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PubMed1981

Twilight effect: initiating dark measurement in photoperiodism of xanthium.

Six experiments studied the effects of low levels of red and far-red light upon the initiation of measurement of the dark period in the photoperiodic induction of flowering in Xanthium strumarium L. (cocklebur), a short-day plant, and compared effects with those of comparable light treatments applied for 2 hours during the middle of a 16-hour inductive dark period. Red light, or red plus far-red, at levels that inhibit flowering when applied during the middle of the inductive dark period, either had no effect on the initiation of dark measurement (i.e., were perceived as darkness), or they delayed the initiation of dark measurement by various times up to the full interval of exposure (2 hours). Far-red light alone had virtually no effect either at the beginning or in the middle of the dark period. These results confirm that time measurement in the photoperiodic response of short-day Xanthium plants is not simply the time required for metabolic dark conversion of phytochrome. Results also suggest that the pigment system (phytochrome?) and/or responses to it may be significantly different as they function during twilight (initiation of dark measurement), and as they function during a light break several hours later. Possible mechanisms by which cocklebur plants detect the change from light to darkness are discussed.COMPARING EXPERIMENTAL RESULTS WITH SPECTRAL LIGHT MEASUREMENTS DURING TWILIGHT AND WITH MEASUREMENTS OF LIGHT FROM THE FULL MOON LED TO TWO CONCLUSIONS: First, light levels pass from values perceived by the plant as full light to values perceived as complete darkness in only about 5.5 to 11.5 minutes, although twilight as perceived by the human eye lasts well over 30 minutes. Second, cocklebur plants probably do not respond to light from the full moon, even when most sensitive, 7 to 9 hours after the beginning of darkness.

Journal Article↗
PubMed1991

Increased neuropeptide Y concentrations in the lateral hypothalamic area of the rat after the onset of darkness: possible relevance to the circadian periodicity of feeding behavior.

Neuropeptide Y (NPY) is a major hypothalamic peptide which powerfully stimulates feeding when injected into the hypothalamus and is implicated in circadian rhythmicity. To investigate whether NPY is involved in the increased feeding that follows the onset of darkness in rats, NPY levels were measured in discrete hypothalamic areas before and after darkness. Four groups of eight adult female Wistar rats were habituated to a 12:12 hour light:dark cycle, with food presented at the onset of darkness (19.00 hours). One group was sacrificed during the 3 hours before darkness and another in the first 2.5 hours after darkness, with food provided as usual. To distinguish any effects of feeding itself, the study was repeated with two further groups, but food was not provided after darkness. Seven hypothalamic regions were microdissected from slices of fresh brain and acid-extracted for radioimmunoassay of NPY. NPY levels (fmol/microgram protein) were significantly higher (p less than 0.01) in the lateral hypothalamic area (LHA) of the dark-phase group in both studies. In the other six regions, NPY levels did not differ between light and dark phases. The LHA regulates the circadian rhythmicity of feeding and NPY injection here stimulates feeding. Alterations in NPY in the LHA around the onset of darkness may be related to the initiation of dark-phase feeding.

Animals↗
PubMed2003

Modulation by sudden darkness of apomorphine-induced behavioral responses.

Sudden darkness increases motor activity and decreases anxiety. In the present study, we focused on the role of dopaminergic mechanisms involved in the effects of sudden darkness. The influence of sudden darkness on the behavioral effects of low (0.05 and 0.1 mg/kg) and high (0.25, 0.45 and 0.6 mg/kg) doses of apomorphine (APO) was tested. We assayed the effects of low APO doses on yawning-penile erection syndrome (YES; 0.05 and 0.1 mg/kg) and on motor activity (0.05 mg/kg), and the effects of high APO doses on motor activity (0.25 mg/kg) and stereotyped behavior (0.45 mg/kg and 0.6 mg/kg). Spontaneous total and genital grooming of male and female rats were also recorded. Sudden darkness modified some spontaneous behaviors and also modulated several APO-induced behavioral effects. It increased spontaneous total grooming and genital grooming in male rats but had no effect on these parameters in female rats. These results show sexual dimorphism for total and genital grooming in both control and sudden darkness conditions. APO was able to induce YES in a dose-dependent manner. Sudden darkness decreased yawning elicited by both 0.05 and 0.1 mg/kg of the drug. No other parameter of YES was modified. In the open-field test, sudden darkness increased total locomotion and rearing and decreased immobility duration. APO at a dose of 0.05 mg/kg had the opposite effect on these parameters under light conditions; none of them were modified by sudden darkness. Animals treated with APO at 0.25 mg/kg, a dose that augmented total locomotion and rearing and diminished immobility duration, were clearly divided into two groups according to their responses, i.e., hypo- and hyper-responsive rats. Sudden darkness improved total locomotion and rearing, reduced immobility duration and total grooming in the hyporesponsive group, and showed no effects on the hyper-responsive group. Sudden darkness caused no modifications of stereotyped behavior. These results may be due to a sudden darkness-induced physiological release of dopamine that diminishes pre-synaptic responses to APO and increases low-intensity post-synaptic responses such as motor activity without modifying high-intensity post-synaptic responses such as stereotyped behavior.

Animals↗
PubMedDate not supplied

Layer differences in the effect of monocular vision in light- and dark-reared kittens.

We compared the effect of 2 days of monocular vision on the ocular dominance of cells in the visual cortex of light-reared kittens with the effect in dark-reared kittens at 6, 9, and 14 weeks of age, and analyzed the results by layer. The size of the ocular-dominance shift declined with age in all layers in light-reared animals. There was not a large change in the ocular-dominance shift with age in dark-reared animals in any layer, suggesting that dark rearing largely keeps the cortex in the immature 6-week state until 14 weeks or longer, although there was a slight decrease in layers II, III, and IV, and a slight increase in layers V and VI. At 14 weeks, the difference between light- and dark-reared animals was smallest in layer IV, larger in layers II/III, and largest in layers V/VI, suggesting that dark rearing has a large effect on intracortical synapses and a small effect on geniculocortical synapses. There was a significant ocular-dominance shift in layer IV at 14 weeks of age in both light- animals and dark-reared animals, showing that the critical period for ocular-dominance plasticity is not ended at this age. While the ocular-dominance shift after 26 h of monocular deprivation in 6-week animals was similar in light- and dark-reared animals, after 14 h it was smaller in dark-reared animals, showing that ocular-dominance changes occur more slowly in dark-reared animals at this age, in agreement with Mower (1991). Increases in selectivity for axis of movement after 26 h of monocular vision were seen in dark-reared animals at 6 weeks of age, but not at 9 or 14 weeks of age, showing that the critical period for axial selectivity ends earlier than the critical period for ocular dominance in dark-reared animals, as it does in light-reared animals.

Aging↗
PubMed1999

Sucrose availability on the aerial part of the plant promotes morphogenesis and flowering of Arabidopsis in the dark.

Conditions to promote dark morphogenesis and flower-ing in Arabidopsis have previously been limited to liquid cultures and to a few laboratory ecotypes. We have obtained development and flowering of Arabidopsis plants under complete darkness by growing them on vertical Petri dishes containing solid agar medium with sucrose. Under these conditions, all the ecotypes tested were able to develop, giving rise to etiolated plants that flowered after producing a certain number of leaves. Dark-grown plants showed similarities with phytochrome-deficient mutants and were different from de-etiolated or constitutive photomorphogenesis mutants such as det and cop. Late- and early-flowering ecotypes, showing large differences in flowering time and leaf number under long days, flowered with a similar number of leaves when grown in the dark. Rapid dark flowering of late-flowering ecotypes was not an effect of darkness but the result of the interaction between dark and sucrose availability at the aerial part of the plant, since sucrose also had an effect when plants were grown in the light. Gibberellin-deficient and insensitive mutants were delayed in the initiation of flowers in the dark, indicating a role for these hormones in flowering promotion in the dark. The late-flowering phenotype of mutants at different loci of the autonomous and long-day-dependent flowering induction pathways was rescued in dark growth conditions. However, the late-flowering phenotype of ft and fwa mutants was not rescued by sucrose either in the dark or in the light, suggesting a different role for these genes in flowering induction.

Arabidopsis↗
PubMed1994

Dark focus of accommodation and vergence posture.

We evaluated the dark focus of accommodation and vergence posture in 20 boys and girls, 10 with and 10 without accommodative esotropia, before and after they wore glasses for at least 1 month. Refractive error was measured by Nidek Autorefractometer AR1600. We used two definitions of the dark focus, DFcus (Non-Cyclo R) and DFcus (Cyclo R). DFcus (Non-Cyclo R) = Dark R - Non-Cyclo R. DFcus (Cyclo R) = Dark R - Cyclo R. (Dark R: refractive state in the dark; Non-Cyclo R: non-cycloplegic refractive error; Cyclo R: cycloplegic refractive error.) Vergence posture was measured by prism cover test during distance fixation while uncorrected. DFcus (Cyclo R) was significantly greater in esotropic subjects than in non-esotropic subjects, although DFcus (Non-Cyclo R) did not differ significantly. A significant hyperopic shift in Dark R was observed after wearing glasses. DFcus (Cyclo R) was significantly decreased after wearing glasses, suggesting that wearing glasses is an important variable of the dark focus. Unlike the dark focus, the vergence posture did not change in either group after wearing glasses. The dissociation of the dark focus from vergence posture seems to be inconsistent with previous findings. It is postulated that change in dark vergence after wearing glasses is responsible for the results.

Accommodation, Ocular↗
PubMed2005

Resetting of the hamster circadian system by dark pulses.

Circadian rhythms of animals are reset by exposure to light as well as dark; however, although the parameters of photic entrainment are well characterized, the phase-shifting actions of dark pulses are poorly understood. Here, we determined the tonic and phasic effects of short (0.25 h), moderate (3 h), and long (6-9 h) duration dark pulses on the wheel-running rhythms of hamsters in constant light. Moderate- and long-duration dark pulses phase dependently reset behavioral rhythms, and the magnitude of these phase shifts increased as a function of the duration of the dark pulse. In contrast, the 0.25-h dark pulses failed to evoke consistent effects at any circadian phase tested. Interestingly, moderate- and long-dark pulses elevated locomotor activity (wheel-running) on the day of treatment. This induced wheel-running was highly correlated with phase shift magnitude when the pulse was given during the subjective day. This, together with the finding that animals pulsed during the subjective day are behaviorally active throughout the pulse, suggests that both locomotor activity and behavioral activation play an important role in the phase-resetting actions of dark pulses. We also found that the robustness of the wheel-running rhythm was weakened, and the amount of wheel-running decreased on the days after exposure to dark pulses; these effects were dependent on pulse duration. In summary, similarly to light, the resetting actions of dark pulses are dependent on both circadian phase and stimulus duration. However, dark pulses appear more complex stimuli, with both photic and nonphotic resetting properties.

Adaptation, Physiological↗
PubMed1996

[Effect of the light-dark cycle over the growth of male rats with different health status].

The light-dark cycle exerts an influence over the lipostatic mechanism in the superior vertebrate animals promoting an acceleration in the lipid synthesis rate during the dark period (in the case of rodents) and a higher lipolitic rate during the day. This cycle, the feeding habits, and the awake-sleep cycle are syncronized. The Growth Hormone is a molecule that facilitates the lipolysis and the protein synthesis. This hormone is released, by the pituitary gland, during the puberty and during the short wave sleep period. Modifying the light-dark cycle and in consequence the sleep-awake cycle and the food consumption, we pretend to study its influence over the growth rate in weight and length in animals that consume 100% of its nutritional requirements. Also want to dilucidate if an alteration of the cycle (18 h light 6 h dark) in the malnourished animals can revert the deficiency of the growth rate expected in these animals. Male Wistar rats (160); 21 days old, adapted to the temperature and to the light-dark cycle (12 h-12 h) during a week. Then, four groups were formed; Group I: Well nourished rats with a light-dark (12 h-12 h) cycle; Group II: Well nourished rats with a light-dark (18 h-6 h) cycle; Group III: Malnourished rats with a light-dark (12 h-12 h) cycle; Group IV: Malnourished rats with a light-dark (18 h-6 h) cycle. Its length and weight were registered weekly, obtained its growth rates, and the results were analysed by the ONE-WAY ANOVA and orthogonal contrasts. It was found a significant difference in the growth rate in weight between the Groups I and II; the growth rate in weight in the Group III had a higher slope than the rate in the Group IV but we did not find a significant difference. The growth rate in length did not show a significant difference between the Groups III and IV. The last weight in the malnourished animals represented 55% of the control animals last weight; and 47% of the Group IV. The last length of the animals of the Group III represented 90% of the control value, and the last length in the Group IV represented 82% of the control animals value. The feeding habits are modified changing the cycle to 18 h light and 6 h dark. Modifying the light-dark cycle appears an accelerating rate in the growth rate in weight in the well nourished animals, but not in the malnourished ones.

Analysis of Variance↗
PubMed1995

Ultrastructural evidence of cell communication between epithelial dark cells and melanocytes in vestibular organs of the human inner ear.

BACKGROUND: The possibility of interaction between epithelial dark cells and melanocytes in the mammalian inner ear has been pointed out because of their morphological and biochemical characteristics, although very few studies have dealt directly with communication between these two types of cells. We investigated the dark cell area of human vestibular organs in order to clarify the ultrastructural evidence for cell interaction between epithelial dark cells and melanocytes. METHODS: All of the material was obtained from vestibular schwannoma operations. Paraffin sections were stained with hematoxylin and eosin (H&E) and by the Fontana-Masson technique. Other paraffin sections were also stained immunohistochemically for S-100 protein. Glutaraldehyde fixed specimens were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). RESULTS: Light microscopy revealed melanin pigment granules in the cytoplasm of epithelial dark cells. Melanocytes in the subepithelial layer stained positively for S-100 protein. The presence of intraepithelial melanocytes was confirmed by the presence of cell profiles with a large number of melanin pigment granules and S-100 protein in the cytoplasm. SEM showed that the dark cells had a pentagonal surface with microvilli on the apical surface edge. They had complicated structures at the basal portion of their cytoplasm. Melanocytes extending cytoplasmic processes to adjacent areas were observed under the dark cells. TEM showed that the dark cells were tightly linked by junctional complexes in the upper lateral portion of their cytoplasmic membrane and interdigitated by lateral infoldings. Compound melanosomes (phagosomes or secondary lysosomes) found in the cytoplasm of the dark cells contained poorly pigmented melanosomes with a periodic internal structure. Gap junctions were clearly showed between adjacent melanocytes in the subepithelial layer. CONCLUSIONS: The characteristic substructures of dark cells and melanocytes suggested the presence of intimate cell interaction between these two types of cells in the vestibular organs of the human inner ear, although it is not clear at this stage whether such cell interaction is specific only for patients with vestibular schwannoma. Dark cells and melanocytes form a cell community that serves to maintain homeostasis in vestibular organs through communication in which cell information obtained by both dark cells and melanocytes serves to facilitate the system.

Adult↗
PubMed1989

The influence of fixation on the relative amount of cytoplasmic ribosomes in mouse epidermal basal keratinocytes. A morphometric study of so-called "dark cells" and their putative role in epidermal carcinogenesis.

The nature and significance of so-called dark keratinocytes in the epidermis during chemical carcinogenesis is still a matter of concern and debate. Based on ultrastructural observations it has been suggested that dark cells most often are shrunken cells. Reports on skin carcinogenesis, however, claim that dark cells are a sign of ongoing tumor promotion and represent those stem cells in the epidermis from which the tumors originate. It is therefore important to find out whether these cells are simply injured and shrunken cells, or vital cells of great importance for carcinogenesis. Dark cells are assumed to be rich in ribosomes. There is evidence, however, that the observed number of dark cells is highly dependent on tissue fixation. In the present ultrastructural study, morphometric methods were used to compare the effects of two different fixation procedures on the amount of cytoplasmic ribosomes in dark cells from both untreated and carcinogen-treated hairless mouse epidermis. The results show that the ultrastructural features of both dark and clear cells vary considerably with different fixation procedures. In acetone-treated controls typical dark cells are only observed when the fixative has a lower osmotic activity than the plasma. With iso-osmolal fixation typical dark cells are not observed. After an abortive two-stage carcinogenesis treatment, in which a single application of 9,10-dimethyl-1,2-benzanthracene (DMBA) in acetone was followed by a single application of 12-O-tetradecanoyl-13-acetate (TPA) in acetone, signs of cell injury could be found after both fixation procedures. With DMBA/TPA and hypo-osmolal fixation the number of dark cells seemed to increase, whereas only signs of cell injury with occurrence of some heavily altered "clear cells" dominated the picture with iso-osmolal fixation. Morphometry showed that both the numerical and the volumetric densities of cytoplasmic ribosomes in basal keratinocytes varied most significantly with the fixation procedure used. The cytoplasmic volumes did not vary in a way that could explain these differences. One might therefore assume that the number of ribosomes depends on the fixative. Large swelling artifacts occurred when a fixative with low osmotic activity was used, leading to compression of neighboring cells. Hence, an increased ribosomal density reported previously in dark cells is probably related to such cell volume artifacts and does not reflect an actually increased quantity of ribosomes.(ABSTRACT TRUNCATED AT 400 WORDS)

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1986

Response properties of single units in the accessory optic system of the dark-reared cat.

The visual responses of single units in the lateral and dorsal terminal nuclei (LTN and DTN) of the accessory optic system (AOS) were studied in adult cats reared in total darkness. In the LTN of the normal cat equal numbers of cells prefer upward and downward vertical stimulus motion (previous results). While direction selectively continued to be a characteristic property of LTN and DTN units in dark-reared animals, the distribution of preferred and non-preferred directions of LTN cells was radically altered such that almost every LTN cell examined in the dark-reared cat preferred downward stimulus motion. In contrast, the distribution of preferred directions among DTN cells was largely unaffected by dark rearing. Both normal and dark-reared cat DTN cells responded best to stimuli moving horizontally toward the recorded hemisphere. The velocity preferences of DTN units of the dark-reared cat were, however, much slower than those of normal DTN units. LTN units responding to downward motion in dark-reared cats showed similar velocity preferences to those downward direction-selective LTN units in normal animals. Unlike the highly binocular responses of AOS cells encountered in the normal cat, the ocular dominance distribution obtained from units in the LTN and DTN of the dark-reared cat is completely monocular, favoring the contralateral eye. Thus, dark rearing renders the distribution of preferred directions most affected in the LTN, velocity preference most affected in the DTN and ocular dominance strongly affected in both nuclei. The physiological response properties of the dark-reared cat presented in this report bear a close resemblance to those we have described in the AOS of acutely decorticated animals (previous results). Data obtained from the dark-reared cat support our earlier suggestion that the visual cortex is a major source of upward direction selectively, high-velocity tuning and ipsilateral eye input for AOS cells. Some of the functional consequences of these findings are discussed in relation to frontal eye placement and optokinetic nystagmus.

Animals↗
PubMed2002

A protein kinase activated by darkness phosphorylates nitrate reductase in Komatsuna (Brassica campestris) leaves.

Although it has been shown that leaf nitrate reductase (NR: EC 1.6.6.1) is phosphorylated by subjecting plants to darkness, there is no evidence for the existence of dark-activated or dark-induced NR kinase. This study was undertaken to investigate the occurrence of a protein kinase phosphorylating NR in response to dark treatments. Immediately after transferring Komatsuna (Brassica campestris L.) plants to darkness, we observed rapid increases in the phosphorylating activity of the synthetic peptide, which is designed for the amino acid sequence surrounding the regulatory serine residue of the hinge 1 region of Komatsuna NR, in crude extracts from leaves. The activity reached a maximum after 10 min of darkness. Inactivation states of NR estimated from relative activities with or without Mg2+ were correlated to activities of the putative dark-activated protein kinase. Using the synthetic peptide as a substrate, we purified a protein kinase from dark-treated leaves by means of successive chromatographies on Q-Sepharose, Blue Sepharose, FPLC Q-Sepharose, and ATP-gamma-Sepharose columns. The purified kinase had an apparent molecular mass of 150 kDa with a catalytic subunit of 55 kDa, and it was Ca2+-independent. The purified kinase phosphorylated a recombinant cytochrome c reductase protein, a partial protein of NR, and holo NR, and inactivated NR in the presence of both 14-3-3 protein and Mg2+. The kinase also phosphorylated synthetic peptide substrates designed for sucrose phosphate synthase and 3-hydroxy-3-methylglutaryl-Coenzyme A reductase. Among inhibitors tested, only K252a, a potent and specific serine/threonine kinase inhibitor, completely inhibited the activity of the dark-activated kinase. The activity of the purified kinase was also specifically inhibited by K252a. Taken together with these findings, results obtained suggest that the putative dark-activated protein kinase may be the purified kinase itself, and may be responsible for in vivo phosphorylation of NR and its inactivation during darkness.

Journal Article↗
PubMed2002

Influence of light-dark schedules and stocking density on behaviour, risk of leg problems and occurrence of chronic fear in broilers.

1. The aims of this study were to determine (1) the effect of light-dark schedules on the walking ability, the risk of tibial dyschondroplasia (TD) as well as the duration of tonic immobility (TI) reactions in commercial broiler flocks and (2) the effect of a daily dark period and reduced density on the behaviour of broiler chickens. 2. Experiment 1. Group 1 had a 2 to 8 h daily dark period from 2 to 26 d of age (light-dark programme A) at a stocking density of 28.4 chicks/m2. Group 2 had 8 h of darkness daily from 2 to 38 d of age (light-dark programme B) at 24 chicks/m2. The control group had 24 h continuous light at 28.4 chicks/m2. 3. Experiment 2. Behaviour was studied with and without a daily 8 h dark period and at high (30 chicks/m2) and low (18 chicks/m2) stocking densities. 4. Programme B reduced the prevalence of impaired walking ability, corresponding to gait score > 2, when compared with controls. The effect on walking ability corresponding to gait score > 0 approached significance. 5. Both light-dark programmes reduced the occurrence of TD. Programme B (combined with reduced stocking density), however, had the greater effect. 6. Both light-dark programmes reduced the duration of TI, compared with controls (mean = 426 s) Programme B resulted in a larger reduction (alpha = -156.9 s) than programme A (alpha = -117.0). 7. The proportions of chicks drinking, eating, pecking, scratching, standing and performing vertical wing-shakes increased--both when the 8 h dark period and the reduced stocking density were applied separately and in combination (experiment 2). 8. For all behaviours, except standing, the effect of the dark period was largest in broilers kept at the high stocking density (d 40).

Animal Husbandry↗
PubMed1994

Dark and Circadian Regulation of mRNA Accumulation in the Short-Day Plant Pharbitis nil.

The developmental transition of the meristem from vegetative to reproductive growth is controlled by the cyclic alternation of light and darkness in photoperiodic plants. Photoperiod is perceived in the leaves or cotyledons, where a flower-inducing signal is produced and transmitted to the apex. To begin to understand the molecular basis of the photoperiodic induction of flowering, we investigated changes in gene expression at the level of mRNA abundance that occur in association with dark induction of flowering in the short-day species Pharbitis nil. Several cDNAs were isolated that corresponded to mRNAs whose abundance is altered after the transition to darkness. The pattern of increase in mRNA levels corresponding to one cDNA clone, PN1, showed a dark-induced maximum at 8 h of darkness, whereas a second clone, PN9, showed a dark-induced accumulation of mRNA with peak levels at 12 to 16 h of darkness. When plants were held in continuous darkness, both PN1 and PN9 exhibited rhythmic patterns of mRNA accumulation with an approximate circadian periodicity, suggesting that their expression is under the control of an endogenous clock. The observed pattern of expression of PN1 and PN9 in cotyledon tissue was unusual in that darkness rather than light promoted mRNA accumulation, which is a temporal pattern of expression distinct from that of several other Pharbitis genes, including Cab, PsaG, and actin, whose mRNAs were most prevalent or equally prevalent in the light. Brief illumination of an inductive dark period by a red light night break strongly inhibited the accumulation of both PN1 and PN9 mRNA. The expression of both PN1 and PN9 was spatially regulated in that mRNA transcripts were detected in the cotyledons and stems, but not the roots, of photoperiodically competent seedlings. Both PN1 and PN9 appeared to be present as single-copy genes in the Pharbitis genome. Sequence analysis has not determined the identity of these genes. Overall, the accumulation of mRNAs corresponding to both PN1 and PN9 closely paralleled the process of photoperiodic floral induction in P. nil, but a clear involvement with this process cannot be established from our findings because of the difficulty of separating photoperiodic events from other light-regulated processes, especially those involved in photosynthesis, such as Cab gene expression. These results identify the products of circadian-regulated genes in photoreceptive tissue of P. nil and support the concept that circadian-regulated gene expression interacting with darkness may be involved in the regulation of photoperiodically controlled physiological processes, including flower induction.

Journal Article↗
PubMed1980

Effect of light, dark, and temperature on root nodule activity (acetylene reduction) of soybeans.

The objectives of this study were (a): to define the effects of light, dark, and temperature on nodule activity (acetylene reduction), and (b) to establish the contributions of reserve carbohydrate and recent photosynthate to the support of nodule function. An in situ assay of nodule activity was developed for use with intact, hydroponically grown soybeans (Glycine max [L.] Merr. cv. Calland).Nodule activity of 35-day-old plants grown in controlled environment chambers decreased during a 10-hour dark period at 18 C, compared with activity during the preceding and subsequent 14-hour light periods at 27 C. In contrast, plants that were maintained at a constant 27 C did not vary in nodule activity during diurnally varying dark and light exposure. Nodules of plants exposed to diurnal 18 and 27 in 24-hour continuous dark were less active at 18 C than at 27 C. At constant 27 C, nodule activity was sustained throughout the 24-hour dark period. Thus, nodule activity was independent of short term dark periods but dependent on temperature; nodule activity was decreased at the lower temperature.Temperature also affected the nodule activity of plants maintained in the light. Exposure of shoots and roots of intact plants to the lower temperature (5 hours at 18 C) during the light period resulted in a marked decrease in nodule activity, compared with that of plants maintained at 27 C. Exposure of only the shoot portion to 18 C (roots were maintained at 27 C) resulted in a similar decrease in nodule activity.Nodules of plants exposed to 10 days of diurnally variable dark, light, and temperature had high activity in the light at 27 C and low activity in the dark at 18 C. Nodule activity of plants at a constant 27 C was not affected by diurnally variable dark and light exposure throughout the 10-day period, although activity generally increased with time due to increased nodule mass. At a constant 27 C, nodules of intact plants in continuous dark sustained activity through 72 hours before declining to zero by 7 days. At diurnally varying 18 and 27 C in continuous dark, peak diurnal nodule activity was sustained through 5 days, and then declined to zero by 8 days. Analyses of the carbohydrate content of tissue harvested from this study suggested use of reserve photosynthate (primarily of shoot origin) in support of nodule activity in the absence of concurrent photosynthesis.

Journal Article↗
PubMed1983

Assimilation of [N]Nitrate and [N]Nitrite in Leaves of Five Plant Species under Light and Dark Conditions.

Light dependency of nitrate and nitrite assimilation to reduced-N in leaves remains a controversial issue in the literature. With the objective of resolving this controversy, the light requirement for nitrate and nitrite assimilation was investigated in several plant species. Dark and light assimilation of [(15)N]nitrate and [(15)N]nitrite to ammonium and amino-N was determined with leaves of wheat, corn, soybean, sunflower, and tobacco. In dark aerobic conditions, assimilation of [(15)N]nitrate as a percentage of the light rate was 16 to 34% for wheat, 9 to 16% for tobacco, 26% for corn, 35 to 76% for soybean, and 55 to 63% for sunflower. In dark aerobic conditions, assimilation of [(15)N]nitrite as a percentage of the light rate was 11% for wheat, 7% for tobacco, 13% for corn, 28 to 36% for soybeans, and 12% for sunflower. It is concluded that variation among plant species in the light requirement for nitrate and nitrite assimilation explains some of the contradictory results in the literature, but additional explanations must be sought to fully resolve the controversy.In dark anaerobic conditions, the assimilation of [(15)N]nitrate to ammonium and amino-N in leaves of wheat, corn, and soybean was 43 to 58% of the dark aerobic rate while dark anaerobic assimilation of [(15)N]nitrite for the same species was 31 to 41% of the dark aerobic rate. In contrast, accumulation of nitrite in leaves of the same species in the dark was 2.5-to 20-fold higher under anaerobic than aerobic conditions. Therefore, dark assimilation of nitrite cannot alone account for the absence of nitrite accumulation in the in vivo nitrate reductase assay under aerobic conditions. Oxygen apparently inhibits nitrate reduction in the dark even in leaves of plant species that exhibit a relatively high dark rate of [(15)N]nitrite assimilation.

Journal Article↗
PubMed1998

Self-trapping of dark incoherent light beams

"Dark beams" are nonuniform optical beams that contain either a one-dimensional (1D) dark stripe or a two-dimensional (2D) dark hole resulting from a phase singularity or an amplitude depression in their optical field. Thus far, self-trapped dark beams (dark solitons) have been observed using coherent light only. Here, self-trapped dark incoherent light beams (self-trapped dark incoherent wavepackets) were observed. Both dark stripes and dark holes nested in a broad partially spatially incoherent wavefront were self-trapped to form dark solitons in a host photorefractive medium. These self-trapped 1D and 2D dark beams induced refractive-index changes akin to planar and circular dielectric waveguides. The experiments introduce the possibility of controlling high-power coherent laser beams with low-power incoherent light sources such as light emitting diodes.

Journal Article↗
PubMed1997

Transcriptional and translational regulation of nitrogenase in light-dark- and continuous-light-grown cultures of the unicellular cyanobacterium Cyanothece sp. strain ATCC 51142.

Cyanothece sp. strain ATCC 51142 is a unicellular, diazotrophic cyanobacterium which demonstrated extensive metabolic periodicities of photosynthesis, respiration, and nitrogen fixation when grown under N2-fixing conditions. N2 fixation and respiration peaked at 24-h intervals early in the dark or subjective-dark period, whereas photosynthesis was approximately 12 h out of phase and peaked toward the end of the light or subjective-light phase. Gene regulation studies demonstrated that nitrogenase is carefully controlled at the transcriptional and posttranslational levels. Indeed, Cyanothece sp. strain ATCC 51142 has developed an expensive mode of regulation, such that nitrogenase was synthesized and degraded each day. These patterns were seen when cells were grown under either light-dark or continuous-light conditions. Nitrogenase mRNA was synthesized from the nifHDK operon during the first 4 h of the dark period under light-dark conditions or during the first 6 h of the subjective-dark period when grown in continuous light. The nitrogenase NifH and NifDK subunits reached a maximum level at 4 to 10 h in the dark or subjective-dark periods and were shown by Western blotting and electron microscopy immunocytochemistry to be thoroughly degraded toward the end of the dark periods. An exception is the NifDK protein (MoFe-protein), which appeared not to be completely degraded under continuous-light conditions. We hypothesize that cellular O2 levels were kept low by decreasing photosynthesis and by increasing respiration in the early dark or subjective-dark periods to permit nitrogenase activity. The subsequent increase in O2 levels resulted in nitrogenase damage and eventual degradation.

Cyanobacteria↗