On the Role of Basic Residues in Adapting the Reaction Centre–LH1 Complex for Growth at Elevated Temperatures in Purple Bacteria

The purple photosynthetic bacterium Thermochromatium tepidum is a moderate thermophile, with a growth optimum of 48–50 °C. The X-ray crystal structure of the reaction centre from this organism has been determined, and compared with that from mesophilic bacteria such as Blastochloris viridis and Rhodobacter sphaeroides (Nogi T et al. (2000) Proc Natl Acad Sci USA 97: 13561–13566). Structural features that could contribute to the enhanced thermal stability of the Thermochromatium tepidum reaction centre were discussed, including three arginine residues exposed at the periplasmic side of the membrane that are not present in reaction centres from mesophilic organisms, and potentially could increase the affinity of the complex for the surrounding membrane. In the present report these arginine residues, plus a histidine identified from an extensive sequence alignment, were engineered into structurally homologous positions in the Rhodobacter sphaeroides reaction centre, and the effect on the thermal stability of the Rhodobacter sphaeroides complex was examined. We find that these residues do not enhance the thermal stability of the reaction centre, as assessed by absorbance spectroscopy of the bacteriochlorin cofactors in membrane-bound reaction centres. Possible roles of these residues in the Thermochromatium tepidum reaction centre are discussed, and it is proposed that they facilitate stronger binding of the reaction centre to the encircling LH1 antenna complex, through ionic interactions with acidic residues at the C-terminal end of the LH1 α-polypeptide. Such an interaction could enhance the stability of the so-called ‘RC–LH1 core’ complex that is formed between the reaction centre and the LH1 antenna, and which represents the minimal functional photosynthetic unit in all known purple photosynthetic bacteria. Stronger bonding interactions between the two complexes could also contribute to an increase in the rigidity of the photosynthetic membrane in Thermochromatium tepidum, in accord with the general finding that the cytoplasmic membrane from thermophilic eubacteria is less fluid than its counterpart in mesophilic bacteria.     

The formation of bacteriochlorophyll · protein complexes of the photosynthetic apparatus of Rhodopseudomonas capsulata during early stages of development

Cells of Rhodopseudomonas capsulata cultivated at an oxygen partial pressure of 400 mmHg in the dark contained 0.1 nmol or less total bacteriochlorophyll per mg membrane protein. The bacteriochlorophyll was found in the reaction center (10 pmol bacteriochlorophyll/mg membrane protein) and in the light harvesting bacteriochlorophyll I but not in the light harvesting bacteriochlorophyll II. Formation of the photosynthetic apparatus in those cells was induced by incubation at a very low oxygen tension in the dark. Reaction center bacteriochlorophyll and light harvesting bacteriochlorophyll increased three fold after 60 min of incubation at 1–2 mmHg (pO₂). Light harvesting bacteriochlorophyll II increased strongly after 60 min and became dominating after 90 min of incubation. The total bacteriochlorophyll content doubled every 30 min, but synthesis of reaction center bacteriochlorophyll proceeded at much lower rates. Consequently the size of the photosynthetic unit (total bacteriochlorophyll/reaction center bacteriochlorophyll) increased from 15 to 52 during 150 min of incubation. The proteins of the photosynthetic apparatus were synthesized concomitantly with bacteriochlorophyll.Cells which were incubated at 0.5 mmHg (pO₂) do not grow but form the photosynthetic apparatus. During the first hours of incubation light harvesting bacteriochlorophyll I and reaction center bacteriochlorophyll were the dominant bacteriochlorophyll species, but light harvesting bacteriochlorophyll II was synthesized only in small amounts. Total bacteriochlorophyll and reaction center bacteriochlorophyll increased from 30 min up until 210 min of incubation more than 10 fold. The final concentrations of total bacteriochlorophyll and reaction center bacteriochlorophyll were 8.6 nmol and 0.26 nmol per mg membrane protein, respectively. The three protein components of the reaction centers (mol. wts. 28 000, 24 000 and 21 000) and the protein of the light harvesting I complex (mol. wt. 12 000) were incorporated simultaneously. The protein of band 1 (mol. wt. 14 000) which was present in the isolated light harvesting complex II, was synthesized only in very small amounts. The proteins of bands 3 and 4 (mol. wt. 10 000 and 8000) however, which were shown to be associated with light harvesting bacteriochlorophyll II, were synthesized in noticeable amounts as was light harvesting bacteriochlorophyll II. In addition a protein with an apparent molecular weight of 45 000 showed a strong incorporation of ¹⁴C-labeled amino acids. This protein comigrates with one protein which was found to be associated with a green pigment excreted during incubation at 0.5 Torr into the medium. The in vivo-absorption maxima of this pigment complex were 660, 590, 540, 417 and 400 nm. The succinate oxidase and the NADH oxidase seemed to be incorporated into the newly formed intracytoplasmic membrane only in very small amounts. Thus, reaction center and light harvesting bacteriochlorophyll and their associated proteins were simultaneously synthesized, whereas light harvesting complex II is the variable part of the photosynthetic apparatus.     

PufQ regulates porphyrin flux at the haem/bacteriochlorophyll branchpoint of tetrapyrrole biosynthesis via interactions with ferrochelatase

Facultative phototrophs such as Rhodobacter sphaeroides can switch between heterotrophic and photosynthetic growth. This transition is governed by oxygen tension and involves the large‐scale production of bacteriochlorophyll, which shares a biosynthetic pathway with haem up to protoporphyrin IX. Here, the pathways diverge with the insertion of Fe²⁺ or Mg²⁺ into protoporphyrin by ferrochelatase or magnesium chelatase, respectively. Tight regulation of this branchpoint is essential, but the mechanisms for switching between respiratory and photosynthetic growth are poorly understood. We show that PufQ governs the haem/bacteriochlorophyll switch; pufQ is found within the oxygen‐regulated pufQBALMX operon encoding the reaction centre–light‐harvesting photosystem complex. A pufQ deletion strain synthesises low levels of bacteriochlorophyll and accumulates the biosynthetic precursor coproporphyrinogen III; a suppressor mutant of this strain harbours a mutation in the hemH gene encoding ferrochelatase, substantially reducing ferrochelatase activity and increasing cellular bacteriochlorophyll levels. FLAG‐immunoprecipitation experiments retrieve a ferrochelatase‐PufQ‐carotenoid complex, proposed to regulate the haem/bacteriochlorophyll branchpoint by directing porphyrin flux toward bacteriochlorophyll production under oxygen‐limiting conditions. The co‐location of pufQ and the photosystem genes in the same operon ensures that switching of tetrapyrrole metabolism toward bacteriochlorophyll is coordinated with the production of reaction centre and light‐harvesting polypeptides.     

On the photosynthetic properties of marine bacterium COL2P belonging to Roseobacter clade

Aerobic anoxygenic phototrophs (AAPs) are prokaryotic microorganisms capable of harvesting light using bacteriochlorophyll-based reaction centres. Marine AAP communities are generally dominated by species belonging to the Roseobacter clade. For this reason, we used marine Roseobacter-related strain COL2P as a model organism to characterize its photosynthetic apparatus, level of pigmentation and expression of photosynthetic complexes. This strain contained functional photosynthetic reaction centres with bacteriochlorophyll a and spheroidenone as the main light-harvesting pigments, but the expression of the photosynthetic apparatus was significantly reduced when compared to truly photoautotrophic species. Moreover, the absence of peripheral light-harvesting complexes largely reduced its light-harvesting capacity. The size of the photosynthetic unit was limited to 35.4 ± 1.0 BChl a molecules supplemented by the same number of spheroidenone molecules. The contribution of oxidative phosphorylation and photophosphorylation was analysed by respiration and fluorometric measurements. Our results indicate that even with a such reduced photosynthetic apparatus, photophosphorylation provides up to three times higher electron fluxes than aerobic respiration. These results suggest that light-derived energy can provide a substantial fraction of COL2P metabolic needs.     

Growth and adaptation to phototrophic conditions of Rhodospirillum rubrum and Rhodopseudomonas sphaeroides at different temperatures

The influence of temperature on yields of cell protein and bacteriochlorophyll as well as on the rates of growth and bacteriochlorophyll synthesis was studied with Rhodospirillum rubrum and Rhodopseudomonas sphaeroides. Under chemotrophic conditions net cell-protein production increased in cultures of both species along with temperature from 14°C up to the optimum at 33°C. Under phototrophic conditions cell-protein yields were largely constant within the range from 21°C to 33°C. At temperatures below 21°C and above 33°C yields decreased. These results are interpreted in terms of coupling between energy yielding or redox equivalent providing metabolisms and cell biosynthesis. Upon adaptation from chemotrophic to phototrophic conditions a direct relationship between temperature increase and bacteriochlorophyll level was observed. Arrhenius plots of both, specific growth rates and rates of bacteriochlorophyll synthesis, revealed discontinuities at about 20°C. Temperature coefficients either above or below those discontinuities were similar in both species. In R. rubrum temperature coefficients of the synthesis of total bacteriochlorophyll were also representative of the synthesis of photochemical reaction center and light harvesting bacteriochlorophylls. But in R. sphaeroides significant differences were observed between temperature coefficients of the syntheses of bacteriochlorophylls of the costantly composed reaction centerlight harvesting complex on one hand and of both, total and the quantitatively variable light harvesting bacteriochlorophylls on the other. The results are interpreted in light of hypotheses on the regulation (a) of cellular bacteriochlorophyll levels as well as (b) of the ratio of functionally different bacteriochlorophylls in the photosynthetic apparatus.Abbreviation Bchl bacteriochlorophyll     

Lipids in photosynthetic reaction centres: structural roles and functional holes

Photosynthetic proteins power the biosphere. Reaction centres, light harvesting antenna proteins and cytochrome b(6)f (or bc(1)) complexes are expressed at high levels, have been subjected to an intensive spectroscopic, biochemical and mutagenic analysis, and several have been characterised to an informatively high resolution by X-ray crystallography. In addition to revealing the structural basis for the transduction of light energy, X-ray crystallography has brought molecular insights into the relationships between these multicomponent membrane proteins and their lipid environment. Lipids resolved in the X-ray crystal structures of photosynthetic proteins bind light harvesting cofactors, fill intra-protein cavities through which quinones can diffuse, form an important part of the monomer-monomer interface in multimeric structures and may facilitate structural flexibility in complexes that undergo partial disassembly and repair. It has been proposed that individual lipids influence the biophysical properties of reaction centre cofactors, and so affect the rate of electron transfer through the complex. Lipids have also been shown to be important for successful crystallisation of photosynthetic proteins. Comparison of the three types of reaction centre that have been structurally characterised reveals interesting similarities in the position of bound lipids that may point towards a generic requirement to reinforce the structure of the core electron transfer domain. The crystallographic data are also providing new opportunities to find molecular explanations for observed effects of different types of lipid on the structure, mechanism and organisation of reaction centres and other photosynthetic proteins.     

Photosynthetic antenna proteins: 100 ps before photochemistry starts

All photosynthetic organisms require a light harvesting system to funnel excitation energy towards the photosynthetic reaction centre, a process which can take 100 ps. Laser spectroscopy allows us to measure rates of energy transfer between pigments of the light harvesting system for the first time. These rates are correlated with models of the light harvesting apparatus.     

光合作用对光和CO2响应模型的研究进展

光合作用对光和CO₂响应模型是研究植物生理和植物生态学的重要工具, 可为植物光合特性对主要环境因子的响应提供科学依据。该文综述了当前光合作用对光和CO₂响应模型的研究进展和存在的问题, 并在此基础上探讨了这些模型的可能发展趋势。光合作用涉及光能的吸收、能量转换、电子传递、ATP合成、CO₂固定等一系列复杂的物理和化学反应过程。光合作用由原初反应、同化力形成和碳同化3个基本过程构成, 任一个过程均可对光合作用速率产生直接的影响。光合作用对光响应模型只涉及光能的转换, 而光合作用的生化模型包含了同化力形成和碳同化这两个基本过程。把光合作用的原初反应, 即把参与光能吸收、传递和转换的捕光色素分子的物理参数(如捕光色素分子数、捕光色素分子光能吸收截面、捕光色素分子处于激发态的平均寿命等)结合到生化模型中, 可能是今后光合作用对光响应机理模型的发展方向。     

Influence of energy flux and quality of light on the molecular organization of the photosynthetic apparatus in Scenedesmus

The photosynthetic apparatus of the unicellular green alga Scenedesmus obliquus adapts to different levels and qualities of light as documented by the fluence-rate curves of photosynthetic oxygen evolution. Cultures adapted to low fluence rates of white light (5W·m^-2) have more chlorophyll (Chl) per cell mass, a higher chlorophyll to carotenoid ratio and a doubling of the chlorophyll to cytochrome f ratio compared with cells adapted to high fluence rates of white light (20W·m^-2). Only small differences can be observed in the halfrise time of fluorescence induction, the electrophoretic profile of the pigment-protein complexes and the Chl a/Chl b-ratio. Scenedesmus cells adapted to blue light of high spectral purity demonstrate, in comparison with those adapted to red light, a higher chlorophyll content, a higher ratio of chlorophyll to carotenoid and a much higher ratio of chlorophyll to cytochrome f. Regarding these parameters and the fluence-rate curves of photosynthesis, the blue light causes the same effects as low levels of white light. In contrast, the action of red light resembles rather that of high levels of white light. Blue-light-adapted Scenedesmus cells have a smaller Chl a to Chl b ratio, a faster half-rise time of fluorescence induction and more chlorophyll in the light-harvesting system than red-light-adapted cells, as shown in the electrophoretic profile of the pigment-protein complexes. Based on these results we propose a model for the adaptation of the photosynthetic apparatus of Scenedesmus to different levels and qualities of light. In this model low as compared with high levels of white light result in an increase in the number of photosystems per electron-transport chain, but not in an increase in the size of these photosystems. The same result is obtained by adaptation to blue light. The lack of sufficient Chl b formation in red-light-adapted cells results in a decrease in the light harvesting chlorophyll-protein complexes and a photosynthetic response like that found in cells adapted to high light levels. The findings reported here confirm our earlier results in comparing blue-and red-light adaptation of the photosynthetic apparatus with adaptation to low and high levels of white light, respectively.Abbreviations Chl chlorophyll - CP chlorophyll-protein complex - DCMU 3-(3,4-dichlorophenyl)-1,1 dimethyl-urea - LHCP light harvesting chlorophyll-protein complex - LiDS lithium dodecyl sulfate - PAGE polyacrylamide gel electrophoresis - PS photosystem     

Photosystem II antenna complexes CP26 and CP29 are essential for nonphotochemical quenching in Chlamydomonas reinhardtii

Photosystems must balance between light harvesting to fuel the photosynthetic process for CO₂ fixation and mitigating the risk of photodamage due to absorption of light energy in excess. Eukaryotic photosynthetic organisms evolved an array of pigment-binding proteins called light harvesting complexes constituting the external antenna system in the photosystems, where both light harvesting and activation of photoprotective mechanisms occur. In this work, the balancing role of CP29 and CP26 photosystem II antenna subunits was investigated in Chlamydomonas reinhardtii using CRISPR-Cas9 technology to obtain single and double mutants depleted of monomeric antennas. Absence of CP26 and CP29 impaired both photosynthetic efficiency and photoprotection: Excitation energy transfer from external antenna to reaction centre was reduced, and state transitions were completely impaired. Moreover, differently from higher plants, photosystem II monomeric antenna proteins resulted to be essential for photoprotective thermal dissipation of excitation energy by nonphotochemical quenching.     

Membrane adhesion in photosynthetic bacterial membranes. Light harvesting complex I (LHI) appears to be the main adhesion factor

We have reconstituted pigment-protein complexes isolated from Rhodopseudomonas palustris photosynthetic membranes into phospholipid liposomes. The various complexes were tested for their ability to promote adhesion of the liposome membrane in the presence and absence of Mg²⁺ ions. Samples containing a reaction center (RC)/light-harvesting I (LHI) complex appeared to stack in a manner resembling control thylakoids in 2 and 5 mM Mg²⁺. We also tested for the effects of Mg²⁺ on detergent extractablity of pigment-protein complexes from intact membranes. Mg²⁺ sharply reduced the amount of LHI solubilized from membranes, while having little effect on the extractability of the light harvesting II complex (LHII) and the RC. Based on these results we suggest that LHI is the principal adhesion factor of R. palustris thylakoids.Abbreviations LHC light harvesting complex - OG octyl glucoside - RC reaction center This paper is dedicated to Professor G. Drews on the occasion of his 60th birthday     

Differentiation of the intracytoplasmic membrane of Rhodopseudomonas palustris induced by variations of oxygen partial pressure or light intensity

The photosynthetic apparatus of Rhodopseudomonas palustris contains, in addition to reaction center bacteriochlorophyll (Bchl) two spectral forms of light harvesting (LH) Bchl, i.e. LH Bchl I, characterized by an infrared absorption maximum at 880 nm (890 nm at 77°K) and LH Bchl II absorbing at 805 and 855 nm (805 and 870 nm at 77°K). LH Bchl I seems to be associated with a single protein species of an apparent mol. wt. of 13000 whereas LH Bchl II is apparently associated with two proteins of mol. wts. of 9000 and 11000.Cells in anaerobic cultures adapt to changes of light intensity 1. by variation of the size of the photosynthetic unit, i.e. the molar ratio of LH Bchl II to reaction center Bchl, 2. by variation of the number of photosynthetic units per unit of membrane area, 3. by regulation of the size of the intracytoplasmic membrane system.During adaptation of changes of oxygen partial pressure cells are able to synthesize reaction center Bchl, LH Bchl and intracytoplasmic membranes at different rates. The synthesis of reaction center Bchl and LH Bchl I are, however, coordinated with each other, while the syntheses of LH Bchl II and reaction center Bchl proceed independently.List of Non-Standard Abbreviations Bchl bacteriochlorophyll - ICM mitracytoplasmic membrane - LDAO lauryldimethyl aminoxide - R Rhodopseudomonas - RC reaction center - SDS sodium dodecylsulfate     

The composition and structure of photosystem I‐associated antenna from Cyanidioschyzon merolae

Red algae contain two types of light‐harvesting antenna systems, the phycobilisomes and chlorophyll a binding polypeptides (termed Lhcr), which expand the light‐harvesting capacity of the photosynthetic reaction centers. In this study, photosystem I (PSI) and its associated light‐harvesting proteins were isolated from the red alga Cyanidioschyzon merolae. The structural and functional properties of the largest PSI particles observed were investigated by biochemical characterization, mass spectrometry, fluorescence emission and excitation spectroscopy, and transmission electron microscopy. Our data provide strong evidence for a stable PSI complex in red algae that possesses two distinct types of functional peripheral light‐harvesting antenna complex, comprising both Lhcr and a PSI‐linked phycobilisome sub‐complex. We conclude that the PSI antennae system of red algae represents an evolutionary intermediate between the prokaryotic cyanobacteria and other eukaryotes, such as green algae and vascular plants.     

The Photosynthetic Reaction Centre from the Purple Bacterium Rhodopseudomonas viridis

We first describe the history and methods of membrane protein crystallization, and show how the structure of the photosynthetic reaction centre from the purple bacterium Rhodopseudomonas viridis was solved. The structure of this membrane protein complex is correlated with its function as a light-driven electron pump across the photosynthetic membrane. Finally we draw conclusions on the structure of the photosystem II reaction centre from plants and discuss the aspects of membrane protein structure.Published in Les Prix Nobel–The Nobel Prizes 1988 (Nobel Foundation, Stockholm, 1989) and republished here with the permission of the Nobel Foundation the copyright holders.     

Competition between annihilation and trapping leads to strongly reduced yields of photochemistry under ps-flash excitation

Excitation of photosynthetic systems with short intense flashes is known to lead to exciton-exciton annihilation processes. Here we quantify the effect of competition between annihilation and trapping for Photosystem II, Photosystem I (thylakoids from peas and membranes from the cyanobacterium Synechocystis sp.), as well as for the purple bacterium Rhodospirillum rubrum. In none of the cases it was possible to reach complete product saturation (i.e. closure of reaction centers) even with an excitation energy exceeding 10 hits per photosynthetic unit. The parameter introduced by Deprez et al. ((1990) Biochim. Biophys. Acta 1015: 295–303) describing the competition between exciton-exciton annihilation and trapping was calculated to range between 4.5 (PS II) and 6 (Rs. rubrum). The rate constants for bimolecular exciton-exciton annihilation ranged between (42 ps)^-1 and (2.5 ps)^-1 for PS II and PS I-membranes of Synechocystis, respectively. The data are interpreted in terms of hopping times (i.e. mean residence time of the excited state on a chromophore) according to random walk in isoenergetic antenna.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - LHC II light harvesting complex II - P primary donor - PS I Photosystem I - PS II Photosystem II - PSU photosynthetic unit - RC reaction center     

The photosynthetic reaction centre from the purple bacteriumRhodopseudomonas viridis

We first describe the history and methods of membrane protein crystallization, and show how the structure of the photosynthetic reaction centre from the purple bacteriumRhodopseudomonas viridis was solved. The structure of this membrane protein complex is correlated with its function as a light-driven electron pump across the photosynthetic membrane. Finally we draw conclusions on the structure of the photosystem II reaction centre from plants and discuss the aspects of membrane protein structure.Published inLes Prix Nobel—The Nobel Prizes 1988 (Nobel Foundation, Stockholm, 1989) and republished here with the permission of the Nobel Foundation the copyright holders.     

Characterization of expressed pigmented core light harvesting complex (LH 1) in a reaction center deficient mutant of Blastochloris viridis

The utility of photosynthetically defective mutants in the purple photosynthetic bacterium Blastochloris viridis (formerly Rhodopseudomonas viridis)was demonstrated with construction of a reaction-center deficient mutant, LH 1-H. This LH 1-H mutant has a photosynthetic apparatus in which most of the puf operon genes were deleted, resulting in an organism containing only the genes for the light harvesting polypeptides and the H subunit of the reaction center. This B. viridisstrain containing a truncation of the puf operon was characterized by gel electrophoresis, lipid-to-protein ratio analysis, optical spectroscopy, electron paramagnetic resonance and transmission electron microscopy. Optical and electron paramagnetic resonance spectroscopies revealed no photoactivity in this LH 1-H mutant consistent with the absence of intact reaction centers. Electron paramagnetic resonance evidence for assembled LH 1 complexes suggested that the interactions between light harvesting polypeptide complexes in membranes were largely unchanged despite the absence of their companion reaction center cores. The observed increase in the lipid-to-protein ratio was consistent with modified interactions between LH 1s, a view supported by transmission electron microscopy analysis of membrane fragments. The results show that B. viridis can serve as a practical system for investigating structure-function relationships in membranes and photosynthesis through the construction of photosynthetically defective mutants. This revised version was published online in August 2006 with corrections to the Cover Date.     

A mechanistic model for the light response of photosynthetic electron transport rate based on light harvesting properties of photosynthetic pigment molecules

Models describing the light response of photosynthetic electron transport rate (ETR) are routinely used to determine how light absorption influences energy, reducing power and yields of primary productivity; however, no single model is currently able to provide insight into the fundamental processes that implicitly govern the variability of light absorption. Here we present development and application of a new mechanistic model of ETR for photosystem II based on the light harvesting (absorption and transfer to the core ‘reaction centres’) characteristics of photosynthetic pigment molecules. Within this model a series of equations are used to describe novel biophysical and biochemical characteristics of photosynthetic pigment molecules and in turn light harvesting; specifically, the eigen-absorption cross-section and the minimum average lifetime of photosynthetic pigment molecules in the excited state, which describe the ability of light absorption of photosynthetic pigment molecules and retention time of excitons in the excited state but are difficult to be measured directly. We applied this model to a series of previously collected fluorescence data and demonstrated that our model described well the light response curves of ETR, regardless of whether dynamic down-regulation of PSII occurs, for a range of photosynthetic organisms (Abies alba, Picea abies, Pinus mugo and Emiliania huxleyi). Inherent estimated parameters (e.g. maximum ETR and the saturation irradiance) by our model are in very close agreement with the measured data. Overall, our mechanistic model potentially provides novel insights into the regulation of ETR by light harvesting properties as well as dynamical down-regulation of PSII.     

Seasonal Effects on Chlorophyll-Protein Complexes Isolated from P-inus silvestris

The seasonal changes in the relative distribution of P700 chlorophyll-protein complex a₁ and light harvesting chlorophyll-protein complex a/b were studied in a natural stand of Pinus silvestris. Similar measurements were made after artificial photobleaching of chlorophyll in pine seedlings or in isolated pine chloroplasts. The chlorophyll-protein complexes were solubilized by sodium dodecyl sulphate and separated by polyacrylamide gel electrophoresis. When autumn and winter destruction of chlorophyll occurs, the chlorophyll a antenna associated with P700 in photosystem 1 (P700-CPa₁) is relatively more affected than the light harvesting complex, which lacks a reaction centre. These results are further supported by low-temperature fluorescence emission properties of isolated chloroplasts presented in this work and elsewhere. The destruction of chlorophyll in stressing autumn and winter climates is most probably caused by photosensitized oxidation of chlorophyll.