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

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.

Frequently asked questions about in vivo chlorophyll fluorescence: practical issues

The aim of this educational review is to provide practical information on the hardware, methodology, and the hands on application of chlorophyll (Chl) a fluorescence technology. We present the paper in a question and answer format like frequently asked questions. Although nearly all information on the application of Chl a fluorescence can be found in the literature, it is not always easily accessible. This paper is primarily aimed at scientists who have some experience with the application of Chl a fluorescence but are still in the process of discovering what it all means and how it can be used. Topics discussed are (among other things) the kind of information that can be obtained using different fluorescence techniques, the interpretation of Chl a fluorescence signals, specific applications of these techniques, and practical advice on different subjects, such as on the length of dark adaptation before measurement of the Chl a fluorescence transient. The paper also provides the physiological background for some of the applied procedures. It also serves as a source of reference for experienced scientists.

Photosynthetic acclimation to drought stress in Agave salmiana Otto ex Salm-Dyck seedlings is largely dependent on thermal dissipation and enhanced electron flux to photosystem I

Agave salmiana Otto ex Salm-Dyck, a crassulacean acid metabolism plant that is adapted to water-limited environments, has great potential for bioenergy production. However, drought stress decreases the requirement for light energy, and if the amount of incident light exceeds energy consumption, the photosynthetic apparatus can be injured, thereby limiting plant growth. The objective of this study was to evaluate the effects of drought and re-watering on the photosynthetic efficiency of A. salmiana seedlings. The leaf relative water content and leaf water potential decreased to 39.6 % and -1.1 MPa, respectively, over 115 days of water withholding and recovered after re-watering. Drought caused a direct effect on photosystem II (PSII) photochemistry in light-acclimated leaves, as indicated by a decrease in the photosynthetic electron transport rate. Additionally, down-regulation of photochemical activity occurred mainly through the inactivation of PSII reaction centres and an increased thermal dissipation capacity of the leaves. Prompt fluorescence kinetics also showed a larger pool of terminal electron acceptors in photosystem I (PSI) as well as an increase in some JIP-test parameters compared to controls, reflecting an enhanced efficiency and specific fluxes for electron transport from the plastoquinone pool to the PSI terminal acceptors. All the above parameters showed similar levels after re-watering. These results suggest that the thermal dissipation of excess energy and the increased energy conservation from photons absorbed by PSII to the reduction of PSI end acceptors may be an important acclimation mechanism to protect the photosynthetic apparatus from over-excitation in Agave plants.

Alterations in photochemical efficiency of photosystem II in wheat plant on hot summer day

In this study the effect of increasing temperature on photochemical efficiency of PS II in wheat plants has been studied on a hot summer day (9:00 AM (Control)-7:00 PM) by measuring Chl a fluorescence. Increasing temperature for a short period of time (2-4 h), in nature affects the efficiency of PS II complex reversibly and does not cause permanent damage to any of the components of photosystem II. A scheme has been provided to demonstrate the sequence and severity of events which get affected maximum by temperature stress.

On the question of the light-harvesting role of β-carotene in photosystem II and photosystem I core complexes

β-Carotene is the only carotenoid present in the core complexes of Photosystems I and II. Its proximity to chlorophyll a molecules enables intermolecular electronic interactions, including β-carotene to chlorophyll a electronic excitation transfers. However, it has been well documented that, compared to chlorophylls and to phycobilins, the light harvesting efficiency of β-carotenes for photosynthetic O2 evolution is poor. This is more evident in cyanobacteria than in plants and algae because they lack accessory light harvesting pigments with absorptions that overlap the β-carotene absorption. In the present work we investigated the light harvesting role of β-carotenes in the cyanobacterium Synechococcus sp. PCC 7942 using selective β-carotene excitation and selective Photosystem detection of photo-induced electron transport to and from the intersystem plastoquinones (the plastoquinone pool). We report that, although selectively excited β-carotenes transfer electronic excitation to the chlorophyll a of both photosystems, they enable only the oxidation of the plastoquinone pool by Photosystem I but not its reduction by Photosystem II. This may suggest a light harvesting role for the β-carotenes of the Photosystem I core complex but not for those of the Photosystem II core complex. According to the present investigation, performed with whole cyanobacterial cells, the lower photosynthesis yields measured with β-Car-absorbed light can be attributed to the different excitation trapping efficiencies in the reaction centers of PSI and PSII.

Light-dependent reversible phosphorylation of the minor photosystem II antenna Lhcb6 (CP24) occurs in lycophytes

Evolution of vascular plants required compromise between photosynthesis and photodamage. We analyzed representative species from two divergent lineages of vascular plants, lycophytes and euphyllophytes, with respect to the response of their photosynthesis and light-harvesting properties to increasing light intensity. In the two analyzed lycophytes, Selaginella martensii and Lycopodium squarrosum, the medium phase of non-photochemical quenching relaxation increased under high light compared to euphyllophytes. This was thought to be associated with the occurrence of a further thylakoid phosphoprotein in both lycophytes, in addition to D2, CP43 and Lhcb1-2. This protein, which showed light intensity-dependent reversible phosphorylation, was identified in S. martensii as Lhcb6, a minor LHCII antenna subunit of PSII. Lhcb6 is known to have evolved in the context of land colonization. In S. martensii, Lhcb6 was detected as a component of the free LHCII assemblies, but also associated with PSI. Most of the light-induced changes affected the amount and phosphorylation of the LHCII assemblies, which possibly mediate PSI-PSII connectivity. We propose that Lhcb6 is involved in light energy management in lycophytes, participating in energy balance between PSI and PSII through a unique reversible phosphorylation, not yet observed in other land plants.

Hierarchical coarse-graining model for photosystem II including electron and excitation-energy transfer processes

We propose a hierarchical reduction scheme to cope with coupled rate equations that describe the dynamics of multi-time-scale photosynthetic reactions. To numerically solve nonlinear dynamical equations containing a wide temporal range of rate constants, we first study a prototypical three-variable model. Using a separation of the time scale of rate constants combined with identified slow variables as (quasi-)conserved quantities in the fast process, we achieve a coarse-graining of the dynamical equations reduced to those at a slower time scale. By iteratively employing this reduction method, the coarse-graining of broadly multi-scale dynamical equations can be performed in a hierarchical manner. We then apply this scheme to the reaction dynamics analysis of a simplified model for an illuminated photosystem II, which involves many processes of electron and excitation-energy transfers with a wide range of rate constants. We thus confirm a good agreement between the coarse-grained and fully (finely) integrated results for the population dynamics.

Chlorophyll fluorescence emission can screen cold tolerance of cold acclimated Arabidopsis thaliana accessions

BACKGROUND

An easy and non-invasive method for measuring plant cold tolerance is highly valuable to instigate research targeting breeding of cold tolerant crops. Traditional methods are labor intensive, time-consuming and thereby of limited value for large scale screening. Here, we have tested the capacity of chlorophyll a fluorescence (ChlF) imaging based methods for the first time on intact whole plants and employed advanced statistical classifiers and feature selection rules for finding combinations of images able to discriminate cold tolerant and cold sensitive plants.

RESULTS

ChlF emission from intact whole plant rosettes of nine Arabidopsis thaliana accessions was measured for (1) non-acclimated (NAC, six week old plants grown at room temperature), (2) cold acclimated (AC, NAC plants acclimated at 4°C for two weeks), and (3) sub-zero temperature (ST) treated (STT, AC plants treated at -4°C for 8 h in dark) states. Cold acclimation broadened the slow phase of ChlF transients in cold sensitive (Co, C24, Can and Cvi) A. thaliana accessions. Similar broadening in the slow phase of ChlF transients was observed in cold tolerant (Col, Rsch, and Te) plants following ST treatments. ChlF parameters: maximum quantum yield of PSII photochemistry (FV/FM) and fluorescence decrease ratio (RFD) well categorized the cold sensitive and tolerant plants when measured in STT state. We trained a range of statistical classifiers with the sequence of captured ChlF images and selected a high performing quadratic discriminant classifier (QDC) in combination with sequential forward floating selection (SFFS) feature selection methods and found that linear combination of three images showed a reasonable contrast between cold sensitive and tolerant A. thaliana accessions for AC as well as for STT states.

CONCLUSIONS

ChlF transients measured for an intact whole plant is important for understanding the impact of cold acclimation on photosynthetic processes. Combinatorial imaging combined with statistical classifiers and feature selection methods worked well for the screening of cold tolerance without exposing plants to sub-zero temperatures. This opens up new possibilities for high-throughput monitoring of whole plants cold tolerance via easy and fully non-invasive means.

Integrative regulatory network of plant thylakoid energy transduction

Highly flexible regulation of photosynthetic light reactions in plant chloroplasts is a prerequisite to provide sufficient energy flow to downstream metabolism and plant growth, to protect light reactions against photodamage, and to ensure controlled cellular signaling from the chloroplast to the nucleus. Such comprehensive regulation occurs via the control of excitation energy transfer to and between the two photosystems (PSII and PSI), of the electrochemical gradient across the thylakoid membrane (ΔpH), and of electron transfer from PSII to PSI electron acceptors. In this opinion article, we propose that these regulatory mechanisms, functioning at different levels of photosynthetic energy conversion, might be interconnected and describe how the concomitant and integrated function of these mechanisms might enable plants to acclimate to a full array of environmental changes.

A model of chlorophyll a fluorescence induction kinetics with explicit description of structural constraints of individual photosystem II units

Chlorophyll a fluorescence induction (FI) kinetics, in the microseconds to the second range, reflects the overall performance of the photosynthetic apparatus. In this paper, we have developed a novel FI model, using a rule-based kinetic Monte Carlo method, which incorporates not only structural and kinetic information on PSII, but also a simplified photosystem I. This model has allowed us to successfully simulate the FI under normal or different treatment conditions, i.e., with different levels of measuring light, under 3-(3',4'-dichlorophenyl)-1,1-dimethylurea treatment, under 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone treatment, and under methyl viologen treatment. Further, using this model, we have systematically studied the mechanistic basis and factors influencing the FI kinetics. The results of our simulations suggest that (1) the J step is caused by the two-electron gate at the Q B site; (2) the I step is caused by the rate limitation of the plastoquinol re-oxidation in the plastoquinone pool. This new model provides a framework for exploring impacts of modifying not only kinetic but also structural parameters on the FI kinetics.

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.

A quick investigation of the detrimental effects of environmental pollutant polycyclic aromatic hydrocarbon fluoranthene on the photosynthetic efficiency of wheat (Triticum aestivum)

The present study describes the effects of polycyclic aromatic hydrocarbon pollutant fluoranthene (FLT) on photochemical efficiency of plants and hence their growth. Chlorophyll a fluorescence measurements were performed in order to get quick and reliable information about the effects of fluoranthene. The number of active reaction centres decreased by more than 20% in FLT treated samples. The results show that FLT affects the overall primary photochemistry by inhibiting the number of active reaction centres (RC) while the efficiency of each of the active reaction centres is not affected. We propose that the effect of fluoranthene is mainly on the absorption and trapping of the light energy by the RC rather than the actual electron transport. The results may be applicable to a wide range of plants provided other environmental and physiological conditions are taken into account carefully.

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.

Salt deliquescence drives photosynthesis in the hyperarid Atacama Desert

Endolithic cyanobacteria are found in halite nodules in the hyperarid core of the Atacama Desert. Using Pulse Amplitude Modulated Fluorometry, we show here that photosynthetic systems of these cyanobacteria become active when the relative humidity rises above 70% and the salt becomes wet by way of deliquescence. This is the first evidence of active metabolism in the hyperarid core of the Atacama, and supports the view of a microbial community sustained by deliquescence. Our results expand the water activity envelope of life on Earth.

Effects of dual stress (high salt and high temperature) on the photochemical efficiency of wheat leaves (Triticum aestivum)

In this study, we have focused on those components of Photosystem (PS) II which are significantly affected by dual stress (high salt and temperature) on wheat as measured by Plant Efficiency Analyser (PEA). It was observed that some of the chlorophyll a fluorescence parameters were temperature dominated, while some other parameters were salt dominated. We have also observed additive effects for parameters like antenna size heterogeneity. An important observation was that in high temperature alone, the K-step was observed at 40 °C, while in case of dual stress, the K-step was observed at 45 °C, while the Chl a fluorescence transient of 40 °C + 0.5 M NaCl was quite similar to 35 °C transient curve. In the presence of salt, K-step was observed at higher temperature suggesting a protection of OEC by salt. Plants are under dual stress, but effect of temperature stress is less severe in presence of salt stress. Thus, we can say that salt stress caused partial prevention from high temperature stress but it did not cause complete protection of PS II.

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.

Post-genomic insight into thylakoid membrane lateral heterogeneity and redox balance

Photosynthetic machinery requires balanced distribution of excitation energy from the light-harvesting complexes to photosystems. The efficiency of light-harvesting is regulated by thermal dissipation of excess energy, while the distribution of energy between photosystems is dependent on STN7 kinase and phosphorylation of thylakoid proteins. The regulation of excitation energy transfer has been linked to the lateral segregation of photosynthetic complexes along the thylakoid membrane. The study of photosynthetic regulation mechanisms using Arabidopsis mutants, which have been available for the last ten years, has challenged traditional views on regulation of excitation energy distribution. Here, we discuss an urgent need to create a holistic view of the dynamics of the thylakoid membrane using systematic research of the mutants available today.

RNA helicase, CrhR is indispensable for the energy redistribution and the regulation of photosystem stoichiometry at low temperature in Synechocystis sp. PCC6803

We investigated the role of a cold-inducible and redox-regulated RNA helicase, CrhR, in the energy redistribution and adjustment of stoichiometry between photosystem I (PSI) and photosystem II (PSII), at low temperature in Synechocystis sp. PCC 6803. The results suggest that during low temperature incubation, i.e., when cells are shifted from 34°C to 24°C, wild type cells exhibited light-induced state transitions, whereas the mutant deficient in CrhR failed to perform the same. At low temperature, wild type cells maintained the plastoquinone (PQ) pool in the reduced state due to enhanced respiratory electron flow to the PQ pool, whereas in ∆crhR mutant cells the PQ pool was in the oxidized state. Wild type cells were in state 2 and ∆crhR cells were locked in state 1 at low temperature. In both wild type and ∆crhR cells, a fraction of PSI trimers were changed to PSI monomers. However, in ∆crhR cells, the PSI trimer content was significantly decreased. Expression of photosystem I genes, especially the psaA and psaB, was strongly down-regulated due to oxidation of downstream components of PQ in ∆crhR cells at low temperature. We demonstrated that changes in the low temperature-induced energy redistribution and regulation of photosystem stoichiometry are acclimatization responses exerted by Synechocystis cells, essentially regulated by the RNA helicase, CrhR, at low temperature.

The slow S to M fluorescence rise in cyanobacteria is due to a state 2 to state 1 transition

In dark-adapted plants and algae, chlorophyll a fluorescence induction peaks within 1s after irradiation due to well documented photochemical and non-photochemical processes. Here we show that the much slower fluorescence rise in cyanobacteria (the so-called "S to M rise" in tens of seconds) is due to state 2 to state 1 transition. This has been demonstrated in particular for Synechocystis PCC6803, using its RpaC(-) mutant (locked in state 1) and its wild-type cells kept in hyperosmotic suspension (locked in state 2). In both cases, the inhibition of state changes correlates with the disappearance of the S to M fluorescence rise, confirming its assignment to the state 2 to state 1 transition. The general physiological relevance of the SM rise is supported by its occurrence in several cyanobacterial strains: Synechococcus (PCC 7942, WH 5701) and diazotrophic single cell cyanobacterium (Cyanothece sp. ATCC 51142). We also show here that the SM fluorescence rise, and also the state transition changes are less prominent in filamentous diazotrophic cyanobacterium Nostoc sp. (PCC 7120) and absent in phycobilisome-less cyanobacterium Prochlorococcus marinus PCC 9511. Surprisingly, it is also absent in the phycobiliprotein rod containing Acaryochloris marina (MBIC 11017). All these results show that the S to M fluorescence rise reflects state 2 to state 1 transition in cyanobacteria with phycobilisomes formed by rods and core parts. We show that the pronounced SM fluorescence rise may reflect a protective mechanism for excess energy dissipation in those cyanobacteria (e.g. in Synechococcus PCC 7942) that are less efficient in other protective mechanisms, such as blue light induced non-photochemical quenching. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Computer reconstruction of plant growth and chlorophyll fluorescence emission in three spatial dimensions

Plant leaves grow and change their orientation as well their emission of chlorophyll fluorescence in time. All these dynamic plant properties can be semi-automatically monitored by a 3D imaging system that generates plant models by the method of coded light illumination, fluorescence imaging and computer 3D reconstruction. Here, we describe the essentials of the method, as well as the system hardware. We show that the technique can reconstruct, with a high fidelity, the leaf size, the leaf angle and the plant height. The method fails with wilted plants when leaves overlap obscuring their true area. This effect, naturally, also interferes when the method is applied to measure plant growth under water stress. The method is, however, very potent in capturing the plant dynamics under mild stress and without stress. The 3D reconstruction is also highly effective in correcting geometrical factors that distort measurements of chlorophyll fluorescence emission of naturally positioned plant leaves.

Low pH induced structural reorganization in thylakoid membranes

By using low temperature fluorescence spectroscopy, it has been shown that exposing chloroplast thylakoid membranes to acidic pH reversibly decreases the fluorescence of photosystem II while the fluorescence of photosystem I increases [P. Singh-Rawal et al. (2010) Evidence that pH can drive state transitions in isolated thylakoid membranes from spinach, Photochem Photobiol Sci, 9 830-837]. In order to shed light on the origin of these changes, we performed circular dichroism (CD) spectroscopy on freshly isolated pea thylakoid membranes. We show that the magnitude of the psi-type CD, which is associated with the presence of chirally ordered macroarrays of the chromophores in intact thylakoid membranes, decreases gradually and reversibly upon gradually lowering the pH of the medium from 7.5 to 4.5 (psi, polymer or salt induced). The same treatment, as shown on thylakoid membranes washed in hypotonic low salt medium possessing no psi-type bands, induces no discernible change in the excitonic CD. These data show that while no change in the pigment-pigment interactions and thus in the molecular organization of the bulk protein complexes can be held responsible for the observed changes in the fluorescence, acidification of the medium significantly alters the macro-organization of the complexes, hence providing an explanation for the pH-induced redistribution of the excitation energy between the two photosystems. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

A tribute to Thomas Roosevelt Punnett, Jr. (1926-2008)

We honor here Thomas (Tom) Roosevelt Punnett, Jr. (May 25, 1926-July 4, 2008), who was a pioneer of Biology, particularly of biochemistry of plants and algae, having specialized in photosynthesis under Robert Emerson of the University of Illinois at Urbana-Champaign. He did exciting work on regulation and control of various metabolic reactions. He was an innovator and raconteur par excellence, and he prized critical thinking. His enthusiasm for basic science questions was matched by his grasp of their "real-world" implications. His last project was a patent for anaerobic sewage treatment that he hoped would lead to solution of waste disposal and energy creation world wide, including the clean-up of Lake Erie, where he had sailed as a boy. On the personal side, he had a strong sense of morality and a great wit and humor.

Adventures with cyanobacteria: a personal perspective

Cyanobacteria, or the blue-green algae as they used to be called until 1974, are the oldest oxygenic photosynthesizers. We summarize here adventures with them since the early 1960s. This includes studies on light absorption by cyanobacteria, excitation energy transfer at room temperature down to liquid helium temperature, fluorescence (kinetics as well as spectra) and its relationship to photosynthesis, and afterglow (or thermoluminescence) from them. Further, we summarize experiments on their two-light reaction - two-pigment system, as well as the unique role of bicarbonate (hydrogen carbonate) on the electron-acceptor side of their photosystem II, PSII. This review, in addition, includes a discussion on the regulation of changes in phycobilins (mostly in PSII) and chlorophyll a (Chl a; mostly in photosystem I, PSI) under oscillating light, on the relationship of the slow fluorescence increase (the so-called S to M rise, especially in the presence of diuron) in minute time scale with the so-called state-changes, and on the possibility of limited oxygen evolution in mixotrophic PSI (minus) mutants, up to 30 min, in the presence of glucose. We end this review with a brief discussion on the position of cyanobacteria in the evolution of photosynthetic systems.