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Govindjee

Publications and source records attributed to Govindjee.

At least 73 records · Page 4Linked to original sources
PubMed1977

Site of bicarbonate effect in Hill reaction. Evidence from the use of artificial electron acceptors and donors.

Using artificial electron donors and acceptors, it is shown here that the major HCO3- effect in the Hill reaction is after the "primary" electron acceptor (Q) of Photosystem II and before the site of action of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (at the plastoquinone pool). Chloroplasts in the presence of both 3-(3',4'-dichlorophenyl)-1,1-dimethylurea, which blocks electron flow from the reduced primary acdeptor Q- to the plastoquinone pool, and silicomolybdate, which accepts electrons from Q-, show no significant bicarbonate stimulation of electron flow. However, a 6-7 fold stimulation is clearly observed when oxidized diaminodurene, as an electron acceptor, and dibromothymoquinone, as an inhibitor of electron flow beyond the plastoquinone pool, are used. In the same chloroplast preparation no measurable effect of bicarbonate is observed in a Photosystem I reaction as monitored by electron flow from reduced diaminodurene to methyl viologen in the presence of 3- (3',4'-dichlorophenyl)-1,1-dimethylurea. The insensitivity of the bicarbonate effect to uncouplers of photophosphorylation and the dependence of this effect on the presence of a weak acid anion and on external pH are also reported.

Bicarbonates↗
PubMed1977

The rise in chlorophyll a fluorescence yield and decay in delayed light emission in tris-washed chloroplasts in the 6-100 microseconds time range after an excitation flash.

Parallel measurements of the rise in chlorophyll a fluorescence yield and delayed light emission decay, after a 10 ns saturating excitation flash, have been made in tris (hydroxymethyl)aminomethane-washed chloroplasts. Various electron donor systems (Mn2+; ascorbate; reduced phenylenediamine and benzidine) were used in conjuction with different preillumination regimes to alter [P+-680], the oxidized form of the Photosystem II reaction center chlorophyll a. Conditions giving rise to high [p+ -680] resulted in only a small rise in fluorescence yield, an inhibition of a 6 microseconds component of delayed light emission. These results confirm the hypothesis that P+-680 acts as a quencher of fluorescence and that delayed light emission in the microsecond time range is due to the back reaction of P+-680 and Q-. (Q is the first "stable" electron acceptor of Photosystem II.) Two preillumination flashes are required before the full effect of Tris washing is observed in the delayed light emission decay and fluorescence yield rise; this suggests that a capacity to hold two charges exists between the Tris block and P+-680. Tris washing has no direct effect on the movement of electrons from Z (the first electron donor to P+-680. Finally, Mn2+ donates electrons to P+-680 via Z.

Ascorbic Acid↗
PubMed1976

Anthroyl stearate as a fluorescent probe of chloroplast membranes.

1. A reversible light-induced enhancement of the fluorescence of a "hydrophobic fluorophore", 12-(9-anthroyl)-stearic acid (anthroyl stearate), is observed with chloroplasts supporting phenazine methosulfate, cyclic or 1,1'-ethylene-2,2'-dipyridylium dibromide (Diquat) pseudo-cyclic electron flow; no fluorescence change is observed when methyl viologen or ferricyanide are used as electron acceptors. The stearic acid moiety of anthroyl stearate is important for its localization and fluorescence response in the thylakoid membrane, since structural analogs of anthroyl stearate lacking this group do not show the same response. 2. This effect is decreased under phosphorylating conditions (presence of ADP, Pi, Mg2+), and completely inhibited by the uncoupler of phosphorylation NH4Cl(5-10mM), as well as the ionophores nigericin and gramicidin-D (both at 5 - 10(-8)M). The MgCl2 concentration dependence of the anthroyl stearate enhancement effect is identical to that previously observed for cyclic photophosphorylation, as well as for the formation of a "high energy intermediate". The anthroyl stearate fluorescence enhancement is inhibited by increasing concentrations of ionophores in parallel with the decrease in ATP synthesis, but is essentially unaffected by specific inhibitors (Dio-9 and phlorizin) of photophosphorylation; thus, it appears that anthroyl stearate monitors a component of the "high energy state" of the thylakoid membrane rather than a terminal phosphorylation step. 3. The light-induced anthroyl stearate fluorescence enhancement is suggested to monitor a proton gradient in the energized chloroplast because (a) similar enhancement can be produced by sudden injection of hydrogen ions in a solution of anthroyl stearate; (b) when the proton gradient is dissipated by gramicidin or nigericin light-induced anthroyl stearate fllorescence is eliminated; (c) when the proton gradient is dissipated by tetraphenylboron, light-induced anthroyl stearate fluorescence decreases, and (d) light-induced anthroyl stearate fluorescence change as a function of pH is qualitatively similar to that observed with other probes for a proton gradient (e.g. 9-aminoacridine). Furthermore, anthroyl stearate does not monitor H+ uptake per se because (a) the pH dependence of H+ transport is different from that of the anthroyl stearate fluorescence change, and (b) tetraphenylboron, which does not inhibit H+ uptake, reduces anthroyl stearate fluorescence. Thus, anthroyl stearate appears to be a useful probe of a proton gradient supported by phenazine methosulfate of Diquat catalyzed electron flow and is the first "non-amine" fluorescence probe utilized for this purpose in chloroplasts.

Anthracenes↗
PubMed1976

Inhibition of the reoxidation of the secondary electron acceptor of photosystem II by bicarbonate depletion.

In bicarbonate-depleted chloroplasts, the chlorophyll a fluorescence decayed with a halftime of about 150 ms after the third flash, and appreciably faster after the first and second flash of a series of flashes given after a dark period. After the fourth to twentieth flashes, the decay was also slow. After addition of bicarbonate, the decay was fast after all the flashes of the sequence. This indicates that the bicarbonate depletion inhibits the reoxidation of the secondary acceptor R2- by the plastoquinone pool; R is the secondary electron acceptor of pigment system II, as it accepts electrons from the reduced form of the primary electron acceptor (Q-). This conclusion is consistent with the measurements of the DCMU (3-(3,4-dichlorophenyl)-),)-dimethylurea)- induced chlorophyll a fluorescence after a series of flashes in the presence and the absence of bicarbonate, if it is assumed that DCMU not only causes reduction of Q if added in the state QR-, but also if added in the state QT2-.

Bicarbonates↗
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

Proton relaxation and charge accumulation during oxygen evolution in photosynthesis.

The water proton spin-spin (transverse) relaxation rate of chloroplast suspensions has been measured after each of a series of 2.4 musec light flashes. The sequence of relaxation rates shows a damped oscillatory pattern with a period of four and peaks after the 3rd, 7th, 11th, and 15th flashes. This result indicates that water proton relaxation can be used to monitor the charge-accumulating states as postulated by Kok and coworkers for the oxygen-evolving mechanism in green plants [(1970) Photochem. Photobiol. 11, 457-475]. Other experiments [Wydrzynski et al. (1975) Biochim. Biophys. Acta 408, 349-354] have shown that the proton relaxation rate is strongly influenced by membrane-bound manganese in various oxidation states, suggesting that manganese participates in the charge accumulation process during oxygen evolution.

Journal Article↗
PubMed1975

Water proton relaxation as a monitor of membrane-bound manganese in spinach chloroplasts.

First measurements of proton relaxation on chloroplast membranes are presented here. Experiments show that the water proton spin-lattice relaxation rate in chloroplast thylakoid membrane suspensions can be used to monitor membrane-bound manganese. The relaxation effect is reduced to 0.4 of its original value upon manganese extraction by washing with either alkaline Tris buffer or NH2OH/EDTA solution. Large increases in the proton relaxation rate are measured in the presence of reductants such as tetraphenylboron and NH2OH; oxidants such as potassium ferricyanide or 2,6-dichlorophenolindophenol lead to an decrease in this rate. These results suggest that manganese exists as a mixture of oxidation states in dark-adapted chloroplasts.

2,6-Dichloroindophenol↗
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

A new site of bicarbonate effect in photosystem II of photosynthesis: evidence from chlorophyll fluorescence transients in spinach chloroplasts.

Recent studies on oxygen evolution of corn chloroplast fragments in flashing light [Stemler, A., Babcock, G.T. and Govindjee (1974) Proc. Natl. Acad. Sci. 71, 4679-4683] have shown that the absence of bicarbonate ions increases the turnover time of the Photosystem II reaction center. The rate limiting steps in Photosystem II turnover can be interpreted in terms of reactions either on the oxidizing (electron donor) or reducing (electron acceptor) side of the reaction center. Experiments are reported here that suggest at least one site of bicarbonate action on the reducing side. In Tris-washed spinach chloroplasts (incapable of O2 evolution), the chlorophyll a fluorescence transient in the presence of various artificial electron donors (hydroquinone, diphenylcarbazide, MnCl2 and NH2OH) and in the absence of bicarbonate ions shows a rapid initial rise; the addition of 10 mM NaHCO3 restores the transient to one characteristic of normal chloroplast. Furthermore, the transients measured as a function of decreasing bicarbonate concentrations are qualitatively similar to those observed with increasing concentrations of 3-(3, 4-dichlorophenyl)-1, 1-dimethyl urea which imposes a block on the reducing side, rather than to transients observed with increasing concentrations of NH2OH or prolonged heat treatments, which impose a block on the oxidizing side.

2,6-Dichloroindophenol↗
PubMed1975

Effects of sodium and magnesium cations on the "dark-" and light-induced chlorophyll a fluorescence yields in sucrose-washed spinach chloroplasts.

The effects of Na plus and Mg-2 plus on the "dark" level (O level) and light-induced (P level) fluorescence in sucrose-washed spinach clhoroplasts were studied. Low concentrations of NaCl (2-10 mM) cause a significant decrease in both the O and P levels in the chlorophyll fluorescence transient. The effect on the O level may reflect changes in the bulk chlorophyll a. At 77 degrees K NaCl increases the F735/F685 emission peak ratio in dark-adapted and preilluminated chloroplasts, but has no significant effect on this ratio in sucrose-washed Photosystem II particles. This evidence is consistent with a sodium-induced excitation-energy distribution in favor of Photosystem I. In the presence of MgCl2, with or without NaCl, there is a slight decrease in the O and P level fluorescence as compared with the salt-free control, but an increase as compared with the NaCl-treated sample. Magnesium appears to override the sodium-induced changes. At low temperatures in chloroplasts and Photosystem II particles, MgCl2 has different effects on the F735/F685 ratio apparently depending on the state of the membrane. Magnesium, however, always induces an increase in the F695/F685 ratio. These results suggest that magnesium may influence Photosystem II reaction centers as well as energy distribution between the two photosystems.

Chlorophyll↗
PubMed1974

The effect of bicarbonate on photosynthetic oxygen evolution in flashing light in chloroplast fragments.

The ability of bicarbonate ion (HCO(3) (-)) to stimulate photosynthetic oxygen evolution in maize chloroplast fragments exposed to continuous light depends on light intensity. Stimulation by HCO(3) (-) is less at low intensities. In HCO(3) (-)-depleted chloroplasts exposed to brief saturating light flashes, period 4 oscillations (in O(2) yield per flash) are damped within three cycles. Readdition of HCO(3) (-) to these preparations restores the oscillatory pattern to higher flash numbers, indicating that HCO(3) (-) reduces the probability of "misses" in the photosystem II reaction center. The rate of the dark relaxation reaction S(n) (') --> S(n+1) (where S refers to the oxidation state of the oxygen-evolving mechanism and n = 0, 1, or 2), after a photoact in the photosystem II reaction center, is retarded in HCO(3) (-)-depleted chloroplasts compared to the rate for this reaction in depleted chloroplasts to which HCO(3) (-) has been resupplied. However, the final oxygen-evolving reaction after the accumulation of four positive charges appears to be independent of HCO(3) (-). Bicarbonate has no effect on the dark deactivation of the higher oxidation states (S(2) and S(3)) of the positive charge-accumulating system. We propose two alternate ways in which the kinetic model of oxygen evolution developed by Kok et al. [(1970) Photochem. Photobiol. 11, 457-475] can be extended to include the action of HCO(3) (-).

Journal Article↗