Structure of the red fluorescence band in chloroplasts

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.

Development of fluorescence quenching in Chlamydomonas reinhardtii upon prolonged illumination at 77 K

Low-temperature fluorescence measurements are frequently used in photosynthesis research to assess photosynthetic processes. Upon illumination of photosystem II (PSII) frozen to 77 K, fluorescence quenching is observed. In this work, we studied the light-induced quenching in intact cells of Chlamydomonas reinhardtii at 77 K using time-resolved fluorescence spectroscopy with a streak camera setup. In agreement with previous studies, global analysis of the data shows that prolonged illumination of the sample affects the nanosecond decay component of the PSII emission. Using target analysis, we resolved the quenching on the PSII-684 compartment which describes bulk chlorophyll molecules of the PSII core antenna. Further, we quantified the quenching rate constant and observed that as the illumination proceeds the accumulation of the quencher leads to a speed up of the fluorescence decay of the PSII-684 compartment as the decay rate constant increases from about 3 to 4 ns^- 1. The quenching on PSII-684 leads to indirect quenching of the compartments PSII-690 and PSII-695 which represent the red chlorophyll of the PSII core. These results explain past and current observations of light-induced quenching in 77 K steady-state and time-resolved fluorescence spectra.

Energy transfer in Anabaena variabilis filaments adapted to nitrogen-depleted and nitrogen-enriched conditions studied by time-resolved fluorescence

Nitrogen is among the most important nutritious elements for photosynthetic organisms such as plants, algae, and cyanobacteria. Therefore, nitrogen depletion severely compromises the growth, development, and photosynthesis of these organisms. To preserve their integrity under nitrogen-depleted conditions, filamentous nitrogen-fixing cyanobacteria reduce atmospheric nitrogen to ammonia, and self-adapt by regulating their light-harvesting and excitation energy-transfer processes. To investigate the changes in the primary processes of photosynthesis, we measured the steady-state absorption and fluorescence spectra and time-resolved fluorescence spectra (TRFS) of whole filaments of the nitrogen-fixing cyanobacterium Anabaena variabilis at 77 K. The filaments were grown in standard and nitrogen-free media for 6 months. The TRFS were measured with a picosecond time-correlated single photon counting system. Despite the phycobilisome degradation, the energy-transfer paths within phycobilisome and from phycobilisome to both photosystems were maintained. However, the energy transfer from photosystem II to photosystem I was suppressed and a specific red chlorophyll band appeared under the nitrogen-depleted condition.

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.

Linking chlorophyll a fluorescence to photosynthesis for remote sensing applications: mechanisms and challenges

Chlorophyll a fluorescence (ChlF) has been used for decades to study the organization, functioning, and physiology of photosynthesis at the leaf and subcellular levels. ChlF is now measurable from remote sensing platforms. This provides a new optical means to track photosynthesis and gross primary productivity of terrestrial ecosystems. Importantly, the spatiotemporal and methodological context of the new applications is dramatically different compared with most of the available ChlF literature, which raises a number of important considerations. Although we have a good mechanistic understanding of the processes that control the ChlF signal over the short term, the seasonal link between ChlF and photosynthesis remains obscure. Additionally, while the current understanding of in vivo ChlF is based on pulse amplitude-modulated (PAM) measurements, remote sensing applications are based on the measurement of the passive solar-induced chlorophyll fluorescence (SIF), which entails important differences and new challenges that remain to be solved. In this review we introduce and revisit the physical, physiological, and methodological factors that control the leaf-level ChlF signal in the context of the new remote sensing applications. Specifically, we present the basis of photosynthetic acclimation and its optical signals, we introduce the physical and physiological basis of ChlF from the molecular to the leaf level and beyond, and we introduce and compare PAM and SIF methodology. Finally, we evaluate and identify the challenges that still remain to be answered in order to consolidate our mechanistic understanding of the remotely sensed SIF signal.

Single-cell confocal spectrometry of a filamentous cyanobacterium Nostoc at room and cryogenic temperature. Diversity and differentiation of pigment systems in 311 cells

The fluorescence spectrum at 298 and 40 K and the absorption spectrum at 298 K of each cell of the filamentous cyanobacterium Nostoc sp. was measured by single-cell confocal laser spectroscopy to study the differentiation of cell pigments. The fluorescence spectra of vegetative (veg) and heterocyst (het) cells of Nostoc formed separate groups with low and high PSII to PSI ratios, respectively. The fluorescence spectra of het cells at 40 K still contained typical PSII bands. The PSII/PSI ratio estimated for the veg cells varied between 0.4 and 1.2, while that of het cells varied between 0 and 0.22 even in the same culture. The PSII/PSI ratios of veg cells resembled each other more closely in the same filament. 'pro-het' cells, which started to differentiate into het cells, were identified from the small but specific difference in the PSII/PSI ratio. The allophycocyanin (APC)/PSII ratio was almost constant in both veg and het cells, indicating their tight couplings. Phycocyanin (PC) showed higher fluorescence in most het cells, suggesting the uncoupling from PSII. Veg cells seem to vary their PSI contents to give different PSII/PSI ratios even in the same culture, and to suppress the synthesis of PSII, APC and PC to differentiate into het cells. APC and PC are gradually liberated from membranes in het cells with the uncoupling from PSII. Single-cell spectrometry will be useful to study the differentiation of intrinsic pigments of cells and chloroplasts, and to select microbes from natural environments.

Relationship between the Absorption and Emission Spectra and the "Red Drop" in the Action Spectra of Fluorescence In Vivo

The Stepanov equation, relating the intensity of emission, f(e)(v), at a given frequency, and that of absorption, k(v), at the same frequency, is applied, in its modified form (see equation 3 in text) to suspensions of Chlorella, Porphyridium, and Anacystis and to chlorophyll solutions. This application can reveal whether the yield of fluorescence, Phi(v), is constant, or changes with frequency. In Chlorella (green alga) a sharp drop of Phi(v) is indicated towards the lower frequencies (longer waves), beginning around v = 1.48 x 10(4)cm(-1) (680 mmu); the Phi(v) function calculated from the Stepanov equation is in fair agreement with the directly determined action spectrum for the excitation of chlorophyll fluorescence in this organism. In Porphyridium (red alga) and Anacystis (blue-green alga) application of the Stepanov equation supports the conclusions, derived from direct measurements, of a much earlier "red drop" of the fluorescence excitation spectra. Direct measurements suggest that the drop in Porphyridium may begin at about 1.53 x 10(4)cm(-1) (654 mmu); in Anacystis, it may begin already above 1.57 x 10(4)cm(-1) (<637mmu). These results confirm the relation, postulated earlier by Duysens and others, between the action spectra of photosynthesis and of chlorophyll a fluorescence in algal cells. The relation of these findings to spectroscopic evidence, suggesting the existence of two main chlorophyll a components in vivo, in green as well as in red and blue-green algae, is discussed.

Fluorescence measurement by a streak camera in a single-photon-counting mode

We describe here a recently developed fluorescence measurement system that uses a streak camera to detect fluorescence decay in a single photon-counting mode. This system allows for easy measurements of various samples and provides 2D images of fluorescence in the wavelength and time domains. The great advantage of the system is that the data can be handled with ease; furthermore, the data are amenable to detailed analysis. We describe the picosecond kinetics of fluorescence in spinach Photosystem (PS) II particles at 4-77 K as a typical experimental example. Through the global analysis of the data, we have identified a new fluorescence band (F689) in addition to the already established F680, F685, and F695 emission bands. The blue shift of the steady-state fluorescence spectrum upon cooling below 77 K can be interpreted as an increase of the shorter-wavelength fluorescence, especially F689, due to the slowdown of the excitation energy transfer process. The F685 and F695 bands seem to be thermally equilibrated at 77 K but not at 4 K. The simple and efficient photon accumulation feature of the system allows us to measure fluorescence from leaves, solutions, single colonies, and even single cells. The 2D fluorescence images obtained by this system are presented for isolated spinach PS II particles, intact leaves of Arabidopsis thaliana, the PS I super-complex of a marine centric diatom, Chaetoceros gracilis, isolated membranes of a purple photosynthetic bacterium, Acidiphilium rubrum, which contains Zn-BChl a, and a coral that contains a green fluorescent protein and an algal endosymbiont, Zooxanthella.

Deriving room temperature excitation spectra for photosystem I and photosystem II fluorescence in intact leaves from the dependence of FV/FM on excitation wavelength

The F(0) and F(M) level fluorescence from a wild-type barley, a Chl b-less mutant barley, and a maize leaf was determined from 430 to 685 nm at 10 nm intervals using pulse amplitude-modulated (PAM) fluorimetry. Variable wavelengths of the pulsed excitation light were achieved by passing the broadband emission of a Xe flash lamp through a birefringent tunable optical filter. For the three leaf types, spectra of F(V)/F(M) (=(F(M) - F(0))/F (M)) have been derived: within each of the three spectra of F(V)/F(M), statistically meaningful variations were detected. Also, at distinct wavelength regions, the (V)/F(M) differed significantly between leaf types. From spectra of F(V)/F (M), excitation spectra of PS I and PS II fluorescence were calculated using a model that considers PS I fluorescence to be constant but variable PS II fluorescence. The photosystem spectra suggest that LHC II absorption results in high values of F(V)/F(M) between 470 and 490 nm in the two wild-type leaves but the absence of LHC II in the Chl b-less mutant barley leaf decreases the F(V)/F(M) at these wavelengths. All three leaves exhibited low values of F(V)/F(M) around 520 nm which was tentatively ascribed to light absorption by PS I-associated carotenoids. In the 550-650 nm region, the F(V)/F(M) in the maize leaf was lower than in the barley wild-type leaf which is explained with higher light absorption by PS I in maize, which is a NADP-ME C(4) species, than in barley, a C(3) species. Finally, low values of F(V)/F(M) at 685 in maize leaf and in the Chl b-less mutant barley leaf are in agreement with preferential PS I absorption at this wavelength. The potential use of spectra of the F(V)/F(M) ratio to derive information on spectral absorption properties of PS I and PS II is discussed.

Spectral signatures of photosynthesis. I. Review of Earth organisms

Why do plants reflect in the green and have a "red edge" in the red, and should extrasolar photosynthesis be the same? We provide (1) a brief review of how photosynthesis works, (2) an overview of the diversity of photosynthetic organisms, their light harvesting systems, and environmental ranges, (3) a synthesis of photosynthetic surface spectral signatures, and (4) evolutionary rationales for photosynthetic surface reflectance spectra with regard to utilization of photon energy and the planetary light environment. We found the "near-infrared (NIR) end" of the red edge to trend from blue-shifted to reddest for (in order) snow algae, temperate algae, lichens, mosses, aquatic plants, and finally terrestrial vascular plants. The red edge is weak or sloping in lichens. Purple bacteria exhibit possibly a sloping edge in the NIR. More studies are needed on pigment-protein complexes, membrane composition, and measurements of bacteria before firm conclusions can be drawn about the role of the NIR reflectance. Pigment absorbance features are strongly correlated with features of atmospheric spectral transmittance: P680 in Photosystem II with the peak surface incident photon flux density at approximately 685 nm, just before an oxygen band at 687.5 nm; the NIR end of the red edge with water absorbance bands and the oxygen A-band at 761 nm; and bacteriochlorophyll reaction center wavelengths with local maxima in atmospheric and water transmittance spectra. Given the surface incident photon flux density spectrum and resonance transfer in light harvesting, we propose some rules with regard to where photosynthetic pigments will peak in absorbance: (1) the wavelength of peak incident photon flux; (2) the longest available wavelength for core antenna or reaction center pigments; and (3) the shortest wavelengths within an atmospheric window for accessory pigments. That plants absorb less green light may not be an inefficient legacy of evolutionary history, but may actually satisfy the above criteria.

A Monte Carlo study of the chlorophyll fluorescence emission and its effect on the leaf spectral reflectance and transmittance under various conditions

Spectral hemispherical reflectance R(lambda) and transmittance T(lambda) are affected by chlorophyll (Chl) fluorescence which may complicate the evaluation of optical parameters of leaves. Measured Chl a fluorescence spectral emission F(lambda) is itself affected by several distortion effects on the leaf level (fluorescence reabsorption, secondary fluorescence, inner filter, surface and subsurface reflections etc.). In this work we propose a Monte Carlo photon transport (MCPT) model capable for treating a variety of optical distortion effects on the leaf level. In the forward mode the model decouples R(lambda), T(lambda) and their fluorescence contributions FR(T)(lambda). To obtain the absorption and scattering spectra of the leaf, utilized in the forward modeling, we have suggested an inversion procedure employing the experimental R(lambda), T(lambda). The attention was paid on the correction of the leaf absorption and scattering spectra caused by the optical effects on the sample level including Chl fluorescence contribution to measured R(lambda), T(lambda).

Photosynthetic activity of far-red light in green plants

We have found that long-wavelength quanta up to 780 nm support oxygen evolution from the leaves of sunflower and bean. The far-red light excitations are supporting the photochemical activity of photosystem II, as is indicated by the increased chlorophyll fluorescence in response to the reduction of the photosystem II primary electron acceptor, Q(A). The results also demonstrate that the far-red photosystem II excitations are susceptible to non-photochemical quenching, although less than the red excitations. Uphill activation energies of 9.8+/-0.5 kJ mol(-1) and 12.5+/-0.7 kJ mol(-1) have been revealed in sunflower leaves for the 716 and 740 nm illumination, respectively, from the temperature dependencies of quantum yields, comparable to the corresponding energy gaps of 8.8 and 14.3 kJ mol(-1) between the 716 and 680 nm, and the 740 and 680 nm light quanta. Similarly, the non-photochemical quenching of far-red excitations is facilitated by temperature confirming thermal activation of the far-red quanta to the photosystem II core. The observations are discussed in terms of as yet undisclosed far-red forms of chlorophyll in the photosystem II antenna, reversed (uphill) spill-over of excitation from photosystem I antenna to the photosystem II antenna, as well as absorption from thermally populated vibrational sub-levels of photosystem II chlorophylls in the ground electronic state. From these three interpretations, our analysis favours the first one, i.e., the presence in intact plant leaves of a small number of far-red chlorophylls of photosystem II. Based on analogy with the well-known far-red spectral forms in photosystem I, it is likely that some kind of strongly coupled chlorophyll dimers/aggregates are involved. The similarity of the result for sunflower and bean proves that both the extreme long-wavelength oxygen evolution and the local quantum yield maximum are general properties of the plants.

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).

Global spectral-kinetic analysis of room temperature chlorophyll a fluorescence from light-harvesting antenna mutants of barley

This study presents a novel measurement, and simulation, of the time-resolved room temperature chlorophyll a fluorescence emission spectra from leaves of the barley wild-type and chlorophyll-b-deficient chlorina (clo) f2 and f104 mutants. The primary data were collected with a streak-camera-based picosecond-pulsed fluorometer that simultaneously records the spectral distribution and time dependence of the fluorescence decay. A new global spectral-kinetic analysis programme method, termed the double convolution integral (DCI) method, was developed to convolve the exciting laser pulse shape with a multimodal-distributed decay profile function that is again convolved with the spectral emission band amplitude functions. We report several key results obtained by the simultaneous spectral-kinetic acquisition and DCI methods. First, under conditions of dark-level fluorescence, when photosystem II (PS II) photochemistry is at a maximum at room temperature, both the clo f2 and clo f104 mutants exhibit very similar PS II spectral-decay contours as the wild-type (wt), with the main band centred around 685 nm. Second, dark-level fluorescence is strongly influenced beyond 700 nm by broad emission bands from PS I, and its associated antennae proteins, which exhibit much more rapid decay kinetics and strong integrated amplitudes. In particular a 705-720 nm band is present in all three samples, with a 710 nm band predominating in the clo f2 leaves. When the PS II photochemistry becomes inhibited, maximizing the fluorescence yield, both the clo f104 mutant and the wt exhibit lifetime increases for their major distribution modes from the minimal 205-500 ps range to the maximal 1500-2500 ps range for both the 685 nm and 740 nm bands. The clo f2 mutant, however, exhibits several unique spectral-kinetic properties, attributed to its unique PS I antennae and thylakoid structure, indicating changes in both PS II fluorescence reabsorption and PS II to PS I energy transfer pathways compared to the wt and clo f104. Photoprotective energy dissipation mediated by the xanthophyll cycle pigments and the PsbS protein was uninhibited in the clo f104 mutant but, as commonly reported in the literature, significantly inhibited in the clo f2; the inhibited energy dissipation is partly attributed to its thylakoid structure and PS II to PS I energy transfer properties. It is concluded that it is imperative with steady-state fluorometers, especially for in vivo studies of PS II efficiency or photoprotective energy dissipation, to quantify the influence of the PS I spectral emission.

Long-wavelength chlorophyll forms in Photosystem I from pea thylakoids

Absorption maximum positions of three LW Chl forms in pea chloroplasts were estimated using 77 K excitation spectra of fluorescence detected in their maxima (720, 732 and 746 nm). The 705, 714 and 723 nm components were revealed in the second derivative plots of the excitation spectra. The same maxima were found in normalized excitation spectra obtained with dividing excitation spectra by absorption spectrum. It was confirmed that the observed maxima belong to absorption of LW fluorescing Chl forms. The same maxima were displayed in an action spectrum of P700 oxidation measured at room temperature. It confirms the energy transfer from LW Chl forms to P700. Close to 50% efficiency of bulk Chl forms in both excitation of LW fluorescence and P700 oxidation was found. Analysis of the shape of normalized excitation spectra suggests that there is no energy exchange among LW Chl forms. Their location and physiological role are discussed.

Model studies of chlorophyll fluorescence reabsorption at chloroplast level under different exciting conditions

A theoretical model is presented describing the distortion of chlorophyll fluorescence spectra of a chloroplast or a group of chloroplasts by the effect of fluorescence reabsorption. Model calculations using the experimental data show that the primary reabsorption effect occurs already within one chloroplast and the spectral distortion depends significantly on the excitation regime of the chloroplast. A theoretical dependence of the distortion function, defined as a change in the F(685)/F(735) fluorescence band ratio, on the mean chlorophyll concentration in a chloroplast is predicted for different light excitation regimes. The distortion of measured chlorophyll fluorescence spectra at 77 K of chloroplast suspension adsorbed on filter papers of two strongly different diffusive reflectivities and at different mean chlorophyll concentrations are discussed with the help of the presented theory.

Chlorophyll fluorescence at 77 K in intact leaves: Characterization of a technique to eliminate artifacts related to self-absorption

Due to the optical density of photosynthetic tissues the spectral characteristics of fluorescence emitted at 77 K directly from frozen plant material are distorted by differential re-absorption of the emitted light: the emission band related to PSII can be lowered by more than 80%, relative to the PSI band and the profile of the excitation spectra becomes flattened. It is demonstrated that such distortion cannot be neglected as its extent varies from sample to sample. A technique is introduced to eliminate sample artifacts related to self-absorption: subcellular small particles are prepared from rapidly cooled leaves and then 'diluted' without re-thawing at a concentration corresponding to about 5 μg chlorophyll·cm(-3) into a matrix consisting of ice and quartz particles. The photochemical pigment apparatus is expected to remain fixed in the in vivo state. Different kinds of plant material is used and it is demonstrated how this preparative approach allows to study the in vivo distribution of energy between the two photosystems from pure 77 K spectrofluorimetry, even when the optical properties of whole leaves or thalli normally would exclude quantitative analysis.

Microspectrofluorometry of carcinogens in living cells

A microspectrofluorometric approach has been used to follow the changes undergone by the carcinogen benzo(a)pyrene in malignant L cells, inducible Buffalo rat liver (BRL) cells and oncogenic mouse embryo C3H/10 T 1/2, clone 8 (CCL 226) cells. Since it is known that benzo(a)pyrene (BP) is converted metabolically to at least 40 metabolites, including phenols, epoxides, quinones, dihydrodiols, diol epoxides, and water-soluble conjugates, the interpretation of blue- and red-spectral shifts in fluorescence emission observed in BP-treated cells, compared to the original BP emission, undoubtedly presents considerable difficulties, but a certain number of facts clearly emerge. The sequence of blue-red shifts expressive of intracellular interactions and detoxification of the carcinogen is accelerated in the induced BRL compared to non-induced, and it is also generally accelerated in the malignant and inducible lines compared to the oncogenic line. The detection of highly reactive molecules (? of ultimate carcinogens) representing a small fraction of bulk fluorescence, still remains elusive, but two promising approaches are described: the use of phase-specific fluorescence quenchers which enable us to probe for the presence of metabolites in aqueous, hydrophobic or membrane phases of the cell, and the matrix analysis based on plotting of excitation-emission at different wavelengths for resolution of complex spectra. The former approach has enabled some separation or enhancement of red-blue emissions, and the second has helped to differentiate between emission of BP per se and its intracellular conversion products. Finally, observations at nuclear and cytoplasmic sites open the possibility of studying carcinogen interactions at different target sites.

Connection between the rate of cooling and fluorescence properties at 77 K or isolated chloroplasts

Cooling of chloroplasts to--196 degrees C can under certain circumstances lead to an erroneous analysis of energy distribution. After minimizing influences of sample geometry and effects of plastid concentration it is shown that externally induced membrane change leads to an increase in the ratio F740/F687 of the fluorescence emission spectrum. Similar alterations can be observed by variation of the rate of cooling the plastids to 77 K, expecially if whole chloroplasts are used. The differences in emission ratios are indicative also of changes in initial energy distribution between the photosystems, given here by the value alphaN. This is inferred from experiments with either osmotically induced thylakoid disturbances or those effected through a slow cooling process. The circumstances and the significance of these observations are discussed.

Fluorescence emission spectra of chloroplasts and subchloroplast preparations at low temperature

A study was made of the chlorophyll fluorescence spectra between 100 and 4.2 K of chloroplasts of various species of higher plants (wild strains and chlorophyll b mutants) and of subchloroplast particles enriched in Photosystem I or II. The chloroplast spectra showed the well known emission bands at about 685, 695 and 715--740 nm; the System I and II particles showed bands at about 675, 695 and 720 nm and near 685 nm, respectively. The effect of temperature lowering was similar for chloroplasts and subchloroplast particles; for the long wave bands an increase in intensity occurred mainly between 100 and 50 K, whereas the bands near 685 nm showed a considerable increase in the region of 50--4.2 K. In addition to this we observed an emission band near 680 nm in chloroplasts, the amplitude of which was less dependent on temperature. The band was missing in barley mutant no. 2, which lacks the light-harvesting chlorophyll a/b-protein complex. At 4.7 K the spectra of the variable fluorescence (Fv) consisted mainly of the emission bands near 685 and 695 nm, and showed only little far-red emission and no contribution of the band at 680 nm. From these and other data it is concluded that the emission at 680 nm is due to the light-harvesting complex, and that the bands at 685 and 695 nm are emitted by the System II pigment-protein complex. At 4.2 K, energy transfer from System II to the light-harvesting complex is blocked, but not from the light-harvesting to the System I and System II complexes. The fluorescence yield of the chlorophyll species emitting at 685 nm appears to be directly modulated by the trapping state of the reaction center.

A picosecond pulse train study of exciton dynamics in photosynthetic membranes

The fluorescence decay time of spinach chloroplasts at 77 degrees K was determined at 735 nm (corresponding to the photosystem I emission) using a train of 10-ps laser pulses spaced 10 ns apart. The fluorescence lifetime is constant at congruent to 1.5 ns for up to the fourth pulse, but then decreases with increasing pulse number within the pulse train. This quenching is attributed to triplet excited states, and it is concluded that triplet excitons exhibit a time lag of about 50 ns in diffusing from light harvesting antenna pigments to photosystem I pigments. The diffusion coefficient of triplet excitons is a least 300--400 times slower than the diffusion coefficient of singlet excitons in chloroplast membranes.

Competition between the 735 nm fluorescence and the photochemistry of Photosystem I in chloroplasts at low temperature

Fluorescence emission spectra of chloroplasts, initially frozen to--196 degrees C, were measured at various temperatures as the sample was allowed to warm. The 735 nm emission band attributed to fluorescence from Photosystem I was approx. 10-fold greater at--196 degrees C than at--78 degrees C. The initial rate of photooxidation of P-700 was also measured at--196 degrees C and--78 degrees C and was found to be approximately twice as large at the higher temperature. It is proposed that the 735 nm emission band is fluorescence from a long wavelength form of chlorophyll, C-705, which acts as a trap for excitation energy in the antenna chlorophyl system of Photosystem I. Furthermore, it is proposed that C-705 only forms on cooling to low temperatures and that the temperature dependence of the 735 nm emission is the temperature dependence for the formation of C-705. C-705 and P-700 compete to trap the excitation energy in Photosystem I. It is estimated from the data that at--78 degrees C P-700 traps approx. 20 times more energy than C-705 while, at--196 degrees C, the two traps are approximately equally effective. By analogy, the 695 nm fluorescence which also appears on cooling to--196 degrees C is attributed to traps in Photosystem II which form only on cooling to temperatures near--196 degrees C.

Effects of sodium and magnesium cations on the "dark-" and light-induced chlorophyll a fluorescence yields in sucrose-washed spinach chloroplasts

The effects of Na plus and Mg-2 plus on the "dark" level (O level) and light-induced (P level) fluorescence in sucrose-washed spinach clhoroplasts were studied. Low concentrations of NaCl (2-10 mM) cause a significant decrease in both the O and P levels in the chlorophyll fluorescence transient. The effect on the O level may reflect changes in the bulk chlorophyll a. At 77 degrees K NaCl increases the F735/F685 emission peak ratio in dark-adapted and preilluminated chloroplasts, but has no significant effect on this ratio in sucrose-washed Photosystem II particles. This evidence is consistent with a sodium-induced excitation-energy distribution in favor of Photosystem I. In the presence of MgCl2, with or without NaCl, there is a slight decrease in the O and P level fluorescence as compared with the salt-free control, but an increase as compared with the NaCl-treated sample. Magnesium appears to override the sodium-induced changes. At low temperatures in chloroplasts and Photosystem II particles, MgCl2 has different effects on the F735/F685 ratio apparently depending on the state of the membrane. Magnesium, however, always induces an increase in the F695/F685 ratio. These results suggest that magnesium may influence Photosystem II reaction centers as well as energy distribution between the two photosystems.