Changes in the In Vivo Absorption and Fluorescence Excitation Spectra with Growth Irradiance in Three Species of Phytoplankton

The fluorescence excitation spectrum is sometimes used as aproxy for the action spectrum of photosynthesis in phytoplankton.The main assumption behind this approximation is that the shapesof absorption and fluorescence excitation spectra are similarexcept for the absorption by photoprotective pigments, whichdo not contribute to the fluorescence spectrum. In this study,we compare the shapes of the absorption and fluorescence spectrain three species of phytoplankton grown at differentirradiances:two diatoms (Thalassiosira weissflogii and Chaetoceros sp.)and a cyanophyte (Synechococcus sp.). The contribution to absorptionby photoprotective pigments was estimated for each experiment.Results showed that the differences between the shapes of absorptionand fluorescence spectra were similar to the estimated absorptionby photoprotective pigments only in the case ofT. weissflogii.In Synechococcus sp., and to a lesser degree in Chaetocerossp., the differences between the two types of spectra were largerthan the absorption by photoprotective pigments. In the caseof Synechococcus sp., the difference between these spectra wasapparently due mainly to the extreme imbalance of chlorophylla distribution between the two photosystems. Chaetoceros sp.seemed to be an intermediate case: a small part of the chlorophylla of the cell appeared to be exclusively associated with photosystemI and therefore did not contribute to fluorescence. Fluorescenceand absorption values were normalized to their values at 545nm, and the ratio of normalized absorption to normalized fluorescencewas computed for the blue (439 nm) and red (676 nm) peaks inthe spectra. The results showed that these peak ratios can beused to distinguish between the effects of photoprotective pigmentsand the arrangement of the photosynthetic apparatus on differencesbetweenfluorescence and absorption spectra.     

The fluorescence spectra of red algae and the transfer of energy from phycoerythrin to phycocyanin and chlorophyll

1. The fluorescence spectra of the alga Porphyridium have been recorded as energy distribution curves for eleven different incident wave lengths of monochromatic incident light between wave lengths 405 and 546 mµ. 2. In these spectra chlorophyll fluorescence predominates when the incident light is in the blue part of the spectrum which is strongly absorbed by chlorophyll. 3. For blue-green and green light the spectrum excited in Porphyridium contains in addition to chlorophyll fluorescence, the fluorescence bands characteristic of phycoerythrin and of phycocyanin. 4. From these spectra the approximate curves for the fluorescence of the individual pigments phycoerythrin, phycocyanin, and chlorophyll in the living material have been derived and the relative intensity of each of them has been obtained for each of the eleven incident wave lengths. 5. The effectiveness spectrum for the excitation of the fluorescence of these three pigments in vivo has been plotted. 6. From comparisons of the effectiveness spectrum for the excitation of each of these pigments it appears that both phycocyanin and chlorophyll receive energy from light which is absorbed by phycoerythrin. 7. It is suggested that phycocyanin may be an intermediate in the resonance transfer of energy from phycoerythrin to chlorophyll. 8. Since phycoerythrin and phycocyanin transfer energy to chlorophyll, it appears probable that chlorophyll plays a specific chemical role in photosynthesis in addition to acting as a light absorber.     

Regulation of the distribution of chlorophyll and phycobilin-absorbed excitation energy in cyanobacteria. A structure-based model for the light state transition

The light state transition regulates the distribution of absorbed excitation energy between the two photosystems (PSs) of photosynthesis under varying environmental conditions and/or metabolic demands. In cyanobacteria, there is evidence for the redistribution of energy absorbed by both chlorophyll (Chl) and by phycobilin pigments, and proposed mechanisms differ in the relative involvement of the two pigment types. We assayed changes in the distribution of excitation energy with 77K fluorescence emission spectroscopy determined for excitation of Chl and phycobilin pigments, in both wild-type and state transition-impaired mutant strains of Synechococcus sp. PCC 7002 and Synechocystis sp. PCC 6803. Action spectra for the redistribution of both Chl and phycobilin pigments were very similar in both wild-type cyanobacteria. Both state transition-impaired mutants showed no redistribution of phycobilin-absorbed excitation energy, but retained changes in Chl-absorbed excitation. Action spectra for the Chl-absorbed changes in excitation in the two mutants were similar to each other and to those observed in the two wild types. Our data show that the redistribution of excitation energy absorbed by Chl is independent of the redistribution of excitation energy absorbed by phycobilin pigments and that both changes are triggered by the same environmental light conditions. We present a model for the state transition in cyanobacteria based on the x-ray structures of PSII, PSI, and allophycocyanin consistent with these results.     

Excitation Energy Transfer in Cyanobacteria Synechocystis Embedded in Polymer Film and Partially Bleached by Polarized Irradiation

The polarized absorption, photoacoustic, fluorescence emission, and fluorescence excitation spectra of whole cells of cyanobacteria Synechocystis sp. embedded in a polymer film were measured. The bacteria cells, as it follows from anisotropy of absorption and fluorescence spectra, were even in a non-stretched polyvinyl alcohol film oriented to a certain extent. The measurements were done for such film in order to avoid the deformation of cyanobacteria shapes. Part of the samples was bleached by irradiation with strong polarized radiation with electric vector parallel to the orientation axis of cells. The anisotropy of photoacoustic spectra was higher than that of absorption spectra and it was stronger changed by the irradiation. Polarized fluorescence was excited in four wavelength regions characterised by different contribution to absorption from various bacteria pigments. The shapes of emission spectra were different depending on wavelength of excitation, polarization of radiation, and previous irradiation of the sample. The fluorescence spectra were analysed on Gaussian components belonging to various forms of pigments from photosystems (PS) 1 and 2. The results inform about excitation energy transfer between pools of pigments, differently oriented in the cells. Energy of photons absorbed by phycobilisomes was transferred predominantly to the chlorophyll of PS2, whereas photons absorbed by carotenoids to chlorophylls of PS1.     

Photoadaptation in marine phytoplankton : changes in spectral absorption and excitation of chlorophyll a fluorescence

The optical properties of marine phytoplankton were examined by measuring the absorption spectra and fluorescence excitation spectra of chlorophyll a for natural marine particles collected on glass fiber filters. Samples were collected at different depths from stations in temperate waters of the Southern California Bight and in polar waters of the Scotia and Ross Seas. At all stations, phytoplankton fluorescence excitation and absorption spectra changed systematically with depth and vertical stability of the water columns. In samples from deeper waters, both absorption and chlorophyll a fluorescence excitation spectra showed enhancement in the blue-to-green portion of the spectrum (470-560 nm) relative to that at 440 nm. Since similar changes in absorption and excitation were induced by incubating sea water samples at different light intensities, the changes in optical properties can be attributed to photoadaptation of the phytoplankton. The data indicate that in the natural populations studied, shade adaptation caused increases in the concentration of photosynthetic accessory pigments relative to chlorophyll a. These changes in cellular pigment composition were detectable within less than 1 day. Comparisons of absorption spectra with fluorescence excitation spectra indicate an apparent increase in the efficiency of sensitization of chlorophyll a fluorescence in the blue and green spectral regions for low light populations.     

In vivo fluorescence excitation and absorption spectra of marine phytoplankton: I. Taxonomic characteristics and responses to photoadaptation

Absorption and fluorescence excitation spectra were measuredfor batch cultures of five species of marine phytoplankton grownunder high and low light. These spectra were examined for propertiescharacteristic of taxonomic position and of photoadaptive response.While regions of absorption and excitation of chlorophyll afluorescence diagnostic of pigment composition were identifiable,photoadaptive response had greater influence on spectral variability.Although reduced growth irradiance caused changes in both theabsorption and fluorescence excitation spectra, the fluorescenceexcitation spectrum appears to be more sensitive to alterationsin the ambient light field for growth than does the absorptionspectrum. For a single species. the fluorescence excitationspectrum for a sample grown at low irradiance showed greaterstructure than that for the sample grown at a high irradiance.Under low light conditions, the excitation of chlorophyll afluorescence by accessory pigments increased relative to theexcitation by chlorophyll a itself The highest fluorescenceyields occur in the blue-green region of the spectrum, correspondingto bands of peak absorption by the accessory pigments. Changesin absorption spectra are less marked, but two features recur.First. in the blue-green region of the spectrum from -500–560nm. absorption is enhanced in the low-light cells relative tothat of the high-light cells. Second, the ratio of absorptionat 435 nm to that at 676 nm was greater for the high-light cells.Correlating changes in pigment concentrations were observed.The influence of photoadaptation on the properties of fluorescenceexcitation spectra is as great or greater than the influenceof pigment complements characteristic of specific algal taxa.     

A quantitative description of fluorescence excitation spectra in intact bean leaves greened under intermittent light

We present a simple approach for the calculation of in vivo fluorescence excitation spectra from measured absorbance spectra of the isolated pigments involved. Taking into account shading of the pigments by each other, energy transfer from carotene to chlorophyll a, and light scattering by the leaf tissue, we arrive at a model function with 6 free parameters. Fitting them to the measured fluorescence excitation spectrum yields good correspondence between theory and experiment, and parameter estimates which agree with independent measurements. The results are discussed with respect to the origin and the interpretation of in vivo excitation spectra in general.     

Both spillover and light absorption cross-section changes are involved in the regulation of excitation energy distribution between the two photosystems during state transitions in wheat leaf

Weak red light-induced changes in chlorophyll fluorescence parameters and in the distribution of PS I and PS II in thylakoid membranes were measured in wheat leaves to investigate effective ways to alter the excitation energy distribution between the two photosystems during state transition in vivo. Both the chlorophyll fluorescence parameter Fm/Fo and F685/F735, the ratio of fluorescence yields of the two photosystems at low temperature (77 K), decreased when wheat leaves were illuminated by weak red light of 640 nm, however, Fm/Fo decreased to its minimum in a shorter time than F685/F735. When Photosystem (PS II) thylakoid (BBY) membranes from adequately dark-adapted leaves (control) and from red light-illuminated leaves were subjected to SDS-polyacrylamide gel electrophoresis under mildly denaturing conditions, PS I was almost absent in the control, but was present in the membranes from the leaves preilluminated with the weak red light. In consonance with this result, the content of Cu, measured by means of the energy dispersive X-ray microanalysis (EDX), increased in the central region, but decreased in the margin of the grana stacks from the leaves preilluminated by the red light as compared with the control. It is therefore suggested that: (i) both spillover and absorption cross-section changes are effective ways to alter the excitation energy distribution between the two photosystems during state transitions in vivo, and the change in spillover has a quicker response to the unbalanced light absorption of the two photosystems than the change in light absorption cross-section, and (ii) the migration of PS I towards the central region of grana stack during the transition to state 2 leads to the enhancement of excitation energy spillover from PS II to PS I.     

Carotenoid-triggered energy dissipation in phycobilisomes of Synechocystis sp. PCC 6803 diverts excitation away from reaction centers of both photosystems

Cyanobacteria are capable of using dissipation of phycobilisome-absorbed energy into heat as part of their photoprotective strategy. Non-photochemical quenching in cyanobacteria cells is triggered by absorption of blue-green light by the carotenoid-binding protein, and involves quenching of phycobilisome fluorescence. In this study, we find direct evidence that the quenching is accompanied by a considerable reduction of energy flow to the photosystems. We present light saturation curves of photosystems’ activity in quenched and non-quenched states in the cyanobacterium Synechocystis sp. PCC 6803. In the quenched state, the quantum efficiency of light absorbed by phycobilisomes drops by about 30-40% for both photoreactions—P700 photooxidation in the photosystem II-less strain and photosystem II fluorescence induction in the photosystem I-less strain of Synechocystis. A similar decrease of the excitation pressure on both photosystems leads us to believe that the core-membrane linker allophycocyanin APC-L_CM is at or beyond the point of non-photochemical quenching. We analyze 77 K fluorescence spectra and suggest that the quenching center is formed at the level of the short-wavelength allophycocyanin trimers. It seems that both chlorophyll and APC-L_CM may dissipate excess energy via uphill energy transfer at physiological temperatures, but neither of the two is at the heart of the carotenoid-binding protein-dependent non-photochemical quenching mechanism.     

Relation between the mode of binding of nitrite and the energy distribution between the two photosystems

The reversibility of nitrite-induced inhibition in relation to energy distribution between the two photosystems was studied in spinach thylakoid membranes. Measurements of electron transfer rate catalyzed by photosystem I (PS I) and photosystem II (PS II), chlorophyll a (Chl a ) fluorescence induction kinetics, S₂ state multiline spectra, and room temperature electron paramagnetic resonance (EPR) signals indicated that nitrite anions bind PS II in two ways: dissociable (loose) and non-dissociable (tight). The inhibition caused by the dissociable binding was reversible in washed (nitrite-treated samples washed with nitrite-free medium) samples, while the inhibition caused by the non-dissociable binding was irreversible. At 77 K, an increase in absorption cross section of PS I (as inferred from the excitation spectra of Chl a fluorescence) and a decrease in absorption cross section of PS II in nitrite-treated sample when compared with sample washed with nitrite-free medium and control sample suggested that nitrite plays a role in regulating the distribution of absorbed excitation energy between the two photosystems. We propose, for the first time, that the removal of loosely bound nitrite leads to migration of light-harvesting complex II back to the PS II, and thus the mode of binding of nitrite regulates the extent of migration of antenna molecules between the two photosystems.     

Photochromic pigments in akinetes and pigment characteristics of akinetes in comparison with vegetative cells of Anabaena variabilis

Since akinete germination is triggered by light and the action spectrum for this process has features in common with the spectra of the two photochromic pigments, phycochromes b and d, a search was made for the presence of these phycochromes in akinetes of the blue-green alga. Anabaena variabilis Kützing. Allophycocyanin-B was also looked for, since the action spectrum for akinete germination points to a possible participation of this pigment too. Isoelectric focusing was used for purification of the pigments. The different fractions were investigated for phycochromes b and d by measuring the absorbance difference spectra: for phycochrome b. 500 nm irradiated minus 570 nm irradiated, and for phycochrome d, 650 nm irradiated minus 610 nm irradiated. For determination of allophycocyanin-B. fourth derivative analysis of absorption spectra was made for some of the fractions from the isoelectric focusing column. Phycochrome b was also assayed for by measuring in vivo absorption difference spectra. The assays were positive for all three pigments. The complete photosynthetic pigment systems were also studied by in vivo fluorescence measurements on both akinetes and vegetative cells of Anabaena variabilis. Fluorescence emission and excitation spectra at selected emission wavelengths were measured at room temperature and liquid nitrogen temperature. The energy transfer from phycoerythrocyanin to phycocyanin is very efficient under all conditions, as is the energy transfer from phycocyanin to allophycocyanin at room temperature. At low temperature, however, phycocyanin is partly decoupled from allophycocyanin, particularly in the akinetes; the energy transfer from allophycocyanin to chlorophyll a is less efficient at low temperature in both types of cells, but especially in akinetes. Delayed light emission was measured for both types of cells and found to be very weak in akinetes compared to vegetative cells. From this study it would seem that akinetes lack an active photosystem II, although the 691 nm peak in the 570 nm excited low temperature fluorescence emission spectrum proves the presence of photosystem II chlorophyll, and also its energetic connection to the phycobilisomes.     

Photosynthetic Acclimation to Temperature in the Desert Shrub, Larrea divaricata: II. Light-harvesting Efficiency and Electron Transport

The response of photosynthetic electron transport and light-harvesting efficiency to high temperatures was studied in the desert shrub Larrea divaricata Cav. Plants were grown at day/night temperatures of 20/15, 32/25, or 45/33 C in rough approximation of natural seasonal temperature variations. The process of acclimation to high temperatures involves an enhancement of the stability of the interactions between the light-harvesting pigments and the photosystem reaction centers. As temperature is increased, the heat-induced dissociation of these complexes results in a decrease in the quantum yield of electron transport at limiting light intensity, followed by a loss of electron transport activity at rate-saturating light intensity. The decreased quantum yield can be attributed to a block of excitation energy transfer from chlorophyll b to chlorophyll a, and changes in the distribution of the excitation energy between photosystems II and I. The block of excitation energy transfer is characterized by a loss of the effectiveness of 480 nm light (absorbed primarily by chlorophyll b) to drive protochemical processes, as well as fluorescence emission by chlorophyll b.     

Biooptical characteristics of PSII and PSI in 33 species (13 pigment groups) of marine phytoplankton,and the relevance for pulse‐amplitude‐modulated and fast‐repetition‐rate fluorometry1

We studied the variability of in vivo absorption coefficients and PSII‐scaled fluorescence excitation (fl‐ex) spectra of high light (HL) and low light (LL) acclimated cultures of 33 phytoplankton species that belonged to 13 different pigment groups (PGs) and 10 different phytoplankton classes. By scaling fl‐ex spectra to the corresponding absorption spectra by matching them in the 540–650 nm range, we obtained estimates for the fraction of total chl a that resided in PSII, the absorption of light by PSII, PSI, and photoprotective carotenoids. The in vivo red peak absorption maxima ranged from 673 to 679 nm, reflecting bonding of chl a to different pigment proteins. A simple approach is presented for quantifying intracellular self‐shading and evaluating the impact of photoacclimation on biooptical characteristics of the different PGs examined. In view of these results, parameters used in the calculation of oxygenic photosynthesis based on pulse‐amplitude‐modulated (PAM) and fast‐repetition‐rate (FRR) fluorometers are discussed, showing that the ratio between light available to PSII and total absorption, essential for the calculation of the oxygen release rate (using the PSII‐scaled fluorescence spectrum as a proxy) was dependent on species and photoacclimation state. Three subgroups of chromophytes exhibited 70%–80%, 60%–80%, and 50%–60% chl a in PSII‐LHCII; the two subgroups of chlorophytes, 70% or 80%; and cyanobacteria, only 12%. In contrast, the mean fraction for chromo‐ and chlorophytes of quanta absorbed by PSII was 73% in LL‐ and 55% in HL‐acclimated cells; thus, the corresponding ratios 0.55 and 0.73 might be used as correction factors adjusting for quanta absorbed by PSII for PAM and FRR measurements.     

Spectral analysis on origination of the bands at 437 nm and 475.5 nm of chlorophyll fluorescence excitation spectrum in Arabidopsis chloroplasts

Chlorophyll fluorescence has been often used as an intrinsic optical molecular probe to study photosynthesis. In this study, the origin of bands at 437 and 475.5 nm in the chlorophyll fluorescence excitation spectrum for emission at 685 nm in Arabidopsis chloroplasts was investigated using various optical analysis methods. The results revealed that this fluorescence excitation spectrum was related to the absorption characteristics of pigment molecules in PSII complexes. Moreover, the excitation band centred at 475.5 nm had a blue shift, but the excitation band at 437 nm changed relatively less due to induction of non‐photochemical quenching (NPQ). Furthermore, fluorescence emission spectra showed that this blue shift occurred when excitation energy transfer from both chlorophyll b (Chl b) and carotenoids (Cars) to chlorophyll a (Chl a) was blocked. These results demonstrate that the excitation band at 437 nm was mainly contributed by Chl a, while the excitation band at 475.5 nm was mainly contributed by Chl b and Cars. The chlorophyll fluorescence excitation spectrum, therefore, could serve as a useful tool to describe specific characteristics of light absorption and energy transfer between light‐harvesting pigments. Copyright © 2015 John Wiley & Sons, Ltd.     

Polarized photoacoustic, absorption and fluorescence spectra of chloroplasts and thylakoids oriented in polyvinyl alcohol films

The polarized photoacoustic, absorption and fluorescence spectra of chloroplasts and thylakoids in unstretched and stretched polyvinyl alcohol films were measured. The intensity ratios of fluorescence bands at 674 nm, 700 nm, 730 nm and 750 nm, and the polarized fluorescence excitation spectra are strongly dependent on light polarization and film stretching. In stretched films, thylakoids exhibit predominantly 674 nm emission. The ratio of photoacoustic signal to absorption is different for light polarized parallel and perpendicular to film stretching. This difference is large in the region of chlorophyll a and carotenoids absorption in which the fluorescence excitation spectra are also strongly dependent on light polarization and film stretching. The observed spectral changes are explained by reorientation of pigment molecules influencing the yield of excitation transfer between different pigments.     

Interaction of phycobilisomes with photosystem II dimers and photosystem I monomers and trimers in the cyanobacterium Spirulina platensis.

Distribution of phycobilisomes between photosystem I (PSI) and photosystem II (PSII) complexes in the cyanobacterium Spirulina platensis has been studied by analysis of the action spectra of H2 and O2 photoevolution and by analysis of the 77 K fluorescence excitation and emission spectra of the photosystems. PSI monomers and trimers were spectrally discriminated in the cell by the unique 760 nm low-temperature fluorescence, emitted by the trimers under reductive conditions. The phycobilisome-specific 625 nm peak was observed in the action spectra of both PSI and PSII, as well as in the 77 K fluorescence excitation spectra for chlorophyll emission at 695 nm (PSII), 730 nm (PSI monomers), and 760 nm (PSI trimers). The contributions of phycobilisomes to the absorption, action, and excitation spectra were derived from the in vivo absorption coefficients of phycobiliproteins and of chlorophyll. Analyzing the sum of PSI and PSII action spectra against the absorption spectrum and estimating the P700:P680 reaction center ratio of 5.7 in Spirulina, we calculated that PSII contained only 5% of the total chlorophyll, while PSI carried the greatest part, about 95%. Quantitative analysis of the obtained data showed that about 20% of phycobilisomes in Spirulina cells are bound to PSII, while 60% of phycobilisomes transfer the energy to PSI trimers, and the remaining 20% are associated with PSI monomers. A relevant model of organization of phycobilisomes and chlorophyll pigment-protein complexes in Spirulina is proposed. It is suggested that phycobilisomes are connected with PSII dimers, PSI trimers, and coupled PSI monomers.     

发状念珠藻藻殖段的分化及其光合特性的研究

Hormogonia of Nostoc flagelliforme is one of the developmental stages in the life cycle of cyanobacterium. High yields of pure hormogonia were obtained by weak light (the filaments were covered by sterilized sand for blocking light), red light, white light plus DCMU (3, 4-dichlorophenyl-1, 1-dimethylurea) in the culture. These pure fractions of hormogonia allowed the study of physiological measurements in comparison to vegetative filaments. The photosynthesis in the hormogonia and the vegetative filaments was characterized by fluorescence emission spectra at 77 K, absorption spectrum and oxygen evolution. Absorption spectrum of the hormogoia and vegetative filaments did not reveal difference. The data indicated the similarity of pigment contents between hormogonia and vegetative filaments. Some differences were observed in oxygen evolution of vegetative filaments and hormogonia in the temperature range of 15 ℃ to 45 ℃ and light intensity around 110 μmol·m-2·s-1 to 1200 μmol·m-2·s-1. The fluorescence emission spectra showed that energy distribution between the two photosystems in mature colonies was more balance than in hormogonia. The absorption of light energy in phycobilisomes and the transfer to the two photosystems in the hormogonia were more effective than in the mature colonies. It may be concluded that the formation of hormogonia affected on the structure and function of phytosynthesis.     

Appearance of a State 1 – State 2 transition during chloroplast development in the wheat leaf: Energetic and structural considerations

The ability of developing chloroplasts to dynamically regulate the distribution of excitation energy between photosystem 1 and photosystem 2, and thus perform a State 1 – State 2 transition, was examined from analyses of chlorophyll fluorescence kinetics in 4- and 8-day-old Triticum aestivum L. cv. Maris Dove leaves grown under a diurnal light regime. Chloroplasts at all stages of development in the two leaf systems could undergo a State 1 – State 2 transition, except those found in the basal 0.5 cm of the 4-day-old leaf. The ability to physiologically modify the excitation energy distribution between the chlorophyll matrices of the two photosystems developed after the development of mature, fully photochemically competent photosystem 2 units and the appearance of excitation energy transfer between photosystem 2 and photosystem 1. Also, changes in the degree of energetic interaction between the two photosystems, in vivo rates of electron transport and the chlorophyll a/b ratio could not be correlated with the appearance of a State 1 – State 2 transition. Ultrastructural studies demonstrated a 32% increase in the degree of thylakoid appression in chloroplasts at the base of the 8-day-old leaf compared to the situation in the basal 0.5 cm of the 4-day-old leaf. This difference in thylakoid stacking can account for the differing abilities of these two tissues to perform a State 1 – State 2 transition when considered in the context of the distribution of the two photosystems within appressed and non-appressed regions of thylakoid membranes.     

Nitrite regulates distribution of excitation energy between the two photosystems by causing state transition.

The mechanism of distribution of absorbed excitation energy between the two photosystems in the presence of nitrite has been investigated in spinach (Spinacia oleracea L.) thylakoid membranes. Nitrite inhibited PS II activity (H(2)O --> DCPIP reaction) and enhanced PS I activity (DCPIPH(2) --> MV reaction). Nitrite decreased the F(v)/F(m) ratio measured at room temperature and increased the F(730)/F(685) ratio measured at low temperature (77 K). These results suggested that nitrite caused a decrease in the excitation energy available to PS II and transferred more energy to PS I by the mechanism of state transition. Measurement of fluorescence excitation spectra at 77 K showed that nitrite increased the absorption cross-section of PS I antenna at the expense of chlorophyll b and LHC II. Based on these observations we have suggested a role of nitrite in causing state transition.     

Seasonal changes of excitation energy transfer and thylakoid stacking in the evergreen tree Taxus cuspidata: How does it divert excess energy from photosynthetic reaction center?

Photosystems must efficiently dissipate absorbed light energy under freezing conditions. To clarify the energy dissipation mechanisms, we examined energy transfer and dissipation dynamics in needles of the evergreen plant Taxus cuspidata by time-resolved fluorescence spectroscopy. In summer and autumn, the energy transfer processes were similar to those reported in other higher plants. However, in winter needles, fluorescence lifetimes became shorter not only in PSII but also in PSI, indicating energy dissipation in winter needles. In addition, almost the same fluorescence spectra were obtained with different excitation wavelengths. In contrast, the fluorescence spectrum showed a large difference due to excitation wavelength in spring needles. The fluorescence spectrum of spring needles in 550-nm excitation showed similar spectra to that of winter needles, however, red-chlorophyll fluorescence was not observed in chlorophyll excitation. These observations suggest that some complexes with some kind of red-shifted carotenoid and red-chlorophyll unlink from the core complex in spring. Seasonal changes of excitation energy dynamics are also discussed in relation to changes in thylakoid stacking.