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.     

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.     

Isolation of Rhodospirillum centenum Mutants Defective in Phototactic Colony Motility by Transposon Mutagenesis

The purple photosynthetic bacterium Rhodospirillum centenum is capable of forming swarm colonies that rapidly migrate toward or away from light, depending on the wavelength of excitation. To identify components specific for photoperception, we conducted mini-Tn5-mediated mutagenesis and screened approximately 23,000 transposition events for mutants that failed to respond to either continuous illumination or to a step down in light intensity. A majority of the ca. 250 mutants identified lost the ability to form motile swarm cells on an agar surface. These cells appeared to contain defects in the synthesis or assembly of surface-induced lateral flagella. Another large fraction of mutants that were unresponsive to light were shown to be defective in the formation of a functional photosynthetic apparatus. Several photosensory mutants also were obtained with defects in the perception and transmission of light signals. Twelve mutants in this class were shown to contain disruptions in a chemotaxis operon, and five mutants contained disruptions of components unique to photoperception. It was shown that screening for photosensory defective R. centenum swarm colonies is an effective method for genetic dissection of the mechanism of light sensing in eubacteria.Behavioral change in response to alterations in the quality and quantity of light in the environment is a ubiquitous trait among motile photosynthetic bacteria. Three distinct types of responses to light have been described in the literature (14, 19, 36, 37). The scotophobic response (fear of darkness) is characterized by a tumbling, stop, or reversal that occurs when a swimming bacterium experiences a temporal, or spatial, step down in light intensity. Photokinesis describes an alteration in the rate of motility caused by differences in light intensity. A phototactic response, which has been studied most extensively in algae and cyanobacteria, involves an oriented movement of a cell toward or away from a light source (19). An important distinction is that the direction of irradiation is not relevant to scotophobic or photokinetic responses, whereas it is a critical determinant in phototaxis. Thus, phototactic organisms are uniquely capable of migrating towards a light source, irrespective of whether they are going up or down a gradient of light intensity (37).The various photosensory behaviors exhibited by anoxygenic photosynthetic bacteria have been studied mainly by physiological and biochemical tests, with little supporting genetic data (3, 4, 8, 9, 13, 16, 27, 38). The few genetic tests that have been undertaken have demonstrated that mutations which functionally impair the photosystem also disrupt the ability of cells to respond to light (3, 20). This indicates that a product of photosynthesis, such as the generation of proton motive force or photosynthesis-driven electron transfer, is most likely the signal that controls photosensory behavior, rather than direct absorption of light by a chromophore-containing receptor. This conclusion is supported by recent physiological studies which have shown that specific inhibitors of cyclic photosynthesis-driven electron transport inhibit photosensory behavior in Rhodobacter sphaeroides (13, 16) and Rhodospirillum centenum (38). By using a site-directed mutational approach, we have shown that the scotophobic and phototactic responses of the purple nonsulfur photosynthetic bacterium R. centenum involve components of the chemotaxis phosphorylation cascade (25, 26). This suggests that a sensor of photosynthetic activity may have features similar to that of chemoreceptors. However, which component of the photosynthesis electron transfer chain is being sensed and what is actually sensing alterations in electron transfer are unknown.To identify components responsible for prokaryotic behavioral responses to light, it is essential that techniques be developed for the isolation of mutants that are specifically defective in photosensory behavior. One of the reasons why screens for photosensory mutants have not been developed is the inherent difficulty of assaying for photosensory behavior. Until recently, screening for such mutants involved the onerous task of microscopically assaying individual cells from liquid-grown cultures for a response to a step up or down in light intensity. Since statistically meaningful results require that multiple cells be assayed, this “brute force” approach is infeasible. A significant advance in the isolation of prokaryotic photosensory mutants was recently provided by our observation that colonies of the purple photosynthetic bacterium R. centenum are capable of macroscopic phototactic motility (36, 37). Cells of R. centenum are dimorphic, existing in liquid medium as swim cells bearing a single polar flagellum or as hyperflagellated swarm cells on solid surfaces (36, 37). A unique feature of R. centenum swarming colonies is that they are capable of migrating rapidly (up to 75 mm/h) toward an infrared light source or away from a visible light source (36, 37). This behavior allows us to rapidly screen for mutants that are deficient in photosensory responses by simply assaying colonies for aberrant light-directed migration. In this study, we have utilized mini-Tn5-mediated mutagenesis to isolate numerous mutants that exhibit defects in light-directed motility. The phenotypes of specific classes of mutants provide some unique observations on photosensory behavior, as well as on the mechanism of swim cell to swarm cell differentiation.     

WAVE LENGTH OF LIGHT AND PHOTIC INHIBITION OF STEREOTROPISM IN TENEBRIO LARV?

A definite intensity of white light is required (about 136 m.c.) to produce negative phototropic orientation of creeping Tenebrio larvæ away from contact with a vertical glass surface. This gives a measure of stereotropism in terms of phototropism, or reciprocally. The effectiveness of light for the suppression of stereotropism varies with wave length. It is therefore simple to obtain a measure of the relation between wave length and stimulating efficiency in this case of phototropic orientation. By determinations of the minimal energy required to inhibit stereotropism with different regions of the spectrum, it is found that the maximum effectiveness is sharply localized in the neighborhood of 535µµ. The curve connecting stimulating efficiency with wave length, while giving a picture of the effective absorption by the photosensory receptors, probably does not permit accurate characterization of the essential photosensitive material.     

Photoreception for circadian, neuroendocrine, and neurobehavioral regulation

In the art and science of lighting, four traditional objectives have been to provide light that: 1) is optimum for visual performance; 2) is visually comfortable; 3) permits aesthetic appreciation of the space; and 4) conserves energy. Over the past 25 years, it has been demonstrated that there are nonvisual, systemic effects of light in healthy humans. Furthermore, light has been used to successfully treat patients with selected affective and sleep disorders as well as healthy individuals who have circadian disruption due to shift work, transcontinental jet travel, or space flight. Recently, there has been an upheaval in the understanding of photoreceptive input to the circadian system of humans and other mammals. Analytical action spectra in rodents, primates, and humans have identified 446-484 nm (predominantly the blue part of the spectrum) as the most potent wavelength region for neuroendocrine, circadian, and neurobehavioral responses. Those studies suggested that a novel photosensory system, distinct from the visual rods and cones, is primarily responsible for this regulation. Studies have now shown that this new photosensory system is based on a small population of widely dispersed retinal ganglion cells that are intrinsically responsive to light, and project to the suprachiasmatic nuclei and other nonvisual centers in the brain. These light-sensitive retinal ganglion cells contain melanopsin, a vitamin A photopigment that mediates the cellular phototransduction cascade. Although light detection for circadian and neuroendocrine phototransduction seems to be mediated principally by a novel photosensory system in the eye, the classic rod and cone photoreceptors appear to play a role as well. These findings are important in understanding how humans adapt to lighting conditions in modern society and will provide the basis for major changes in future architectural lighting strategies.     

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.     

Phy meets ERFs to regulate seed germination

The complex process of seed germination is impacted heavily by environmental cues, such as light, mediated via photosensory systems and phytochromes. This pathway was discovered a long time ago, but the underlying molecular mechanisms are not fully understood. Li et al. recently showed how ETHYLENE RESPONSE FACTORs (ERFs) modulate phytochrome-mediated regulation of germination.     

The 14-3-3 proteins of Arabidopsis regulate root growth and chloroplast development as components of the photosensory system

The 14-3-3 proteins specifically bind a number of client proteins to influence important pathways, including flowering timing via the photosensory system. For instance, 14-3-3 proteins influence the photosensory system through interactions with Constans (CO) protein. 14-3-3 associations with the photosensory system were further studied in this investigation using 14-3-3 T-DNA insertion mutants to study root and chloroplast development. The 14-3-3 μ T-DNA insertion mutant, 14-3-3μ-1, had shorter roots than the wild type and the difference in root length could be influenced by light intensity. The 14-3-3 ν T-DNA insertion mutants also had shorter roots, but only when grown under narrow-bandwidth red light. Five-day-old 14-3-3 T-DNA insertion and co mutants all had increased root greening compared with the wild type, which was influenced by light wavelength and intensity. However, beyond 10 d of growth, 14-3-3μ-1 roots did not increase in greening as much as wild-type roots. This study reveals new developmental roles of 14-3-3 proteins in roots and chloroplasts, probably via association with the photosensory system.     

Photosensory input pathways in the medicinal leech

Summary The medicinal leech,Hirudo medicinalis possesses two types of photosensory organs: five bilateral pairs of eyes embedded in two longitudinal rows in the dorsal surface of the head, and seven bilateral pairs of sensilla situated in both the dorsal and the ventral surface of each of the 21 body segments. The photoreceptor cells of each eye or sensillum project their axons centrally via a characteristic cephalic or segmental nerve which carries the photosensory input to the brain or to the segmental ganglion. In response to a pulse of light the photoreceptors produce a train of impulses whose frequency first rises to anearly peak and then declines to asteady state plateau at which it remains until the end of the pulse. The amplitude of the early peak response and the level of the steady state plateau rise linearly with the log of the light pulse intensity, but the dynamic range of the early peak response is much narrower than that of the plateau. Both ocular and sensillar photoreceptors adapt to the intensity of interpulse background illumination; the ocular receptors adapt so completely that their level of background activity is nearly independent of the background light intensity, whereas the ventral sensillar photoreceptors adapt incompletely, so that their background activity rises with the background light intensity. Ocular and sensillar photoreceptors make their maximal response to green light at a wavelength of about 540 nm. They are almost insensitive to red and violet light at both extremes of the visible spectrum. The photosensory response of a single eye is directionally selective, whereas that of a single sensillum has much less directional selectivity. Several higher order sensory neurons were identified in the segmental ganglion that receive photosensory input from the sensilla. One of these neurons has the sensillum in the ipsilateral dorso-medial body wall of the same segment as its receptive field and another neuron the bilateral set of ventral sensilla in the body wall of the next posterior segment.We are indebted to Frank S. Werblin for valuable advice and discussions. We thank Kenneth L. Carlock for designing and constructing much of the special electronic equipment used in this study. We also thank Alexander Petruncola for his helpful suggestions regarding the computational analysis of the experimental results and for writing the computer programs used in the processing of the data.This research was supported by Grant No. GB 31933X from the National Science Foundation, and NIH research grant No. GM 17866 and Training Grant No. GM 00829 from the Institute for General Medical Sciences.     

细菌的光响应及其机制研究进展

光作为一种环境信号,对细菌的生长和代谢有广泛的调节作用。对于光合细菌来讲,一方面,感光蛋白可以协助光合细菌游向最适的光环境,以利于其细胞内的光系统进行光合作用;另一方面,一些光合细菌可以感受并捕获光能为代谢提供能量。目前发现有些非光合细菌也有光响应,感光蛋白在细菌基因组内是普遍存在的,而且与细菌的一些生理功能有关。本文以非光合细菌为主介绍了目前在细菌中发现的趋光现象及其响应机制。     

Photosynthetic Acclimation to Temperature in the Desert Shrub, Larrea divaricata: II. Light-harvesting Efficiency and Electron Transport

The response of photosynthetic electron transport and light-harvesting efficiency to high temperatures was studied in the desert shrub Larrea divaricata Cav. Plants were grown at day/night temperatures of 20/15, 32/25, or 45/33 C in rough approximation of natural seasonal temperature variations. The process of acclimation to high temperatures involves an enhancement of the stability of the interactions between the light-harvesting pigments and the photosystem reaction centers. As temperature is increased, the heat-induced dissociation of these complexes results in a decrease in the quantum yield of electron transport at limiting light intensity, followed by a loss of electron transport activity at rate-saturating light intensity. The decreased quantum yield can be attributed to a block of excitation energy transfer from chlorophyll b to chlorophyll a, and changes in the distribution of the excitation energy between photosystems II and I. The block of excitation energy transfer is characterized by a loss of the effectiveness of 480 nm light (absorbed primarily by chlorophyll b) to drive protochemical processes, as well as fluorescence emission by chlorophyll b.     

Blue light sensory systems in plants

Plant development is influenced by many environmental stimuli, including light, temperature and gravity. Of these stimuli, light is of particular importance because plants depend on it for energy and, thus, for survival. Moreover, virtually all stages of plant development are regulated in part by light through the action of various photosensory systems. Examples of light-regulated processes include germination, stem growth, leaf and root development, tropic responses and flower induction. This review provides an analysis of recent investigations of blue light sensory systems in plants. Current results suggest that plants respond to blue light through a complex photosensory network that incorporates the action of multiple blue light perception systems.     

Rhodopsin(s) in eubacteria

While the biochemical basis of photosynthesis by bacteriochlorophyll-based reaction centres in purple phototrophic Eubacteria and retinal-based bacteriorhodopsin in the Archaebacterium Halobacterium salinarium has been elucidated in great detail, much less is known about photosensory signal transduction; this is especially the case for Eubacteria. Recent findings on two different photosensory proteins in two different Eubacteria, which both show clear resemblances to the rhodopsins, will be presented. The photoactive yellow protein (PYP) from the purple phototrophic organism Ectothiorhodospira halophila probably functions as the photoreceptor for a new type of negative phototaxis response and has been studied in some detail with respect to its structural and photochemical characteristics. On basis of crystallographic an photochemical data it has been proposed that PYP contains retinal as a chromophore. However, we have unambiguously demonstrated that the PYP chromophore is different from retinal, in spite of the fact that PYP's photochemical properties show striking similarities with the rhodopsins. The cyanobacterium Calothrix sp. displays complementary chromatic adaptation, a process in which the pigment composition of the phycobilisomes is adjusted to the spectral characteristics of the incident light. In orange light the blueish chromophore phycocyanin is present, in green light the reddish phycoerythrin is synthesized. On the basis of the action spectrum of this adaptation process, we hypothesized that a rhodopsin is the photosensor in this process. In line with this, we found that nicotine, an inhibitor of the biosynthesis of beta-carotene (which is the precursor of retinal), abolishes chromatic adaptation. Direct proof of the involvement of a photosensory rhodopsin was obtained in experiments in which the chromatic adaptation response was restored by the addition of retinal to the cultures. The two photosensory proteins mentioned above represent the first examples of eubacterial photoreceptors that can be studied at a molecular level. Our current knowledge on these two proteins and their status as retinal proteins will be reviewed.     

Physiological regulation and functional significance of shade avoidance responses to neighbors

Plants growing in dense vegetations compete with their neighbors for resources such as water, nutrients and light. The competition for light has been particularly well studied, both for its fitness consequences as well as the adaptive behaviors that plants display to win the battle for light interception. Aboveground, plants detect their competitors through photosensory cues, notably the red:far-red light ratio (R:FR). The R:FR is a very reliable indicator of future competition as it decreases in a plant-specific manner through red light absorption for photosynthesis and is sensed with the phytochrome photoreceptors. In addition, also blue light depletion is perceived for neighbor detection. As a response to these light signals plants display a suite of phenotypic traits defined as the shade avoidance syndrome (SAS). The SAS helps to position the photosynthesizing leaves in the higher zones of a canopy where light conditions are more favorable. In this review we will discuss the physiological control mechanisms through which the photosensory signals are transduced into the adaptive phenotypic responses that make up the SAS. Using this mechanistic knowledge as a starting point, we will discuss how the SAS functions in the context of the complex multi-facetted environments, which plants usually grow in.Key words: competition, shade avoidance, hormones, cell wall, adaptive plasticity, photoreceptor, light     

Reflector based chlorophyll production by Spirulina platensis through energy save mode

The present study suggests that the effect of silver coated polyester film fixed in culture racks serves as a reflector of light intensity on Spirulina platensis cultivation, using of KNO₃ and urea as nitrogen sources. The use of light reflector (LR) gave light intensity of 4.8-6.0 klux and the reduction in number of tube light with reflector gave 2.5 klux of light intensity and its effect was studied on S. platensis. Total chlorophyll productions were observed for the cultivation at light intensity of two lights with reflector. This improvement is simple, inexpensive and saves 50% electric energy by reducing the number of lights, and thus contributes to energy conservation.     

Conformational changes in photosynthetic pigment proteins on thylakoid membranes can lead to fast non-photochemical quenching in cyanobacteria

A high non-photochemical quenching (NPQ) appeared below the phase transition temperature when Microcystis aeruginosa PCC7806 cells were exposed to saturated light for a short time. This suggested that a component of NPQ, independent from state transition or photo-inhibition, had been generated in the PSII complex; this was a fast component responding to high intensity light. Glutaraldehyde (GA), commonly used to stabilize membrane protein conformations, resulted in more energy transfer to PSII reaction centers, affecting the energy absorption and dissipation process rather than the transfer process of phycobilisome (PBS). In comparison experiments with and without GA, the rapid light curves (RLCs) and fluorescence induction dynamics of the fast phase showed that excess excitation energy was dissipated by conformational change in the photosynthetic pigment proteins on the thylakoid membrane (PPPTM). Based on deconvolution of NPQ relaxation kinetics, we concluded that the fast quenching component (NPQf) was closely related to PPPTM conformational change, as it accounted for as much as 39.42% of the total NPQ. We hypothesize therefore, that NPQf induced by PPPTM conformation is an important adaptation mechanism for Microcystis blooms under high-intensity light during summer and autumn.     

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.     

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.     

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.