Light-induced changes in the fluorescence yield of chlorophyll a in vivo. II. Chlorella pyrenoidosa

Comparative modeling of fluorescence and P700 induction kinetics for alga Scenedesmus sp. obliques and cyanobacterium Synechocystis sp. PCC 6803. Role of state 2-state 1 transitions and redox state of plastoquinone pool

The model of thylakoid membrane system (T-M model) (Belyaeva et al. Photosynth Res 2019, 140:1-19) has been improved in order to analyze the induction data for dark-adapted samples of algal (Scenedesmus obliques) and cyanobacterial (Synechocystis sp. PCC 6803) cells. The fluorescence induction (FI) curves of Scenedesmus were measured at light exposures of 5 min, while FI and P700 redox transformations of Synechocystis were recorded in parallel for 100 s intervals. Kinetic data comprising the OJIP-SMT fluorescence induction and OABCDEF P700+ absorbance changes were used to study the processes underlying state transitions qT2→1 and qT1→2 associated with the increase/decrease in Chl fluorescence emission. A formula with the Hill kinetics (Ebenhöh et al. Philos Trans R Soc B 2014, 369:20130223) was introduced into the T-M model, with a new variable to imitate the flexible size of antenna AntM(t) associated with PSII. Simulations revealed that the light-harvesting capacity of PSII increases with a corresponding decrease for that of PSI upon the qT2→1 transition induced by plastoquinone (PQ) pool oxidation. The complete T-M model fittings were attained on Scenedesmus or Synechocystis fast waves OJIPS of FI, while SMT wave of FI was reproduced at intervals shorter than 5 min. Also the fast P700 redox transitions (OABC) for Synechocystis were fitted exactly. Reasonable sets of algal and cyanobacterial electron/proton transfer (ET/PT) parameters were found. In the case of Scenedesmus, ET/PT traits remained the same irrespective of modeling with or without qT2→1 transitions. Simulations indicated a high extent (20%) of the PQ pool reduction under dark conditions in Synechocystis compared to 2% in Scenedesmus.

Model quantification of the light-induced thylakoid membrane processes in Synechocystis sp. PCC 6803 in vivo and after exposure to radioactive irradiation

Measurements of OJIP-SMT patterns of fluorescence induction (FI) in Synechocystis sp. PCC 6803 (Synechocystis) cells on a time scale up to several minutes were mathematically treated within the framework of thylakoid membrane (T-M) model (Belyaeva et al., Photosynth Res 140:1-19, 2019) that was renewed to account for the state transitions effects. Principles of describing electron transfer in reaction centers of photosystems II and I (PSII and PSI) and cytochrome b₆f complex remained unchanged, whereas parameters for dissipative reactions of non-radiative charge recombination were altered depending on the oxidation state of Q_B-site (neutral, reduced by one electron, empty, reduced by two electrons). According to our calculations, the initial content of plastoquinol (PQH₂) in the total quinone pool of Synechocystis cells adapted to darkness for 10 min ranged between 20 and 40%. The results imply that the PQ pool mediates photosynthetic and respiratory charge flows. The redistribution of PBS antenna units responsible for the increase of Chl fluorescence in cyanobacteria (qT_{2 → 1}) upon state 2 → 1 transition or the fluorescence lowering (qT_{1 → 2}) due to state 1 → 2 transition were described in the model by exponential functions. Parameters of dynamically changed effective cross section were found by means of simulations of OJIP-SMT patterns observed on Synechocystis cells upon strong (3000 μmol photons m^-2s^-1) and moderate (1000 μmol photons m^-2s^-1) actinic light intensities. The corresponding light constant values kL_ΣAnt = 1.2 ms^-1 and 0.4 ms^-1 define the excitation of total antenna pool dynamically redistributed between PSII and PSI reaction centers. Although the OCP-induced quenching of antenna excitation is not involved in the model, the main features of the induction signals have been satisfactorily explained. In the case of strong illumination, the effective cross section decreases by approximately 33% for irradiated Synechocystis cells as compared to untreated cells. Under moderate light, the irradiated Synechocystis cells showed in simulations the same cross section as the untreated cells. The thylakoid model renewed with state transitions description allowed simulation of fluorescence induction OJIP-SMT curves detected on time scale from microseconds to minutes.

Photosynthesis: basics, history and modelling

BACKGROUND

With limited agricultural land and increasing human population, it is essential to enhance overall photosynthesis and thus productivity. Oxygenic photosynthesis begins with light absorption, followed by excitation energy transfer to the reaction centres, primary photochemistry, electron and proton transport, NADPH and ATP synthesis, and then CO2 fixation (Calvin-Benson cycle, as well as Hatch-Slack cycle). Here we cover some of the discoveries related to this process, such as the existence of two light reactions and two photosystems connected by an electron transport 'chain' (the Z-scheme), chemiosmotic hypothesis for ATP synthesis, water oxidation clock for oxygen evolution, steps for carbon fixation, and finally the diverse mechanisms of regulatory processes, such as 'state transitions' and 'non-photochemical quenching' of the excited state of chlorophyll a.

SCOPE

In this review, we emphasize that mathematical modelling is a highly valuable tool in understanding and making predictions regarding photosynthesis. Different mathematical models have been used to examine current theories on diverse photosynthetic processes; these have been validated through simulation(s) of available experimental data, such as chlorophyll a fluorescence induction, measured with fluorometers using continuous (or modulated) exciting light, and absorbance changes at 820 nm (ΔA820) related to redox changes in P700, the reaction centre of photosystem I.

CONCLUSIONS

We highlight here the important role of modelling in deciphering and untangling complex photosynthesis processes taking place simultaneously, as well as in predicting possible ways to obtain higher biomass and productivity in plants, algae and cyanobacteria.

Eugene I. Rabinowitch: A prophet of photosynthesis and of peace in the world

More than 45 years have passed since Eugene I. Rabinowitch died, on May 15, 1973, at the age of 75, but many still remember him as a photosynthesis giant, the author of a 2000-page "Bible" on photosynthesis, a great chemist and physicist, a discoverer of several basic photoreactions, one of the founders of modern biophysics, a peacemaker, a poet, an architect, an artist, a wonderful human being, and above all a great mentor. Sir John Rotblatt cited Eugene Rabinowitch, together with Bertrand Russell, for their key contributions that led to the Nobel Peace Prize awarded in 1995 jointly to Rotblatt and the Pugwash Conferences on Science and World Affairs "for their efforts to diminish the part played by nuclear arms in international politics and, in the longer run, to eliminate such arms." Already in 1965, Eugene Rabinowitch had received the prestigious Kalinga Prize from UNESCO "in recognition of his work to encourage international cooperation among scientists and to bring to light the potential dangers of science to the public."

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.

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.

Links between peptides and Mn oxide: nano-sized manganese oxide embedded in a peptide matrix

We report on a poly-peptide/Mn oxide nanocomposite as a model for the water-oxidizing catalyst in Photosystem II.

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.

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.

Role of Ions in the Regulation of Light-Harvesting

Regulation of photosynthetic light harvesting in the thylakoids is one of the major key factors affecting the efficiency of photosynthesis. Thylakoid membrane is negatively charged and influences both the structure and the function of the primarily photosynthetic reactions through its electrical double layer (EDL). Further, there is a heterogeneous organization of soluble ions (K+, Mg2+, Cl-) attached to the thylakoid membrane that, together with fixed charges (negatively charged amino acids, lipids), provides an electrical field. The EDL is affected by the valence of the ions and interferes with the regulation of "state transitions," protein interactions, and excitation energy "spillover" from Photosystem II to Photosystem I. These effects are reflected in changes in the intensity of chlorophyll a fluorescence, which is also a measure of photoprotective non-photochemical quenching (NPQ) of the excited state of chlorophyll a. A triggering of NPQ proceeds via lumen acidification that is coupled to the export of positive counter-ions (Mg2+, K+) to the stroma or/and negative ions (e.g., Cl-) into the lumen. The effect of protons and anions in the lumen and of the cations (Mg2+, K+) in the stroma are, thus, functionally tightly interconnected. In this review, we discuss the consequences of the model of EDL, proposed by Barber (1980b) Biochim Biophys Acta 594:253-308) in light of light-harvesting regulation. Further, we explain differences between electrostatic screening and neutralization, and we emphasize the opposite effect of monovalent (K+) and divalent (Mg2+) ions on light-harvesting and on "screening" of the negative charges on the thylakoid membrane; this effect needs to be incorporated in all future models of photosynthetic regulation by ion channels and transporters.

International conference on "Photosynthesis research for sustainability-2015" in honor of George C. Papageorgiou", September 21-26, 2015, Crete, Greece

During September 21-26, 2015, an international conference entitled ''Photosynthesis Research for Sustainability-2015'' was held in honor of George C. Papageorgiou at the Conference Center of the Orthodox Academy of Crete, an exceptionally beautiful location right on the Mediterranean Sea coast, Kolymvari, Chania, Crete, (Greece) (see http://photosynthesis2015.cellreg.org/ ). The meeting was held under the auspices of the Greek "General Secretariat for Research and Technology" (GSRT). We first provide a brief introduction and key contributions of George C. Papageorgiou, the honored scientist, and then information on the conference, on the speakers, and the program. A special feature of this conference was awards given to 13 young investigators, who are recognized in this Report. Several photographs are also included; they show the pleasant ambience at this conference. We invite the readers to the next conference on "Photosynthesis Research for Sustainability-2016," which will honor Nathan Nelson and T. Nejat Veziroglu; it will be held during June 19-25, 2016, in Pushchino, Moscow Region, Russia (see http://photosynthesis2016.cellreg.org/ ).

The slow phase of chlorophyll a fluorescence induction in silico: Origin of the S-M fluorescence rise

In higher plants, algae, and cyanobacteria, chlorophyll (Chl) a fluorescence induction (ChlFI) has a fast (under a second) increasing OJIP phase and a slow (few minutes) PS(M)T phase, where O is for origin, the minimum fluorescence, J and I for intermediate levels, P for peak, S for a semi-steady state, M for a maximum (which is sometimes missing), and T for the terminal steady-state level. We have used a photosynthesis model of Ebenhöh et al. (Philos Trans R Soc B, 2014, doi: 10.1098/rstb.2013.0223 ) in an attempt to simulate the slow PS(M)T phase and to determine the origin of the S-M rise in Chlamydomonas (C.) reinhardtii cells. Our experiments in silico show that a slow fluorescence S-M rise (as that observed, e.g., by Kodru et al. (Photosynth Res 125:219-231, 2015) can be simulated only if the photosynthetic samples are initially in a so-called "state 2," when the absorption cross section (CS) of Photosystem II (PSII) is lower than that of PSI, and Chl a fluorescence is low (see, e.g., a review by Papageorgiou and Govindjee (J Photochem Photobiol B 104:258-270, 2011). In this case, simulations show that illumination induces a state 2 (s2) to state 1 (s1) transition (qT₂₁), and a slow S-M rise in the simulated ChlFI curve, since the fluorescence yield is known to be higher in s1, when CS of PSII is larger than that of PSI. Additionally, we have analyzed how light intensity and several photosynthetic processes influence the degree of this qT₂₁, and thus the relative amplitude of the simulated S-M phase. A refinement of the photosynthesis model is, however, necessary in order to obtain a better fit of the simulation data with the measured ChlFI curves.

The slow S to M rise of chlorophyll a fluorescence reflects transition from state 2 to state 1 in the green alga Chlamydomonas reinhardtii

The green alga Chlamydomonas (C.) reinhardtii is a model organism for photosynthesis research. State transitions regulate redistribution of excitation energy between photosystem I (PS I) and photosystem II (PS II) to provide balanced photosynthesis. Chlorophyll (Chl) a fluorescence induction (the so-called OJIPSMT transient) is a signature of several photosynthetic reactions. Here, we show that the slow (seconds to minutes) S to M fluorescence rise is reduced or absent in the stt7 mutant (which is locked in state 1) in C. reinhardtii. This suggests that the SM rise in wild type C. reinhardtii may be due to state 2 (low fluorescence state; larger antenna in PS I) to state 1 (high fluorescence state; larger antenna in PS II) transition, and thus, it can be used as an efficient and quick method to monitor state transitions in algae, as has already been shown in cyanobacteria (Papageorgiou et al. 1999, 2007; Kaňa et al. 2012). We also discuss our results on the effects of (1) 3-(3,4-dichlorophenyl)-1,4-dimethyl urea, an inhibitor of electron transport; (2) n-propyl gallate, an inhibitor of alternative oxidase (AOX) in mitochondria and of plastid terminal oxidase in chloroplasts; (3) salicylhydroxamic acid, an inhibitor of AOX in mitochondria; and (4) carbonyl cyanide p-trifluoromethoxyphenylhydrazone, an uncoupler of phosphorylation, which dissipates proton gradient across membranes. Based on the data presented in this paper, we conclude that the slow PSMT fluorescence transient in C. reinhardtii is due to the superimposition of, at least, two phenomena: qE dependent non-photochemical quenching of the excited state of Chl, and state transitions.

Parameters of photosynthetic energy partitioning

Almost every laboratory dealing with plant physiology, photosynthesis research, remote sensing, and plant phenotyping possesses a fluorometer to measure a kind of chlorophyll (Chl) fluorescence induction (FLI). When the slow Chl FLI is measured with addition of saturating pulses and far-red illumination, the so-called quenching analysis followed by the so-called relaxation analysis in darkness can be realized. These measurements then serve for evaluation of the so-called energy partitioning, that is, calculation of quantum yields of photochemical and of different types of non-photochemical processes. Several theories have been suggested for photosynthetic energy partitioning. The current work aims to summarize all the existing theories, namely their equations for the quantum yields, their meaning and their assumptions. In the framework of these theories it is also found here that the well-known NPQ parameter ( [Formula: see text] ; Bilger and Björkman, 1990) equals the ratio of the quantum yield of regulatory light-induced non-photochemical quenching to the quantum yield of constitutive non-regulatory non-photochemical quenching (ΦNPQ/Φf,D). A similar relationship is also found here for the PQ parameter (ΦP/Φf,D).

Modeling chlorophyll a fluorescence transient: relation to photosynthesis

To honor Academician Alexander Abramovitch Krasnovsky, we present here an educational review on the relation of chlorophyll a fluorescence transient to various processes in photosynthesis. The initial event in oxygenic photosynthesis is light absorption by chlorophylls (Chls), carotenoids, and, in some cases, phycobilins; these pigments form the antenna. Most of the energy is transferred to reaction centers where it is used for charge separation. The small part of energy that is not used in photochemistry is dissipated as heat or re-emitted as fluorescence. When a photosynthetic sample is transferred from dark to light, Chl a fluorescence (ChlF) intensity shows characteristic changes in time called fluorescence transient, the OJIPSMT transient, where O (the origin) is for the first measured minimum fluorescence level; J and I for intermediate inflections; P for peak; S for semi-steady state level; M for maximum; and T for terminal steady state level. This transient is a real signature of photosynthesis, since diverse events can be related to it, such as: changes in redox states of components of the linear electron transport flow, involvement of alternative electron routes, the build-up of a transmembrane pH gradient and membrane potential, activation of different nonphotochemical quenching processes, activation of the Calvin-Benson cycle, and other processes. In this review, we present our views on how different segments of the OJIPSMT transient are influenced by various photosynthetic processes, and discuss a number of studies involving mathematical modeling and simulation of the ChlF transient. A special emphasis is given to the slower PSMT phase, for which many studies have been recently published, but they are less known than on the faster OJIP phase.

Govindjee at 80: more than 50 years of free energy for photosynthesis

We provide here a glimpse of Govindjee and his pioneering contributions on the two light reactions and the two pigment systems, particularly on the water-plastoquinone oxido-reductase, Photosystem II. His focus has been on excitation energy transfer; primary photochemistry, and the role of bicarbonate in electron and proton transfer. His major tools have been kinetics and spectroscopy (absorption and fluorescence), and he has provided an understanding of both thermoluminescence and delayed light emission in plants and algae. He pioneered the use of lifetime of fluorescence measurements to study the phenomenon of photoprotection in plants and algae. He, however, is both a generalist and a specialist all at the same time. He communicates very effectively his passion for photosynthesis to the novice as well as professionals. He has been a prolific author, outstanding lecturer and an editor par excellence. He is the founder not only of the Historical Corner of Photosynthesis Research, but of the highly valued Series Advances in Photosynthesis and Respiration Including Bioenergy and Related Processes. He reaches out to young people by distributing Z-scheme posters, presenting Awards of books, and through tri-annual articles on "Photosynthesis Web Resources". At home, at the University of Illinois at Urbana-Champaign, he has established student Awards for Excellence in Biological Sciences. On behalf of all his former graduate students and associates, I wish him a Happy 80th birthday. I have included here several tributes to Govindjee by his well-wishers. These write-ups express the high regard the photosynthesis community holds for "Gov" and illuminate the different facets of his life and associations.

Excitonic connectivity between photosystem II units: what is it, and how to measure it?

In photosynthetic organisms, light energy is absorbed by a complex network of chromophores embedded in light-harvesting antenna complexes. In photosystem II (PSII), the excitation energy from the antenna is transferred very efficiently to an active reaction center (RC) (i.e., with oxidized primary quinone acceptor Q(A)), where the photochemistry begins, leading to O2 evolution, and reduction of plastoquinones. A very small part of the excitation energy is dissipated as fluorescence and heat. Measurements on chlorophyll (Chl) fluorescence and oxygen have shown that a nonlinear (hyperbolic) relationship exists between the fluorescence yield (Φ(F)) (or the oxygen emission yield, (Φ(O2)) and the fraction of closed PSII RCs (i.e., with reduced Q(A)). This nonlinearity is assumed to be related to the transfer of the excitation energy from a closed PSII RC to an open (active) PSII RC, a process called PSII excitonic connectivity by Joliot and Joliot (CR Acad Sci Paris 258: 4622-4625, 1964). Different theoretical approaches of the PSII excitonic connectivity, and experimental methods used to measure it, are discussed in this review. In addition, we present alternative explanations of the observed sigmoidicity of the fluorescence induction and oxygen evolution curves.

Excitonic connectivity between photosystem II units: what is it, and how to measure it?

In photosynthetic organisms, light energy is absorbed by a complex network of chromophores embedded in light-harvesting antenna complexes. In photosystem II (PSII), the excitation energy from the antenna is transferred very efficiently to an active reaction center (RC) (i.e., with oxidized primary quinone acceptor Q(A)), where the photochemistry begins, leading to O2 evolution, and reduction of plastoquinones. A very small part of the excitation energy is dissipated as fluorescence and heat. Measurements on chlorophyll (Chl) fluorescence and oxygen have shown that a nonlinear (hyperbolic) relationship exists between the fluorescence yield (Φ(F)) (or the oxygen emission yield, (Φ(O2)) and the fraction of closed PSII RCs (i.e., with reduced Q(A)). This nonlinearity is assumed to be related to the transfer of the excitation energy from a closed PSII RC to an open (active) PSII RC, a process called PSII excitonic connectivity by Joliot and Joliot (CR Acad Sci Paris 258: 4622-4625, 1964). Different theoretical approaches of the PSII excitonic connectivity, and experimental methods used to measure it, are discussed in this review. In addition, we present alternative explanations of the observed sigmoidicity of the fluorescence induction and oxygen evolution curves.

Experimental in vivo measurements of light emission in plants: a perspective dedicated to David Walker

This review is dedicated to David Walker (1928-2012), a pioneer in the field of photosynthesis and chlorophyll fluorescence. We begin this review by presenting the history of light emission studies, from the ancient times. Light emission from plants is of several kinds: prompt fluorescence (PF), delayed fluorescence (DF), thermoluminescence, and phosphorescence. In this article, we focus on PF and DF. Chlorophyll a fluorescence measurements have been used for more than 80 years to study photosynthesis, particularly photosystem II (PSII) since 1961. This technique has become a regular trusted probe in agricultural and biological research. Many measured and calculated parameters are good biomarkers or indicators of plant tolerance to different abiotic and biotic stressors. This would never have been possible without the rapid development of new fluorometers. To date, most of these instruments are based mainly on two different operational principles for measuring variable chlorophyll a fluorescence: (1) a PF signal produced following a pulse-amplitude-modulated excitation and (2) a PF signal emitted during a strong continuous actinic excitation. In addition to fluorometers, other instruments have been developed to measure additional signals, such as DF, originating from PSII, and light-induced absorbance changes due to the photooxidation of P700, from PSI, measured as the absorption decrease (photobleaching) at about 705 nm, or increase at 820 nm. In this review, the technical and theoretical basis of newly developed instruments, allowing for simultaneous measurement of the PF and the DF as well as other parameters is discussed. Special emphasis has been given to a description of comparative measurements on PF and DF. However, DF has been discussed in greater details, since it is much less used and less known than PF, but has a great potential to provide useful qualitative new information on the back reactions of PSII electron transfer. A review concerning the history of fluorometers is also presented.

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

The fast (up to 1 s) chlorophyll (Chl) a fluorescence induction (FI) curve, measured under saturating continuous light, has a photochemical phase, the O-J rise, related mainly to the reduction of Q(A), the primary electron acceptor plastoquinone of Photosystem II (PSII); here, the fluorescence rise depends strongly on the number of photons absorbed. This is followed by a thermal phase, the J-I-P rise, which disappears at subfreezing temperatures. According to the mainstream interpretation of the fast FI, the variable fluorescence originates from PSII antenna, and the oxidized Q(A) is the most important quencher influencing the O-J-I-P curve. As the reaction centers of PSII are gradually closed by the photochemical reduction of Q(A), Chl fluorescence, F, rises from the O level (the minimal level) to the P level (the peak); yet, the relationship between F and [Q(A) (-)] is not linear, due to the presence of other quenchers and modifiers. Several alternative theories have been proposed, which give different interpretations of the O-J-I-P transient. The main idea in these alternative theories is that in saturating light, Q(A) is almost completely reduced already at the end of the photochemical phase O-J, but the fluorescence yield is lower than its maximum value due to the presence of either a second quencher besides Q(A), or there is an another process quenching the fluorescence; in the second quencher hypothesis, this quencher is consumed (or the process of quenching the fluorescence is reversed) during the thermal phase J-I-P. In this review, we discuss these theories. Based on our critical examination, that includes pros and cons of each theory, as well mathematical modeling, we conclude that the mainstream interpretation of the O-J-I-P transient is the most credible one, as none of the alternative ideas provide adequate explanation or experimental proof for the almost complete reduction of Q(A) at the end of the O-J phase, and for the origin of the fluorescence rise during the thermal phase. However, we suggest that some of the factors influencing the fluorescence yield that have been proposed in these newer theories, as e.g., the membrane potential ΔΨ, as suggested by Vredenberg and his associates, can potentially contribute to modulate the O-J-I-P transient in parallel with the reduction of Q(A), through changes at the PSII antenna and/or at the reaction center, or, possibly, through the control of the oxidation-reduction of the PQ-pool, including proton transfer into the lumen, as suggested by Rubin and his associates. We present in this review our personal perspective mainly on our understanding of the thermal phase, the J-I-P rise during Chl a FI in plants and algae.

Photosystem II fluorescence: slow changes--scaling from the past

With the advent of photoelectric devices (photocells, photomultipliers) in the 1930s, fluorometry of chlorophyll (Chl) a in vivo emerged as a major method in the science of photosynthesis. Early researchers employed fluorometry primarily for two tasks: to elucidate the role in photosynthesis, if any, of other plant pigments, such as Chl b, Chl c, carotenoids and phycobilins; and to use it as a convenient inverse measure of photosynthetic activity. In pursuing the latter task, it became apparent that Chl a fluorescence emission is influenced (i) by redox active Chl a molecules in the reaction center of photosystem (PS) II (photochemical quenching); (ii) by an electrochemical imbalance across the thylakoid membrane (high energy quenching); and (iii) by the size of the peripheral antennae of weakly fluorescent PSI and strongly fluorescent PSII in response to changes in the ambient light (state transitions). In this perspective we trace the historical evolution of our awareness of these concepts, particularly of the so-called 'State Transitions'.

On the relation between the Kautsky effect (chlorophyll a fluorescence induction) and Photosystem II: basics and applications of the OJIP fluorescence transient

Chlorophyll a fluorescence is a highly sensitive, non-destructive, and reliable tool for measuring, rather quickly, photosynthetic efficiency, particularly of Photosystem II (PSII), the water-plastoquinone oxidoreductase. We briefly review here the connection between the fast (up to 2 s) chlorophyll fluorescence rise and PSII, as well as the empirical use of the fluorescence rise kinetics in understanding photosynthetic reactions, particularly of PSII. When dark-adapted photosynthetic samples are exposed to light, a fluorescence induction is observed, known as the Kautsky effect, after Hans Kautsky, the discoverer of the phenomenon showing the existence of variable fluorescence. The chlorophyll fluorescence intensity rises from a minimum level (the O level), in less than 1 s, to a maximum level (the P-level) via two intermediate steps labeled J and I. This is followed by a decline to a lower semi-steady state level, the S level, which is reached in about one minute. We provide here an educational review on how this phenomenon has been exploited through analysis of the fast OJIP fluorescence transient, by discussing basic assumptions, derivation of equations, as well as application to PSII-related questions.

The fast and slow kinetics of chlorophyll a fluorescence induction in plants, algae and cyanobacteria: a viewpoint

The light-induced/dark-reversible changes in the chlorophyll (Chl) a fluorescence of photosynthetic cells and membranes in the mus-to-several min time window (fluorescence induction, FI; or Kautsky transient) reflect quantum yield changes (quenching/de-quenching) as well as changes in the number of Chls a in photosystem II (PS II; state transitions). Both relate to excitation trapping in PS II and the ensuing photosynthetic electron transport (PSET), and to secondary PSET effects, such as ion translocation across thylakoid membranes and filling or depletion of post-PS II and post-PS I pools of metabolites. In addition, high actinic light doses may depress Chl a fluorescence irreversibly (photoinhibitory lowering; q(I)). FI has been studied quite extensively in plants an algae (less so in cyanobacteria) as it affords a low resolution panoramic view of the photosynthesis process. Total FI comprises two transients, a fast initial (OPS; for Origin, Peak, Steady state) and a second slower transient (SMT; for Steady state, Maximum, Terminal state), whose details are characteristically different in eukaryotic (plants and algae) and prokaryotic (cyanobacteria) oxygenic photosynthetic organisms. In the former, maximal fluorescence output occurs at peak P, with peak M lying much lower or being absent, in which case the PSMT phases are replaced by a monotonous PT fluorescence decay. In contrast, in phycobilisome (PBS)-containing cyanobacteria maximal fluorescence occurs at M which lies much higher than peak P. It will be argued that this difference is caused by a fluorescence lowering trend (state 1 --> 2 transition) that dominates the FI pattern of plants and algae, and correspondingly by a fluorescence increasing trend (state 2 --> 1 transition) that dominates the FI of PBS-containing cyanobacteria. Characteristically, however, the FI pattern of the PBS-minus cyanobacterium Acaryochloris marina resembles the FI patterns of algae and plants and not of the PBS-containing cyanobacteria.

Fluorescence induction in the phycobilisome-containing cyanobacterium Synechococcus sp PCC 7942: Analysis of the slow fluorescence transient

At room temperature, the chlorophyll (Chl) a fluorescence induction (FI) kinetics of plants, algae and cyanobacteria go through two maxima, P at approximately 0.2-1 and M at approximately 100-500 s, with a minimum S at approximately 2-10 s in between. Thus, the whole FI kinetic pattern comprises a fast OPS transient (with O denoting origin) and a slower SMT transient (with T denoting terminal state). Here, we examined the phenomenology and the etiology of the SMT transient of the phycobilisome (PBS)-containing cyanobacterium Synechococcus sp PCC 7942 by modifying PBS-->Photosystem (PS) II excitation transfer indirectly, either by blocking or by maximizing the PBS-->PS I excitation transfer. Blocking the PBS-->PS I excitation transfer route with N-ethyl-maleimide [NEM; A. N. Glazer, Y. Gindt, C. F. Chan, and K.Sauer, Photosynth. Research 40 (1994) 167-173] increases both the PBS excitation share of PS II and Chl a fluorescence. Maximizing it, on the other hand, by suspending cyanobacterial cells in hyper-osmotic media [G. C. Papageorgiou, A. Alygizaki-Zorba, Biochim. Biophys. Acta 1335 (1997) 1-4] diminishes both the PBS excitation share of PS II and Chl a fluorescence. Here, we show for the first time that, in either case, the slow SMT transient of FI disappears and is replaced by continuous P-->T fluorescence decay, reminiscent of the typical P-->T fluorescence decay of higher plants and algae. A similar P-->T decay was also displayed by DCMU-treated Synechococcus cells at 2 degrees C. To interpret this phenomenology, we assume that after dark adaptation cyanobacteria exist in a low fluorescence state (state 2) and transit to a high fluorescence state (state 1) when, upon light acclimation, PS I is forced to run faster than PS II. In these organisms, a state 2-->1 fluorescence increase plus electron transport-dependent dequenching processes dominate the SM rise and maximal fluorescence output is at M which lies above the P maximum of the fast FI transient. In contrast, dark-adapted plants and algae exist in state 1 and upon illumination they display an extended P-->T decay that sometimes is interrupted by a shallow SMT transient, with M below P. This decay is dominated by a state 1-->2 fluorescence lowering, as well as by electron transport-dependent quenching processes. When the regulation of the PBS-->PS I electronic excitation transfer is eliminated (as for example in hyper-osmotic suspensions, after NEM treatment and at low temperature), the FI pattern of Synechococcus becomes plant-like.

Fluorescence lifetime imaging microscopy of Chlamydomonas reinhardtii: non-photochemical quenching mutants and the effect of photosynthetic inhibitors on the slow chlorophyll fluorescence transient

Fluorescence lifetime-resolved images of chlorophyll fluorescence were acquired at the maximum P-level and during the slower transient (up to 250 s, including P-S-M-T) in the green photosynthetic alga Chlamydomonas reinhardtii. At the P-level, wild type and the violaxanthin-accumulating mutant npq1 show similar fluorescence intensity and fluorescence lifetime-resolved images. The zeaxanthin-accumulating mutant npq2 displays reduced fluorescence intensity at the P-level (about 25-35% less) and corresponding lifetime-resolved frequency domain phase and modulation values compared to wild type/npq1. A two-component analysis of possible lifetime compositions shows that the reduction of the fluorescence intensity can be interpreted as an increase in the fraction of a short lifetime component. This supports the important photoprotection function of zeaxanthin in photosynthetic samples, and is consistent with the notion of a 'dimmer switch'. Similar, but quantitatively different, behaviour was observed in the intensity and fluorescence lifetime-resolved imaging measurements for cells that were treated with the electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, the efficient PSI electron acceptor methyl viologen and the protonophore nigericin and. Lower fluorescence intensities and lifetimes were observed for all npq2 mutant samples at the P-level and during the slow fluorescence transient, compared to wild type and the npq1 mutant. The fluorescence lifetime-resolved measurements during the slow fluorescence changes after the P level up to 250 s for the wild type and the two mutants, in the presence and absence of the above inhibitors, were analyzed with a graphical procedure (polar plots) to determine lifetime compositions. At higher illumination intensity, wild type and npq1 cells show a rise in fluorescence intensity and corresponding rise in the species concentration of the slow lifetime component after the initial decrease following the P level. This reversal is absent in the npq2 mutant, and for all samples in the presence of the inhibitors. Lifetime heterogeneities were observed in experiments averaged over multiple cells as well as within single cells, and these were followed over time. Cells in the resting state (induced by several hours of darkness), instead of the normal swimming state, show shortened lifetimes. The above results are discussed in terms of a superposition of effects on electron transfer and protonation rates, on the so-called 'State Transitions', and on non-photochemical quenching. Our data indicate two major populations of chlorophyll a molecules, defined by two 'lifetime pools' centred on slower and faster fluorescence lifetimes.

Passage of a Young Indian Physical Chemist through the World of Photosynthesis Research at Urbana, Illinois, in the 1960s: A Personal Essay

In September 1963, I came to the famous Photosynthesis Laboratory of Eugene Rabinowitch (1901-1973) at the University of Illinois at Urbana, Illinois, after submitting my doctoral thesis, under Professor Pasupati Mukerjee, in physical chemistry, then at the Indian Association for the Cultivation of Science, Calcutta, India. I present here my personal impressions, my research and interactions at the then International Center of Photosynthesis at Urbana, Illinois. A brief mention is made of research of others at this center, my collaboration with Govindjee (Urbana) and with John Olson (at Brookhaven National Laboratory).

Chlorophyll a fluorescence rise induced by high light illumination of dark-adapted plant tissue studied by means of a model of photosystem II and considering photosystem II heterogeneity

Chlorophyll a fluorescence rise (FLR) measured in vivo in dark-adapted plant tissue immediately after the onset of high light continuous illumination shows complex O-K-J-I-P transient. The steps typically appear at about 400 micros (K), 2 ms (J), 30 ms (I), and 200 - 500 ms (P) and a transient decrease of fluorescence to local minima (dips D) can be observed after the K, J, and I steps. As the FLR reflects a function of photosystem II (PSII) and to more understand the FLR, a PSII reactions model was formulated comprising equilibrium of excited states among all light harvesting and reaction centre pigments and P680, reversible radical pair formation and the donor and acceptor side functions. Such a formulated model is the most detailed and complex model of PSII reactions used so far for simulations of the FLR. By varying of selected model parameters (rate constants and initial conditions) several conclusions can be made as for the origin of and changes in shape of the theoretical FLR and compare them with in-literature-reported results. For homogeneous population of PSII and using standard in-literature-reported values of the model parameters, the simulated FLR is characterized by reaching the minimal fluorescence F(0) at about 3 ns after the illumination is switched on lasting to about 1 micros, followed by fluorescence rise to a plateau located at about 2 ms and subsequent fluorescence rise to a global maximum that is reached at about 60 ms. Varying of the values of rate constants of fast processes that can compete for utilization of the excited states with fluorescence emission does not change qualitatively the shape of the FLR. However, primary photochemistry of PSII (the charge separation, recombination and stabilization), non-radiative loss of excited states in light harvesting antennae and excited states quenching by oxidized plastoquisnone (PQ) molecules from the PQ pool seem to be the main factors controlling the maximum quantum yield of PSII photochemistry as expressed by the F(V)/F(M) ratio. The appearance of the plateau at about 2 ms in the FLR is affected by several factors: the height of the plateau in the FLR increases when the fluorescence quenching by oxidized P680(+) is not considered in the simulations or when the electron transfer from Q(A)(-) to Q(B)((-)) is slowed down whereas the height of the plateau decreases and its position is shifted to shorter times when OEC is initially in higher S state. The plateau at about 2 ms is changed into the local fluorescence maximum followed by a dip when the fluorescence quenching by oxidized PQ molecules or the charge recombination between P680(+) and Q(A)(-) is not considered in the simulations or when all OEC is initially in the S(0) state or when the S -state transitions of OEC are slowed down. Slowing down of the S -state transitions of OEC as well as of the electron transfer from Q(A)(-) to Q(B)((-)) also causes a decrease of maximal fluorescence level. In the case of full inhibition of the S -state transitions of OEC as well as in the case of full inhibition of the electron donation to P680(+) by Y(Z), the local fluorescence maximum becomes the global fluorescence maximum. Assuming homogeneous PSII population, theoretical FLR curve that only far resembles experimentally measured O-J-I-P transient at room temperature can be simulated when slowly reducing PQ pool is considered. Assuming heterogeneous PSII population (i.e. the alpha/beta and the Q(B) -reducing/Q(B)-non-reducing heterogeneity and heterogeneity in size of the PQ pool and rate of its reduction) enables to simulate the FLR with two steps between minimal and maximal fluorescence whose relative heights are in agreement with the experiments but not their time positions. A cause of this discrepancy is discussed as well as different approaches to the definition of fluorescence signal during the FLR.

Photosynthesis research in Greece: a historical snapshot (1960-2001)

The origin of photosynthesis research in Greece can be traced to the early 1960s, and the first dedicated laboratory was established by George Akoyunoglou in the Nuclear Reseach Center (now National Center for Scientific Research) Demokritos, in Athens. More photosynthesis groups subsequently emerged, in Demokritos and in the universities. Research in Greece benefited greatly from the links of Greek scientists with laboratories and personalities, primarily in the USA and western Europe. The local research output is a proportional part of global research and, more or less, in tune with the shifting priorities of the latter. The list of references provided includes only a sample of publications: it is not inclusive.

Some effects of CO 2 concentration and decreased O 2 concentration on induction fluorescence in leaves

A procedure is described that permits spinach leaves to display secondary fluorescence kinetics when illuminated in air at 20 °C. The initial peak in chlorophyll a fluorescence is then followed by a fall to a quasi-steady state (S), a rise to a new peak (M) and a final fall to a terminal steady-state value (T). These kinetics can be modified by changing the periods of light and darkness before measurement. The M peak is abolished by exposure to CO 2 -free air and greatly modified by exposure to 5 % CO 2 . In 2% O 2 the period of darkness immediately before illumination needs to be lengthened if secondary kinetics are to be observed. The results are discussed in relation to the probable impact of photosynthetic carbon assimilation on fluorescence-quenching mechanisms.

State 1/State 2 changes in higher plants and algae

Current ideas regarding the molecular basis of State 1/State 2 transitions in higher plants and green algae are mainly centered around the view that excitation energy distribution is controlled by phosphorylation of the light-harvesting complex of photosystem II (LHC-II). The evidence supporting this view is examined and the relationship of the transitions occurring in these systems to the corresponding transitions seen in red and blue-green algae is explored.

Sudden changes in the rate of photosynthetic oxygen evolution and chlorophyll fluorescence in intact isolated chloroplasts: the role of orthophosphate

Simultaneous "ripples" (sudden changes in rate) in CO2 dependent O2 evolution and associated chlorophyll a fluorescence were followed in isolated, largely intact, spinach chloroplasts. These ripples could only be observed under conditions in which the supply of inorganic phosphate was limiting. This limitation was achieved either by 1) omission of phosphate in the assay medium, 2) use of inhibitors of the phosphate translocator, or 3) the addition of triose phosphate, a competitive inhibitor of Pi for the same translocator.The possible relation of these ripples to the dampening oscillations that can be observed in leaves, leaf pieces, isolated cells and protoplasts, is discussed.

Chlorophyll a fluorescence and light-scattering kinetics displayed by leaves during induction of photosynthesis

Light-scattering, which can be taken as an indicator of the transthylakoid proton-gradient, and chlorophyll a fluorescence, have been followed simultaneously during re-illumination of spinach leaves at different energy fluence rates and carbon dioxide concentrations. The slow fluorescence transient ("M peak"), which has been associated with photosynthetic induction, was observed in air only at the lower fluence rates used. Data are presented that indicate that M peaks in chlorophyll fluorescence kinetics can only be observed if there is also a simultaneous transient in light-scattering and that these transients are observed when the dark period is relatively long, fluence rate relatively low, and CO2 concentration relatively high.The results are discussed in relation to the varying demands on ATP by carbon assimilation during induction of photosynthesis at different carbon dioxide concentrations and the manner in which these variations influence the quenching of chlorophyll a fluorescence.

The relationship between state II to state I transitions and cyclic electron flow around photosystem I

The effects of electron acceptors, inhibitors of electron flow and uncouplers and inhibitors of photophosphorylation on a state II to I transition were studied. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) did not inhibit the state II to I transition. By contrast, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), methyl viologen and antimycin A inhibited the transition indicating that the cyclic electron flow around photosystem I, but not the oxidation of electron carriers (such as plastoquinone), induced the state II to I transition. Uncouplers, but not inhibitors of photophosphorylation, inhibited the state transition suggesting that the proton transport through the cyclic electron flow was related to the transition.

A new mechanism for adaptation to changes in light intensity and quality in the red alga, Porphyra perforata. II. Characteristics of state II-state III transitions

Characteristics of state II-state III transitions in the red alga, Porphyra perforata, were studied by measuring the fluorescence time course at room temperature and fluorescence spectra at 77 K. The state II to III transition was induced by system II light and was sensitive to uncouplers of photophosphorylation. This state II to III transition has a dark step(s) that could be easily separated from the light process. A state III to II transition occurred in the dark, but system I light accelerated the transition. The accelerating effect of system I light was not sensitive to uncouplers of photophosphorylation, but was inhibited by the addition of valinomycin + KCl or antimycin A. Compared to state I-state II transitions, the state II-state III transitions occurred more rapidly. The state II to state III transitions are different from the state I to state II transitions in that in state III the activity of photosystem II is changed without having any effect on photosystem I activity (Satoh and Fork, Biochim. Biophys, Acta, in press, 1982). It is suggested that the state II-state III transition represents a mechanism by which the alga can avoid photodamage resulting from absorption of excess light energy by photosystem II.

The relationship between state II to state I transitions and cyclic electron flow around photosystem I

The effects of electron acceptors, inhibitors of electron flow and uncouplers and inhibitors of photophosphorylation on a state II to I transition were studied. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) did not inhibit the state II to I transition. By contrast, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), methyl viologen and antimycin A inhibited the transition indicating that the cyclic electron flow around photosystem I, but not the oxidation of electron carriers (such as plastoquinone), induced the state II to I transition. Uncouplers, but not inhibitors of photophosphorylation, inhibited the state transition suggesting that the proton transport through the cyclic electron flow was related to the transition.

Mechanism of photoactivation of electron transport in intact bryopsis chloroplasts

The mechanism of photoactivation of photosystem I electron transport was studied in intact Bryopsis corticulans chloroplasts. The evidence from chemical and photochemical studies suggests that photoactivation is a consequence of a reduction of an electron transport component, presumably ferredoxin-NADP(+) reductase. O(2) does not act as a mediator of the process but rather acts as an electron acceptor after photoactivation has occurred. We suggest that the initial function of the chloroplasts in a transition from dark to light is to initiate pseudocyclic electron flow.

Light-dependent quenching of chlorophyll fluorescence in pea chloroplasts induced by adenosine 5'-triphosphate

Addition of ATP to chloroplasts causes a reversible 25-30% decrease in chlorophyll fluorescence. This quenching is light-dependent, uncoupler insensitive but inhibited by DCMU and electron acceptors and has a half-time of 3 minutes. Electron donors to Photosystem I can not overcome the inhibitory effect of DCMU, suggesting that light activation depends on the reduced state of plastoquinone. Fluorescence emission spectra recorded at -196 degrees C indicate that ATP treatment increases the amount of excitation energy transferred to Photosystem I. Examination of fluorescence induction curves indicate that ATP treatment decreases both the initial (F0) and variable (Fv) fluorescence such that the ratio of Fv to the maximum (Fm) yield is unchanged. The initial sigmoidal phase of induction is slowed down by ATP treatment and is quenched 3-fold more than the exponential slow phase, the rate of which is unchanged. A plot of Fv against area above the induction curve was identical plus or minus ATP. Thus ATP treatment can alter quantal distribution between Photosystems II and I without altering Photosystem II-Photosystem II interaction. The effect of ATP strongly resembles in its properties the phosphorylation of the light-harvesting complex by a light activated, ATP-dependent protein kinase found in chloroplast membranes and could be the basis of physiological mechanisms which contribute to slow fluorescence quenching in vivo and regulate excitation energy distribution between Photosystem I and II. It is suggested that the sensor for this regulation is the redox state of plastoquinone.

Slow fluorescence fluctuations following high light to low light or dark transitions in Chlamydomonas reinhardi

Slow fluorescence transients in Chlamydomonas reinhardi arise after transitions from high light intensities to low light or dark conditions. Characteristics of the newly described transient phenomena include: (a) A slow biphasic decrease in fluorescence yield occurs in the dark, followed by an even slower, hour long, increase in fluorescence. (b) A similar, but faster, fluorescence yield decrease and subsequent increase also occurs during low intensity illumination periods separating high light intervals, or after transitions from high intensity to low intensity light. (c) Short (several seconds) flashes of light given during a dark period have no effect on the dark fluorescence decay, regardless of the flash frequency. Such flash regimes accurately monitor the dark decline of the M2 level by tracing the parallel decay of flash-generated P2 (Kautsky) peaks. However, flashes during a low light illumination period do influence the decay kinetics. Frequent flashes allow decay similar to that occurring in dark, but less frequent flashes inhibit the decrease in fluorescence yield.

Evidence that circularly dichroic chlorophyll forms a-682 and a-710 are oriented at right angles to the thylakoid membranes of whole chloroplasts, and that the circular dichroism is light-dependent

Chloroplasts from the pea (Pisum sativum L.) suspended in iso-osmotic buffered medium were oriented by flow in specially constructed cuvettes and examined for circular dichroism (c.d.). In one cuvette the flow was transverse to the direction of the light-path, but the other cuvette was designed so that flow and the light-path were coaxial. The induced orientation is such that the chloroplasts appear to move edgewise. C.d. was maximum when the light-path lay in the plane of the chloroplast disc. The intense c.d. of intact chloroplasts ascribed by Gregory & Raps [Biochem. J. (1974) 142, 193-201] to bulk chlorophyll a was found to contain two components, one the split-exciton type centred at 682nm and the other a simple maximum at 700-710 nm. The chlorophyll a-710 form was distinguished by its greater dependence on chloroplast orientation. The preferred direction of the transition moment in both chlorophyll forms was at right angles to the plane of the chloroplast, that is, at right angles to the plane of the thylakoids. This is in conflict with several reports based on polarization of fluorescence. It is suggested that the present effect is due to thylakoid-thylakoid interaction. Evidence for this is the reversible diminution in the c.d. signal caused by illumination in the presence of electron-transport reagents. It is argued that the c.d. is an indicator of chlorophyll movement, or changes in the thylakoid-thylakoid distance, possibly related to ion movement, affecting energy transfer between photosynthetic units.