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PubMed1975

Silicomolybdate and silicotungstate mediated dichlorophenyldimethylurea-insensitive photosystem II reaction: electron flow, chlorophyll a fluorescence and delayed light emission changes.

We have investigated the possible role of silicomolybdate and silicotungstate as acceptors of electrons in chloroplasts directly from Q, the primary electron acceptor of Photosystem II. Our data show: 1. Either of these compounds can accept electrons directly from Q in a 3-(3', 4'-dichlorophenyl)-1, 1-dimethylurea (DCMU)-insensitive electron transport; however, the DCMU insensitivity is only short-lived, so initial rates must be used exclusively. 2. High concentrations of these silico compounds act as direct chemical quenchers of chlorophyll a fluorescence, but lower concentrations which also mediate O2 evolution affect only the variable component of fluorescence in a manner suggestive of their electron-accepting capabilities. 3. Measurements of delayed light emission confirm the conclusions made from the fluorescence data. Also, they show the role of Q in delayed light emission as hydroxylamine data of other investigations have shown the role of Z, the electron donor of Photosystem II. 4. Silico compounds appear to be acting as electron acceptors and not as simple membrane modifiers allowing other acceptors to support a DCMU-insensitive electron transport.

Chlorophyll↗
PubMed1975

Identification of the 120 mus phase in the decay of delayed fluorescence in spinach chloroplasts and subchloroplast particles as the intrinsic back reaction. The dependence of the level of this phase on the thylakoids internal pH.

After a 500 mus laser flash a 120 mus phase in the decay of delayed fluorescence is visible under a variety of circumstances in spinach chloroplasts and subchloroplast particles enriched in Photosystem II prepared by means of digitonin. The level of this phase is high in the case of inhibition of oxygen evolution at the donor side of Photosystem II. Comparison with the results of Babcock and Sauer (1975) Biochim. Bio-phys. Acta 376, 329-344, indicates that their EPR signal IIf which they suppose to be due to Z+, the oxidized first secondary donor of Photosystem II, is well correlated with a large amplitude of our 120 mus phase. We explain our 120 mus phase by the intrinsic back reaction of the excited reaction center in the presence of Z+, as predicted by Van Gorkom and Donze (1973) Photochem. Photobiol. 17, 333-342. The redox state of Z+ is dependent on the internal pH of the thylakoids. The results on the effect of pH in the mus region are compared with those obtained in the ms region.

Carbonyl Cyanide m-Chlorophenyl Hydrazone↗
PubMed1975

Light-induced changes of absorbance and electron spin resonance in small photosystem II particles.

Photosystem II reaction center components have been studied in small system II particles prepared with digitonin. Upon illumination the reduction of the primary acceptor was indicated by absorbance changes due to the reduction of a plastoquinone to the semiquinone anion and by a small blue shifts of absorption bands near 545 nm (C550) and 685 nm. The semiquinone to chlorophyll ratio was between 1/20 and 1/70 in various preparations. The terminal electron donor in this reaction did not cause large absorbance changes but its oxidized form was revealed by a hitherto unknown electron spin resonance (ESR) signal, which had some properties of the well-known signal II but a linewidth and g-value much nearer to those of signal I. Upon darkening absorbance and ESR changes decayed together in a cyclic or back reaction which was stimulated by 3-(3,4 dichlorophenyl)-1,1-dimethylurea. The donor could be oxidized by ferricyanide in the dark. Illumination in the presence of ferricyanide induced absorbance and ESR changes, rapidly reversed upon darkening, which may be ascribed to the oxidation of a chlorophyll a dimer, possibly the primary electron donor of photosystem II. In addition an ESR signal with 15 to 20 gauss linewidth and a slower dark decay was observed, which may have been caused by a secondary donor.

Binding Sites↗
PubMed1975

Chlorophyll radical cation in photosystem II of chloroplasts. Millisecond decay at low temperature.

We compare the absorption changes, in the near infrared and in the green part of the spectrum, induced in spinach chloroplasts suspensions, at -- 170 degrees C, by continuous light and by flashes. (1) Following flash excitation, an absorption increase peaking at 825 nm which reverses rapidly (t 1/2 = 3.0 ms) is not affected by ferricyanide; it is suppressed when chloroplasts are preilluminated in the presence of 3-(3',4'-dichlorophenyl)-1,1'-dimethylurea (DCMU) and hydroxylamine. The reversion of that signal is simultaneous with a partial back reoxidation of C-550 (fully reduced by the flash) and with partial (about 25%) oxidation of cytochrome b559. The magnitude of the signal peaking at 825 nm (that we attribute to the radical cation of the trap chlorophyll of Photosystem II, acting as a primary electron donor) decreases progressively within a series of successive flashes. (2) An absorption increase (40% of which is slowly reversible) with a broad peak around 810 nm is induced by continuous light or by a flash. It is suppressed by pretreatment with ferricyanide, but it is little affected by the treatment with 3-(3',4'-dichlorophenyl)-1,1'-dimethylurea and hydroxylamine. We attribute it to oxidized P700. (3) With chloroplasts pretreated with 10 mM ferricyanide, an absorption increase, whose magnitude is nearly independent of wavelength between 790 and 870 nm, can be induced by continuous light. One saturating flash produces only 20% of the signal. This absorption change (20% of which is reversible in 30 s) might be due to a secondary donor of Photosystem II.

Chlorophyll↗
PubMed1975

Vanadium compounds and ferrocyanide as ionic redox agents in photosynthesis.

The effect of such ionic redox agents as ferrocyanide and several vanadium compounds was determined on photosynthetic reactions of spinach chloroplasts. It was found that: 1. Vanadyl sulfate like ferrocyanide in moderately high concentrations (0.03 M) donates electrons to Photosystem II. 2. Decavanadate in the presence of 2,5-dibromothymoquinone accepts electrons in Photosystem II. 3. In the absence of a block between the two photosystems, decavanadate accepts electrons in Photosystem I in the vicinity of plastocyanin or beyond. 4. Vanadite and ferrocyanide in high concentrations (0.32 M) donate electrons to Photosystem I. 5. On the basis of chelator inhibition and polyoxyethylene sorbitan monolaureate treatment, the vanadite oxidation site is located near plastocyanin while the ferrocyanide site is between plastocyanin and P-700.

Chloroplasts↗
PubMed1975

Fluorescence induction in intact spinach chloroplasts.

Under conditions in which the Photosystem II quencher is rapidly reduced upon illumination, either after a preillumination or following treatment with dithionite, the fluorescence-induction curve of intact spinich chloroplasts (class I type) displays a pronounced dip. This dip is probably identical with that observed after prolonged anaerobic incubation of whole algal cells ("I-D dip"). It is inhibited by 3(3,4-dichlorophenyl)-1,1-dimethylurea and occurs in the presence of dithionite, sufficient to reduce the plastoquinone pool. It is influenced by far red light, methylviologen, anaerobiosis and uncouplers in a manner consistent with the interpretation that it represents a photochemical quenching of fluorescence by an electron transport component situated between the Photosystem II quencher and plastoquinone. Glutaraldehyde inhibition may indicate that protein structural changes are involved.

Anaerobiosis↗
PubMed1975

Evidence for energy migration from photosystem I to photosystem II and the effect of magnesium.

1. With Euglena chloroplasts 7722 nm light excites a fluorescence induction pattern, measured at l,u nm, typical of the Photosystem II reduction of Q. 2. Both methylviologen and phenazine methosulfate partially quench this fluorescence rise in the absence of electron transport between Photosystem II and Photosystem I, and in a manner consistent with their known role of stimulating Photosystem I photochemistry. 3. The variable fluorescence excited with 22 nm light is reduced to a greater extent than that excited by 638 nm light upon chlorophyll dilution during division in the dark. 4. These observations are interpreted to indicate that in Euglena chloroplasts light absorbed by Photosystem I is transferred energetically "uphill" to Photosystem II, where it can perform Photosystem II photochemistry. 5. Magnesium ions stimulate the variable fluorescence with both 638 nm and 722 nm light to a similar extent, which argues against the concept of magnesium ion interrupting energy transfer between Photosystem II and Photosystem I.

Chlorophyll↗
PubMed1975

The effect of 1-ethyl-3(3-dimethylaminopropyl)carbodiimide on calcium binding and associated changes in chloroplast structure and chlorophyll a fluorescence in spinach chloroplasts.

1. Chemical modification of carboxyl groups on the chloroplast membrane with a water-soluble carbodiimide plus a nucleophile caused inhibition of Ca-2plus binding. 2. Both binding sites were affected and showed a decrease in the number of binding sites and an increase in the dissociation constant. 3. Cation-induced changes in chlorophyll a fluorescence and structural changes (deltaA540) were inhibited at the same carbodiimide concentrations as Ca-2plus binding, emphasizing the relationship between these processes. 4. Chloroplasts that were illuminated with high intensity light for short time periods showed a decrease in the carbodiimide-mediated inhibition of Ca-2plus binding.

Calcium↗
PubMed1975

Temperature and preillumination dependence of delayed fluorescence of spinach chloroplasts.

Delayed fluorescence (luminescence) from spinach chloroplasts, induced by short saturating flashes, was studied in the temperature region between 0 and minus 40 degrees C. At these temperatures, in contrast to what is observed at room temperature, luminescence at 40 ms after a flash was strongly dependent, with period four, on the number of preilluminating flashes (given at room temperature, before cooling). At minus 35 degrees C luminescence of chloroplasts preilluminated with two flashes (the optimal preillumination) was about 15 times larger than that of dark-adapted chloroplasts. The intensity of luminescence obtained with preilluminated chloroplasts increased steeply below minus 10 degrees C, presumably partly due to accumulation of reduced acceptor (Q minus), and reached a maximum at minus 35 degrees C. In the presence of 50 mM NH4Cl; at temperatures below minus 20 degrees C luminescence at 40 ms was decreased by NH4C1. At room temperature a strongly enhanced 40-ms luminescence was observed after the third and following flashes. The results indicate that both the S2 to S3 and the S3 to S4 conversion are affected by NlH4Cl. Inhibitors of Q minus reoxidation, like 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea, did only slightly affect the preillumination dependence of luminescence at sub-zero temperatures if they were added after the preillumination. This indicates that these substances by themselves do not accelerate the deactivation of S2 and S3.

Ammonium Chloride↗
PubMed1976

Phosphorylation in isolated chloroplasts coupled to dichlorophenyldimethylurea-insensitive silicomolybdate reduction.

1. The electron transport in isolated chloroplasts with silicomolybdate as electron acceptor has been reinvestigated. The silicomolybdate reduction has been directly measured as deltaA750 or indirectly as O2 evolution (in the presence or absence of ferricyanide). 2. Silicomolybdate-dependent O2 evolution is inhibited to a similar extent by 3-(3,4-dichlorophenyl) 1, 1-dimethylurea (DCMU) or dibromothymoquinone (DBMIB), indicating the existence of two different sites of silicomolybdate reduction: one before the DCMU block (i.e. at Photosystem II) and one after the DBMIB block (i.e. at Photosystem I). 3. Silicomolybdate-dependent O2 evolution is coupled to ATP synthesis with an ATP/2e- ratio of 1.0 to 1.1. The presence of ferricyanide inhibits this ATP synthesis (ATP/2e- ratio then is about 0.3). 4. Silicomolybdate-dependent O2 evolution is also coupled to ATP-synthesis in the presence of DCMU with an ATP/2e- ratio of 0.6-0.8 characteristic of Site II; in this case the electron transport itself is not affected by uncouplers or energy-transfer inhibitors. 5. The data are interpreted as a further demonstration that the water-splitting reaction is responsible for the conservation of energy at Photosystem II.

Adenosine Triphosphate↗
PubMed1976

Concentration-dependent effects of salicylaldoxime on chloroplast reactions.

Salicylaldoxime has been found to have a variety of concentration-dependent effects on chloroplast activities. At low concentrations (less than 10 mM), salicyladoxime reversibly inhibits all reactions which involve Photosystem II. Since the DCMU-insensitive silicomolybdate Hill reaction is also inhibited, one site of inhibition is definitely located before the DCMU-sensitive site, possibly before the photoact. The inhibition kinetics and the response of chloroplast fluorescence may indicate another site in the DCMU-sensitive region. At almost exactly the same concentrations (less than 10 mM), salicylaldoxime uncouples phosphorylation reversibly, whether it is supported by Photosystem II or by Photosystem I. At higher concentrations (approx. 20 mM) salicylaldoxime inhibits Photosystem II irreversibly, uncouples irreversibly, and begins to cause changes in chloroplast light scattering which could be manifestations of membrane damage. At very high concentrations (approx. 45 mM) salicylaldoxime irreversibly inhibits Photosystem I activity in the region of plastocyanin. This is indicated by the ability of salicylaldoxime to inhibit the photooxidation of cytochrome f but not the photooxidation of P-700.

Chloroplasts↗
PubMed1976

Light-dependent inhibitory action of p-nitrothiophenol on photosystem II in relation to the redox state of electron carriers.

The photosystem-II activity of chloroplasts was inhibited by the treatment with p-nitrothiophenol (NphSH) in the light, and the inhibition was accompanied by a change of the fluorescence spectrum. Aromatic mercaptans examined were active in causing this inhibition and fluorescence change. These effects of p-nitrothiophenol were highly accelerated by blocking the electron transport of the oxidation side of photosystem II by carbonyl-cyanide-m-chlorophenylhydrazone (CCCP) or Tris-HC1 or heat pre-treatment, whereas these were suppressed by blocking the transport on the reduction side by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). It was deduced that the site of NphSH action in the electron transport chain is closer to the reaction center of photosystem II that the blocking site of CCCP or Tris-HC1 or heat, and that such a site in photosystem II is exposed to be modified with NphSH when electron carriers on the oxidation side of photosystem II are oxidized by illumination.

Carbonyl Cyanide m-Chlorophenyl Hydrazone↗
PubMed1976

A highactivity ATP translocator in mesophyll chloroplasts of Digitaria sanguinalis, a plant having the C-4 dicarboxylic acid pathway of photosynthesis.

The effect of exogenous adenine nucleotides on CO2 fixation and oxygen evolution was studied with mesophyll protoplast extracts of the C4 plant Digitaria sanguinalis. Exogenous ATP was found to stimulate the rate of pyruvate and pyruvate+oxalacetate induced CO2 fixation, as well as reverse the inhibition of CO2 fixation of carbonyl cyanide m-chlorophenyl hydrazone and several electron transport inhibitors. The ATP-dependent stimulation of CO2 fixation varied from 40 to 70 mumol CO2 fixed/mg chlorophyll per h, suggesting that ATP was crossing the chloroplast membranes at rates of 80-140 mumol/mg chlorophyll per h, since 2 ATP are required for each CO2 fixed. Fixation of CO2 could also be induced in the dark by exogenous ATP, in which case ADP accumulated outside the chloroplasts...

Adenosine Diphosphate↗
PubMed1976

Studies on the structural and functional organization of system II of photosynthesis. The use of trypsin as a structurally selective inhibitor at the outer surface of the thylakoid membrane.

The effect of trypsin on the photosynthetic electron transport has been investigated in the presence of various electron acceptors (benzyl viologen, p-benzo-quinone, K3[Fe(CN)6]) by measurements of flash-induced oxygen evolution and of the absorption changes at 334 nm, indicating the primary electron acceptor of System II, X 320, and at 515 nm, indicating via electrochromism the electrical potential gradient across the thylakoid membrane. It was found that the effect of trypsin is strongly dependent on the nature of the electron acceptor: (1) Oxygen evolution is completely inhibited in the presence of p-benzo-quinone, but remains nearly unaffected by K3[Fe(CN)]6. (2) The initial amplitude deltaAO of the 334 nm absorption change is insensitive to trypsin in the presence of K3[Fe(CN)6], but the absorption change is abolished if benzyl viologen is used as acceptor. (3) The initial amplitude deltaAO of the 515 nm absorption change decreases by trypsin down to 50% with K3[Fe(CN)6] and is completely suppressed with benzyl viologen. (4) In trypsinated chloroplasts, the above-mentioned activities appear to be rather insensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea, in contrast to normal chloroplasts. On the basis of these results it is inferred that the primary electron acceptor of System II, X 320, is covered by a proteinaceous component susceptible to tryptic digestion. In addition, it is postulated that this component acts as well as an allosteric protein responsible for the regulation of the electronic interaction between X 320 and the plastoquinone pool, as for the inhibitory effect of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Various other possible effects caused by the proteinaceous shield and its modification by trypsin are discussed. The present results are in complete agreement with asymmetric membrane models postulating a zig-zag arrangement of the electron transport chain with the reducing side located towards the outer phase and the oxidizing side near the inner phase of the thylakoids.

2,6-Dichloroindophenol↗
PubMed1976

A major site of bicarbonate effect in system II reaction. Evidence from ESR signal IIvf, fast fluorescence yield changes and delayed light emission.

In order to determine the major site of bicarbonate action in the electron transport complex of Photosystem II, the following experimental techniques were used: electron spin resonance measurements of Signal IIvf, measurements of chlorophyll a fluorescence yield rise and decay kinetics, and delayed light emission decay. From data obtained using these experimental techniques the following conclusions were made: (1) absence of bicarbonate causes a reversible inactivation of up to 40% of Photosystem II reaction center activity; (2) there is no significant effect of bicarbonate on electron flow from the charge accumulating S state to Z; (3) there is no significant effect of bicarbonate on electron flow from Z to P-680+; (4) electron flow from Q-- to the intersystem electron transport pool is inhibited by from 4- to 6-fold under bicarbonate depletion conditions.

Bicarbonates↗
PubMed1976

A demonstration of energy transfer from photosystem II to photosystem I in chloroplasts.

Photosystem I activity of Tris-washed chloroplasts was measured at room temperature as the rate of photoreduction of NADP and as the rate of oxygen uptake mediated by methyl viologen in both cases using dichlorophenolindophenol plus ascorbate as the source of electrons for Photosystem I. With both assay systems the rate of electron transport by Photosystem I was stimulated approx. 20% by the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea which caused the Photosystem II reaction centers to close. Photosystem I activity of chloroplasts was measured at low temperature as the rate of photooxidation of P-700. Chloroplasts suspended in the presence of hydroxylamine and 3-(3,4-dichlorophenyl)-1,1-dimethylurea were frozen to -196 degrees C after adaptation to darkness or after a preillumination at room temperature. The Photosystem II reaction centers of the frozen dark-adapted sample were all open; those of the preilluminated sample were all closed. The rate of photooxidation of P-700 at -196 degrees C with the preilluminated sample was approx. 25% faster than with the dark-adapted sample. We conclude from both the room temperature and the low temperature experiments that there is greater energy transfer from Photosystem II to Photosystem I when the Photosystem II reaction centers are closed and that these results are a direct demonstration of spillover.

Chloroplasts↗
PubMed1976

Photoinduction kinetics of electrical potential in a single chloroplast as studied with micro-electrode technique.

1. Using single chloroplasts of Peperomia metallica the kinetics of light-induced potential changes were studied. Three kinetic components (the initial fast rise, the decay in the light and the decay in the dark) were found to be characterized by time constants 4, 220 and 60 ms, respectively at light intensity 5000 1x and temperature 18 degrees C. After flash excitation the potential kept on rising for about 10 ms. Cooling of the medium down to 5 degrees C had no effect on the duration of potential rise after the flash. 2. Variations in the medium temperature in the range 2-23 degrees C had little effect on photoresponse magnitude but resulted in significant acceleration of decay in the light. 3. Addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (5-)0(-6) M) resulted in suppression of the magnitude of the photoresponse but was not accompanied by any change in the rate of initial rise of potential. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea-inhibited photoresponse could be restored and even enhanced by subsequent addition of N-methylphenazonium methosulfate (10(-4) M). N-Methylphenazonium methosulfate essentially influenced the time course and light-intensity curves of photoresponse. 4. The chloroplast photoresponses were of different time-courses when elicited by red (640 nm) or far red (712 nm) light. This fact as well as an enhancement effect of combined illumination by two intermittent light beams indicate on the interaction of two photosynthetic pigment systems when the photoelectric response was formed. 5. An imposed electrical field resulted in stimulation or suppression of chloroplast photoresponse depending on the polarity of the field. No indications for the existance of "reversal potential" for photoelectric response were obtained. 6. A kinetic scheme of photoresponse formation is proposed, which includes two sequential photochemical reactions of photosynthesis.

Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone↗
PubMed1976

Rates and properties of endogenous cyclic photophosphorylation of isolated intact chloroplasts measured by CO2 fixation in the presence of dihydroxyacetone phosphate.

1. Dihydroxyacetone phosphate in concentrations greater than or equal to 2.5 mM completely inhibits CO2-dependent O2 evolution in isolated intact spinach chloroplasts. This inhibition is reversed by the addition of equimolar concentrations of Pi, but not by addition of 3-phosphoglycerate. In the absence of Pi, 3-phosphoglycerate and dihydroxyacetone phosphate, only about 20% of the 14C-labelled intermediates are found in the supernatant, whereas in the presence of each of these substances the percentage of labelled intermediates in the supernatant is increased up to 70-95%. Based on these results the mechanism of the inhibition of O2 evolution by dihydroxyacetone phosphate is discussed with respect to the function of the known phosphate translocator in the envelope of intact chloroplasts. 2. Although O2 evolution is completely suppressed by dihydroxyacetone phosphate, CO2 fixation takes place in air with rates of up to 65 mu mol-mg1 chlorophyll-h1. As non-cyclic electron transport apparently does not occur under these conditions, these rates must be due to endogenous pseudocyclic and/or cyclic photophosphorylation. 3. Under anaerobic conditions, the rates of CO2 fixation in presence of dihydroxyacetone phosphate are low (2.5-7 mumol-mg1 chlorophyll-h1), but they are strongly stimulated by addition of dichlorophenyl-dimethylurea (e.g. 2-10(-7) M) reaching values of up to 60 mumol-mg1 chlorophyll-h1. As under these conditions the ATP necessary for CO2 fixation can be formed by an endogenous cyclic photophosphorylation, the capacity of this process seems to be relatively high, so it might contribute significantly to the energy supply of the chloroplast. As dichlorophenyl-dimethylurea stimulates CO2 fixation in presence of dihydroxyacetone phosphate under anaerobic but not under aerobic conditions, it is concluded t-at only under anaerobic conditions an "overreduction" of the cyclic electron transport system takes place, which is removed by dichlorophenyl-dimethylurea in suitable concentrations. At concentrations above 5-10(-7) M dichlorophenyl-dimethylurea inhibits dihydroxyacetone phosphate-dependent CO2 fixation under anaerobic as well as under aerobic conditions in a similar way as normal CO2 fixation. Therefore, we assume that a properly poised redox state of the electron transport chain is necessary for an optimal occurrence of endogenous cyclic photophosphorylation. 4. The inhibition of dichlorophenyl-dimethylurea-stimulated CO2 fixation in presence of dihydroxyacetone phoshate by dibromothymoquinone under anaerobic conditions indicated that plastoquinone is an indispensible component of the endogenous cyclic electron pathway.

Carbon Dioxide↗