Ferricyanide Photoreduction Research Articles

Structural Changes of D1 C-terminal α-Carboxylate during S-state Cycling in Photosynthetic Oxygen Evolution

Changes in the chemical structure of alpha-carboxylate of the D1 C-terminal Ala-344 during S-state cycling of photosynthetic oxygen-evolving complex were selectively measured using light-induced Fourier transform infrared (FTIR) difference spectroscopy in combination with specific [(13)C]alanine labeling and site-directed mutagenesis in photosystem II core particles from Synechocystis sp. PCC 6803. Several bands for carboxylate symmetric stretching modes in an S(2)/S(1) FTIR difference spectrum were affected by selective (13)C labeling of the alpha-carboxylate of Ala with l-[1-(13)C]alanine, whereas most of the isotopic effects failed to be induced in a site-directed mutant in which Ala-344 was replaced with Gly. Labeling of the alpha-methyl of Ala with l-[3-(13)C]alanine had much smaller effects on the spectrum to induce isotopic bands due to a symmetric CH(3) deformation coupled with the alpha-carboxylate. The isotopic bands for the alpha-carboxylate of Ala-344 showed characteristic changes during S-state cycling. The bands appeared prominently upon the S(1)-to-S(2) transition and to a lesser extent upon the S(2)-to-S(3) transition but reappeared at slightly upshifted frequencies with the opposite sign upon the S(3)-to-S(0) transition. No obvious isotopic band appeared upon the S(0)-to-S(1) transition. These results indicate that the alpha-carboxylate of C-terminal Ala-344 is structurally associated with a manganese ion that becomes oxidized upon the S(1)-to-S(2) transition and reduced reversely upon the S(3)-to-S(0) transition but is not associated with manganese ion(s) oxidized during the S(0)-to-S(1) (and S(2)-to-S(3)) transition(s). Consistently, l-[1-(13)C]alanine labeling also induced spectral changes in the low frequency (670-350 cm(-1)) S(2)/S(1) FTIR difference spectrum.

Effect of white and red light on the pigment content and functional activity in pine chloroplasts.

The pigment content and rates of primary photosynthetic reactions were determined in chloroplasts of 14-day-old pine (Pinus silvestris L.) seedlings grown in light and darkness. In addition, the functional activities were investigated in chloroplasts from dark-grown seedlings exposed to white, red (λ = 670 nm), and red + far-red (λ = 748 nm) light. Dark-grown seedlings were capable of performing the Hill reaction, noncyclic photophosphorylation, and phenazine methosulfate–supported photophosphorylation, although the reaction rates in chloroplasts from dark-grown plants were considerably lower than in preparations from light-grown plants. Light treatment of dark-grown seedlings rapidly activated the photoreduction of ferricyanide and photophosphorylation, while the additional accumulation of green pigments started only after a lag period of two hours. Preirradiation of dark-grown seedlings with red light stimulated the formation of pigments, especially chlorophyll b, as well as the functional activity of chloroplasts. When far-red light was applied after red-light exposure, the processes examined were inhibited. It is concluded that accumulation of the light-harvesting complex and functional activities of chloroplasts at the photosystem II level in pine seedlings are controlled by the phytochrome.

Differential Photosynthetic Electron Transport and Oxidative Stress in Paraquat-Resistant and Sensitive Biotypes ofSolanum americanum

Electron transport in a paraquat-resistant (R) biotype ofSolanum americanumMill. (American black nightshade), when measured as the photoreduction of ferricyanide by thylakoid membranes, was significantly lower than that of a sensitive (S) biotype. The discrepancy in the rate of photosynthesis was confirmed by measuring carbon dioxide fixation at the whole plant level. Dry matter accumulation by the R biotype was exceeded by the S biotype. Superoxide production by nontreated, illuminated thylakoid membranes was also lower in the R biotype, and the increase in oxidative stress that resulted from paraquat treatment was greater in the S biotype. Paraquat-treated thylakoids of the R biotype and nontreated thylakoids of the S biotype had similar rates of O−2production. The low rates of electron transport and correspondingly low levels of oxidative stress in nontreated thylakoids of the R biotype imply that paraquat resistance inS. americanumis due to reduced photosynthetic electron flow.

.ALPHA.-Azohydroperoxide, A New Photosynthetic Reaction Inhibitor.

The azo-compound, 1-(p-bromophenylazo)-1-phenyl-1-hydroperoxymethane, is also called α-azohydroperoxide or AHPO. AHPO differs in structure from other herbicides, and at 10-50μM inhibites O2 evolution and photoreduction of ferricyanide, NADP+ and methyl viologen (O2) which represent photosystem II (PS II), cytochrome-b6/f complex and photosystem I (PS I) reactions. But PS II specific O2 evolution and PS I specific reactions (NADP+ and methyl viologen (O2) photoreduction, cytochrome-f photooxidation and cyclic photophosphorylation), however, were not inhibited. The AHPO inhibition site was presumed to be the electron transport system between PS II and PS I, perhaps at the plastoquinone or the cytochrome-b6/f complex site. Photosynthesis of chloroplasts, green algae and leaf cells were easily inhibited by AHPO. Thus AHPO is an unique inhibitor possibly applicable as a new type of herbicide.

Inhibition of photooxidation of α-tocopherol by quercetin in human blood cell membranes in the presence of hematoporphyrin as a photosensitizer

Photooxidation of alpha-tocopherol (alpha-TH) and photoperoxidation of lipids in blood cell membranes in the presence of hematoporphyrin (HP) as a photosensitizer were inhibited by quercetin. Half maximal inhibition for the photooxidation of alpha-TH was obtained at about 0.3 mM quercetin and that for the lipid photoperoxidation at about 1.5 microM quercetin. The difference of the half maximal inhibition may be due to the difference of mechanism of the inhibition between the two reactions. O2- and H2O2 hardly participated in the photooxidation of alpha-TH and 1O2 participated in the photooxidation only partially (about 5%). The electron transfer reaction from alpha-TH to excited HP was indicated by measuring ferricyanide photoreduction in the suspensions of alpha-TH in PBS solution in the presence of HP. The photooxidation of alpha-TH in PBS solution was inhibited by quercetin and vice versa. In the presence of linoleic acid in PBS solution, quercetin inhibited the photooxidation of alpha-TH and alpha-TH stimulated the photooxidation of quercetin. Based on the above data, as possible mechanisms of the inhibition of photooxidation of alpha-TH in blood cell membranes by quercetin, competition of quercetin with alpha-TH for excited HP and for radicals generated during lipid peroxidation and reduction of oxidized alpha-TH by quercetin are proposed. The antioxidative function of quercetin was enhanced by ascorbate even under conditions in which ascorbate functioned as a prooxidant when it was added alone. The enhancement is attributed to the functions of ascorbate to reduce the oxidized quercetin and of quercetin to inhibit ascorbate photooxidation.

Effect of localized and systemic tobacco mosaic virus infection on some photochemical and enzymatic activities of isolated tobacco chloroplasts

Certain photochemical and enzymic activities of isolated chloroplast preparations were evaluated in tobacco plants susceptible ( Nicotiana tabacum cv. Bright BC 60) and hypersensitive ( N. tabacum cv. Havana 425) to tobacco mosaic virus (TMV) infection. The main results were as follows: 1. (i) Chloroplasts isolated from inoculated (asymptomatic) and systemically infected (with mosaic symptoms) leaves of the susceptible plants did not exhibit any change in the rates of nicotinamide adenine dinucleotide phosphate (NADP) and ferricyanide photoreduction expressed as μmol mg −1 chlorophyll. In contrast, chloroplasts isolated from inoculated leaves of the hypersensitive plants, starting at the beginning of local lesion expression (collapsed areas, 2 days after inoculation; necrotic areas, 3 to 4 days after inoculation) exhibited a strong inhibition of electron transport reaching values about 60–70% of the control. This effect was observed when water was the electron donor, irrespective of whether NADP or ferricyanide was the electron acceptor; no differences were observed when NADP reduction occurred via Photosystem I after 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) inhibition, using ascorbate and 2,6-dichlorophenol indophenol (DCPIP) as an artificial electron donor. 2. (ii) The rate of oxygen photoreduction, expressed as μmol mg −1 chlorophyll, was slightly-increased in the chloroplasts from hypersensitive leaves 1 day after inoculation and then reduced to values about 40% of the control 2–4 days after inoculation. Less oxygen photoreduction was also observed in chloroplasts from systemically infected leaves, while no variation was observed in the chloroplasts from inoculated leaves of the susceptible plants. 3. (iii) Ribulose-1,5-bisphosphate carboxylase activity (measured in chloroplast extracts) was decreased in inoculated hypersensitive leaves during differentiation of local lesions (2 to 4 days after inoculation), reaching values of 30–40% of the control. A less intense decrease of this activity was observed in uninoculated leaves of the susceptible plants displaying mosaic symptoms which did not exhibit any significant variation in photochemical activity (NADP and ferricyanide photoreduction). 4. (iv) Other enzymatic activities measured in chloroplast extracts and/or total-leaf extracts, including malate dehydrogenase nicotinamide adenine dinucleotide-dependent (NADH-dependent) glutamate oxalacetate transaminase, superoxide dismutase, and phosphoenolpyruvate carboxylase, were either unchanged or little affected by TMV infection both in susceptible and hypersensitive combinations. These results are discussed with respect to the hypothesis that the hypofunctionality of the photosynthetic apparatus, characteristic of chloroplasts from hypersensitive leaves, may contribute, in part, to the localization of infection and thence to host resistance.

Uncoupling Effect of Hydroxycinnamic Acid Derivatives on Pea Chloroplasts

Summary The effect of p-coumaric acids as well as the glucoside of caffeic acid at concentrations of 10-7 to 10-4 M on ferricyanide photoreduction, ATP synthesis and proton exchange parameters in pea chloroplasts has been studied. All the 3 compounds were shown to activate electron transport and to inhibit hotophosphorylation. Caffeic acid has the greatest effect, since it inhibits proton uptake by 40% at 10-4 M, whereas the dissipation constant of proton gradient has the greatest value under the effect of pcoumaric acid (140% at 10-4 M). The protolytic properties of the substances under study have been determined by the buffer capacity method. The ρssssK values of p-coumaric acid (4.8 and 9.15), caffeic acid (4.9; 8.85 and > 10) and caffeic acid glucoside (8.6 and > 10) were obtained. It is suggested that the uncoupling effect of hydroxycinnamic acids is due to their acid-base properties and the ability to form complexes with divalent metal ions.

Photoelectrochemical Characterization of Novel Rhodium Iodide Photoconductors

The physical and photoelectrochemical (PEC) properties of three rhodium iodide semiconducting materials have been investigated; crystalline , amorphous , and a new material that may be formulated as . Although the former two materials have been previously prepared, their semiconducting properties have not been examined. A previously unreported semiconducting material results from reacting with triiodide ion in aqueous solutions. Its chemical and physical properties are substantially different from the known rhodium iodides (either crystalline or amorphous). The new material has been isolated as a powder and spontaneously deposits onto substrates including conducting coated glass and freshly etched titanium metal. Chemical reactivity, ESCA, and PEC properties are reported for this new material. The new photoconductor is found to be p‐type with a bandgap of 1.17 eV (indirect) and a direct transition at 1.67 eV. Doping densities depend on the stoichiometry of the original solution phase reactants. The anisotropic photoresponse and the nonstoichiometry of are explained with a chain‐like structure of edge sharing octahedra found in . is photostable for the photoreduction of triiodide, protons, and ferricyanide in aqueous solutions at potentials positive of 0.0 V (vs. SCE).

Functional linkage between phycobilisome and reaction center in two phycobilisome oxygen-evolving photosystem II preparations isolated from the thermophilic cyanobacterium Synechococcus sp

Photosystem II oxygen-evolving preparations with attached phycobilisomes were isolated from the thermophilic cyanobacterium Synechococcus sp. with β-octylglucoside or digitonin. Fluorescence emission spectra of the two preparations determined at 77 K largely lacked a far red band which originates from photosystem I. The spectrum of the digitonin preparation was otherwise similar to that of intact cells, whereas the β-oc-tylglucoside preparation showed a pronounced band at 687 nm, which is considered to be emitted from phycobilisomes. The relative yield of phycobilin fluorescence was similar between the digitonin preparations and the cells but was considerably larger in the β-octylglucoside preparations at room temperature. The quantum yield of ferricyanide photoreduction determined with light which is absorbed mainly by phycobiliproteins was 0.85 for the digitonin preparation and 0.57 for the β-octylglucoside preparation. The results indicate that excitation energy is transferred from phycobilisomes to photosystem II reaction centers in the digitonin preparation as efficiently as in intact cells, while a significant portion of light energy harvested by phycobilisomes is not utilized by the primary photochemistry in the β-octylglucoside preparation. Digitonin and β-octylglucoside preparations had 65 and 48 chlorophyll a molecules per photosystem II reaction center, respectively. The β-octylglucoside preparation contained twice as much phycocyanin and allophycocyanin per photosystem II reaction center as the digitonin preparation, which has a phycobiliprotein-to-photosystem II reaction center ratio very similar to that of cells. It is concluded that whereas the β-octylglucoside preparation contains a considerable amount of free phycobilisomes, all phycobilisomes present in the digitonin preparation are physically and functionally linked to photosystem II reaction center complexes.

Uncoupling of detectable O 2 evolution from the apparent S-state transitions in Photosystem II by lauroylcholine chloride: Possible implications in the photosynthetic water-splitting mechanism

Recently we have introduced the use of choline / fatty acid derived compounds, in particular lauroylcholine chloride (LCC), to probe selectively Photosystem II (PS II) structure and function (Wydrzynski, T. and Huggins, B.J. (1983) in The Oxygen-Evolving System of Photosynthesis (Inoue, Y., Crofts, A.R., Govindjee, Murata, N., Renger, G. and Satoh, K., eds.), pp. 265–272, Academic Press Tokyo, Japan). In this paper we report an unusual condition in thylakoid membrane samples at relatively low amounts of LCC in which detectable O 2 evolution cannot be measured, yet electron flow through PS II is near normal without added electron donors. LCC does not appear to interfere with the O 2 yield measurements directly nor act as an electron donor itself after the Tris block. Under this condition, steady state and flash O 2 yield measurements show no O 2 release or uptake, while steady-state ferricyanide photoreduction and the variable component of the chlorophyll a fluorescence transient remains at more than 50% of the control. The photoreduction of the primary quinone acceptor, Q A, measured by microsecond range chlorophyll a fluorescence continues for a minimum of 200 single turnover excitation light flashes. Most importantly, the yield of the 35 |gms component of the chlorophyll a delayed fluorescence remains at approx. 65% of the control and oscillates with a normal period four over two cycles, indicating the normal cycling of the S-state transitions in PS II. Thus, it appears that PS II can operate normally without detectable O 2 evolution. The question remains as to whether water is still being photooxidized under this condition without the release of the dioxygen product, or whether there is another source of electrons. The results are interpreted in terms of the possible existence of an additional water binding component (termed ‘H’) in PS II and a concerted oxidation reaction mechanism for photosynthetic water splitting.

Photosynthetic Electron Transfer in Preparations of the Cyanobacterium Spirulina platensis

Electron transfer activity in intact trichomes of Spirulina platensis (Nordst.) Geitl. can be observed with either CO(2) or methylviologen as the Hill acceptor. Ferricyanide cannot penetrate the intact trichomes, but photoreduction of this oxidant can be observed when mediated by lipophilic oxidants such as p-phenylenediamine or 2,5-dimethyl-p-benzoquinone. The insensitivity of these reactions to dibromothymoquinone indicates that they are due largely to the activity of photosystem II. Direct photoreduction of ferricyanide can be observed in spheroplasts of Spirulina, indicating that such preparations have altered permeability properties when compared with intact trichomes. Preparation of these spheroplasts, which are osmotically fragile, requires that intact trichomes be washed with KCl and EDTA to induce lysozyme sensitivity and thereby allow digestion of the cell wall. The KCl/EDTA washing procedure used for spheroplast preparation alters the permeability of Spirulina trichomes, as evidenced by the ability of these preparations to photoreduce ferricyanide. This photoreduction reaction is insensitive to dibromothymoquinone, and is stimulated by high concentrations of divalent cations. During assays, the reaction is inhibited by the inclusion of polyethyleneglycol as an osmotic protectant. Photoreduction of methylviologen and NADP(+) is also observed in the washed trichomes, along with an endogenously catalyzed photoreduction of O(2) to H(2)O(2). Photophosphorylation cannot be observed in the washed preparations, but cyclic photophosphorylation with phenazinemethosulfate is observed after mild sonication. These results indicate that KCl/EDTA-washed trichomes of S. platensis retain the full range of energy transducing capacities associated with thylakoid membranes of the intact trichomes; the washing procedure facilitates spheroplast formation and alters, but does not abolish, permeability barriers in these preparations.

Cation-induced, inhibitor-resistant photosystem II reactions in cyanobacterial membranes

Ferricyanide-supported oxygen evolution in sonic vesicles from the cyanobacterium Spirulina platensis is only partially sensitive to inhibition by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), and addition of cations to inhibited membranes stimulates the rate of oxygen evolution. The order of cation effectiveness (M 3+ > M 2+ > M +) suggests that this stimulation is due at least in part to surface charge screening effects which permit freer access of anionic ferricyanide to the vesicle membrane surface; La 3+, Ca 2+, and K + are most effective in this regard. Ferricyanide photoreduction is completely sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and neither mono- nor divalent cations affect this inhibition. Addition of La 3+, on the other hand, causes a nearly complete restoration of ferricyanide-supported oxygen evolution. We conclude that the membrane surfaces of these vesicles are uniquely different from those o higher plants; sites of ferricyanide reduction associated with the interphotosystem chain are surface localized, and the primary acceptor region of photosystem II is susceptible to a trivalent cation-specific reaction in which ferricyanide may directly oxidize the primary acceptor.

The effect on photosynthetic electron transport of temperature-dependent changes in the fluidity of the thylakoid membrane in a thermophilic blue-green alga

Various electron transport reactions in cell or isolated thylakoid membranes of the thermophilic blue-green alga, Synechococcus sp. were measured at a different temperatures between 72 and 3°C. They are classified into two groups with respect to their temperature dependency. The first group involves cytochrome 553 photooxidation, methyl viologen photoreduction with reduced 2,6-dichlorophenolindophenol as electron donor and 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea-resistant ferricyanide photoreduction determined in the presence or absence of silicomolybdate. The Arrhenius plot of these reactions showed a single straight line with the activation energy of about 10 kcal/mol throughout wide temperature ranges studied. Methyl viologen photoreduction with water as electron donor, reduction of flash-oxidized cytochrome 553, ferricyanide photoreduction and photosynthetic O 2 evolution form the second group. Their Arrhenius plots are characterized by discontinuities or breaks at about 30 and 10°C, which respectively correspond to the upper and lower boundaries of the lateral phase separation of the membrane lipids. The first group reactions represent short spans of electron transport which are mediated either by Photosystem I or Photosystem II alone and not related to plastoquinone, whereas all the reactions of the second group involve plastoquinone. It is concluded therefore that the membrane fluidity affect electron transport specifically at the region of plastoquinone. It is proposed that the reaction center chlorophyll-protein complexes of both Photosystems I and II are closely associated with related electron carrier proteins to form functional supramolecular assemblies so that electron transfer within such a cluster of proteins proceeds independently of the phase changes in the membrane lipids. On the other hand, the role of plastoquinone as a mobile electron carrier mediating electron trnasfer from the protein assembly of Photosystem II to that of Photosystem I through the fluid hydrophobic matrix of the membranes is highly sensitive to the physical state of the membrane lipids. It is proposed that the reaction center chlorophyll-protein complexes of both Photosystems I and II are closely associated with related electron carrier proteins to form functional supramolecular assemblies so that electron transfer within such a cluster of proteins proceeds independently of the phase changes in the membrane lipids. On the other hand, the role of plastoquinone as a mobile electron carrier mediating electron transfer from the protein assembly of Photosystem II to that of Photosystem I through the fluid hydrophobic matrix of the membranes is highly sensitive to the physical state of the membrane lipids.

The rapidly metabolized 32,000-dalton polypeptide of the chloroplast is the "proteinaceous shield" regulating photosystem II electron transport and mediating diuron herbicide sensitivity.

Mild trypsin treatment of Spirodela oligorrhiza thylakoid membranes leads to partial digestion of the rapidly metabolized, surface-exposed, 32,000-dalton protein. Under these conditions, photoreduction of ferricyanide becomes insensitive to diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], an inhibitor of photosystem II electron transport. Preincubation of thylakoids with diuron leads to a conformational change in the 32,000-dalton protein, modifying its trypsin digestion and preventing expression of diuron insensitivity. Finally, light affects the susceptibility of the 32,000-dalton protein to digestion by trypsin. In other experiments, thylakoids specifically depleted in the 32,000-dalton protein were found to be deficient in electron transport at the reducing side of photosystem II but not at the oxidizing side or in photosystem I activities. Thus, the rapidly metabolized 32,000-dalton thylakoid protein in Spirodela chloroplasts fulfills the requirements of the hypothesized "proteinaceous shield" [Renger, G. (1976) Biochim. Biophys. Acta 440, 287-300] regulating electron flow through photosystem II and mediating diuron sensitivity.

Inhibition of electron flow and energy transduction in isolated spinach chloroplasts by the herbicide dinoseb

The mode of action of dinoseb (2-sec-butyl-4-6-dinitrophenol) on chloroplast reactions was studied. Electron flow from water or from an artificial electron donor, diphenylcarbazide (DPC), to dichlorophenol indophenol (DCPIP) was inhibited at low concentrations of the herbicide (5–10 μM) suggesting a site for dinoseb inhibition at the oxidizing side of photosystem II (PS II). Ferricyanide photoreduction was also inhibited by dinoseb. Cyclic and non-cyclic photophosphorylation and Mg 2+-ATPase activity were inhibited by dinoseb, which indicates that this herbicide also acts as an energy transfer inhibitor. Among the above mentioned activities, non-cyclic photophosphorylation was the most sensitive to the inhibition by dinoseb. Ca 2+-ATPase activity of solubilized heat activated chloroplast coupling factor 1 (CF 1) was stimulated by dinoseb. However, the same activity was inhibited in chloroplasts, which perhaps reflect a difference in the mode of interaction of dinoseb with solubilized and membrane bound coupling factor.

Composition and Function of Thylakoid Membranes from Grana-rich and Grana-deficient Chloroplast Mutants of Barley

Chlorophyll-deficient barley (Hordeum vulgare) mutants were studied that had chlorophyll a/b ratios either higher or lower than the wild type. Mutants with high ratios (>5.2) had a reduced proportion of their photosynthetic lamellae appressed into grana ("grana-deficient" mutants) compared with wild type (chlorophyll a/b = 3.2), while the majority of lamellae in the chloroplasts with low chlorophyll a/b ratios (2.0-2.4) were organized into grana ("grana-rich" mutants).All mutants catalyzed photosystem I and photosystem II electron transport, were tightly coupled as evidenced by increased rates of electron transport in the presence of methylamine, and were able to generate a light-dependent transmembrane proton gradient. Differences were evident in rates of electron transport per mole of chlorophyll. The mutants having high chlorophyll a/b ratios catalyzed 15- to 50-fold higher rates of ferricyanide photoreduction than the mutants having low chlorophyll a/b ratios, and 5- to 7-fold higher than the wild type.Low temperature absorption spectra of chloroplast fragments showed that the grana-deficient mutant with a high a/b ratio had a chlorophyll spectrum characteristic of a PSI preparation while mutants with the low ratio had a spectrum typical of a PSII preparation.The temperature fluorescence emission spectra of thylakoid membrane fragments from the two types of mutants were also strikingly different from one another, as were the electrophoretic patterns of the thylakoid polypeptides.

Photosynthetic activities of a thermophilic blue-green alga

Photosynthetic activities of a thermophilic blue-green alga, a species of Synechococcus, were studied with special reference to its growth at high temperatures. A rapid algal growth occurred in the temperature range between 50 and 60°C, showing the maximum rate, six doublings per day, at about 57°C. Photosynthetic oxygen evolution and methyl viologen photoreduction in the cells were also active at high temperatures and the optimum temperatures for these activities agreed with that of the algal growth. The growth and photosynthetic activities were very low at room temperature and irreversibly inactivated at temperatures above 60°C. The thylakoid membranes isolated from the alga were also photochemically active at high temperatures. The membranes mediated ferricyanide photoreduction coupled with a stoichiometric oxygen evolution at a rate comparable to that of photosynthetic oxygen evolution in the cells. The optimum temperature for the reaction was as high as 50°C. The membranes also showed a photosystem I-mediated reaction at high temperatures. These observations indicate that the thylakoid membranes are intrinsically thermophilic in this organism. Thus the growth of the alga at high temperatures can be well correlated to thermophilic properties of the photosynthetic apparatus.

The site of ferricyanide photoreduction in pea chloroplasts pretreated by silicomolybdic acid

The site of ferricyanide photoreduction in pea chloroplasts pretreated by silicomolybdic acid

Effect of Temperature on Photoreductive Activity of Chloroplasts From Passionfruit Species of Different Chilling Sensitivity

The Hill reaction activity of chloroplast membranes isolated from five Passiflora species of different chilling sensitivity was determined as a function of temperature over the range 0 - 40°C. Activity was measured by the photoreduction of ferricyanide in the presence of gramicidin D. In two lowland tropical and chilling-sensitive species, P. quadrangularis and P. edulis forma flavicarpa, the apparent Arrhenius activation energy (Ea) for ferricyanide photoreduction was more than doubled at temperatures below 16 and 10°C, respectively. This increase in Ea was also characteristic of more chilling- resistant species. P. cincinnata, which is intermediate in chilling sensitivity between the two lowland tropical and the other two species studied, showed an increase in Ea below 15°C. A hybrid of this species with P. edulis forma flavicarpa behaved similarly. In chloroplasts from P. edulis, a species relatively resistant to chilling temperatures, there was an increase in Ea below 16°C, like that shown by P. quadrangularis, as well as another increase below 4°C. P. caerulea, the most chilling-resistant of the species studied, showed changes in Ea at 7 and 19°C. Thus, there was no clear correlation between the chilling sensitivity or resistance of a species and the presence or absence of a change in the temperature dependence of the Hill activity at low temperatures. The heat stability of the photoreductive activity of chloroplasts isolated from the various Passiflora species also did not correlate with their chilling sensitivity. The heat stability of photoreductive activity in the most cold-tolerant species, P. caerulea, was comparable with that found for the tropical species, while chloroplasts from P. cincinnata were more heat-labile than chloroplasts from the other species.

The Photochemical Activity of Chloroplasts and Subchloroplast Particles Isolated from Etiolated Bean Leaves Exposed to Continuous Light

The photochemical activity of chloroplasts and subchloroplast particles isolated from primary bean leaves between the 4th and 24th hour of illumination of etiolated seedlings is the subject of this paper. The photosystem I activity (oxygen uptake in the presence of MV, DCIP, ascorbate and DCMU), expressed on a unit chlorophyll basis, decreased approximately 10-fold between 4 and 8 h of greening. At the same time the photosystem II activity (DCIP photoreduction in the presence of DPC) was reduced to a half. The photosystem I activity also decreased in all hitherto investigated fractions which were isolated from the digitonin-treated chloroplasts. However, at the initial phase of greening this decrease was the most significant in the fraction containing heavy particles. After 24 h of greening DCMU, at concentrations higher than 10~10 M, limited the rate of ferricyanide photoreduction by isolated chloroplasts, whereas after 6 h of greening this effect was observable even in the presence of 10~12 M DCMU. The results obtained demonstrated that under those conditions both photosystems were active after 4 h of greening and PS I activity developed more rapidly than that of PS II. It also follows from the presented data that the water splitting reactions were delayed in development as compared to the other reactions investigated, and that PS