THE KINETICS OF DARK ADAPTATION

1. Data are presented for the dark adaptation of four species of animals. They show that during dark adaptation the reaction time of an animal to light of constant intensity decreases at first rapidly, then slowly, until it reaches a constant minimum. 2. On the assumption that at all stages of adaptation a given response to light involves a constant photochemical effect, it is possible to describe the progress of dark adaptation by the equation of a bimolecular reaction. This supposes, therefore, that dark adaptation represents the accumulation within the sense cells of a photosensitive material formed by the chemical combination of two other substances. 3. The chemical nature of the process is further borne out by the fact that the speed of dark adaptation is affected by the temperature. The velocity constant of the bimolecular process describing dark adaptation bears in Mya a relation to the temperature such that the Arrhenius equation expresses it with considerable exactness when µ = 17,400. 4. A chemical mechanism is suggested which can account not only for the data of dark adaptation here presented, but for many other properties of the photosensory process which have already been investigated in these animals. This assumes the existence of a coupled photochemical reaction of which the secondary, "dark" reaction is catalyzed by the products of the primary photochemical reaction proper. This primary photochemical reaction itself is reversible in that its main products combine to form again the photosensitive material, whose concentration controls the behavior of the system during dark adaptation.     

DARK ADAPTATION AND THE LIGHT-GROWTH RESPONSE OF PHYCOMYCES

1. A single-celled, elongating sporangiophore of Phycomyces responds to a sufficient increase in intensity of illumination by a brief increase in growth rate. This is the "light-growth response" of Blaauw. 2. The reaction time is compound, consisting of an exposure period and a latent period (this comprising both the true latent period resulting from photochemical action and any "action time" necessary for the response). During the latter period the plant may be in darkness, responding nevertheless at the end of the latent period. 3. Both light adaptation and dark adaptation occur in the sporangiophore. The kinetics of dark adaptation can be accounted for on the basis of a bimolecular reaction, perhaps modified by autocatalysis. Attention is called to the bimolecular nature of the "dark" reaction in all other photosensory systems that have been studied, in spite of the diversity of the photosensitive substances themselves and of the different forms of the responses to light.     

DARK ADAPTATION IN DINEUTES

The level of dark adaptation of the whirligig beetle can be measured in terms of the threshold intensity calling forth a response. The course of dark adaptation was determined at levels of light adaptation of 6.5, 91.6, and 6100 foot-candles. All data can be fitted by the same curve. This indicates that dark adaptation follows parts of the same course irrespective of the level of light adaptation. The intensity of the adapting light determines the level at which dark adaptation will begin. The relation between log aI ₀ (instantaneous threshold) and log of adapting light intensity is linear over the range studied.     

SENSORY EQUILIBRIUM AND DARK ADAPTATION IN MYA ARENARIA

1. The reaction time of Mya to light is composed of two parts. The first, a sensitization period, is an exceedingly short interval of the order of magnitude associated with photographic processes. The second is a latent period of about 1.3 seconds, during which Mya need not remain exposed to the stimulating light. 2. The process of dark adaptation in Mya is orderly. Its progress may be represented by the formation of a photosensitive substance according to the dynamics of a bimolecular reaction. See PDF for Structure 3. Photosensory equilibrium as represented by the light- and dark-adapted conditions finds a rational explanation in terms of the "stationary state" of a reversible photochemical reaction involving a photosensitive substance and its two precursors. 4. There are two corollaries to this hypothesis. The first requires that the reaction time at sensory equilibrium for a given intensity should vary inversely with the temperature; the second, that the rate of dark adaptation should vary directly with the temperature. Experiments verified both of these requirements.     

DARK ADAPTATION IN AGRIOLIMAX

A method is described which measures the excitation of Agriolimax by light, during the progress of light adaptation, by assuming that the orientating effect of continuous excitation is expressed as a directly proportionate tension difference in the orienting muscles of the two sides of the body. The tendency toward establishment of such a tension difference is caused to work against a similar geotropic effect at right angles to the phototropic one. This enables one to study the kinetics of light adaptation, and of dark adaptation as well. The situation in the receptors is adequately described by the paradigm See PDF for Equation similar to that derived by Hecht for the differential sensitivity of various forms, but with the difference that the "dark" reaction is not only "bimolecular" but also autocatalysed by the reaction product S. The progress of dark adaptation is reflected (1) in the recovery of the amplitude of the orientation and (2) in the rates of light adaptation at different levels of the recovery; each independently supports these assumptions, for which the necessary equations have been provided. These equations also account for the relative variabilities of the angles of orientation, and, more significantly, for the two quite different kinds of curves of dark adaptation which are obtained in slightly different types of tests.     

THE NATURE OF FOVEAL DARK ADAPTATION

1. After a discussion of the sources of error involved in the study of dark adaptation, an apparatus and a procedure are described which avoid these errors. The method includes a control of the initial light adaptation, a record of the exact beginning of dark adaptation, and an accurate means of measuring the threshold of the fovea after different intervals in the dark. 2. The results show that dark adaptation of the eye as measured by foveal vision proceeds at a very precipitous rate during the first few seconds, that most of the adaptation takes place during the first 30 seconds, and that the process practically ceases after 10 minutes. These findings explain much of the irregularity of the older data. 3. The changes which correspond to those in the fovea alone are secured by correcting the above results in terms of the movements of the pupil during dark adaptation. 4. On the assumption that the photochemical effect of the light is a linear function of the intensity, it is shown that the dark adaptation of the fovea itself follows the course of a bimolecular reaction. This is interpreted to mean that there are two photolytic products in the fovea; that they are disappearing because they are recombining to form anew the photosensitive substance of the fovea; and that the concentration of these products of photolysis in the sense cell must be increased by a definite fraction in order to produce a visual effect. 5. It is then suggested that the basis of the initial event in foveal light perception is some mechanism that involves a reversible photochemical reaction of which the "dark" reaction is bimolecular. Dark adaptation follows the "dark" reaction; sensory equilibrium is represented by the stationary state; and light adaptation by the shifting of the stationary state to a fresh point of equilibrium toward the "dark" side of the reaction.     

The Duplex Nature of the Retina of the Nocturnal Gecko as Reflected in the Electroretinogram

The effect of light and dark adaptation on the electrical activity in two species of nocturnal gecko, Hemidactylus turcicus and Tarentola mauritanica was studied. The electroretinogram of both species changes from the scotopic type in the dark-adapted state to the photopic type after strong light adaptation. For the scotopic response fusion frequencies up to 18 flashes per sec. are obtained in both species. For the photopic response fusion frequencies up to 50 flashes per sec. are seen in Tarentola, and up to 25 flashes per sec. in Hemidactylus. Proceeding from dark to light adaptation the increment threshold (dI) is measured at different levels of adaptive illumination (I). At low levels of illumination the dI/I ratio is found to be small and at high levels of illumination to be large. No difference in the dI/I ratio is obtained for test lights of 462 and 605 mµ. During dark adaptation the change of threshold after exposure to moderate and weak lights (up to 10³ times dark threshold) is rather fast. After light adaptation to strong light (10⁶ times dark threshold) duplex dark adaptation curves are seen with a break separating a fast and a slow phase of dark adaptation. The significance of these results from a retina which possesses sense cells of only one type is discussed.     

CRITICAL ILLUMINATION AND CRITICAL FREQUENCY FOR RESPONSE TO FLICKERED LIGHT,IN DRAGONFLY LARVAE

Curves relating flicker frequency (F) to mean critical illumination (I_m) for threshold response to flickered light, with equal durations of light and no light intervals, and relating illumination (I) to mean critical flicker frequency (F_m) for the same response, have been obtained from homogeneous data based upon the reactions of dragonfly larvae (Anax junius). These curves exhibit the properties already described in the case of the fish Lepomis. The curve for F_m lies above the curve of I_m by an amount which, as a function of I, can be predicted from a knowledge either of the variation of I_m or of F_m. The law of the observable connection between F and I is properly expressed as a band, not as a simple curve. The variation of I_m (and of F_m) is not due to "experimental error," but is an expression of the variable character of the organism''s capacity to exhibit the reaction which is the basis of the measurements. As in other series of measurements, P.E. _I is a rectilinear function of I_m; P.E. _F passes through a maximum as F (or I) increases. The form of P.E. _F as a function of I can be predicted from the measurements of P.E. _I. It is pointed out that the equations which have been proposed for the interpretation of curves of critical flicker frequency as a function of intensity, based upon the balance of light adaptation and dark adaptation, have in fact the character of "population curves;" and that their contained constants do not have the properties requisite for the consistent application of the view that the shape of the F - I curve is governed by the steady state condition of adaptation. These curves can, however, be understood as resulting from the achievement of a certain level of difference between the average effect of a light flash and its average after effect during the dark interval.     

Diminution and enlargement of the mosquito rhabdom in light and darkness

The rhabdoms of the larval ocelli of the mosquito Aedes aegypti undergo morphological light and dark adaptation over periods of hours. The rhabdom enlarges during dark adaptation and grows smaller during light adaptation. Diminution is exponential, enlargement linear, and rates of change are proportional to log light intensity. Rhabdoms maintained at a constant intensity level off at a constant volume proportional to log intensity. We argue that changes in rhabdom volume after changes in light intensity reflect an influence of light on the turnover of photoreceptro membrane, and that the volumes at which rhabdoms level off represent equilibria between opposed processes of membrane loss and renewal.     

THE DARK ADAPTATION OF THE HUMAN EYE

During the dark adaptation of the human eye, its visual threshold decreases to a small fraction of its original value in the light. An analysis of the quantitative data describing this adaptation shows that it follows the course of a bimolecular chemical reaction. On the basis of these findings it is suggested that visual reception in dim light is conditioned by a reversible photochemical reaction involving a photosensitive substance and its two products of decomposition. Accordingly, dark adaptation depends on the course of the "dark" reaction during which the two products of decomposition reunite to synthesize the original photosensitive substance.     

TEMPERATURE AND CRITICAL ILLUMINATION FOR REACTION TO FLICKERING LIGHT : I. ANAX LARVAE

The curve of mean critical illumination (I_m) for response to flicker as a function of flicker frequency (F) for the larvae of the dragonfly Anax junius is progressively shifted toward higher intensities the lower the temperature. The maximum flicker frequency (one half the cycle time of light and no light) and the maximum intensity with which it is associated are very little if at all affected by change of temperature. These facts are in agreement with the requirements of the conception that recognition of critical illumination for reaction to flicker involves and depends upon a kind of intensity discrimination, namely between the effects of flashes and the after effects of these flashes during the intervals of no light. The shift of the F-I_m curve with change of temperature is quite inconsistent with the stationary state conception of the determination of the shape of the curve. The dispersion (P.E. _II1) of the measurements of I ₁ is directly proportional to I_m, but the factor of proportionality is less at high and at low temperature than at an intermediate temperature; the scatter of the values of P.E. _II1 is also a function of the temperature. These facts can also be shown to be concordant with the intensity discrimination basis for marginal recognition of flicker.     

Functional Analysis of the Photosynthetic Apparatus of Prochlorothrix hollandica (Prochlorales), a Chlorophyll b Containing Procaryote

Light-shade adaptation of the chlorophyll a/b containing procaryote Prochlorothrix hollandica was studied in semicontinuous cultures adapted to 8, 80 and 200 μmole quanta per square meter per second. Chlorophyll a contents based on dry weight differed by a factor of 6 and chlorophyll b by a factor of 2.5 between the two extreme light conditions. Light utilization efficiencies determined from photosynthesis response curves were found to decrease in low light grown cultures due to lower light harvesting efficiencies; quantum requirements were constant at limiting and saturating irradiances for growth. At saturating growth irradiances, changes in light saturated oxygen evolution rate originated from changes in chlorophyll a antenna relative to the number of reaction centers II. At light-limiting conditions both the number of reaction centers II and the antenna size changed. The amount of chlorophyll b relative to reaction center II remained constant. As in cyanobacteria, the ratio of reaction center I to reaction center II was modulated during light-shade adaptation. On the other hand, time constants for photosynthetic electron transport (4 milliseconds) were low as observed in green algae and diatoms. The occurrence of state one to two and state two to one transitions is reported here. Another feature linking photosynthetic electron transport in P. hollandica to that in the eucaryotic photosynthetic apparatus was blockage of the state one to two transition by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Although chlorophyll b was reported in association with photosystem I, the 630 nanometer light effect does not exclude that chlorophyll b is the photoreceptor for the state one to two transition.     

THE PHOTIC SENSITIVITY OF CIONA INTESTINALIS

1. Ciona possesses two means of responding to an increase in the intensity of illumination. One is by means of a local reaction; the other is by a retraction reflex of the body as a whole. 2. The "ocelli" are not photoreceptors. The photosensitive area is in the intersiphonal region containing the neural mass. This area contains no pigment. 3. The reaction time to light is composed of a sensitization period during which Ciona must be exposed to the light, and of a latent period during which it need not be illuminated in order to react to the stimulus received during the sensitization period. 4. The duration of the reaction time varies inversely as the intensity. Analysis shows the latent period to be constant. The relation between the sensitization period and the intensity follows the Bunsen-Roscoe rule. 5. During dark adaptation the reaction time is at first large, then it decreases until a constant minimum is reached. 6. A photochemical system consisting of a reversible reaction is suggested in order to account for the phenomena observed. This system includes a photosensitive substance and its precursor, the dynamics of the reaction following closely the peculiarities of the photosensitivity of Ciona. 7. It is shown that in order to produce a reaction, a constant ratio must be reached between the amount of sensitive substance broken down by the stimulus and the amount previously broken down. 8. From the chemical system suggested certain experimental predictions were made. The actual experiments verified these predictions exactly. 9. The results obtained with regularly repeated stimulation not only fail to show any basis for a learning process or for the presence of a "higher behavior," but follow the requirements of the photochemical system suggested before.     

THE INFLUENCE OF LIGHT ADAPTATION ON SUBSEQUENT DARK ADAPTATION OF THE EYE

The course of dark adaptation of the human eye varies with the intensity used for the light adaptation which precedes it. Preadaptation to intensities below 200 photons is followed only by rod adaptation, while preadaptation to intensities above 4000 photons is followed first by cone adaptation and then by rod adaptation. With increasing intensities of preadaptation, cone dark adaptation remains essentially the same in form, but covers an increasing range of threshold intensities. At the highest preadaptation the range of the subsequent cone dark adaptation covers more than 3 log units. Rod dark adaptation appears in two types—a rapid and a delayed. The rapid rod dark adaptation is evident after preadaptations to low intensities corresponding to those usually associated with rod function. The delayed rod dark adaptation shows up only after preadaptation to intensities which are hundreds of times higher than those which produce the maximal function of the rods in flicker, intensity discrimination, and visual acuity. The delayed form remains essentially constant in shape following different intensities of preadaptation. However, its time of appearance increases with the preadaptation intensity; after the highest preadaptation, it appears only after 12 or 13 minutes in the dark. These two modes of rod dark adaptation are probably the expression of two methods of formation of visual purple in the rods after its bleaching by the preadaptation lights.     

Sensitivity changes of photoreceptor cells of Hirudo medicinalis caused by changes in extracellular calcium concentration

Extracellular recordings from the vacoule of photoreceptor cells of Hirudo medicinalis L. were performed using microelectrodes. The cells were adapted by white light flashes given at constant intervals (20 s). Response height versus relative intensity curves obtained from the same cell in physiological saline (PS) and in bathing solutions of either a) lowered calcium contents (2 ΜM/1 or less) or b) raised calcium contents (15 mM/1) were compared. The cells' adaptation state in PS was operationally defined by the ratio Q=h _A /h _S where h _A is the response height evoked by the adapting flashes, and h _S is the corresponding saturation response height. Sensitivity changes were measured by the half saturation intensity shift. Lowering extracellular calcium resulted in:

1. The response height increased and the shape of the response became more rounded and prolonged.
2. The total resistance between the vacuole and outside decreased from 8.2±1.4 MΩ (n=6) in PS to 4.6±0.4 MΩ (n=5). The resistance was independent of the cells' adaptation state.
3. A change of the cells' sensitivity occured either in direction to light adaptation or in direction to dark adaptation. It depended functionally on the ratio Q:

a) if Q was less or equal to about 0.6 the cells' sensitivity increased. b) if Q was greater than 0.6 the cells' sensitivity diminished. Raising extracellular calcium decreased the sensitivity of all cells tested independent of their adaptation states in PS. The results can be interpreted under the assumptions that 1. the sensitivity of leech photoreceptor cells is inversely proportional to the intracellular free calcium concentration and Z. intracellular calcium can interact with extracellular calcium in relatively dark adapted cells whereas in relatively light adapted cells the raise of intracellular free calcium is mainly effected by a release from intracellular stores. It is assumed that a Q value of about 0.6 separates relatively light adapted cells from relatively dark adapted cells.     

THE COURSE OF ROD DARK ADAPTATION AS INFLUENCED BY THE INTENSITY AND DURATION OF PRE-ADAPTATION TO LIGHT

An increase in the degree of light adaptation causes a decrease in the slope of the subsequent rod dark adaptation function and a displacement of the function to the right on the time axis. Over a wide range, these changes occur to the same extent whether the increase in the degree of light adaptation is produced by raising the intensity or by prolonging the exposure. Within these limits, the Bunsen-Roscoe reciprocity law applies to the intensity and duration of pre-exposure. Over a still wider range, dark adaptation has the same course following brief exposure to a bright light as it has following prolonged exposure to a dim light, provided the degree of light adaptation is the same in both instances (as indicated by identical initial dark adaptation thresholds).     

Ecological Consequences of Long-Term Exposure of Anabaena variabilis (Cyanophyceae) to Shifts in Environmental Factors

Cultures of Anabaena variabilis were exposed to different light intensities, and the time course of photoadaptation was measured by photosynthetic rate and changes in pigmentation. A shift down in intensity of 284 μEin · m⁻² · sec⁻¹ caused a temporary decrease in the photosynthetic response followed by gradual adaptation to the new conditions. Final chlorophyll a and carotenoid concentrations were reached after 1 day, although other physiological indicators showed that adaptation required 4 days. The parameter I_k was shown to be the best indicator of photoadaptation. A shift up in light intensity of the same magnitude also required 4 days for complete photoadaptation by the culture, although chlorophyll and carotenoid concentrations stabilized within 1 day. A shift down in light intensity of 392 μEin · m⁻² · sec⁻¹ resulted in a temporary attempt to adapt followed by collapse of the population. This demonstrates an apparent threshold in the magnitude of the shift in light intensity which will permit successful adaptation. Simultaneous changes in light intensity and temperature also adversely affected culture populations. Our observations present a possible cause for the decline or prevention of an algal bloom under a fluctuating light regime and suggest that it may be possible to predict this decline as a result of synoptic weather patterns or hydrodynamic influences.     

TIME AND INTENSITY IN PHOTOSENSORY STIMULATION

In its photosensory effect, the action of light depends on two variables,—intensity and time. If the intensity alone is varied, the photochemical effect is proportional to the logarithm of the intensity. If the time alone is varied, the effect is proportional to the time. Experiments here reported show that when both the intensity and the time are varied, the photochemical effect is equal to the product of their separate activities: E = kt log I. These results furnish the means of expressing directly the relation between the intensity of illumination and the reaction time of Mya.     

Photosynthesis dynamics and regulation sensed in the frequency domain

Foundations of photosynthesis research have been established mainly by studying the response of plants to changing light, typically to sudden exposure to a constant light intensity after dark acclimation or light flashes. This approach remains valid and powerful, but can be limited by requiring dark acclimation before time-domain measurements and often assumes that rate constants determining the photosynthetic response do not change between dark and light acclimation. We show that these limits can be overcome by measuring plant responses to sinusoidally modulated light of varying frequency. By its nature, such frequency-domain characterization is performed in light-acclimated plants with no need for prior dark acclimation. Amplitudes, phase shifts, and upper harmonic modulation extracted from the data for a wide range of frequencies can target different kinetic domains and regulatory feedbacks. The occurrence of upper harmonic modulation reflects nonlinear phenomena, including photosynthetic regulation. To support these claims, we measured chlorophyll fluorescence emission of the green alga Chlorella sorokiniana in light that was sinusoidally modulated in the frequency range 1000–0.001 Hz. Based on these experimental data and numerical as well as analytical mathematical models, we propose that frequency-domain measurements can become a versatile tool in plant sensing.

Characterizing photosynthesis in the frequency domain removes the need for dark adaptation and, thus, assumptions about the dark-to-light transition.     

Light and dark adaptation in Phycomyces light-growth response

Sporangiophores of the fungus Phycomyces exhibit adaptation to light stimuli over a dynamic range of 10(10). This range applies to both phototropism and the closely related light-growth response; in the latter response, the elongation rate is modulated transiently by changes in the light intensity. We have performed light- and dark- adaptation experiments on growing sporangiophores using an automated tracking machine that allows a continuous measurement of growth velocity under controlled conditions. The results are examined in terms of the adaptation model of Delbruck and Reichardt (1956, Cellular Mechanisms in Differentiation and Growth, 3-44). The "level of adaptation," A, was inferred from responses to test pulses of light by means of a series of intensity-response curves. For dark adaptation to steps down in the normal intensity range (10(-6)-10(-2) W/m2), A decays exponentially with a time constant b = 6.1 +/- 0.3 min. This result is in agreement with the model. Higher-order kinetics are indicated, however, for dark adaptation in the high-intensity range (10(-2)-1 W/m2). Adaptation in this range is compared with predictions of a model relating changes in A to the inactivation and recovery of a receptor pigment. In response to steps up in intensity in the normal range, A was found to increase rapidly, overshoot the applied intensity level, and then relax to that level within 40 min. These results are incompatible with the Delbruck-Reichardt model or any simple generalizations of it. The asymmetry and overshoot are similar to adaptation phenomena observed in systems as diverse as bacterial chemotaxis and human vision. It appears likely that light and dark adaptation in Phycomyces are mediated by altogether different processes.