Light-induced isomerization of the LHCII-bound xanthophyll neoxanthin: possible implications for photoprotection in plants

Light-harvesting pigment-protein complex of Photosystem II (LHCII) is the largest photosynthetic antenna complex of plants and the most abundant membrane protein in the biosphere. Plant fitness and productivity depend directly on a balance between excitations in the photosynthetic apparatus, generated by captured light quanta, and the rate of photochemical processes. Excess excitation energy leads to oxidative damage of the photosynthetic apparatus and entire organism and therefore the balance between the excitation density and photosynthesis requires precise and efficient regulation, operating also at the level of antenna complexes. We show that illumination of the isolated LHCII leads to isomerization of the protein-bound neoxanthin from conformation 9'-cis to 9',13- and 9',13'-dicis forms. At the same time light-driven excitation quenching is observed, manifested by a decrease in chlorophyll a fluorescence intensity and shortened fluorescence lifetimes. Both processes, the neoxanthin isomerization and the chlorophyll excitation quenching, are reversible in dim light. The results of the 77K florescence measurements of LHCII show that illumination is associated with appearance of the low-energy states, which can serve as energy traps in the pigment-protein complex subjected to excess excitation. Possible sequence of the molecular events is proposed, leading to a protective excess excitation energy quenching: neoxanthin photo-isomerization→formation of LHCII supramolecular structures which potentiate creation of energy traps→excitation quenching.     

Invariants of spatial summation for S (short wavelength) cone vision

Spatial summation in the human visual system was studied as a function of retinal eccentricity upon selective stimulation of the short-wavelength sensitive cones. The area of complete spatial summation (Ricco's area) was found to increase with retinal eccentricity while the threshold of stimuli equal in size with Ricco's area remained constant. Comparisons with known morphology of the small bistratified retinal ganglion cells, the only cells known to be excited by S-one ON stimulation, showed that Ricco's area included 2-4 such cells and is up to 1.5 times larger than the dendritic field of a single cell. These relationships were relatively constant within the eccentricity range tested (5-20 deg along the temporal horizontal meridian) and might be the source of threshold invariance of stimuli matching Ricco's area.     

Auslösung von Elementarprozessen durch einzelne Lichtquanten im Fliegenauge

Summary Light quanta impinging upon the photopigments located in the rhabdomeric receptor structures of the fly's compound eyes trigger photochemical reactions which in turn elicit miniature receptor potentials (bumps). The paper mainly deals with the problem whether a single quantum of light is sufficient, or whether a coincidence of quanta and/or elementary photochemical events is necessary to trigger a miniature receptor potential.The experiments were based on tests of the optomotor responses of fixed flying flies suspended in a rotating patterned cylinder with periodic distributions of inner surface brightness. The tests were made under two different light programs: 1) Illumination constant in time 2) Illumination by periodic light pulse sequences with various frequencies. Average light fluxes absorbed by the receptors were equal in both programs. Theoretical considerations lead to the following conclusions: The strength of the optomotor responses to the light programs 1 and 2 should not differ from each other in the case of single quantum processes. However for multiquantum processes light program 2 should be more effective than light program 1 as it favours the coincidence of quantum absorptions per unit time. But these theoretical conclusions are valid only if two conditions are fulfilled in the experiments: a) The pulse frequency of light program 2 has to be kept below a certain limit which is determined by the kinetics of the photochemical systems. Otherwise light program 2 gets averaged in time and in principle can be not more effective than light program 1. b) The rates of quanta absorbed by the receptors have to be kept low enough to guarantee that the concentration of unbleached pigment molecules remains practically unchanged as compared with the concentration in darkness. Accordingly the test experiments were carried out with light pulse frequencies ranging from 500 to 1/120 cycles per second. Intensities were used which corresponded to an average quantum flux effective for one rhabdomeric structure ranging between 10 and 250 quanta per second.The interpretation of the experimental results is in accordance with the hypothesis that one single quantum of light is sufficient to trigger an elementary photochemical reaction and that in turn one single photochemical event can elicit a miniature receptor potential. At present time the experiments do not allow conclusions about the possible occurrence of coincidence-functions of synapses at the level of the first optical ganglion which receive their information via fibers leading off from the receptors.In one of the appendices of the paper, the transinformation flux into a receptor is calculated, taking into consideration the Poisson noise of the quanta disrupting the signal at extremely low quantum rates.

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Ein Teil der in dieser Arbeit abgedruckten Ergebnisse wurde bereits in zwei vorläufigen Mitteilungen publiziert, Reichardt (1965, 1966).     

ANOXIA AND BRIGHTNESS DISCRIMINATION

1. Brightness discrimination has been studied with individuals breathing oxygen concentrations corresponding to 7 altitudes between sea level and 17,000 feet. The brightnesses were 0.1, 0.01, and 0.001 millilambert involving only daylight (cone) vision. 2. At these light intensities, brightness discrimination begins to deteriorate at fairly low altitudes. The deterioration is obvious at 8,000 feet, and becomes marked at 15,000 feet, where at low brightness, the contrast must be increased 100 per cent over the sea level value before it can be recognized. 3. The impairment of brightness discrimination with increase in altitude is greater at higher altitudes than at lower. The impairment starts slowly and becomes increasingly rapid the higher the altitude. 4. Impairment of brightness discrimination varies inversely with the light intensity. It is most evident under the lowest light intensities studied, but shows in all of them. However, it decreases in such a way that the deterioration is negligible in full daylight and sunlight. 5. The thresholds of night (rod) vision and day (cone) vision are equally affected by anoxia. 6. The quantitative form of the relation between brightness discrimination ΔI/I and the prevailing brightness I remains the same at all oxygen concentrations. The curve merely shifts along the log I axis, and the extent of the shift indicates the visual deterioration. 7. The data are described in terms of retinal chemistry. Since anoxia causes only a shift in log I it is shown that the photochemical receptor system cannot be affected. Instead the conversion of photochemical change into visual function is impaired in such a way that the conversion factor varies as the fourth power of the arterial oxygen saturation.     

Spatial summation in taste: NaCl thresholds and stimulated area on the anterior human tongue

Spatial summation has been demonstrated in several sensory modalities.In this study, spatial summation of sensitivity to NaCl wasinvestigated as a function of stimulated area. In a two-alternativeforced choice procedure, a filter paper stimulation method wasused to determine the sensitivity to, and the detection probabilitiesof, five low-intensity NaCl stimuli. Each of the stimuli waspresented 32 times in two conditions, a circular area (ø9 mm) and half that circle in four counterbalanced orientations.Only one side of the tongue was used. The results showed that(a) the summation found was partial; (b) the observed relationshipbetween threshold intensity and area obeyed Piper's Law, and(c) detection probabilities increased according to chance. Adhoc analysis of orientation yielded sensitivity differencesalong the vertical but not the horizontal axis.     

ENERGY,QUANTA, AND VISION

1. Direct measurements of the minimum energy required for threshold vision under optimal physiological conditions yield values between 2.1 and 5.7 x 10^–10 ergs at the cornea, which correspond to between 54 and 148 quanta of blue-green light. 2. These values are at the cornea. To yield physiologically significant data they must be corrected for corneal reflection, which is 4 per cent; for ocular media absorption, which is almost precisely 50 per cent; and for retinal transmission, which is at least 80 per cent. Retinal transmission is derived from previous direct measurements and from new comparisons between the percentage absorption spectrum of visual purple with the dim-vision luminosity function. With these three corrections, the range of 54 to 148 quanta at the cornea becomes as an upper limit 5 to 14 quanta actually absorbed by the retinal rods. 3. This small number of quanta, in comparison with the large number of rods (500) involved, precludes any significant two quantum absorptions per rod, and means that in order to produce a visual effect, one quantum must be absorbed by each of 5 to 14 rods in the retina. 4. Because this number of individual events is so small, it may be derived from an independent statistical study of the relation between the intensity of a light flash and the frequency with which it is seen. Such experiments give values of 5 to 8 for the number of critical events involved at the threshold of vision. Biological variation does not alter these numbers essentially, and the agreement between the values measured directly and those derived from statistical considerations is therefore significant. 5. The results clarify the nature of the fluctuations shown by an organism in response to a stimulus. The general assumption has been that the stimulus is constant and the organism variable. The present considerations show, however, that at the threshold it is the stimulus which is variable, and that the properties of its variation determine the fluctuations found between response and stimulus.     

Delayed fluorescence in photosynthesis

Photosynthesis is a very efficient photochemical process. Nevertheless, plants emit some of the absorbed energy as light quanta. This luminescence is emitted, predominantly, by excited chlorophyll a molecules in the light-harvesting antenna, associated with Photosystem II (PS II) reaction centers. The emission that occurs before the utilization of the excitation energy in the primary photochemical reaction is called prompt fluorescence. Light emission can also be observed from repopulated excited chlorophylls as a result of recombination of the charge pairs. In this case, some time-dependent redox reactions occur before the excitation of the chlorophyll. This delays the light emission and provides the name for this phenomenon—delayed fluorescence (DF), or delayed light emission (DLE). The DF intensity is a decreasing polyphasic function of the time after illumination, which reflects the kinetics of electron transport reactions both on the (electron) donor and the (electron) acceptor sides of PS II. Two main experimental approaches are used for DF measurements: (a) recording of the DF decay in the dark after a single turnover flash or after continuous light excitation and (b) recording of the DF intensity during light adaptation of the photosynthesizing samples (induction curves), following a period of darkness. In this paper we review historical data on DF research and recent advances in the understanding of the relation between the delayed fluorescence and specific reactions in PS II. An experimental method for simultaneous recording of the induction transients of prompt and delayed chlorophyll fluorescence and decay curves of DF in the millisecond time domain is discussed.     

Optomotorische Reaktionen der Fliege Musca Domestica

Summary The torque exerted by the housefly Musca domestica during fixed flight was used as a measure of the optomotor reaction of the insect elicited by the rotation of cylindrical patterns with periodic distributions of surface brightness. Measurements were made of the dependence of the reaction on the wave length, speed of rotation, contrast, and mean brightness of the stimulus patterns. The effect on the reaction of modulation of the light illuminating the stimulus pattern was examined. Further experiments indicated that stimulation of only one of the two complex eyes is sufficient to elicit an optomotor reaction, and that there is overlap between the visual fields of neighboring photoreceptor units in the complex eye. Estimates of the rates of absorption of light quanta by individual ommatidia in the complex eye indicated that these rates are low enough that the Poisson statistics of the light quanta results in a significant level of noise in the light signals received by the photoreceptors, when the brightness of the stimulus pattern is low but still sufficient to elicit a measurable reaction. The contrast that is required of a rotating stimulus pattern in order to elicit a just-measurable reaction was found to depend upon the mean brightness of the pattern in a manner that is consistent with the hypothesis that the noise due to the statistics of the light quanta absorbed by the photoreceptors in the complex eye is a principle cause of the breakdown of the optomotor reaction at low values of the contrast and mean brightness of the stimulus pattern.

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Mit Unterstützung durch ein Fellowship des National Institute of Neurological Diseases and Blindness, US Public Health Service und ein Fellowship der National Science Foundation, USA.

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Herrn Freiberg danken wir für das Anfertigen der Abbildungen.     

The relation between visual acuity and brightness discrimination

1. Visual acuity depends on the brightness contrast between test object and background; and conversely, brightness discrimination depends on the target size. Both functions vary with the brightness of the background. Measurements with rectangular targets of length-width ratio 2 were made over a range of sizes, contrasts, and brightnesses sufficient to determine the relations among these three variables. The rectangles were from 2' to 50' wide; the contrast fraction, DeltaI/I, ranged from 0.01 to 40; the background brightness varied from 0.0001 to 2500 millilamberts. 2. When DeltaI/I or visual acuity is plotted as a function of brightness the data do, in general, follow Hecht's equation. The departure from a simple photochemical theory which the larger targets show is probably due to changes in the functional retinal mosaic with changing brightness. 3. In general also, the relation between visual acuity and brightness, at selected contrasts, fits Hecht's derivation. At low contrasts, as the brightness is reduced a point is reached at which the test object becomes invisible at any size. 4. No simple relation emerges from the data relating visual acuity to contrast, at set levels of illumination. Over only a very short range are visual acuity and contrast directly related. At high contrasts, visual acuity reaches a maximum, whereas at low visual acuity, DeltaI/I reaches a minimum which cannot be passed regardless of size. 5. The shape of the curves relating DeltaI/I to brightness is not significantly altered by changing the exposure time. There is some evidence to show that a 3 second exposure of the target is equivalent to two looks of 0.2 second each. 6. In all these studies the thresholds were determined by a frequency of seeing method, and the data have been considered in terms of a quantum theory of threshold seeing. It was found that a threshold response involves between four and eight independent critical events, which are largely independent of size, brightness, and criterion of seeing.     

Lysozyme Photochemistry as a Function of Temperature. The Protective Effect of Nanoparticles on Lysozyme Photostability

The presence of aromatic residues and their close spatial proximity to disulphide bridges makes hen egg white lysozyme labile to UV excitation. UVB induced photo-oxidation of tryptophan and tyrosine residues leads to photochemical products, such as, kynurenine, N–formylkynurenine and dityrosine and to the disruption of disulphide bridges in proteins. We here report that lysozyme UV induced photochemistry is modulated by temperature, excitation power, illumination time, excitation wavelength and by the presence of plasmonic quencher surfaces, such as gold, and by the presence of natural fluorescence quenchers, such as hyaluronic acid and oleic acid. We show evidence that the photo-oxidation effects triggered by 295 nm at 20°C are reversible and non-reversible at 10°C, 25°C and 30°C. This paper provides evidence that the 295 nm damage threshold of lysozyme lies between 0.1 μW and 0.3 μW. Protein conformational changes induced by temperature and UV light have been detected upon monitoring changes in the fluorescence emission spectra of lysozyme tryptophan residues and SYPRO^® Orange. Lysozyme has been conjugated onto gold nanoparticles, coated with hyaluronic acid and oleic acid (HAOA). Steady state and time resolved fluorescence studies of free and conjugated lysozyme onto HAOA gold nanoparticles reveals that the presence of the polymer decreased the rate of the observed photochemical reactions and induced a preference for short fluorescence decay lifetimes. Size and surface charge of the HAOA gold nanoparticles have been determined by dynamic light scattering and zeta potential measurements. TEM analysis of the particles confirms the presence of a gold core surrounded by a HAOA matrix. We conclude that HAOA gold nanoparticles may efficiently protect lysozyme from the photochemical effects of UVB light and this nanocarrier could be potentially applied to other proteins with clinical relevance. In addition, this study confirms that the temperature plays a critical role in the photochemical pathways a protein enters upon UV excitation.     

Simultaneous gas exchange and fluorescence measurements indicate differences in the response of sunflower,bean and maize to water stress

Gas exchange and fluorescence measurements of attached leaves of water stressed bean, sunflower and maize plants were carried out at two light intensities (250 mol quanta m^-2s^-1 and 850 mol quanta m^-2s^-1). Besides the restriction of transpiration and CO₂ uptake, the dissipation of excess light energy was clearly reflected in the light and dark reactions of photosynthesis under stress conditions. Bean and maize plants preferentially use non-photochemical quenching for light energy dissipation. In sunflower plants, excess light energy gave rise to photochemical quenching. Autoradiography of leaves after photosynthesis in ¹⁴CO₂ demonstrated the occurrence of leaf patchiness in sunflower and maize but not in bean. The contribution of CO₂ recycling within the leaves to energy dissipation was investigated by studies in 2.5% oxygen to suppress photorespiration. The participation of different energy dissipating mechanisms to quanta comsumption on agriculturally relevant species is discussed.Abbreviations F_o minimal fluorescence - F_m maximal fluorescence - F_p peak fluorescence - g leaf conductance - P_N net CO₂ uptake - q_N coefficient of non-photochemical quenching - q_P coefficient of photochemical quenching     

基于计算机视觉技术评价光照度对斜纹夜蛾幼虫体色的影响

【目的】为揭示光照度对斜纹夜蛾Prodenia litura(Fabricius)幼虫体色变化的影响,建立一套新的昆虫体色评价体系。【方法】采用计算机视觉检测技术,将斜纹夜蛾幼虫的颜色数值化,评价了斜纹夜蛾幼虫体色随日龄的变化规律及光照度对斜纹夜蛾幼虫体色变化的影响。【结果】随着日龄的增加,斜纹夜蛾幼虫的明度(B)和彩色指数(CI)逐渐减少,体色偏差系数(BDV)逐渐增加,色泽从开始的绿色占主导逐渐转变为红色占主导,不同日龄间主观色各颜色指标都差异显著(P<0.01);在低光照度(0~1 000 lx)下,明度(B)和体色偏差系数(BDV)主要受光照影响,而彩色指数(CI)和归一化RGB值主要受发育日龄的影响,在高光照度下(1 000 lx以上),所有颜色指标均受发育日龄影响较大,受光照处理相对较小,光照度对斜纹夜蛾幼虫体色影响的阈值为1 000 lx,在阈值下,明度(B)、彩色指数(CI)及体色分化系数(BDC)都随光照度的增加显著增加,体色偏差系数(BDV)、RGB色差则随光照度的增加而减少。【结论】光照度主要影响斜纹夜蛾幼虫体色的明暗程度,对色彩程度影响不大,且光照度的影响阈值大约为1 000 lx。     

The chloroplast NADPH thioredoxin reductase C,NTRC, controls non‐photochemical quenching of light energy and photosynthetic electron transport in Arabidopsis

High irradiances may lead to photooxidative stress in plants, and non‐photochemical quenching (NPQ) contributes to protection against excess excitation. One of the NPQ mechanisms, qE, involves thermal dissipation of the light energy captured. Importantly, plants need to tune down qE under light‐limiting conditions for efficient utilization of the available quanta. Considering the possible redox control of responses to excess light implying enzymes, such as thioredoxins, we have studied the role of the NADPH thioredoxin reductase C (NTRC). Whereas Arabidopsis thaliana plants lacking NTRC tolerate high light intensities, these plants display drastically elevated qE, have larger trans‐thylakoid ΔpH and have 10‐fold higher zeaxanthin levels under low and medium light intensities, leading to extremely low linear electron transport rates. To test the impact of the high qE on plant growth, we generated an ntrc–psbs double‐knockout mutant, which is devoid of qE. This double mutant grows faster than the ntrc mutant and has a higher chlorophyll content. The photosystem II activity is partially restored in the ntrc–psbs mutant, and linear electron transport rates under low and medium light intensities are twice as high as compared with plants lacking ntrc alone. These data uncover a new role for NTRC in the control of photosynthetic yield.     

Photosynthetic activity of far-red light in green plants

We have found that long-wavelength quanta up to 780 nm support oxygen evolution from the leaves of sunflower and bean. The far-red light excitations are supporting the photochemical activity of photosystem II, as is indicated by the increased chlorophyll fluorescence in response to the reduction of the photosystem II primary electron acceptor, Q(A). The results also demonstrate that the far-red photosystem II excitations are susceptible to non-photochemical quenching, although less than the red excitations. Uphill activation energies of 9.8+/-0.5 kJ mol(-1) and 12.5+/-0.7 kJ mol(-1) have been revealed in sunflower leaves for the 716 and 740 nm illumination, respectively, from the temperature dependencies of quantum yields, comparable to the corresponding energy gaps of 8.8 and 14.3 kJ mol(-1) between the 716 and 680 nm, and the 740 and 680 nm light quanta. Similarly, the non-photochemical quenching of far-red excitations is facilitated by temperature confirming thermal activation of the far-red quanta to the photosystem II core. The observations are discussed in terms of as yet undisclosed far-red forms of chlorophyll in the photosystem II antenna, reversed (uphill) spill-over of excitation from photosystem I antenna to the photosystem II antenna, as well as absorption from thermally populated vibrational sub-levels of photosystem II chlorophylls in the ground electronic state. From these three interpretations, our analysis favours the first one, i.e., the presence in intact plant leaves of a small number of far-red chlorophylls of photosystem II. Based on analogy with the well-known far-red spectral forms in photosystem I, it is likely that some kind of strongly coupled chlorophyll dimers/aggregates are involved. The similarity of the result for sunflower and bean proves that both the extreme long-wavelength oxygen evolution and the local quantum yield maximum are general properties of the plants.     

Photosynthetic activity of far-red light in green plants

We have found that long-wavelength quanta up to 780 nm support oxygen evolution from the leaves of sunflower and bean. The far-red light excitations are supporting the photochemical activity of photosystem II, as is indicated by the increased chlorophyll fluorescence in response to the reduction of the photosystem II primary electron acceptor, Q_A. The results also demonstrate that the far-red photosystem II excitations are susceptible to non-photochemical quenching, although less than the red excitations. Uphill activation energies of 9.8 ± 0.5 kJ mol⁻¹ and 12.5 ± 0.7 kJ mol⁻¹ have been revealed in sunflower leaves for the 716 and 740 nm illumination, respectively, from the temperature dependencies of quantum yields, comparable to the corresponding energy gaps of 8.8 and 14.3 kJ mol⁻¹ between the 716 and 680 nm, and the 740 and 680 nm light quanta. Similarly, the non-photochemical quenching of far-red excitations is facilitated by temperature confirming thermal activation of the far-red quanta to the photosystem II core. The observations are discussed in terms of as yet undisclosed far-red forms of chlorophyll in the photosystem II antenna, reversed (uphill) spill-over of excitation from photosystem I antenna to the photosystem II antenna, as well as absorption from thermally populated vibrational sub-levels of photosystem II chlorophylls in the ground electronic state. From these three interpretations, our analysis favours the first one, i.e., the presence in intact plant leaves of a small number of far-red chlorophylls of photosystem II. Based on analogy with the well-known far-red spectral forms in photosystem I, it is likely that some kind of strongly coupled chlorophyll dimers/aggregates are involved. The similarity of the result for sunflower and bean proves that both the extreme long-wavelength oxygen evolution and the local quantum yield maximum are general properties of the plants.     

Light-dependent ATP synthesis in mitochondria.

Light-dependent ATP synthesis was studied in an illuminated suspension of rat liver mitochondria. The action of light was shown to lead to an increase in the ATP content in the absence of oxidisable substrates and in the presence of high (hundreds of microM) ADP concentrations in the medium. At a relatively low (50 microM) ADP concentration, efficient light-dependent phosphorylation was observed in the presence of alpha-ketoglutarate. Prolonged illumination stimulated ATP hydrolysis. Rotenone, antimycin, azide, dicyclohexylcarbodiimide, and oligomycin inhibited the light-dependent phosphorylation almost completely. The level of ATP decreased under the action of 2,4-dinitrophenol in the dark but was restored by high light intensities. Blue light, 436 nm, was most efficient to produce light-dependent phosphorylation. It is assumed that quanta of vibrational excitation formed in the course of vibrational relaxation and the internal conversion of photoexcited flavoproteins and cytochromes are transferred to the ATP-synthetase and "eject" ATP from the active center, thus shifting the enzymatic reaction to ATP production.     

Fluorescence and oxygen evolution from Chlorella pyrenoidosa

The process of photosynthetic energy conversion in Chlorella pyrenoidosa was investigated by simultaneous measurement of transient and steady-state rates of O₂ evolution and fluorescence.

1. 1. Alternation or superimposition of light 1 and light 2 illumination induces both fast and slow changes in fluorescence and rate of O₂ evolution. The fast changes are ascribed to changes in conditions of the reaction centers in the context of the ¹ model and the kinetic analysis of ². The slow changes are interpreted as adaptations to the intensity and wavelength of illumination. The adaptive mechanism is described in terms of slow variation in fraction () of total absorbed quanta delivered to System 2. At low intensities, the calculated value of for cells adapted to light 2 illumination (light 2 state) is approx. 0.9 of for cells adapted to light 1 illumination (light 1 state).

2. 2. An increase in fluorescence yield was found to accompany the decrease in O₂ yield at the onset of light saturation with either light 1 or light 2 excitation. Variation in is proposed to account for the differences between the maximum fluorescence yield observed in steady-state conditions and the 1.5 times higher maximum yield observed in transient conditions or in cells inhibited by 3(3,4-dichlorophenyl)-1,1-dimethylurea. Variation in can also explain the observation of a higher rate of fluorescence emission with light 1 excitation than with light 2 excitation for a given steady-state rate of O₂ evolution.

3. 3. A model for energy conversion by System 2 is proposed to account for our observations. The model proposes competitive dissipation of absorbed energy by photochemical trapping at reaction centers and by fluorescence and radiationless de-excitation from both the pigment bed and reaction centers of System 2.

Energy transfer and the distribution of excitation energy in the photosynthetic apparatus of spinach chloroplasts

Equations are derived from our model of the photochemical apparatus of photosynthesis to show that the yield of energy transfer from Photosystem II to Photosystem I, ?_T(II→Iz), can be obtained from measurements on an individual sample of chloroplasts frozen to ?196 °C by comparing the sum of two specifically defined fluorescence excitation spectra with the absorption spectrum of the sample. Then, given that value of ?_T(II→I), the fraction of the quanta absorbed by the photochemical apparatus which is distributed initially to Photosystem I, α, can be determined as a function of the wavelength of excitation from the same fluorescence excitation spectra. The results obtained in this study of individual samples of chloroplasts frozen to ?196 °C in the absence of divalent cations, namely, that ?_T(II→I) varies from a minimum value of 0.10 when the Photosystem II reaction centers are all open to a maximum value of 0.25 when the centers are all closed and that α has a value of about 0.30 which is almost independent of wavelength for wavelengths shorter than 675 nm (α increases rapidly toward unity at wavelengths longer than 675 nm), agrees quite well with results obtained previously from comparative measurements of chloroplasts frozen to ?196 °C in the presence and absence of divalent cations.     

Color and polarization vision in foraging Papilio

This paper gives an overview of behavioral studies on the color and polarization vision of the Japanese yellow swallowtail butterfly, Papilio xuthus. We focus on indoor experiments on foraging individuals. Butterflies trained to visit a disk of certain color correctly select that color among various other colors and/or shades of gray. Correct selection persists under colored illumination, but is systematically shifted by background colors, indicating color constancy and simultaneous color contrast. While their eyes contain six classes of spectral receptors, their wavelength discrimination performance indicates that their color vision is tetrachromatic. P. xuthus innately prefers brighter targets, but can be trained to select dimmer ones under certain conditions. Butterflies trained to a dark red stimulus select an orange disk presented on a bright gray background over one on dark gray. The former probably appears darker to them, indicating brightness contrast. P. xuthus has a strong innate preference for vertically polarized light, but the selection of polarized light changes depending on the intensity of simultaneously presented unpolarized light. Discrimination of polarization also depends on background intensity. Similarities between brightness and polarization vision suggest that P. xuthus perceive polarization angle as brightness, such that vertical polarization appears brighter than horizontal polarization.