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PubMed2003

Daytime exposure to bright light, as compared to dim light, decreases sleepiness and improves psychomotor vigilance performance.

STUDY OBJECTIVES: This study examined the effects of bright light exposure, as compared to dim light, on daytime subjective sleepiness, incidences of slow eye movements (SEMs), and psychomotor vigilance task (PVT) performance following 2 nights of sleep restriction. DESIGN: The study had a mixed factorial design with 2 independent variables: light condition (bright light, 1,000 lux; dim light, < 5 lux) and time of day. The dependent variables were subjective sleepiness, PVT performance, incidences of SEMs, and salivary melatonin levels. SETTING: Sleep research laboratory at Monash University. PARTICIPANTS: Sixteen healthy adults (10 women and 6 men) aged 18 to 35 years (mean age 25 years, 3 months). INTERVENTIONS: Following 2 nights of sleep restriction (5 hours each night), participants were exposed to modified constant routine conditions. Eight participants were exposed to bright light from noon until 5:00 pm. Outside the bright light exposure period (9:00 am to noon, 5:00 pm to 9:00 pm) light levels were maintained at less than 5 lux. A second group of 8 participants served as controls for the bright light exposure and were exposed to dim light throughout the entire protocol. MEASUREMENTS AND RESULTS: Bright light exposure reduced subjective sleepiness, decreased SEMs, and improved PVT performance compared to dim light. Bright lights had no effect on salivary melatonin. A significant positive correlation between PVT reaction times and subjective sleepiness was observed for both groups. Changes in SEMs did not correlate significantly with either subjective sleepiness or PVT performance. CONCLUSIONS: Daytime bright light exposure can reduce the impact of sleep loss on sleepiness levels and performance, as compared to dim light. These effects appear to be mediated by mechanisms that are separate from melatonin suppression. The results may assist in the development of treatments for daytime sleepiness.

Adolescent↗
PubMed2005

The effect of infection-control barriers on the light intensity of light-cure units and depth of cure of composite.

AIM: Dental curing lights are vulnerable to contamination with oral fluids during routine intra-oral use. This controlled study aimed to evaluate whether or not disposable transparent barriers placed over the light-guide tip would affect light output intensity or the subsequent depth of cure of a composite restoration. METHODS: The impact on light intensity emitted from high-, medium- and low-output light-cure units in the presence of two commercially available disposable infection-control barriers was evaluated against a no-barrier control. Power density measurements from the three intensity light-cure units were recorded with a radiometer, then converted to a digital image using an intra-oral camera and values determined using a commercial computer program. For each curing unit, the measurements were repeated on ten separate occasions with each barrier and the control. Depth of cure was evaluated using a scrape test in a natural tooth model. RESULTS: At each level of light output, the two disposable barriers produced a significant reduction in the mean power density readings compared to the no-barrier control (P<0.005). The cure sleeve inhibited light output to a greater extent than either the cling film or the control (P<0.005). Only composite restorations light-activated by the high level unit demonstrated a small but significant decrease in the depth of cure compared to the control (P<0.05). CONCLUSION: Placing disposable barriers over the light-guide tip reduced the light intensity from all three curing lights. There was no impact on depth of cure except for the high-output light, where a small decrease in cure depth was noted but this was not considered clinically significant. Disposable barriers can be recommended for use with light-cure lights.

Analysis of Variance↗
PubMed2001

Protection of photoreceptor cells in adult rats from light-induced degeneration by adaptation to bright cyclic light.

Light history has been shown to affect the susceptibility of the albino rat retina to the damaging effects of constant light exposure. Retinas of animals raised in relatively bright cyclic light are protected against light-induced degeneration compared with dim-reared animals. These effects were observed in animals raised from birth in bright cyclic light and are part of an adaptive response that protects photoreceptors from stress-induced degeneration. To determine if retinas of adult animals are capable of such adaptive changes or flexibility by switching between different light environments which do not pathologically damage photoreceptor cells, albino rats were maintained in less than 250 lux cyclic light for more than 3 weeks. At 12-13 weeks of age, they were placed into 800 lux cyclic light for 1 week, after which they were exposed to constant illumination of 1500-lux for 1, 3 or 7 days. Retinal function was evaluated by electroretinography and photoreceptor cell death was quantified by measuring outer nuclear layer thickness. After 1 week in bright cyclic light, the retinas were completely protected against 1 day constant light exposure that significantly damaged retinas of animals without 800 lux cyclic light adaptation. Significant protection was also observed in 3 day constant light exposed animals; limited protection occurred after 7 days exposure. These results indicate that the retinas of adult rats adapted to bright cyclic light within certain ranges that did not significantly damage photoreceptor cells are protected from constant light challenge. This phenomenon is a post-developmental response that demonstrates a remarkable plasticity of the retina. The mechanism(s) underlying the ability of this adaptation/flexibility in protecting photoreceptors could involve endogenous molecules that encompass many aspects of retinal cell and molecular biology and physiology. Identification of these molecules may provide insight into the development of therapeutic approaches to treat retinal degeneration.

Adaptation, Ocular↗
PubMed1981

Functional and structural organization of chlorophyll in the developing photosynthetic membranes of Euglena gracilis Z. IV. Light-harvesting properties of system II photosynthetic units and thylakoid ultrastructure during greening under intermittent light.

Dark-grown, non-dividing Euglena gracilis Z cells were exposed for 100 h to intermittent light (15 s every 15 min darkness) and were then transferred to continuous light. During chloroplast differentiation, the development of light harvesting and trapping properties of Photosystem II was analyzed mainly with fluorescence induction measurements in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and was associated with observations on ultrastructural organisation of developing thylakoids using thin section and freeze-fracture methods. Results showed that: (a) the synthesis of chlorophyll b and probably that of the light-harvesting chlorophyll a/b-protein complex was more reduced by intermittent light than the formation of active system II reaction centers; (b) the size of the overall photosynthetic units, i.e. the number of chlorophyll molecules per O2 molecule evolved under a regime of repetitive saturating short flashes were reduced by 2-3 compared to those developed under continuous light; (c) the lack of chlorophyll induced by intermittent light affected more specifically the size of light-harvesting antennae of system II units, the optical cross-section of which was reduced by 3-4; (d) energy transfers did not occur between these small system II units in spite of high concentrations of PS II reaction centers and of a high trapping efficiency of the absorbed energy; (e) thylakoids developed under intermittent light were not stacked; (f) particles on exoplasmic fracture faces were significantly smaller than those developed under continuous light; (g) rapid synthesis of chlorophyll (Chl a and Chl b) upon exposure to continuous light of cells first greened under intermittent light are concomittant with rapid recovery of light-harvesting properties and structural characteristics of thylakoids developed under continuous light. These structural and functional observations are consistent with the hypothesis that system II units are organized in the photosynthetic membrane as individual and discrete entities, the morphological expression of which correspond to exoplasmic fracture face particles. They also support the model whereby energy transfers between physically connected system II units could occur across the partition between exoplasmic fracture face particles brought into contact in stacked regions.

Chlorophyll↗
PubMed1988

Timing of oviposition in mixed systems using bright light, dim light and darkness.

1. Two experiments were conducted to examine time of oviposition for hens exposed to continuous dim lighting, to dim lighting alternating with bright lighting in a 24 h cycle or to a mixed system using bright light, dim light and darkness. 2. Under continuous dim lighting (0.3 lux), the pattern of ovipositions was the same as that reported previously for constant darkness, more eggs being laid around midnight than around noon. 3. With alternating bright and dim phases, mean time of lay was approximately 16 h after the transition from bright to dim lighting, which was 3 h earlier than under the corresponding cycle of light and dark. This phase advance was the same whether the bright:dim ratio was 16:1 or 160:1. 4. Dim lighting (1.25 lux) preceded by a period of normal lighting (5 lux or 50 lux) and followed by 8 h darkness was treated as part of the photoperiod. 5. It is concluded that, when there is no darkness, a period of dim lighting is treated as darkness, provided the contrast between bright and dim phases is sufficient. However, when darkness, dim light and bright light are all included in a cycle, the dim light is treated as part of the photoperiod, even though there may be a contrast between the brightly lit and dimly lit phases which, in the absence of darkness, would cause the dim phase to be treated as dark.

Animals↗
PubMed2006

Reversal by green light of blue light-stimulated stomatal opening in intact, attached leaves of Arabidopsis operates only in the potassium-dependent, morning phase of movement.

Green light reversal of blue light-stimulated stomatal opening was discovered in isolated stomata. The present study shows that the response also occurs in stomata from intact leaves. Arabidopsis thaliana plants were grown in a growth chamber under blue, red and green light. Removal of the green light opened the stomata and restoration of green light closed them to baseline values under experimental conditions that rule out a mesophyll-mediated effect. Assessment of the response to green light over a daily time course showed that the stomatal sensitivity to green light was observed only in the morning, which coincided with the use of potassium as a guard cell osmoticum. Sensitivity to green light was absent during the afternoon phase of stomatal movement, which was previously shown to be dominated by sucrose osmoregulation in Vicia faba. Hence, the shift away from potassium-based osmoregulation in guard cells is further postulated to entail a shift from blue light to photosynthesis as the primary component of the stomatal response to light. Stomata from intact leaves of the zeaxanthin-less, npq1 mutant of Arabidopsis failed to respond to the removal or restoration of green light in the growth chamber, or to short, high fluence pulses of blue or green light. These data confirm previous studies showing that npq1 stomata are devoid of a specific blue light response. In contrast, stomata from intact leaves of phot1 phot2 double mutant plants had a reduced but readily detectable response to the removal of green light and to blue and green pulses.

Arabidopsis↗
PubMed1991

Light-activated heterotrophic growth of the cyanobacterium Synechocystis sp. strain PCC 6803: a blue-light-requiring process.

A glucose-tolerant strain of Synechocystis sp. strain 6803 will not grow on glucose under complete darkness unless given a daily pulse of white light, typically 5 min of 40 mumol m-2 s-1 (light-pulsed conditions). The light pulse is insufficient for photoautotrophy, as glucose is required and growth yield is dependent on glucose concentration. Growth rate is independent of fluence, but growth yield is dependent on fluence, saturating at 40 to 75 mumol m-2 s-1. A Synechocystis strain 6803 psbA mutant strain grows under light-pulsed conditions at rates similar to those for the glucose-tolerant strain, indicating that photosystem II is not required for growth. The relative spectral sensitivity of the growth of light-pulsed cultures (growth only in blue light, 400 to 500 nm, maximum at 450 nm) precludes energetic contribution from cyclic electron transport around photosystem I. Pulses of long-wavelength light (i.e., 550 and 650 nm) did not support the growth of Synechocystis strain 6803 and, when supplied before or after a blue-light pulse, did not inhibit blue-light-stimulated growth of Synechocystis strain 6803. We conclude that the required blue-light pulse does not support growth via photosynthetic electron transport but appears instead to function as an environmental signal regulating heterotrophic metabolism, cell division, or other photomorphogenic processes. We have termed the growth of Synechocystis strain 6803 pulsed with light and kept otherwise in complete darkness light-activated heterotrophic growth. This observation of a blue-light requirement for the growth of Synechocystis strain 6803 represents a novel blue light effect on the growth of a cyanobacterium.

Cyanobacteria↗
PubMed1990

The protective effect of ascorbic acid in retinal light damage of rats exposed to intermittent light.

Retinal light damage in dark-reared rats supplemented with ascorbic acid and exposed to multiple doses of intermittent light was studied and compared with damage in unsupplemented dark-reared and cyclic-light-reared rats. The extent of photoreceptor cell loss from intense light exposure was determined by whole-eye rhodopsin levels and retinal DNA measurements two weeks after light treatment. Two weeks after 3 or 8 hr of intermittent light, ascorbate-supplemented animals had rhodopsin and retinal DNA levels that were two to three times higher than in unsupplemented dark-reared rats. In both types of rats rhodopsin levels were influenced by the number of light doses, the duration of light exposure, and to a lesser extent, by the length of the dark period between exposures. Rhodopsin levels in the dark-reared ascorbate-supplemented rats were significantly higher than in unsupplemented dark-reared rats, and were similar to the levels in unsupplemented cyclic-light-reared animals. Ascorbate treatment had no effect on the rate of rhodopsin bleaching. However, regeneration was greater in supplemented rats after multiple 1-hr light exposures. Intermittent light also resulted in lower ascorbate levels in the retinas of supplemented and unsupplemented rats, with dramatic losses from the retinal pigment epithelium (RPE)-choroid in both types of animals. We conclude that ascorbic acid protects the eye by reducing the irreversible Type I form of light damage in dark-reared rats. Ascorbate appears to shift light damage to the Type II form typical of cyclic-light-reared animals.

Animals↗
PubMed1998

Smokers' misperceptions of light and ultra-light cigarettes may keep them smoking.

INTRODUCTION: This study examined smokers' understanding of the relative tar deliveries of Ultra-light, Light, and Regular cigarettes, reasons for smoking Ultra-light/Light cigarettes, and the likelihood of both quitting smoking and switching to Regular cigarettes if they came to learn that one Ultra-light/Light cigarette gave the same amount of tar as one Regular cigarette. DESIGN: Ten- to fifteen-minute random-digit-dialed, computer-assisted telephone interviews (CATI) were conducted with both a national probability sample (n = 788) and a state random sample (n = 266) of daily smokers over the age of 18. RESULTS: Less than 10% of smokers in the national sample and only 14% of smokers in the state sample knew that one Light cigarette could give the same amount of tar as one Regular cigarette. Less than 10% of smokers in the state sample knew that one Ultra-light cigarette could give the same amount of tar as one Regular cigarette. Thirty-two percent of the Light and 26% of the Ultra-light smokers in the national sample, and 27% of Light and 25% of Ultra-light smokers in the state sample, said they would be likely to quit smoking if they learned one Light/Ultra-light equaled one Regular. CONCLUSION: Many Light and Ultra-light smokers are smoking these cigarettes to reduce the risks of smoking and/or as a step toward quitting. However, these smokers are unaware that one Ultra-light/Light cigarette can give them the same amount of tar and nicotine as one Regular cigarette. Many of the Ultra-light/Light smokers sampled in this study stated that they would be likely to quit if they knew this information. Mistaken beliefs about low-yield brands are reducing intentions to quit smoking.

Adult↗
PubMed1985

Protein-protein interactions in contact activation of blood coagulation. Binding of high molecular weight kininogen and the 5-(iodoacetamido) fluorescein-labeled kininogen light chain to prekallikrein, kallikrein, and the separated kallikrein heavy and light chains.

Binding of the 5-(iodoacetamido)fluorescein (IAF)-labeled high molecular weight (HMW) kininogen light chain to prekallikrein and D-Phe-Phe-Arg-CH2Cl-inactivated kallikrein was monitored by a 0.040 +/- 0.002 increase in fluorescence anisotropy. Indistinguishable average dissociation constants and stoichiometries of 14 +/- 3 nM and 1.1 +/- 0.1 mol of prekallikrein/mol of IAF-light chain and 17 +/- 3 nM and 0.9 +/- 0.1 mol of kallikrein/mol of IAF-light chain were determined for these interactions at pH 7.4, mu 0.14 and 22 degrees C. Prekallikrein which had been reduced and alkylated in 6 M guanidine HCl lost the ability to increase the fluorescence anisotropy of the IAF-kininogen light chain, suggesting that the native tertiary structure was required for tight binding. The kallikrein heavy and light chains were separated on the basis of the affinity of the heavy chain for HMW-kininogen-Sepharose, after mild reduction and alkylation of kallikrein under nondenaturing conditions. Under these conditions, alkylation with iodo [14C]acetamide demonstrated that only limited chemical modification had occurred. Binding of the IAF-kininogen light chain to the isolated alkylated kallikrein heavy chain, when compared to prekallikrein and kallikrein, was characterized by an indistinguishable increase in fluorescence anisotropy, average dissociation constant of 14 +/- 3 nM, and stoichiometry of 1.2 +/- 0.1 mol of kallikrein heavy chain/mol of IAF-light chain. In contrast, no binding of the D-Phe-Phe-Arg-CH2Cl-inactivated kallikrein light chain was detected at concentrations up to 500 nM. Furthermore, 300 nM kallikrein light chain did not affect IAF-kininogen light chain binding to prekallikrein, kallikrein, or the kallikrein heavy chain. The binding of monomeric single chain HMW-kininogen to prekallikrein, kallikrein, and the kallikrein heavy and light chains was studied using the IAF-kininogen light chain as a probe. Analysis of the competitive binding of HMW-kininogen gave average dissociation constants and stoichiometries of 12 +/- 2 nM and 1.2 +/- 0.1 mol of prekallikrein/mol of HMW-kininogen, 15 +/- 2 nM and 1.3 +/- 0.1 mol of kallikrein/mol of HMW-kininogen, 14 +/- 3 nM and 1.4 +/- 0.2 mol of kallikrein heavy chain/mol of HMW-kininogen, and no detectable effect of 300 nM kallikrein light chain on these interactions. We conclude that a specific, nonenzymatic interaction between sites located exclusively on the light chain of HMW-kininogen and the heavy chain of kallikrein or prekallikrein is responsible for the formation of 1:1 noncovalent complexes between these proteins.

Binding, Competitive↗
PubMed2000

Early light reduction for preventing retinopathy of prematurity in very low birth weight infants.

BACKGROUND: This section is under preparation and will be included in the next issue. OBJECTIVES: To answer the question: "Among very low birth weight infants, what is the effect of reducing early environmental light exposure on the incidence of any "Acute ROP", or "Poor ROP Outcomes"? SEARCH STRATEGY: Searches were made of the Cochrane Neonatal Group Register of Controlled Trials, Medline, EMBASE, the Cochrane Library, previous reviews including cross references, abstracts, conference and symposia proceedings, and expert informants. The search terms used were [retrolental fibroplasia or retinopathy of prematurity] and [light or light/ae or lighting or lighting/ae or light/tu or lighting/st]. SELECTION CRITERIA: Randomized or quasi-randomized controlled trials that reduced light exposure to premature infants within the first 7 days following birth were considered for this review. DATA COLLECTION AND ANALYSIS: Data on clinical outcomes including any Acute ROP and Poor ROP Outcome were excerpted by both reviewers independently and consensus reached. Data analysis was conducted according to the standards of the Neonatal Cochrane Review Group. MAIN RESULTS: Data from four recent randomized trials, and one much older quasi-randomized trial failed to show any reduction in Acute ROP, or Poor ROP Outcome with the reduction of light to premature infants' retinas. The number of infants studied to date allows 95% confidence that IF there were a true difference being missed, it would be smaller than 7 percentage points on a background of 54% of all infants under 2 kg developing ROP. REVIEWER'S CONCLUSIONS: Decreasing retinal light exposure in premature infants is very unlikely to reduce the incidence of ROP.

Humans↗
PubMed2001

Blue light adaptation and desensitization of light signal transduction in Neurospora crassa.

The ascomycete Neurospora crassa has the capacity of adapting to a given light quantity, leading to transient blue light responses under continuous light conditions. Here, we present an investigation of this photoadaptation phenomenon. We demonstrated previously that two proteins of the Neurospora blue light signal transduction chain, WC1 and WC2, are subject to light-dependent phosphorylation. WC1 was phosphorylated in parallel with the transient increase in transcript levels of light-regulated genes. Using the light-dependent phosphorylation of WC1 as a marker for an active signalling state of WC1, we show that the transiency of Neurospora blue light responses results from desensitization of the photoreceptor and/or the signalling cascade. Furthermore, a Neurospora mutant was characterized that revealed a specific defect in photoadaptation. In this mutant, the transient expression of light-regulated genes under continuous light, the temporary insensitivity after a light pulse and the capability of differentiating between and adapting to low and high light intensities were abolished. The corresponding protein seems to represent a central component of a negative feedback desensitization mechanism. This negative feedback regulation requires continuous and light-dependent protein de novo biosynthesis.

Adaptation, Physiological↗
PubMed1981

Biosynthesis of the light-harvesting chlorophyll a/b protein. Control of messenger RNA activity by light.

1. Antibodies raised against the 26000-Mr polypeptides of the light-harvesting chlorophyll a/b proteins of pea leaves specifically immunoprecipitated two 32000-Mr polypeptides synthesized when pea leaf poly(A)-containing RNA was translated in vitro. On the basis of immunochemical relatedness and by comparison of their partial tryptic digestion products, the 32000-Mr products formed in vitro are identified as precursors to the authentic polypeptides of the light-harvesting chlorophyll a/b complex. 2. The specificity of the immunoprecipitation permitted the development of an assay for the cellular levels of translationally active light-harvesting protein mRNA in plants exposed to different light regimes. Low levels of the mRNAs were detectable in dark-grown plants. Exposure to continuous illumination caused these levels to increase by at least ten-fold and led to the appearance of large quantities of the light-harvesting chlorophyll a/b complex. In plants exposed to intermittent illumination (2 min of white light every 2 h for 2 days), the light-harvesting complex did not accumulate, although levels of mRNA specifying the polypeptides of the complex were high (50% of those in continuously illuminated plants). 3. Messenger RNAs encoding the light-harvesting proteins were detected in polysomes of intermittently illuminated leaves. These polysomes were active in a wheat-germ 100 000 X g supernatant "run-off" system, to form light-harvesting protein precursors, under conditions when only nascent polypeptide chains initiated in vivo were elongated and terminated. These results demonstrate that the inability of intermittently illuminated leaves to accumulate the light-harvesting proteins is not due to a selective inhibition of the translation of the corresponding mRNAs. 4. Intermittently illuminated leaves were labelled with [35S]methionine in darkness, and incorporation of radioisotope into the light-harvesting proteins and their precursors was assayed immunologically. No pool of untransported or unprocessed 32000-Mr precursor polypeptides could be detected in the soluble fraction (cytoplasm and stroma). However, low levels of the mature 26000-Mr polypeptides were detected in the membrane fraction. It is concluded that the newly synthesized light-harvesting chlorophyll a/b protein fail to accumulate in intermittently illuminated leaves because they undergo rapid turnover. The site of light-harvesting protein breakdown is probably the thylakoid membrane, and the cause of breakdown is probably the absence of chlorophyll a and chlorophyll b molecules that are required for eventual stabilization of the proteins within the photosynthetic membrane.

Chlorophyll↗
PubMed2000

An in vitro study of microleakage of occlusal composite restorations polymerized by a conventional curing light and a PAC curing light.

With the increase in usage of bonded dentistry procedures, so comes the development of advanced technology to polymerize it. The purpose of this in vitro study was to evaluate the microleakage of sealants and resin restorations utilizing two different curing lights. The conventional Ortholux curing light (OCL) and the Plasma Arc Curing (PAC) light attached to the KCP air abrasion unit of American Dental Technologies were utilized to polymerize sealants and resin restorations on extracted third molars and premolars. Forty-eight caries-free teeth were divided into 4 groups of 12 specimens. Occlusal sealants were polymerized on groups 1 and 3. Class I composite resin restorations were polymerized on groups 2 and 4. The PAC light polymerized group 2 and the OCL was used for groups 3 and 4. Therefore a comparison between the two light sources' polymerization could be measured by evaluating the microleakage of the two sets of specimens. The PAC light polymerized the Z100 adhesive for 5 seconds and the OCL polymerized it for 10 seconds. The Z100 A-1 composite placed in two 1 mm increments was polymerized with the PAC light for 10 seconds and the OCL for 40 seconds. The teeth were thermocycled 200 times between 5 degrees C and 55 degrees C. The teeth, coated with fingernail polish excluding the occlusal surface, were soaked in 5% methylene blue for 4 hours, removed, and rinsed with water. The teeth were invested in clear resin (Castin Craft) and sectioned longitudinally using an Isomet diamond saw. Microleakage was scored "0" if no leakage was present. Scored "1" if the leakage was present to 1/2 the preparation depth. Scored "2" if the leakage was present past the 1/2 way point but not to the pulpal floor of the preparation. A score of "3" was used if the leakage reached the pulpal floor. Unpaired t-Tests were used to statistically analyze the data. A significant difference (p < 0.05) was found between groups 1 and 3. The PAC light used to polymerize the sealants in group 1 produced no microleakage. A significant difference (p < .005) was found between groups 2 and 4. The OCL produced less microleakage (16 and 20 out of 24 sections) than the resin restorations polymerized with the PAC (only 8 of 24 sections with no microleakage. The 10-second cure by the PAC light appears to be insufficient in polymerizing a class I composite resin restoration. Possibly longer exposures with the PAC would produce less microleakage. Further research needs to be performed to evaluate if the PAC light, with an intensity of 1196 mW/cm2, may be producing high levels of strain within the setting composite. This strain may be responsible for the statistically significant increase in microleakage of the restorations polymerized with the PAC light.). It appears the PAC light would be best utilized to cure sealants and/or possibly polymerize orthodontic brackets. The conventional curing light appears to remain the best choice for polymerizing class I composite restorations.

Bicuspid↗
PubMed2000

Light quantity controls leaf-cell and chloroplast development in Arabidopsis thaliana wild type and blue-light-perception mutants.

Plants acclimate to changes in light quantity by altering leaf-cell development and the accumulation of chloroplast components, such that light absorption is favoured under limiting illumination, and light utilisation under non-limiting conditions. Previous evidence suggests an involvement of a high-light photosynthetic redox signal in the down-regulation of the accumulation of the light-harvesting complexes of photosystem 11 (Lhcb) and the expression of the Lhcb genes. and of a blue-light signal in the control of leaf development and in the increase in photosynthetic capacity, as affected by the accumulation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). We examined the internal anatomy of leaves, the ultrastructure of chloroplasts and accumulation of light-harvesting complexes and Rubisco in wild-type Arabidopsis thaliana (L.) Heynh. and in mutants in each of the three known blue-light photoreceptors, cryptochrome 1, cryptochrome 2 and phototropin, as well as a mutant in both cryptochromes. Our results indicate an extensive capacity of the Arabidopsis mesophyll cells to adapt to high light fluence rate with an increase in palisade elongation. Under high light, chloroplasts showed increased starch accumulation and reductions in the amount of granal thylakoids per chloroplast, in the proportion of chlorophyll b relative to chlorophyll a, and in the accumulation of the major Lhcb polypeptides. The responses were similar for all four mutants, with respect to their wild types. The results are consistent with either a complete redundancy in function between cryptochromes and phototropin, or their absence of involvement in the light-quantity responses tested. We observed minimal effects of light quantity on Rubisco accumulation over the range of fluence rates used, and conclude that elongation of palisade mesophyll cells and accumulation of Rubisco are controlled separately. This indicates that light acclimation must be the result of a number of individual elementary responses. Quantitative differences in the acclimatory responses were observed between the Landsberg erecta and Columbia wild-type ecotypes used.

Arabidopsis↗
PubMed2002

The effects of prior light history on the suppression of melatonin by light in humans.

We investigated the impact of light exposure history on light sensitivity in humans, as assessed by the magnitude of the suppression of melatonin secretion by nocturnal light. The hypothesis was that following a week of increased daytime bright-light exposure, subjects would become less sensitive to light, and that after a week of restriction to dimmer light they would become more sensitive. During the bright week, subjects (n = 12) obtained 4.3 +/- 0.4 hr of bright light per day (by going outside and using light boxes indoors). During the dim week, they wore dark goggles (about 2% light transmission) when outside during daylight and spent 1.4 +/- 0.9 hr per day outside. Saliva samples were obtained every 30 min for 7 hr in dim light (<15 lux) on two consecutive nights (baseline and test night) at the end of each week. On the test night, 500 lux was presented for 3 hr in the middle of the collection period to suppress melatonin. There was significantly more suppression after the dim week compared with after the bright week (to 53 versus 41% of the baseline night values, P < 0.05). However, there were large individual differences, and the difference between the bright and dim weeks was most pronounced in seven of the 12 subjects. Possible reasons for these individual differences are discussed, including the possibility that 1 wk was not long enough to change light sensitivity in some subjects. In conclusion, this study suggests that the circadian system's sensitivity to light can be affected by a recent change in light history.

Adult↗
PubMed1977

A light-activated GTPase in vertebrate photoreceptors: regulation of light-activated cyclic GMP phosphodiesterase.

We have been studying the mechanism by which light and nucleoside triphosphates activate the discmembrane phosphodiesterase (oligonucleate 5'-nucleotidohydrolase; EC 3.1.4.1) in frog rod outer segments. GTP is orders of magnitude more effective than ATP as a cofactor in the light-dependent activation step. GTP and the analogue guanylyl-imidodiphosphate function equally as allosteric activators of photoreceptor phosphodiesterase rather than participating in the formation of a phosphorylated activator. Moreover, we have found a light-activated (5-fold) GTPase which participates in the modulation of photoreceptor phosphodiesterase. This GTPase activity appears necessary for the reversal of phosphodiesterase activation in vitro and may play a critical role in the in vivo regulation of light-sensitive phosphodiesterase. The K(m) for GTP in the light-activated GTPase reaction is <1 muM. The light sensitivity of this GTPase (number of photons required for half-maximal activation) is identical to that of light-activated phosphodiesterase. The GTPase action spectrum corresponds to the absorption spectrum of rhodopsin. There is, in addition, a light-insensitive GTPase activity with a K(m) for GTP of 90 muM. At GTP concentrations above 5 muM, there is no appreciable activation of GTPase activity by light. The substrate K(m) values for guanylate cyclase, light-activated GTPase, and light-activated phosphodiesterase order an enzyme array that might permit light to simultaneously cause the hydrolysis of both the substrate and product of guanylate cyclase. These findings reveal yet another facet of light regulation of photoreceptor/cyclic GMP levels and also provide a striking analogy to the GTP regulation of nonphotoreceptor, hormone-sensitive adenylate cyclase.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed2006

Growth towards light as an adaptation to high light conditions in Chara branches.

Growth of plants or plant organs towards more light is commonly interpreted as an adaptation to low light conditions. Here, we show for the first time, in a study of charophyte branches, a growth-based orientation towards light functioning as a mechanism to protect the plant from excessive light. Two Chara species were exposed to five different intensities of photosynthetically active radiation and species traits and pigmentation were measured. Branches of plants exposed to higher light intensities were convergent and pointed steeply upwards, whereas those exposed to lower light intensities grew nearly straight and were less inclined. Only branches that increased in length during the experiments reacted to differences in light intensity. This indicates that branch orientation is determined by a light-dependent growth reaction. Orientation of charophyte branches towards light is accompanied by a decrease in chlorophyll a (Chla) content and a lower Chla : carotenoid ratio, which clearly indicates that the plant is taking protective measures against potentially damaging excess light conditions. We suggest that the growth-based orientation of Chara branches towards light may protect sexual organs, which grow on adaxial branch sides, from light damage. In addition, the upward orientation of branches might lead to increased light transmission within dense charophyte beds, thus enabling an enhanced gross production.

Adaptation, Physiological↗