Photoperiod modifies daily maps of light and dark sensitivity for N-acetyltransferase activity in pineal glands of 3-week oldGallus domesticus

Summary N-acetyltransferase (NAT) activity in pineal glands exhibits a circadian rhythm with peak activity occurring in the dark-time. We previously showed that inGallus domesticus chicks pretreated with LD12:12, NAT activity was increased by dark exposure (peak dark sensitivity occurred during the expected dark-time) or decreased by light at night (peak light sensitivity occurred early in the night during the time of dark sensitivity). In this study we mapped dark sensitivity vs time (for NAT activity increase in response to 2 h dark pulses), and light sensitivity vs time (for NAT activity decrease in response to 10 min or 30 min light pulses) over a cycle for 3-week old chicks,Gallus domesticus, pretreated with long (LD16:8) or short photoperiod (LD8:16). Sensitivity to light was increased in the second 8 h after L/D by LD8:16. Sensitivity to dark was increased in the first 8 h after L/D by LD16:8.Abbreviations LD16:8 a light-dark cycle consisting of 16 h of light alternating with 8 h of dark - LD8:16 a light-dark cycle consisting of 8 h of light alternating with 16 h of dark - DD constant dark - LL constant light - L/D lights-off - D/L lights-on - NAT pineal serotonin N-acetyltransferase - NAT activity is given in nmoles/pineal gland/h - chick used here to denote a young bird of either sex of the speciesGallus domesticus from hatching to three weeks of age     

Extent of mismatch between the period of circadian clocks and light/dark cycles determines time‐to‐emergence in fruit flies

Circadian clocks time developmental stages of fruit flies Drosophila melanogaster, while light/dark (LD) cycles delimit emergence of adults, conceding only during the “allowed gate.” Previous studies have revealed that time‐to‐emergence can be altered by mutations in the core clock gene period (per), or by altering the length of LD cycles. Since this evidence came from studies on genetically manipulated flies, or on flies maintained under LD cycles with limited range of periods, inferences that can be drawn are limited. Moreover, the extent of shortening or lengthening of time‐to‐emergence remains yet unknown. In order to pursue this further, we assayed time‐to‐emergence of D. melanogaster under 12 different LD cycles as well as in constant light (LL) and constant dark conditions (DD). Time‐to‐emergence in flies occurred earlier under LL than in LD cycles and DD. Among the LD cycles, time‐to‐emergence occurred earlier under T4–T8, followed by T36–T48, and then T12–T32, suggesting that egg‐to‐emergence duration in flies becomes shorter when the length of LD cycles deviates from 24 h, bearing a strong positive and a marginally negative correlation with day length, for values shorter and longer than 24 h, respectively. These results suggest that the extent of mismatch between the period of circadian clocks and environmental cycles determines the time‐to‐emergence in Drosophila.     

Biosorption of di(2-ethylhexyl)phthalate by seaweed biomass

Samples of various Sargassum species were collected in the Hong Kong marine environment and used for studies on biosorption of di(2-ethylhexyl)phthalate (DEHP). Batch adsorption experiments were carried out to determine the removal capacity and removal efficiency of the biosorbents. The DEHP removal ability was similar among beached seaweed and three freshly collected Sargassum species. Different physico-chemical factors were evaluated in order to enhance the performance of the biosorbents. Under optimized conditions (25 mg biomass, initial pH 4, 25 °C, 40 mg L^–1 DEHP), the mean removal capacity of beached seaweed and Sargassum siliquastrum was 5.68 and 6.54 mg g^–1, respectively. Examination of the Langmuir and Freundlich adsorption isotherms showed that the biosorption phenomenon by these biosorbents could well be described by these models. Desorption of DEHP was also assessed with methanol, which showed the most satisfactory desorbing ability. Further study in multiple adsorption–desorption of DEHP by the biosorbents demonstrated the reusability of both beached seaweed and S. siliquastrum for biosorption of DEHP.     

Isolation of Two Plasticizers,Bis(2-ethylhexyl) Terephthalate and Bis(2-ethylhexyl) Phthalate,from Capparis spinosa L. Leaves

Many plants have been known to be contaminated and accumulate plasticizers from the environment, including water sources, soil, and atmosphere. Plasticizers are used to confer elasticity and flexibility to various fiber and plastic products. Consumption of plasticizers can lead to many adverse effects on human health, including reproductive and developmental toxicity, endocrine disruption, and cancer. Herein, we report for the first time that two plasticizers, bis(2-ethylhexyl) terephthalate (DEHT) and bis(2-ethylhexyl) phthalate (DEHP), have been isolated from the leaves of Capparis spinosa L. (the caper bush), a plant that is widely used in food seasonings and traditional medicine. 297 mg/kg of DEHT and 48 mg/kg of DEHP were isolated from dried and grounded C. spinosa L. leaves using column chromatography and semi-preparative high-performance liquid chromatography. Our study adds to the increase in the detection of plasticizers in our food and medicinal plants and to the alarming concern about their potential adverse effects on human health.     

Dim light at night increases body mass of female mice

During the past century, the prevalence of light at night has increased in parallel with obesity rates. Dim light at night (dLAN) increases body mass in male mice. However, the effects of light at night on female body mass remain unspecified. Thus, female mice were exposed to a standard light/dark (LD; 16?h light at ～150?lux/8?h dark at ～0?lux) cycle or to light/dim light at night (dLAN; 16?h light at ～150?lux/8?h dim light at ～5?lux) cycles for six weeks. Females exposed to dLAN increased the rate of change in body mass compared to LD mice despite reduced total food intake during weeks five and six, suggesting that dLAN disrupted circadian rhythms resulting in deranged metabolism.     

Calcium-induced formation of chlorophyll b and light-harvesting chlorophyll complex in cucumber cotyledons in the dark

Dark-grown cucumber seedlings were exposed to intermittent light (2 min light and 98 min dark) and then cotyledons were incubated with 50 mM CaCl₂ in the dark. Chlorophyll (Chl) a was selectively accumulated under intermittent light and Chl b was accumulated during the subsequent dark incubation with CaCl₂. The change in chlorophyll-protein complexes during Chl b accumulation induced by CaCl₂ in the dark was investigated by SDS-polyacrylamide gel electrophoresis. Chlorophyll-protein complex I and free chlorophyll were major chlorophyll-containing bands of the cotyledons intermittently illuminated 10 times. When these cotyledons were incubated with CaCl₂ in the dark, the light-harvesting Chl complex was formed. When the number of intermittent illumination periods was extended to 55, small amounts of Chl b and light-harvesting Chl complex were recognized at the end of intermittent light treatment, and these two pigments were further increased during the subsequent incubation of the cotyledons with CaCl₂ in the dark compared to water controls.     

Locomotor activity rhythm in four wrasse species under varying light conditions

Under controlled laboratory conditions, the locomotor activity rhythms of four species of wrasses (Suezichthys gracilis, Thalassoma cupido, Labroides dimidiatus andCirrhilabrus temminckii) were individually examined using an actograph with infra-red photo-electric switches in a dark room at temperatures of 21.3–24.3°C, for 7 to 14 days. The locomotor activity ofS. gracilis occurred mostly during the light period under a light-dark cycle regimen (LD 12:12; 06:00-18:00 light, 18:00-06:00 dark). The locomotor activity commenced at the beginning of the light period and continued until a little before the beginning of dark period. The diel activity rhythm of this species synchronizes with LD. Under constant illumination (LL) this species shows distinct free-running activity rhythms varying in length from 23 hrs. 39 min. to 23 hrs. 47 min. Therefore,S. gracilis appears to have a circadian rhythm under LL. However, in constant darkness (DD), the activity of this species was greatly suppressed. All the fish showed no activity rhythms in DD conditions. After DD, the fish showed the diel activity rhythm with the resumption of LD, but this activity began shortly after the beginning of light period. The fish required several days to synchronize with the activity in the light period. Therefore,S. gracilis appeared to continue the circadian rhythm under DD. InT. cupido, the locomotor activity commenced somewhat earlier than the beginning of the light period and continued until the beginning of the dark period under LD. The diel activity rhythm of this species synchronizes with LD. Under LL, four of the five specimens of this species tested showed free-running activity rhythms for the first 5 days or longer varying in length from 22 hrs. 54 min. to 23 hrs. 39 min. Although the activity of this species was suppressed under DD, two of five fish showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 38 min. to 23 hrs. 50 min. under DD. Therefore, it was ascertained thatT. cupido has a circadian rhythm. InL. dimidiatus, the locomotor activity rhythm under LD resembled that observed inT. cupido. The diel activity rhythm of this species synchronizes with LD. Under LL, four of seven of this species showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 07 min. to 25 hrs. 48 min. Although the activity of this species was suppressed under DD, three of five fish showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 36 min. to 23 hrs. 41 min. under DD. Therefore, it was ascertained thatL. dimidiatus has a circadian rhythm. Almost all locomotor activity of C.temminckii occurred during the light period under LD. The diel activity rhythm of this species coincides with LD. Under LL, two of four of this species showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 32 min. to 23 hrs. 45 min. Although the activity of this species was suppressed under DD, one of the four fish showed free-running activity rhythms throughout the experimental period. The length of the free-running period was 23 hrs. 21 min. under DD. Therefore,C. temminckii appeared to have a circadian rhythm. According to field observations,S. gracilis burrows and lies in the sandy bottom whileT. cupido, L. dimidiatus, andC. temminckii hide and rest in spaces among piles of boulders or in crevices of rocks during the night. It seems that the differences in nocturnal behavior among the four species of wrasses mentioned above are closely related to the intensity of endogenous factors in their locomotor activity rhythms.     

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.     

Pineal-dependent locomotor activity of lamprey,Lampetra japonica,measured in relation to LD cycle and circadian rhythmicity

Summary Locomotor activity of the river lamprey, Lampetra japonica, was investigated under a light-dark (LD 1212) cycle and under continuous dark conditions. Intact lampreys were entrained to the light:dark cycle. They were active mainly in the early half of the dark period and inactive in light period. The light:dark entrainment continued in 72.7% of lampreys after the removal of bilateral eyes, but additional pinealectomy made the entrainment disappear in all lampreys. When lampreys were pinealectomized with their eyes intact, light: dark entrainment was abolished in most cases. The results indicate that the pineal organ of the lamprey is a photoreceptive organ responsible for synchronizing locomotor activity to LD cycle. Under continuous dark conditions, the locomotor activity began to free-run with a period of 21.3 ± 0.9 h (mean ± SD, n = 53). This circadian rhythmicity was not affected by the removal of lateral eyes but was abolished by pinealectomy. The pineal organ appears to function as an oscillator, or as one of the oscillators, for the circadian locomotor rhythm of lampreys.Abbreviations DD continuous dark - LD light:dark     

The effects of pinealectomy and blinding on the circadian locomotor activity rhythm in the Japanese newt,Cynops pyrrhogaster

We examined the effects of pinealectomy and blinding (bilateral ocular enucleation) on the circadian locomotor activity rhythm in the Japanese newt, Cynops pyrrhogaster. The pinealectomized newts were entrained to a light-dark cycle of 12 h light and 12 h darkness. After transfer to constant darkness they showed residual rhythmicity for at least several days which was gradually disrupted in prolonged constant darkness. Blinded newts were also entrained to a 12 h light/12 h dark cycle. In subsequent constant darkness they showed free-running rhythms of locomotor activity. However, the freerunning periods noticeably increased compared with those observed in the previous period of constant darkness before blinding. In blinded newts entrained to the light/dark cycle the activity rhythms were gradually disrupted after pinealectomy even in the presence of the light/dark cycle. These results suggest that both the pineal and the eyes are involved in the newt's circadian system, and also suggest that the pineal of the newt acts as an extraretinal photoreceptor which mediates the entrainment of the locomotor activity rhythm.Abbreviations circadian period - DD constant darkness - LD cycle, light-dark cycle - LD 12:12 light-dark cycle of 12 h light and 12 h darkness     

The adult eclosion rhythm in Hyphantria cunea (Lepidoptera: Arctiidae): Endogenous and exogenous light effects

Abstract

Endogenous and exogenous effects of light on adult eclosion in Hyphantria cunea were tested by exposure to various light regimes. Regression analysis showed that the position of the eclosion peak after lights on was proportional to the length of the photophase, and that the peak was influenced by the timing of both lights‐on and lights‐off. Under photoperiods of 2–12 hours LD cycle, the eclosion peak was situated after lights‐off, but moved into the light phase as the photophase increased to 22 hours. Pupae were exposed to 3 “skeleton”; photoperiods of LDLD2:2:6:14, 4:2:4:14 and 6:2:2:14. Under the first of these, most adults emerged at the start or just before the longest dark period. Under the second and third skeleton regimes, 20% and 70% respectively of pupae emerged during the shorter dark period. When the compound eyes of the pharate adults were covered, adults smerged 1–4 hours before lights‐off under LD10:14, compared to a control group which emerged just after lights‐off. When pupae were transferred from LD to LL or DD conditions, the eclosion peak occurred approximately every 24 hours after the last LD peak. Results suggest that light received by the compound eyes influences the eclosion rhythm, either through an exogenous masking effect, or by altering the phase of the pacemaker controlling eclosion.     

Enhanced growth and total fatty acid production of microalgae under various lighting conditions induced by flashing light

Microalgae are gaining importance as a source of high‐value bioproducts. However, data regarding optimization of algal productivity via variation of environmental factors are lacking. Here, we evaluated a novel lighting method for the enhancement of biomass and total fatty acid (TFA) productivities during algal cultivation. We cultivated six different algal strains (Chlorella vulgaris KCTC AG10002, Acutodesmus obliquus KGE18, Uronema sp. KGE03, Micractinium reisseri KGE19, Fragilaria sp., and Spirogyra sp.) under various lighting conditions—continuous light (CL), light‐dark cycle (LD), and continuous dark (CD)—with or without additional flashing light. We monitored dry cell weight (DCW) and TFA concentrations during cultivation. For each algal strain, the growth rate showed markedly different responses to the various lighting modes. The growth rates of C. vulgaris KCTC AG10002 (1.34‐fold DCW increase, LD with flash), A. obliquus KGE18 (5.16‐fold DCW increase, LD with flash), Uronema sp. KGE03 (2.77‐fold DCW increase, CL with flash), and M. reisseri KGE19 (1.52‐fold DCW increase, CL with flash) markedly increased in response to flashing light. Additionally, in some algal strains cultivated under the LD mode, the flashing light treatment induced increased TFA concentrations (C. vulgaris, 1.19‐fold increase; A. obliquus, 2.59‐fold increase; and M. reisseri, 3.31‐fold increase). Phytohormone analysis of M. reisseri revealed increases in growth rate and TFA concentrations, associated with phytohormone induction via flashing light (e.g. 2.93‐fold increase in gibberellic acid); hence, flashing light can promote substantial alterations in algal metabolism.     

Biological adaptability under seasonal variation of light/dark cycles

3A substantial amount of experimental models designed to understand rhythms entrainment and the effects of different regimens of light exposure on health have been proposed. However, many of them do not relate to what occurs in real life. Our objective was to evaluate the influence of “seasonal-like” variation in light/dark cycles on biological rhythms. Twenty adult male Wistar rats were assigned to three groups: control (CT), kept in 12:12 light/dark (LD) cycle; long photoperiod/short photoperiod (LP/SP), kept in 16.5:7.5 LD cycle for 18 days (phase A), then 17 days of gradual reductions in light time (phase B), then 18 days of shorter exposure (7.5:16.5 LD cycle, phase C); short photoperiod/long photoperiod (SP/LP) group, with same modifications as the LP/SP group, but in reverse order, starting phase A in 7.5:16.5 LD cycle. Activity and temperature were recorded constantly, and melatonin and cortisol concentrations were measured twice. Activity and temperature acrophases of all groups changed according to light. The correlation between activity and temperature was, overall, significantly lower for SP/LP group compared with LP/SP and CT groups. Regarding melatonin concentration, LP/SP group showed significant positive correlation between phase A and C (p = 0.018). Animals changed temperature and activity according to photoperiod and demonstrated better adaptability in transitioning from long to short photoperiod. Since this model imitates seasonal variation in light in a species that is largely used in behavioral experiments, it reveals promising methods to improve the reliability of experimental models and of further environmental health research.     

Disruption of circadian rhythms accelerates development of diabetes through pancreatic beta-cell loss and dysfunction

Type 2 diabetes mellitus (T2DM) is complex metabolic disease that arises as a consequence of interactions between genetic predisposition and environmental triggers. One recently described environmental trigger associated with development of T2DM is disturbance of circadian rhythms due to shift work, sleep loss, or nocturnal lifestyle. However, the underlying mechanisms behind this association are largely unknown. To address this, the authors examined the metabolic and physiological consequences of experimentally controlled circadian rhythm disruption in wild-type (WT) Sprague Dawley and diabetes-prone human islet amyloid polypeptide transgenic (HIP) rats: a validated model of T2DM. WT and HIP rats at 3 months of age were exposed to 10 weeks of either a normal light regimen (LD: 12:12-h light/dark) or experimental disruption in the light-dark cycle produced by either (1) 6-h advance of the light cycle every 3 days or (2) constant light protocol. Subsequently, blood glucose control, beta-cell function, beta-cell mass, turnover, and insulin sensitivity were examined. In WT rats, 10 weeks of experimental disruption of circadian rhythms failed to significantly alter fasting blood glucose levels, glucose-stimulated insulin secretion, beta-cell mass/turnover, or insulin sensitivity. In contrast, experimental disruption of circadian rhythms in diabetes-prone HIP rats led to accelerated development of diabetes. The mechanism subserving early-onset diabetes was due to accelerated loss of beta-cell function and loss of beta-cell mass attributed to increases in beta-cell apoptosis. Disruption of circadian rhythms may increase the risk of T2DM by accelerating the loss of beta-cell function and mass characteristic in T2DM.     

Circadian rhythms and the induction of flowering in the long-day grass Lolium temulentum L.

Plants of Lolium temulentum L. strain Ceres were grown in 8-h short day (SD) for 45 d before being exposed either to a single long day (LD) or to a single 8-h SD given during an extended dark period. For LD induction, the critical photoperiod was between 12 and 14 h, and more than 16 h were needed for a maximal flowering response. During exposure to a single 24-h LD, the translocation of the floral stimulus began between the fourteenth and the sixteenth hours after the start of the light period, and was completed by the twenty-fourth hour. Full flowering was also induced by one 8-h SD beginning 4 or 28 h after the start of a 40-h dark period, i.e. by shifting 12 h forward or beyond the usual SD. The effectiveness of a so-called ‘displaced short day’ (DSD) was not affected by light quality and light intensity. With a mixture of incandescent and fluorescent lights at a total photosynthetic photon flux density of 400 μmol m⁻² s⁻¹, a 4-h light exposure beginning 4 h after the start of a 40-h dark period was sufficient to induce 100% flowering. The flower-inducing effect of a single 8-h DSD was also assessed during a 64-h dark period. Results revealed two maxima at a 20-h interval. This fluctuation in light sensitivity suggests that a circadian rhythm is involved in the control of flowering of L. temulentum.     

Photoperiod,stress and the distribution of leukocytes in the peripheral blood ofNotophthalmus viridescens

Summary Adult newts,Notophthalmus viridescens, were maintained for 8 days at a constant temperature of 11.0±0.5 °C. In one series, the control animals were kept in constant darkness (DD), while the experimental newts were exposed to alternating 12-hour periods of light and dark (LD). In a second series, controlNotophthalmus lived in DD, and experimental animals lived in constant light (LL). In both series, the newts were sacrificed on the ninth day when blood smears were prepared. Differential counts of the leukocytes of animals that lived under the LD regimen were the same as those of controlNotophthalmus (Table 1). However, in newts that were maintained in LL, the neutrophils increased and the lymphocytes decreased relative to those types of cells in the controls (Table 1). Those changes indicate that continuous light constitutes stress for this species.Supported in part by a grant from the Committee on Research, Travel and Sabbatical Leaves, Colby College     

Daily rhythms of lipid metabolic gene expression in zebra fish liver: Response to light/dark and feeding cycles

Despite numerous studies about fish nutrition and lipid metabolism, very little is known about the daily rhythm expression of lipogenesis and lipolysis genes. This research aimed to investigate the existence of daily rhythm expressions of the genes involved in lipid metabolism and their synchronization to different light/dark (LD) and feeding cycles in zebra fish liver. For this purpose, three groups of zebra fish were submitted to a 12:12?h LD cycle. A single daily meal was provided to each group at various times: in the middle of the light phase (ML); in the middle of the dark phase (MD); at random times. After 20 days of acclimation to these experimental conditions, liver samples were collected every 4?h in one 24-h cycle. The results revealed that most genes displayed a significant daily rhythm with an acrophase of expression in the dark phase. The acrophase of lipolytic genes (lipoprotein lipase – lpl, peroxisome proliferator-activated receptor – pparα and hydroxyacil CoA dehydrogenase – hadh) was displayed between ZT 02:17?h and ZT 18:31?h. That of lipogenic genes (leptin-a – lepa, peroxisome proliferator-activated receptor – pparγ, liver X receptor – lxr, insulin-like growth factor – igf1, sterol regulatory element-binding protein – srebp and fatty acid synthase – fas) was displayed between ZT 15:25?h and 20:06?h (dark phase). Feeding time barely influenced daily expression rhythms, except for lxr in the MD group, whose acrophase shifted by about 14?h compared with the ML group (ZT 04:31?h versus ZT 18:29?h, respectively). These results evidence a strong synchronization to the LD cycle, but not to feeding time, and most genes showed a nocturnal acrophase. These findings highlight the importance of considering light and feeding time to optimize lipid metabolism and feeding protocols in fish farming.     

Activation of latent origins of DNA replication in florally determined shoot meristems of long-day and short-day plants: Silene coeli-rosa and Pharbitis nil

Replicon spacing was measured during the S-phase of the cell cycle in shoot meristems of Silene coeli-rosa L., a long-day (LD) plant, and Pharbitis nil Chois, a short-day (SD) plant to examine the hypothesis that activation of latent origins of DNA replication is a feature of floral determination. Silene coeli-rosa was germinated and grown in SD for 28 d and then exposed to either a florally inductive combination of 7 LD + 2 SD, the last day of which coincides with determination of the sepal and stamen whorls, or was germinated and grown in 37 non-inductive SD. Pharbitis nil was germinated and grown in continuous light (CL) for 5 d and then given either 48 h of inductive darkness followed by 1 d of CL, the last day of which coincides with determination of the sepal, petal and stamen whorls, or given one of two independent non-inductive treatments: 48 h dark interrupted by red light (R) + 1 d of CL, or 8 d of CL. Following these treatments, each batch of plants was exposed to tritiated [methyl-³H]thymidine for 30, 60, 90 or 120 min. Apical domes were dissected, nuclei lysed and prepared as fibre autoradiographs from which replicon size was recorded. In S. coeli-rosa, replicon size was in the range 10–15 μm in SD (non-inductive) and 0–5 μm in LD (inductive) while in P. nil it was 10–15 μm in the 48 h dark interrupted by R, 5–10 μm in CL (both non-inductive) but was reduced to 0–5 μm in the 48 h dark treatment (inductive). Therefore, the recruitment of additional initiation points for DNA replication occurred in both a LD and a SD plant immediately before the appearance of floral organs. The data are consistent in showing that a shortening of S-phase, which is a characteristic feature of florally determined shoot meristems for both species, is brought about by the activation of latent origins of DNA replication. Received: 14 May 1998 / Accepted: 20 August 1998