Thermoluminescence study of charge recombination in Photosystem II at low temperatures. II. Oscillatory properties of the Zv and A thermoluminescence bands in chloroplasts dark-adapted for various time periods

The oscillations of the Z_V and A thermoluminescence bands were investigated in spinach chloroplasts which had been dark-adapted for various time periods and subjected to a series of flashes at +2°C before continuous illumination at various low temperatures. When excited with continuous light below −65°C, the Z_V band exhibited period-4 oscillation, with maxima on preflashes 0, 4 and 8. Above −65°C, the oscillation pattern depended greatly on the dark-adaptation period of the chloroplasts. In preilluminated samples (15 s light followed by 3 min dark), when the Q_B pool is half oxidized, the oscillation of the thermoluminescence intensity measured at −50°C was similar to that observed below −65°C. However, after the thorough dark-adaptation of the chloroplasts (6 h), when the major fraction of the Q_B pool is assumed to be oxidized, a binary oscillation appeared in the oscillation pattern, with maxima at odd flash numbers. Below −65°C, period-2 oscillation of the Z_V band could not be induced by the dark-adaptation of the chloroplasts, suggesting an inhibition of electron exchange between Q_A and Q_B. Upon excitation of the chloroplasts with continuous light at −30°C, the A band oscillated with a periodicity of 4 with maxima at preflash numbers 2 and 6. At pH 7.5, the period-4 oscillation was converted into a period-2 oscillation by thorough dark-adaptation of the chloroplasts (24 h). Model calculations of the oscillatory patterns suggest that the period-4 oscillations of the Z_V and A bands are determined by the concentrations [S₀] + [S₁] and [S₂] + [S₃], respectively, which are present after the preflashes prior to the low-temperature continuous illumination. The period-2 oscillations in the amplitudes of the Z_V and A bands reflect the changes occurring in the redox state of the Q_B pool in a sequence of flashes. The possible relationship between the characteristics of the Z_V and A bands and the temperature-dependence of the S state transitions was investigated. Comparison of the amplitudal changes of the B (S₂Q⁻_B and S₃Q⁻_B recombination) and Q (S₂Q⁻_A recombination) thermoluminescence bands as a function of the excitation temperature suggests that the S₂ → S₃ and S₃ → S₄ transitions are blocked at about −65 and −40°C, respectively. It is also concluded that the thermoluminescence intensity emitted by the reaction center is about twice as high in the S₃ state as in the S₂ state.     

Effect of pH on the thermoluminescence of spinach chloroplasts in the presence and absence of Photosystem II inhibitors

The glow curve of chloroplasts excited by continuous light of high intensity (500 W · m⁻²) at pH 7.5 during cooling from +2 to −80°C consisted of seven bands appearing at about −30°C (TL₋₃₀), −15°C (TL₋₁₀), +10°C (TL₊₁₀), +30°C (TL₊₃₀), +50°C (TL₊₅₀), +65°C (TL₊₆₅) and +85°C (TL₊₈₀), in which TL stands for thermoluminescence. In the pH range from 5.5 to 9.0 the peak positions of the TL₋₃₀, TL₋₁₀, TL₊₅₀, TL₊₆₅ and TL₊₈₀ bands were independent of pH. On the other hand the peak positions of the TL₊₁₀ and TL₊₃₀ bands were gradually shifted from +25 to −5°C and from +20 to +40°C, respectively, as the pH was decreased from 9.0 to 5.5. The same pH-induced shift (from +25 to −5°C) was observed for the TL₊₁₀ band when electron transport was inhibited by DCMU. In dinoseb-treated chloroplasts the peak position of the main thermoluminescence band also exhibited pH dependency, and shifted from +20 to −20°C upon lowering the pH from 9.0 to 5.5. After the water-splitting system had been inactivated by Tris or NH₂OH treatment no pH-induced shifts were observed in the peak positions of the thermoluminescence bands of DCMU and dinoseb-treated chloroplasts. The results suggest that the effect of pH on the thermoluminescence of untreated and inhibitor-treated chloroplasts is associated with protonation/deprotonation reactions occurring at the donor and acceptor sides of Photosystem II during the S₁ → S₂ transition of the water-splitting system.     

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 ?40 °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 ?35 °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 ?10 °C, presumably partly due to accumulation of reduced acceptor (Q^?), and reached a maximum at ?35 °C.In the presence of 50 mM NH₄Cl the temperature optimum was at ?15 °C; at this temperature luminescence was increased by NH₄Cl; at temperatures below ?20 °C luminescence at 40 ms was decreased by NH₄Cl. 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^? 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.     

Heat sensitivity of chloroplasts and leaves: Leakage of protons from thylakoids and reversible activation of cyclic electron transport

In illuminated intact spinach chloroplasts, warming to and beyond 40 °C increased the proton permeability of thylakoids before linear electron transport through Photosystem II was inhibited. Simultaneously, antimycin A-sensitive cyclic electron transport around Photosystem II was activated with oxygen or CO2, but not with nitrite as electron acceptors. Between 40 to 42 °C, activation of cyclic electron transport balanced the loss of protons so that a sizeable transthylakoid proton gradient was maintained. When the temperature of darkened spinach leaves was slowly increased to 40°C, reduction of the quinone acceptor of Photosystem II, Q_A, increased particularly when respiratory CO2 production and autoxidation of plastoquinones was inhibited by decreasing the oxygen content of the atmosphere from 21 to 1%. Simultaneously, Photosystem II activity was partially lost. The enhanced dark Q_A reduction disappeared after the leaf temperature was decreased to 20 °C. No membrane energization was detected by light-scattering measurements during heating the leaf in the dark. In illuminated spinach leaves, light scattering and nonphotochemical quenching of chlorophyll fluorescence increased during warming to about 40 °C while Photosystem II activity was lost, suggesting extra energization of thylakoid membranes that is unrelated to Photosystem II functioning. After P700 was oxidized by far-red light, its reduction in the dark was biphasic. It was accelerated by factors of up to 10 (fast component) or even 25 (slow component) after short heat exposure of the leaves. Similar acceleration was observed at 20 °C when anaerobiosis or KCN were used to inhibit respiratory oxidation of reductants. Methyl viologen, which accepts electrons from reducing side of Photosystem II, completely abolished heat-induced acceleration of P700+ reduction after far-red light. The data show that increasing the temperature of isolated chloroplasts or intact spinach leaves to about 40 °C not only inhibits linear electron flow through Photosystem II but also activates Photosystem I-driven cyclic electron transport pathways capable of contributing to the transthylakoid proton gradient. Heterogeneity of the kinetics of P700+ reduction after far-red oxidation is discussed in terms of Photosystem I-dependent cyclic electron transport in stroma lamellae and grana margins.     

Fractional control of photosynthesis by the QB protein,the cytochrome f/b 6 complex and other components of the photosynthesic apparatus

In order to obtain information on fractional control of photosynthesis by individual catalysts, catalytic activities in photosynthetic electron transport and carbon metabolism were modified by the addition of inhibitors, and the effect on photosynthetic flux was measured using chloroplasts of Spinacia oleracea L. In thylakoids with coupled electron transport, light-limited electron flow to ferricyanide was largely controlled by the Q_B protein of the electron-transport chain. Fractional control by the cytochrome f/b ₆ complex was insignificant under these conditions. Control by the cytochrome f/b ₆ complex dominated at high energy fluence rates where the contribution to control of the Q_B protein was very small. Uncoupling shifted control from the cytochrome f/b ₆ complex to the Q_B protein. Control of electron flow was more complex in assimilating chloroplasts than in thylakoids. The contributions of the cytochrome f/b ₆ complex and of the Q_B protein to control were smaller in intact chloroplasts than in thylakoids. Thus, even though the transit time for an electron through the electron-transport chain may be below 5 ms in leaves, oxidation of plastohydroquinone was only partially responsible for limiting photosynthesis under conditions of light and CO₂ saturation. The energy fluence rate influenced control coefficients. Fractional control of photosynthesis by the ATP synthetase, the cytochrome f/b ₆ complex and by ribulose-1,5-bisphosphate carboxylase increased with increasing fluence rates, whereas the contributions of the Q_B protein and of enzymes sensitive to SH-blocking agents decreased. The results show that the burdens of control are borne by several components of the photosynthetic apparatus, and that burdens are shifted as conditions for photosynthesis change.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DNP-INT 2,4-dinitro phenylether of 2-iodo-4-nitrothymol - pCMBS p-chloromercuribenzosulfonate     

Occurrence of Cu,Zn-Superoxide Dismutase in the Intrathylakoid Space of Spinach Chloroplasts

Thylakoids obtained from intact spinach chloroplasts showedno superoxide dismutase (SOD) activity, but Cu,Zn- and Mn-SODactivities were detected in the presence of Triton X-100. Thylakoidmembranes and the lumen fraction were separated by centrifugationafter treatment of the thylakoids with a Yeda pressure cell.Cu,Zn-SOD was found in the lumen fraction. Mn-SOD was detectedin the thylakoid fraction only after addition of 1% Triton X-100.Antibody against spinach Cu,Zn-SOD did not interact with thelatent Cu,Zn-SOD in the thylakoids unless Triton was added.These results indicate that Cu,Zn-SOD occurs in the lumen inaddition to the stroma of spinach chloroplasts, and Mn-SOD bindsto the thylakoid membranes. (Received February 29, 1984; Accepted May 28, 1984)     

Thermoluminescence and fluorescence study of changes in Photosystem II photochemistry in desiccating barley leaves

An effect of desiccation (a decrease of relative water content from 97% to 10% within 35 h) on Photosystem II was studied in barley leaf segments (Hordeum vulgare L. cv. Akcent) using chlorophyll a fluorescence and thermoluminescence (TL). The O-J-I-P fluorescence induction curve revealed a decrease of F_P and a slight shift of the J step to a shorter time with no change in its height. The analysis of the fluorescence decline after a saturating light flash revealed an increased portion of slow exponential components with increasing desiccation. The TL bands obtained after excitation by continuous light were situated at about –27°C (Z_v band – recombination of P680⁺Q_A ⁻), –14 °C (A band – S₃Q_A ⁻), +12 °C (B band – S_2/3Q_B ⁻) and +45 °C (C band – TyrD⁺Q_A ⁻). The bands related to the S-states of oxygen evolving complex (A and B) were reduced by desiccation and shifted to higher and lower temperatures, respectively. In accordance with this, the band observed at about +27 °C (S₂Q_B ⁻) after excitation by 1 flash fired at –10 °C and band at about +20 °C (S_2/3Q_B ⁻) after 2 flashes decreased with increasing water deficit and shifted to lower temperatures. A new band around 5 °C appeared in both regimes of TL excitation for a relative water content of under 42% and was attributed to the Q band (S₂Q_A ⁻). It is suggested that under desiccation, an inhibition of the formation of S₂- and S₃-states in OEC occurred simultaneously with a lowering of electron transport on the acceptor side of PS II. The temperature down-shift of the TL bands obtained after the flash excitation was induced at the initial phases of water stress, indicating a decrease of the activation energy for the S_2/3Q_B ⁻recombination. This revised version was published online in June 2006 with corrections to the Cover Date.     

Measurement of the relative electron-transport capacity of Photosystem I and Photosystem II in spinach chloroplasts

The ratio of Photosystem (PS) II to PS I electron-transport capacity in spinach chloroplasts was compared from reaction-center and steady-state rate measurements. The reaction-center electron-transport capacity was based upon both the relative concentrations of the PS II_α, PS II_β and PS I centers, and the number of chlorophyll molecules associated with each type of center. The reaction-center ratio of total PS II to PS I electron-transport capacity was about 1.8:1. Steady-state electron-transport capacity data were obtained from the rate of light-induced absorbance-change measurements in the presence of ferredoxin-NADP⁺, potassium ferricyanide and 2,5-dimethylbenzoquinone (DMQ). A new method was developed for determining the partition of reduced DMQ between the thylakoid membrane and the surrounding aqueous phase. The ratio of membrane-bound to aqueous DMQH₂ was experimentally determined to be 1.3:1. When used at low concentrations (200 μM), potassium ferricyanide is shown to be strictly a PS I electron acceptor. At concentrations higher than 200 μM, ferricyanide intercepted electrons from the reducing side of PS II as well. The experimental rates of electron flow through PS II and PS I defined a PS II/PS I electron-transport capacity ratio of 1.6:1.     

Oxidation-reduction properties of the electron acceptors of Photosystem II I. Redox titration of the flash-induced carotenoid band shift,of C550 and of the variable fluorescence yield in spinach chloroplasts

Redox titration of the electrochromic carotenoid band shift, detected at 50 μs after a saturating actinic flash, in spinach chloroplasts, shows that only one electron acceptor in Photosystem II participates in a transmembrane primary electron transfer. This species, the primary quinone acceptor, Q, shows only one midpoint potential (E_m,7.5) of approx. 0 V and is undoubtedly equivalent to the fluorescence quencher, Q_H. A second titration wave is observed at low potential ($\text{E}{}_{\mathrm{<mtext>m</mtext><mtext>,7.5</mtext>}}\text{? ? 240}\text{mV}$) and at greater than 3 ms after a saturating actinic flash. This wave has an action spectrum different from that of Photosystem II centers containing Q and could arise from a secondary but not primary electron transfer. A low-potential fluorescence quencher is observed in chloroplasts which largely disappears in a single saturating flash at ? 185 mV and which does not participate in a transmembrane electron transfer. This low-potential quencher (probably equivalent to fluorescence quencher, Q_L) and Q are altogether different species. Redox titration of C550 shows that if electron acceptor Q_β is indeed characterized by an E_m,7 of + 120 mV, then this acceptor does not give rise to a C550 signal upon reduction and does not participate in a transmembrane electron transfer. This titration also shows that C550 is not associated with Q_L.     

Thermoluminescence study of charge recombination in Photosystem II at low temperatures. I. Characterization of the Zv and A thermoluminescence bands

The characteristics of the Z_v and A thermoluminescence bands appearing in the glow curve at about -75 and -30°C, respectively, were investigated in spinach chloroplasts. Inhibitory concentrations of DCMU decreased the amplitude of the Z_v band by half and completely abolished the A band. On the other hand, after two preflashes at +2°C before freezing, the A band could be charged by low-temperature illumination even when the electron transport was interrupted between Q_A and Q_B by DCMU addition after the preflashes. Two-flash preillumination greatly enhanced the amplitude of the A band, but diminished that of the Z_v band. Tris washing and NH₂OH treatment, which inactivated the oxygen-evolving system, almost completely abolished the Z_v band, but did not affect the A band. Severe trypsin treatment, which also impaired the oxygen-evolving system, resulted in a very large intensification of the Z_v band. The half-times of the A and Z_v bands, determined by theoretical analysis of the thermoluminescence data, proved to be about 4 ms and 200–500 μs, respectively. These results, taken together with EPR data from the literature, suggest that the A band arises from charge recombination between a negatively charged acceptor located before the DCMU block (most probably Q⁻_A) and the oxidized donor Z⁺ (which accounts for the EPR Signal II_vf and Signal II_f). The electron carrier responsible for the Z_v band is also a component located prior to the inhibitory site of DCMU (Q⁻_A); its interacting counterpart is an unidentified donor which is involved in charge exchange with the S states.     

Interference by luteolin,quercetin, and taxifolin with chloroplast-mediated electron transport and phosphorylation

Summary The effects of luteolin, quercetin, and taxifolin on light induced phosphorylation and electron transport in isolated, greenhouse-grown, spinach (Spinacia oleracea L.) thylakoids were investigated. Luteolin and quercetin interacted with components associated with both the ATP-generating pathway and the electron-transport pathway. However, the action of taxifolin involved only the phosphorylation pathway. Interference with the phosphorylation pathway was evidenced by the greater sensitivity of phosphorylation than oxygen uptake in coupled whole-chain electron transport, inhibition of the light-activated Mg²⁺-ATPase, and inhibition of the Ca²⁺-ATPase associated with CF₁. The following order of decreasing inhibitory effectiveness was exhibited: luteolin > quercetin >>> taxifolin. On the electron-transport pathway, luteolin and quercetin interfered with the activity of the Q_B-protein complex as evidenced by inhibition of the partial reaction with diphenylcarbazide as the electron donor and 2,6-dichlorophenolindophenol as electron acceptor; alteration of the chlorophyll fluorescence transients; and competitive displacement of radiolabeled atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine].     

Decrease lysyl oxidase activity in hearts of copper-deficient bovines

BackgroundLysyl oxidase (LOX) is a metalloenzyme that requires Cu as a cofactor and it is responsible for the formation of collagen and elastin cross-linking. The objective of this work was to measure the LOX enzyme activity in the heart of bovines with Cu deficiency induced by high molybdenum and sulfur levels in the diet.MethodsEighteen myocardial samples were obtained from Cu-deficient (n = 9) and control (n = 9) Holstein bovines during two similar assays. The samples were frozen in liquid nitrogen and stored at −70 °C to measure enzymatic activity. A commercial kit was used, following producer instructions.ResultsThe results showed that LOX activity from the hearts of Cu-deficient bovines is 29 % lower than the ones of control bovines, being this difference statistically significant (p = 0.03).ConclusionTo our knowledge, this is the first report that determined LOX enzymatic activity in bovine heart of Cu-deficient animals. The microscopic alterations found in these animals in our previous work, could be explained by a diminished LOX activity. The results are in agreement with other authors, who found a relationship between LOX activity and dietary Cu intake. The information provided by this work could help to clarify the pathogenesis of cardiac lesions in cattle with dietary Cu deficiency.     

Sulfhydryl reagents inhibit electron transport in Photosystem II of spinach chloroplasts

A 120 min incubation period with sulfhydryl reagents, such as p-chloromercuribenzoic acid, shows greater than 50% loss of electron-transport activity in Photosystem (PS) II of spinach chloroplasts. Since p-chloromercuriphenylsulfonic acid, a nonpenetrating sulfhydryl reagent, and 4,4′-dithiopyridine, a bifunctional sulfhydryl reagent, show greater inhibition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea-insensitive silicomolybdate reduction than of dibromothymoquinone-insensitive indophenol reduction, it is postulated that two different sulfhydryl reagent-sensitive sites are involved in the PS II electron-transport chain of spinach chloroplasts.     

Gas-liquid Chromatographic Separation of Amine Enantiomers as Diastereoisomeric Chrysanthemoylamide Derivatives

An immobilized chloroplast film, prepared by immobilizing spinach chloroplasts in 2 wt% agar gel, was attached to a SnO₂ optically transparent electrode to obtain the immobilized chloroplast film electrode. The immobilized chloroplast film electrode worked as a photoanode under illumination in the presence of methyl viologen, which was an electron carrier from chloroplasts to the SnO₂ optically transparent electrode. Water photolysis for producing hydrogen by a photoelectrochemical cell using the immobilized chloroplasts film electrode was successfully achieved. A smooth platinum electrode was used as a cathode to produce hydrogen. The pH and temperature of the anolyte were kept at 7.8 and 25°C. Optimizations of the concentrations of methyl viologen and chlorophyll in the immobilized chloroplast film were studied. The optimum thickness for the immobilized chloroplast film was about 0.8 mm. The immobilized chloroplasts had higher storage stability than that of isolated chloroplasts and they retained more than 50% of the initial activities of photosystem I and photosystem II after 10 days when they were stored at 4°C in the dark. It was conceived from the relationship between the photocurrent and the photosystem I and II activities that the main cause for the decrease in the photocurrent was the photochemical inactivation of photosystem II.     

Identification of a g equals 1.90 high-potential iron-sulfur protein in chloroplasts.

A new bound iron-sulfur protein has been identified in spinach chloroplasts. In the reduced form, this protein has an electron paramagnetic resonance spectrum at 20°K with g-values of 2.02 and 1.90. The midpoint oxidation-reduction potential (E_m) of the protein, which is pH-independent, is +290 mV. These properties are similar to those of the “Rieske” g = 1.90 iron-sulfur protein of mitochondrial Complex III.     

Synthesis and uptake of cytoplasmically synthesized pyruvate, pi dikinase polypeptide by chloroplasts

Polyadenylated RNA was isolated from maize leaves and translated in vitro. In agreement with a previous report by others, we found among the translation products a 110-kilodalton pyruvate orthophosphate dikinase (PPDK) precursor that is about 16 kilodaltons larger than the polypeptide isolated from cells. This maize PPDK precursor polypeptide was taken up from the translation product mixture by intact spinach chloroplasts and yielded a mature PPDK polypeptide (94 kilodaltons). The uptake and processing support the proposal that the extra 16-kilodalton size of the polypeptide from in vitro translation of maize leaf mRNA represents a transit sequence which is cleaved after its entry into chloroplasts. Moreover, these results provide additional evidence that in vivo in maize leaf cells PPDK polypeptide is synthesized in the cytoplasm and is transported into the chloroplasts.

Location of PPDK in C₃ plant leaves was investigated by immunochemical analysis. Intact chloroplasts were isolated from leaves of spinach, wheat, and maize. A protein blot of stromal protein in each case gave rise to bands corresponding to authentic PPDK polypeptide. This result indicates that PPDK is present in chloroplasts of C₃ plant leaves as it is in the case of C₄ plants.

     

Variation in photosynthetic pigments and plastoquinone contents in sugar beet chloroplasts with changes in leaf copper content

The changes in the contents of chlorophyll (Chl) a, Chl b, carotenoid, and plastoquinone in sugar beet (Beta vulgaris) chloroplasts were investigated during Cu deficiency and resupply. With the onset of Cu deficiency, extrachloroplastic Cu decreased more rapidly than chloroplast Cu, which eventually accounted for all of the Cu present in the leaf. The ultrastructure of Cu-deficient chloroplasts did not differ from the control except in very young, severely deficient leaves. During Cu depletion and resupply, Chl a, Chl b, carotenoid, and plastoquinone contents changed concomitantly. Because of the concurrent changes in the contents of terpenoid-containing pigments and plastoquinone with Cu depletion and resupply, we suggest that Cu might be involved in the regulation of the early steps of terpenoid biosynthesis prior to the formation of geranyl-geranyl-pyrophosphate.     

Charge accumulation and recombination in Photosystem II studied by thermoluminescence. I. Participation of the primary acceptor Q and secondary acceptor B in the generation of thermoluminescence of chloroplasts

In the glow curves of chloroplasts excited by a series of flashes at +1°C the intensity of the main thermoluminescence band appearing at +30°C (B band; B, secondary acceptor of Photosystem II) exhibits a period-4 oscillation with maxima on the 2nd and 6th flashes indicating the participation of the S₃ state of the water-splitting system in the radiative charge recombination reaction. After long-term dark adaptation of chloroplasts (6 h), when the major part of the secondary acceptor pool (B pool) is oxidized, a period-2 contribution with maxima occurring at uneven flash numbers appears in the oscillation pattern. The B band can even be excited at ?160°C as well as by a single flash in which case the water-splitting system undergoes only one transition (S₁ → S₂). The experimental observations and computer simulation of the oscillatory patterns suggest that the B band originates from charge recombination of the S₂B^? and S₃B^? redox states. The half-time of charge recombination responsible for the B band is 48 s. When a major part of the plastoquinone pool is reduced due to prolonged excitation of the chloroplasts by continuous light, a second band (Q band; Q, primary acceptor of Photosystem II) appears in the glow curve at +10°C which overlaps with the B band. In chloroplasts excited by flashes prior to DCMU addition only the Q band can be observed showing maxima in the oscillation pattern at flash numbers 2, 6 and 10. The Q band can also be induced by flashes after DCMU addition which allows only one transition of the water-splitting system (S₁ → S₂). In the presence of DCMU, electrons accumulate on the primary acceptor Q, thus the Q band can be ascribed to the charge recombination of either the S₂Q^? or S₃Q^? states depending on whether the water-splitting system is in the S₂ or the S₃ state. The half-time of the back reaction of Q^? with the donor side of PS II (S₂ or S₃ states) is 3 s. It was also observed that in a sequence of flashes the peak positions of the Q and B bands do not depend on the advancement of the water-splitting system from the S₂ state to the S₃ state. This result implies that the midpoint potential of the water-splitting system remains unmodified during the S₂ → S₃ transition.     

Effect of temperature on senescing detached rice leaves: I. Photoelectron transport activity of chloroplasts

Detached rice(Oryza sativaL cv Mousouri)leaves were induced to senesce in darkness at 0°C(cold), 27°C(room temperature) and 40°C(heat). 2,6-dichlorophenol indophenol (DCPIP) Hill reaction activity of chloroplasts isolated from senescing leaves under all experimental temperatures with H₂O, Mn²⁺ or diphenyl carbazide (DPC) as electron donor declined during the period of incubation. Since DPC and Mn²⁺ augmented 2,6-dichlorophenol indophenol photoreduction by chloroplasts from senescing leaves, damage and/or change in the conformation of a site between H₂O and DCPIP in photosystem II (PS II) is suggested. Heat caused a faster decline of the Hill activity compared to cold or room temperature. However, cold treatment showed no significant effect on the photoelectron transport from H₂O to DCPIP compared with room temperature.