Chlorophyll a fluorescence induction: a personal perspective of the thermal phase, the J-I-P rise

Dissecting the interaction of photosynthetic electron transfer with mitochondrial signalling and hypoxic response in the Arabidopsis rcd1 mutant

The Arabidopsis mutant rcd1 is tolerant to methyl viologen (MV). MV enhances the Mehler reaction, i.e. electron transfer from Photosystem I (PSI) to O₂, generating reactive oxygen species (ROS) in the chloroplast. To study the MV tolerance of rcd1, we first addressed chloroplast thiol redox enzymes potentially implicated in ROS scavenging. NADPH-thioredoxin oxidoreductase type C (NTRC) was more reduced in rcd1. NTRC contributed to the photosynthetic and metabolic phenotypes of rcd1, but did not determine its MV tolerance. We next tested rcd1 for alterations in the Mehler reaction. In rcd1, but not in the wild type, the PSI-to-MV electron transfer was abolished by hypoxic atmosphere. A characteristic feature of rcd1 is constitutive expression of mitochondrial dysfunction stimulon (MDS) genes that affect mitochondrial respiration. Similarly to rcd1, in other MDS-overexpressing plants hypoxia also inhibited the PSI-to-MV electron transfer. One possible explanation is that the MDS gene products may affect the Mehler reaction by altering the availability of O₂. In green tissues, this putative effect is masked by photosynthetic O₂ evolution. However, O₂ evolution was rapidly suppressed in MV-treated plants. Transcriptomic meta-analysis indicated that MDS gene expression is linked to hypoxic response not only under MV, but also in standard growth conditions. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Photosynthetic cyclic electron transport provides ATP for homeostasis during trap closure in Dionaea muscipula

BACKGROUND AND AIMS

The processes connected with prey capture and the early consumption of prey by carnivorous Dionaea muscipula require high amounts of energy. The aim of the present study was to identify processes involved in flytrap energy provision and ATP homeostasis under these conditions.

METHODS

We determined photosynthetic CO2 uptake and chlorophyll fluorescence as well as the dynamics of ATP contents in the snap traps upon closure with and without prey.

KEY RESULTS

The results indicate that upon prey capture, a transient switch from linear to cyclic electron transport mediates a support of ATP homeostasis. Beyond 4 h after prey capture, prey resources contribute to the traps' ATP pool and, 24 h after prey capture, export of prey-derived resources to other plant organs may become preferential and causes a decline in ATP contents.

CONCLUSIONS

Apparently, the energy demand of the flytrap for prey digestion and nutrient mining builds on both internal and prey-derived resources.

Variable fluorescence of closed photochemical reaction centers

Chlorophyll fluorescence induction during 0.4 to 200 ms multiple-turnover pulses (MTP) was measured in parallel with O₂ evolution induced by the MTP light. Additionally, a saturating single-turnover flash (STF) was applied at the end of each MTP and the total MTP +STF O₂ evolution was measured. Quantum yield of O₂ evolution during the MTP transients was calculated and related to the number of open PSII centers, found from the STF O₂ evolution. Proportionality between the number of open PSII and their running photochemical activity showed the quantum yield of open PSII remained constant independent of the closure of adjacent centers. During the induction, total fluorescence was partitioned between F_o of all the open centers and F_c of all the closed centers. The fluorescence yield of a closed center was 0.55 of the final F_m while less than a half of the centers were closed, but later increased, approaching F_m to the end of the induction. In the framework of the antenna/radical pair equilibrium model, the collective rise of the fluorescence of centers closed earlier during the induction is explained by an electric field, facilitating return of excitation energy from the Pheo^- P680⁺ radical pair to the antenna.

Linking physiological parameters with visible/near-infrared leaf reflectance in the incubation period of vascular wilt disease

The photosynthetic pigments are mainly responsible for absorbing the light intended to promote photosynthesis on the chloroplast of the leaves. Different studies have related the spectral response in the leaves of plants with the biotic stress generated by pathogens. In general, maximum differences in reflectance have been found in the range of 380-750 nm between plants subjected to biotic stress and healthy plants. In this study, it was possible to characterize and relate the spectral variance in leaves of S. lycopersicum infected with F. oxysporum with this physiological variation and pathogen concentration in tomato plants during the asymptomatic period of vascular wilt. Photosynthetic parameters derived from gaseous exchange analysis in the tomato leaves correlated related with four bands in the visible range (Vis). Additionally, five specific bands also present a high correlation with the increase in the concentration of F. oxysporum conidia measured at the root: 448-523 nm, 624-696 nm, 740-960 nm, 973-976 nm, and 992-995 nm. These wavelengths allowed a 100% correct classification of the plants inoculated with F. oxysporum from the plants subjected to hydric stress and the control plants in the asymptomatic period of the disease. The spectral response to biotic and abiotic stress in the measured Vis/NIR range can be explained by the general tendency to change the concentration of chlorophyll and carotene in tomato leaves. These studies also highlight the importance of the implementation of robust multivariate analysis over the multiple univariate analysis used in the applied biological sciences and specifically in the agricultural sciences. These results demonstrate that specific wavelength responses are due to physiological changes in plants subjected to stress, and can be used in indexes and algorithms applied to the early detection of diseases in plants on different pathosystems.

Measurement of Fluorescence for Monitoring Algal Growth and Health

Measuring fluorescence is a noninvasive, inexpensive, and quick way of determining biomass concentration and health of the algae. Fluorescence is generally positively correlated with chlorophyll a and can as such be used as a proxy for biomass. In addition, the proportion variable fluorescence of maximal fluorescence is a measure of photochemical efficiency, which is affected by stress in a very early stage and can as such be used as a proxy for algal health.

Slow induction of chlorophyll a fluorescence excited by blue and red light in Tradescantia leaves acclimated to high and low light

Tradescantia is a good model for assaying induction events in higher plant leaves. Chlorophyll (Chl) fluorescence serves as a sensitive reporter of the functional state of photosynthetic apparatus in chloroplasts. The fluorescence time-course depends on the leaf growth conditions and actinic light quality. In this work, we investigated slow induction of Chl a fluorescence (SIF) excited by blue light (BL, λ_max = 455 nm) or red light (RL, λ_max = 630 nm) in dark-adapted leaves of Tradescantia fluminensis acclimated to high light (~ 1000 µmol photons m^-2 s^-1; HL) or low light (~ 100 µmol photons m^-2 s^-1; LL). Our special interest was focused on the contribution of the avoidance response to SIF kinetics. Bearing in mind that BL and RL have different impacts on photoreceptors that initiate chloroplast movements within the cell (accumulation/avoidance responses), we have compared the SIF patterns during the action of BL and RL. The time-courses of SIF and kinetics of non-photochemical quenching (NPQ) of Chl a fluorescence revealed a certain difference when leaves were illuminated by BL or RL. In both cases, the yield of fluorescence rose to the maximal level P and then, after the lag-phase P-S-M₁, the fluorescence level decreased toward the steady state T (via the intermediate phases M₁-M₂ and M₂-T). In LL-acclimated leaves, the duration of the P-S-M₁ phase was almost two times longer that in HL-grown plants. In the case of BL, the fluorescence decay included the transient phase M₁-M₂. This phase was obscure during the RL illumination. Non-photochemical quenching of Chl a fluorescence has been quantified as [Formula: see text], where [Formula: see text] and [Formula: see text] stand for the fluorescence response to saturating pulses of light applied to dark-adapted and illuminated samples, respectively. The time-courses of such a formally determined NPQ value were markedly different during the action of RL and BL. In LL-grown leaves, BL induced higher NPQ as compared to the action of RL. In HL-grown plants, the difference between the NPQ responses to BL and RL illumination was insignificant. Comparing the peculiarities of Chl a fluorescence induced by BL and RL, we conclude that the avoidance response can provide a marked contribution to SIF and NPQ generation. The dependence of NPQ on the quality of actinic light suggests that chloroplast movements within the cell have a noticeable impact on the formally determined NPQ value. Analyzing kinetics of post-illumination decay of NPQ in the context of solar stress resistance, we have found that LL-acclimated Tradescantia leaves are more vulnerable to strong light than the HL-grown leaves.

Chromium effects on photosynthetic electron transport in pea (Pisum sativum L.)

MAIN CONCLUSION

Components of the photosynthetic electron transport chain in pea (Pisum sativum L.) leaves under in vivo conditions showed the following sensitivity to the inhibitory action of chromium(VI): intersystem electron transport > photosystem I > photosystem II. Inhibitory effects of chromium (VI) (K2Cr2O7, Cr) on the light reactions of photosynthesis were studied in vivo in Pisum sativum L. by using Multi-function Plant Efficiency Analyser (M-PEA-2). Photosynthetic parameters related to photosystem (PS) II, PSI and intersystem electron carriers were calculated from the light-induced kinetics of prompt chlorophyll a fluorescence (OJIP transient), delayed chlorophyll a fluorescence (DF), and 820 nm modulated reflection (MR). We showed that the I2 step of DF induction is sensitive to inhibition of the Q0 site of the cytochrome b6f complex. Such parameters as δRo of the JIP test related to the functional state of photosynthetic reactions beyond the PQ pool, Vred of the MR induction assigned to the overall rate of P700+ and plastocyanin reduction, and I2 step of the DF induction were significantly altered in the presence of low-dose Cr(VI). Moderate doses of Cr affected mainly PSI-related parameters including Vox and ΔMR parameters of the MR induction, whereas high-dose treatment influenced JIP test parameters φPo(= FV/FM) and ψEo related to PSII. The obtained results showed that the earliest Cr(VI) effect on the photosynthetic electron transport chain manifests itself by inhibition of the intersystem electron transport, rather, at the level of the cytochrome b6f complex. Inhibitory effects of Cr on PSI were more pronounced than those on PSII. Sensitivity of the used kinetic parameters toward the functional state of photosynthetic reactions makes this approach suitable for early diagnostics of toxic action of pollutants on plants.

CO2 uptake and chlorophyll a fluorescence of Suaeda fruticosa grown under diurnal rhythm and after transfer to continuous dark

Although only 2-4% of absorbed light is emitted as chlorophyll (Chl) a fluorescence, its measurement provides valuable information on photosynthesis of the plant, particularly of Photosystem II (PSII) and Photosystem I (PSI). In this paper, we have examined photosynthetic parameters of Suaeda fruticosa L. (family: Amaranthaceae), surviving under extreme xerohalophytic conditions, as influenced by diurnal rhythm or continuous dark condition. We report here CO₂ gas exchange and the kinetics of Chl a fluorescence of S. fruticosa, made every 3 hours (hrs) for 3 days, using a portable infra-red gas analyzer and a Handy PEA fluorimeter. Our measurements on CO₂ gas exchange show the maximum rate of photosynthesis to be at 08:00 hrs under diurnal condition and at 05:00 hrs under continuous dark. From the OJIP phase of Chl a fluorescence transient, we have inferred that the maximum quantum yield of PSII photochemistry must have increased during the night under diurnal rhythm, and between 11:00 and 17:00 hrs under constant dark. Overall, our study has revealed novel insights into how photosynthetic reactions are affected by the photoperiodic cycles in S. fruticosa under high salinity. This study has further revealed a unique strategy operating in this xero-halophyte where the repair mechanism for damaged PSII operates during the dark, which, we suggest, contributes to its ecological adaptation and ability to survive and reproduce under extreme saline, high light, and drought conditions. We expect these investigations to help in identifying key genes and pathways for raising crops for saline and dry areas.

Rapidly reversible chlorophyll fluorescence quenching induced by pulses of supersaturating light in vivo

The saturation pulse method provides a means to distinguish between photochemical and non-photochemical quenching, based on the assumption that the former is suppressed by a saturating pulse of light (SP) and that the latter is not affected by the SP. Various types of non-photochemical quenching have been distinguished by their rates of dark relaxation in the time ranges of seconds, minutes, and hours. Here we report on a special type of non-photochemical quenching, which is rapidly induced by a pulse of high-intensity light, when PS II reaction centers are closed, and rapidly relaxes again after the pulse. This high-intensity quenching, HIQ, can be quantified by pulse-amplitude-modulation (PAM) fluorimetry (MULTI-COLOR-PAM, high sensitivity combined with high time resolution) via the quasi-instantaneous post-pulse fluorescence increase that precedes recovery of photochemical quenching in the 100-400-µs range. The HIQ amplitude increases linearly with the effective rate of quantum absorption by photosystem II, reaching about 8% of maximal fluorescence yield. It is not affected by DCMU, is stimulated by anoxic conditions, and is suppressed by energy-dependent non-photochemical quenching (NPQ). The HIQ amplitude is close to proportional to the square of maximal fluorescence yield, Fm', induced by an SP and varied by NPQ. These properties are in line with the working hypothesis of HIQ being caused by the annihilation of singlet excited chlorophyll a by triplet excited carotenoid. Significant underestimation of maximal fluorescence yield and photosystem II quantum yield in dark-acclimated samples can be avoided by use of moderate SP intensities. In physiologically healthy illuminated samples, NPQ prevents significant lowering of effective photosystem II quantum yield by HIQ, if excessive SP intensities are avoided.

Probing functional and optical cross-sections of PSII in leaves during state transitions using fast repetition rate light induced fluorescence transients

Plants adjust the relative sizes of PSII and PSI antennae in response to the spectral composition of weak light favouring either photosystem by processes known as state transitions (ST), attributed to a discrete antenna migration involving phosphorylation of light-harvesting chlorophyll-protein complexes in PSII. Here for the first time we monitored the extent and dynamics of ST in leaves from estimates of optical absorption cross-section (relative PSII antenna size; aPSII). These estimates were obtained from in situ measurements of functional absorption cross-section (σPSII) and maximum photochemical efficiency of PSII (φPSII); i.e. aPSII = σPSII/φPSII (Kolber et al. 1998) and other parameters from a light induced fluorescence transient (LIFT) device (Osmond et al. 2017). The fast repetition rate (FRR) QA flash protocol of this instrument monitors chlorophyll fluorescence yields with reduced QA irrespective of the redox state of plastoquinone (PQ), as well as during strong ~1 s white light pulses that fully reduce the PQ pool. Fitting this transient with the FRR model monitors kinetics of PSII → PQ, PQ → PSI, and the redox state of the PQ pool in the 'PQ pool control loop' that underpins ST, with a time resolution of a few seconds. All LIFT/FRR criteria confirmed the absence of ST in antenna mutant chlorina-f2 of barley and asLhcb2-12 of Arabidopsis, as well as STN7 kinase mutants stn7 and stn7/8. In contrast, wild-type barley and Arabidopsis genotypes Col, npq1, npq4, OEpsbs, pgr5 bkg and pgr5, showed normal ST. However, the extent of ST (and by implication the size of the phosphorylated LHCII pool participating in ST) deduced from changes in a'PSII and other parameters with reduced QA range up to 35%. Estimates from strong WL pulses in the same assay were only ~10%. The larger estimates of ST from the QA flash are discussed in the context of contemporary dynamic structural models of ST involving formation and participation of PSII and PSI megacomplexes in an 'energetically connected lake' of phosphorylated LHCII trimers (Grieco et al. 2015). Despite the absence of ST, asLhcb2-12 displays normal wild-type modulation of electron transport rate (ETR) and the PQ pool during ST assays, reflecting compensatory changes in antenna LHCIIs in this genotype. Impaired LHCII phosphorylation in stn7 and stn7/8 accelerates ETR from PSII →PQ, over-reducing the PQ pool and abolishing the yield difference between the QA flash and WL pulse, with implications for photochemical and thermal phases of the O-J-I-P transient.

Unsupervised classification of PSII with and without water-oxidizing complex samples by PARAFAC resolution of excitation-emission fluorescence images

The potential of excitation-emission fluorescence spectroscopy combined with three-way analysis was investigated for discriminating the photosystem II (PSII) (with the water-oxidizing complex) and without the water-oxidizing complex (wPSII) using unsupervised classification methods. The water-oxidizing complex within PSII carry out the reaction of water splitting which is as a vital process on the earth. Therefore, discriminating the presence of the water-oxidizing complex in protein samples is crucial. Low cost and accurate spectroscopic determination of the amount of clusters inside PSII or any other protein containing species are important when investigating the inclusion and exclusion of such clusters into and from species. Fluorescence data of samples were similar, and we showed the potential usefulness of multivariate methods, such as parallel factor analysis (PARAFAC) and principal component analysis (PCA) for recognition of the two types of samples. Both techniques were applied to the excitation-emission fluorescence matrices (EEM) of solutions at two of different pH values (2.0 and 12.0). Three fluorescent components were found for all samples that are related to tyrosine (Tyr), tryptophan (Trp) and phenylalanine (Phe) amino acids. These three amino acids are representative of all datasets and indicate their similarities and differences. We then found the effectual wavelengths for separation of samples in a specific acidity, including the excitation wavelengths of 220 and 230 nm and the emission wavelengths of 300 and 305 nm. The acidity of the solutions has various influences on the conformation of proteins. In PSII and PSII the without water-oxidizing complex samples conformational changes can change their spectra which was applied for discrimination purpose. This separation was better in pH = 12.0. We also showed the effect of time on small conformational changes within datasets were higher in pH = 2.0. In the end, for indicating the high distribution of spectral data from proteins which is the result of conformational changes, we compared the distribution of measured spectral data with that from a simple organic molecule, fluorescein. Altogether, we could distinguish between the two groups of protein samples properly at pH = 12.0 using low-cost EEM spectral images and PARAFAC.

Unresolved quenching mechanisms of chlorophyll fluorescence may invalidate MT saturating pulse analyses of photosynthetic electron transfer in microalgae

Chlorophyll a fluorescence is a powerful tool for estimating photosynthetic efficiency, but there are still unanswered questions that hinder the use of its full potential. The present results describe a caveat in estimation of photosynthetic performance with so-called rapid light curves (RLCs) with pulse amplitude modulation fluorometers. RLCs of microalgae show a severe decrease in photosynthetic performance in high light, although a similar decrease cannot be seen with other methods. We show that this decrease cannot be assigned to energy-dependent non-photochemical quenching or photoinhibition or to the geometry of the algal sample. The measured decrease in electron transfer rate is small in the tested siphonaceuous algae and higher plants, but very notable in all planktonic species, exhibiting species-dependent variation in extent and reversibility. We performed in-depth analysis of the phenomenon in the diatom Phaeodactylum tricornutum, in which the decrease is the most pronounced and reversible among the tested organisms. The results suggest that quenching of fluorescence by oxidized plastoquinone alone cannot explain the phenomenon, and alternative quenching mechanisms within PSII need to be considered.

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 2 ns (∼60%) and 10 ns (∼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 Q_A ; 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.

Tissue specific disruption of photosynthetic electron transport rate in pigeonpea (Cajanus cajan L.) under elevated temperature

Nutritional rich pigeonpea (Cajanus cajan [L.]), a perennial shrub member of family Fabaceae is the sixth most important grain legume of the world. Continuous rise of temperature and current global climate scenario limits plant growth and performance but photosynthetic machineries are adversely affected. The aim of this study was the analysis of tissue specific photosynthetic pigments and photosynthetic electron transport rate under elevated temperature. Two different stages of both leaf (young and mature), and pod (young and mature) were chosen, and photosynthetic pigment and J-I-P tests were measured. Leaves and pods were detached and incubated in normal temperature (25°C) for 24 h in two different condition one light irradiance (100 µmol m-2 s-1) and another darkness as control, and treated with high temperature 45°C for 24 h and repeated previous measurements were taken. Tissue specific photosynthetic pigments variation were observed; J-I-P parameters clearly revealed that elevated temperature has greater influence on photosystem II (PSII) electron transport than photosystem I (PSI), and significant changes were observed in pods than leaves. Young tissues were adversely affected by elevated temperature. In addition, the J-I-P tests and energy pipeline model indicated that PSI electron transport rate of leaves and pods appeared to be more thermo-tolerance than those in PSII. Only a minor drop in pigments pool and photosynthetic performance was observed after 24 h of darkness. O-J-I-P transients can be used as a sensitive, nondestructive method for measuring heat stress damage and a special tool for investigating action sites of high temperature stress. Findings of this study will contribute to basic understanding of photosynthetic performance, and to screen potential thermo-tolerant genotypes of pigeonpea to sustain in either current scenario of climate change or/and erratic future climatic conditions.

Analyzing both the fast and the slow phases of chlorophyll a fluorescence and P700 absorbance changes in dark-adapted and preilluminated pea leaves using a Thylakoid Membrane model

The dark-to-light transitions enable energization of the thylakoid membrane (TM), which is reflected in fast and slow (OJIPSMT or OABCDE) stages of fluorescence induction (FI) and P700 oxidoreduction changes (ΔA810). A Thylakoid Membrane model (T-M model), in which special emphasis has been placed on ferredoxin-NADP+-oxidoreductase (FNR) activation and energy-dependent qE quenching, was applied for quantifying the kinetics of FI and ΔA810. Pea leaves were kept in darkness for 15 min and then the FI and ΔA810 signals were measured upon actinic illumination, applied either directly or after a 10-s light pulse coupled with a subsequent 10-s dark interval. On the time scale from 40 µs to 30 s, the parallel T-M model fittings to both FI and ΔA810 signals were obtained. The parameters of FNR activation and the buildup of qE quenching were found to differ for dark-adapted and preilluminated leaves. At the onset of actinic light, photosystem II (PSII) acceptors were oxidized (neutral) after dark adaptation, while the redox states with closed and/or semiquinone QA(-)QB(-) forms were supposedly generated after preillumination, and did not relax within the 10 s dark interval. In qE simulations, a pH-dependent Hill relationship was used. The rate constant of heat losses in PSII antenna kD(t) was found to increase from the basic value kDconst, at the onset of illumination, to its maximal level kDvar due to lumenal acidification. In dark-adapted leaves, a low value of kDconst of ∼ 2 × 106 s-1 was found. Simulations on the microsecond to 30 s time scale revealed that the slow P-S-M-T phases of the fluorescence induction were sensitive to light-induced FNR activation and high-energy qE quenching. Thus, the corresponding time-dependent rate constants kD(t) and kFNR(t) change substantially upon the release of electron transport on the acceptor side of PSI and during the NPQ development. The transitions between the cyclic and linear electron transport modes have also been quantified in this paper.

The ratio of single-turnover to multiple-turnover fluorescence varies predictably with growth rate and cellular chlorophyll in the green alga Dunaliella tertiolecta

Marine phytoplankton experience a wide range of nutrient and light conditions in nature and respond to these conditions through changes in growth rate, chlorophyll concentration, and other physiological properties. Chlorophyll fluorescence is a non-invasive and efficient tool for characterizing changes in these physiological properties. In particular, the introduction of fast repetition rate fluorometry (FRRf) into studies of phytoplankton physiology has enabled detailed studies of photosynthetic components and kinetics. One property retrieved with an FRRf is the 'single-turnover' maximum fluorescence (Fm^ST) when the primary electron acceptor, Qa, is reduced but the plastoquinone (PQ) pool is oxidized. A second retrieved property is the 'multiple-turnover' fluorescence (F^MT) when both Qa and PQ are reduced. Here, variations in Fm^ST and F^MT were measured in the green alga Dunaliella tertiolecta grown under nitrate-limited, light-limited, and replete conditions. The ratio of Fm^ST to F^MT (ST/MT) showed a consistent relationship with cellular chlorophyll in D. tertiolecta across all growth conditions. However, the ST/MT ratio decreased with growth rate under nitrate-limited conditions but increased with growth rate under light-limited conditions. In addition, cells from light-limited conditions showed a high accumulation of Qb-nonreducing centers, while cells from nitrate-limited conditions showed little to none. We propose that these findings reflect differences in the reduction and oxidation rates of plastoquinone due to the unique impacts of light and nitrate limitation on the stoichiometry of light-harvesting components and downstream electron acceptors.

Photo-reducible plastoquinone pools in chloroplasts of Tradescentia plants acclimated to high and low light

In photosynthetic systems of oxygenic type, plastoquinone (PQ) molecules are reduced by photosystem II (PSII). The turnover of PQ determines the rate of PSII operation. PQ molecules are present in surplus with respect to PSII. In this work, using the pulse amplitude modulation-fluorometry technique, we quantified photo-reducible PQ pools in chloroplasts of two contrasting ecotypes of Tradescantia, acclimated either to low light (~ 100 μmol photons·m^-2 ·s^-1 , LL) or to high light (~ 1000 μmol photons·m^-2 ·s^-1 , HL). The LL-grown plants are characterized by higher capacity of rapidly reducible PQ pool ([PQ]₀ /[PSII] ≈ 8) as compared to HL-grown plants of both species ([PQ]₀ /[PSII] ≈ 4). The elevated content of PQ in LL plants favours photosynthetic electron flow at low-solar irradiance.

Light acclimation of shade-tolerant and sun-resistant Tradescantia species: photochemical activity of PSII and its sensitivity to heat treatment

In this work, we have compared photosynthetic characteristics of photosystem II (PSII) in Tradescantia leaves of two contrasting ecotypes grown under the low light (LL) and high light (HL) regimes during their entire growth period. Plants of the same genus, T. fluminensis (shade-tolerant) and T. sillamontana (sun-resistant), were cultivated at 50-125 µmol photons m^-2 s^-1 (LL) or at 875-1000 µmol photons m^-2 s^-1 (HL). Analyses of intrinsic PSII efficiency was based on measurements of fast chlorophyll (Chl) a fluorescence kinetics (the OJIP test). The fluorescence parameters F_v/F_m (variable fluorescence) and F₀ (the initial level of fluorescence) in dark-adapted leaves were used to quantify the photochemical properties of PSII. Plants of different ecotypes showed different sustainability with respect to changes in the environmental light intensity and temperature treatment. The sun-resistant species T. sillamontana revealed the tolerance to variations in irradiation intensity, demonstrating constancy of maximum quantum efficiency of PSII upon variations of the growth light. In contrast to T. sillamontana, facultative shade species T. fluminensis demonstrated variability of PSII photochemical activity, depending on the growth light intensity. The susceptibility of T. fluminensis to solar stress was documented by a decrease in F_v/F_m and a rise of F₀ during the long-term exposition of T. fluminensis to HL, indicating the loss of photochemical activity of PSII. The short-term (10 min) heat treatment of leaf cuttings caused inactivation of PSII. The temperature-dependent heating effects were different in T. fluminensis and T. sillamontana. Sun-resistant plants T. sillamontana acclimated to LL and HL displayed the same plots of F_v/F_m versus the treatment temperature (t), demonstrating a decrease in F_v/F_m at t ≥ 45 °C. The leaves of shadow-tolerant species T. fluminensis grown under the LL and HL conditions revealed different sensitivities to heat treatment. Plants grown under the solar stress conditions (HL) demonstrated a gradual decline of F_v/F_m at lower heating temperatures (t ≥ 25 °C), indicating the "fragility" of their PSII as compared to T. fluminensis grown at LL. Different responses of sun and shadow species of Tradescantia to growth light and heat treatment are discussed in the context of their biochemical and ecophysiological properties.

A sixty-year tryst with photosynthesis and related processes: an informal personal perspective

After briefly describing my early collaborative work at the University of Allahabad, that had laid the foundation of my research life, I present here some of our research on photosynthesis at the University of Illinois at Urbana-Champaign, randomly selected from light absorption to NADP⁺ reduction in plants, algae, and cyanobacteria. These include the fact that (i) both the light reactions I and II are powered by light absorbed by chlorophyll (Chl) a of different spectral forms; (ii) light emission (fluorescence, delayed fluorescence, and thermoluminescence) by plants, algae, and cyanobacteria provides detailed information on these reactions and beyond; (iii) primary photochemistry in both the photosystems I (PS I) and II (PS II) occurs within a few picoseconds; and (iv) most importantly, bicarbonate plays a unique role on the electron acceptor side of PS II, specifically at the two-electron gate of PS II. Currently, the ongoing research around the world is, and should be, directed towards making photosynthesis better able to deal with the global issues (such as increasing population, dwindling resources, and rising temperature) particularly through genetic modification. However, basic research is necessary to continue to provide us with an understanding of the molecular mechanism of the process and to guide us in reaching our goals of increasing food production and other chemicals we need for our lives.

Changes in the photosynthetic apparatus and lipid droplet formation in Chlamydomonas reinhardtii under iron deficiency

The unicellular photosynthetic alga Chlamydomonas reinhardtii was propagated in iron deficiency medium and patterns of growth, photosynthetic efficiency, lipid accumulation, as well as the expression of lipid biosynthetic and photosynthesis-related proteins were analysed and compared with iron-sufficient growth conditions. As expected, the photosynthetic rate was reduced (maximally after 4 days of growth) as a result of increased non-photochemical quenching (NPQ). Surprisingly, the stress-response protein LHCSR3 was expressed in conditions of iron deficiency that cause NPQ induction. In addition, the protein contents of both the PSI and PSII reaction centres were gradually reduced during growth in iron deficiency medium. Interestingly, the two generations of Fe deficiency cells could be able to recover the photosynthesis but the second generation cells recovered much slower as these cells were severely in shock. Analysis by flow cytometry with fluorescence-activated cell sorting and thin layer chromatography showed that iron deficiency also induced the accumulation of triacylglycerides (TAG), which resulted in the formation of lipid droplets. This was most significant between 48 and 72 h of growth. Dramatic increases in DGAT2A and PDAT1 levels were caused by iron starvation, which indicated that the biosynthesis of TAG had been increased. Analysis using gas chromatography mass spectrometry showed that levels of 16:0, 18:0, 18:2 and 18:3^Δ9,12,15 fatty acids were significantly elevated. The results of this study highlight the genes/enzymes of Chlamydomonas that affect lipid synthesis through their influence on photosynthesis, and these represent potential targets of metabolic engineering to develop strains for biofuel production.

Influence of nitrogen source on photochemistry and antenna size of the photosystems in marine green macroalgae, Ulva lactuca

Ulva lactuca is regarded as a prospective energy crop for biorefinery owing to its affluent biochemical composition and high growth rate. In fast-growing macroalgae, biomass development strictly depends on external nitrogen pools. Additionally, nitrogen uptake rates and photosynthetic pigment content vary with type of nitrogen source and light conditions. However, the combined influence of nitrogen source and light intensity on photosynthesis is not widely studied. In present study, pale green phenotype of U. lactuca was obtained under high light (HL) condition when inorganic nitrogen (nitrate) in the media was substituted with organic nitrogen (urea). Further, pale green phenotype survived the saturating light intensities in contrast to the normal pigmented control which bleached in HL. Detailed analysis of biochemical composition and photosynthesis was performed to understand functional antenna size and photoprotection in pale green phenotype. Under HL, urea-grown cultures exhibited increased growth rate, carbohydrate and lipid content while substantial reduction in protein, chlorophyll content and PSII antenna size was observed. Further, in vivo slow and polyphasic chlorophyll a (Chl a) fluorescence studies revealed reduction in excitation pressure on PSII along with low non-photochemical quenching thus, transmitting most of the absorbed energy into photochemistry. The results obtained could be correlated to previous report on cultivation of U. lactuca through saturating summer intensities (1000 µmole photons m^-2 s^-1) in urea based: poultry litter extract (PLE). Having proved critical role of urea in conforming photoprotection, the application PLE was authenticated for futuristic, sustainable and year-round biomass cultivation.

Modulation in light utilization by a microalga Asteracys sp. under mixotrophic growth regimes

This study is the first to explore the influence of incident light intensity on the photosynthetic responses under mixotrophic growth of microalga Asteracys sp. When grown mixotrophically, there was an enhanced regulation of non-photochemical quenching (NPQ) of the excited state of chlorophyll (Chl) a within the cells in response to white cool fluorescent high light (HL; 600 µmol photons m^-2 s^-1). Simultaneous measurement of reactive oxygen species (ROS) production as malondialdehyde (MDA) and ascorbate peroxidase (APX), an ROS scavenger, showed improved management of stress within mixotrophic cells under HL. Despite the observed decrease in quantum yield of photosynthesis measured through the Chl a fluorescence transient, no reduction in biomass accumulation was observed under HL for mixotrophy. However, biomass loss owing to photoinhibition was observed in cells grown phototrophically under the same irradiance. The measurements of dark recovery of NPQ suggested that "state transitions" may be partly responsible for regulating overall photosynthesis in Asteracys sp. The partitioning of photochemical and non-photochemical processes to sustain HL stress was analysed. Collectively, this study proposes that mixotrophy using glucose leads to a change in the photosynthetic abilities of Asteracys sp. while enhancing the adaptability of the alga to high irradiances.

Low temperature induced modulation of photosynthetic induction in non-acclimated and cold-acclimated Arabidopsis thaliana: chlorophyll a fluorescence and gas-exchange measurements

Cold acclimation modifies the photosynthetic machinery and enables plants to survive at sub-zero temperatures, whereas in warm habitats, many species suffer even at non-freezing temperatures. We have measured chlorophyll a fluorescence (ChlF) and CO₂ assimilation to investigate the effects of cold acclimation, and of low temperatures, on a cold-sensitive Arabidopsis thaliana accession C24. Upon excitation with low intensity (40 µmol photons m^- 2 s^- 1) ~ 620 nm light, slow (minute range) ChlF transients, at ~ 22 °C, showed two waves in the SMT phase (S, semi steady-state; M, maximum; T, terminal steady-state), whereas CO₂ assimilation showed a linear increase with time. Low-temperature treatment (down to - 1.5 °C) strongly modulated the SMT phase and stimulated a peak in the CO₂ assimilation induction curve. We show that the SMT phase, at ~ 22 °C, was abolished when measured under high actinic irradiance, or when 3-(3, 4-dichlorophenyl)-1, 1- dimethylurea (DCMU, an inhibitor of electron flow) or methyl viologen (MV, a Photosystem I (PSI) electron acceptor) was added to the system. Our data suggest that stimulation of the SMT wave, at low temperatures, has multiple reasons, which may include changes in both photochemical and biochemical reactions leading to modulations in non-photochemical quenching (NPQ) of the excited state of Chl, "state transitions," as well as changes in the rate of cyclic electron flow through PSI. Further, we suggest that cold acclimation, in accession C24, promotes "state transition" and protects photosystems by preventing high excitation pressure during low-temperature exposure.

Three phases of energy-dependent induction of [Formula: see text] and Chl a fluorescence in Tradescantia fluminensis leaves

In plants, the short-term regulation (STR, seconds to minute time scale) of photosynthetic apparatus is associated with the energy-dependent control in the chloroplast electron transport, the distribution of light energy between photosystems (PS) II and I, activation/deactivation of the Calvin-Benson cycle (CBC) enzymes, and relocation of chloroplasts within the plant cell. In this work, using a dual-PAM technique for measuring the time-courses of P₇₀₀ photooxidation and Chl a fluorescence, we have investigated the STR events in Tradescantia fluminensis leaves. The comparison of Chl a fluorescence and [Formula: see text] induction allowed us to investigate the contribution of the trans-thylakoid pH difference (ΔpH) to the STR events. Two parameters were used as the indicators of ΔpH generation: pH-dependent component of non-photochemical quenching of Chl a fluorescence, and pH_in-dependent rate of electron transfer from plastoquinol (PQH₂) to [Formula: see text] (via the Cyt b₆f complex and plastocyanin). In dark-adapted leaves, kinetics of [Formula: see text] induction revealed three phases. Initial phase is characterized by rapid electron flow to [Formula: see text] (τ_1/2 ~ 5-10 ms), which is likely related to cyclic electron flow around PSI, while the outflow of electrons from PSI is restricted by slow consumption of NADPH in the CBC. The light-induced generation of ΔpH and activation of the CBC promote photooxidation of P₇₀₀ and concomitant retardation of [Formula: see text] reduction (τ_1/2 ~ 20 ms). Prolonged illumination induces additional slowing down of electron transfer to [Formula: see text] (τ_1/2 ≥ 30-35 ms). The latter effect is not accompanied by changes in the Chl a fluorescence parameters which are sensitive to ΔpH generation. We suggest the tentative explanation of the latter results by the reversal of Q-cycle, which causes the deceleration of PQH₂ oxidation due to the back pressure of stromal reductants.

Unique photosynthetic electron transport tuning and excitation distribution in heterokont algae

Heterokont algae are significant contributors to marine primary productivity. These algae have a photosynthetic machinery that shares many common features with that of Viridiplantae (green algae and land plants). Here we demonstrate, however, that the photosynthetic machinery of heterokont algae responds to light fundamentally differently than that of Viridiplantae. While exposure to high light leads to electron accumulation within the photosynthetic electron transport chain in Viridiplantae, this is not the case in heterokont algae. We use this insight to manipulate the photosynthetic electron transport chain and demonstrate that heterokont algae can dynamically distribute excitation energy between the two types of photosystems. We suggest that the reported electron transport and excitation distribution features are adaptations to the marine light environment.

Exogenous citrate and malate alleviate cadmium stress in Oryza sativa L.: Probing role of cadmium localization and iron nutrition

Organic acids play an important role in metal uptake and trafficking in plants. Therefore, the role of exogenous citrate and malate on Cd tolerance was studied in the seedlings of Oryza sativa L. cv MTU 7029. Seedlings were exposed to Cd plus organic acids in hydroponics and monitored changes in Cd accumulation, expression of metal transporters, chlorophyll fluorescence, and antioxidants. It found that organic acid supplements decrease Cd accumulation in leaf because of up-regulation of tonoplast localized heavy metal ATPase (OsHMA3) which allows vacuolar sequestration of Cd in the root. Malic acid helped Cd exclusion in the root too. A shift in Cd speciation from sulphhydryl to the carboxylic group also noticed in the roots of plants exposed to organic acids. Treatment of organic acids was effective to prevent Cd inducible Fe deficiency via up-regulation of the iron-regulated transporter (OsIRT1), increase in ferric chelate reductase activity, and formation of Cd stabilized Fe³⁺ - organic acid complex respectively. Also, exposure to organic acids increased the accumulation of antioxidants such as anthocyanin and glutathione (GSH) under Cd stress. Above changes assisted in upholding of photosynthetic electron transport and biomass productivity during the course of Cd treatment with organic acid supplements.

Radioprotective role of cyanobacterial phycobilisomes

Cyanobacteria are thought to be responsible for pioneering dioxygen production and the so-called "Great Oxygenation Event" that determined the formation of the ozone layer and the ionosphere restricting ionizing radiation levels reaching our planet, which increased biological diversity but also abolished the necessity of radioprotection. We speculated that ancient protection mechanisms could still be present in cyanobacteria and studied the effect of ionizing radiation and space flight during the Foton-M4 mission on Synechocystis sp. PCC6803. Spectral and functional characteristics of photosynthetic membranes revealed numerous similarities of the effects of α-particles and space flight, which both interrupted excitation energy transfer from phycobilisomes to the photosystems and significantly reduced the concentration of phycobiliproteins. Although photosynthetic activity was severely suppressed, the effect was reversible, and the cells could rapidly recover from the stress. We suggest that the actual existence and the uncoupling of phycobilisomes may play a specific role not only in photo-, but also in radioprotection, which could be crucial for the early evolution of Life on Earth.

Simulation of chlorophyll fluorescence rise and decay kinetics, and P700-related absorbance changes by using a rule-based kinetic Monte-Carlo method

A model of primary photosynthetic reactions in the thylakoid membrane was developed and its validity was tested by simulating three types of experimental kinetic curves: (1) the light-induced chlorophyll a fluorescence rise (OJIP transients) reflecting the stepwise transition of the photosynthetic electron transport chain from the oxidized to the fully reduced state; (2) the dark relaxation of the flash-induced fluorescence yield attributed to the Q_A^- oxidation kinetics in PSII; and (3) the light-induced absorbance changes near 820 or 705 nm assigned to the redox transitions of P₇₀₀ in PSI. A model was implemented by using a rule-based kinetic Monte-Carlo method and verified by simulating experimental curves under different treatments including photosynthetic inhibitors, heat stress, anaerobic conditions, and very high light intensity.

The artificial humic substance HS1500 does not inhibit photosynthesis of the green alga Desmodesmus armatus in vivo but interacts with the photosynthetic apparatus of isolated spinach thylakoids in vitro

Humic substances (HSs) can influence the growth and composition of freshwater phytoplankton assemblage. Since HSs contain many phenolic and quinonic moieties and cause growth reductions in eco-physiological field experiments, HSs are considered photosystem II herbicides. To test this specific mode of action in vivo and in vitro, respectively, we used intact cells of the green alga Desmodesmus armatus, as well as thylakoids isolated from spinach (Spinacia oleracea) as a model system for the green algal chloroplast. Photosynthetic electron transport was measured as oxygen evolution and variable chlorophyll fluorescence. The in vivo effect of the artificial humic substance HS1500 on algae consisted of no impact on photosynthesis-irradiance curves of intact green algae compared to untreated controls. In contrast, addition of HS1500 to isolated thylakoids resulted in light-induced oxygen consumption (Mehler reaction) as an in vitro effect. Fluorescence induction kinetics of HS-treated thylakoids revealed a large static quenching effect of HS1500, but no inhibitory effect on electron transport. For the case of intact algal cells, we conclude that the highly hydrophilic and rather large molecules of HS1500 are not taken up in effective quantities and, therefore, cannot interfere with photosynthesis. The in vitro tests show that HS1500 has no inhibitory effect on photosystem II but operates as a weak, oxygen-consuming Hill acceptor at photosystem I. Hence, the results indicate that eco-physiological field experiments should focus more strongly on effects of HSs on extracellular features, such as reducing and red-shifting the underwater light field or influencing nutrient availability by cation exchange within the plankton network.

Chlorophyll a fluorescence and transcriptome reveal the toxicological effects of bisphenol A on an invasive cyanobacterium, Cylindrospermopsis raciborskii

Bisphenol A has attracted worldwide attention due to its harmful effects on humans, animals and plants. In this study, the toxicological effects of BPA on Cylindrospermopsis raciborskii were assessed based on chlorophyll a fluorescence and transcriptome analyses. The results showed that the growth of C. raciborskii was significantly inhibited when BPA exceeded 0.1 mg L^-1. A marked rise of phase J was observed at a concentration greater than 0.1 mg L^-1, while a K phase appeared at 20 mg L^-1. The chlorophyll a fluorescence parameters of RC/CS₀, F₀, φ_P0, φ_E0, and ψ₀, underwent a significant decline under all treatments of BPA, whereas a significant increase in both V_J and M₀ occurred under all concentrations of BPA. Additionally, ABS/RC and DIo/RC markedly increased at 10 mg L^-1 and 20 mg L^-1. The transcriptome analysis revealed that the genes of photosynthesis, including psbA, psbB, psbC, psbD, apcA, apcB, cpcA, and cpcB, as well as those of chlorophyll and carotenoid biosynthesis, namely hemN, acsF, chlL, chlN, chlP, crtB, pds, were all down-regulated. Moreover, BPA also inhibited the oxidative phosphorylation, glycolysis/gluconeogenesis, citrate cycle (TCA cycle), and fatty acid metabolism in C. raciborskii. Taken together, these results suggest BPA can negatively affect the expression of multiple genes and the vital energy metabolism process to arrest the growth and photosynthesis of C. raciborskii.

Kinetics of photosystem II electron transport: a mathematical analysis based on chlorophyll fluorescence induction

The OJDIP rise in chlorophyll fluorescence during induction at different light intensities was mathematically modeled using 24 master equations describing electron transport through photosystem II (PSII) plus ordinary differential equations for electron budgets in plastoquinone, cytochrome f, plastocyanin, photosystem I, and ferredoxin. A novel feature of the model is consideration of electron in- and outflow budgets resulting in changes in redox states of Tyrosine Z, P680, and Q_A as sole bases for changes in fluorescence yield during the transient. Ad hoc contributions by transmembrane electric fields, protein conformational changes, or other putative quenching species were unnecessary to account for primary features of the phenomenon, except a peculiar slowdown of intra-PSII electron transport during induction at low light intensities. The lower than F _m post-flash fluorescence yield F _f was related to oxidized tyrosine Z. The transient J peak was associated with equal rates of electron arrival to and departure from Q_A and requires that electron transfer from Q_A^- to Q_B be slower than that from Q_A^- to Q_B^-. Strong quenching by oxidized P680 caused the dip D. Reduced plastoquinone, a competitive product inhibitor of PSII, blocked electron transport proportionally with its concentration. Electron transport rate indicated by fluorescence quenching was faster than the rate indicated by O₂ evolution, because oxidized donor side carriers quench fluorescence but do not transport electrons. The thermal phase of the fluorescence rise beyond the J phase was caused by a progressive increase in the fraction of PSII with reduced Q_A and reduced donor side.

Rice intermediate filament, OsIF, stabilizes photosynthetic machinery and yield under salinity and heat stress

Cytoskeleton plays a vital role in stress tolerance; however, involvement of intermediate filaments (IFs) in such a response remains elusive in crop plants. This study provides clear evidence about the unique involvement of IFs in cellular protection against abiotic stress in rice. Transcript abundance of Oryza sativa intermediate filament (OsIF) encoding gene showed 2-10 fold up-regulation under different abiotic stress. Overexpression of OsIF in transgenic rice enhanced tolerance to salinity and heat stress, while its knock-down (KD) rendered plants more sensitive thereby indicating the role of IFs in promoting survival under stress. Seeds of OsIF overexpression rice germinated normally in the presence of high salt, showed better growth, maintained chloroplast ultrastructure and favourable K+/Na+ ratio than the wild type (WT) and KD plants. Analysis of photosynthesis and chlorophyll a fluorescence data suggested better performance of both photosystem I and II in the OsIF overexpression rice under salinity stress as compared to the WT and KD. Under salinity and high temperature stress, OsIF overexpressing plants could maintain significantly high yield, while the WT and KD plants could not. Further, metabolite profiling revealed a 2-4 fold higher accumulation of proline and trehalose in OsIF overexpressing rice than WT, under salinity stress.

Rate-limiting steps in the dark-to-light transition of Photosystem II - revealed by chlorophyll-a fluorescence induction

Photosystem II (PSII) catalyses the photoinduced oxygen evolution and, by producing reducing equivalents drives, in concert with PSI, the conversion of carbon dioxide to sugars. Our knowledge about the architecture of the reaction centre (RC) complex and the mechanisms of charge separation and stabilisation is well advanced. However, our understanding of the processes associated with the functioning of RC is incomplete: the photochemical activity of PSII is routinely monitored by chlorophyll-a fluorescence induction but the presently available data are not free of controversy. In this work, we examined the nature of gradual fluorescence rise of PSII elicited by trains of single-turnover saturating flashes (STSFs) in the presence of a PSII inhibitor, permitting only one stable charge separation. We show that a substantial part of the fluorescence rise originates from light-induced processes that occur after the stabilisation of charge separation, induced by the first STSF; the temperature-dependent relaxation characteristics suggest the involvement of conformational changes in the additional rise. In experiments using double flashes with variable waiting times (∆τ) between them, we found that no rise could be induced with zero or short ∆τ, the value of which depended on the temperature - revealing a previously unknown rate-limiting step in PSII.

Rate-limiting steps in the dark-to-light transition of Photosystem II - revealed by chlorophyll-a fluorescence induction

Photosystem II (PSII) catalyses the photoinduced oxygen evolution and, by producing reducing equivalents drives, in concert with PSI, the conversion of carbon dioxide to sugars. Our knowledge about the architecture of the reaction centre (RC) complex and the mechanisms of charge separation and stabilisation is well advanced. However, our understanding of the processes associated with the functioning of RC is incomplete: the photochemical activity of PSII is routinely monitored by chlorophyll-a fluorescence induction but the presently available data are not free of controversy. In this work, we examined the nature of gradual fluorescence rise of PSII elicited by trains of single-turnover saturating flashes (STSFs) in the presence of a PSII inhibitor, permitting only one stable charge separation. We show that a substantial part of the fluorescence rise originates from light-induced processes that occur after the stabilisation of charge separation, induced by the first STSF; the temperature-dependent relaxation characteristics suggest the involvement of conformational changes in the additional rise. In experiments using double flashes with variable waiting times (∆τ) between them, we found that no rise could be induced with zero or short ∆τ, the value of which depended on the temperature - revealing a previously unknown rate-limiting step in PSII.

Electron Transfer through the Acceptor Side of Photosystem I: Interaction with Exogenous Acceptors and Molecular Oxygen

This review considers the state-of-the-art on mechanisms and alternative pathways of electron transfer in photosynthetic electron transport chains of chloroplasts and cyanobacteria. The mechanisms of electron transport control between photosystems (PS) I and II and the Calvin-Benson cycle are considered. The redistribution of electron fluxes between the noncyclic, cyclic, and pseudocyclic pathways plays an important role in the regulation of photosynthesis. Mathematical modeling of light-induced electron transport processes is considered. Particular attention is given to the electron transfer reactions on the acceptor side of PS I and to interactions of PS I with exogenous acceptors, including molecular oxygen. A kinetic model of PS I and its interaction with exogenous electron acceptors has been developed. This model is based on experimental kinetics of charge recombination in isolated PS I. Kinetic and thermodynamic parameters of the electron transfer reactions in PS I are scrutinized. The free energies of electron transfer between quinone acceptors A_1A/A_1B in the symmetric redox cofactor branches of PS I and iron-sulfur clusters F_X, F_A, and F_B have been estimated. The second-order rate constants of electron transfer from PS I to external acceptors have been determined. The data suggest that byproduct formation of superoxide radical in PS I due to the reduction of molecular oxygen in the A₁ site (Mehler reaction) can exceed 0.3% of the total electron flux in PS I.

Redox changes of ferredoxin, P700, and plastocyanin measured simultaneously in intact leaves

Properties and performance of the recently introduced Dual/KLAS-NIR spectrophotometer for simultaneous measurements of ferredoxin (Fd), P700, and plastocyanin (PC) redox changes, together with whole leaf chlorophyll a (Chl) fluorescence (emission >760, 540 nm excitation) are outlined. Spectral information on in vivo Fd, P700, and PC in the near-infrared region (NIR, 780-1000 nm) is presented, on which the new approach is based. Examples of application focus on dark-light and light-dark transitions, where maximal redox changes of Fd occur. After dark-adaptation, Fd reduction induced by moderate light parallels the Kautsky effect of Chl fluorescence induction. Both signals are affected analogously by removal of O₂. A rapid type of Fd reoxidation, observed after a short pulse of light before light activation of linear electron transport (LET), is more pronounced in C4 compared to C3 leaves and interpreted to reflect cyclic PS I (CET). Light activation of LET, as assessed via the rate of Fd reoxidation after short light pulses, occurs at very low intensities and is slowly reversed (half-time ca. 20 min). Illumination with strong far-red light (FR, 740 nm) reveals two fractions of PS I, PS I (LET), and PS I (CET), differing in the rates of Fd reoxidation upon FR-off and the apparent equilibrium constants between P700 and PC. Parallel information on oxidation of Fd and reduction of P700 plus PC proves essential for identification of CET. Comparison of maize (C4) with sunflower and ivy (C3) responses leads to the conclusion that segregation of two types of PS I may not only exist in C4 (mesophyll and bundle sheath cells), but also in C3 photosynthesis (grana margins plus end membranes and stroma lamellae).

Opposite domination of cyclic and pseudocyclic electron flows in short-illuminated dark-adapted leaves of angiosperms and gymnosperms

The present work was aimed to explain the recently reported higher O₂-dependent electron flow capacity in gymnosperms than in angiosperms and to search for other differences in the electron transport processes by simultaneous characterization of the relative capacities of pseudocyclic (direct or Flavodiiron proteins (Flv)-mediated O₂-reduction, Mehler(-like) reactions) and cyclic electron flows around photosystem I (CEF-PSI). To this end, a comparative multicomponent analysis was performed on the fluorescence decay curves of dark-adapted leaves after illumination with a 1-s saturating light pulse. In both gymnosperms and angiosperms, two or three exponential decay components were resolved: fast (t _1/21 ~ 170-260 ms), middle (~1.0-2.3 s), and slow (>4.2 s). The sensitivity of the decay parameters (amplitudes A1-3, halftimes t _1/2 1-3) to the alternative electron flows was assessed using Arabidopsis pgr5 and ndhM mutants, defective in CEF-PSI, Synechocystis sp. PCC 6803 Δflv1 mutant, defective in Flv-mediated O₂-photoreduction, different O₂ concentrations, and methyl viologen treatment. A1 reflected the part of electrons involved in linear and O₂-photoreduction pathways after PSI. The middle component appeared in pgr5 (but not in ndhM), in gymnosperms under low O₂, and in Δflv1, and reflected limitations at the PSI acceptor side. The slow component was sensitive to CEF-PSI. The comparison of decay parameters provided evidence that Flv mediate O₂-photoreduction in gymnosperms, which explains their higher O₂-dependent electron flow capacity. The concomitant quantification of relative electrons branching in O₂-photoreduction and CEF-PSI pathways under the applied non-steady-state photosynthetic conditions reveals that CEF-PSI capacity significantly exceeds that of O₂-photoreduction in angiosperms while the opposite occurs in gymnosperms.

Relative functional and optical absorption cross-sections of PSII and other photosynthetic parameters monitored in situ, at a distance with a time resolution of a few seconds, using a prototype light induced fluorescence transient (LIFT) device

The prototype light-induced fluorescence transient (LIFT) instrument provides continuous, minimally intrusive, high time resolution (~2s) assessment of photosynthetic performance in terrestrial plants from up to 2m. It induces a chlorophyll fluorescence transient by a series of short flashes in a saturation sequence (180 ~1μs flashlets in <380μs) to achieve near-full reduction of the primary acceptor QA, followed by a relaxation sequence (RQA; 90 flashlets at exponentially increasing intervals over ~30ms) to observe kinetics of QA re-oxidation. When fitted by the fast repetition rate (FRR) model (Kolber et al. 1998) the QA flash of LIFT/FRR gives smaller values for FmQA from dark adapted leaves than FmPAM from pulse amplitude modulated (PAM) assays. The ratio FmQA/FmPAM resembles the ratio of fluorescence yield at the J/P phases of the classical O-J-I-P transient and we conclude that the difference simply is due to the levels of PQ pool reduction induced by the two techniques. In a strong PAM-analogous WL pulse in the dark monitored by the QA flash of LIFT/FRR φPSIIWL ≈ φPSIIPAM. The QA flash also tracks PQ pool reduction as well as the associated responses of ETR QA → PQ and PQ → PSI, the relative functional (σPSII) and optical absorption (aPSII) cross-sections of PSII in situ with a time resolution of ~2s as they relax after the pulse. It is impractical to deliver strong WL pulses at a distance in the field but a longer PQ flash from LIFT/FRR also achieves full reduction of PQ pool and delivers φPSIIPQ ≈ φPSIIPAM to obtain PAM-equivalent estimates of ETR and NPQ at a distance. In situ values of σPSII and aPSII from the QA flash with smaller antenna barley (chlorina-f2) and Arabidopsis mutants (asLhcb2-12, ch1-3 Lhcb5) are proportionally similar to those previously reported from in vitro assays. These direct measurements are further validated by changes in antenna size in response to growth irradiance. We illustrate how the QA flash facilitates our understanding of photosynthetic regulation during sun flecks in natural environments at a distance, with a time resolution of a few seconds.

Acclimation of shade-tolerant and light-resistant Tradescantia species to growth light: chlorophyll a fluorescence, electron transport, and xanthophyll content

In this study, we have compared the photosynthetic characteristics of two contrasting species of Tradescantia plants, T. fluminensis (shade-tolerant species), and T. sillamontana (light-resistant species), grown under the low light (LL, 50-125 µmol photons m^-2 s^-1) or high light (HL, 875-1000 µmol photons m^-2 s^-1) conditions during their entire growth period. For monitoring the functional state of photosynthetic apparatus (PSA), we measured chlorophyll (Chl) a emission fluorescence spectra and kinetics of light-induced changes in the heights of fluorescence peaks at 685 and 740 nm (F ₆₈₅ and F ₇₄₀). We also compared the light-induced oxidation of P₇₀₀ and assayed the composition of carotenoids in Tradescantia leaves grown under the LL and HL conditions. The analyses of slow induction of Chl a fluorescence (SIF) uncovered different traits in the LL- and HL-grown plants of ecologically contrasting Tradescantia species, which may have potential ecophysiological significance with respect to their tolerance to HL stress. The fluorometry and EPR studies of induction events in chloroplasts in situ demonstrated that acclimation of both Tradescantia species to HL conditions promoted faster responses of their PSA as compared to LL-grown plants. Acclimation of both species to HL also caused marked changes in the leaf anatomy and carotenoid composition (an increase in Violaxanthin + Antheraxantin + Zeaxanthin and Lutein pools), suggesting enhanced photoprotective capacity of the carotenoids in the plants grown in nature under high irradiance. Collectively, the results of the present work suggest that the mechanisms of long-term PSA photoprotection in Tradescantia are based predominantly on the light-induced remodeling of pigment-protein complexes in chloroplasts.

A four state parametric model for the kinetics of the non-photochemical quenching in Photosystem II

The phenomenon of non-photochemical quenching (NPQ) was studied in spinach chloroplasts using pulse amplitude modulated (PAM) fluorometry. We present a new analysis method which describes the observed fluorescence quantum yield as the sum of the product of four different states of PSII and their corresponding quantum yields. These four distinct states are PSII in the quenched or unquenched state, and with its reaction center either open or closed depending upon the reduction of the Q_A site. With this method we can describe the dynamics of the NPQ induction and recovery as well as quantify the percentage of photoinactivated RC throughout the measurement. We show that after one cycle of quenching followed by a period of recovery, approximately 8-9% of the RC are photoinactivated, after two cycles of illumination this number becomes 15-17%. The recovery from the quenching appeared with rates of (50s)^-1 and (1h)^-1. The new analysis method presented here is flexible, allowing it to be applied to any type of PAM fluorometry protocol. The method allows to quantitatively compare qualitatively different PAM curves on the basis of statistically relevant fitting parameters and to quantify quenching dynamics and photoinactivation. Moreover, the results presented here demonstrate that the analysis of a single PAM fluorometry quenching experiment can already provide information on the relative quantum yield of the four different states of PSII for the intact chloroplasts - something no other form of spectroscopy could provide in a single measurement.

Ascorbic Acid Alleviates Damage from Heat Stress in the Photosystem II of Tall Fescue in Both the Photochemical and Thermal Phases

L-Ascorbate (Asc) plays important roles in plant development, hormone signaling, the cell cycle and cellular redox system, etc. The higher content of Asc in plant chloroplasts indicates its important role in the photosystem. The objective of this study was to study the roles of Asc in tall fescue leaves against heat stress. After a heat stress treatment, we observed a lower value of the maximum quantum yield for primary photochemistry (φPo), which reflects the inhibited activity of the photochemical phase of photosystem II (PSII). Moreover, we observed a higher value of efficiency of electron transfer from QB to photosystem I acceptors (δR0), which reflects elevated activity of the thermal phase of the photosystem of the tall fescue. The addition of Asc facilitate the behavior of the photochemical phase of the PSII by lowering the ROS content as well as that of the alternative electron donor to provide electron to the tyrosine residue of the D1 protein. Additionally, exogenous Asc reduces the activity of the thermal phase of the photosystem, which could contribute to the limitation of energy input into the photosystem in tall fescue against heat stress. Synthesis of the Asc increased under heat stress treatment. However, under heat stress this regulation does not occur at the transcription level and requires further study.

Differential Response of Floating and Submerged Leaves of Longleaf Pondweed to Silver Ions

In this study, we have investigated variations in the potential of floating and submerged leaves of longleaf pondweed (Potamogeton nodosus) to withstand silver ion (Ag+)-toxicity. Both floating and submerged leaves changed clear colorless AgNO3 solutions to colloidal brown in the presence of light. Transmission electron microscopy revealed the presence of distinct crystalline Ag-nanoparticles (Ag-NPs) in these brown solutions. Powder X-ray diffraction pattern showed that Ag-NPs were composed of Ag0 and Ag2O. Photosystem (PS) II efficiency of leaves declined upon exposure to Ag+ with a significantly higher decline in the submerged leaves than in the floating leaves. Similarly, Ag+ treatment caused a significant reduction in the carboxylase activity of the ribulose bisphosphate carboxylase/oxygenase in leaves. The reduction in this carboxylase activity was significantly higher in the submerged than in the floating leaves. Ag+ treatment also resulted in a significant decline in the levels of non-enzymatic and enzymatic antioxidants; the decline was significantly lower in the floating than in submerged leaves. X-ray photoelectron spectroscopy revealed the presence of Ag2O in these leaves. Inductively coupled plasma mass spectrometry analysis revealed a three-fold higher Ag content in the submerged than in floating leaves. Our study demonstrates that floating leaves of longleaf pondweed have a superior potential to counter Ag+-toxicity compared with submerged leaves, which could be due to superior potential of floating leaves to reduce Ag+ to less/non-toxic Ag0/Ag2O-nanoparticles/nanocomplexes. We suggest that modulating the genotype of longleaf pondweed to bear higher proportion of floating leaves would help in cleaning fresh water bodies contaminated with ionic forms of heavy metals.

Frederick Yi-Tung Cho (1939-2011) : His PhD days in Biophysics, the Photosynthesis Lab, and his patents in engineering physics

We present here a Tribute to Frederick Yi-Tung Cho (1939-2011), an innovative and ingenious biophysicist and an entrepreneur. He was one of the 4 earliest PhD students [see: Cederstrand (1965)-Carl Nelson Cederstrand; coadvisor: Eugene Rabinowitch; Papageorgiou (1968)-George C. Papageorgiou (coauthor of this paper); and Munday (1968)-John C. Munday Jr. (also a coauthor of this paper)] of one of us (Govindjee) in Biophysics at the University of Illinois at Urbana-Champaign (UIUC) during the late 1960s (1963-1968). Fred was best known, in the photosynthesis circle for his pioneering work on low temperature (down to liquid helium temperature, 4 K) absorption and fluorescence spectroscopy of photosynthetic systems; he showed temperature independence of excitation energy transfer from (i) chlorophyll (Chl) b to Chl a and (ii) from Chl a 670 to Chl a 678; and temperature dependence of energy transfer from the phycobilins to Chl a and from Chl a 678 to its suggested trap. After doing research in biophysics of photosynthesis, Fred shifted to do research in solid-state physics/engineering in the Government Electronics Division (Group) of the Motorola Company, Scottsdale, Arizona, from where he published research papers in that area and had several patents granted. We focus mainly on his days at the UIUC in context of the laboratory in which he worked. We also list some of his papers and most of his patents in engineering physics. His friends and colleagues have correctly described him as an innovator and an ingenious scientist of the highest order. On the personal side, he was a very easy-going and amiable individual.

The plastoquinone pool of Nannochloropsis oceanica is not completely reduced during bright light pulses

The lipid-producing model alga Nannochloropsis oceanica has a distinct photosynthetic machinery. This organism possesses chlorophyll a as its only chlorophyll species, and has a high ratio of PSI to PSII. This high ratio of PSI to PSII may affect the redox state of the plastoquinone pool during exposure to light, and consequently may play a role in activating photoprotection mechanisms. We utilized pulse-amplitude modulated fluorometry to investigate the redox state of the plastoquinone pool during and after bright light pulses. Our data indicate that even very intense (5910 μmol photons s-1m-2 of blue light having a wavelength of 440 nm) light pulses of 0.8 second duration are not sufficient to completely reduce the plastoquinone pool in Nannochloropsis. In order to achieve extensive reduction of the plastoquinone pool by bright light pulses, anaerobic conditions or an inhibitor of the photosynthetic electron transport chain has to be utilized. The implication of this finding for the application of the widely used saturating pulse method in algae is discussed.

Frequently asked questions about chlorophyll fluorescence, the sequel

Using chlorophyll (Chl) a fluorescence many aspects of the photosynthetic apparatus can be studied, both in vitro and, noninvasively, in vivo. Complementary techniques can help to interpret changes in the Chl a fluorescence kinetics. Kalaji et al. (Photosynth Res 122:121-158, 2014a) addressed several questions about instruments, methods and applications based on Chl a fluorescence. Here, additional Chl a fluorescence-related topics are discussed again in a question and answer format. Examples are the effect of connectivity on photochemical quenching, the correction of F V /F M values for PSI fluorescence, the energy partitioning concept, the interpretation of the complementary area, probing the donor side of PSII, the assignment of bands of 77 K fluorescence emission spectra to fluorescence emitters, the relationship between prompt and delayed fluorescence, potential problems when sampling tree canopies, the use of fluorescence parameters in QTL studies, the use of Chl a fluorescence in biosensor applications and the application of neural network approaches for the analysis of fluorescence measurements. The answers draw on knowledge from different Chl a fluorescence analysis domains, yielding in several cases new insights.

The Acceptor Side of Photosystem II Is the Initial Target of Nitrite Stress in Synechocystis sp. Strain PCC 6803

Nitrite, a common form of inorganic nitrogen (N), can be used as a nitrogen source through N assimilation. However, high levels of nitrite depress photosynthesis in various organisms. In this study, we investigated which components of the photosynthetic electron transfer chain are targeted by nitrite stress in Synechocystis sp. strain PCC 6803 cells. Measurements of whole-chain and photosystem II (PSII)-mediated electron transport activities revealed that high levels of nitrite primarily impair electron flow in PSII. Changes in PSII activity in response to nitrite stress occurred in two distinct phases. During the first phase, which occurred in the first 3 h of nitrite treatment, electron transfer from the primary quinone acceptor (Q_A) to the secondary quinone acceptor (Q_B) was retarded, as indicated by chlorophyll (Chl) a fluorescence induction, S-state distribution, and Q_A^- reoxidation tests. In the second phase, which occurred after 6 h of nitrite exposure, the reaction center was inactivated and the donor side of photosystem II was inhibited, as revealed by changes in Chl fluorescence parameters and thermoluminescence and by immunoblot analysis. Our data suggest that nitrite stress is highly damaging to PSII and disrupts PSII activity by a stepwise mechanism in which the acceptor side is the initial target. IMPORTANCE In our previous studies, an alga-based technology was proposed to fix the large amounts of nitrite that are released from NO_X-rich flue gases and proved to be a promising industrial strategy for flue gas NO_X bioremediation (W. Chen et al., Environ Sci Technol 50:1620-1627, 2016, https://doi.org/10.1021/acs.est.5b04696; X. Zhang et al., Environ Sci Technol 48:10497-10504, 2014, https://doi.org/10.1021/es5013824). However, the toxic effects of high concentrations of nitrite on algal cells remain obscure. The analysis of growth rates, photochemistry, and protein profiles in our study provides important evidence that the inhibition by nitrite occurs in two phases: in the first phase, electron transfer between Q_A^- and Q_B is retarded, whereas in the second, the donor side of PSII is affected. This is an excellent example of investigating the "early" inhibitory effects (i.e., within the first 6 h) on the PSII electron transfer chain in vivo This paper provides novel insights into the mechanisms of nitrite inhibition of photosynthesis in an oxygenic phototrophic cyanobacterium.

Culture density influence on the photosynthetic efficiency of microalgae growing under different spectral compositions of light

A density in algal suspension causes a significant change in the intensity and spectral composition of light reaching individual cells. Measurements of chlorophyll fluorescence allow us to observe any general changes in the bioenergetic status of photosynthesis. The aim of the study was to determine the effect of cultivation density on the PSII photochemical efficiency of three species of algae (Chlorella vulgaris, Botryococcus braunii and Chlorella emersonii), each with a different rate of growth - high, medium and low - respectively. The cell density of algae in suspension differentiated through the cultivation time (2, 4, and 8days) and the spectral composition of light. The results showed that the density of cultivation led to change in the photosynthetic apparatus of algae. The differences described between each day of cultivation (2, 4, and 8) in the kinetics of chlorophyll a fluorescence intensity in cells of the algal strains under study probably resulted from the different phases of growth of these cultures. In addition the results showed the beneficial effect of far red light on the photosynthetic apparatus and the growth of biomass in investigated algal strains.