Fluence Response Research Articles

The track interaction model for alpha particle induced thermoluminescence supralinearity: dependence of the supralinearity on the vector properties of the alpha particle radiation field

Thermoluminescence (TL) - fluence response characteristics for peaks 5, 7, 8 and 9 in LiF:Mg,Ti (TLD-100; Harshaw/Bicron) were measured for 5 MeV alpha particles, in both `non-parallel' and `near-parallel' geometries,and for 1 MeV alpha particles in `near-parallel' geometry. The onset of supralinearity in the non-parallel configuration is always at a significantly lower fluence (by approximately a factor of five) than in the nearly parallel configuration. This dependence of the onset of supralinearity on the vector properties of the alpha particle radiation field is interpreted as `proof positive' of the dominant importance of track interaction effects in the linear/supralinear behaviour of the glow peaks of TLD-100. A mathematical expression for the linear/supralinear behaviour for heavy charged particles in near-parallel geometry has been developed and fitted to the TL - fluence response curves. The model incorporates both possibilities of electron and hole diffusion in the glow curve heating stage as well as the contribution to the supralinearity of all the participating nearest-neighbour track interactions. The model is capable of yielding excellent fits to the experimental data; the inclusion of hole diffusion and retrapping is preferred to predict the very abrupt transition from linear to supralinear behaviour for the high-temperature peaks.

Induction and repair of (6-4) photoproducts in normal human and xeroderma pigmentosum variant cells during the cell cycle.

The reduced rate of (6-4) photoproduct repair observed in some cell lines may represent a more severe repair deficiency in some cohort of the cell cycle, such as S-phase. Radioimmunoassay was used to determine the kinetics of (6-4) photoproduct repair in normal human fibroblasts and xeroderma pigmentosum variant cells fractionated into different phases of the cell cycle by counterflow centrifugal elutriation. Ultraviolet fluence response curves indicated that the same amount of (6-4) photoproduct damage was induced at all phases of the cell cycle. The extent of (6-4) photoproduct repair in asynchronous XP variant cells was significantly reduced compared to normal human cells. However, the rate and extent of (6-4) photoproduct repair was constant throughout the cell cycle in both normal and XP variant cells. Hence, the UV hypersensitive and hypermutable phenotypes observed in XP variant cells are not attributable to cell cycle-dependent deficiencies in excision repair nor the yield of photodamage through the cell cycle.

Neutron Energy Response of a Modified Andersson-Braun REM Counter

The fluence response of three models of Andersson-Braun (A-B) rem counters in neutron radiation fields with different neutron energies has been investigated. In three monoenergetic neutron fields of higher energy and two high energy stray radiation fields the response of the modified A-B rem counters is better than that of conventional A-B counters. In the field of high energy neutron radiation, underestimations of the neutron dose equivalent will be unavoidable with the conventional A-B rem counter.

Phytochrome-mediated germination control of Hygrophila auriculata seeds following dry storage augmented by temperature pulse, hormones, anaerobiosis or osmoticum imbibition

A pulse of red light ( R) stimulates, and far-red light ( FR) inhibits germination of Hygrophila auriculata seeds with primary dormancy. These seeds lose their light requirement for germination after 4 months of dry storage in air. A secondary dormancy can then be induced by 5-day dark imbition in osmoticum in air (21% O 2). Red light will break this dormancy. Seeds stored in nitrogen gas (0% O 2) neither lose the light requirement, nor is a secondary dormancy imposed. However, FR stimulated some germination in osmoticum-induced dormant seeds is evident when such seeds are given a 1–6 hr 40°C pulse, gibberellin A 3 (GA 3), or ethylene treatments. These results suggest that changes of photosensitivity from a low fluence response to a very low fluence response may occur concurrently with the status of dormancy in seeds. The processes of dry after-ripening, the induction of secondary dormancy or of photosensitivity all require oxygen.

Mechanism of Photosensory Adaptation inHalobacterium salinarium

Phototaxis inHalobacterium salinariumis the result of an interplay of sensory rhodopsin excitation and adaptation to the stimulus background. Adaptation to orange light, received by sensory rhodopsin I was probed by measuring the behavioral response of cells to a step-like decrease in intensity. Cells were able to adapt to an intensity range of more than four orders of magnitude. The data were analysed on the basis of theoretical fluence rate response relationships calculated from the photocycle kinetics of the complex of sensory rhodopsin I with its transducer Htrl. Independent of the stimulus background, the cellular response was shown to be a function of the absolute number of photoreceptor complex molecules turned over the the light stimulus. Receptor deactivation was identified as the underlying mechanism of adaptation and was sufficient to account for the experimental results. We suggest that reversible methylation of the transducer protein HtrI provides the chemical mechanism of sensory adaptation inH. salinariumand also explains the different sensitivity of the cells to orange and UV light.

Individual Members of the Cab Gene Family Differ Widely in Fluence Response.

Chlorophyll a/b-binding protein genes (Cab genes) can be extremely sensitive to light. Transcript accumulation following a red light pulse increases with fluence over 8 orders of magnitude (L.S. Kaufman, W.F. Thompson, W.R. Briggs [1984] Science 226: 1447-1449). We have constructed fluence-response curves for individual Cab genes. At least two Cab genes (Cab-8 and AB96) show a very low fluence response to a single red light pulse. In contrast, two other Cab genes (AB80 and AB66) fail to produce detectable transcript following a single pulse of either red or blue light but are expressed in continuous red light. Thus, very low fluence responses and high irradiance responses occur in the same gene family.

CR-39 track neutron dosemeter/spectrometer: Investigations in the thermal, epithermal and intermediate energy region

The fluence response of a recently developed CR-39 neutron dosemeter with spectrometric properties has been calculated with the MCNP code in the neutron energy region from 2 meV to 24 keV and is compared with measurements. In addition to measurements already carried out at 186 eV, 2 keV and 24 keV, the response was measured with thermal neutrons having different mean energies. The shapes of the measured and calculated response curves agree well. Remaining differences in the absolute response values of roughly 10% may be attributed to insufficient knowledge of the detailed detection properties of the CR-39 detector. Since no measurements of critical angles of detection exist for the low-energy 4He and 7Li particles emitted in the 10B(n, α) reaction, those angles have been calculated in this work using the knowledge of the behaviour of other charged particles.

Action spectrum for subliminal light control of adaptation in Phycomyces phototropism.

Adaptation processes enable phototropism and other blue light responses of Phycomyces to operate over a 10-decade range of fluence rate. Phototropic latency, used routinely to monitor the kinetics of sensitivity recovery after a step down in fluence rate, can be shortened by application of dim light for 35 min during the early part of the latency period. This light is termed subliminal, because it does not elicit phototropism under these experimental conditions; rather, it exerts its influence on the underlying adaptation kinetics. Fluence rate-response data for this latency reduction, obtained at 17 wavelengths of subliminal light from 347 to 742 nm, showed a variety of shapes that could be fit by zero, one, or two sigmoidal components, plus a constant term. At most wavelengths, the fluence-rate threshold for latency reduction by subliminal light tended to be well below the absolute threshold for phototropism, indicating that this effect is highly sensitive. An action spectrum for the sensitivity of the subliminal light effect, derived from the fluence rate-response curves, shows major peaks around 400 and 500 nm and a broad band from 570 to 670 nm, followed by a steep absorption edge. The sensitivity in the near ultraviolet region is relatively very low. The magnitude of the latency reduction also depends strongly on wavelength with a maximum at about 450 nm. The fluence-rate response data and the action spectrum--which is markedly different from that for phototropism and other blue-light responses of Phycomyces--indicate the participation of multiple pigments, or pigment states, in the photocontrol of adaptation.

Arabidopsis HY8 locus encodes phytochrome A.

hy8 long hypocotyl mutants of Arabidopsis defective in responsiveness to prolonged far-red light (the so-called "far-red high-irradiance response") are selectively deficient in functional phytochrome A. To define the molecular lesion in these mutants, we sequenced the phytochrome A gene (phyA) in lines carrying one or other of two classes of hy8 alleles. The hy8-1 and hy8-2 mutants that express no detectable phytochrome A each have a single nucleotide change that inserts a translational stop codon in the protein coding sequence. These results establish that phyA resides at the HY8 locus. The hy8-3 mutant that expresses wild-type levels of photochemically active phytochrome A has a glycine-to-glutamate missense mutation at residue 727 in the C-terminal domain of the phyA sequence. Quantitative fluence rate response analysis showed that the mutant phytochrome A molecule produced by hy8-3 exhibited no detectable regulatory activity above that of the phyA-protein-deficient hy8-2 mutant. This result indicates that glycine-727, which is invariant in all sequenced phytochromes, has a function important to the regulatory activity of phytochrome A but not to photoperception.

Light‐ and sucrose‐dependent gene expression in photomixotrophic cell suspension cultures and protoplasts of rape (Brassica napus L.)

Light‐dependent gene expression was analysed in photomixotrophic cell suspension cultures of rape (Brassica napus L.) growing in media containing either 2.0% or 0.6% sucrose. During growth in darkness phytochrome type I and NADPH‐protochlorophyllide oxidoreductase (Pchlide reductase) accumulated in both cell culture lines to a similar extent. Illumination with continuous white, blue or red light, but not with far‐red light, resulted in disappearance of both chromoproteins within 24 h in both cell cultures. Further analysis showed that the phytochrome system of rape cell cultures reacts in a similar way to that of re‐etiolated dicotyledonous plants, showing rapid Pfr destruction and rapid Pfr dark reversion. In contrast, the light‐dependent expression of genes encoding the major chlorophyll a‐ and b‐binding protein (CAB) and the re‐accumulation of chlorophyll were found to be strongly dependent on sucrose concentration in culture media. Whereas cells grown in darkness in medium containing 2.0% sucrose showed, after exposure to continuous white light, a very weak re‐induction of CAB mRNA, CAB protein and chlorophyll accumulation, the cells in medium containing 0.6% sucrose reacted very strongly. It was also possible to demonstrate that phytochrome (by high irradiance response, HIR, and by low fluence response, LF) and the blue/UV‐A receptor are involved in the light‐dependent gene expression of CAB. Similar to complete cells, protoplasts derived from the two different cell cultures showed an almost identical sucrose concentration‐dependent and light‐quality‐dependent regulation of CAB mRNA accumulation. As the dark‐grown photomixotrophic cells and protoplasts reflect some typical photoregulatory characteristics known from dark‐grown plants it is supposed that this system will be an excellent tool for studying biochemical and molecular biological aspects of light‐dependent signal transduction in cells of higher plants.

The circadian oscillator is regulated by a very low fluence response of phytochrome in wheat.

Expression of genes encoding the light-harvesting chlorophyll a/b binding proteins of photosystem II (Cab) in etiolated wheat seedlings is controlled by phytochrome and a circadian clock. Even photoconversion of <1% of phytochrome to its active form, which can be achieved by moonlight, induces the expression of the Cab genes, particularly that of the Cab-1 gene, in circadian fashion. Thus, this reaction shows the characteristics of a low and a very low fluence response. A single far-red light pulse given to an etiolated seedling is sufficient for a persistence of the circadian oscillation of the Cab-1 mRNA level for at least 100 h. Subsequent red (R) or long-wavelength far-red (RG9) light irradiations alter the free running rhythm. These observations indicate a change in sensitivity to phytochrome and/or a control by stable phytochrome. The latter hypothesis is supported by the observation that the level of Cab-1 mRNA is increased or decreased by a second R or RG9 light pulse, respectively.

Transduction of Blue-Light Signals.

BUT IS IT AN ELEPHANT? In the old tale, several blind men are taken to an elephant. In turn, each is asked to examine and identify the mystery object. One blind man examines a leg and claims it to be a tree, another, examining the trunk, claims it to be a snake, and yet another, feeling a tusk, claims it to be a smooth rock. The pattern continues: each describes a component, none identifies the gestalt. There are a wealth of blue-light responses in higher plants. It is clear that many are biochemically, physiologically, or genetically independent, and others are linked. The mechanism by which a photon of blue light is converted into a biochemical signal and transduced into a biological response is under investigation for several of these responses. In many cases, investigators have successfully defined one or more components of the signaling mechanism. Taken together, the identified components have the potentia1 to define an entire signal transduction mechanism; receptor, G-protein, diacylglycerol, inositol triphosphate, calcium, calmodulin, several kinases, ion channels, and so forth. However, direct demonstration of a11 the components has not been achieved in any one system. Indeed, in some cases specific carriers are known to be absent. Further, the locations of the respective carriers differ in accordance with the location of the specific response (Fig. l). Perhaps the mechanisms responsible for transducing the blue-light signals are best represented by a herd of elephants. Signal transduction paradigms are well established. However, recent data suggest that plants may have several nove1 signal carriers (Deng et al., 1992), and it is not certain that the paradigms will be transferable to plants. The majority of studies of signal carriers for blue-light responses in higher plants make use of standard biochemical tools: direct identification based on biochemical properties or antibody crossreactivity, or indirect identification based on inhibitor/activator studies. These biochemical activities are correlated with the respective blue-light response by the photobiological activity (fluence response and/or time course), physical location, or involvement with the response as demonstrated by activator/inhibitor studies. The biochemical approach has recently been joined by a vigorous genetic approach. There are four blue-light responses in higher plants cur' Supported in part by the National Science Foundation and the

Changing sensitivity to light and nitrate but not to gibberellins regulates seasonal dormancy patterns in Sisymbrium officinale seeds

ABSTRACTSeeds of Sisymbrium officinale (L.) Scop, that are buried under natural conditions in soil pass annually through a seasonal pattern of changes in dormancy. Dormancy is broken in autumn‐winter and re‐induced in summer. To elucidate dormancy regulation in this species under natural conditions, a detailed analysis of the changes in sensitivity to some relevant germination factors was carried out Germination data fitted as logistic dose response curves showed that sensitivity to light and nitrate, both indispensable stimuli for germination of this species, varied with the seasons. Patterns of shifts in requirement for light and nitrate were remarkably similar. Sensitivity increased when both primary and secondary dormancy were alleviated, and it was reversed during induction of secondary dormancy. During alleviation of primary dormancy in spring 1991, the fluence response curves exhibited a biphasic character with responses occurring both in the very‐low‐fluence‐range and in the low‐fluence‐range. The nitrate dose response data could all be fitted as monophasic curves, although responses might have occurred in two distinct ranges as well. From interpretation of curve parameters, it is postulated that dormancy is regulated by changes in the number of phytochrome and nitrate receptors, in shifts in the binding characteristics of the receptors and/or in shifts in the response chain initiated by the ligand‐receptor interaction. Somewhere in this response chain, biosynthesis of gibberellins (GAs) is stimulated. By use of the GA biosynthesis inhibitor tetcyclasis, it was indirectly proven that the capacity to synthesize GAs indeed varied with the seasons. Sensitivity to GAs gradually increased from burial onwards and was not particularly related to changes in dormancy. Thus, except for the first few months of burial, GA sensitivity may not be regarded as a limiting factor in controlling dormancy in this species.

Rapid, Blue-Light-Induced Acidifications at the Surface of Ectocarpus and Other Marine Macroalgae.

In most brown algae, photosynthesis saturated with red light can be stimulated by continuous blue light. Pulses of blue light lead to transient increases in photosynthetic rate. When a CO2-sensitive electrode was used, occasionally blue light was observed to cause an apparent increase of CO2 instead of the expected decrease. This was changed by buffering the seawater medium and, under these conditions, blue light caused stimulation of CO2 consumption. These results led to investigations of blue-light-dependent pH changes at the outer surface of the plants. Shifts of the pH were recorded in the presence of the photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea. In all brown algae tested and in the green algae Ulva and Enteromorpha, blue-light pulses caused transient acidification of 0.03 to 0.18 pH units, depending on the species. The kinetics showed lag phases of a few seconds and the minimum was reached after 5 to 9 min. Fluence response relationships indicated that the sensitivity (threshold) to blue light was very similar in all species. The responses in Ectocarpus changed with time, and about 5 h after the beginning of red light or darkness, a second component became evident, which peaked 20 min after the blue-light pulse. The refractory period of the whole system was about 3 h in Ectocarpus. The blue-light-dependent pH changes show striking similarities to those of higher plant guard cells, and it is possible that similar responses may occur in other tissues of higher plants. In red algae, however, no blue-light-dependent acidifications could be detected. The possible role of the observed pH shifts in a mechanism of CO2 acquisition is discussed.

Photoperiodism and photocontrol of stem elongation in two photomorphogenic mutants of Pisum sativum L.

The photomorphogenic mutation lv in the garden pea (Pisum sativum L.), which appears to reduce the response to light-stable phytochrome, has been isolated on a tall, late photoperiodic genetic background and its effects further characterised. Plants possessing lv have a reduced flowering response to photoperiod relative to wild-type plants, indicating that light-stable phytochrome may have a flower-inhibitory role in the flowering response of long-day plants to photoperiod. In general, lv plants are longer and have reduced leaf development relative to Lv plants. These differences are maximised under continuous light from fluorescent lamps (containing negligible far-red (FR) light), and decrease with addition of FR to the incident light. Enrichment of white light from fluorescent lamps with FR promotes stem elongation in the wild type but causes a reduction in elongation in the lv mutant. This “negative” shade-avoidance response appears to be the consequence of a strong inhibitory effect of light rich in FR, revealed in lv plants in the absence of a normal response to red (R) light. These results indicate that the wild-type response to the R: FR ratio may be comprised of two distinct photoresponses, one in which FR supplementation promotes elongation by reducing the inhibitory effect of R, and the other in which light rich in FR actively inhibits elongation. This hypothesis is discussed in relation to functional differentiation of phytochrome types in the light-grown plant. Gene lw has been reported previously to reduce internode length and the response to gibberellin A1, and to delay flowering. The present study shows that the lw mutation confers an increased response to photoperiod. In all these responses the lw phenotype is superficially “opposite” to the lv phenotype. The possibility that the mutation might primarily affect light perception was therefore considered. The degree of dwarfing of lw plants was found to depend upon light quality and quantity. Dwarfing is more extreme in plants grown under continuous R light than in those grown in continuous FR or blue light or in darkness. Studies of the fluence-rate response show that the lw mutation imparts a lower fluence requirement for inhibition of elongation by white light from fluorescent lamps. Dark-grown lw plants are more strongly inhibited by a R pulse than are wild-type plants but, as in the wild type, this inhibition remains reversible by FR. Light-grown lw plants show an exaggerated elongation response to end-of-day FR light. Taken together, these findings indicate that the lw mutant may be hypersensitive to phytochrome action.

Light pulses induce "singular" behavior and shorten the period of the circadian phototaxis rhythm in the CW15 strain of Chlamydomonas.

While measuring action spectra for phase-shifting the circadian clock of Chlamydomonas, we observed that light pulses started near the phase response curve (PRC) "breakpoint" caused a reduction of the amplitude of the phototactic rhythm and two unexpected effects: (1) nonmonotonic fluence response curves (FRCs), and (2) shortening of the period of the subsequent free-running rhythm. The reduction of the rhythm's amplitude is dependent upon both the fluence and wavelength of the light pulse. The results are consistent with the amplitude being dependent upon the perceived "strength" of the stimulus, and with the nonmonotonic FRCs and reduced amplitude reflecting a light-induced change of the pacemaker's state variables to a region of the phase plane close to the "singularity." The period change that is evoked by single stimuli exhibits novel characteristics: large changes in period and a phase specificity that correlates with "singular" behavior. These period changes also appear to be a function of the stimulus strength, but indirectly; the magnitude of the period change is most strongly correlated with the magnitude of the light-induced phase shift. These results are interpreted in the context of limit cycle models of circadian clocks, and are used to suggest new tactics for measuring action spectra of light-induced clock resetting.

Type Testing and Routine Calibration of Neutron Personal Dosemeters: Phantoms and Phantom Backscatter

It is proposed that personal dosemeters be type tested and routinely calibrated in terms of the ICRU quantities Hp(10) and Hp(0.07): these quantities are defined as the dose equivalent at 10 mm and 0.07 mm respectively, in a 300 × 300 × 150 mm3 slab of ICRU 4-element tissue. In practice ICRU tissue cannot be fabricated so phantoms of various surrogate materials are used; for calibration and routine performance tests irradiation is sometimes performed free-in-air to facilitate the simultaneous exposure of large numbers of dosemeters to the same fluence. It may then be necessary to apply dosemeter-specific corrections to take into account the difference between the backscattered radiation field in the practical situation and that for which the calibration quantity is defined. It has been suggested that if a standard phantom of dimensions 300 × 300 × 150 mm3 composed of poly-methyl methacrylate (PMMA) is used, then no corrections for differences in backscatter are required. Results are presented for the differences in fluence response of routinely used neutron personal dosemeters irradiated on 300 × 300 × 150 mm3 phantoms composed of a tissue substitute material (MS20), PMMA and polyethylene. These are compared with free-air irradiations corrected for room scatter by the shadow cone technique. The contribution to the response due to backscatter from the phantom is thus derived and is discussed in terms of the appropriateness of phantom selection.

Phytochrome-Mediated Germination of Very Sensitive Oospores

The light receptor and its mode of operation were studied in photosensitive oospores of Nitella furcata subsp. megacarpa (Allen emend. Wood). Brief pulses of light activated maximal germination of post-secondary dormant oospores removed from lake sediments. Fluence response data at 12 wavelengths were used to construct an action spectrum for germination. The shape of the action spectrum with its maximum at 669 nm provides evidence for the involvement of phytochrome. Germination was induced with photon fluences that established as little as 0.01% of the phytochrome in the far red-absorbing form, which suggests that phytochrome was operating in the very low-fluence response mode. The functioning of phytochrome in the very low-fluence response mode in Nitella is similar to that in higher plants.

Pigment of the imagination: a history of phytochrome research

Daylength and flowering the 1930s spectral studies action spectra for floral induction action spectra for elongation growth photoreversibility the 1950s high-energy responses detection partial purification the properties of phytochrome phototransformation in vivo spectrophotometric studies seed germination and flowering photoperiodic timing mougeotia ode of action the Hartmann model large phytochrome the gene regulation hypothesis dark transformations localization properties of large phytochrome the membrane hypothesis high-irradiance responses the very low fluence response - a dimer model shade avoidance the chromophore native phytochrome native pr and pfr sequestered and ubiquitinated phytochrome multiple phytochromes gene regulation autoregulation phytochrome and flowering. Epiloque: weed control coloured mulch.

ANALYSIS OF MULTIPLE PHOTORECEPTOR PIGMENTS FOR PHOTOTROPISM IN A MUTANT OF Arabidopsis thaliana

Abstract— The shape of the fluence‐response relationship for the phototropic response of the JK224 strain of Arabidopsis thaliana depends on the fluence rate and wavelength of the actinic light. At low fluence rate (0.1 μmol m‐2s‐1), the response to 450‐nm light is characterized by a single maximum at about 9 μmol m‐2. At higher fluence rate (0.4 μmol m‐2s‐1), the response shows two maxima, at 4.5 and 9 μmol m‐2. The response to 510‐nm light shows a single maximum at 4.5 μmol m‐2. Unilateral preirradiation with high fluence rate (25 μmol m‐2s‐1) 510‐nm light eliminates the maximum at 4.5 μmol m‐2 in the fluence response curve to a subsequent unilateral 450‐nm irradiation, while the second maximum at 9 μmol m‐2 is unaffected. Based on these results, it is concluded that a single photoreceptor pigment has been altered in the JK224 strain of Arabidopsis thaliana.