Cyanobacterial Acclimation to Photosystem I or Photosystem II Light

The organization and function of the photochemical apparatus of Synechococcus 6301 was investigated in cells grown under yellow and red light regimes. Broadband yellow illumination is absorbed preferentially by the phycobilisome (PBS) whereas red light is absorbed primarily by the chlorophyll (Chl) pigment beds. Since PBSs are associated exclusively with photosystem II (PSII) and most of the Chl with photosystem I (PSI), it follows that yellow and red light regimes will create an imbalance of light absorption by the two photosystems. The cause and effect relationship between light quality and photosystem stoichiometry in Synechococcus was investigated. Cells grown under red light compensated for the excitation imbalance by synthesis/assembly of more PBS-PSII complexes resulting in high PSII/PSI = 0.71 and high bilin/Chl = 1.30. The adjustment of the photosystem stoichiometry in red light-grown cells was necessary and sufficient to establish an overall balanced absorption of red light by PSII and PSI. Cells grown under yellow light compensated for this excitation imbalance by assembly of more PSI complexes, resulting in low PSII/PSI = 0.27 and low bilin/Chl = 0.42. This adjustment of the photosystem stoichiometry in yellow light-grown cells was necessary but not quite sufficient to balance the absorption of yellow light by the PBS and the Chl pigment beds. A novel excitation quenching process was identified in yellow light-grown cells which dissipated approximately 40% of the PBS excitation, thus preventing over-excitation of PSII under yellow light conditions. It is hypothesized that State transitions in O₂ evolving photosynthetic organisms may serve as the signal for change in the stoichiometry of photochemical complexes in response to light quality conditions.     

The effect of norflurazon on protein composition and chlorophyll organization in pigment–protein complex of Photosystem II

The pyridazinone-type herbicide norflurazon SAN 9789 inhibiting the biosynthesis of long-chain carotenoids results in significant decrease in PS II core complexes and content of light-harvesting complex (LHC) polypeptides in the 29.5–21 kDa region. The Chl a forms at 668, 676, and 690 nm that belong to LHC and antenna part of PS I disappear completely after treatment. The intensity of the Chl b form at 648 nm is sharply decreased in treated seedlings grown under 30 or 100 lx light intensity. The bands of carotenoid absorption at 421, 448 (Chl a), 452, 480, 492, 496 (β-carotene), and 508 nm also disappear. The band shift from 740 to 720 nm and decrease in its intensity relative to the 687 nm emission peak in the low-temperature fluorescence spectrum (77 K) suggests a disturbance of energy transfer from LHC to the Chla form at 710–712 nm.     

Assembly of Pigment–Protein Complexes in Carotenoid-Deficient Membranes of Plastids from Wheat Seedlings Treated with Norflurazon

The pyridazinone-type herbicide norflurazon SAN 9789 inhibiting the biosynthesis of long-chain carotenoids results in significant decrease in PS II core complexes and content of light-harvesting complex (LHC) polypeptides. At the same time, early light-induced proteins (ELIP) with molecular masses of 20.5-16.5 and 13.5 kD disappear in norflurazon-treated seedlings grown under intermittent (pulsed) light, confirming the hypothesis that they are carotenoid-binding proteins. Full disappearance of Chl a forms at 668, 676, and 690 nm and a sharp decrease in Chl b form at 648 nm in treated seedlings grown under 30 or 100 lx light intensity shows close contact of these forms with carotenoids in the thylakoid membrane. The band shift from 740 to 720 nm in the low-temperature fluorescence spectrum (77 K) suggests a disturbance of energy transfer from LHC to the Chl a form at 710-712 nm.     

Effects of Chromatic Adaptation on the Photochemical Apparatus of Photosynthesis in Porphyridium cruentum

Cells of Porphyridium cruentum were grown in different colors of light which would be absorbed primarily by chlorophyll (Chl) (red and blue light) or by the phycobilisomes (green or two intensities of cool-white fluorescent light), and samples of these cells were frozen to −196 C for measurements of absorption and fluorescence emission spectra. Cells grown in the high intensity white light had least of all of the photosynthetic pigments, a higher ratio of carotenoid/Chl, but essentially the same ratio of phycobilin to Chl as cells grown in the low intensity white light. The ratio of photosystem II (PSII) to photosystem I (PSI) pigments was affected by light quality; the ratios of phycobilin to Chl and of short wavelength (PSII) Chl to long wavelength (PSI) Chl were both greater in the cells grown in red or blue light.     

Interaction of exogenous quinones with membranes of higher plant chloroplasts: modulation of quinone capacities as photochemical and non-photochemical quenchers of energy in Photosystem II during light-dark transitions

Light modulation of the ability of three artificial quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duroquinone), to quench chlorophyll (Chl) fluorescence photochemically or non-photochemically was studied to simulate the functions of endogenous plastoquinones during the thermal phase of fast Chl fluorescence induction kinetics. DBMIB was found to suppress by severalfold the basal level of Chl fluorescence (F(o)) and to markedly retard the light-induced rise of variable fluorescence (F(v)). After irradiation with actinic light, Chl fluorescence rapidly dropped down to the level corresponding to F(o) level in untreated thylakoids and then slowly declined to the initial level. DBMIB was found to be an efficient photochemical quencher of energy in Photosystem II (PSII) in the dark, but not after prolonged irradiation. Those events were owing to DBMIB reduction under light and its oxidation in the dark. At high concentrations, DCBQ exhibited quenching behaviours similar to those of DBMIB. In contrast, duroquinone demonstrated the ability to quench F(v) at low concentration, while F(o) was declined only at high concentrations of this artificial quinone. Unlike for DBMIB and DCBQ, quenched F(o) level was attained rapidly after actinic light had been turned off in the presence of high duroquinone concentrations. That finding evidenced that the capacity of duroquinone to non-photochemically quench excitation energy in PSII was maintained during irradiation, which is likely owing to the rapid electron transfer from duroquinol to Photosystem I (PSI). It was suggested that DBMIB and DCBQ at high concentration, on the one hand, and duroquinone, on the other hand, mimic the properties of plastoquinones as photochemical and non-photochemical quenchers of energy in PSII under different conditions. The first model corresponds to the conditions under which the plastoquinone pool can be largely reduced (weak electron release from PSII to PSI compared to PSII-driven electron flow from water under strong light and weak PSI photochemical capacity because of inactive electron transport on its reducing side), while the second one mimics the behaviour of the plastoquinone pool when it cannot be filled up with electrons (weak or moderate light and high photochemical competence of PSI).     

The development of antenna complexes of barley (Hordeum vulgare cv. Akcent) under different light conditions as judged from the analysis of 77 K chlorophyll a fluorescence spectra

Using 77 K chlorophyll a (Chl a) fluorescence spectra in vivo, the development was studied of Photosystems II (PS II) and I (PS I) during greening of barley under intermittent light followed by continuous light at low (LI, 50 μmol m⁻² s⁻¹) and high (HI, 1000 μmol m⁻² s⁻¹) irradiances. The greening at HI intermittent light was accompanied with significantly reduced fluorescence intensity from Chl b excitation for both PS II (F685) and PS I (F743), in comparison with LI plants, indicating that assembly of light-harvesting complexes (LHC) of both photosystems was affected to a similar degree. During greening at continuous HI, a slower increase of emission from Chl b excitation in PS II as compared with PS I was observed, indicating a preferred reduction in the accumulation of LHC II. The following characteristics of 77 K Chl a fluorescence spectra documented the photoprotective function of an elevated content of carotenoids in HI leaves: (1) a pronounced suppression of Soret region of excitation spectra (410–450 nm) in comparison with the red region (670–690 nm) during the early stage of greening indicated a strongly reduced excitation energy transfer from carotenoids to the Chl a fluorescing forms within PS I and PS II; (2) changes in the shape of the excitation band of Chl b and carotenoids (460–490 nm) during greening under continuous light confirmed that the energy transfer from carotenoids to Chl a within PS II remained lower as compared with the LI plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

A model for the 77 K excited state dynamics in Chlamydomonas reinhardtii in state 1 and state 2

The regulatory mechanism of state transitions was studied in Chlamydomonas reinhardtii (C.r.) wild type (WT) as well as mutant strains deficient in the photosystem I (PSI) or the photosystem II (PSII) core. Time-resolved fluorescence measurements were obtained on instantly frozen cells incubated beforehand in the dark in aerobic or anaerobic conditions which leads to state 1 (S1) or state 2 (S2). WT data contains information on the light-harvesting complex (LHC) connected to PSI and PSII. The mutants' data contain information on either LHCII-LHCI-PSI or LHCII-PSII, plus information on LHC antennas devoid of a PS core. In a simultaneous analysis of the data from all strains under S1 or S2 conditions a unified model for the excited state dynamics at 77 K was created. This yielded the completely resolved LHCII-LHCI-PSI and LHCII-PSII dynamics and quantified the state transitions. In WT cells the fraction of light absorbed by LHCII connected to PSII decreases from 45% in S1 to 29% in S2, while it increases from 0% to 16% for LHCII connected to PSI. Thus (16/45 =) 36% of all LHCII is involved in the state transition. In the mutant strains deficient in the PSI core, the red most species peaking at 716 nm disappears completely, indicating that this far red Chl pigment is located in the PSI core. In the mutant strain deficient in the PSII core, red shifted species with maxima at 684 and 686 nm appear in the LHCII antenna. LHCII-684 is quenched and decays with a rate of (310 ps)^? 1.     

Effects of different excitation and detection spectral regions on room temperature chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence parameters

The effects of different spectral region of excitation and detection of chlorophyll (Chl) a fluorescence at room temperature on the estimation of excitation energy utilization within photosystem (PS) 2 were studied in wild-type barley (Hordeum vulgare L. cv. Bonus) and its Chl b-less mutant chlorina f2 grown under low and high irradiances [100 and 1 000 μmol(photon) m⁻² s⁻¹]. Three measuring spectral regimes were applied using a PAM 101 fluorometer: (1) excitation in the red region (maximum at the wavelength of 649 nm) and detection in the far-red region beyond 710 nm, (2) excitation in the blue region (maximum at the wavelength of 461 nm) and detection beyond 710 nm, and (3) excitation in the blue region and detection in the red region (660– 710 nm). Non-photochemical quenching of maximal (NPQ) and minimal fluorescence (SV₀), determined by detecting Chl a fluorescence beyond 710 nm, were significantly higher for blue excitation as compared to red excitation. We suggest that this results from higher non-radiative dissipation of absorbed excitation energy within light-harvesting complexes of PS2 (LHC2) due to preferential excitation of LHC2 by blue radiation and from the lower contribution of PS1 emission to the detected fluorescence in the case of blue excitation. Detection of Chl a fluorescence originating preferentially from PS2 (i.e. in the range of 660–710 nm) led to pronounced increase of NPQ, SV₀, and the PS2 photochemical efficiencies (F_V/F_M and F_V′/F_M′), indicating considerable underestimation of these parameters using the standard set-up of PAM 101. Hence PS1 contribution to the minimal fluorescence level in the irradiance-adapted state may reach up to about 80 %.     

Heat stress induces an inhibition of excitation energy transfer from phycobilisomes to photosystem II but not to photosystem I in a cyanobacterium Spirulina platensis.

The effects of high temperature (30-52.5 degrees C) on excitation energy transfer from phycobilisomes (PBS) to photosystem I (PSI) and photosystem II (PSII) in a cyanobacterium Spirulina platensis grown at 30 degrees C were studied by measuring 77 K chlorophyll (Chl) fluorescence emission spectra. Heat stress had a significant effect on 77 K Chl fluorescence emission spectra excited either at 436 or 580 nm. In order to reveal what parts of the photosynthetic apparatus were responsible for the changes in the related Chl fluorescence emission peaks, we fitted the emission spectra by Gaussian components according to the assignments of emission bands to different components of the photosynthetic apparatus. The 643 and 664 nm emissions originate from C-phycocyanin (CPC) and allophycocyanin (APC), respectively. The 685 and 695 nm emissions originate mainly from the core antenna complexes of PSII, CP43 and CP47, respectively. The 725 and 751 nm band is most effectively produced by PSI. There was no significant change in F725 and F751 during heat stress, suggesting that heat stress had no effects on excitation energy transfer from PBS to PSI. On the other hand, heat stress induced an increase in the ratio of Chl fluorescence yield of PBS to PSII, indicating that heat stress inhibits excitation energy transfer from PBS to PSII. However, this inhibition was not associated with an inhibition of excitation energy transfer from CPC to APC since no significant changes in F643 occurred at high temperatures. A dramatic enhancement of F664 occurring at 52.5 degrees C indicates that excitation energy transfer from APC to the PSII core complexes is suppressed at this temperature, possibly due to the structural changes within the PBS core but not to a detachment of PBS from PSII, resulting in an inhibition of excitation energy transfer from APC to PSII core complexes (CP47 + CP43). A decrease in F685 and F695 in heat-stressed cells with excitation at 436 nm seems to suggest that heat stress did not inhibit excitation energy transfer from the Chl a binding proteins CP47 and CP43 to the PSII reaction center and the decreased Chl fluorescence yields from CP43 and CP47 could be explained by the inhibition of the energy transfer from APC to PSII core complexes (CP47 + CP43).     

Red antenna states of photosystem I from Synechocystis PCC 6803

Single-molecule spectroscopy at low temperatures was used to elucidate spectral properties, heterogeneities, and dynamics of the red-shifted chlorophyll a (Chl a) molecules responsible for the fluorescence from photosystem I (PSI). Emission spectra of single PSI complexes from the cyanobacterium Synechocystis PCC 6803 show zero-phonon lines (ZPLs) as well as broad intensity distributions without ZPLs. ZPLs are found most frequently on the blue side of the broad intensity distributions. The abundance of ZPLs decreases almost linearly at longer wavelengths. The distribution of ZPLs indicates the existence of at least two pools with maxima at 699 and 710 nm. The pool with the maximum at 710 nm is assigned to chlorophylls absorbing around 706 nm (C706), whereas the pool with the maximum at 699 nm (F699) can be assigned to chlorophylls absorbing at 692, 695, or 699 nm. The broad distributions dominating the red side of the spectra are made up of a low number of emitters assigned to the red-most pool C714. The properties of F699 show close relation to those of F698 in Synechococcus PCC 7002 and C708 in Thermosynechococcus elongatus. Furthermore, a high similarity is found between the C714 pool in Synechocystis PCC 6803 and C708 in Synechococcus PCC 7002 as well as C719 in T. elongatus.     

Effects of Light on Chlorophyll Formation in Cultured Tobacco Cells I. Chlorophyll Accumulation and Phototransformation of Protochlorophyll(ide) in Callus Cells under Blue and Red light

A marked accumulation of chlorophyll was observed in calluscells of Nicotiana glutinosa when they were grown under bluelight, while under strong red light no chlorophyll accumulated.This blue light effect saturated at an intensity of about 500mW.m^–2. The effects of white, blue and red light on the transformationof protochlorophyll (ide) (Pchl) accumulated in dark-grown calluscells were studied by following the changes in the intensityof fluorescence emitted by Pchl and different forms of chlorophyll(ide) (Chi). Pchl with a fluorescence maximum at 633 nm (absorptionmaximum: 630 nm) decreased slowly, concomitant with an increasein Chl having a fluorescence maximum at 677 nm (absorption maximum:675 nm), which was subsequently transformed, independently oflight, to Chi with a fluorescence maximum at 683 nm (absorptionmaximum: 680 nm). Both blue and red light of low intensitieswere effective for the phototransformation, while red light,but not blue light, of high intensities caused significant destructionof Pchl. An action spectrum for this photodestruction showedthat the maximum destruction took place at 630 nm. White lightof high intensities was effective for the photoreduction withonly slight destruction of Pchl, suggesting that blue lightcounteracts the destructive effect of red light. At low temperatures,however, blue light as well as red light of low intensitiescaused photodestruction of Pchl. It was inferred that blue lightenhances a certain step or steps involved in the productionof a reductant required for the photoreduction of Pchl to Chl. (Received July 3, 1981; Accepted November 11, 1981)     

Far‐red light enhances photochemical efficiency in a wavelength‐dependent manner

Linear electron transport depends on balanced excitation of photosystem I and II. Far‐red light preferentially excites photosystem I (PSI) and can enhance the photosynthetic efficiency when combined with light that over‐excites photosystem II (PSII). The efficiency of different wavelengths of far‐red light exciting PSI was quantified by measuring the change in quantum yield of PSII (Φ_PSII) of lettuce (Lactuca sativa) under red/blue light with narrowband far‐red light added (from 678 to 752 nm, obtained using laser diodes). The Φ_PSII of lettuce increased with increasing wavelengths of added light from 678 to 703 nm, indicating longer wavelengths within this region are increasingly used more efficiently by PSI than by PSII. Adding 721 nm light resulted in similar Φ_PSII as adding 703 nm light, but Φ_PSII tended to decrease as wavelength increased from 721 to 731 nm, likely due to decreasing absorptance and low photon energy. Adding 752 nm light did not affect Φ_PSII. Leaf chlorophyll fluorescence light response measurements showed lettuce had higher Φ_PSII under halogen light (rich in far‐red) than under red/blue light (which over‐excites PSII). Far‐red light is more photosynthetically active than commonly believed, because of its synergistic interaction with light of shorter wavelengths.     

Comparison of chlorophyll a spectra in wild-type and mutant barley chloroplasts grown under day or intermittent light

The absorption (640–710 nm) and fluorescence emission (670–710 nm) spectra (77 K) of wild-type and Chl b-less, mutant, barley chloroplasts grown under either day or intermittent light were analysed by a RESOL curve-fitting program. The usual four major forms of Chl a at 662, 670, 678 and 684 nm were evident in all of the absorption spectra and three major components at 686, 693 and 704 nm in the emission spectra. A broad Chl a component band at 651 nm most likely exists in all chlorophyll spectra in vivo. The results show that the mutant lacks not only Chl b, but also the Chl a molecules which are bound to the light-harvesting, Chl a/b, protein complex of normal plants. It also appears that the absorption spectrum of this antenna complex is not modified appreciably by its isolation from thylakoid membranes.Abbreviations Chl chlorophyll - DL daylight - ImL intermittent light - WT wildtype - LHC light-harvesting Chl a/b protein complex - S.E. standard error of the mean DBP-CIW No. 763.     

Photoinhibition of photosystem II in vitro: Spectral and kinetic analyses

Two different preparations of photosystem II (PSII) (BBY-type membrane fragments and PSII core complexes) were isolated from 14-day-old pea seedlings (Pisum sativum L.) and used for spectral and kinetic study of photobleaching of chlorophyll (Chl) and amino acids under photoinhibitory conditions. A short-term (2–4 min) illumination of PSII preparations with high-intensity red light (λ > 610 nm, 800 W/m²) resulted in irreversible photobleaching of Chl at 672 and 682 nm under conditions of both acceptor- and donor-side photoinhibition. At longer illumination exposures (> 10 min) the photobleaching maximum at 682 nm was predominant. The calculated kinetic constants for Chl photobleaching in both absorption bands at temperatures of 20 and 4°C had similar values under different photoinhibitory conditions. The shape of action spectrum for Chl photooxidation indicates that photoinhibition of PSII was sensitized by two spectral forms of Chl with absorption maxima at 670 and 680 nm. The photobleaching of amino acids in PSII membrane fragments was only observed during acceptor-side photoinhibition and displayed the photobleaching peaks at 220 and 274 nm. The photogeneration of superoxide anion radical during donor-side photoinhibition was 4–6 times larger than during acceptor-side photoinhibition. Nevertheless, the kinetics of Chl and amino acid photobleaching in PSII preparations showed no appreciable differences. The activation energies for Chl photooxidation were estimated around 3.5 and 9 kcal/mol during acceptor- and donor-side photoinhibition, respectively, providing evidence for the involvement of biochemical stages in PSII photoinhibition. Based on the data obtained, it is proposed that the antenna Chl, rather than Chl of the reaction center, is the sensitizer for both acceptor- and donor-side photoinhibition of PSII in vitro.     

Chromatic variation of the abundance of PSII complexes observed with the red alga Prophyridium cruentum.

Chromatic regulation of photosystem stoichiometry in cyanophytes, green algae and probably vascular plants is achieved by regulation of the abundance of PSI in response to thylakoid electron transport state at least under our experimental conditions [cf. Fujita (1997) Photosyn. Res. 53: 83]. However, variation of not only PSI but also PSII, in reverse of each other, is characteristic of the stoichiometry regulation in red algae and some of marine cyanophytes. Our previous study with the red alga Porphyridium cruentum has revealed that PSII is inactivated by 50% upon a light shift from the light absorbed by Chl a, PSI light, to that mainly absorbed by phycobilisomes (PBS), PSII light [Fujita (1999) Plant Cell Physiol. 40: 924]. To evaluate the contribution of the photoinactivation to the chromatic variation of PSII, variation of the abundance of PSI, PSII and PBS, together with the fluorescence parameter and the activity of PSII, was followed after a light shift from PSI light to PSII light. Upon a light shift to PSII light, PSII, determined as Cyt b(559) per PBS, decreased rapidly, following the photoinactivation, down to the level a half of that before the light shift, and remained constant. Since the increase in PBS was not significant during this period, a rapid decrease of PSII/PBS led us to tentatively conclude that the degradation of PSII is a main cause for variation of the abundance of PSII. Photoinactivation of PSII, and also decrease in Cyt b(559), was accelerated, but only slightly, by the addition of chloramphenicol (CAP) at a moderate concentration while CAP at the same concentration significantly suppressed the increment of PSI determined as P700. A selective effect of CAP supports the above conclusion.     

Organization of the pigment molecules in the chlorophyll a/b/c containing alga Mantoniella squamata (Prasinophyceae) studied by means of absorption, circular and linear dichroism spectroscopy

In order to obtain information on the organization of the pigment molecules in chlorophyll (Chl) a/b/c-containing organisms, we have carried out circular dichroism (CD), linear dichroism (LD) and absorption spectroscopic measurements on intact cells, isolated thylakoids and purified light-harvesting complexes (LHCs) of the prasinophycean alga Mantoniella squamata. The CD spectra of the intact cells and isolated thylakoids were predominated by the excitonic bands of the Chl a/b/c LHC. However, some anomalous bands indicated the existence of chiral macrodomains, which could be correlated with the multilayered membrane system in the intact cells. In the red, the thylakoid membranes and the LHC exhibited a well-discernible CD band originating from Chl c, but otherwise the CD spectra were similar to that of non-aggregated LHC II, the main Chl a/b LHC in higher plants. In the Soret region, however, an unusually intense (+) 441 nm band was observed, which was accompanied by negative bands between 465 and 510 nm. It is proposed that these bands originate from intense excitonic interactions between Chl a and carotenoid molecules. LD measurements revealed that the Q(Y) dipoles of Chl a in Mantoniella thylakoids are preferentially oriented in the plane of the membrane, with orientation angles tilting out more at shorter than at longer wavelengths (9 degrees at 677 nm, 20 degrees at 670 nm and 26 degrees at 662 nm); the Q(Y) dipole of Chl c was found to be oriented at 29 degrees with respect to the membrane plane. These data and the LD spectrum of the LHC, apart from the presence of Chl c, suggest an orientation pattern of dipoles similar to those of higher plant thylakoids and LHC II. However, the tendency of the Q(Y) dipoles of Chl b to lie preferentially in the plane of the membrane (23 degrees at 653 nm and 30 degrees at 646 nm) is markedly different from the orientation pattern in higher plant membranes and LHC II. Hence, our CD and LD data show that the molecular organization of the Chl a/b/c LHC, despite evident similarities, differs significantly from that of LHC II.     

Light-dependent reversal of dark-chilling induced changes in chloroplast structure and arrangement of chlorophyll-protein complexes in bean thylakoid membranes

Changes in chloroplast structure and rearrangement of chlorophyll-protein (CP) complexes were investigated in detached leaves of bean (Phaseolus vulgaris L. cv. Eureka), a chilling-sensitive plant, during 5-day dark-chilling at 1 degrees C and subsequent 3-h photoactivation under white light (200 mumol photons m(-2) s(-1)) at 22 degrees C. Although, no change in chlorophyll (Chl) content and Chl a/b ratio in all samples was observed, overall fluorescence intensity of fluorescence emission and excitation spectra of thylakoid membranes isolated from dark-chilled leaves decreased to about 50%, and remained after photoactivation at 70% of that of the control sample. Concomitantly, the ratio between fluorescence intensities of PSI and PSII (F736/F681) at 120 K increased 1.5-fold upon chilling, and was fully reversed after photoactivation. Moreover, chilling stress seems to induce a decrease of the relative contribution of LHCII fluorescence to the thylakoid emission spectra at 120 K, and an increase of that from LHCI and PSI, correlated with a decrease of stability of LHCI-PSI and LHCII trimers, shown by mild-denaturing electrophoresis. These effects were reversed to a large extent after photoactivation, with the exception of LHCII, which remained partly in the aggregated form. In view of these data, it is likely that dark-chilling stress induces partial disassembly of CP complexes, not completely restorable upon photoactivation. These data are further supported by confocal laser scanning fluorescence microscopy, which showed that regular grana arrangement observed in chloroplasts isolated from control leaves was destroyed by dark-chilling stress, and was partially reconstructed after photoactivation. In line with this, Chl a fluorescence spectra of leaf discs demonstrated that dark-chilling caused a decrease of the quantum yield PSII photochemistry (F(v)/F(m)) by almost 40% in 5 days. Complete restoration of the photochemical activity of PSII required 9 h post-chilling photoactivation, while only 3 h were needed to reconstruct thylakoid membrane organization and chloroplast structure. The latter demonstrated that the long-term dark-chilled bean leaves started to suffer from photoinhibition after transfer to moderate irradiance and temperature conditions, delaying the recovery of PSII photochemistry, independently of photo-induced reconstruction of PSII complexes.     

The Accumulation of Protochlorophyllide in Cells of Synechocystis PCC 6714 with a Low PSI/PSII Stoichiometry

Changes in intracellular levels of Chl a precursors were examinedin relation to changes in the PSI/PSII stoichiometry in thecyanophyte Synechocystis PCC 6714. Protochlorophyllide (Pchlide)accumulated markedly in cells with a low PSI/PSII stoichiometrygrown under light that is absorbed by Chl a (PSI light) whereasno accumulation occurred in cells with a high PSI/PSII stoichiometrygrown under light absorbed by phycobilisomes (PSII light). Levelsof Pchlide in cells grown under PSI light decreased rapidlyupon a shift to PSII light. The rapid decrease in Pchlide accompanieda transient increase in chlorophyllide a, indicating that reductionof Pchlide was enhanced by shift to PSII light. The action spectrumindicated that the Pchlide decrease upon the shift to PSII lightdepended on excitation of Pchlide, suggesting that the accumulationof Pchllide was due to limited excitation of Pchlide, so thatPchlide photoreduction, under PSI light. However, comparisonof levels of Pchlide and the photosystem complexes in wild-typePlectonema boryanum with those in a mutant that lacked the darkPchlide reductase (YFC 1004) indicated that dark reduction compensatedfor the limited photoreduction under PSI light. Similar compensationby dark reduction was confirmed with Synechocystis PCC 6714.In cultures of Synechocystis under conditions where Pchlidecould not be photoreduced, accumulation of Pchlide and low PSI/PSIIstoichiometry occurred only when cells were illuminated withlight that preferentially excited PSI. The results indicatethat the low PSI/PSII stoichiometry in cells grown under PSIlight is not a result of inefficient synthesis of Chl a witha reduced rate of Pchlide photoreduction. They suggest furtherthat accumulation of Pchlide under PSI light results from retardationof the Chl a synthesis due to suppression of PSI synthesis. ¹Present address: Tsurukawa 5-15-11, Machida, Tokyo, 195 Japan.     

Structure and composition of the photochemical apparatus of the antarctic green alga,Chlamydomonas subcaudata

The green alga, Chlamydomonas subcaudata, collected from a perennially ice-covered Antarctic lake, was able to grow at temperatures of 16°C or lower, but not at temperatures of 20°C or higher, which confirmed its psychrophilic nature. Low temperature (77 K) Chl a fluorescence emission spectra of whole cells of the mesophile, C. reinhardtii, indicated the presence of major emission bands at 681 and 709 nm associated with PS II and PS I, respectively. In contrast, emission spectra of whole cells of C. subcaudata exhibited major emission bands at 681 and 692 nm associated with PS II, but the absence of a major PS I emission band at 709 nm. These results for C. subcaudata were consistent with: (1) low ratio of Chl a/b (1.80); (2) low levels of PsaA/PsaB heterodimer as well as specific Lhca polypeptides as determined by immunoblotting, (3) decreased levels of the Chl-protein complexes CP1 and LHC I associated with PS I; and (4) an increased stability of the oligomeric form of LHC II as assessed by non-denaturing gel electrophoresis in the psychrophile compared to the mesophile. Furthermore, immunoblotting indicated that the stoichiometry of PS II:PS I:CF₁ is significantly altered in C. subcaudata compared to the mesophile. Even though the psychrophile is adapted to growth at low irradiance, it retained the capacity to adjust the total xanthophyll cycle pool size as well as the epoxidation state of the xanthophyll cycle. Despite these differences, the psychrophile and mesophile exhibited comparable photosynthetic efficiency for O₂ evolution regardless of growth conditions. P^max for both Chlamydomonas species was similar only when grown under identical conditions. We suggest that these photosynthetic characteristics of the Antarctic psychrophile reflect the unusual light and low temperature regime to which it is adapted.