Composition of photosynthetic pigments in thylakoid membrane vesicles from spinach

Comprehensive genetic diversity and genome-wide association studies revealed the genetic basis of avocado fruit quality traits

Introduction

Avocado (Persea americana) is a highly nutritious fruit gaining worldwide popularity. However, its cultivation is currently reliant on a limited number of cultivars with restricted genetic diversity. This study aims to investigate the genetic diversity and population structure of avocado germplasm and identify genetic loci associated with key fruit quality traits that influence customer preference.

Methods

A diversity panel of 110 avocado accessions was analyzed using 4,706 high-quality single nucleotide polymorphisms (SNPs). Genetic diversity and population structure were analyzed using pairwise FST, AMOVA, admixture analysis, and phylogenetic analysis. Genome-wide association studies (GWAS) were conducted targeting nine fruit quality traits using two models: General Linear Model (GLM) with Principal Component Analysis (PCA) and Mixed Linear Model (MLM) with PCA and kinship (PCA + K).

Results

The analysis revealed three distinct populations corresponding to the three avocado ecotypes: Guatemalan, West Indian, and Mexican. Phylogenetic analysis indicated a closer relationship between the Guatemalan and West Indian races compared to the Mexican race in our Florida germplasm collection. GWAS led to identification of 12 markers within 11 genomic regions significantly associated with fruit quality traits such as fruit color, shape, taste, and skin texture. These markers explained between 14.84% to 43.96% of the phenotypic variance, with an average of 24.63%. Annotation of these genomic regions unveiled candidate genes potentially responsible for controlling these traits.

Discussion

The findings enhance our understanding of genetic diversity and population structure in avocado germplasm. The identified genetic loci provide valuable insights into the genetic basis of fruit quality traits, aiding breeding programs in developing improved avocado cultivars. Marker-assisted selection can accelerate the development of new varieties, promoting a more diverse and resilient avocado market.

Accelerated wound healing induced by spinach extract in experimental model diabetic rats with streptozotocin

Patients with diabetes often have difficult-to-heal wounds. Spinacia oleracea extract comprises anti-inflammatory and anti-oxidative compounds; this research, therefore, studied the impact of Spinacia oleracea extracts on ulcer regeneration. This study was conducted on 72 adult Wistar rats (200 [Formula: see text] 20 g). They were randomly divided into six groups of twelve. A: Diabetic group receiving normal saline. B: Non-diabetic group receiving normal saline. C: Diabetic group receiving spinach aqueous extract. D: Diabetic group receiving spinach alcoholic extract. E: preventive group that received aqueous extract for 2 months. F: preventive group that received alcoholic extract for 2 months. Ulcer regeneration, vascular endothelium growth factor, blood sugar, and weight changes were measured on days 3, 7, 14, 21, and 30. Macroscopic investigation of the wounds non-diabetic control group, diabetic group, as well as spinach aqueous and alcoholic extract groups, were compared and there were significant changes (P < 0.05). Pathologic examination in the spinach aqueous and alcoholic extract groups, and nondiabetic group than in the diabetic group revealed significant advances (P <  0.05). On the third and seventh days, Vascular endothelium growth factor detected significant differences between groups (P < 0.05). Results indicate that, in regenerating diabetic ulcers, Spinacia oleracea may be effective. It influences the ulcer structure and speed.

Effects of thylakoid intake on appetite and weight loss: a systematic review

Background

Previous studies have shown thylakoids, the membrane proteins which are extracted from green leaves like spinach, can induce satiety through homeostatic and non-homeostatic pathways. In this study, we reviewed the current human literature on thylakoids' characteristics and their relationship to satiety regulation and weight loss.

Methods

A systematic search of literature published between January 1990 and May 2019 was conducted on the electronic databases; including WEB OF SCIENCE, Cochrane Library, MEDLINE, Scopus, and EMBASE databases. We included all clinical trials that addressed the effects of thylakoids or chloroplast intake on satiety and weight loss.

Results

After excluding non-human studies, non-RCTs, duplications, studies with irrelevant data and interventions, eight studies were included in the qualitative synthesis. All studies supported this hypothesis that thylakoids reduce the feeling of hunger by increasing postprandial cholecystokinin and leptin and decreasing serum ghrelin, but the consequences of thylakoid intake on anthropometric characteristics were controversial.

Conclusion

In conclusion, our results may approve this postulation that receiving a thylakoid-enriched meal can decrease appetite and probably food intake in short term; however, more studies are needed to explore the effects of long term supplementation with thylakoids on weight loss in human subjects.

Effects of Storage Conditions on Degradation of Chlorophyll and Emulsifying Capacity of Thylakoid Powders Produced by Different Drying Methods

Thylakoid membranes isolated from spinach have previously been shown to inhibit lipase/co-lipase and prolong satiety in vivo. There is a need to develop thylakoid products that not only have the desired characteristics and functionality after processing, but also are stable and provide equivalent effect on appetite over the promised shelf life. The aim of the present study was therefore to evaluate how the thylakoid powders' characteristics and functionality were affected by moisture during storage. Thylakoids produced by drum-drying, spray-drying, and freeze-drying were incubated in controlled atmosphere with different relative humidity (10 RH%, 32 RH%, 48 RH% and 61 RH%) for 8 months. The water content in all powders was increased during storage. The water absorption was moisture-dependent, and the powders were considered hygroscopic. Relative humidity showed a definite influence on the rate of chlorophyll degradation and loss of green color in thylakoid powders after storage which correlated with impaired emulsifying capacity. Spray-dried powder had the overall highest chlorophyll content and emulsifying capacity at all RH-levels investigated. Spray drying was therefore considered the most suitable drying method yielding a powder with best-maintained functionality after storage. The results can be applied towards quality control of high-quality functional foods with appetite suppressing abilities.

The effects of hydroalcoholic extract of spinach on prevention and treatment of some metabolic and histologic features in a rat model of nonalcoholic fatty liver disease

BACKGROUND

This study evaluated the effects of hydroalcoholic extract of spinach (HES) on nonalcoholic fatty liver disease (NAFLD). In the prevention phase, 18 Sprague-Dawley rats were fed a high-fat diet, a high-fat diet plus 400 mg kg^-1 HES, or a chow diet for 7 weeks. For the treatment phase, after the induction of NAFLD, they were fed a high-fat diet, a high-fat diet plus 400 mg kg^-1 HES, a chow diet, or chow diet plus 400 mg kg^-1 HES for 4 weeks (n = 6).

RESULTS

Weight gain (P = 0.01), food intake (P < 0.01), serum glucose (P = 0.01), triglyceride (TG) (P = 0.02), low-density lipoprotein cholesterol (LDL-c) (P = 0.01), aspartate aminotransferase (AST) (P = 0.02), liver steatosis, and the nonalcoholic fatty liver disease (NAFLD) activity score (NAS) (P < 0.01) in the high-fat group were statistically higher than in the other groups at the end of the prevention phase. Feeding spinach extract to rats on a high-fat diet decreased serum glucose (P = 0.01), total cholesterol (TCh) (P < 0.01), AST (P = 0.01), alkaline phosphatase (ALP) (P < 0.01), and liver steatosis (P < 0.01) in the treatment phase.

CONCLUSION

Overall, spinach extract showed beneficial effects in the prevention and treatment of NAFLD. © 2019 Society of Chemical Industry.

Dietary green-plant thylakoids decrease gastric emptying and gut transit, promote changes in the gut microbial flora, but does not cause steatorrhea

Green-plant thylakoids increase satiety by affecting appetite hormones such as ghrelin, cholecystokinin (CCK) and glucagon-like peptide-1 (GLP-1). The objective of this study was to investigate if thylakoids also affect gastrointestinal (GI) passage and microbial composition. To analyse the effects on GI passage, 16 rats were gavage-fed a control or thylakoid-supplemented high-fat diet (HFD) 30 min before receiving Evans blue. Another 16 rats were fed a control HFD or thylakoid HFD for two weeks prior to the intragastric challenge with Evans blue. The amount of Evans blue in the stomach and the distance of migration in the intestines after 30 min were used as a measurement of gastric emptying and intestinal transit. These were reduced by thylakoid supplementation in the acute study, and however not significantly also after the two-week diet study.

The second aim of the study was to investigate if thylakoid-supplementation affects the gut microbiota and amount of faecal fat in healthy human volunteers (n = 34) receiving thylakoid or placebo treatments for three months. Microbiota was analysed using 16S rRNA gene sequencing and qPCR, and faecal fat was extracted by dichloromethane. The total bacteria, and specifically the Bacteriodes fragilis group, were increased by thylakoid treatment versus placebo, while thylakoids did not cause steatorrhea. Dietary supplementation with thylakoids thus affects satiety both via appetite hormones and GI fullness, and affects the microbial composition without causing GI adverse effects such as steatorrhea. This suggests thylakoids as a novel agent in prevention and treatment of obesity.

Molecular Dynamics of Photosystem II Embedded in the Thylakoid Membrane

Photosystem II (PSII) is one of the key protein complexes in photosynthesis. We introduce a coarse grained model of PSII and present the analysis of 60 μs molecular dynamics simulations of PSII in both monomeric and dimeric form, embedded in a thylakoid membrane model that reflects its native lipid composition. We describe in detail the setup of the protein complex and the many natural cofactors and characterize their mobility. Overall we find that the protein subunits and cofactors are more flexible toward the periphery of the complex as well as near the PLQ exchange cavity and at the dimer interface. Of all cofactors, β-carotenes show the highest mobility. Some of the β-carotenes diffuse in and out of the protein complex via the thylakoid membrane. In contrast with the PSII dimer, the monomeric form adopts a tilted conformation in the membrane, with strong interactions between the soluble PsbO subunit and the glycolipid headgroups. Interestingly, the tilted conformation causes buckling of the membrane. Together, our results provide an unprecedented view of PSII dynamics on a microsecond time scale. Our data may be used as basis for the interpretation of experimental data as well as for theoretical models describing exciton energy transfer.

The effect of heat treatment of thylakoids on their ability to inhibit in vitro lipase/co-lipase activity

Thylakoids has been shown to prolong lipolysis by the inhibition of lipase/co-lipase, which makes thylakoids suitable as a functional food ingredient with satiating properties. The components of thylakoids that provide its function as a lipolysis modulator are primarily photosystems I and II, which are structurally stabilised by chlorophyll. However, chlorophyll is known to be heat sensitive yet the enzymatic inhibiting capacity after heat treatment has not been previously studied. It was hypothesised that the retained function of thylakoids after heat treatment could be correlated to the degree of degradation. Heat treatment at either 60 °C, 75 °C or 90 °C for time interval ranging from 15 s to 120 min induced a color shift from bright green to olive brown which was attributed to degradation. The ability of heat-treated thylakoids to inhibit lipolysis in vitro was also reduced. A correlation between chlorophyll a degradation and the enzymatic inhibiting capacity could be established which opens possibilities to use a spectrophotometric method to quantify the ability of thylakoids to inhibit lipase/co-lipase in a more rapid and cost effective way to complement the pH-stat method used today. With the degradation pattern investigated, it is then possible to design a thermal treatment process to ensure a microbiological safe appetite-reducing product and at the same time minimize the loss of functionality.

Heat-induced aggregation of thylakoid membranes affect their interfacial properties

Heat treatment of thylakoid membranes induce loss of chlorophyll, reduced emulsifying capacity and reduced lipase/co-lipase inhibition capacityin vitro.

Chloroplast thylakoids reduce glucose uptake and decrease intestinal macromolecular permeability

Thylakoid membranes, derived from chloroplasts, have previously been shown to retard fat digestion and lower blood glucose levels after oral intake. The purpose of the present study was to investigate the effect of thylakoid membranes on the passage of methyl-glucose, dextran and ovalbumin over rat intestine in vitro using Ussing chambers. The results show that thylakoids retard the passage of each of the test molecules in a dose-dependent way. The thylakoids appear to be attached on the mucosal surface and a mechanism is suggested that the thylakoids delay the passage of the test molecules by sterical hindrance. The present results indicate that thylakoid membranes may be useful both to control intestinal absorption of glucose and to enhance the barrier function of the intestine.

Chloroplast thylakoid membrane-stabilised emulsions

BACKGROUND

Thylakoid-stabilised emulsions have been reported to possess satiety-promoting effects and inhibit pancreatic lipase-colipase activity in vitro, which prompted the investigation of their interfacial properties.

RESULTS

Thylakoid membranes isolated from spinach were used as an emulsifier/stabiliser in oil (triglyceride)-in-water emulsions. Emulsions were characterised with respect to droplet size, interfacial tension, creaming, surface load and electron microscopy. The effects of pH and thylakoid concentration were also considered. Droplet size decreased with increasing thylakoid concentration, reaching a plateau around 15 microm beyond concentrations of 2 mg protein mL(-1) oil. The resulting emulsions were stable against coalescence but were subject to creaming. The surface pressure (air/water interface) of the thylakoid isolate was 44 mN m(-1) and the surface load 13 mg m(-2) at 10 mg protein mL(-1) oil. Electron micrographs showed thylakoids adsorbed as bunched vesicles on the drop surfaces. The stabilisation mechanism can be described as a combined effect of surface-active molecules, mainly membrane proteins but also membrane lipids, exposed on surfaces of thylakoid membrane vesicles adsorbed as particles.

CONCLUSION

Thylakoid membranes effectively stabilise oil-in-water emulsions, which should facilitate their incorporation in food with satiety-promoting effects. To the authors' knowledge, this is the first study on the emulsifying properties of an isolated biological membrane as a functional ingredient.

The constant proportion of grana and stroma lamellae in plant chloroplasts

The relative proportion of stroma lamellae and grana end membranes was determined from electron micrographs of 58 chloroplasts from 21 different plant species. The percentage of grana end membranes varied between 1 and 21% of the total thylakoid membrane indicating a large variation in the size of grana stacks. By contrast the stroma lamellae account for 20.3 +/- 2.5 (sd)% of the total thylakoid membrane. A plot of percentage stroma lamellae against percentage of grana end membranes fits a straight line with a slope of zero showing that the proportion of stroma lamellae is independent of the size of the grana stacks. That stroma lamellae account for about 20% of the thylakoid membrane is in agreement with fragmentation and separation analysis (Gadjieva et al. Biochim. Biophys. Acta 144: 92-100, 1999). Chloroplasts from spinach, grown under high or low light, were fragmented by sonication and separated by countercurrent distribution into two vesicle populations originating from grana and stroma lamellae plus end membranes, respectively. The separation diagrams were very similar lending independent support for the notion that the proportion of stroma lamellae is constant. The results are discussed in relation to the composition and function of the chloroplast in plants grown under different environmental conditions, and in relation to a recent quantitative model for the thylakoid (Albertsson, Trends Plant Sci. 6: 349-354, 2001).

Quantification of photosystem I and II in different parts of the thylakoid membrane from spinach

Electron paramagnetic resonance (EPR) was used to quantify Photosystem I (PSI) and PSII in vesicles originating from a series of well-defined but different domains of the thylakoid membrane in spinach prepared by non-detergent techniques. Thylakoids from spinach were fragmented by sonication and separated by aqueous polymer two-phase partitioning into vesicles originating from grana and stroma lamellae. The grana vesicles were further sonicated and separated into two vesicle preparations originating from the grana margins and the appressed domains of grana (the grana core), respectively. PSI and PSII were determined in the same samples from the maximal size of the EPR signal from P700(+) and Y(D)( .-), respectively. The following PSI/PSII ratios were found: thylakoids, 1.13; grana vesicles, 0.43; grana core, 0.25; grana margins, 1.28; stroma lamellae 3.10. In a sub-fraction of the stroma lamellae, denoted Y-100, PSI was highly enriched and the PSI/PSII ratio was 13. The antenna size of the respective photosystems was calculated from the experimental data and the assumption that a PSII center in the stroma lamellae (PSIIbeta) has an antenna size of 100 Chl. This gave the following results: PSI in grana margins (PSIalpha) 300, PSI (PSIbeta) in stroma lamellae 214, PSII in grana core (PSIIalpha) 280. The results suggest that PSI in grana margins have two additional light-harvesting complex II (LHCII) trimers per reaction center compared to PSI in stroma lamellae, and that PSII in grana has four LHCII trimers per monomer compared to PSII in stroma lamellae. Calculation of the total chlorophyll associated with PSI and PSII, respectively, suggests that more chlorophyll (about 10%) is associated with PSI than with PSII.

Carotenoid levels during the period of growth and ripening in fruits of different olive varieties (Hojiblanca, Picual and Arbequina)

During fruit growth and development, carotenoid accumulation follows the same qualitative pattern in three olive varieties (Olea europaea L.). In the stage of ripening, the Arbequina variety is differentiated from Hojiblanca and Picual by its possession of esterified xanthophylls. The Chl a/b ratio is higher in Arbequina than in Hojiblanca and Picual throughout the life cycle of the fruit, while the percentage of lutein is always lower, and that of beta-carotene higher. Independent of the high (Hojiblanca and Picual) or low (Arbequina) pigment content, the chlorophyll/carotenoid ratio (a + b)/(x + c) is similar for the three varieties. There is evident carotenoid breakdown at the onset of ripening in the fruits of the Hojiblanca and Picual varieties, while in Arbequina there is a new period of carotenoid accumulation. As ripening proceeds in Arbequina fruits, a slow carotenoid-breakdown process is initiated.

Geometrical E/Z isomers of (6R)- and (6S)-neoxanthin and biological implications

9'Z-(3S,5R,6R,3'S,5'R,6'S)-Neoxanthin reisolated from spinach (Spinacea oleracea) and characterized by HPLC, VIS, MS, and 2D (1)H NMR, has been submitted to photoinduced stereomutation in the presence of iodine or diphenyl diselenide at conditions not involving isomerization of the allenic bond. The six individual geometrical isomers, all-E,9Z,9'Z,13Z,13'Z,15Z and three minor di-Z-isomers, presumably 9,9'-di-Z,9',13-di-Z and 9',13'-di-Z, present in the equilibrium mixture have been characterized by HPLC, VIS data, 1H NMR and reversibility tests. Judged by the quantitative composition of the equilibrium mixture the naturally occurring 9'Z-isomer is thermodynamically less stable than the all-E-isomer. The availability of these isomers facilitates future search in natural sources. 9'Z-(6R90% of total neoxanthin in spinach and broccoli (Brassica oleracea var. italica), consistent with previous findings of its abundance in chloroplasts. all-E90% of total violaxanthin in the same sources. It is postulated that a neoxanthin Delta9'-isomerase is present and involved in the biosynthesis of abscisic acid in higher plants. Allenic S-isomers are of interest as postulated biosynthetic precursors of R-allenes. All-E-(6S)- and 9'Z-(6S)-neoxanthin were available as semi-synthetic model compounds. The allenic (6S)-diastereomers could not be detected in spinach or broccoli.

The topology of phosphatidylglycerol populations is essential for sustaining photosynthetic electron flow activities in thylakoid membranes

The transmembrane distribution of phosphatidylglycerol (PG) was determined in rightside-out (RO) and inside-out vesicles (IO) obtained by fragmentation of spinach thylakoids in a Yeda press, followed by partition in an aqueous dextran-polyethyleneglycol two-phase system. Using the phospholipase A(2) from porcine pancreas to digest selectively PG molecules in the outer monolayer (exposed to the incubation medium) of the membrane, we found the molar outside/inside distribution to be 70/30+/-5 in RO and 40/60+/-3 in IO. The transmembrane distribution of PG in IO was the opposite of that in intact thylakoids (molar ratio 58/42+/-3). The phospholipid population which sustained most of the uncoupled photosystem II electron flow activity was localized in the inner monolayer (exposed to the thylakoid lumen) of both thylakoid and RO membranes. In contrast, the activity in IO membranes was highly dependent on the PG population located in the outer monolayer. This finding brings the first direct demonstration of the dependence of the photosynthetic electron flow activity on the integrity of the inner topological pool of PG in the thylakoid membrane.

Fractionation of the thylakoid membranes from tobacco. A tentative isolation of 'end membrane' and purified 'stroma lamellae' membranes

Thylakoids isolated from tobacco were fragmented by sonication and the vesicles so obtained were separated by partitioning in aqueous polymer two-phase systems. By this procedure, grana vesicles were separated from stroma exposed membrane vesicles. The latter vesicles could be further fractionated by countercurrent distribution, with dextran-polyethylene glycol phase systems, and divided into two main populations, tentatively named 'stroma lamellae' and 'end membrane'. Both these vesicle preparations have high chlorophyll a/b ratio, high photosystem (PS) I and low PS II content, suggesting their origin from stroma exposed regions of the thylakoid. The two vesicle populations have been compared with respect to biochemical composition and photosynthetic activity. The 'end membrane' has a higher chlorophyll a/b ratio (5.7 vs. 4.7), higher P700 content (4.7 vs. 3.3 mmol/mol of chlorophyll). The 'end membrane' has the lowest PS II content, the ratio PS I/PS II being more than 10, as shown by EPR measurements. The PS II in both fractions is of the beta-type. The decay of fluorescence is different for the two populations, the 'stroma lamellae' showing a very slow decay even in the presence of K3Fe(CN)6 as an acceptor. The two vesicle populations have very different surface properties: the end membranes prefer the upper phase much more than the stroma lamellae, a fact which was utilized for their separation. Arguments are presented which support the suggestion that the two vesicle populations originate from the grana end membranes and the stroma lamellae, respectively.

The structure and function of the chloroplast photosynthetic membrane - a model for the domain organization

Recent work on the domain organization of the thylakoid is reviewed and a model for the thylakoid of higher plants is presented. According to this model the thylakoid membrane is divided into three main domains: the stroma lamellae, the grana margins and the grana core (partitions). These have different biochemical compositions and have specialized functions. Linear electron transport occurs in the grana while cyclic electron transport is restricted to the stroma lamellae. This model is based on the following results and considerations. (1) There is no good candidate for a long-range mobile redox carrier between PS II in the grana and PS I in the stroma lamellae. The lateral diffusion of plastoquinone and plastocyanin is severely restricted by macromolecular crowding in the membrane and the lumen respectively. (2) There is an excess of 14±18% chlorophyll associated with PS I over that of PS II. This excess is assumed to be localized in the stroma lamellae where PS I drives cyclic electron transport. (3) For several plant species, the stroma lamellae account for 20±3% of the thylakoid membrane and the grana (including the appressed regions, margins and end membranes) for the remaining 80%. The amount of stroma lamellae (20%) corresponds to the excess (14-18%) of chlorophyll associated with PS I. (4) The model predicts a quantum requirement of about 10 quanta per oxygen molecule evolved, which is in good agreement with experimentally observed values. (5) There are at least two pools of each of the following components: PS I, PS II, cytochrome bf complex, plastocyanin, ATP synthase and plastoquinone. One pool is in the grana and the other in the stroma compartments. So far, it has been demonstrated that the PS I, PS II and cytochrome bf complexes each differ in their respective pools.

Atomic model of plant light-harvesting complex by electron crystallography

The structure of the light-harvesting chlorophyll a/b-protein complex, an integral membrane protein, has been determined at 3.4 A resolution by electron crystallography of two-dimensional crystals. Two of the three membrane-spanning alpha-helices are held together by ion pairs formed by charged residues that also serve as chlorophyll ligands. In the centre of the complex, chlorophyll a is in close contact with chlorophyll b for rapid energy transfer, and with two carotenoids that prevent the formation of toxic singlet oxygen.

The 28 kDa apoprotein of CP 26 in PS II binds copper

Photosystem II (PS II) particles isolated from spinach in the presence of 10 μM CuSO4 contained 1.2 copper/300 Chl that was resistant to EDTA. When CuSO4 was not added during the isolation, PS II particles contained variable amounts of copper resistant to EDTA (0.1-1.1 copper/300 Chl). No correlation was found between copper content and oxygen evolving capacity of the PS II particles. To identify the copper binding protein, we developed a fractionation procedure which included solubilisation of PS II particles followed by precipitation with polyethylene glycol. A 22-fold purification of copper with respect to protein was achieved for a 28 kDa protein. Partial amino acid sequence of a 13 kDa fragment, obtained after V8 (endo Glu-C) protease treatment, showed identity with CP 26 over a 14 amino acid stretch. EPR measurements on the purified protein suggest oxygen and/or nitrogen as ligands for copper but tend to exclude sulfur. We conclude that the 28 kDa apoprotein of CP 26 from spinach binds one copper per molecule of CP 26. A possible function for this copper protein in the xanthophyll cycle is discussed.

Characterization of a non-detergent PS II-cytochrome b/f preparation (BS)

A non-detergent photosystem II preparation, named BS, has been characterized by countercurrent distribution, light saturation curves, absorption spectra and fluorescence at room and at low temperature (-196°C). The BS fraction is prepared by a sonication-phase partitioning procedure (Svensson P and Albertsson P-Å, Photosynth Res 20: 249-259, 1989) which removes the stroma lamellae and the margins from the grana and leaves the appressed partition region intact in the form of vesicles. These are closed structures of inside-out conformation. They have a chlorophyll a/b ratio of 1.8-2.0, have a high oxygen evolving capacity (295 μmol O2 per mg chl h), are depleted in P700 and enriched in the cytochrome b/f complex. They have about 2 Photosystem II reaction centers per 1 cytochrome b/f complex.The plastoquinone pool available for PS II in the BS vesicles is 6-7 quinones per reaction center, about the same as for the whole thylakoid. It is concluded, therefore, that the plastoquinone of the stroma lamellae is not available to the PS II in the grana and that plastoquinone does not act as a long range electron transport shuttler between the grana and stroma lamellae.Compared with Photosystem II particles prepared by detergent (Triton X-100) treatment, the BS vesicles retain more cytochrome b/f complex and are more homogenous in their surface properties, as revealed by countercurrent distribution, and they have a more efficient energy transfer from the antenna pigments to the reaction center.