Consecutive Flashes Research Articles

The flash droughts across the south-central United States in 2022: Drivers, predictability, and impacts

A rare subseasonal-to-seasonal phenomenon – two consecutive flash drought events interrupted by a period of recovery – occurred across eastern Oklahoma, Arkansas, and southern Missouri, spanning the summer and early fall of 2022. These flash drought events (the first in June–July, the second in August–September) led to severe (D2) and extreme (D3) drought conditions via the United States Drought Monitor across much of the region following the first period of rapid drought intensification, and extreme (D3) and exceptional (D4) drought conditions by the end of the second event. A notable driver of both flash drought events included a persistent upper-level ridge either centered over or shifted west and upstream of the flash drought region, leading to broad-scale subsidence and reduced mid-level moisture which acted to limit precipitation development and increase evaporative demand. In addition, several heatwave events developed during the warm season in 2022 and either (1) acted to drive flash drought development via increased evaporative demand or (2) were enhanced by land surface desiccation and land-atmosphere feedbacks following rapid drought intensification. Furthermore, S2S composite forecasts predicted drought development for both events. However, only 20% of the ensembles predicted rapid drought development associated with flash drought for the first event and 16% of the ensembles predicted rapid drought development during the second event. This result highlights a key challenge in S2S prediction of rapidly developing drought conditions versus that of more conventional and slower drought development. The ensembles that did predict rapid drought intensification were associated with the forecasting of positive 500 hPa geopotential height anomalies over the south-central United States (first event) or an amplified wave pattern centered over the west-central United States (second event). Lastly, the compounding effects of two flash droughts in a single warm season led to substantial impacts on agricultural, environmental, and hydrologic sectors across the region.

Unusual flash cooling-induced phase in a crystal of triphenylsilanol.

The unique phase in a crystal of triphenylsilanol (Ph3SiOH) is reported. It is observed that after the crystal is flash-cooled from room temperature to 100 K, a new stable ordered phase II occurs with an increase in the unit-cell parameters compared to the earlier reported phase I. The new phase II occurs upon fast cooling while on slow cooling the disordered phase I is present. Gradual heating from 100 K (phase II) causes the crystal to return to the original phase I at about 150 K. The crystal undergoes the observed transformation in a reversible manner in many consecutive flash cooling/heating cycles without cracking.

A new instability domain of CNO-flashing low-mass He-core stars on their early white-dwarf cooling branches

Context. Before reaching their quiescent terminal white-dwarf cooling branch, some low-mass helium-core white dwarf stellar models experience a number of nuclear flashes which greatly reduce their hydrogen envelopes. Just before the occurrence of each flash, stable hydrogen burning may be able to drive global pulsations that could be relevant in shedding some light on the internal structure of these stars through asteroseismology, similarly to what occurs with other classes of pulsating white dwarfs. Aims. We present a pulsational stability analysis applied to low-mass helium-core stars on their early white-dwarf cooling branches going through CNO flashes in order to study the possibility that the ε mechanism is able to excite gravity-mode pulsations. We assess the ranges of unstable periods and the corresponding instability domain in the log g − Teff plane. Methods. We carried out a nonadiabatic pulsation analysis for low-mass helium-core white-dwarf models with stellar masses between 0.2025 and 0.3630 M⊙ going through CNO flashes during their early cooling phases. Results. We found that the ε mechanism due to stable hydrogen burning can excite low-order (ℓ = 1, 2) gravity modes with periods between ∼80 and 500 s for stars with 0.2025 ≲ M⋆/M⊙ ≲ 0.3630 located in an extended region of the log g − Teff diagram, with effective temperature and surface gravity in the ranges 15 000 ≲ Teff ≲ 38 000 K and 5.8 ≲ log g ≲ 7.1, respectively. For the sequences that experience multiple CNO flashes, we found that with every consecutive flash, the region of instability becomes wider and the modes are more strongly excited. The magnitudes of the rate of period change for these modes are in the range of ∼10−10–10−11 [s/s]. Conclusions. Since the timescales required for these modes to reach amplitudes large enough to be observable are shorter than their corresponding evolutionary timescales, the detection of pulsations in these stars is feasible. Given the current problems in distinguishing some stars that populate the same region of the log g − Teff plane, the eventual detection of short-period pulsations may help in the classification of such stars. Furthermore, if a low-mass white dwarf star were found to pulsate with low-order gravity modes in this region of instability, it would confirm our result that such pulsations can be driven by the ε mechanism. In addition, confirming a rapid rate of period change in these pulsations would support the idea that these stars actually experience CNO flashes, as has been predicted by evolutionary calculations.

Visual representations of time elicit early responses in human temporal cortex

Time perception is inherently part of human life. All human sensory modalities are always involved in the complex task of creating a temporal representation of the external world. However, when representing time, people primarily rely on auditory information. Since the auditory system prevails in many audio-visual temporal tasks, one may expect that the early recruitment of the auditory network is necessary for building a highly resolved and flexible temporal representation in the visual modality. To test this hypothesis, we asked 17 healthy participants to temporally bisect three consecutive flashes while we recorded EEG. We demonstrated that visual stimuli during temporal bisection elicit an early (50–90 ​ms) response of an extended area of the temporal cortex, likely including auditory cortex too. The same activation did not appear during an easier spatial bisection task. These findings suggest that the brain may use auditory representations to deal with complex temporal representation in the visual system.

Spatial Cues Influence Time Estimations in Deaf Individuals.

SummaryRecent studies have reported a strong interaction between spatial and temporal representation when visual experience is missing: blind people use temporal representation of events to represent spatial metrics. Given the superiority of audition on time perception, we hypothesized that when audition is not available complex temporal representations could be impaired, and spatial representation of events could be used to build temporal metrics. To test this hypothesis, deaf and hearing subjects were tested with a visual temporal task where conflicting and not conflicting spatiotemporal information was delivered. As predicted, we observed a strong deficit of deaf participants when only temporal cues were useful and space was uninformative with respect to time. However, the deficit disappeared when coherent spatiotemporal cues were presented and increased for conflicting spatiotemporal stimuli. These results highlight that spatial cues influence time estimations in deaf participants, suggesting that deaf individuals use spatial information to infer temporal environmental coordinates.

Redox transients of P680 associated with the incremental chlorophyll-a fluorescence yield rises elicited by a series of saturating flashes in diuron-treated photosystem II core complex of Thermosynechococcus vulcanus.

Recent chlorophyll-a fluorescence yield measurements, using single-turnover saturating flashes (STSFs), have revealed the involvement of a rate-limiting step in the reactions following the charge separation induced by the first flash. As also shown here, in diuron-inhibited PSII core complexes isolated from Thermosynechococcus vulcanus the fluorescence maximum could only be reached by a train of STSFs. In order to elucidate the origin of the fluorescence yield increments in STSF series, we performed transient absorption measurements at 819 nm, reflecting the photooxidation and re-reduction kinetics of the primary electron donor P680. Upon single flash excitation of the dark-adapted sample, the decay kinetics could be described with lifetimes of 17 ns (∼50%) and 167 ns (∼30%), and a longer-lived component (∼20%). This kinetics are attributed to re-reduction of P680•+ by the donor side of PSII. In contrast, upon second-flash (with Δt between 5 μs and 100 ms) or repetitive excitation, the 819 nm absorption changes decayed with lifetimes of about 2ns (∼60%) and 10ns (∼30%), attributed to recombination of the primary radical pair P680•+ Pheo•- , and a small longer-lived component (∼10%). These data confirm that only the first STSF is capable of generating stable charge separation-leading to the reduction of QA ; and thus, the fluorescence yield increments elicited by the consecutive flashes must have a different physical origin. Our double-flash experiments indicate that the rate-limiting steps, detected by chlorophyll-a fluorescence, are not correlated with the turnover of P680.

Euphorbia dendroides Latex as a Source of Jatrophane Esters: Isolation, Structural Analysis, Conformational Study, and Anti-CHIKV Activity.

An efficient process was used to isolate six new jatrophane esters, euphodendroidins J (3), K (5), L (6), M, (8), N (10), and O (11), along with seven known diterpenoid esters, namely, euphodendroidins A (4), B (9), E (1), and F (2), jatrophane ester (7), and 3α-hydroxyterracinolides G and B (12 and 13), and terracinolides J and C (14 and 15) from the latex of Euphorbia dendroides. Their 2D structures and relative configurations were established by extensive NMR spectroscopic analysis. The absolute configurations of compounds 1, 11, and 15 were determined by X-ray diffraction analysis. Euphodendroidin F (2) was obtained in 18% yield from the diterpenoid ester-enriched extract after two consecutive flash chromatography steps, making it an interesting starting material for chemical synthesis. Euphodendroidins K and L (5 and 6) showed an unprecedented NMR spectroscopic behavior, which was investigated by variable-temperature NMR experiments and molecular modeling. The structure-conformation relationships study of compounds 1, 5, and 6, using DFT-NMR calculations, indicated the prominent role of the acylation pattern in governing the conformational behavior of these jatrophane esters. The antiviral activity of compounds 1-15 was evaluated against Chikungunya virus (CHIKV) replication.

Efficient access of mobile flows to heterogeneous networks under flash crowds

Future wireless networks need to offer orders of magnitude more capacity to address the predicted growth in mobile traffic demand. Operators to enhance the capacity of cellular networks are increasingly using WiFi to offload traffic from their core networks. This paper deals with the efficient and flexible management of a heterogeneous networking environment offering wireless access to multimode terminals. This wireless access is evaluated under disruptive usage scenarios, such as flash crowds, which can mean unwanted severe congestion on a specific operator network whilst the remaining available capacity from other access technologies is not being used. To address these issues, we propose a scalable network assisted distributed solution that is administered by centralized policies, and an embedded reputation system, by which initially selfish operators are encouraged to cooperate under the threat of churn. Our solution after detecting a congested technology, including within its wired backhaul, automatically offloads and balances the flows amongst the access resources from all the existing technologies, following some quality metrics. Our results show that the smart integration of access networks can yield an additional wireless quality for mobile flows up to thirty eight percent beyond that feasible from the best effort standalone operation of each wireless access technology. It is also evidenced that backhaul constraints are conveniently reflected on the way the flow access to wireless media is granted. Finally, we have analyzed the sensitivity of the handover decision algorithm running in each terminal agent to consecutive flash crowds, as well as its centralized feature that controls the connection quality offered by a heterogeneous access infrastructure owned by distinct operators.

Grey matter volume in early human visual cortex predicts proneness to the sound-induced flash illusion

Visual perception can be modulated by sounds. A drastic example of this is the sound-induced flash illusion: when a single flash is accompanied by two bleeps, it is sometimes perceived in an illusory fashion as two consecutive flashes. However, there are strong individual differences in proneness to this illusion. Some participants experience the illusion on almost every trial, whereas others almost never do. We investigated whether such individual differences in proneness to the sound-induced flash illusion were reflected in structural differences in brain regions whose activity is modulated by the illusion. We found that individual differences in proneness to the illusion were strongly and significantly correlated with local grey matter volume in early retinotopic visual cortex. Participants with smaller early visual cortices were more prone to the illusion. We propose that strength of auditory influences on visual perception is determined by individual differences in recurrent connections, cross-modal attention and/or optimal weighting of sensory channels.

Superoxide Flashes Reveal Novel Properties of Mitochondrial Reactive Oxygen Species Excitability in Cardiomyocytes

Superoxide flash represents quantal and bursting production of mitochondrial reactive oxygen species (ROS) instigated by transient opening of the mitochondrial permeability transition pore (mPTP). Given their critical role in metabolism, ischemia-reperfusion injury, and apoptosis, characterization of flash properties would be valuable to further mechanistic and physiological studies of this newly discovered mitochondrial phenomenon. Here we developed the flash detector FlashSniper based on segmentation of two-dimensional feature maps extracted from time-lapse confocal image stacks, and on the theory for correcting optical distortion of flash-amplitude histograms. Through large-scale analysis of superoxide flashes in cardiomyocytes, we demonstrated uniform mitochondrial ROS excitability among subsarcolemmal and intermyofibrillar mitochondria, and exponential distribution of intervals between consecutive flash events. Flash ignition displayed three different patterns: an abrupt rise from quiescence (44%), a rise with an exponential foot (27%), or a rise occurring after a pedestal precursor (29%), closely resembling action-potential initiation in excitable cells. However, the optical blurring-corrected amplitudes of superoxide flashes were highly variable, as were their durations, indicating stochastic automaticity of single-mitochondrion ROS excitation. Simultaneous measurement of mitochondrial membrane potential revealed that graded, rather than all-or-none, depolarization mirrored the precursor and the primary peak of the flash. We propose that superoxide flash production is a regenerative process dominated by stochastic, autonomous recruitment of a limited number of units (e.g., mPTPs) in single mitochondria.

Ferredoxin:NADP+ Oxidoreductase Association with Phycocyanin Modulates Its Properties

In photosynthetic organisms, ferredoxin:NADP(+) oxidoreductase (FNR) is known to provide NADPH for CO(2) assimilation, but it also utilizes NADPH to provide reduced ferredoxin. The cyanobacterium Synechocystis sp. strain PCC6803 produces two FNR isoforms, a small one (FNR(S)) similar to the one found in plant plastids and a large one (FNR(L)) that is associated with the phycobilisome, a light-harvesting complex. Here we show that a mutant lacking FNR(L) exhibits a higher NADP(+)/NADPH ratio. We also purified to homogeneity a phycobilisome subcomplex comprising FNR(L,) named FNR(L)-PC. The enzymatic activities of FNR(L)-PC were compared with those of FNR(S). During NADPH oxidation, FNR(L)-PC exhibits a 30% decrease in the Michaelis constant K(m)((NADPH)), and a 70% increase in K(m)((ferredoxin)), which is in agreement with its predicted lower activity of ferredoxin reduction. During NADP(+) reduction, the FNR(L)-PC shows a 29/43% decrease in the rate of single electron transfer from reduced ferredoxin in the presence/absence of NADP(+). The increase in K(m)((ferredoxin)) and the rate decrease of single reduction are attributed to steric hindrance by the phycocyanin moiety of FNR(L)-PC. Both isoforms are capable of catalyzing the NADP(+) reduction under multiple turnover conditions. Furthermore, we obtained evidence that, under high ionic strength conditions, electron transfer from reduced ferredoxin is rate limiting during this process. The differences that we observe might not fully explain the in vivo properties of the Synechocystis mutants expressing only one of the isoforms. Therefore, we advocate that FNR localization and/or substrates availability are essential in vivo.

Organization of multisynaptic inputs from LGN to MT and V4 of macaques

Temporal analysis of the multifocal cortical visual evoked potential (VEP) was studied using pseudo-random (m-sequence) achromatic stimulation. The effects of variation of luminance contrast on the first-order response were complex. At low to mid contrasts (<60%), a wave doublet (P100-N115) predominated. A second wave complex (N100-P120-N160) dominated at high contrasts. The second-order responses, however, showed an extremely simple variation with luminance contrast. Intrinsic differences in the adaptation time of the generators of these two components caused a distinct separation in the slices of the second-order response. A rapidly adapting nonlinearity saturating at low contrasts was only observable when measuring the responses from two consecutive flashes. Its latency coincided with the contrast saturating first-order response component. By comparison, the nonlinearity derived from the responses to the stimuli with longer interstimulus intervals (second and third slices) yielded a much more linear contrast response-function with lower contrast gain and latencies, which clearly corresponded to the longer latency component of the first-order response. Thus, the second-order responses show a first slice which is predominantly driven by neural elements that have a latency and contrast function that mimic those of the magnocellular neurons of the primate LGN and a second slice which is dominated by a generator whose properties resemble primate parvocellular function. This division into magno and parvocellular contribution to the VEP is based on function (interaction time) as distinct from other currently available analyses, with potential for neural analysis of visual disease.

Evolution of lightning in an isolated hailstorm of moderate size in the tropics

Evolution of lightning activity in a tropical hailstorm of moderate size that developed in the premonsoon season at Pune (18°32′N, 73°51′E, 559 m above sea level) is studied from the measurements of surface electric field, the Maxwell current and thunder. Total flash rate is counted from the electric field record, and the cloud‐to‐ground (CG) flash rate is estimated from the visual observations. Precise timings of their occurrence were confirmed from the observations of overshoot in the Maxwell current records. The storm exhibited an almost constant rate of one CG flash every 1 to 2 min over the whole life time of the storm. The ratio of intracloud (IC) to CG flashes (IC/CG) increased with the increase in total flash rate. In the convective stage of the storm, field changes from consecutive flashes were generally found to alternate in polarity. Moreover, in this stage, field changes occur in pairs, the first field change of each pair being of negative polarity and the second one of positive polarity. The two field changes in a pair occur with an average time difference of 14.3 ± 8.4 s while two consecutive pairs appear after 29.3 ± 9.1 s. In between the convective and mature stages, our observations suggest the occurrence of the phenomenon of rain gush and the field excursion associated with falling precipitation. Development of the mature stage was marked with rapid transitions in the surface electric field and the Maxwell current polarities from negative to positive. Further, total flash rate and IC/CG ratio sharply increase, and the lightning‐induced electric field changes become almost exclusively of negative polarity. Observations suggest possibly a lifting up of the charging region in mature stage of the storm. The dissipating stage of the storm witnessed hail and rain showers, sharp transition of electric field and the Maxwell current from positive to negative polarity and occurrence of a few positive CG discharges. Our observations are consistent with the general belief that that some lightning flashes, by neutralizing and depositing charge in the region of opposite polarity, change the charge distribution so as to trigger another discharge in the storm.

Light-induced FTIR difference spectroscopy as a powerful tool toward understanding the molecular mechanism of photosynthetic oxygen evolution

The molecular mechanism of photosynthetic oxygen evolution remains a mystery in photosynthesis research. Although recent X-ray crystallographic studies of the photosystem II core complex at 3.0-3.5 A resolutions have revealed the structure of the oxygen-evolving center (OEC), with approximate positions of the Mn and Ca ions and the amino acid ligands, elucidation of its detailed structure and the reactions during the S-state cycle awaits further spectroscopic investigations. Light-induced Fourier transform infrared (FTIR) difference spectroscopy was first applied to the OEC in 1992 as detection of its structural changes upon the S(1)-->S(2) transition, and spectra during the S-state cycle induced by consecutive flashes were reported in 2001. These FTIR spectra provide extensive structural information on the amino acid side groups, polypeptide chains, metal core, and water molecules, which constitute the OEC and are involved in its reaction. FTIR spectroscopy is thus becoming a powerful tool in investigating the reaction mechanism of photosynthetic oxygen evolution. In this mini-review, the measurement method of light-induced FTIR spectra of OEC is introduced and the results obtained thus far using this technique are summarized.

The spatial pattern of bioluminescent flashes in the polychaete Eusyllis blomstrandi (Annelida)

Each of the trunk segments of the polychaete Eusyllis blomstrandi is equipped with paired epidermal luminescent domains. They luminesce upon mechanical or electrical stimulation. Light emission can be rapidly turned on and off, appears intracellular and is highly coordinated among the trunk segments. Luminescent light is typically emitted in series of flashes. Light emission in a flash starts locally in a group of segments and recruits adjacent segments at a rate as fast as ≤1 ms/segment. The collapse of light emission at the end of a flash is almost simultaneous in all of the segments involved. In the intact worm, the luminescent reaction usually involves only a posterior group of segments. Facilitation becomes manifest as the consecutive flashes in a series increase in brightness and duration and recruit additional anterior segments that were not active in earlier flashes. The flash series stops abruptly instead of decreasing asymptotically in brightness. In posterior fragments, all the segments participate in flashing luminescence, indicating the loss of an inhibitory effect exerted by the anterior end in the case of whole animals. Posterior fragments survive and are still capable of luminescence weeks after fragmentation although they do not regenerate a head. Immediately upon fragmentation of the worm, the posterior fragment luminesces continuously for some seconds while the anterior part quickly stops light emission. This suggests a decoy and/or a predator-alerting function of prolonged, strong luminescence by the moribund posterior fragment to the benefit of the survival of the anterior fragment.

Structural Perturbation of the Carboxylate Ligands to the Manganese Cluster upon Ca2+/Sr2+ Exchange in the S-State Cycle of Photosynthetic Oxygen Evolution As Studied by Flash-Induced FTIR Difference Spectroscopy

A Ca(2+) ion is an indispensable element in the oxygen-evolving Mn cluster in photosystem II (PSII). To investigate the structural relevance of Ca(2+) to the Mn cluster, the effects of Sr(2+) substitution for Ca(2+) on the structures and reactions of ligands to the Mn cluster during the S-state cycle were investigated using flash-induced Fourier transform infrared (FTIR) difference spectroscopy. FTIR difference spectra representing the four S-state transitions, S(1) --> S(2), S(2) --> S(3), S(3) --> S(0), and S(0) --> S(1), were recorded by applying four consecutive flashes either to PSII core complexes from Thermosynechococcus elongatus or to PSII-enriched membranes from spinach. The spectra were also recorded using biosynthetically Sr(2+)-substituted PSII core complexes from T. elongatus and biochemically Sr(2+)-substituted PSII membranes from spinach. Several common spectral changes upon Sr(2+) substitution were observed in the COO(-) stretching region of the flash-induced spectra for both preparations, which were best expressed in Ca(2+)-minus-Sr(2+) double difference spectra. The significant intensity changes in the symmetric COO(-) peaks at approximately 1364 and approximately 1418 cm(-)(1) at the first flash were reversed as opposite intensity changes at the third flash, and the slight shift of the approximately 1446 cm(-)(1) peak at the second flash corresponded to the similar but opposite shift at the fourth flash. Analyses of these changes suggest that there are at least three carboxylate ligands whose structures are significantly perturbed by Ca(2+)/Sr(2+) exchange. They are (1) the carboxylate ligand having a bridging or unidentate structure in the S(2) and S(3) states and perturbed in the S(1) --> S(2) and S(3) --> S(0) transitions, (2) that with a chelating or bridging structure in the S(1) and S(0) states and perturbed also in the S(1) --> S(2) and S(3) --> S(0) transitions, and (3) that with a chelating structure in the S(3) and S(0) states and changes in the S(2) --> S(3) and S(0) --> S(1) transitions. Taking into account the recent FTIR studies using site-directed mutagenesis and/or isotope substitution [Chu et al. (2004) Biochemistry 43, 3152-3116; Kimura et al. (2005) J. Biol. Chem. 280, 2078-2083; Strickler et al. (2006) Biochemistry 45, 8801-8811], it was concluded that these carboxylate groups do not originate from either D1-Ala344 (C-terminus) or D1-Glu189, which are located near the Ca(2+) ion in the X-ray crystallographic model of the Mn cluster. It was thus proposed that if the X-ray model is correct, the above carboxylate groups sensitive to Sr(2+) substitution are ligands to the Mn ions strongly coupled to the Ca(2+) ion rather than direct ligands to Ca(2+).

Amplitude calculation in multifocal ERG: comparison of repeatability in 30 Hz flicker and first order kernel stimulation

In multifocal flicker stimulation, each step of the M-sequence consists of four consecutive flashes with a frequency of 30 Hz. The resulting amplitudes can be calculated by means of a discrete fourier transformation (DFT). With this method, amplitudes can be calculated without having to localise peaks and troughs and set cursors. The purpose of this study is to compare the re-test stability of this method to conventional mfERG stimulation. We examined 27 healthy subjects using a RETI-scan device (Roland Consult, Wiesbaden). We used 61 hexagons within a 30 deg. visual field. We compared the classic first order kernel (FOK) stimulation with the multifocal 30 Hz Flicker (mfFlicker-ERG) stimulation. Repeatability was calculated using coefficients of variation. Both methods had coefficients of 15% for the sum P1-amplitude and the DFT results, respectively. The amplitudes calculated by flicker and DFT were approximately 25% smaller than the FOK amplitudes. This study showed no difference of re-test repeatability between the mfFlicker-ERG and the conventional first order kernel method. Since the mfFlicker-ERG method does not require a definition of peaks and troughs in order to calculate the amplitudes, we believe that a common source of error is eradicated, especially when dealing with distorted or atypical curves.

Transients of delayed fluorescence induction signal and photosynthetic antennas: A possible relationship. Mathematical modeling approach

A mathematical model was developed for resolved temporal transients of experimentally recorded delayed fluorescence (DF) induction signal. During an intermittent light regime, antennas of the photosynthetic apparatus were treated as targets, repeatedly hit by potentially absorbable photons within a series of consecutive light flashes. Formulas were derived for the number of antennas, cumulatively hit by a specific number of photons, as a function of the flash serial number (time). Model parameters included number of absorbable photons in one flash, antenna sizes, and their number. A series of induction curves were analyzed, obtained from a Zea mays leaf segment and differing in the previous dark period (t d). Each curve, consisting of the two most prominent DF transients (C and D), was fitted with several model types, differing in the number of absorbed photons. For both transients, the best fitting result was achieved when DF induction was linked to the second absorbed photon. As expected, model parameters related to antenna sizes showed weaker dependence on t d than those referring to antenna number. With restrictions applied to this model, the two DF induction transients may be related to two classes of photosynthetic antennas. Their different sizes may have a predominant influence on the efficiency of photon absorption and possibly time-dependent appearance of DF transients.

Kinetics of Reactions Around the Cytochrome bf Complex Studied in Intact Leaf Disks

Electron transfers in the photosynthetic electron transport chain including the cytochrome (cyt) bf and Photosystem (PS) I complexes were studied in leaves of several plant species by measuring flash-induced absorbancy changes at specific wavelengths. The electrochromic signal (ECS), indicative of a trans-thylakoid membrane electric field, consisted of a fast phase arising from charge separation in both photosystems, and a slow rise usually interpreted as charge transfer in the cyt bf complex (part of the Q-cycle). The amplitude of the slow phase of the ECS was frequently greater than could be accounted for by the withdrawal of an electron from cyt bf via plastocyanin (PC) by oxidised P700 in PS I. The ‘extra’ slow ECS, variable depending on the number of turnovers and plant species, can be attributed to a variable operation of proton-pumping activity of the cyt bf complex. The redox kinetics of cyt f and b were obtained by deconvolution of the signals at three or four wavelengths. Rates of cyt b reduction were very high, and never the same as the onset kinetics of the slow ECS. The cyt f signal suggests that a fraction of the oxidised cyt f was re-reduced only slowly in the time of 5 s between consecutive flashes. Leaf discs in far-red light were given single-turnover flashes to measure the rates of P700ox reduction and reoxidation. To simulate the redox kinetics of the ECS, cyt f, cyt b and P700 it was assumed that a Q-cycle normally operated in bf complexes; reasonable values for the appropriate rate coefficients, and for the equilibrium constants for the cyt f/PC and P700/PC reactions were chosen. Close similarity of the observed data with those predicted from the simulation was obtained for cyt b, P700 (far-red light experiments) and the ECS, but not for cyt f. The results contribute to an understanding of photosynthetic electron transfers in vivo.

MfERG response dynamics of the aging retina.

To determine age-related changes in retinal response dynamics derived from multifocal electroretinograms (mfERGs). MfERG data were obtained from 70 subjects with normal phakic eyes, age 9 to 80 years. Whereas the first- and higher-order kernels resulting from the mfERG contain detailed information regarding the nonlinear response dynamics of the retina, they do not lend themselves directly to an easy and intuitive interpretation. To achieve a better appreciation of fast adaptive mechanisms and their changes with aging, regional averages of the kernel series were translated at different retinal eccentricities (0 degrees -5 degrees, 5 degrees -15 degrees, and 15 degrees -25 degrees ) into responses generated in different contexts. Specifically, the effect of aging on responses to stimuli presented in isolation was compared with the effect on responses adapted by preceding stimuli ("forward" effect). The interference of the immediately following stimuli with the response generation ("backward effect") was also considered. Age-related changes were found in the isolated flash response as well as in the backward and forward interactions between consecutive flash responses. Larger fractional changes with age were found in response density than in implicit time, and the rate of change with age was larger for responses to isolated flashes than for responses adapted by preceding flashes. Senescent changes in the isolated flash response and in consecutive flash interactions derived from the binary kernel series indicate an aging process at an early stage in the visual system. Mechanisms of retinal adaptation may partially compensate for age-related reductions in the isolated flash response.