Constant High Light Research Articles

Flavodiiron proteins enhance the rate of CO2 assimilation in Arabidopsis under fluctuating light intensity.

The proton concentration gradient (ΔpH) and membrane potential (Δψ) formed across the thylakoid membrane contribute to ATP synthesis in chloroplasts. Additionally, ΔpH downregulates photosynthetic electron transport via the acidification of the thylakoid lumen. K+ exchange antiporter 3 (KEA3) relaxes this downregulation by substituting ΔpH with Δψ in response to fluctuation of light intensity. In the Arabidopsis (Arabidopsis thaliana) line overexpressing KEA3 (KEA3ox), the rate of electron transport is elevated by accelerating the relaxation of ΔpH after a shift from high light (HL) to low light. However, the plant cannot control electron transport toward photosystem I (PSI), resulting in PSI photodamage. In this study, we crossed the KEA3ox line with the line (Flavodiiron [Flv]) expressing the Flv proteins of Physcomitrium patens. In the double transgenic line (Flv-KEA3ox), electrons overloading toward PSI were pumped out by Flv proteins. Consequently, photodamage of PSI was alleviated to the wild-type level. The rate of CO2 fixation was enhanced in Flv and Flv-KEA3ox lines during HL periods of fluctuating light, although CO2 fixation was unaffected in any transgenic lines in constant HL. Upregulation of CO2 fixation was accompanied by elevated stomatal conductance in fluctuating light. Consistent with the results of gas exchange experiments, the growth of Flv and Flv-KEA3ox plants was better than that of WT and KEA3ox plants under fluctuating light.

Mobility Performance in Retinitis Pigmentosa Under Different Lighting Simulation Conditions

This study examined the mobility performance in retinitis pigmentosa (RP) under different simulation lighting conditions. A total of twenty-one subjects with RP and twenty-one age-matched controls were enrolled. Preferred Walking Speed (PWS) were determined using a simple mobility course at 61 cd/m2 while Percentage of Preferred Walking Speed (PPWS) and error score were determined at five different illumination level which were 62, 47, 20, 6 and 1 cd/m2 using a complex mobility course. RP and normal people had similar mobility performances in simple mobility situations at a constant high light level. In lower light levels as well as in complex mobility situations, RP subjects demonstrated markedly reduced mobility performance. The relationship between PPWS and luminance was linear, with the PPWS decreasing significantly when the mobility course luminance dropped below the illumination of 20 cd/m2 . The Error score was also noted to be linearly related to log luminance. A luminance level of 20 cd/m2 may provide a useful decision point in setting indoor light levels for clinical mobility courses.

Mobility Performance in Retinitis Pigmentosa Under Different Lighting Simulation Conditions

This study examined the mobility performance in retinitis pigmentosa (RP) under different simulation lighting conditions. A total of twenty-one subjects with RP and twenty-one age-matched controls were enrolled. Preferred Walking Speed (PWS) were determined using a simple mobility course at 61 cd/m2 while Percentage of Preferred Walking Speed (PPWS) and error score were determined at five different illumination level which were 62, 47, 20, 6 and 1 cd/m2 using a complex mobility course. RP and normal people had similar mobility performances in simple mobility situations at a constant high light level. In lower light levels as well as in complex mobility situations, RP subjects demonstrated markedly reduced mobility performance. The relationship between PPWS and luminance was linear, with the PPWS decreasing significantly when the mobility course luminance dropped below the illumination of 20 cd/m2 . The Error score was also noted to be linearly related to log luminance. A luminance level of 20 cd/m2 may provide a useful decision point in setting indoor light levels for clinical mobility courses.

Mobility Performance in Retinitis Pigmentosa Under Different Lighting Simulation Conditions

This study examined the mobility performance in retinitis pigmentosa (RP) under different simulation lighting conditions. A total of twenty-one subjects with RP and twenty-one age-matched controls were enrolled. Preferred Walking Speed (PWS) were determined using a simple mobility course at 61 cd/m2 while Percentage of Preferred Walking Speed (PPWS) and error score were determined at five different illumination level which were 62, 47, 20, 6 and 1 cd/m2 using a complex mobility course. RP and normal people had similar mobility performances in simple mobility situations at a constant high light level. In lower light levels as well as in complex mobility situations, RP subjects demonstrated markedly reduced mobility performance. The relationship between PPWS and luminance was linear, with the PPWS decreasing significantly when the mobility course luminance dropped below the illumination of 20 cd/m2 . The Error score was also noted to be linearly related to log luminance. A luminance level of 20 cd/m2 may provide a useful decision point in setting indoor light levels for clinical mobility courses.

Mobility Performance In Retinitis Pigmentosa Under Different Lighting Simulation Conditions

This study examined the mobility performance in retinitis pigmentosa (RP) under different simulation lighting conditions. A total of twenty-one subjects with RP and twenty-one age-matched controls were enrolled. Preferred Walking Speed (PWS) were determined using a simple mobility course at 61 cd/m2 while Percentage of Preferred Walking Speed (PPWS) and error score were determined at five different illumination level which were 62, 47, 20, 6 and 1 cd/m2 using a complex mobility course. RP and normal people had similar mobility performances in simple mobility situations at a constant high light level. In lower light levels as well as in complex mobility situations, RP subjects demonstrated markedly reduced mobility performance. The relationship between PPWS and luminance was linear, with the PPWS decreasing significantly when the mobility course luminance dropped below the illumination of 20 cd/m2 . The Error score was also noted to be linearly related to log luminance. A luminance level of 20 cd/m2 may provide a useful decision point in setting indoor light levels for clinical mobility courses.

Roles of alternative electron flows in response to excess light in Ginkgo biloba

Ginkgo biloba L., the only surviving species of Ginkgoopsida, is a famous relict gymnosperm, it may provide new insight into the evolution of photosynthetic mechanisms. Flavodiiron proteins (FDPs) are conserved in nonflowering plants, but the role of FDPs in gymnosperms has not yet been clarified. In particular, how gymnosperms integrate FDPs and cyclic electron transport (CET) to better adapt to excess light is poorly understood. To elucidate these questions, we measured the P700 signal, chlorophyll fluorescence and electrochromic shift signal under fluctuating and constant light in G. biloba. Within the first seconds after light increased, G. biloba could not build up a sufficient proton gradient (ΔpH). Concomitantly, photo-reduction of O2 mediated by FDPs contributed to the rapid oxidation of P700 and protected PSI under fluctuating light. Therefore, in G. biloba, FDPs mainly protect PSI under fluctuating light at acceptor side. Under constant high light, the oxidation of PSI and the induction of non-photochemical quenching were attributed to the increase in ΔpH formation, which was mainly caused by the increase in CET rather than linear electron transport. Therefore, under constant light, CET finely regulates the PSI redox state and non-photochemical quenching through ΔpH formation, protecting PSI and PSII against excess light. We conclude that, in G. biloba, FDPs are particularly important under fluctuating light while CET is essential under constant high light. The coordination of FDPs and CET fine-tune photosynthetic apparatus under excess light.

Photosynthetic acclimation strategies in response to intermittent exposure to high light intensity in wheat (Triticum aestivum L.)

Plants are intermittently exposed to high intensity light; however, it is unknown how the photosynthetic apparatus of cultivated crops, such as wheat (Triticum aestivum L.), acclimatizes to intermittent high light conditions. To address this question, we grew wheat plants under constant high light (800 μmol m−2s−1; 100%H), constant low light (100 μmol m−2s−1; 0%H), 1 day high light followed by 3 days low light (25%H), or 1 day high light followed by 1 day low light (50%H) for 4 weeks. We analyzed seedling growth and the activity and photosensitivity of the photosynthetic apparatus in leaves. Compared with 0%H plants, there was no difference in photosynthetic electron transfer rate or thylakoid membrane protein levels in 25%H plants, but the Rubisco protein levels and photosynthetic carbon fixing capacity were higher. A higher proportion of exposure to high light (50%H) did not further improve the photosynthetic carbon fixing capacity, but it increased the photosynthetic electron transfer rate and photosynthetic protein levels in thylakoid membranes and enhanced the tolerance of the photosynthetic apparatus to high light. Therefore, the acclimation of photosynthesis to intermittent high light exposure depends on the proportion and duration of the high light exposure. Furthermore, during intermittent high light exposure, wheat preferentially invested in photosynthetic carbon assimilation vs the electron transfer reaction and photoprotection. This photosynthetic acclimation strategy maximizes carbon assimilation under intermittent high light conditions.

Photoacclimation to Constant and Changing Light Conditions in a Benthic Diatom

ABSTRACT Photoacclimation to constant and changing light conditions was studied in the benthic diatom Nitzschia cf. aurariae isolated from the littoral zone of the Baltic Sea. The diatom was grown under a wide range of irradiances, i.e., 15-350 mol photons m-2 s-1 with the photoperiod of 16h of light and 8h of darkness. In the first experiment, three levels of a constant light were applied, i.e. 30, 115 and 350 mol photons m-2 s-1. In the second experiment, the diatom was exposed to two ranges of changing light conditions, i.e., the lower-range of variable light, i.e., 15-30-150 mol photons m-2 s-1, and the higher-range of variable light, i.e., 30-60-350 mol photons m-2 s-1. The cellular content of photosynthetic (chlorophyll a and fucoxanthin), as well as photoprotective pigments (diadinoxanthin and diatoxanthin), was determined by the total daily light doses. The de-epoxidation state of the xanthophyll cycle reached higher values in cultures maintained under variable light regimes. The analysis of photosynthesis-irradiance curves suggested that N. cf. aurariae acclimated primarily through the changes in the number of photosynthetic units (PSU). Higher photosynthetic rates observed under variable irradiance indicated the maximization of photosynthesis at lower light intensities. In constant high light, the diatom accumulated more photoprotective pigments, however, the activity of the xanthophyll cycle was limited. Under variable light regimes wide changes in the de-epoxidation state allowed for efficient photoprotection, depending on the light intensities applied. Photoprotection appeared to represent an interplay between long-term photoacclimation and rapid adjustment to ambient light conditions within the constraints set by the former. Prolonged exposure to high light caused a decrease in photosynthetic rates. However, the stable growth of the diatom across the applied light intensities showed that it can survive periods of potentially stressful light conditions. Acclimation mechanisms observed in the studied diatom were consistent with those observed in microalgae present in habitats characterized by high irradiance and rapid changes in light conditions.

The Effect of a Common Daily Schedule on Human Circadian Rhythms During the Polar Day in Svalbard: A Field Study.

All Arctic visitors have to deal with extreme conditions, including a constant high light intensity during the summer season or constant darkness during winter. The light/dark cycle serves as the most potent synchronizing signal for the biological clock, and any Arctic visitor attending those regions during winter or summer would struggle with the absence of those entraining signals. However, the inner clock can be synchronized by other zeitgebers such as physical activity, food intake, or social interactions. Here, we investigated the effect of the polar day on the circadian clock of 10 researchers attending the polar base station in the Svalbard region during the summer season. The data collected in Svalbard was compared with data obtained just before leaving for the expedition (in the Czech Republic 49.8175°N, 15.4730°E). To determine the circadian functions, we monitored activity/rest rhythm with wrist actigraphy followed by sleep diaries, melatonin rhythm in saliva, and clock gene expression (Per1, Bmal1, and Nr1D1) in buccal mucosa samples. Our data shows that the two-week stay in Svalbard delayed melatonin onset but did not affect its rhythmic secretion, and delayed the activity/rest rhythm. Furthermore, the clock gene expression displayed a higher amplitude in Svalbard compared to the amplitude detected in the Czech Republic. We hypothesize that the common daily schedule at the Svalbard expedition strengthens circadian rhythmicity even in conditions of compromised light/dark cycles. To our knowledge, this is the first study to demonstrate peripheral clock gene expression during a polar expedition.

Modification of Activity of the Thylakoid H+/K+ Antiporter KEA3 Disturbs ∆pH-Dependent Regulation of Photosynthesis.

The thylakoid K+ efflux antiporter 3 (KEA3) is required for regulating components of the proton motive force (pmf), proton concentration gradient (ΔpH), and membrane potential (Δψ). The Arabidopsis (Arabidopsis thaliana) disturbed proton gradient regulation mutant (dpgr) is a dominant allele of KEA3, conferring disturbed transport activity. Here, we show that overexpressing the DPGR-type KEA3 (DPGRox) retarded plant growth, whereas overexpressing the wild-type KEA3 (KEA3ox) did not. In KEA3ox lines, the contribution of Δψ to pmf was enhanced, but in DPGRox lines, the size of pmf was reduced. In DPGRox plants, proton conductivity of the thylakoid membrane (g H +) was elevated under high light, implying disturbed stoichiometry of H+/K+ antiport through DPGR-type KEA3 rather than simply enhanced activity. The ΔpH-dependent regulation consisting of thermal dissipation of excessively absorbed light energy and downregulation of cytochrome b 6 f complex activity was severely and mildly affected in DPGRox and KEA3ox plants, respectively. Consequently, photosystem I was sensitive to fluctuating light in both transgenic plants. Both photosystems were sensitive to constant high light and were slightly photodamaged even at standard growth light intensity in DPGRox plants. KEA3 regulates the components of pmf and optimizes the operation of ∆pH-dependent regulation of electron transport.

Concomitant production of fatty acid methyl ester (biodiesel) and exopolysaccharides using efficient harvesting technology in flat panel photobioreactor with special sparging system via Scenedesmus abundans

Current study focusses on the concomitant production of fatty acid methyl ester (FAME, biodiesel) and exopolysaccharides (EPS) from Scenedesmus abundans cell factory in flat panel photobioreactor using cost effective harvesting strategy. Parallel mini and medium scale flat panel photobioreactors (PBRs) with special gas sparging system enabling high gas to liquid mass transfer and efficient mixing were designed. Biomass titer of 6.9 g/l with overall biomass productivity of 1.2 g/l/day was achieved with constant high light intensity of 2162 µE/m2/s in growth phase (134 h) using optimum nutrient concentration. FAME concentration of 1.53 g/l was achieved after 15 days of nitrogen deprivation condition with productivity of 67 mg/l/day. The EPS production of 236 mg/l with a yield of 37 mg/g biomass was achieved. The strain proved its capability to produce multiproducts simultaneously in a single stage PBR by natural autoflocculation harvesting technology.

PGR5-Dependent Cyclic Electron Flow Protects Photosystem I under Fluctuating Light at Donor and Acceptor Sides.

In response to a sudden increase in light intensity, plants must cope with absorbed excess photon energy to protect photosystems from photodamage. Under fluctuating light, PSI is severely photodamaged in the Arabidopsis (Arabidopsis thaliana) proton gradient regulation5 (pgr5) mutant defective in the main pathway of PSI cyclic electron transport (CET). Here, we aimed to determine how PSI is protected by two proposed regulatory roles of CET via transthylakoid ΔpH formation: (1) reservation of electron sink capacity by adjusting the ATP/NADPH production ratio (acceptor-side regulation) and (2) down-regulation of the cytochrome b 6 f complex activity called photosynthetic control for slowing down the electron flow toward PSI (donor-side regulation). We artificially enhanced donor- and acceptor-side regulation in the wild-type and pgr5 backgrounds by introducing the pgr1 mutation conferring the hypersensitivity of the cytochrome b 6 f complex to luminal acidification and moss Physcomitrella patens flavodiiron protein genes, respectively. Enhanced photosynthetic control partially alleviated PSI photodamage in the pgr5 mutant background but restricted linear electron transport under constant high light, suggesting that the strength of photosynthetic control should be optimized. Flavodiiron protein-dependent oxygen photoreduction formed a large electron sink and alleviated PSI photoinhibition, accompanied by the induction of photosynthetic control. Thus, donor-side regulation is essential for PSI photoprotection but acceptor-side regulation also is important to rapidly induce donor-side regulation. In angiosperms, PGR5-dependent CET is required for both functions.

Analytical approach for characterization of morphology and chemistry of a CH3NH3PbI3/TiO2 solar cell layered system

Manufacturing of new perovskite layered solar cells with constant high light conversion efficiency over time may be hampered by the loss of efficiency caused by structural and/or chemical alterations of the complex layered system. SEM/EDX combined with XPS were chosen as an appropriate methodical approach to characterize perovskite laboratory cells in depth and at surface, before and after light exposure. The layered perovskite system investigated here is based on glass covered with fluorine doped tin oxide, followed by thin films of TiO2, ZrO2, and a thick monolithic carbon. TiO2 film is subdivided into a dense layer covered by porous one constituted of nanoparticles of truncated bipyramidal shape. This layered system serves as the matrix for the perovskite. EDX spectral maps on cross sections of specimen have shown that Pb and I are distributed homogeneously throughout the porous layers C, ZrO2, and TiO2. SEM/EDX data show that 20 weeks of ambient daylight did not change significantly the in‐depth distribution of the elemental composition of Pb and I throughout the entire solar cell system. It was confirmed with EDX that nanoparticles identified in high‐resolution SEM micrographs contain mainly Pb and I, indicating these to be the perovskite crystals. However, a compositional and chemical altering began in the near‐surface region of the outermost ~10 nm after 2 months of illumination which was observed with XPS.

Photoinhibition of photosystem I in Nephrolepis falciformis depends on reactive oxygen species generated in the chloroplast stroma.

We studied how high light causes photoinhibition of photosystem I (PSI) in the shade-demanding fern Nephrolepis falciformis, in an attempt to understand the mechanism of PSI photoinhibition under natural field conditions. Intact leaves were treated with constant high light and fluctuating light. Detached leaves were treated with constant high light in the presence and absence of methyl viologen (MV). Chlorophyll fluorescence and P700 signal were determined to estimate photoinhibition. PSI was highly oxidized under high light before treatments. N. falciformis showed significantly stronger photoinhibition of PSI and PSII under constant high light than fluctuating light. These results suggest that high levels of P700 oxidation ratio cannot prevent PSI photoinhibition under high light in N. falciformis. Furthermore, photoinhibition of PSI in N. falciformis was largely accelerated in the presence of MV that promotes the production of superoxide anion radicals in the chloroplast stroma by accepting electrons from PSI. From these results, we propose that photoinhibition of PSI in N. falciformis is mainly caused by superoxide radicals generated in the chloroplast stroma, which is different from the mechanism of PSI photoinhibition in Arabidopsis thaliana and spinach. Here, we provide some new insights into the PSI photoinhibition under natural field conditions.

Superoxide generated in the chloroplast stroma causes photoinhibition of photosystem I in the shade-establishing tree species Psychotria henryi.

Our previous studies indicated that high light induced significant photoinhibition of photosystem I (PSI) in the shade-establishing tree species Psychotria henryi. However, the underlying mechanism has not been fully clarified. In the present study, in order to investigate the mechanism of PSI photoinhibition in P. henryi, we treated detached leaves with constant high light in the presence of methyl viologen (MV) or a soluble α-tocopherol analog, 2,2,5,7,8-pentamethyl-6-chromanol (PMC). We found that MV significantly depressed photochemical quantum yields in PSI and PSII when compared to PMC. On condition that no PSI photoinhibition happened, although cyclic electron flow (CEF) was abolished in the MV-treated samples, P700 oxidation ratio was maintain at higher levels than the PMC-treated samples. In the presence of PMC, PSI photoinhibition little changed but PSII photoinhibition was significantly alleviated. Importantly, PSI photoinhibition was largely accelerated in the presence of MV, which stimulates the production of superoxide and subsequently other reactive oxygen species at the chloroplast stroma by accepting electrons from PSI. Furthermore, MV largely aggravated PSII photoinhibition when compared to control. These results suggest that high P700 oxidation ratio cannot prevent PSI photoinhibition in P. henryi. Furthermore, the superoxide produced in the chloroplast stroma is critical for PSI photoinhibition in the higher plant P. henryi, which is opposite to the mechanism underlying PSI photoinhibition in Arabidopsis thaliana and spinach. These findings highlight a new mechanism of PSI photoinhibition in higher plants.

Responses of Photosystem I Compared with Photosystem II to Fluctuating Light in the Shade-Establishing Tropical Tree Species Psychotria henryi.

Shade-establishing plants growing in the forest understory are exposed to constant high light or fluctuating light when gaps are created by fallen trees. Our previous studies indicate that photosystem I (PSI) is sensitive to constant high light in shade-establishing tree species, however, the effects of fluctuating light on PSI and photosystem II (PSII) in shade-establishing species are little known. In the present study, we examined the responses of PSI and PSII to fluctuating light in comparison to constant high light in the shade-establishing species Psychotria henryi. Accompanying with significant activation of cyclic electron flow (CEF), the P700 oxidation ratio was maintained at high levels when exposed to strong light either under fluctuating light or constant high light. Under moderate fluctuating light, PSI and PSII activities were remained stable in P. henryi. Interestingly, PSI was insusceptible to fluctuating light but sensitive to constant high light in P. henryi. Furthermore, both PSI and PSII were more sensitive to constant high light than fluctuating light. These results suggest that CEF is essential for photoprotection of PSI under fluctuating light in P. henryi. Furthermore, photoinhibition of PSI under high light in P. henryi is more related to the accumulation of reactive oxygen species rather than to P700 redox state, which is different from the mechanisms of PSI photoinhibition in Arabidopsis thaliana and rice. Taking together, PSI is a key determiner of photosynthetic responses to fluctuating light and constant high light in the shade-establishing species P. henryi.

A physiological role of cyclic electron transport around photosystem I in sustaining photosynthesis under fluctuating light in rice.

Plants experience a highly variable light environment over the course of the day. To reveal the molecular mechanisms of their photosynthetic response to fluctuating light, we examined the role of two cyclic electron flows around photosystem I (CEF-PSI)—one depending on PROTON GRADIENT REGULATION 5 (PGR5) and one on NADH dehydrogenase-like complex (NDH)—in photosynthetic regulation under fluctuating light in rice (Oryza sativa L.). The impairment of PGR5-dependent CEF-PSI suppressed the photosynthetic response immediately after sudden irradiation, whereas the impairment of NDH-dependent CEF-PSI did not. However, the impairment of either PGR5-dependent or NDH-dependent CEF-PSl reduced the photosynthetic rate under fluctuating light, leading to photoinhibition at PSI and consequently a reduction in plant biomass. The results highlight that (1) PGR5-dependent CEF-PSI is a key regulator of rapid photosynthetic responses to high light intensity under fluctuating light conditions after constant high light; and (2) both PGR5-dependent and NDH-dependent CEF-PSI have physiological roles in sustaining photosynthesis and plant growth in rice under repeated light fluctuations. The highly responsive regulatory system managed by CEF-PSI appears able to optimize photosynthesis and plant growth under naturally fluctuating light conditions.

Influence of the Quantity and Quality of Light on Photosynthetic Periodicity in Coral Endosymbiotic Algae

Symbiotic corals, which are benthic organisms intimately linked with their environment, have evolved many ways to deal with fluctuations in the local marine environment. One possible coping mechanism is the endogenous circadian clock, which is characterized as free running, maintaining a ∼24 h periodicity of circuits under constant stimuli or in the absence of external cues. The quantity and quality of light were found to be the most influential factors governing the endogenous clock for plants and algae. Unicellular dinoflagellate algae are among the best examples of organisms that exhibit circadian clocks using light as the dominant signal. This study is the first to examine the effects of light intensity and quality on the rhythmicity of photosynthesis in the symbiotic dinoflagellate Symbiodinium sp., both as a free-living organism and in symbiosis with the coral Stylophora pistillata. Oxygen production measurements in Symbiodinium cultures exhibited rhythmicity with a periodicity of approximately 24 h under constant high light (LL), whereas under medium and low light, the cycle time increased. Exposing Symbiodinium cultures and corals to spectral light revealed different effects of blue and red light on the photosynthetic rhythm, specifically shortening or increasing the cycle time respectively. These findings suggest that the photosynthetic rhythm is entrained by different light cues, which are wired to an endogenous circadian clock. Furthermore, we provide evidence that mRNA expression was higher under blue light for two potential cryptochrome genes and higher under red light for a phytochrome gene isolated from Symbiodinium. These results offer the first evidence of the impact of the intensity and quality of light on the photosynthetic rhythm in algal cells living freely or as part of a symbiotic association. Our results indicate the presence of a circadian oscillator in Symbiodinium governing the photosynthetic apparatus through a light-induced signaling pathway that has yet to be described.

Light-induced degeneration and microglial response in the retina of an epibenthonic pigmented teleost: age-dependent photoreceptor susceptibility to cell death

Constant intense light causes apoptosis of photoreceptors in the retina of albino fish. However, very few studies have been performed on pigmented species. Tench (Tinca tinca) is a teleost inhabiting dimly lit environments that has a predominance of rods within the photoreceptor layer. To test the hypothesis that constant high intensity light can result in retinal damage in such pigmented epibenthonic teleost species, photodegeneration of the retina was investigated in the larvae and in juveniles of tench to assess whether any damage may also be dependent on fish age. We exposed both groups of animals to 5 days of constant darkness, followed by 4 days of constant 20,000 lx light, and then by 6 days of recovery in a 14 h light:10 h dark cycle. The results showed that the retina of the larvae group exhibited abundant photoreceptor cell apoptosis during the time of exposition to intense light, whereas that of juveniles was indifferent to it. Damaged retinas showed a strong TUNEL signal in photoreceptor nuclei, and occasionally a weak cytoplasmic TUNEL signal in Müller glia. Specific labelling of microglial cells with Griffonia simplicifolia lectin (GSL) histochemistry revealed that photoreceptor cell death alerts microglia in the degenerating retina, leading to local proliferation, migration towards the injured outer nuclear layer (ONL), and enhanced phagocytosis of photoreceptor debris. During the first days of intense light treatment, Müller cells phagocytosed dead photoreceptor cells but, once microglial cells became activated, there was a progressive increase in the phagocytic capacity of the microglia.

Responses to light changes in tropical seedlings of the Brazilian Atlantic rainforest tree species Cecropia glazioui (Cecropiaceae) and Cedrela fissilis (Meliaceae)

This study analysed the behaviour of Cecropia glazioui Sneth. and Cedrela fissilis Vell. seedlings across a 2–50% full sun light gradient. The morphological adjustments of the seedlings to the different light regimes were interpreted as means of light interception in low light and avoidance of water stress in high light conditions. Cecropia seedlings showed greater trait plasticity at the 2–15% light interval, whereas Cedrela seedlings showed a slight tendency to present greater trait plasticity at the 15–30% light interval than the 2–15% interval. The seedlings of both species did not fully acclimate to decreasing light, showing lower values for the mean relative growth rate (RGR) components, leaf area ratio (LAR) and net assimilation rate (NAR) than seedlings under constant low light, but acclimated to increasing light, showing higher values for RGR components than seedlings under constant high light. The general responses to light gradient in these two species indicate their relative tolerance and ability to grow at low and high light and provide traits used in coping with gap dynamics. The results also indicated that the two species differed in their shade tolerance.