoperiod signals (i.e., its photoperiodic history). This report examined in male Siberian hamsters which of two aspects of photoperiod history – prior melatonin exposure or entrainment state of the circadian system – is critical for generating contingent responses to a common photoperiodic signal.

Results

In Experiment #1, daily melatonin infusions of 5 or 10 h duration stimulated or inhibited gonadal growth, respectively, but had no effect on entrainment of the locomotor activity rhythm to long or short daylengths, thereby demonstrating that melatonin history and entrainment status could be experimentally dissociated. These manipulations were repeated in Experiment #2, and animals were subsequently exposed to a 12 week regimen of naturalistic melatonin signals shown in previous experiments to reveal photoperiodic history effects. Gonadal responses differed as a function of prior melatonin exposure but were unaffected by the circadian entrainment state. Experiment #3 demonstrated that a new photoperiodic history could be imparted during four weeks of exposure to long photoperiods. This effect, moreover, was blocked in animals treated concurrently with constant release melatonin capsules that obscured the endogenous melatonin signal: Following removal of the implants, the gonadal response depended not on the immediately antecedent circadian entrainment state, but on the more remote photoperiodic conditions prior to the melatonin implant.

Conclusions

The interpretation of photoperiodic signals as a function of prior conditions depends specifically on the history of melatonin exposure. The photoperiodic regulation of circadian entrainment state contributes minimally to the interpretation of melatonin signals.

     

Photoperiodic counter of diapause induction in Pseudopidorus fasciata (Lepidoptera: Zygaenidae)

Induction of larval diapause is a photoperiodically controlled event in the life history of the moth Pseudopidorus fasciata. In the present study, the photoperiodic counter of diapause induction has been systematically investigated. The required day number (RDN) for a 50% response was determined by transferring from a short night (LD 16:8) to a long night (LD 12:12) or vice versa at different times after hatching, The RND differed significantly between short- and long-night cycles at different temperatures. The RDN for long-night cycles at 20, 22, 25 and 28 degrees C was 11.5, 9.5, 7.5 and 8.5 days, respectively. The RDN for short-night cycles was 3 days at 22 degrees C and 5 days at 20 degrees C indicating that the effect of one short night was equivalent to the effect of 2-3 long nights effect. Night-interruption experiments of 24h photoperiods by a 1 h light pulse showed that the most crucial event for the photoperiodic time measurement in this moth was whether the length of pre-interruption (D(1)) or the post-interruption (D(2)) scotophases exceeded the critical night length (10.5 h). If D(1) or D(2) exceeded 10.5 h diapause was induced. The diapause-averting effect of a single short-night cycle (LD 16:8) against a background of long nights (LD 12:12) showed that the photoperiodic sensitivity was greatest during the first 7 days of the larval period and the highest sensitivity was on the fourth day. Both non-24 and 24 h light-dark cycle experiments revealed that the photoperiodic counter in P. fasciata is able to accumulate both long and short nights during the photosensitive period, but in different ways. The information from short-night cycles seems to be accumulated one by one in contrast to long-night cycles where six successive cycles were necessary for about 50% diapause induction and eight cycles for about 90% diapause. These results suggest the accumulation of long-night and short-night cycles may be based on different mechanisms.     

Photoperiodic and temperature responses in the growth and tetrasporogenesis ofGigartina acicularis (Rhodophyta) from Ireland

Gigartina acicularis is an intertidal, perennial red alga which reaches its northern distributional limit in the north-eastern Atlantic on the Irish coast. It has only rarely been found with reproductive structures in the British Isles. Plants isolated vegetatively from field-collected plants near the northern distributional limit in Ireland formed tetrasporangia, the tetraspores of which gave rise to plants which formed gametangia and carposporophytes at 16°C, . Sporelings grown from the carpospores of these plants formed tetrasporangia at all daylengths tested (16-8 h) at 13–20°C; but there was a quantitative photoperiodic response in the numbers of plants forming tetrasporangia, and in the numbers of sori formed, at 13–16°C. Only one in 20 plants became fertile at 16°C, and , but 16 in 20 plants reproduced at 16°C, . At 20°C, and , all plants formed tetrasporangia, and formation was most rapid the long-day regime. No tetrasporangia were formed at 9–10°C, regardless of daylength. Apical elongation of these plants also appeared to show a quantitative photoperiodic response at 16°C, 1 h light breaks in a 16 h night giving more or less a long-day response. This effect was apparent about 4 weeks before the formation of the first tetrasporangia. There would therefore appear to be quantitative photoperiodic control of tetrasporogenesis in the Irish strain ofG. acicularis. Temperature and photoperiodic control of reproduction in this species only partially accounts for the observed rarity of reproductive structures in populations of this species near the northern limit of its distribution.Paper presented at the Seaweed Biogeography Workshop of the International Working Group on Seaweed Biogeography, held from 3–7 April 1984 at the Department of Marine Biology, Rijksuniversiteit Groningen (The Netherlands). Convenor: C. van den Hoek.     

Contrasting types of photoperiodic response in the control of dormancy

Abstract The effect of night-break or day-extension treatments with red (R) and/or far-red (FR) light were examined in the control of dormancy and extension growth in Weigela florida and Picea abies. Dormancy in Weigela (as assessed by the continued production of new leaves) was completely prevented by a 30 min night-break treatment with R: the effect of R was prevented by a subsequent exposure to 30 min FR. A day-extension treatment for 8.25 h with R or with R+FR also completely prevented dormancy, irrespective of whether it was given before or after a short day (SD) in sunlight. There was no significant difference between any dayextension treatment, nor between day-extension treatments and a night-break with R. Dormancy in Picea (as assessed by the maintenance of shoot growth) was also delayed by a night-break with R but less effectively than the most effective day-extension. A day-extension treatment for 9 h with R light was more effective when it preceded than when it followed a SD in sunlight. The addition of FR in the second but not in the first 9 h of a 17 h photoperiod increased its effectiveness. It is concluded that the mechanisms of the control of dormancy must differ in the two species. A day-extension for 16 h with light from tungstenfilament lamps increased stem elongation in Weigela when compared with an R night-break. This resulted from an increase in internode length and there was no significant difference between the two treatments in their effect on leaf production.     

The Involvement of Cyclic ADPR in Photoperiodic Flower Induction of Pharbitis nil

Cyclic adenosine diphosphate ribose (cADPR) is a potent endogenous calcium-mobilizing agent synthesized from NAD⁺ by ADP-ribosyl cyclases described for several animal cells. Pharmacological studies suggest that cADPR is an endogenous modulator of Ca²⁺-induced Ca²⁺ release channels. There is also information about the sub-micromolar concentration of cADPR in plant cells. Whether cADPR can act as a Ca²⁺-mobilizing intracellular messenger in plant tissue is an unresolved question. Despite the obvious importance of monitoring cADPR cellular levels under various physiological conditions in plants, its measurement has been technically difficult and requires specialized reagents. In the present study a widely applicable sensitivity assay for cADPR is described. We show that Pharbitis nil tissue from cotyledons contains a certain cADPR level. To explain the possible roles of this second messenger in photoperiodic flower induction, some physiological experiments were also performed. The exogenous applications of cADPR to Pharbitis nil plants, which were exposed to a 12-h-long subinductive night, significantly increased flowering response. Nevertheless 8-Br-cADPR inhibited flowering when these compounds were applied during a 16-h-long inductive night. The effect of ruthenium red, a calcium channel blocker and ryanodine, a calcium channel stimulator, on the photoperiodic induction of flowering was also studied. Ruthenium red, when applied before and during an inductive 16-h dark period, slightly inhibited flowering, whereas ryanodine, when applied before and during a 12-h long subinductive night, stimulated flower bud formation. We also confirmed evidence that Ca²⁺ ions are involved in the photoperiodic induction of flowering. Thus, the obtained results may suggest the involvement of cyclic ADPR-activated Ca²⁺ mobilization in the photoperiodic flower induction process in Pharbitis nil.     

CIRCANNUAL GROWTH RHYTHM AND PHOTOPERIODIC SORUS INDUCTION IN THE KELP LAMINARIA SETCHELLII (PHAEOPHYTA)1

Growth and reproduction of laboratory-grown sporophytes of Laminaria setchellii Silva were investigated in a tank system with controlled conditions of daylength, temperature, and nutrients (N and P). A circannual growth rhythm of the frond was detected under constant laboratory conditions. In continuous long-day and night-break conditions the period τ of the free-running rhythm varied between 11.3 and 17.3 months; in short-day conditions the frond grew indefinitely. The growth rhythm of individual plants could be synchronized by a simulated annual cycle of day-length with a period of T = 12 months. The four seasons of the year were simultaneously simulated by phase shifting the annual cycle of daylength by 3, 6, or 9 months in three out of four tanks. The annual growth cycle followed these phase shifts, and initiation of the new blade always started just after the winter daylength minimum. The formation of sori was induced by a genuine photoperiodic short-day reaction in 1- to 2-year-old plants. Sori became, visible 9–14 weeks after transfer of individual plants from long-day to short-day conditions, whereas plants cultured in continuous long-day or night-break conditions remained sterile. Sporophytes with or without blades were able to continue growth or produce new blades in continuous darkness.     

Photoperiodic induction of diapause in the spider mite Tetranychus urticae: qualitative or quantitative time measurement?

Abstract. Insects and mites may measure photoperiods eitfier by classifying them as long or short relative to a critical value (qualitative time measurement) or by using the absolute value (quantitative time measurement). The spider mite Tetranychus urticae is thought to use a qualitative mechanism of time measurement. In this paper we present the results of experiments with an inbred line of the spider mite (to keep genetic variation in photoperiodic responses small), to test whether quantitative aspects also play a role. Differences in diapause incidence in different long-night photoperiods at different temperatures may be an indication of quantitative responses to photoperiod. The effect of temperature on the photoperiodic response curve was studied at 16^oC, 19^oC and 22^oC. The response curves appeared to be similar at 16^oC and 19^oC, with a critical nightlength between 10 and 11 h. At 22^oC, diapause induction was less than 100% in all long-night regimens and die critical nightlength had shifted to 12 h. Maximum diapause induction (93%) occurred in a light-dark cycle with a 16 h dark phase (LD 8:16 h). Diapause induction was lowest in long-night photoperiods with dark phases of 20 h and longer. The number of light-dark cycles needed for 50% diapause induction at 19^oC varied. between 12.1 and 14.7 for LD 6:18 h, between 10.9 and 12.5 for LD 8:16 h, between 10.6 and 11.6 for LD 10:14 h, and between 10.1 and 10.7 for LD 12:12 h. Independent of die light-dark regimen, diapause induction took place in some individuals after receiving 8 cycles and virtually all individuals entered diapause after 16 cycles. No effect was found of the photoperiodic treatment during prediapause development (LD 6:18 h, LD 8:16 h, LD 10:14 h, LD 12:12 h) on diapause duration. The average diapause duration at LD 10:14 h and 19^oC was 61 days over all four treatments. We explained the results by hypothesising that nightlengths are assessed qualitatively and mat the photoperiodic clock operates more accurately near the critical nightlength.