Screening Pigment Research Articles

A three‐dimensional representation of the relationship between penetration of u.v.‐B radiation and u.v.‐screening pigments in leaves of Brassica napus

summaryA three‐dimensional (3–D) representation of the penetration of u.v.‐B (280–320 nm) radiation using fibre‐optic microprobes, and of the spectrophotometric analysis of u.v.‐.screening pigments in leaves of Brassira napus, L. ev. Ceres is presented. In this way the results from radiation measurements were directly related to the distribution of u.v.‐screening pigments. Brasssica napus was grown in a greenhouse chamber with a photon flux density of 740μmol m−2 s−1 photosynthetieally active radiation (400–700 nm) with or without the addition of u.v.‐B radiation. The amount of u.v‐B added was equivalent to 9 kJ m−2 d−1 biologically effective u.v.‐B. Growth parameters measured lncluded leaf thickness and dry weight. Ultraviolet‐B radiation stimulated the accumulation of increased amounts of screening pigments, located mostly beneath the adaxial surface of the leaf. Leaf dry weight decreased by 15% in u.v.‐treated plants and, on a dry weight basis, chlorophyll content increased by 19% in treated leaves. The chlorophyll profiles within leaves from both control and u.v.‐treated plants showed an increase from the adaxial surface to a plateau in the middle of the leaf, at a leaf depth of c. 150–320μm. Relatively large amounts of 285 nm radiation penetrated into leaves of u.v.‐B treated plants to sites located within the initial 40μm of the leaf. Thus, despite the presence of additional screening pigments, u.v.‐B radiation was capable of reaching potential targets within the epidermis and mesophyll.

The morphology of the dorsal eye of the hydrothermal vent shrimp,Rimicaris exoculata

The bresiliid shrimp, Rimicaris exoculata, lives in large masses on the sides of hydrothermal vent chimneys at two sites on the Mid-Atlantic Ridge. Although essentially no daylight penetrates to depths of 3500 m, very dim light is emitted from the hydrothermal vents themselves. To exploit this light, R. exoculata has evolved a modified compound eye on its dorsal surface that occupies about 0.5% of the animal's body volume. The eye's morphology suggests that it is extremely sensitive to light. The cornea of the dorsal eye is smooth with no dioptric apparatus. The retina consists of two wing-shaped lobes that are fused across the midline anteriorly. The rhabdomeral segments of the 7000 ommatidia form a compact layer of photosensitive membrane with an entrance aperture of more than 26 mm2. Within this layer, the volume density of rhabdom is more than 70%. Below the rhabdomeral segments, a thick layer of white diffusing cells scatters light upward into the photoreceptors. The arhabdomeral segments of the five to seven photoreceptors of each ommatidium are mere strands of cytoplasm that expand to accommodate the photoreceptor nuclei. The rhabdom is comprised of well-organized arrays of microvilli, each with a cytoskeletal core. The rhabdomeral segment cytoplasm contains mitochondria, but little else. The perikaryon contains a band of mitochondria, but has only small amounts of endoplasmic reticulum. There is no ultrastructural indication of photosensitive membrane cycling in these photoreceptors. Vestigial screening pigment cells and screening pigment granules within the photoreceptors are both restricted to the inner surface of the layer of the white diffusing cells. Below the retina, photoreceptor axons converge in a fanshaped array to enter the dorsal surface of the brain. The eye's size and structure are consistent with a role for vision in shrimp living at abyssal hydrothermal vents.

Spectral efficiency measured by heterochromatic flicker photometry is similar in human infants and adults

Spectral efficiency functions based on heterochromatic flicker photometry (HFP) were measured for three adults and 42 infants using a rapid visually-evoked potential (VEP) method. A 5°-diameter, broadband standard (0.6 cd/m 2) was presented in square-wave counterphase (15 Hz) with one of 13 monochromatic lights (420–660 nm; 20 nm steps). The intensity of the monochromatic light was continuously varied while extracting the phase-locked VEP amplitude of the fundamental component. HFP functions measured psychophysically by the method of adjustment were also obtained for the adults. Adult HFP functions from the two methods were found to be essentially the same. Both of these functions were compared to Vos'-modified 2° V(λ) function and to the 10 ° CIE V(λ) function. The mean adult data were slightly better fit to the 2° V(λ) function than to the 10° CIE V(λ) function, although there was an elevation in sensitivity at 420 and 440 nm. Infant HFP functions were similar to Vos' modified V(λ) except for an elevation in efficiency at short wavelengths. The mean infant HFP function agreed better with the 10°CIE V(λ) function than Vos'-modified V(λ) function, but infant sensitivity was elevated by 0.4 log units at 420 nm compared to the 10° CIE observer. The elevation found at short wavelengths for both adults and infants is attributed to individual and age-related variation in the density of the ocular media, and to reduced macular pigment screening resulting from use of a 5° field size. An infant spectral efficiency function based on the HFP data collected in this study was modeled. This function may serve to equate stimuli with respect to luminosity for the average infant.

The dorsal eye of the dragonfly Sympetrum: specializations for prey detection against the blue sky

Dragonflies of the genus Sympetrum have compound eyes conspicuously divided into dorsal and ventral regions. Using anatomical, optical, electrophysiological, in-vivo photochemical and microspectrophotometrical methods, we have investigated the design and physiology of the dorsal part which is characterized by a pale yellow-orange screening pigment and extremely large facets. The upper part of the yellow dorsal region is a pronounced fovea with interommatidial angles approaching 0.3°, contrasting to the much larger values of 1.5°–2° in the rest of the eye. The dorsal eye part is exclusively sensitive to short wavelengths (below 520 nm). It contains predominantly blue-receptors with a sensitivity maximum at 420 nm, and a smaller amount of UV-receptors. The metarhodopsin of the blue-receptors absorbs maximally at 535 nm. The yellow screening pigment transmits longwavelength light (cut-on 580 nm), which increases the conversion rate from metarhodopsin to rhodopsin (see Fig. 11a). We demonstrate that because of the yellow pigment screen nearly all of the photopigment is in the rhodopsin state under natural conditions, thus maximizing sensitivity. Theoretical considerations show that the extremely long rhabdoms (1.1 mm) in the dorsal fovea are motivated for absorption reasons alone. A surprising consequence of the long rhabdoms is that the sensitivity gain, caused by pumping photopigment into the rhodopsin state, is small. To explain this puzzling fact we present arguments for a mechanism producing a gradient of rhodopsin concentration along the rhabdom, which would minimize saturation of transduction units, and hence improve the signal-to-noise ratio at high intensities. The latter is of special importance for the short integration time and high contrast sensitivity these animals need for spotting small prey at long distances.

INTRACEREBRAL OCELLI IN THE GIANT ANTARCTIC SLATER GLYPTONOTUS ANTARCTICUS (ISOPODA: VALVIFERA)

ABSTRACT Details on the location and ultrastructural organization of 2 intracerebral ocellar regions in the giant Antarctic slater Glyptonotus antarcticus are given. Each ocellar region measures approximately 120 x 45 μm and consists of 2 overlapping ocellar subunits inserted under the neurilemma in the median part of the protocerebrum. One subunit comprises 2 retinula cells; the second 3. Small rhabdoms, made up of typical microvilli, are formed by both subunits. Lipid droplets, but no screening pigment granules, were present in all 5 retinula cells together with a host of intracellular organelles characteristic of membrane-turnover processes. It is concluded that direct photostimulation of the intracerebral ocelli is likely and that functionally they could be analogous to the pineal organ of lower vertebrates. Evidence is presented that, at least anatomically, the intracerebral ocelli are independent from other sensory complexes in the head of G. antarcticus, such as the compound eyes, the organ of Bellonci, and the sensory pores.

AMELIORATION OF UV‐B DAMAGE UNDER HIGH IRRADIANCE. II: ROLE OF BLUE LIGHT PHOTORECEPTORS

Abstract Sensitivity of plants to UV‐B radiation (280–315 nm) is often reduced at high background irradiance. Interpretation of plant responses to potential increases in solar UV‐B requires improved understanding of interactions between UV‐B and other environmental parameters. In this study, photosynthetically active radiation (PAR, 400–700 nm) was kept approximately constant (38 mol m‐2 per day) while the daily blue light fluence (BL, 400–500 nm) was varied between 0.23 and 2.68 mol m‐2. Two lines of cucumber (cvs Ashley and Poinsett) with differential sensitivity to UV‐B were compared. At low BL, 3 days of UV‐B treatment (21 kJ m‐2 biologically effective radiation per 10 h per day) caused severe inhibition of growth in a developing leaf in both cultivars. Growth effects were detectable sooner and were accompanied by chlorotic lesions in the sensitive cultivar (cv Poinsett). Supplemental BL progressively reduced symptoms, consistent with an important role for BL photoreceptor(s) in prevention or repair of UV‐B damage. Ultraviolet‐induced increases in UV‐absorbing compounds on an area basis were significant within 24 h of the start of the treatment but were independent of BL fluence over the range tested, suggesting that bulk accumulation of screening pigments did not contribute to BL‐dependent amelioration of UV damage. However, BL did stimulate net increases in extractable UV‐absorbing compounds on a total leaf busis, while high‐performance liquid chromatography analysis indicated that BL and UV‐B acted synergistically to increase specific components. Thus, the data do not necessarily exclude UV‐absorbing compounds from an important role in overall UV‐B protection nor do they rule out some more specific function for these compounds (e.g. antioxidants). Finally, BL effects on UV‐B alteration of leaf growth and accumulation of UV‐absorbing compounds were not saturated under the conditions used here, suggesting that BL may contribute to interactions between UV‐B and natural levels of background irradiance. Caution is urged in the interpretation of data on UV‐B effects obtained under conditions of low BL irradiance.

Total reflection X-ray fluorescence in the ultramicro analysis of artists' pigments

The analytical characterization of artists' pigments is very important for art history and for restoration and conservation. Total reflection X-ray fluorescence analysis makes possible the non-destructive analysis of pigments. This new approach complements existing techniques and is unique in being at the same time accurate, fast, inexpensive, and well suited to the screening of pigments used in historical works of art.

Visual pigment, dark adaptation and rhodopsin renewal in the eye of Pontoporeia affinis (Crustacea, Amphipoda)

The benthic amphipod Pontoporeia affinis lives in the Baltic sea and in northern European lakes in an environment where very little light is available for vision. The eyes, consisting of 40–50 ommatidia, are correspondingly modified. Microspectrophotometric recordings on isolated eyes show the presence of at least two kinds of screening pigments in the ommatidia with maxima at 540–580 nm and 460–500 nm. Difference spectra obtained from the rhabdoms after exposure to red and blue light, respectively, give evidence of a single rhodopsin with its maximum at 548 nm and a 500-nm metarhodopsin. In ERG recordings sensitivity in the dark-adapted state, after saturating exposures to blue and to red light, stabilizes at levels determined by the rhodopsin concentration. No change is observed during 10–14 h after the beginning of dark adaptation. However, using animals pre-exposed with a strong red light and then kept in darkness, it is found that after a delay of 20–40 h sensitivity of the dark-adapted eye begins to increase and finally, after 5–6 days reaches a level corresponding to 100% rhodopsin. Thus, a slow renewal of rhodopsin appears to occur in darkness, where a photoisomerization of metarhodopsin is excluded.

The eye of the New Zealand freshwater crabHalicarcinus lacustris, and some eco‐physiological predictions based on eye anatomy

In specimens of Halicarcinus lacustries of 6 mm body length and width, the two compound eyes each comprise about 400 ommatidia, and anatomically resemble those of other brachyuran crabs. All ommatidia are of the apposition type with short, proximally tapering, quadripartite crystalline cones as well as a cornea (maximum thickness 18 μm) with con vexly‐curved inner surfaces. The mean distance from cornea to basement membrane is 125 μm, and interommatidial angles average 9.5°. Each ommatidium possesses a horizontally‐banded rhabdom, 35–40 μm long and 2.5 μm wide, with seven contributing retinula cells. An eighth axon can be traced to a small distally‐placed retinula cell. We conclude that the photoreceptors of H. lacustris are useful in detecting small environmental changes in brightness, as well as the plane of polarized light, but have poor spatial and temporal resolution. High densities of screening pigment granules throughout the eye, a decrease in microvillus diameter from an average of 61.1 nm distall...

Screening-Pigment Migration in the Octopus Retina Includes Control by Dopaminergic Efferents

ABSTRACT The extent of screening-pigment (SP) migration in the intact octopus retina and the amplitude of the early receptor potential (ERP) correspond with the degree of adaptation to light or darkness. The light-adapted retina has SP granules concentrated in an apical layer, at the tips of the photoreceptor rhabdoms and supporting cells, and the ERP is barely detectable. In the fully dark-adapted retina, the SP granules are mostly at the base of the rhabdoms, and the ERP is at its maximum. Retinae at intermediate stages, between the fully dark-and light-adapted states, show corresponding intermediate stages of SP migration and ERP amplitude. A series of experiments demonstrates the effects on SP migration of the efferent nerves, which form a subset of fibres in the optic nerves. When the optic nerves to one half of the retina have been severed, there is a dramatic difference in the distribution of SP in areas of the retina (of the dark-adapted eye) connected with severed or intact nerves: apical versus basal, respectively. On incubation of a light-adapted retina with 5 μmol l−1 dopamine, but not with other catecholamines or other putative neurotransmitter substances, SP migrates basally and the ERP is significantly larger than for controls. In octopuses treated with reserpine, SP stays in an apical location and the ERP remains very small, regardless of the state of adaptation and of whether the optic nerves are intact. It is concluded that dopaminergic efferents from the optic lobes effect dark-adaptational SP migration in the cephalopod retina. The arrival in the retina of efferent signals that effect adaptational changes through the mediation of dopamine is a remarkable analogue of the vertebrate system.

Daily and circadian rhythms of synaptic frequency in the first visual neuropile of the housefly’s ( Musca domestica L.) optic lobe

Photoreceptors of the fly's compound eye generally show no very obvious daily or circadian rhythms, a lack which prompted us to examine whether their function might be regulated not in the retina, but at the site of transmission in the first visual neuropile, or lamina. Here, photoreceptor terminals (R1-R6) are reciprocally interconnected with one class of lamina monopolar cell, L2: L2 receives input from R1-R6 at so-called tetrad synapses, and in turn is presynaptic to R1-R6 at feedback synapses. We have calculated the mean frequencies of these synaptic profiles in electron micrographs of single lamina sections. L2 feedback synapses were more numerous at night than during the day, whereas the number of tetrads showed only small modulations between day and night. These changes persisted amongst feedback synapses in flies held in constant darkness, and are thus circadian. In contrast to the slow modulations during a 24 h cycle, the number of L2 feedback synapses after 1 h light pulse in flies held in constant darkness showed no clear change, whereas it increased the number of tetrad profiles. These findings support the occurrence of cyclical daily and circadian changes amongst the two lamina synaptic populations, with tetrads showing rather weak modulations in frequency, but more pronounced responses to the light pulse than feedback synapses.

Cytoskeleton of retinular cells from the stomatopod, Gonodactylus oerstedii: possible roles in pigment granule migration

Retinular cells of the compound eyes of stomatopods (mantis shrimps) contain screening pigment granules that migrate radially in response to light. To clarify the role of the cytoskeleton in these movements, we have performed light microscopy and ultrastructural analyses of cytoskeletal organelles in retinular cells. Rhodamine phalloidin staining indicates that filamentous actin is a component of microvillar rhabdomeres and zonula adherens between retinular cells. Ultrastructural studies reveal three populations of microtubules in retinular cells that differ in their orientations and labilities to fixation. Two of these populations are oriented longitudinally in cells: the soma microtubules, found primarily in a column in the cell soma, and the more labile palisade microtubules, which extend alongside the palisade vacuole near the rhabdomere. The third, most labile microtubule population, and filaments 9–30 nm in diameter, are oriented radially in retinular cells, some within cytoplasmic bridges that span the palisade. The radial microtubules and filaments are appropriately oriented for participating in pigment granule migration. Determination of microtubule polarities in retinular cells by decoration with endogenous tubulin indicates that palisade and soma microtubules contain subpopulations having opposite polarity orientations, as has been observed in neuronal dendrites. In contrast, neighboring pigment cells contain microtubules uniformly oriented with minus ends towards the nucleus, as has been observed in most cell types studied.

Circadian changes in cockroach ommatidial structure.

1. Morphological correlates of circadian changes in eye sensitivity to light measured electrophysiologically were sought in the cockroach, Leucophaea maderae. Cross sections of ommatidia removed at subjective midday and subjective midnight on 3 successive days from roaches held under constant darkness (DD) at 25 +/- 2 degrees C were examined using a transmission electron microscope for morphological differences related to sampling time. 2. The temporal difference in submicrovillar cisternae (SMC) area appeared to exhibit a circadian rhythm, however, the amplitude of this temporal difference measured under DD was less than that observed under LD 12:12 conditions. SMC areas characteristic at nighttime were achieved at subjective midnights but the area diminished only partially toward the daytime state on subjective middays. 3. Rhabdom area remained constant and the daily rhythm of screening pigment granules (SPG) arrangement about the rhabdom was not observed under conditions of constant darkness. 4. Results of this study indicate that a pacemaker(s) actively influences the change in the SMC toward the nighttime state, whereas, the change toward the daytime state results from a passive mechanism that possibly could be accelerated by light.

Flavone photoreactivity. UV-induced reactions in organic solvents and micellar systems

Flavonoid photochemistry is a subject of interest in studies dealing with the role of phenolic compounds as screening pigments in plants. In order to contribute to the understanding of the processes involved in the interaction between flavonoids and UV radiation, we have studied UV-induced flavone photodegradation in both organic solvents and micellar systems. The results obtained show that flavone photosensitivity depends on the characteristics of the reaction environment and is influenced by the medium polarity and the charges on the micellar surface. Qualitative and quantitative differences in the photodegradation products were demonstrated by high performance liquid chromatography (HPLC) analysis.

Cephalic structures in the nemertine Parborlasia corrugatus—Are they really eyes?

The opinion as to whether tiny, approximately 0.1 mm large spots around the innermost margin of the cephalic slits in the Antarctic nemertine worm Parborlasia corrugatus represent photoreceptors or not has fluctuated over the years. This first electron microscope study of the enigmatic spots fails to detect any screening pigment granules, rhabdomeres, or lamellae, but reveals that the structure in question is principally made up of two types of cell, characterized by vesicular and vacuolar material of approximately 80 nm and 0.3 μm in diameter, respectively. Filamentous connective tissue strands with gaps for axons surrounds the ‘eye-spot’ and it is suggested that either exposure to the bright Antarctic summer light has led to a total disintegration of all visual membranes or these structures do not represent eyes at all.

Behavioral selection in crayfish correlates with movement of a screening pigment in the eye

Light induces two contrasting behavioral responses in crayfish: attraction at low intensities and withdrawal at high intensities. The aim of our experiments has been to study whether screening pigments of the eye influence the selection of attraction or withdrawal responses. During illumination, screening pigments mask photoreceptor cells, reducing the gain of the visual system. Comparison of the time and light-intensity functions of pigment migration and of attraction and withdrawal responses suggest that pigment migration might influence the selection as well as the latency of the response.

Degeneration of photoreceptors in rhodopsin mutants of Drosophila.

Five different, well-characterized mutants of the R1-6 rhodopsin gene (ninaE), which corresponds to the rod opsin gene of vertebrates, have been examined morphologically as a function of age (up to 9 weeks) to determine whether or not the photoreceptors degenerate and to assess the pattern of degeneration. Structural deterioration of R1-6 photoreceptors with age has been found in all five mutants. The structural pattern of degeneration is similar in the five mutants, but the time course of degeneration is allele dependent and varies greatly among the five, with the strongest alleles causing the fastest degeneration. The degeneration appears to be independent of either the illumination cycle to which the animals are exposed or the presence of screening pigments in the eye. Although the degeneration first appears in R1-6 photoreceptors, eventually R7/8 photoreceptors, which correspond to cones of vertebrates, are also affected. In many of these mutants, striking proliferations of membrane processes have been observed in the subrhabdomeric region of R1-6 photoreceptors. It is hypothesized that (1) this accumulation of membranes may be caused by the failure of newly synthesized membranes that are inserted into the base of microvilli to be assembled into R1-6 rhabdomeres and (2) this failure may be caused by the extremely low concentration of normal R1-6 rhodopsin in the ninaE mutants.

Evidence for a pathway of distal screening pigment granules across the basement membrane of the crayfish photoreceptor.

The distal pigment cells of Orconectes limosus contain two layers of large electron lucent vacuoles that are separated by layers of small right-angled platelets adjacent to the crystalline cones. The crystalline cones of the dioptric apparatus of this species have evaginations into the distal pigment cell cytoplasm. In photoreceptors of Orconectes limosus and Procambarus clarkii a dark pigment accumulation site was detected just distal to the basement membrane at the edges of each retina. These pigment accumulations occurred independent of the state of light adaptation. Ultrastructurally the pigment granules at this accumulation site resemble distal screening pigment granules according to their size (up to 1.2 microns in diameter) and fibrous structure. Distal screening pigment granules were also found in tube-like cell processes or extracellularly within and proximal to the retinal basement membrane, indicating pigment transport to and across the basement membrane. Proximal to the basement membrane screening pigment granules were also observed disintegrated to a gravel-like electron dense material in widely branched cells. Evidence was found that an electron dense material, probably resulting from disintegrating screening pigment granules, was incorporated in the integument of the eyestalk. Four hours after injection of gold particles into the eye stalk distal to the retina they were detected inside and proximal to the retinal basement membrane.

Effects of chloroquine on the photosensory membrane turnover and the ultrastructure of lysosome-related bodies of the crayfish photoreceptor.

The effect of chloroquine in combination with bright light on the ultrastructure of crayfish (Orconectes limosus R.) photoreceptors was investigated in vivo. Chloroquine had several effects upon the crayfish retina. Multivesicular bodies (MVB) that are involved in lysosomal degradation of photosensory membrane were altered in ultrastructure. MVB frequently contained smaller MVB in a state of more advanced membrane degradation. Additionally screening pigment-like granules appeared in MVB. MVB accumulated in and filled the retinular cell cytoplasm just proximal or distal to the basement membrane which indicated inhibition of photosensory membrane degradation. Under chloroquine treatment rhabdom degradation appeared to be inhibited, as rhabdom diameter was less reduced under these conditions. Also chloroquine caused accumulation of screening pigment granules in glial cells within the lamina ganglionaris.

Conditional inhibition of screening-pigment aggregation by lidocaine in crayfish photoreceptors and frog retinal pigment epithelium.

Lidocaine, at concentrations equal to or lower than those that inhibit fast axoplasmic transport, was found to interfere with the dark-adapting migration of the screening pigments along crayfish photoreceptors and within the cells of the frog retinal pigment epithelium (RPE). The effects of the anesthetic on pigment movements were studied in isolated eyes incubated under light or dark conditions in media of different ionic compositions. Treatment of crayfish eyes with 25 mmol l-1 lidocaine in normal Van Harreveld's saline arrested pigment migration to the dark-adapted position or caused migration towards the light-adapted position in the dark. Similar results were obtained with frog eyecups exposed to 5 mmol l-1 lidocaine in Ringer's solution. In each case, the inhibition of dark adaptation was reversible and dependent on the levels of Na+ and Ca2+ in the incubation medium. A dark-adapted position of both pigments was compatible with lidocaine treatment provided that low-Na+, or high-Ca2+ or Co(2+)-containing solutions were used. These results indicate that light-adapted and dark-adapted pigment positions in both types of retinal cells can occur in the absence of local nervous input. Further, the data suggest a direct effect of lidocaine upon the photoreceptors or RPE cells. The inhibition of pigment aggregation is interpreted to be a consequence of an anesthetic-induced increase in the permeability of the plasma membrane, which in turn affects the intracellular ionic balance that controls pigment position.