Increased photoreversal of ultraviolet injury by flashing light

1. Photoreversal of ultraviolet (UV) injury was studied in the ciliate protozoan Tetrahymena pyriformis (geleii) strain W, cultured in the absence of other living organisms. The division pattern of progeny of single animals was followed in hanging drop preparations. 2. A sublethal dose of 450 ergs/mm.² of monochromatic UV of wave length 2654 A produces a lag before the first division followed by a period of cessation of fission after the second division. This cessation sometimes lasts as long as 6 weeks, during which time the animals become smaller and rounder and more opaque. Organisms about to resume division increase in size and transparency; after a few divisions the animals regain their normal division rate. 3. The effect of UV ranging in intensity from 5 to 15 ergs/mm.²/sec. was found to obey the reciprocity law quite well for the UV effect on the division pattern of T. pyriformis. However, the same dose at lower and at higher intensities was less effective. 4. The effect of a dose of UV delivered at high intensity (19 ergs/mm.²/sec.) could be increased by flashing the light, indicating that the system became saturated in the continuous light. 5. A photoreversing dose of monochromatic blue light of wave length 4350 A was found to be more effective when delivered as continuous light at a low intensity, or as intermittent light at a high intensity, rather than as continuous light at the high intensity—indicating that a dark mechanism participates in photoreversal. 6. The time for the dark reaction was determined to be of the order of a few hundredths of a second in experiments in which different lengths of dark period were used while maintaining a constant light period of 0.0025 second. 7. For Colpidium colpoda the efficiency of a given dose of photoreversing light was increased by flashing the light. 8. The present experiments are interpreted in terms of data available in the literature.     

Quantum relations in photoreactivation of Colpidium

1. The amount of visible or long ultraviolet light (UV) required to photoreactivate Colpidium colpoda injured with known dosages of short UV (2654 A) was determined. 2. The effect of the short UV was tested by the delay in division of exposed animals compared to controls. Photoreactivation was tested by the effect of postillumination on the delay of division of treated colpidia compared to controls. 3. Colpidia were used in two physiological states: well fed and starved in balanced medium for 48 hours. The latter are much more sensitive to short UV although less susceptible of photoreactivation. 4. Photoreactivation occurred over the entire span from 3350 A to 4350 A for the well fed colpidia, from 3130 A to 5490 (green) for starved colpidia. 5. The photoreactivating effect of a single quantum of blue (4350 A) or long UV (3660 A) delivered per quantum of 2654 A used to injure colpidia was too slight to be considered significant. The effect of 10 quanta was usually more pronounced, but only after 100 quanta had been delivered was the photoreactivation nearly maximal for well fed colpidia. 6. The quantum requirement for maximal photoreactivation of the starved animals was greater at all wave lengths tried: 3660, 4050, 4350, and 5460 A being of the order of 800 incident quanta per incident quantum of 2654 A. 7. The transmission of UV(2654 A), blue, yellow, and red light by a suspension of colpidia was determined. 8. Large dosages of blue, violet, or long UV were slightly injurious to starved colpidia. In a few cases large dosages of 3660 A killed starved colpidia, especially after a non-lethal dose of short UV(2654 A). 9. Photoreactivation seems to be a balance between the slight injurious effect produced by the visible light or UV of long wave lengths and the injury produced by short wave length UV. 10. Possible reasons for the large number of quanta of photoreactivating light required per quantum of short UV are discussed.     

Photoreversal of nuclear and cytoplasmic effects of short ultraviolet radiation on Paramecium caudatum

1. Irradiation with three short ultraviolet (UV) wave lengths, 226, 233, and 239 mµ rapidly immobilizes Paramecium caudatum, the dosage required being smaller the shorter the wave length. 85 per cent of paramecia immobilized with wave length 226 mµ recover completely. Recovery from immobilizing doses is less the longer the wave length. 2. Irradiation continued after immobilization kills the paramecia in a manner which is markedly different for very short (226, 233, and 239 mµ) and longer (267 mµ) wave lengths. 3. An action spectrum for immobilization in P. caudatum was determined for the wave lengths 226, 233, 239, 248, and 267 mµ, and found to resemble the absorption of protein and lipide in the wave length region below 248 mµ. Addition of these data to those of Giese (1945 b) gives an action spectrum resembling the absorption by albumin-like protein. 4. Division of P. caudatum is delayed by doses of wave lengths 226, 233, and 239 mµ which cause immobilization, the longest wave length being most effective. 5. Immobilization at any of the wave lengths tested (226, 233, 239, 248, 267 mµ) is not photoreversible when UV-treated paramecia are concurrently illuminated. 6. Division delay resulting from immobilizing doses of 226, 233, and 239 mµ is photoreversible by exposure to visible light concurrently with the UV. 7. Division delay induced by exposure to wave length 267 mµ is reduced by exposure to visible light applied concurrently with UV or immediately afterwards. 8. The data suggest that the shortest UV wave length tested (226 mµ) affects the cytoplasm selectively, because it is absorbed superficially as indicated by unilateral fluorescence in UV. Consequently it immobilizes paramecia rapidly but has little effect on the division rate because little radiation reaches the nucleus. 9. The data support the view that nuclear effects of UV are readily photoreversed but cytoplasmic effects are not.     

Efficacy of UV Irradiation in Inactivating Cryptosporidiumparvum Oocysts

To evaluate the effectiveness of UV irradiation in inactivating Cryptosporidium parvum oocysts, the animal infectivities and excystation abilities of oocysts that had been exposed to various UV doses were determined. Infectivity decreased exponentially as the UV dose increased, and the required dose for a 2-log₁₀ reduction in infectivity (99% inactivation) was approximately 1.0 mWs/cm² at 20°C. However, C. parvum oocysts exhibited high resistance to UV irradiation, requiring an extremely high dose of 230 mWs/cm² for a 2-log₁₀ reduction in excystation, which was used to assess viability. Moreover, the excystation ability exhibited only slight decreases at UV doses below 100 mWs/cm². Thus, UV treatment resulted in oocysts that were able to excyst but not infect. The effects of temperature and UV intensity on the UV dose requirement were also studied. The results showed that for every 10°C reduction in water temperature, the increase in the UV irradiation dose required for a 2-log₁₀ reduction in infectivity was only 7%, and for every 10-fold increase in intensity, the dose increase was only 8%. In addition, the potential of oocysts to recover infectivity and to repair UV-induced injury (pyrimidine dimers) in DNA by photoreactivation and dark repair was investigated. There was no recovery in infectivity following treatment by fluorescent-light irradiation or storage in darkness. In contrast, UV-induced pyrimidine dimers in the DNA were apparently repaired by both photoreactivation and dark repair, as determined by endonuclease-sensitive site assay. However, the recovery rate was different in each process. Given these results, the effects of UV irradiation on C. parvum oocysts as determined by animal infectivity can conclusively be considered irreversible.     

Studies of Chloroplast Development in Euglena: XIV. Sequential Interactions of Ultraviolet Light and Photoreactivating Light in Green Colony Formation

Photoreactivation (PR) of green colony-forming ability in Euglena is pH-insensitive from pH 6.0 to 8.0 and temperature-sensitive with a maximum rate at 35°C. There is no PR at 0°C. The rate of PR varies with the growth stage of the cells; PR of exponential phase cells is slower than that of stationary phase cells. The reciprocity rule holds for PR over a 6-fold range of intensity. The shape of PR curves is a function of the UV dose; there appears to be a progressive increase in multiplicity until a limiting multiplicity is reached as indicated by the fact that curves for high doses are superposable. Dark-grown and light-grown cells give the same PR response for comparable UV doses. UV inactivation of cells which have been treated with UV and then with PR light shows that, if the PR dose is sufficiently large, the same UV-inactivation curve is obtained as for nonpretreated control cells. Doses of PR lower than the saturating dose produce UV-inactivation curves, the ultimate slopes of which are parallel to the slope of the control curve, but which show reduced multiplicity. The multiplicity of these curves increases with increasing PR dose. The UV inactivation of green colony-forming ability in Euglena is completely photoreactivable at the doses studied, in contrast with the UV inactivation of colony-forming ability, which occurs at considerably higher UV doses and behaves like most other photoreactivable systems, showing a photoreactivable sector of 0.32.     

Studies on the vegetative life cycle of Chlamydomonas reinhardi Dangeard in synchronous culture I. Some characteristics of the cell cycle

Cells of Chlamydomonas reinhardi Dangeard were grown synchronouslyunder a 12 hr light-12 hr dark regime. Time courses of nucleardivision, chloroplast division, "apparent cytokinesis" and zoosporeliberation were followed during the vegetative cell cycle inthe synchronous culture. Liberation of zoospores occurred atabout 23–24 hr after the beginning of the light periodat 25°C. Four zoospores were produced per mother cell underthe conditions used. At lower temperatures, the process of zoosporeliberation as well as length of the cell cycle was markedlyprolonged, but the number of zoospores produced per mother cellwas approximately the same. At different light intensities,lengths of the cell cycle were virtually the same, while thenumber of zoospores liberated was larger at higher rather thanat lower light intensities. During the dark period, nuclear division, chloroplast divisionand apparent cytokinesis took place, in diis order, and proceededless synchronously than did the process of zoospore liberation.When the 12 hr dark period was replaced with a 12 hr light periodduring one cycle, the time of initiation as well as the durationof zoospore liberation was litde affected in most cases, whereasnuclear division, chloroplast division and apparent cytokinesiswere considerably accelerated by extended illumination. Whenalgal cells which had been exposed to light for 24 hr were furtherincubated in the light, zoospore liberation started much earlierand proceeded far less synchronously, compared with that under12 hr light-12 hr dark alternation. (Received October 12, 1970; )     

Increased inactivation of Saccharomyces cerevisiae by protraction of UV irradiation.

The principle of equi-effectivity of the product of intensity and exposure time (principle of Bunsen-Roscoe) of UV irradiation has been assumed to be valid for the inactivation of microorganisms in general. Earlier studies claimed higher survival of Escherichia coli B/r with fractionated irradiation compared with single-exposure survival. However, data on the inactivation effect of protraction of UV irradiation are not available. By means of a specially designed UV irradiation apparatus which secured absolute UV dose measurements throughout the experiments, the effects of variation of UV irradiation intensities (253.7 nm) and exposure times were tested on the inactivation of a bacterial virus (Staphylococcus aureus phage A994), a vegetative bacterial strain (E. coli ATCC 25922), and bacterial spores (Bacillus subtilis ATCC 6633) as well as three haploid laboratory strains (RC43a, YNN281, and YNN282) and two diploid strains (commercial bakery yeast strain and laboratory strain YNN281 x YNN282) or yeast (Saccharomyces cerevisiae) and spores of the latter diploid yeast strain. Each test organism was exposed to three UV intensities (0.02, 0.2, and 2 W/m2), with corresponding exposure times resulting in three dose levels for each intensity. Differences in inactivation rates were tested by analyses of variance and Newman-Keuls tests. Virus and bacteria showed no differences in inactivation rates by variation of intensities and exposure times within selected UV doses; hence, the principle of Bunsen-Roscoe could not be rejected for these strains. However, in the eukaryotic test strains of S. cerevisiae longer exposure times with lower intensities led to enhanced inactivation in both haploid and diploid strains, with a more pronounced effect in the diploid yeast strains, whereas in yeast spores in this dose rate effect could not be observed.     

Light Adaptation in the Ventral Photoreceptor of Limulus

Light adaptation in both the ventral photoreceptor and the lateral eye photoreceptor is a complex process consisting of at least two phases. One phase, which we call the rapid phase of adaptation, occurs whenever there is temporal overlap of the discrete waves that compose a light response. The recovery from the rapid phase of adaptation follows an exponential time-course with a time constant of approximately 75 ms at 21°C. The rapid phase of adaptation occurs at light intensities barely above discrete wave threshold as well as at substantially higher light intensities with the same recovery time-course at all intensities. It occurs in voltage-clamped and unclamped photoreceptors. The kinetics of the rapid phase of adaptation is closely correlated to the photocurrent which appears to initiate it after a short delay. The rapid phase of adaptation is probably identical to what is called the "adapting bump" process. At light intensities greater than about 10 times discrete wave threshold another phase of light adaptation occurs. It develops slowly over a period of ½ s or so, and decays even more slowly over a period of several seconds. It is graded with light intensity and occurs in both voltage-clamped and unclamped photoreceptors. We call this the slow phase of light adaptation.     

The effect of nutritional state on photoreversal of ultraviolet injuries in Didinium nasutum

1. The effect of the nutritional state of Didinium nasutum on its resistance to short ultraviolet (UV) radiation (2654 A) and its recovery from the injury following illumination with visible light (4350 A, blue) was studied. 2. The resistance of a didinium to UV is considerably increased by feeding it a paramecium 15 to 60 minutes before exposure to UV. If fed just before exposure to UV, the resistance is less than that of an unfed control. 3. Photoreversal is only slightly greater in didinia fed after irradiation with UV but before exposure to visible light as compared to those fed after exposure to visible light. 4. Irradiated paramecia are eaten by didinia, provided they have not started to cytolyze. Didinia fed on irradiated paramecia divide at about the same rate as controls or slightly faster. 5. The available stock of Didinium declines in vigor with lapse of time after excystment, as measured by the time required for division. The sensitivity of Didinium to UV did not change essentially during the 5 month period over which tests were made. 6. The theoretical implications of the results are considered.     

Effect of Light Intensity on Adaptation of Dunaliella to Very High Salt Concentrations

Light intensity was found to have a strong effect on the adaptationto high salt concentrations of a green microalga, Dunaliellaparva, normally grown at low and medium salt concentrations.At high light intensities (200 µmol m^–2 s^–1)the cell glycerol content increased in parallel with an increasein external salt concentration; protein synthesis and cell divisioncontinued with no period of arrest. At low light intensitiesno glycerol synthesis occurred as the external salt concentrationwas raised; protein synthesis and cell division were equallyarrested. The importance of high light intensity during theinitial phase of increase of salt concentration was demonstratedand was found to be a requirement for protein synthesis andcell division. In experiments designed to discover the effectof light intensity on cells growing in media in which the saltconcentration was kept constant, it was confirmed that the lightintensity needed for growth increases as the salt concentrationof the medium is increased. Key words: Dunaliella, salt concentration, light intensity, growth rates     

Retardation of Division of Three Ciliates by Intermittent and Continuous Ultraviolet Radiations at Different Temperatures

The same dosage of ultraviolet (UV) radiation retards division of several protozoans more effectively when the light is intermittent than when it is continuous, and especially at temperatures of 25–35°C. At lower temperatures the difference between the effects of intermittent and continuous radiations is less marked. Somewhat similar results were obtained with the ciliates Paramecium caudatum, Blepharisma japonicum, and Colpidium colpoda, the disparity between intermittent and continuous light decreasing in the order given. The data are taken to indicate that thermochemical dark reactions succeed the absorption of UV radiations by the cells. In Blepharisma, besides initial delay in division, the cells stop dividing after one or two divisions, a "stasis" ensuing. Stasis is marked when the cells are irradiated at higher temperatures but is slight when they are irradiated at low temperatures, as if the temperature-sensitive reaction involved stasis (in all cases cultures are grown at 25°C). The data are related to findings in the literature.     

Some observations on the carbon dioxide burst in chlorella and chlamydomonas

In the course of mass spectrometer studies with the algae Chlorella and Chlamydomonas, data were obtained which indicate that the CO₂ burst and gulp are sensitive to oxygen. Furthermore, the CO₂ burst was found to be strongly suppressed when wave lengths shorter than 460 nanometers were blocked at intensities adequate to saturate photosynthesis. Under appropriate conditions at 30°, the CO₂ burst was interrupted by a brief CO₂ gulp and the post illumination gulp by a brief burst of CO₂. The post illumination gulp of CO₂ could be induced during illumination by interposition of a filter blocking wave lengths shorter than 460 nanometers. These data are discussed in relation to earlier reports of the phenomenon and briefly as they affect the detection of photorespiration.     

Effects of changes in ambient PAR and UV radiation on the nutritional quality of an Arctic diatom (Thalassiosira antarctica var. borealis)

Polyunsaturated fatty acids (PUFAs) are essential macromolecules that are synthesized by phytoplankton during spring bloom, and they play a key role in the Arctic food web. They are, however, considered to be sensitive to oxidation by UV radiation (280-400 nm). Changes in the food quality of primary producers may affect the transport of biomass and energy in the whole ecosystem. Using a common Arctic diatom, we looked at the effect of ambient and increased UV radiation on its nutritional quality, specifically, the fatty acid composition and elemental ratios. In May 2004, in the archipelago of Svalbard (79° N), a unialgal culture of Thalassiosira antarctica var. borealis was subjected to a 17-day experiment in outdoor aquaria. The diatoms were kept in semi-continuous culture (40 1) and exposed to three treatments with different levels of UV radiation: none (UV-shielded), ambient, and enhanced. Fatty acid composition, C:N:P ratios, photosynthetic pigment composition, optimum quantum yield of PSII, and cell numbers were analysed over the experimental period. An initial increase in PAR (photosynthetically active radiation, 400-700 nm) intensities profoundly affected the fatty acid composition and substantially inhibited the synthesis of PUFAs, but the relative amounts of PUFAs were not reduced by UV radiation. Enhanced UV radiation did, however, cause a significant reduction in optimum quantum yield of PSII and affected some fatty acids, mainly 18:0 and 16:1 n-7, during the first week of the experiment. Both ambient and enhanced UV radiation caused significantly lower C:P and N:P ratios. At the same time, these treatments elicited a higher relative content of the photoprotective pigments diadinoxanthin and diatoxanthin. After acclimation to the new light levels these effects faded off. Thus, brief periods with high light exposure may cause significant changes in photosynthetic activity and food quality, but the capacity for photo-acclimation seems high. The impact of UV radiation seems to be less important for food quality than that of PAR during a sudden rise in total light intensity.     

Studies of Chloroplast Development in Euglena: XV. Factors Influencing the Decay of Photoreactivability of Green Colony Formation

When UV-treated cells of Euglena gracilis var. bacillaris are incubated in the dark in a nutrient medium which permits cell division, they lose the ability to be photoreactivated. The rate of this loss increases with the UV dose. For any given UV dose, the rate of decay increases with increasing growth rate. The same phenomena are observed in light-grown and in dark-grown cells, although the sensitivity to UV of the light-grown cells is smaller by a factor of 1.7. The kinetics of photoreactivation (PR) change during the decay of photoreactivability only if the cells are incubated in growth medium. A UV-inactivation curve for cells photoreactivated only after appreciable PR shows the same slope as that for untreated cells (number of UV-sensitive targets). These results are discussed from the point of view of possible models.     

SYNCHRONOUS CULTURE OF CHLORELLA: I. KINETIC ANALYSIS OF THE LIFE CYCLE OF CHLORELLA ELLIPSOIDEA AS AFFECTED BY CHANGES OF TEMPERATURE AND LIGHT INTENSITY

1. Using the technique of synchronous culture, investigationsweremade of the effects of temperature and light-intensityon cellularlife cycle of Chlorella ellipsoidea. Some improvementsin theculture technique for obtaining a good synchrony of algalgrowthwere described.
2. By following the changes of averagecell volume and cell numberoccurring during culturing, therates of the following processesof life cycle were determined:(i) "growth" (or the increasein cell mass) occurring from thestage of smaller cells (D_a)to the stage of ripened cell (L₃),(ii) "ripening" (or processofformation of "nuclear substances"as estimated from the averagenumber of daughter cells formedfrom single mother cell), and(iii) " maturing and division" which leads to the full maturationof mother cells (L-cells)and their division into separate daughtercells (D-cells).
3. "Growth"and "ripening" were found to be dependent in light,"maturingand division" light-independent. The time requiredfor "growth"and "ripening" (C) is dependent on temperaturebut independentof light intensity, the onset of "maturing anddivision" occurringat the same time (D) of culturing undervaried light intensities.The average cell volume at this stage(L₃),however, was foundto be markedly modified by light intensity;larger with highertemperatures (see Fig. 4).
4. Changes in incubation temperature(under the condition of saturatinglight intensities) were foundto affect the life cycle in thefollowing way: (i) The timeof onset of "maturing and division"(D), varies markedly withculturing temperature; earlier athigher temperatures, (ii)The average cell volume at this stagealso depends on temperature; smaller at higher temperatures.
5. The average number of daughtercells (n) emerging from singlemother cells, was found to beuninfluenced by culturing temperature;(4.0–4.1 underthe conditions of the present study). Itwas found that thedivision number n is remarkably varied bychanging the lightintensity in the "growth" and "ripening"phases; 2.0 at 1 kilolux,3.7 at 5 kilolux, 4.2 at saturatinglight intensities (10 and25 kilolux). This finding was explainedby assuming a light-dependentformation of "nuclear substances"during the "growth" and "ripening"phases, the quantity of thesubstances in the cell at L₃ stagedeterminig the division number.
6. The experimental data wereanalyzed reaction kinetically, therate constants and othercharacteristics of the reactions constitutingthe processesof life cycle were determined, and values forthe apparent activationenergy for each reaction were computed.The reactions were discussedwith special reference to theirrelationship with photosyntheticprocess was discussed.

(Received November 7, 1959; )     

Enhancement models at high light intensities

The separate package and spillover models of photosynthetic enhancement are generalized and extended to light saturation assuming hyperbolic or exponential rate versus intensity curves. The available data are more consistent with the exponential model at high light intensities. Two significant conclusions follow: (a) the decreases in enhancement observed at high light intensities are accounted for by the curvatures of the light intensity curves for photosynthesis, and (b) nonlinearity of rate versus intensity curves may be sufficient to reduce enhancement greatly even when the intensity curves appear linear.     

Interplay between traveling wave propagation and amplification at the apex of the mouse cochlea

Sounds entering the mammalian ear produce waves that travel from the base to the apex of the cochlea. An electromechanical active process amplifies traveling wave motions and enables sound processing over a broad range of frequencies and intensities. The cochlear amplifier requires combining the global traveling wave with the local cellular processes that change along the length of the cochlea given the gradual changes in hair cell and supporting cell anatomy and physiology. Thus, we measured basilar membrane (BM) traveling waves in vivo along the apical turn of the mouse cochlea using volumetric optical coherence tomography and vibrometry. We found that there was a gradual reduction in key features of the active process toward the apex. For example, the gain decreased from 23 to 19 dB and tuning sharpness decreased from 2.5 to 1.4. Furthermore, we measured the frequency and intensity dependence of traveling wave properties. The phase velocity was larger than the group velocity, and both quantities gradually decrease from the base to the apex denoting a strong dispersion characteristic near the helicotrema. Moreover, we found that the spatial wavelength along the BM was highly level dependent in vivo, such that increasing the sound intensity from 30 to 90 dB sound pressure level increased the wavelength from 504 to 874 μm, a factor of 1.73. We hypothesize that this wavelength variation with sound intensity gives rise to an increase of the fluid-loaded mass on the BM and tunes its local resonance frequency. Together, these data demonstrate a strong interplay between the traveling wave propagation and amplification along the length of the cochlea.     

Metabolic conditions in Chlorella

1. The effect of nitrate reduction and assimilation on the CO(2)/O(2) quotient of gas exchange has been used as an index of the relative rates of carbon and nitrogen assimilation in Chlorella pyrenoidosa. Changes in over-all metabolism induced by starvation, high light intensity, and nitrogen deficiency have been studied in comparison with the metabolism of cells growing at light-limiting intensities. 2. Starvation, which results in depletion of carbohydrate reserves, gives rise to a high CO(2)/O(2) quotient ( approximately 0.9) during photosynthesis and, therefore, a high C/N assimilation ratio. Starved cells apparently restore their normal C/N ratio before becoming growing cells. 3. Under photosynthesis-saturating light intensities cells show the high CO(2)/O(2) quotient (0.9) indicative of a high C/N assimilation ratio. Return to low light intensities is followed by the abnormally low CO(2)/O(2) quotient ( approximately 0.4) of a low C/N assimilation ratio. High light intensity apparently gives rise to a condition of a limiting rate of nitrogen assimilation and to an overflow metabolism analogous to that found in other microorganisms. 4. Nitrogen deficiency leads to a completely carbohydrate metabolism in short time experiments and makes still more pronounced the effects characteristic of high light intensity alone. 5. Considerations of nutritional economy sustain the experimental evidence in establishing the metabolism of cells growing under light-limiting intensities as the normal or reference metabolic condition in Chlorella.     

LIGHT-DEPENDENT INHIBITION OF CELL DIVISION AND RHIZOID ELONGATION IN GEMMAE OF THE FERN,VITTARIA GRAMINIFOLIA

Gametophytes of the shoe-string fern Vittaria graminifolia produce linear, six-celled propagules called gemmae. The terminal cells of each gemma elongate into primary rhizoids in culture, and the inner body cells divide asymmetrically to produce prothallial or rhizoid initials. The initiation of both asymmetric cell division and rhizoid elongation is delayed by light intensities greater than 2 w/m². The maximal rates of cell division and rhizoid elongation are unaltered. A 24-hr pulse of high light intensity delays cell division and rhizoid elongation to the same extent, whenever applied during the first 3 d of culture. The model we propose for cell division hypothesizes the existence of a preparatory phase of finite duration prior to mitosis that is sensitive to light intensity. If a cell is irradiated by light intensities greater than 2 w/m² while in the preparatory phase, its entrance into mitosis is delayed. A similar model is proposed for the initiation of rhizoid elongation. Despite the fact that both cell division and rhizoid elongation are dependent on photosynthesis, direct measurements of CO₂-uptake rates show that the inhibitory effects of high light intensities are not due to an inhibition of photosynthesis.     

The synchronizing effect of slight oscillations of light intensity on activity period of birds

Summary To settle the question whether the slight daily oscillations of light intensity in high arctic summer have a synchronizing effect or not, experiments with sine wave and square wave light-dark cycles were done. Within a wide range (0.5 till 600 lux) many LD-cycles were tested. All cycles had a maximum:minimum ratio of light intensity of 2:1 resp. 3:2 and a period of 24 h. Between sine wave and square wave light-dark cycles hardly a difference in the synchronizing effect was found. In contrast to this the synchronizing effect of cycles with a ratio of 2:1 was considerably greater than with a ratio of 3:2. Since under 3:2 conditions only a very small number of birds showed a synchronized activity period, it is supposed that, at least in song-birds, this ratio is near the limit of synchronizing capacity. The absolute value of the amplitude of the light cycle seems to have only a small influence on the synchronizing effect, whereas the maximum:minimum ratio has a great influence. With higher light intensities however more and more test-birds showed continuous activity. Because the light intensities in high arctic summer oscillate in a range far exceeding 10⁴ lux, one have to assume that these oscillations of light intensity do not have any synchronizing effect, although at lower light intensities slight oscillations definitely can be successfully, as the experiments have shown.