A THEORY OF VISUAL INTENSITY DISCRIMINATION

1. A theory of visual intensity discrimination is proposed in terms of the photochemical events which take place at the moment when a photosensory system already adapted to the intensity I is exposed to the just perceptibly higher intensity I+ΔI. Unlike previous formulations this theory predicts that the fraction ΔI/I, after rapidly decreasing as I increases, does not increase again at high intensities, but reaches a constant value which is maintained even at the highest intensities. 2. The theory describes quantitatively the intensity discrimination data of Drosophila, of the bee, and of Mya. 3. With some carefully considered exceptions the intensity discrimination data of the human eye fall into two classes: those with small test areas or with red light, which form a single continuous curve describing the function of the retinal cones alone, and those with larger areas, and with white, orange, and yellow light, which form a double curve showing a clear inflection point, and represent the separate function of the rods at intensities below the inflection point and of the cones at intensities above it. 4. The theory describes all these data quantitatively by treating the rods and cones as two independently functioning photosensory systems in accordance with the well established duplicity idea. 5. In terms of the theory the data of intensity discrimination give critical information about the order of both the photochemical and dark reactions in each photosensory system. The reactions turn out to be variously monomolecular and bimolecular for the different animals.     

The quenching effect of blue light on halorhodopsin

A mutant of Halobacterium halobium which contains halorhodopsin was isolated from strain S₉. An absorbance change at 380 nm caused by steady orange light illumination (λ ?530 nm) was observed. This change depended upon the intensity of the actinic light. The bleached envelope vesicles and vesicles derived from nicotine-grown cells showed a small or no absorbance change at 380 nm, suggesting that the change stemmed from the photochemical intermediate of halorhodopsin (referred to as P-380). When blue light was superimposed on orange background illumination, the membrane potential (Δψ) of the envelope vesicles decreased. Δψ was determined from the tetraphenylphosphonium cation (TPP⁺) distribution by means of a TPP⁺ electrode. When blue light intensity was increased, both Δψ and the amount of P-380 were decreased. An equation was derived which showed that Δψ is proportional to the concentration of P-380 formed by illumination under the assumption that the ionic composition is not significantly changed upon illumination. This equation was checked experimentally from the following three points: The blue light effect, the relationship between Δψ and light intensity, and the effect of gramicidin. The data obtained accorded well with the theoretical relationship.     

THE VISUAL DISCRIMINATION OF INTENSITY AND THE WEBER-FECHNER LAW

1. A study of the historical development of the Weber-Fechner law shows that it fails to describe intensity perception; first, because it is based on observations which do not record intensity discrimination accurately, and second, because it omits the essentially discontinuous nature of the recognition of intensity differences. 2. There is presented a series of data, assembled from various sources, which proves that in the visual discrimination of intensity the threshold difference ΔI bears no constant relation to the intensity I. The evidence shows unequivocally that as the intensity rises, the ratio See PDF for Equation first decreases and then increases. 3. The data are then subjected to analysis in terms of a photochemical system already proposed for the visual activity of the rods and cones. It is found that for the retinal elements to discriminate between one intensity and the next perceptible one, the transition from one to the other must involve the decomposition of a constant amount of photosensitive material. 4. The magnitude of this unitary increment in the quantity of photochemical action is greater for the rods than for the cones. Therefore, below a certain critical illumination—the cone threshold—intensity discrimination is controlled by the rods alone, but above this point it is determined by the cones alone. 5. The unitary increments in retinal photochemical action may be interpreted as being recorded by each rod and cone; or as conditioning the variability of the retinal cells so that each increment involves a constant increase in the number of active elements; or as a combination of the two interpretations. 6. Comparison with critical data of such diverse nature as dark adaptation, absolute thresholds, and visual acuity shows that the analysis is consistent with well established facts of vision.     

The Spatial Variability of the Fluorescent Characteristics of Phytoplankton in the Black Sea

The spatial variability of the photochemical efficiency of phytoplankton photosystem II was analyzed in two contrasting regions of the Black Sea: the western part of the deepwater region and a region influenced by the Danube River discharge. The fluorescence values for open (F₀) and closed (F_m) photosystem II reaction centers in the investigated areas varied by an order of magnitude and correlated closely. The potential photochemical efficiency of phytoplankton photosystem II (F_m ? F₀)/F_m varied from 0.16 to 0.70. Three types of the vertical distribution of this index were found. In the first type, the values increased from the surface to the top of the thermocline and then remained stable down to the bottom of the euphotic zone; in the second type, they increased from the surface to the bottom of the euphotic zone; in shallow areas, they were stable within the euphotic zone. With an increase in light intensity, the phytoplankton photochemical efficiency decreased. The light inhibition of the photosystem II efficiency was more intense in the deeper than in the upper layers of the euphotic zone.     

THE EFFECT OF EXPOSURE PERIOD AND TEMPERATURE ON THE PHOTOSENSORY PROCESS IN CIONA

1. Experiments are presented which show that the latent period in the photosensory response of Ciona is inversely proportional to the duration of the exposure period to light. From this it is found that the velocity of the chemical reaction which determines the latent period is directly proportional to the concentration of photochemical products formed during the exposure period. This is interpreted as showing that the two processes form a coupled photochemical reaction, of which the secondary reaction proceeds only in the presence of products from the primary reaction. This coupling may be a catalysis or a direct chemical relation. 2. Further experiments show that the relation between temperature and the latent period is accurately described by the Arrhenius equation in which µ = 16,200. The precise numerical value of µ tentatively identifies the latent period process as an oxidation reaction which is catalyzed by iron. 3. The photocatalytic properties of certain iron compounds are used as a model for the coupled photochemical reaction suggested for the photosensory mechanism of Ciona and Mya.     

Evidences from Action Spectra for a Specific Participation of Chlorophyll b in Photosynthesis

Rate of oxygen evolution in photosynthesis was measured as the current from a polarized platinum electrode covered by a thin layer of Chlorella. The arrangement gave a reproducibly measurable rate of photosynthesis proportional to light intensity at the low levels used and gave rapid response to changes in illumination. Two phenomena have been explored. The Emerson effect was observed as an enhancement of photosynthesis in long wavelength red light (700 mµ) when shorter wavelengths were added. Two light beams of wavelengths 653 and 700 mµ when presented together gave a photosynthetic rate about 25 per cent higher than the sum of the rates obtained separately. Large and reproducible transients in rate of oxygen evolution were observed accompanying change in illumination between two wavelengths adjusted in intensity to support equal steady rates of photosynthesis. The transients were found not to be specifically related to long wavelength red light. Both enhancement and the transients have identical action spectra which are interpreted as demonstrating a specific photochemical participation of chlorophyll b.     

Flash-induced photophosphorylation in Rhodospirillum rubrum chromatophores I. The relationship between cytochrome c-420 content and photophosphorylation

The content of cytochrome c-420 in Rhodospirillum rubrum chromatophores prepared by grinding with alumina is 5–10% of that in whole cells, and 20–40% in chromatophores by ‘French’ pressing.Flash-induced phosphorylation of various chromatophores which varied in cytochrome content from 7 to 40% is proportional to the cytochrome content. Extrapolating the cytochrome c-420 content to that observed in whole cells, a ratio $\text{ATP}\text{P}{}^{\mathrm{+}}\text{X}{}^{\mathrm{?}}$ near 1 is calculated. At low flash intensity the phosphorylation per flash is proportional to flash energy.Photophosphorylation in flashes given after a time of several minutes is only slightly dependent on the number of flashes. If the flashes are spaced from 0.1 to 10 s, relative phosphorylation in the first flash is about 70% and in the second 90% of that observed in the following flashes. Proton binding is not affected by the cytochrome c-420 content and a ratio of $\text{H}{}^{\mathrm{+}}\text{P}{}^{\mathrm{+}}\text{X}{}^{\mathrm{?}}$ of 2.3 was found.These results can be explained by a working hypothesis in which charge separation occurring at one reaction centre and the resulting electron transport mediated amongst others by c-420, results in the injection of two protons into an ATPase, this in contrast to a chemiosmotic mechanism, where the protons are released in the chromatophore inner space.     

THE INFLUENCE OF LIGHT AND CARBON DIOXIDE ON PHOTOSYNTHESIS

1. An optical system is described which furnishes an intensity of 282,000 meter candles at the bottom of a Warburg manometric vessel. With such a high intensity available it was possible to measure the rate of photosynthesis of single fronds of Cabomba caroliniana over a large range of intensities and CO₂ concentrations. 2. The data obtained are described with high precision by the equation KI = p/(p ² _max. – p ²)^½ where p is the rate of photosynthesis at light intensity I, K is a constant which locates the curve on the I axis, and p _max. is the asymptotic maximum rate of photosynthesis. With CO₂ concentration substituted for I, this equation describes the data of photosynthesis for Cabomba, as a function of CO₂ concentration. 3. The above equation also describes the data obtained by other investigators for photosynthesis as a function of intensity, and of CO₂ concentration where external diffusion rate is not the limiting factor. This shows that for different species of green plants there is a fundamental similarity in kinetic properties and therefore probably in chemical mechanism. 4. A derivation of the above equation can be made in terms of half-order photochemical and Blackman reactions, with intensity and CO₂ concentration entering as the first power, or if both sides of the equation are squared, the photochemical and Blackman reactions are first order and intensity and CO₂ enter as the square. The presence of fractional exponents or intensity as the square suggests a complex reaction mechanism involving more than one photochemical reaction. This is consistent with the requirement of 4 quanta for the reduction of a CO₂ molecule.     

REACTIONS OF LIMULUS TO ILLUMINATED FIELDS OF DIFFERENT AREA AND FLICKER FREQUENCY

In phototropic tests with young Limulus, the phototropic reactions to flickering fields were studied. If the two fields are equal in area and brightness but different in flicker frequency, the number of animals going to the two fields is proportional to their flicker frequencies. Equal stimulating effects of two fields differing in flicker frequency are obtained by reduction of the area of the faster flickering field. The areas for equal effect must be inversely proportional to their flicker frequencies. It seems that equal effects are dependent upon equality of the number of active excitation elements per unit of time.     

Sequential Photochemical and Microbial Degradation of Organic Molecules Bound to Humic Acid

We studied the effects of photochemical processes on the mineralization by soil microorganisms of [2-¹⁴C]glycine bound to soil humic acid. Microbial mineralization of these complexes in the dark increased inversely with the molecular weight of the complex molecules. Sunlight irradiation of glycine-humic acid complexes resulted in loss of absorbance in the UV range and an increase in the amount of ¹⁴C-labeled low-molecular-weight photoproducts and the rate and extent of mineralization. More than half of the radioactivity in the low-molecular-weight photoproducts appears to be associated with carboxylic acids. Microbial mineralization of the organic carbon increased with solar flux and was proportional to the loss of A₃₃₀. Mineralization was proportional to the percentage of the original complex that was converted to low-molecular-weight photoproducts. Only light at wavelengths below 380 nm had an effect on the molecular weight distribution of the products formed from the glycine-humic acid complexes and on the subsequent microbial mineralization. Our results indicate that photochemical processes generate low-molecular-weight, readily biodegradable molecules from high-molecular-weight complexes of glycine with humic acid.     

THE QUANTUM YIELD OF HYDROGEN AND CARBON DIOXIDE ASSIMILATION IN PURPLE BACTERIA

1. The effect of H₂ tension, CO₂ tension, pH, time, light intensity, density of suspension, salt content of the medium, and certain spectral regions on the rate of photoassimilation of H₂ and CO₂ by Streptococcus varians has been studied. 2. The method of making light absorption measurements with thin suspensions of bacteria is described. 3. A light source, optical system, and filter for isolating 852 mµ with 894 mµ in sufficient intensity for photochemical work and an improved design of thermostat are given. 4. The photoassimilation of 2H₂ with 1CO₂ apparently involves little over all energy change but nevertheless requires 4 quanta.     

SENSORY ADAPTATION AND THE STATIONARY STATE

1. Experiments are described which measure the sensitivity of animals exposed to continued illumination to which they have become adapted. It is shown that the amount of outside light energy necessary to stimulate an adapted animal increases with the intensity of the adapting illumination. 2. The data are analyzed quantitatively in terms of the reversible reaction S ⇌ P + A shown previously to account for the photic sensitivity of these animals. This analysis demonstrates that, though the amount of incident energy necessary for a minimal response varies with the adapting intensity, the actual amount of photochemical decomposition required to set off the sensory mechanism is a constant quantity. 3. The ability of these animals to come into sensory equilibrium with any sustained illumination is accounted for quantitatively by the presence of a stationary state in the reversible photochemical reaction S ⇌ P + A during which the concentrations of the three components are constant. 4. It is shown that the concentrations of these substances at the stationary state are automatically controlled by the outside intensity. Therefore, given the sensory mechanism as a basis, the adaptation of the animals to light and the consequent changes in sensitivity, are determined entirely by the light to which the animals are exposed. 5. Because of the properties of the stationary state, and of the constancy of photochemical decomposition for a minimal effect, it is suggested that the sensory system is not only the traditional receptor system, but is also a protecting layer which stabilizes and buffers the relation between the nervous system and the environment.     

Fluorescence Properties of Wild-Type Chlamydomonas reinhardi and Three Mutant Strains Having Impaired Photosynthesis

The wild-type strain of Chlamydomonas reinhardi and 3 mutant strains ac-21, ac-141, and ac-115 have been compared for their fluorescence (and luminescence) properties. The different fluorescence levels, the rapid and slow photochemical responses affecting fluorescence, and the intensity of luminescence have been studied under various conditions: air, nitrogen, 3(p-chlorophenyl)-1,1-dimethylurea. The strain ac-21 exhibits fluorescence properties only quantitatively different from those of the wild-type strain, and it is believed to be affected in some component of the electron transport chain between the 2 light reactions. Both ac-141 and ac-115 have an abnormally high initial fluorescence level; ac-115 does not show the normal photochemical response associated with System II and has a very low luminescence. Mutant strains ac-141 and ac-115 both seem to be modified in the System II photochemical center. These conclusions are compared with a previous analysis based on absorbance changes of cytochrome 559.     

Photodynamic Alteration of Sodium Currents in Lobster Axons

Photodynamic alteration of lobster giant axons drastically changed the magnitude and kinetics of sodium currents seen under voltage clamp using the sucrose gap technique. Illumination of axons following treatment with acridine orange or eosin Y decreased the maximum sodium conductance to a zero asymptote as an exponential function of illumination time. Normal sodium inactivation was slowed, with τ_h more than doubled depending on experimental conditions. A second slower inactivation rate developed occasionally. τ_h was altered little, if at all. Sodium current "tails" were not prolonged. At maximum light intensity and with eosin Y as sensitizer leakage current increased after 4–10 sec in light. These changes were irreversible. Decreases in maximum sodium conductance correlated highly with increases in time to peak sodium current. The magnitude of change varied linearly with light intensity. The action spectra for eosin Y and acridine orange peaked near 545 and 505 nm, respectively. The magnitude of change varied with preillumination dye exposure time in a quasi-exponential approach to a maximum effect. Sodium dithionite protected the axon from photodynamic change.     

DARK ADAPTATION AND THE DARK GROWTH RESPONSE OF PHYCOMYCES

Light-adapted sporangiophores of the fungus Phycomyces respond to sudden darkening by a temporary decrease in the rate of elongation, after a latent period of several minutes. The reaction time of this "dark growth" response is compound like that of the "light growth" response. It is, moreover, shorter the more intense the previous illumination. The rate of dark adaptation following adaptation to a very large range of light intensities is found to be proportional to the logarithm of the preceding light intensity. It is shown that a constant amount of dark adaptation takes place before the response occurs. On the assumption that changes in the rate of growth reflect changes in the concentration of a substance which at constant light intensity is in equilibrium with a light-sensitive material, possible equations for such a photostationary state are examined. The most reasonable formulation requires that the partial velocity of the "light" reaction be taken proportional to log I instead of to I directly.     

Photosynthetic response of poikilochlorophyllous desiccation-tolerant Pleurostima purpurea (Velloziaceae) to dehydration and rehydration

The poikilochorophyllous, desiccation-tolerant (PDT) angiosperm, Pleurostima purpurea, normally occurs in less exposed rock faces and slightly shady sites. Our aim was to evaluate the light susceptibility of the photosynthetic apparatus during dehydration-rehydration cycle in P. purpurea. In a controlled environment, the potted plants were subjected to water deficit under two different photosynthetic photon flux densities [PPFD, 100 and 400 μmol(photon) m^?2 s^?1]. In the higher PPFD, net photosynthetic rate (P _N) become undetectable after stomata closure but photochemical efficiency of photosystem II, electron transport rate, and photochemical quenching coefficient were maintained relatively high, despite a partial decrease. The photochemical activity was inhibited only after the complete loss of chlorophylls, when leaf relative water content dropped below 72% and total carotenoids reached maximal accumulation. Nonphotochemical energy dissipation increased earlier in response to dehydration under higher PPFD. P _N and photochemical activity were fully recovered after rehydration under both light treatments. Our results suggested that the natural occurrence of P. purpurea should not be restricted by the light intensity during the complete desiccation-rehydration cycles.     

INTENSITY AND THE PROCESS OF PHOTORECEPTION

1. In the photosensory process of Mya the latent period varies inversely as the intensity of the stimulating light. 2. Quantitative analysis of the data shows that the photochemical effect of the light is a logarithmic function of its intensity, the two variables being related to each other according to the well known "compound interest" law. 3. Comparison with previous experiments demonstrates that the Reciprocity Law of Bunsen and Roscoe applies to the photosensory process not only for the minimum energy required for a response, but for a much greater range of energy application as well.     

THE ORIENTATION OF ANIMALS BY OPPOSED BEAMS OF LIGHT

When orientation is attained under the influence of beams of parallel light opposed at 180° the deflection θ from a path at right angles to the beams is given by tan See PDF for Equation, where I ₁ and I ₂ are the photic intensities and H is the average angle between the photoreceptive surfaces. This expression is independent of the units in which I is measured, and holds whether the primary photosensory effect is proportional to I or to log I. When photokinetic side-to-side motions of the head occur, H decreases with increasing total acting light intensity, but increases if higher total light intensity restricts the amplitude of random movements; in each case, H is very nearly proportional to log I ₁ I ₂. For beams of light at 90°, See PDF for Equation. The application of these equations to some particular instances is discussed, and it is shown why certain simpler empirical formulæ previously found by others yield fair concordance with the experimental data. The result is thus in complete accord with the tropism theory, since the equations are based simply on the assumption that when orientation is attained photic excitation is the same on the two sides.