Nitrogenase activity in the non-heterocystous cyanobacterium Oscillatoria sp. grown under alternating light-dark cycles

The non-heterocystous cyanobacterium Oscillatoria sp. strain 23 fixes nitrogen under aerobic conditions. If nitrate-grown cultures were transferred to a medium free of combined nitrogen, nitrogenase was induced within about 1 day. The acetylene reduction showed a diurnal variation under conditions of continuous light. Maximum rates of acetylene reduction steadily increased during 8 successive days. When grown under alternating light-dark cycles, Oscillatoria sp. fixes nitrogen preferably in the dark period. For dark periods longer than 8 h, nitrogenase activity is only present during the dark period. For dark periods of 8 h and less, however, nitrogenase activity appears before the beginning of the dark period. This is most pronounced in cultures grown in a 20 h light – 4 h dark cycle. In that case, nitrogenase activity appears 3–4 h before the beginning of the dark period. According to the light-dark regime applied, nitrogenase activity was observed during 8–11 h. Oscillatoria sp. grown under 16 h light and 8 h dark cycle, also induced nitrogenase at the usual point of time, when suddenly transferred to conditions of continuous light. The activity appeared exactly at the point of time where the dark period used to begin. No nitrogenase activity was observed when chloramphenicol was added to the cultures 3 h before the onset of the dark period. This observation indicated that for each cycle, de novo nitrogenase synthesis is necessary.     

Different circadian rhythms regulate photoperiodic flowering response and leaf movement in Pharbitis nil (L.) Choisy

The phase shifting effect of red light on both the leaf movement rhythm, and on the rhythm of responsiveness of photoperiodic flower induction towards short light breaks (10 min red light), has been studied in Pharbitis nil, strain Violet, and comparisons between the two rhythms have been made. The phase angle differences between the rhythms after a phase shift with 2 or 6 h of red light given at different times during a long dark period were not constant. The results indicate the involvement of two different clocks controlling leaf movement and photoperiodic flower induction.Abbreviations DD continuous darkness - l:D x:y light/dark cycles with x hours of light and y hours of darkness - PPR rhythm of photoperiodic responsiveness towards light break     

Further observations on rhythmic emission of fragrance in flowers

Observations regarding floral fragrance and the rhythmicity of its emission in four plant species are reported. In the case of flowers of Hoya carnosa R. Br. which are characterized by circadian rhythmicity of scentedness (R. Altenburger and P. Matile, 1988, Planta 174, 248–252), temperature compensation of the free-running period as well as persistence of oscillations in permanent darkness have been demonstrated. A hitherto unidentified component of fragrance turned out to be identical to an unusual sesquiterpene recently discovered in cardamom oil (B. Maurer et al., 1986, Tetrahedron Lett. 27, 2111–2112). In Stephanotis floribunda Brongs. the rhythmic emission of fragrance is circadian in nature, but in a constant environment the oscillations of individual components are increasingly asynchronous. In excised flowers of Odontoglossum constrictum Lindl. the diurnal oscillations observed in a natural photoperiod are abolished under constant environmental conditions. They are resumed upon the return to a 1212-h photoperiod. The absence of circadian control could also be demonstrated in excised flowers of Citrus medica L. In this species, too, the daily maxima of scent emission reappear upon the transfer of flowers to a 1212-h light/ dark cycle. Results obtained upon the comparative analysis of volatiles in the headspace above the flowers and in petal extract indicate that the relative abundance of an individual compound in the floral fragrance is not a function of differential volatility.Abbreviations DL 1212 h photoperiod - DD continuous darkness - GLC gas-liquid chromatography - LL continuous illumination The authors are indebted to R. Kaiser, Givaudon Corp.for kindly carrying out the identifikation of volatiles by gas-liquid chromalography/mass spectrometry.     

Investigation of the endogenous rhythm of flowering inChenopodium rubrum L.

Under the conditions applied in our laboratory 4 1/2 days old plants ofChenopodium rubrum require 2–3 photoperiodic cycles for maximal flowering response, whereas 2 1/2 days old plants are able to flower after having obtained a single inductive cycle. The period length of the free-running rhythm of flowering observed in 2 1/2 days old plants after a single transfer from light to darkness is 30h and the first peak of flowering occurs at about hour 12 in darkness. When a cycle consisting of 16h darkness and 8h light or of 8h darkness and 8h light precedes the long dark period the rhythm is rephased. Rephasing is greater when the light commenced to act on the positive slope of the first peak of the free running rhythm than when it impinged on the negative slope. With an 8h interruption of darkness by light rhythm phase is controlled by the light-on, as well as by the light-off signal. Feeding 0.4 M glucose during the long period of darkness enhanced the amplitude of the flowering response and, moreover, substituted for one photoperiodic cycle.     

Rhythmicity of Flowering in Pharbitis nil

When young seedlings of Pharbitis nil are grown under continuous light, except for a single inductive dark period, they flower to a varying degree, depending on when this dark period is given. Plants become sensitive to this induction approximately three days after the seedlings emerge from the soil. The expression of flowering varies in a rhythmic fashion for three or more cycles, when an inductive dark period is given at progressively later times. The time between maximum expression of flowering is 24 hours or somewhat longer. It appears necessary that the inductive dark period be of sufficient duration, to only partially induce the plants to flower for this rhythm to be expressed. Under the conditions employed in this study, this duration is 12 hours. If this rhythm is endogenous, it exists at least from when the plants emerged from the soil since no environmental cues are given after that time, and it raises questions of the interpretations of data from previous studies with this organism.     

Influence of temperature on the rhythmic emission of volatiles from Ribes nigrum flowers in situ

The floral volatiles of blackcurrant (Ribes nigrum) were collected in situ using the headspace technique. Eleven compounds were identified, including monoterpenes hydrocarbons and monoterpene ethers. The fragrance was emitted in a rhythmic manner, the maximum being in the middle of the photoperiod. Emission at 20°C was significantly higher than at 10°C. The rhythmic nature of emission seemed to cease under conditions of constant light. However, a slight rise in emission during the second period in continuous light may indicate a circadian control of emission. The significance of flower volatiles in relation to pollination is discussed.     

Influence of light quality and photoperiod on flowering of Cyclamen persicum Mill. cv. ‘Dixie White’

The effect of light quality (spectral quality) and photoperiod (day length) were studied on flowering of Cyclamen persicum cv. Dixie White. Light generated from light emitting diodes (LED) i.e. monochromatic blue (10 or 12 h per day), monochromatic red (10 or 12 h per day), blue plus red (10 or 12 h per day) and fluorescent lights were used in these studies. It was found that blue plus red LEDs improved flower induction in cyclamen, the number flower buds and open flowers being highest in the plants grown under blue plus red LED (10 h per day). Blue and red LEDs alone reduced the flowering response. Peduncle length (flower stalk length) and blooming period of flowers were also influenced by light qualities and photoperiod treatments. Peduncle length was 23.8 cm on plants grown under red LED (12 h per day) treatment but 14 cm on plants grown under fluorescent light. Blooming period of flowers grown under fluorescent light was 20 d, whereas it was 40 d with the plants grown under red LEDs (10 h per day). The results indicate that flowering and subsequent growth of cyclamen can be controlled by manipulating light quality and lighting period.     

Light- and temperature-entrained circadian regulation of activity and mRNA accumulation of 1-aminocyclopropane-1-carboxylic acid oxidase in Stellaria longipes

Stem and leaf tissues of Stellaria longipes Goldie (prairie ecotype) exhibit circadian rhythmicity in the activity and mRNA abundance for 1-aminocyclopropane-1-carboxylic acid oxidase (EC 1.4.3). The steady-state mRNA levels and enzymatic activity levels fluctuated with a period of approximately 24 h and reached their maxima by the middle of the light phase and minima by the middle of the dark phase. The oscillations showed damping under constant light, constant dark and constant temperature conditions, indicating that the rhythm is entrained by an external signal. The results indicate that light/dark cycles have greater entraining effects than temperature cycles. A 15-min red light pulse, but not a blue light pulse, could reset rhythm in continuous darkness, suggesting the possible role of a red-light signal transduction pathway in the circadian regulation of 1-aminocyclopropane-1-carboxylic acid oxidase.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - DD continuous dark - LD light-dark - LL continuous light - ZT Zeitgeber time (start of light period for circadian entrainment) This study was supported by operating grants to C.C.C., and D.M.R. from the Natural Sciences and Engineering Research Council of Canada.The authors gratefully acknowledge the award of a Bettina Bahlsen memorial Graduate Scholarship by University of Calgary to A.K. We are grateful to Dr. M.M. Moloney for allowing the use of his laboratory facilities.     

Endogenous circannual rhythms and photorefractoriness of testis activity, moult and prolactin concentrations in mink (Mustela vison).

Mink are seasonal photosensitive breeders; testis activity is triggered when days have less than 10 h light. Increasing and decreasing plasma concentrations of prolactin induce the spring and autumn moults. In a 5 year experiment, males were maintained under short days (8 h light:16 h dark) at 13 degrees C or long days (16 h light:8 h dark) at 21 degrees C, winter and summer conditions, respectively. Under winter and summer conditions, circannual cycles of prolactin secretion and moulting were observed at intervals of about 11 months. Recurrence of testis cycles was not evident. In a second experiment, males were maintained under an 8 h light:16 h dark cycle from the winter solstice or under 10 h light:14 h dark, 12 h light:12 h dark or 14 h light:10 h dark cycles from 10 February. Under 8 h light:16 h dark cycle, testis regression was slightly later than under natural conditions, indicating photorefractoriness. However, mink remained sensitive to light: the longer the photoperiod, the faster the testis regression. In a third experiment, males were transferred under 8 h light:16 h dark or 16 h light:8 h dark from 15 May (group 1), 12 June (group 2) or 4 July (group 3); males submitted to long days received melatonin capsules on the day of transfer. Increasing concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and testis volume were shown by half the males in group 2 and nearly all the males in group 3; the constant release of melatonin from implants was more efficient than short days; but in the three groups, prolactin concentrations decreased in the few days after short-day or melatonin treatment. Overall, the results demonstrate endogenous circannual rhythms of prolactin secretion, body weight and moulting. Although a refractory period to short days was observed, the annual cycle of testis activity totally relies on the annual changes in daylength.     

Ethylene production and metabolism of 1-aminocyclopropane-1-carboxylic acid inChenopodium rubrum L. as influenced by photoperiodic flower induction

Chenopodium rubrum plants, induced to flower by three cycles of 12 h darkness and 12 h light, produced 42% less ethylene than vegetative plants kept under continuous light. Plants that had each dark cycle broken by 2 h light in the middle did not flower and produced almost as much ethylene as the vegetative plants. Shoots and roots of plants of all three experimental treatments had a similar content of 1-aminocyclopropane-1-carboxylic acid (ACC), the mean amounting to about 2 nmol · g^–1 dry weight. Also the content of N-malonyl-ACC (MACC) was similar in shoots of all three treatments. MACC content in roots was shown to be much higher, especially in the treatments with three dark periods (about 85 nmol · g^–1 dry weight). When labeled [2,3-¹⁴C] ACC was administered, the relative contents of ACC and MACC were very similar among all three treatments. The only process influenced by flower induction was ACC conversion to ethylene. Induced plants converted 36% less ACC than the vegetative ones. Plants subjected to night-break converted almost as much ACC to ethylene as vegetative plants. It is concluded that flower induction in the short-day plantChenopodium rubrum decreases ethylene production by decreasing their capability of converting ACC to ethylene.     

Temperature-induced phase shifting of circadian rhythms in cotton seedlings as related to variations in chilling resistance

The relationship between the degree of chilling resistance and phase shifting caused by low-temperature pulses was examined in two circadian rhythms in cotton (Gossypium hirsutum L. cv. Deltapine 50) seedlings grown under light-dark cycles of 1212 h at 33° C. The seedlings showed a circadian rhythm of chilling resistance and of cotyledon movement. A pulse of 19° C for 12 h during the chilling-sensitive phase (light period) caused a phase delay of 6 h, while a similar temperature pulse during the chilling-resistant phase (dark period) did not cause any phase shift. Exposure to 19° C, 85% RH (relative humidity) for 12 h during the dark period induced chilling resistance in the following otherwise chilling-sensitive light period. In this light period a 12-h 19° C pulse did not cause a phase shift of chilling resistance. Pulses of low temperatures (5–19° C) were more effective in causing phase delays in the rhythm of cotyledon movement when given during the chilling-sensitive phase than when given during the chilling-resistant phase. A 12-h pulse of 5° C, 100% RH during the light period caused a phase delay of cotyledon movement of 12 h. However, when that pulse had been preceded by a chill-acclimating exposure to 19° C, 85% RH for 12 h during the dark period the phase delay was shortened to 6 h. The correlation between higher degree of chilling resistance and the prevention or shortening of the phase delay caused by low temperatures indicates that the mechanism that increases chilling resistance directly or indirectly confers greater ability for prevention of phase shifting by low temperatures in circadian rhythms.Abbreviations CT circadian time - LDC light-dark cycle of 24 h - RH relative humidity     

Flowering of Pharbitis nil is controlled by an endogenous semidian rhythm

Abstract Flowering of Pharbitis nil after an inductive dark period is greatly influenced by far-red (FR) irradiation during the preceding light period. The response to FR is rhythmic in otherwise constant conditions, and the period of the oscillation is approximately 12 h (i.e. semidian). The rhythm also appears to operate under daily light-dark cycles. The expression of this novel rhythm depends on the time from the beginning of FR pretreatment to the onset of the inductive dark period. The cotyledons are the site of response to both the pretreatment and inductive darkness, and both these conditions must be perceived by the same cotyledon.     

Light/dark‐induced effects on behavioral rhythms in suprachiasmatic nucleus‐lesioned rats irrespective of the presence of functional suprachiasmatic nucleus brain implants

Summary

Suprachiasmatic nucleus (SCN)‐lesioned rats which had received a fetal SCN graft were kept in constant red light for three months. After this period it was examined whether those rats that showed a recovered free‐running circadian rhythm could be entrained to light/dark cycles. To this end, they were subjected to a 12 h light/12 h dark schedule, followed by a 12 h light shift and again to dark conditions. In addition, the same regime was imposed on SCN‐grafted rats without recovered circadian rhythms and on sham‐grafted animals with a lesion, which were studied as controls. The presence of an SCN graft was identified immunocytochemically by the presence of vasopressin, vasoactive intestinal polypeptide and somatostatin cells.

Drinking, eating and wheel‐running rhythms were found to synchronize to the light/dark cycles in all rats, not with standing the presence of an SCN graft was. A 12 h light shift was immediately followed by a shift in the three rhythms. Under final dark conditions, free‐running patterns reappeared in rhythm‐recovered animals, without any convincing evidence for entrainment of the rhythms in the pattern of transition.

Behavioral rhythms in SCN‐lesioned rats are apparently masked by 12 h light/dark schedules via other visual pathways than the direct projection from the retina to the SCN.     

Stem elongation and floral initiation on in vitro chicory root explants: influence of photoperiod

Chicory root explants (Cichorium intybus L.) were cultured in vitro under different photoperiods. In complete darkness, strong stem elongation, but no flowering induction was observed. We suggest that this stem elongation could be homologous to the pit growth in chicory heads in vivo. Under a photoperiod of 12 h (LI=±40 E m^–2 s^–1), only vegetative growth was observed. Photoperiods of 16 h or more light a day induced the in vitro explants to develop stems bearing flower buds. When the in vitro cultures were kept in the dark for different durations starting from the first day of culture and afterwards transferred to long-day conditions, 4 days dark were sufficient to cause a decrease in flowering induction. We suggest that during the dark culture, a flowering inhibitory process was started.     

An investigation of circadian rhythm in Escherichia coli

Background: Persuasive evidence for circadian programs in non-photosynthetic bacteria other than cyanobacteria is still lacking, we aimed to investigate the circadian rhythm of specific growth rate in Escherichia coli ATCC 25922, one of the important prokaryotes. Methods: To grow E. coli under different light and dark conditions. When the growth entered into the stationary phase, we stopped the culture and obtained the viable counts by MTT assay every 3 h. The specific growth rates (SGRs) were calculated and analyzed with cosinor method for potential rhythms. Results: Single cosinor method revealed that the SGR of E. coli displayed rhythmic variations with a period of around 24 h both under light/dark cycles and under constant darkness. The best-fitting periods and best-fitting cosine curves were acquired. Conclusions: The SGR of E. coli (ATCC 25922) in a culture medium with limiting substrates in the stationary and death phases displayed rhythmic variations with a period of around 24 h under light/dark cycles and constant darkness conditions.     

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     

Cyclic variations of photosynthetic activity under nitrogen fixing conditions in Synchococcus RF-1

When nitrogen fixing cell cultures of Synechococcus RF-1 were subjected to an alternating lightdark regime (12 h:12 h), a cyclic decrease in the photosynthetic oxygen evolution potential was observed during the dark periods. This rhythm of net photosynthesis rate was maintained for at least two days after transition to continuous light. The decrease in net photosynthesis was accompanied by a stimulation of dark respiration. However, the magnitude of oxygen uptake was considerably smaller than the observed decrease in oxygen evolution. The photosynthetic activity of cells taken from the dark period was characterized by (i) a significantly lower quantum yield and (ii) a strong reduction in the light-saturated rate of photosynthesis. Growing the cultures on nitrate or under continuous light completely suppressed this rhythm. Protein synthesis was not necessary for the recovery of the light-saturated rate of photosynthesis during the light period. The cellular content of chlorophyll a and of phycobiliproteins did not vary between light and dark period, indicating that quantitative changes in the composition of the photosynthetic apparatus are not the basis for the observed oscillations. Regulatory modifications of the photosynthetic efficiency are proposed as an adaptation mechanism to adjust the intracellular oxygen concentration to the needs for nitrogenase activity.Abbreviation Chl chlorophyll