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PubMed1977

Uptake of ATP analogs by isolated pea chloroplasts and their effect on CO2 fixation and electron transport.

1. The ATP analog, adenylyl-imidodiphosphate rapidly inhibited CO2-dependent oxygen evolution by isolated pea chloroplasts. Both alpha, beta- and beta, gamma-methylene adenosine triphosphate also inhibited oxygen evolution. The inhibition was relieved by ATP but only partially relieved by 3-phosphoglycerate. Oxygen evolution with 3-phosphoglycerate as substrate was inhibited by adenylyl-imidodiphosphate to a lesser extent than CO2-dependent oxygen evolution. The concentration of adenylylimidodiphosphate required for 50% inhibition of CO2-dependent oxygen evolution was 50 micronM. 2. Although non-cyclic photophosphorylation by broken chloroplasts was not significantly affected by adenylyl-imidodiphosphate, electron transport in the absence of ADP was inhibited by adenylyl-imidodiphosphate to the same extent as by ATP, suggesting binding of the ATP analog to the coupling factor of phosphorylation. 3. The endogenous adenine nucleotides of a chloroplast suspension were labelled by incubation with [14C]ATP and subsequent washing. Addition of adenylyl-imidodiphosphate to the labelled chloroplasts resulted in a rapid efflux of adenine nucleotides suggesting that the ATP analog was transported into the chloroplasts via the adenine nucleotide translocator. 4. It was concluded that uptake of ATP analogs in exchange for endogenous adenine nucleotides decreased the internal ATP concentration and thus inhibited CO2 fixation. Oxygen evolution was inhibited to a lesser extent in spinach chloroplasts which apparently have lower rates of adenine nucleotide transport than pea chloroplasts.

Adenosine Triphosphate↗

PubMed1978

Requirement of the light-harvesting pigment.protein complex for magnesium ion regulation of excitation energy distribution in chloroplasts.

Cation regulation of excitation energy distribution was examined in chloroplasts isolated from (a) pea seedlings, grown in intermittent illumination, which contain no light-harvesting complex, (b) a barley mutant which is deficient in the major polypeptide component of the light-harvesting complex, and (c) a soybean mutant which contains a reduced amount of light-harvesting complex. It was found that: (1) Mg2+-induced increase in Photosystem II fluorescence at room temperature is small in the chloroplasts of the soybean mutant, smaller in the barley mutant, and almost absent in the light-harvesting complex-less chloroplasts of pea as compared to their respective controls. (2) Mg2+-induced increase in the F685/F730 emission peak ratio at 77 K is not detected in the isolated chloroplasts of the intermittent light-grown pea and the barley mutant. (3) Pre-illumination induced State 1-State 2 and adaptation in vivo is absent in the barley mutant and is less pronounced in the soybean mutant as compared to their respective controls. (4) Increase of slow fluorescence decay upon addition of Mg2+ observed in control chloroplasts was not detected in chloroplasts of intermittent-light grown peas. These results confirm earlier conclusions (Armond, P.A., Arntzen, C.J., Briantais, J.M. and Vernotte, C. (1976) Arch. Biochem. Biophys. 175, 54--63; Davis, D.J., Armond, P.A., Gross, E.L. and Arntzen, C.J. (1976) Arch. Biochem. Biophys. 175, 64--70) that light-harvesting complex is required for the Mg2+-induced regulation of the excitation energy distribution between Photosystems I and II. The characteristic P-S decay and I-D dip of the in vivo fluorescence inductions (Kautsky effect) were not significantly altered in the light-harvesting complex-less and the light-harvesting complex-deficient chloroplasts as compared to their respective controls. These results indicate that light-harvesting complex is not obligatorily required to observe the P-S decay or the I-D dip.

Cell-Free System↗

PubMed1985

Synthetic abilities of Euglena chloroplasts in darkness.

Protein synthesis, normally a light-dependent process in isolated mature chloroplasts of Euglena gracilis var. bacillaris will take place in darkness if ATP and Mg2+ (ATP/Mg) are supplied. Either 5 or 10 mM ATP plus 15 mM MgCl2 are optimal and rates equal to those in the light can be obtained. Since ATP and Mg2+ are not stoichiometrically related, and since the optimal Mg2+ concentration is similar to that which stabilizes chloroplast ribosomes in vitro, it is suggested that the chloroplast is freely permeable to Mg2+ under these conditions. Protein synthesis under these conditions is not inhibited appreciably by DCMU, FCCP, cycloheximide, or by the addition of ribonuclease, but is highly sensitive to chloramphenicol. Carbon dioxide fixation is also a light-dependent process in isolated mature chloroplasts from Euglena, but addition of ATP (5 mM) and fructose bisphosphate (5 mM) plus aldolase (1.0 unit/ml) (fructose-1,6-bisphosphate/aldolase) yields CO2 fixation rates in darkness that are 43% of those normally obtained in the light. Mg2+ higher than 1.0 mM (e.g., 16 mM) is somewhat inhibitory. Chlorophyll synthesis from 5-aminolevulinate in 36 h developing chloroplasts from Euglena is also light-dependent, but addition of ATP/Mg and fructose-1,6-bis-phosphate/aldolase in darkness brings about the accumulation of a compound having the same RF on chromatography as protochlorophyllide from Barley; a subsequent brief illumination of the chloroplasts converts this compound to a compound with the RF of chlorophyll. Thus Euglena chloroplasts supplied with appropriate additions can carry out protein synthesis, carbon dioxide fixation and most of chlorophyll synthesis in darkness. This versatility is appropriate in photosynthetic organelles isolated from photo-organotrophic cells.

Adenosine Triphosphate↗

PubMed1976

Hydrophobic affinity partition of spinach chloroplasts in aqueous two-phase systems.

The surface properties of spinach chloroplasts, both of intact chloroplasts with surrounding envelope and broken chloroplasts consisting of the inner lamellar system, have been studied by partitioning them between two aqueous phases, especially using counter-current distribution technique. The two-phase system consists of poly(ethyleneglycol), dextran and water. The two polymers are enriched in opposite phases and by binding deoxycholate or palmitate to one of the polymers the affinity of chloroplasts for the corresponding phase is strongly enhanced. The partition of the two classes of chloroplasts, however, is not affected to the same degree and the affinity of the chloroplast envelope for deoxycholate and palmitate is stronger than that of the lamellar system. This has been correlated to the chemical composition of the two types of membranes. By studying the effect of salts on the partition it has been found that the lamellar system bears a larger number of negative charges as compared to the envelope of the intact chloroplast.

Binding Sites↗

PubMed1980

A mechanism for the formation of inside-out membrane vesicles. Preparation of inside-out vesicles from membrane-paired randomized chloroplast lamellae.

Inside-out thylakoid membrane vesicles can be isolated by aqueous polymer two-phase partition of Yeda press-fragmented spinach chloroplasts (Andersson, B. and Akerlund, H.-E. (1978) Biochim. Biophys. Acta 503, 462-472). The mechanism for their formation has been investigated by studying the yield of inside-out vesicles after various treatments of the chloroplasts prior to fragmentation. No inside-out vesicles were isolated during phase partitioning if the chloroplasts had been destacked in a low-salt medium prior to the fragmentation. Only in those cases where the chloroplast lamellae had been stacked by cations of membrane-paired by acidic treatment did we get any yield of inside-out vesicles. Thus, the intrinsic properties of chloroplast thylakoids seem to be such that they seal into right-side out vesicles after disruption unless they are in an appressed state. This favours the following mechanism for the formation of inside-out thylakoids. After press treatment, a ruptured membrane still remains appressed with an adjacent membrane. Resealing of such an appressed membrane pair would result in an inside-out vesicle. If the compartmentation of chloroplast lamellae into appressed grana and unappressed stroma lamellae is preserved by cations before fragmentation, the inside-out vesicles are highly enriched in photosystem II. This indicates a granal origin which is consistent with the proposed model outlined. Inside-out vesicles possessing photosystem I and II properties in approximately equal proportions could be obtained by acid-induced membrane-pairing of chloroplasts which had been destacked and randomized prior to fragmentation. Since this new preparation of inside-out thylakoid vesicles also exposes components derived from the stroma lamellae it complements the previous preparation. It is suggested that fragmentation of paired membrane followed by phase partitioning should be a general method of obtaining inside-out vesicles from membranes of various biological sources.

Cell Fractionation↗

PubMed1991

Chloroplast biogenesis. Cell-free transfer of envelope monogalactosylglycerides to thylakoids.

An ATP- and temperature-dependent transfer of monogalactosylglycerides from the chloroplast envelope to the chloroplast thylakoids was reconstituted in a cell-free system prepared from isolated chloroplasts of garden pea (Pisum sativum) or spinach (Spinacia oleracea). Isolated envelope membranes, in which the label was present exclusively in monogalactosylglycerides, were prepared radiolabeled in vitro with [14C]galactose from UDP-[14C]galactose to label galactolipids as the donor. ATP-dependent transfer of radioactivity from donor to unlabeled acceptor thylakoids, immobilized on nitrocellulose strips, was observed. In some experiments linear transfer for longer than 30 min of incubation was facilitated by the addition of stroma proteins but in other experiments stroma was without effect or inhibitory suggesting no absolute requirements for a soluble protein carrier. Transfer was donor specific. No membrane fraction tested (plasma membrane, tonoplast, endoplasmic reticulum, nuclei, Golgi apparatus, mitochondria or thylakoids) (isolated from tissue radiolabeled in vivo with [14C]acetate) other than chloroplast envelopes demonstrated any significant ability to transfer labeled membrane lipids to immobilized thylakoids. Acceptor specificity, while not absolute, showed a 3-10-fold greater ATP-dependent transfer of labeled galactolipids from chloroplast envelopes to immobilized thylakoids than to other leaf membranes. The results provide independent confirmation of the potential for transfer of galactolipids between chloroplast envelopes and thylakoids suggested previously from ultrastructural studies and of the known location of thylakoid galactolipid biosynthetic activities in the chloroplast envelope.

Adenosine Triphosphate↗

PubMed1975

Size, conformation and purity of chloroplast DNA of some higher plants.

1. Chloroplast DNA of Antirrhinum majus, Oenothera hookeri, Beta vulgaris and Spinacia oleracea band at the same buoyant density of 1.697 g-cm-3 in neutral CsCl equilibrium gradients. The corresponding nuclear DNAs band at 1.691, 1.703, 1.695 and 1.695 g-cm-3, respectively. The purity of chloroplast and nuclear DNA can be assessed objectively only in the cases of Antirrhinum and Oenothera. 2. Electron microscopic analysis of chloroplast DNA, purified in CsCl or CsCl/ethidium bromide gradients, revealed up to 80% circular molecules. Of these about 15% were of supertwisted conformation. Best yields of circular molecules were recovered when populations of unbroken chloroplasts were subjected to DNAase and phosphodiesterase treatment, and when the DNA was purified from viscous lysates by centrifugation into a CsCl cushion. Treatment of plastids with DNAase alone did not guarantee complete degradation of nuclear DNA. 3. The average contour length of the open circular chloroplast DNA molecules was basically similar for all four plants. They were 45.9 plus or minus 2.1 mum for Antirrhinum, 45.7 plus or minus 1.9 mum for Spinacia, 44.9 plus or minus 1.7 mum for Beta and 45.2 mum for Oenothera. This is comparable to the size derived for the coding capacity of chloroplast DNA from reassociation experiments. As much as 15% of the total population of circles in chloroplast DNA of Spinacia were circular dimers.

Centrifugation, Density Gradient↗

PubMed1975

Amino acid incorporation into protein by ribosomes bound to chloroplast thylakoid membranes: formation of discrete products.

A system which incorporates amino acids into proteins of chloroplast membranes of Chlamydomonas reinhardti is described. It consists of chloroplast ribosomes bound to thylakoid membranes and cell extract. mRNA is present in this thylakoid-ribosome complex, since neither initiation nor RNA synthesis seems to be required for amino acid incorporation. Incorporation requires ATP, GTP and a soluble portion of cell extract. It is inhibited by chloramphenicol, but not cycloheximide. Most incorporated radioactivity remains bound to the membranes. Although a large portion of this labeled membrane-bound protein occurs as nascent polypeptides, a portion appears at least four products of discrete molecular weights. The major in vitro product migrates as a polypeptide of 23 000 daltons. We conclude that a portion of chloroplast membrane proteins is not only made within the chloroplast, but directly on the membranes. We had previously observed that release of membrane-bound ribosomes is partially dependent on puromycin, and concluded that some membrane-bound ribosomes were attached to the membranes through nascent protein chains. Thus, our results suggest that some chloroplast membrane proteins are inserted into the membranes as they are synthesized. This chloroplast membrane amino acid incorporation system offers a promising tool for studying biosynthesis of membrane proteins, and how they become inserted into chloroplast thylakoids to form functional membranes.

Amino Acids↗

PubMed1976

Light-stimulated synthesis of chloroplast DNA.

Light-stimulated chloroplast DNA synthesis was studied during chloroplast development in the absence of cell division and nuclear DNA synthesis. Incorporation of 32Pi was stimulated 10-15 fold, however, the ratio of chloroplast DNA to nuclear DNA remained constant. Isotope dilution experiments suggested that stimulated labeling of chloroplast DNA represented more efficient utilization of exogenously supplied Pi rather than stimulated turnover of chloroplast DNA. The low level of DNA synthesis and chloroplast development were resistant to nalidixic acid which inhibits semiconservative replication of chloroplast DNA.

Animals↗

PubMed2006

Acclimation of mesophyll and bundle sheath chloroplasts of maize to different irradiances during growth.

The regulation by light of the photosynthetic apparatus, and composition of light-harvesting complexes in mesophyll and bundle sheath chloroplasts was investigated in maize. Leaf chlorophyll content, level of plastoquinone, PSI and PSII activities and Lhc polypeptide compositions were determined in plants grown under high, moderate and low irradiances. Photochemical efficiency of PSII, photochemical fluorescence quenching and non-photochemical fluorescence quenching over a range of actinic irradiances were also determined, using chlorophyll a fluorescence analysis. Acclimation of plants to different light conditions caused marked changes in light-harvesting complexes, LHCI and LHCII, and antenna complexes were also reorganized in these types of chloroplasts. The level of LHCII increased in plants grown in low light, even in agranal bundle sheath chloroplasts where the amount of PSII was strongly reduced. Irradiance also affected LHCI complex and the number of structural polypeptides, in this complex, generally decreased in chloroplasts from plants grown under lower light. Surprisingly moderate and low irradiances during growth do not affect the light reaction and fluorescence parameters of plants but generated differences in composition of light-harvesting complexes in chloroplasts. On the other hand, the changes in photosynthetic apparatus in plants acclimated to high light, resulted in a higher efficiency of photosynthesis. Based on these observations we propose that light acclimation to high light in maize is tightly coordinated adjustment of light reaction components/activity in both mesophyll and bundle sheath chloroplasts. Acclimation is concerned with balancing light utilization and level of the content of LHC complexes differently in both types of chloroplasts.

Biochemistry↗

PubMed2005

Genomics-based dissection of the cross-talk of chloroplasts with the nucleus and mitochondria in Arabidopsis.

Post-endosymbiotic evolution of chloroplasts was characterized by a massive transfer of cyanobacterial genes to the nucleus, followed by re-routing of many of their encoded proteins. In consequence, most plastid proteins are nucleus-encoded, enabling an anterograde (nucleus-to-plastid) control of the organelle. The regulation of chloroplast functions includes also cross-talk between chloroplasts and mitochondria, as well as retrograde (plastid-to-nucleus) signalling. Genetic analyses reveal that redox state, flux through the chlorophyll biosynthetic pathway, sugar sensing and reactive oxygen species contribute to retrograde signalling. However, the identity of the messenger molecule(s) remains largely unknown. Novel facets of the chloroplast-mitochondrion cross-talk have been revealed by the characterization of mitochondrial mutants affected in chloroplast properties. Studies of the nuclear chloroplast transcriptome imply the existence of at least three distinct types of transcriptional regulation: a master switch, acting in a binary mode by either inducing or repressing the same large set of genes; a "mixed response" with about equal numbers of up- and down-regulated genes; and mechanisms supporting the specific co-regulation of nuclear genes for photosynthesis and for plastid gene expression. The recent discovery of the latter mode of control highlights a possibly ancient route to co-ordinate chloroplast and nuclear genome expression.

Arabidopsis↗

PubMed2003

Cytochrome f and subunit IV, two essential components of the photosynthetic bf complex typically encoded in the chloroplast genome, are nucleus-encoded in Euglena gracilis.

The photosynthetic protist Euglena gracilis contains chloroplasts surrounded by three membranes which arise from secondary endosymbiosis. The genes petA and petD, encoding cytochrome f and subunit IV of the cytochrome bf complex, normally present in chloroplast genomes, are lacking from the chloroplast DNA (cpDNA) of E. gracilis. The bf complex of E. gracilis was isolated, and the identities of cytochrome f and subunit IV were established immunochemically, by heme-specific staining, and by Edman degradation. Based on N-terminal and conserved internal protein sequences, primers were designed and used for PCR gene amplification and cDNA sequencing. The complete sequence of the petA cDNA and the partial sequence of the petD cDNA from E. gracilis are described. Evidence is provided that in this protist, the petA and petD genes have migrated from the chloroplast to the nucleus. Both genes exhibit a typical nuclear codon usage, clearly distinct from the usage of chloroplast genes. The petA gene encodes an atypical cytochrome f, with a unique insertion of 62 residues not present in other f-type cytochromes. The petA gene also acquired a region that encodes a large tripartite chloroplast transit peptide (CTP), which is thought to allow the import of apocytochrome f through the three-membrane envelope of E. gracilis chloroplasts. This is the first description of petA and petD genes that are nucleus-localized.

Amino Acid Sequence↗

PubMed1988

Some characteristics of cyclic photophosphorylation in maize bundle sheath chloroplasts.

PMS-dependent photophosphorylation in bundle sheath chloroplasts isolated from Zea mays was monitored by using a continuous method. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and venturicidin were shown to inhibit the ATP-synthesis. Venturicidin has been shown to inhibit ATP-formation in both mesophyll and bundle sheath chloroplasts. In contrast to the case in mesophyll chloroplasts, FMN was not able to promote photophosphorylation in bundle sheath chloroplasts. The effects of other cofactors and inhibitors on the ATP-synthesis in bundle sheath chloroplasts are shown. No photoinduced synthesis of inorganic pyrophosphate was seen, neither in bundle sheath chloroplasts, nor in mesophyll chloroplasts.

Adenosine Triphosphate↗

PubMedDate not supplied

Translation in chloroplasts.

The discovery that chloroplasts have semi-autonomous genetic systems has led to many insights into the biogenesis of these organelles and their evolution from free-living photosynthetic bacteria. Recent developments of our understanding of the molecular mechanisms of translation in chloroplasts suggest selective pressures that have maintained the 100-200 genes of the ancestral endosymbiont in chloroplast genomes. The ability to introduce modified genes into chloroplast genomes by homologous recombination and the recent development of an in vitro chloroplast translation system have been exploited for analyses of the cis-acting requirements for chloroplast translation. Trans-acting translational factors have been identified by genetic and biochemical approaches. Several studies have suggested that chloroplast mRNAs are translated in association with membranes.

Chloroplasts↗

PubMedDate not supplied

Chloroplast proteases: possible regulators of gene expression?

A wide range of proteolytic processes in the chloroplast are well recognized. These include processing of precursor proteins, removal of oxidatively damaged proteins, degradation of proteins missing their prosthetic groups or their partner subunit in a protein complex, and adjustment of the quantity of certain chloroplast proteins in response to changing environmental conditions. To date, several chloroplast proteases have been identified and cloned. The chloroplast processing enzyme is responsible for removing the transit peptides of newly imported proteins. The thylakoid processing peptidase removes the thylakoid-transfer domain from proteins translocated into the thylakoid lumen. Within the lumen, Tsp removes the carboxy-terminal tail of the precursor of the PSII D1 protein. In contrast to these processing peptidases which perform a single endo-proteolytic cut, processive proteases that can completely degrade substrate proteins also exist in chloroplasts. The serine ATP-dependent Clp protease, composed of the proteolytic subunit ClpP and the regulatory subunit ClpC, is located in the stroma, and is involved in the degradation of abnormal soluble and membrane-bound proteins. The ATP-dependent metalloprotease FtsH is bound to the thylakoid membrane, facing the stroma. It degrades unassembled proteins and is involved in the degradation of the D1 protein of PSII following photoinhibition. DegP is a serine protease bound to the lumenal side of the thylakoid membrane that might be involved in the chloroplast response to heat. All these peptidases and proteases are homologues of known bacterial enzymes. Since ATP-dependent bacterial proteases and their mitochondrial homologues are also involved in the regulation of gene expression, via their determining the levels of key regulatory proteins, chloroplast proteases are expected to play a similar role.

Chloroplasts↗

PubMed1998

Eubacterial origin of nuclear genes for chloroplast and cytosolic glucose-6-phosphate isomerase from spinach: sampling eubacterial gene diversity in eukaryotic chromosomes through symbiosis.

Higher plants possess two distinct nuclear-encoded glucose-6-phosphate isomerase (GPI) isoenzymes, a cytosolic enzmye of the Embden-Meyerhof pathway and a chloroplast enzyme essential to storage and mobilization of carbohydrate fixed by the Calvin cycle. We have purified spinach chloroplast GPI to homogeneity, determined amino acid sequences from the active enzyme, and cloned cDNAs for chloroplast and cytosolic GPI isoenzymes from spinach. Sequence comparisons reveal three distantly related families of GPI genes that are non-uniformly distributed among contemporary eubacteria and archaebacteria, suggesting that ancient gene diversity existed for this glycolytic enzyme. Spinach chloroplast GPI is much more similar to its homologue from the cyanobacterium Synechocystis PCC6803 than it is to the enzyme from any other source, providing strong evidence that the gene for chloroplast GPI was acquired by the nucleus via endosymbiotic gene transfer from the cyanobacterial antecedants of chloroplasts. Eukaryotic nuclear genes for cytosolic GPI are more similar to eubacterial than to archaebacterial homologues, suggesting that these too were acquired by eukaryotes from eubacteria, probably during the course of the endosymbiotic origin of mitochondria. Chloroplast and cytosolic GPI provide evidence for a eubacterial origin of yet another component of the eukaryotic glycolytic pathway.

Amino Acid Sequence↗

PubMed1995

Hypervariable microsatellites provide a general source of polymorphic DNA markers for the chloroplast genome.

BACKGROUND: The study of plant populations is greatly facilitated by the deployment of chloroplast DNA markers. Asymmetric inheritance, lower effective population sizes and perceived lower mutation rates indicate that the chloroplast genome may have different patterns of genetic diversity compared to nuclear genomes. Convenient assays that would allow intraspecific chloroplast variability to be detected are required. RESULTS: Eukaryote nuclear genomes contain ubiquitous simple sequence repeat (microsatellite) loci that are highly polymorphic in length; these polymorphisms can be rapidly typed by the polymerase chain reaction (PCR). Using primers flanking simple mononucleotide repeat motifs in the chloroplast DNA of annual and perennial soybean species, we demonstrate that microsatellites in the chloroplast genome also exhibit length variation, and that this polymorphism is due to changes in the repeat region. Furthermore, we have observed a nonrandom geographic distribution of variations at these loci, and have examined the number and location of such repeats within the chloroplast genomes of other species. CONCLUSIONS: PCR-based analysis of mononucleotide repeats may be used to detect both intraspecific and interspecific variability in the chloroplast genomes of seed plants. The analysis of polymorphic microsatellites thus provides an important experimental tool to examine a range of issues in plant genetics.

Base Sequence↗

PubMed2000

Reinvestigation of the triplet-minus-singlet spectrum of chloroplasts.

A comparison of the triplet-minus-singlet (TmS) absorption spectrum of spinach chloroplasts, recorded some thirty years ago, with the more recently published TmS spectrum of isolated Chla/b LHCII (light-harvesting complexes associated with photosystem II of higher plants) shows that the two spectra are very similar, which is to be expected, since only the carotenoid pigments contribute to each spectrum. Be that as it may, the comparison also reveals a dissimilarity: photoexcitation of the sample does, or does not, affect the absorbance in the Qy region (650-700 nm), depending on whether the sample is a suspension of chloroplasts or of isolated LHCII. The Qy-signal in the TmS spectrum of LHCII decays, it should be noted, at the same rate as the rest of the difference spectrum, and its most prominent feature is a negative peak. As the carotenoids do not absorb in the Qy region, the presence of a signal in this region calls for an explanation: van der Vos, Carbonera and Hoff, the first to find as well as fathom the phenomenon, attributed the Qy-signal to a change, in the absorption spectrum of a chlorophyll a (Chla) molecule, brought about by the presence of triplet excitation on a neighbouring carotenoid (Car). The difference in the behaviours of chloroplasts and LHCII, if reproducible, would imply that the Car triplets which give rise to the TmS spectrum of chloroplasts do not influence the absorption spectra of their Chla neighbours. With a view to reaching a firm conclusion about this vexed issue, spinach chloroplasts and thylakoids have been examined with the aid of the same kinetic spectrometer as that used for investigating LHCII; the TmS spectra of both chloroplasts and thylakoids contain prominent bleaching signals centred at 680 nm, and the triplet decay time in each case is comparable to that of the Chla/b LHCII triplets. Results pertaining to other closely related systems are recalled, and it is concluded that, so far as the overall appearance of the TmS spectrum is concerned, spinach chloroplasts are by no means abnormal.

Carotenoids↗