IDENTITY OF THE PHOTORECEPTORS FOR THE PERCEPTION OF IRRADIANCE IN PHOTOSYNTHETIC LIGHT ACCLIMATION IN TOMATO*

The photoreceptors involved in the photosynthetic acclimation of tomato (Lycopersicon esculentum Mill.) to increased irradiance were investigated. Plants were transferred from 100 p.mol m^?2 s^?1 cool white fluorescent light to higher irradiances of white light or white light supplemented with blue, red, green or yellow light. In these experiements light of all wavelengths tested was capable of causing acclimation as measured by the rate of light-saturated photosynthesis. It was concluded that the photosynthetic system rather than the blue-absorbing photoreceptor or phytochrome system acts as the photoreceptor for increased irradiance. No acclimation was observed in response to increased CO₂ levels, but increasing light integral at a constant irradiance was effective in bringing about acclimation. We conclude that acclimation is a response to increased photosynthetic light capture rather than increased photosynthetic carbon fixation, and involves a photon counting mechanism.     

EXTRARETINAL PHOTORECEPTION IN ENTRAINMENT OF CRUSTACEAN ORCADIAN RHYTHMS*

Abstract— An attempt was made to determine whether entrainment of the circadian rhythms of locomotor activity and ERG amplitude of the crayfish involved extraretinal photoreception. The results of a variety of experiments involving surgical lesions and localized illumination provided evidence that both rhythms can be entrained via an extraretinal pathway. The data also demonstrate that the caudal photoreceptor is unnecessary for entrainment. Our evidence to date suggests that the extraretinal photoreceptor is located in the supraesophageal ganglion; however, the possibility of photoreceptive input from other regions of the CNS, particularly the optic lobe, has not been eliminated. It is also tentatively concluded that the circadian oscillators for both the locomotor and the ERG amplitude rhythms are located within the supraesophageal ganglion, but more data are needed to confirm this conclusion.     

THE PHOTORECEPTORS INVOLVED IN ANTHOCYANIN SYNTHESIS

Abstract— Experiments with irradiation sequences where red precedes far-red lead to the conclusion that, in turnip, phytochrome is the only pigment mediating anthocyanin synthesis in red and far-red. Results from experiments where far-red precedes red, however, suggest that more than one reaction is involved. A possible interpretation is that the 'high-energy' reaction in far-red and the low energy red/far-red reversible reaction are mediated by two different forms of phytochrome.
The 'high-energy' reaction in blue light does not appear to depend on phytochrome.     

EXCITATION AND ADAPTATION IN LIMULUS VENTRAL PHOTORECEPTORS

Abstract— Currents induced by illumination can be recorded from Limulus ventral photoreceptors by a voltage-clamp technique. Several assumptions underlying the analysis of these currents are discussed and recent experiments designed to examine these assumptions are reviewed.     

INTERACTION OF RHODOPSIN, G-PROTEIN AND KINASE IN OCTOPUS PHOTORECEPTORS

Light induced phosphorylation of octopus rhodopsin was greatly enhanced by guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), suggesting that the kinases are involved in regulating interaction between rhodopsin and G-protein. We determined phosphorylated peptides of octopus rhodopsin in the presence or absence of GTP gamma S. Possible phosphorylation sites for octopus rhodopsin enhanced by GTP gamma S were Thr329, Thr330 and/or Thr336, which suggest that the G-protein associates with cytoplasmic loops including C-terminal peptide in the seventh helix of octopus rhodopsin.     

REDOX DYES AS ARTIFICIAL PHOTORECEPTORS IN LIGHT-DEPENDENT CAROTENOID SYNTHESIS*

Abstract— The photodynamically active dyes methylene blue, toluidine blue, and neutral red act as artificial photoreceptors in light-dependent carotenoid synthesis in Fusarium aquaeductuum. Normally, carotenoid production is only induced by light of wavelengths shorter than 520 nm, but when mycelia are incubated with neutral red, methylene blue, and toluidine blue, red light is also effective in inducing carotenogenesis. Experiments with methylene blue and red light showed that pigments induced under these conditions are qualitatively the same as those induced with white light, and also that, in accord with the results found for photoinduction with white light, the amount of pigment synthesized was proportional to the logarithm of illumination time. In addition to their role in photoinduction, the dyes were also shown to interfere with the biosynthesis of carotenoids, whereas addition after irradiation caused an increase in pigment production that involves a quantitative change in the more unsaturated components.     

BLUE AND ULTRAVIOLET-B LIGHT PHOTORECEPTORS IN PARSLEY CELLS

Abstract— Ultraviolet-B (UV-B) and blue light photoreceptors have been shown to regulate chalcone synthase and flavonoid synthesis in parsley cell cultures. These photoreceptors have not yet been identified. In the present work, we studied UV-B photoreception with physiological experiments involving temperature shifts and examined the possible role of flavin in blue and UV-B light photoreception. Cells irradiated with UV-B light (0.5–15 min) at 2°C have the same fluence requirement for chalcone synthase and flavonoid induction as controls irradiated at 25°C. This is indicative of a purely photochemical reaction. Cells fed with riboflavin and irradiated with 6 h of UV-containing white light synthesize higher levels of chalcone synthase and flavonoid than unfed controls. This effect did not occur with blue light. These results indicate that flavin-sensitization requires excitation of flavin and the UV-B light photoreceptor. The in vivo kinetics of flavin uptake and bleaching indicate that the added flavin may act at the surface of the plasma membrane. In view of the likely role of membrane-associated flavin in photoreception, we measured in vitro flavin binding to microsomal membranes. At least one microsomal flavin binding site was solubilized by resuspension of a microsomal pellet in buffer with high KPi and NaCl concentrations and centrifugation at 38000 g. The 38000 g insoluble fraction had much greater flavin binding and contained a receptor with an apparent K_D of about 3.6 μM and an estimated in vivo concentration of at least 6.7 × 10–⁸M. Flavin mononucleotide, roseoflavin, and flavin adenine dinucleotide can compete with riboflavin for this binding site(s), although each has lower affinity than riboflavin. Most microsomal protein was solubilized by resuspension of the microsomal pellet in non-denaturing detergents and centrifugation at 38 000 g ; however, this inhibited flavin binding, presumably because of disruption of the environment of the flavin receptor. The parsley microsomal flavin binding receptor(s) have a possible role in physiological photoreception.     

QUANTUM EFFICIENCY AND PHOTOSENSITIVITY OF THE RHODOPSIN ⇌ METARHODOPSIN CONVERSION IN CRAYFISH PHOTORECEPTORS

Photosensitivity (K_λ) of a visual pigment is the product of the molecular absorption coefficient (α_λ) and the quantum efficiency for photoconversion (γ). Among the invertebrates, many visual pigments are stable not only in the rhodopsin (R) conformation but also as the photoproduct, metarhodopsin (M), We here employ a method for determining the photosensitivities of the two stable pigments of a rhodopsin-metarhodopsin pair, using kinetic analysis of fluorescence from metarhodopsin combined with measurements of spectral absorption made before and after saturation at the isosbestic wavelength of the pigment pair. A curve fitting technique, in which a theoretical function is scaled for best fit to the measured absorption spectrum of the photosteady-state mixture, yields values for the photosensitivity of rhodopsin at λ._max, the ratio of quantum efficiencies for rhodopsin—metarhodopsin interconversion, and the fractional composition of the steady-state mixture. With knowledge of the molecular extinction coefficient, the absolute values of quantum efficiency can be calculated. For crayfish ( Orconectes, Procambarus ) rhodopsin, measured in isolated rhabdoms, K_max= 1.05 x 10^-16 cm² at 535 nm with >7λR→M0.69. These values are similar to the photosensitivity and quantum efficiency of bleaching of vertebrate rhodopsins in digitonin solution (Dartnall, 1972). For the metarhodopsin, K_max= 1.02 x 10^-16 cm² at 510 nm, and λ_M-R= 0.49.     

ACTION SPECTRUM AND THRESHOLD SENSITIVITY OF ENTRAINMENT OF ORCADIAN RUNNING ACTIVITY IN THE COCKROACH PERIPLANETA AMERICANA

Abstract— The interaction between the compound eye and the circadian timing system for running activity in the cockroach, Periplaneta americana , has been investigated by measuring the action spectrum for entrainment of such behavior by imposed photic stimuli. Test wavelengths were selected which spanned the spectral range of the dichromatic retina of this insect and intensities of such lights for which half of a population of animals failed to be entrained (threshold) were determined. Stimuli were presented either with a 12:12 light–dark regime or for 12h/day on a continuous background of orange light. The latter procedure altered the balance of sensitivity in the two groups of color receptors in the eye. A comparison of the resulting action spectra and the spectral sensitivities of receptors and visual interneurons measured under similar conditions suggest that the entrainment mechanism is dominated by the receptor group most sensitive to long wavelength light. Furthermore, the eye-clock mechanism shows an extraordinarily high sensitivity to such stimuli. We estimate that a mean flux of about 5 photons/eye/s is sufficient to entrain the behavior. The implications of these results are discussed.     

THE EFFECT OF LIGHT QUALITY IN Prunus cerasus. I. PHOTORECEPTORS INVOLVED IN INTERNODE ELONGATION AND LEAF EXPANSION IN JUVENILE PLANTS

Evidence is presented to suggest that stem elongation in this woody species is controlled not only by phytochrome as previously suggested for other woody species, but also by a specific blue absorbing pigment (BAP). In contrast, the BAP seems to have no role to play in the development of total leaf area in these juvenile specimens.     

EFFECTS OF PHASIC AND TONIC LIGHT INPUTS ON THE ORCADIAN ORGANIZATION OF THE SQUIRREL MONKEY

Abstract— The organization of the circadian timing system in Saimiri sciureus was probed using the phasic (abrupt transition) and tonic (continuous action) effects of light intensity. The behavior of the simultaneously monitored circadian rhythms of feeding behavior, colonic temperature, and urinary potassium excretion was studied in response to the phasic effects of (a) an abrupt 8-h phase delay in the light–dark (LD) cycle and (b) a series of non-24 h LD cycles ( T = 18 to 30 h). These studies demonstrated that the feeding and temperature rhythms were more tightly coupled to the light-dark cycle than was the rhythm of urinary potassium excretion. The tonic effects of constant levels of illumination confirmed this conclusion. In constant light, internal desynchronization spontaneously occurred in 25% of animals with the potassium rhythm exhibiting a period quite different from that of the feeding and colonic temperature rhythms. Thus, the response of the internal circadian timekeeping system to phasic and tonic light inputs shows that the system in this species comprises at least two potentially independent oscillators with differential light sensitivities.     

THE SENSORY PHOTORECEPTORS OF Halobacterium halobium REVISITED: ACTION SPECTRA and INFLUENCE OF BACKGROUND LIGHT*

Abstract– Action spectra of the light-dependent behavior of Halobacterium and the effect of background light have been measured with regard to the current hypothesis of Spudich and Bogomolni [Nature 312 ,509–513 (1984)], which proposes sensory rhodopsin I (sRI₅₈₇) to be the receptor for long-wavelength light, and its photoproduct S₃₇₃ to be the receptor for UV light. The action spectrum shows three maxima for attractant responses (prolonged swimming intervals) at 565, 590, and 610 nm, and two maxima for repellent responses (shortened intervals) at 370 and 480 nm. The latter is assigned to sensory rhodopsin II (P-480). All peaks are red-shifted after substitution of the endogeneous retinal by 3, 4-dehydroretinal. The peaks at 590 and 610 nm are suppressed by long-wavelength background light. Ultraviolet background light converts all attractant peaks into repellent peaks. The response at 370 nm is strongly activated by visible background light, the maximal effect occurring with 510 nm. The activated state declines with a half-life of about 1.2 s. In a growing culture, full sensitivity to UV and blue light is restored about 10 h earlier than sensitivity to long-wavelength light. Some of the results cannot easily be explained by the sRI₅₈₇/S₃₇₃ hypothesis. Explanations for the three maxima in the long-wavelength range and for the maximal activation of the UV response by 510 nm light are discussed.     

LIGHT DISTRIBUTION AND ELECTRON DONATION IN THE Z SCHEME*

Abstract— Several recent reports indicate that the photosynthetic machinery possesses a device whereby the relative distribution of absorbed light energy into the two reaction centers can be varied in response to the environment in which the photosynthesizing cells or chloro-plasts are placed. Conditions reported which affect this distribution include the electron acceptor or electron donor system employed, the concentration of magnesium ions, and the preillumination history of the preparation. These observations are described and discussed in terms of their relevance to the interpretation of quantum yield and enhancement data. Several laboratories have presented evidence recently that chloroplasts, after undergoing a pretreatment which destroys their capacity to evolve oxygen, can still transfer electrons via both photosystems, provided an appropriate electron donor is added. It is shown that electron donors like manganous ions and ascorbate can compete effectively with water as electron donors to Photosystem II, in untreated chloroplasts.     

ARE THERE SEVERAL PHOTORECEPTORS INVOLVED IN PHOTOTROPISM OF Phycomyces blakesleeunus? KINETIC STUDIES OF DICHROMATIC IRRADIATION*

Abstract— Photogeotropic equilibrium angles were measured for Phycomyces blakesleeanus wild type firstly by means of dichromatic fluence rate response curves using simultaneous irradiation with near threshold 450 nm reference light (constant at 1.2 × 10^?8 W m^?2) and variable fluence rates of test light (498–630 nm) from the same side. These curves showed minima for test light fluence rates that were close to the photogeotropic threshold for these wavelengths. Secondly, the time course of this inhibitory effect was studied with both the inductive reference 450 nm light (2 × 10^?-⁷ W m^?2) and the test light (606 or 450 nm) given as light pulses of 2 s duration (2 s light/48 s dark periods for 6 h). The dark period between the onset of the inductive reference light and test light pulses was varied between 0 and 48 s. No inhibitory effects were observed for simultaneous pulses; however, inhibitory effects were demonstrated for delay times of 2 s and 20 s for 606 nm as well as 450 nm test light. If the test light pulses were given immediately before the inductive reference light, only 606 nm test light was effective in producing a significant inhibitory effect. The results are discussed with regard to a multichromophoric photoreceptor system and to the wavelength dependence of the effects observed. The data and conclusions favor a photoreceptor system with at least two separate chromophoric absorptions of the blue light receptor type, one acting positively, the other acting inhibitorily, and at least one other photoreceptor of presumably minor influence.     

BLUE LIGHT PHOTORECEPTION IN Neurospora ORCADIAN RHYTHM: EVIDENCE FOR INVOLVEMENT OF THE FLAVIN TRIPLET STATE

The mechanism of the photoreceptor acting on the circadian conidiation rhythm of Neurospora crassa was studied, with the following results: (1) the efficiency of 8-haloflavins as sensitizers increased with their triplet yields. (2) Phase shifts were not abolished by removal of oxygen prior to illumination. (3) Oxygen inhibited phase shifts when introduced into the cultures after light treatment. It is proposed that the blue light photoreceptor for the circadian clock of Neurospora crassa acts (1) from its triplet state, but (2) not via singlet oxygen; (3) signal transduction involves (an) oxygen-sensitive intermediate(s).     

THE ROLE OF ARGININES 82 AND 227 IN THE BACTERIORHODOPSIN PROTON PUMP*

Abstract— Arginine residues 82 and 227 in bacteriorhodopsin were replaced by glutamine residues, using the site-directed mutagenesis techniques. Mutant bacteriorhodopsins were found to be competent in formation and decomposition of the photocycle M412 intermediate as well as in generation of photoelectric potential provided that pH of the medium is sufficiently high. Lowering of pH results in transition of bacteriorhodopsin into a blue acidic form which cannot produce M412 and photo-potential. The pK values of these transitions for Arg-227 → Gln and Arg-82 → Gln mutants are shifted correspondently for 1 and 4 pH units to a higher pH region in comparison with native bacteriorhodopsin. The rate of the M412 formation in both mutants was similar to that in the native protein. As to M412 decay, it is much slower in Arg-227 → Gln mutant than in native and Arg-82 → Gln bacteriorhodopsins. In all cases, the decay depends only slightly upon pH. It is concluded that Arg-82 is involved in maintenance of a bacteriorhodopsin structure that is resistant to the pH decrease down to 4 whereas Arg-227 is required first of all for the process of Schiff base reprotonation.