Enhancement of cyclic electron flow around PSI at high light and its contribution to the induction of non-photochemical quenching of chl fluorescence in intact leaves of tobacco plants

Non-photochemical quenching (NPQ) of Chl fluorescence is a mechanism for dissipating excess photon energy and is dependent on the formation of a DeltapH across the thylakoid membranes. The role of cyclic electron flow around photosystem I (PSI) (CEF-PSI) in the formation of this DeltapH was elucidated by studying the relationships between O2-evolution rate [V(O2)], quantum yield of both PSII and PSI [Phi(PSII) and Phi(PSI)], and Chl fluorescence parameters measured simultaneously in intact leaves of tobacco plants in CO2-saturated air. Although increases in light intensity raised V(O2) and the relative electron fluxes through both PSII and PSI [Phi(PSII) x PFD and Phi(PSI) x PFD] only Phi(PSI) x PFD continued to increase after V(O2) and Phi(PSII) x PFD became light saturated. These results revealed the activity of an electron transport reaction in PSI not related to photosynthetic linear electron flow (LEF), namely CEF-PSI. NPQ of Chl fluorescence drastically increased after Phi(PSII) x PFD became light saturated and the values of NPQ correlated positively with the relative activity of CEF-PSI. At low temperatures, the light-saturation point of Phi(PSII) x PFD was lower than that of Phi(PSI) x PFD and NPQ was high. On the other hand, at high temperatures, the light-dependence curves of Phi(PSII) x PFD and Phi(PSI) x PFD corresponded completely and NPQ was not induced. These results indicate that limitation of LEF induced CEF-PSI, which, in turn, helped to dissipate excess photon energy by driving NPQ of Chl fluorescence.     

Photoinactivation of ascorbate peroxidase in isolated tobacco chloroplasts: Galdieria partita APX maintains the electron flux through the water-water cycle in transplastomic tobacco plants

We evaluated the H2O2-scavenging activity of the water-water cycle (WWC) in illuminated intact chloroplasts isolated from tobacco leaves. Illumination under conditions that limited photosynthesis [red light (>640 nm), 250 micromol photons m(-2) s(-1) in the absence of HCO3-] caused chloroplasts to take up O2 and accumulate H2O2. Concomitant with the O2 uptake, both ascorbate peroxidase (APX) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) lost their activities. However, superoxide dismutase (SOD), monodehydroascorbate radical reductase (MDAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) activities remained unaffected. The extent to which the photosynthetic linear electron flow decreased was small compared with the decline in APX activity. Therefore, the loss of APX activity lowered the electron flux through the WWC, as evidenced by a decrease in relative electron flux through PSII [Phi(PSII)xPFD]. To verify these interpretations, we created a transplastomic tobacco line in which an H2O2-insensitive APX from the red alga, Galdieria partita, was overproduced in the chloroplasts. In intact transplastomic chloroplasts which were illuminated under conditions that limited photosynthesis, neither O2 uptake nor H2O2 accumulation occurred. Furthermore, the electron flux through the WWC and the activity of GAPDH were maintained. The present work is the first report of APX inactivation by endogenous H2O2 in intact chloroplasts.     

Cyclic electron flow within PSII protects PSII from its photoinhibition in thylakoid membranes from spinach chloroplasts

Cyclic electron flow within PSII (CEF-PSII) was proven to alleviate the photoinhibition of PSII. We set the conditions where CEF-PSII functioned or did not, by adding nigericin to the reaction mixture for the dissipation of DeltapH across thylakoid membranes, and then the thylakoids were illuminated. When CEF-PSII did not function and the activity of linear electron flow (LEF) was low, light-treated thylakoid membranes largely lost the activity of LEF. The inactivation of LEF was due to the loss of the activity of PSII, but not that of PSI. The inactivation of PSII was suppressed, when CEF-PSII functioned or LEF was enhanced. These results imply that CEF-PSII contributes to the protection of PSII from its photoinhibition with LEF, as an electron sink.     

Cyclic flow of electrons within PSII in thylakoid membranes

In photosynthesis, the electrons released from PSII are considered to be shared mainly by carbon metabolism and the water-water cycle. We demonstrated previously that some electrons are utilized in a CO2- and O2-independent manner in leaves of wild watermelon [Miyake and Yokota (2000) Plant Cell Physiol: 41: 335]. In the present study, we examined the mechanism of this alternative flow of electrons in thylakoid membranes, isolated from fresh spinach leaves, by simultaneously measuring the quantum yield of PSII and the flux of the linear flow of electrons. In the presence of the protonophore nigericin, which eliminates the pH gradient across thylakoid membranes, the quantum yield and the flux of the linear flow of electrons were directly proportional to one another. The quantum yield at a given linear flux of electrons was much higher in the absence of nigericin than in its presence, indicating that an additional or alternative flow of electrons can occur independently of the linear flow in the absence of nigericin. In the presence of nigericin, the alternative flux of electrons increased with decreasing pH and with increasing reduction of the plastoquinone pool. Cyclic flow of electrons in PSII appears to be the most plausible candidate for the alternative flow of electrons. The flux reached 280 micromol x e(-) (mg Chl)(-1) x h(-1) and was similar to that of the CO2- and O2-independent alternative flow of electrons that we found in leaves of wild watermelon. The cyclic, alternative flow of electrons in PSII provides a possible explanation for the alternative flow of electrons observed in vivo.     

Dissipation of the Proton Electrochemical Potential in Intact Chloroplasts (II. The pH Gradient Monitored by Cytochrome f Reduction Kinetics)

The potency of various uncouplers for collapsing the light-induced pH gradient across thylakoid membranes in intact chloroplasts was investigated by time-resolved optical spectroscopy. The thylakoid transmembrane pH gradient ([delta]pH) was monitored indirectly by measuring the rate of cytochrome (Cyt) f reduction following a light flash of sufficient duration to create a sizable [delta]pH. The results show that the rate of Cyt f reduction is controlled in part by the internal pH of the thylakoid inner aqueous space. At pH values from 6.5 to 8.0, the Cyt f reduction rate was maximal, whereas at lower pH values from 6.5 to 5.5 the reduction rate decreased to 25% of the maximal rate. The ability of three uncouplers, nigericin, carbonylcyanide m-chlorophenylhydrazone, and gramicidin, to accelerate the rate of Cyt f reduction was determined for intact chloroplasts isolated from spinach (Spinacia oleracea). The efficacy of the uncouplers for collapsing the [delta]pH was determined using the empirical relationship between the [delta]pH and the Cyt f reduction rate. For intact chloroplasts, nigericin was the most effective uncoupler, followed by carbonylcyanide m-chlorophenylhydrazone, which interacted strongly with bovine serum albumin. Gramicidin D, even at high gramicidin:chlorophyll ratios, did not completely collapse the pH gradient, probably because it partitions in the envelope membranes and does not enter the intact chloroplast.     

Distinct roles of the cytochrome pathway and alternative oxidase in leaf photosynthesis

In illuminated leaves, mitochondria are thought to play roles in optimizing photosynthesis. However, the roles of the cytochrome pathway (CP) and alternative oxidase (AOX) in photosynthesis, in particular in the redox state of the photosynthetic electron transport chain, are not separately characterized. We examined the effects of specific inhibition of two respiratory pathways, CP and AOX, on photosynthetic oxygen evolution and the redox state of the photosynthetic electron transport chain in broad bean (Vicia faba L.) leaves under various light intensities. Under saturating photosynthetic photon flux density (PPFD; 700 micromol photon m(-2) s(-1)), inhibition of either pathway caused a decrease in the steady-state levels of the photosynthetic O(2) evolution rate and the PSII operating efficiency, Phi(II). Because these inhibitors, at the concentrations applied to the leaves, had little effect on photosynthesis in the intact chloroplasts, two respiratory pathways are essential for maintenance of high photosynthetic rates at saturating PPFD. CP or AOX inhibition affected to Chl fluorescence parameters (e.g. photochemical quenching and non-photochemical quenching) differently, suggesting that CP and AOX contribute to photosynthesis in different ways. At low PPFD (100 micromol photon m(-2) s(-1)), only the inhibition of AOX, not CP, lowered the photosynthetic rate and Phi(II). AOX inhibition also decreased the Phi(II)/Phi(I) ratio even at low PPFD levels. These data suggest that AOX inhibition caused the over-reduction of the photosynthetic electron transport chain and induced the cyclic electron flow around PSI (CEF-PSI) even at the low PPFD. Based on these results, we discuss possible roles for CP and AOX in the light.     

Alternative electron flows (water-water cycle and cyclic electron flow around PSI) in photosynthesis: molecular mechanisms and physiological functions

An electron flow in addition to the major electron sinks in C(3) plants [both photosynthetic carbon reduction (PCR) and photorespiratory carbon oxidation (PCO) cycles] is termed an alternative electron flow (AEF) and functions in the chloroplasts of leaves. The water-water cycle (WWC; Mehler-ascorbate peroxidase pathway) and cyclic electron flow around PSI (CEF-PSI) have been studied as the main AEFs in chloroplasts and are proposed to play a physiologically important role in both the regulation of photosynthesis and the alleviation of photoinhibition. In the present review, I discuss the molecular mechanisms of both AEFs and their functions in vivo. To determine their physiological function, accurate measurement of the electron flux of AEFs in vivo are required. Methods to assay electron flux in CEF-PSI have been developed recently and their problematic points are discussed. The common physiological function of both the WWC and CEF-PSI is the supply of ATP to drive net CO(2) assimilation. The requirement for ATP depends on the activities of both PCR and PCO cycles, and changes in both WWC and CEF-PSI were compared with the data obtained in intact leaves. Furthermore, the fact that CEF-PSI cannot function independently has been demonstrated. I propose a model for the regulation of CEF-PSI by WWC, in which WWC is indispensable as an electron sink for the expression of CEF-PSI activity.     

Physiological functions of the water-water cycle (Mehler reaction) and the cyclic electron flow around PSI in rice leaves

Changes in chlorophyll fluorescence, P700(+)-absorbance and gas exchange during the induction phase and steady state of photosynthesis were simultaneously examined in rice (Oryza sativa L.), including the rbcS antisense plants. The quantum yield of photosystem II (PhiPSII) increased more rapidly than CO(2) assimilation in 20% O(2). This rapid increase in PhiPSII resulted from the electron flux through the water-water cycle (WWC) because of its dependency on O(2). The electron flux of WWC reached a maximum just after illumination, and rapidly generated non-photochemical quenching (NPQ). With increasing CO(2) assimilation, the electron flux of WWC and NPQ decreased. In 2% O(2), WWC scarcely operated and PhiPSI was always higher than PhiPSII. This suggested that cyclic electron flow around PSI resulted in the formation of NPQ, which remained at higher levels in 2% O(2). The electron flux of WWC in the rbcS antisense plants was lower, but these plants always showed a higher NPQ. This was also caused by the operation of the cyclic electron flow around PSI because of a higher ratio of PhiPSI/PhiPSII, irrespective of O(2) concentration. The results indicate that WWC functions as a starter of photosynthesis by generating DeltapH across thylakoid membranes for NPQ formation, supplying ATP for carbon assimilation. However, WWC does not act to maintain a high NPQ, and PhiPSII is down-regulated by DeltapH generated via the cyclic electron flow around PSI.     

Suppression of flash-induced PSII-dependent electrogenesis caused by proton pumping in chloroplasts

Pre-illumination of the thylakoid membrane of Peperomia metallica chloroplasts leads to a reversible suppression of the flash-induced electrical potential as measured either with the electrochromic bandshift (P515), microelectrode impalement or patch-clamp technique. The energization-dependent potential suppression was not observed in the presence of 1 μ M nigericin suggesting the involvement of proton and/or cation gradients. Energization in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and N,N,N',N'-tetramethylphenylenediamine (TMPD), i.e. cyclic electron flow around photosystem (PS) I, results in the accumulation of TMPD⁺ in the thylakoid lumen. The reversible suppression of the flash-induced membrane potential was not observed in these conditions indicating that it is not a general cation-induced increase of membrane capacitance. Cyclic electron flow around PSI in the presence of DCMU and phenazine methosulfate (PMS) results in the accumulation of PMS⁺ and H⁺ in the thylakoid lumen. The absence of reversible suppression of the flash-induced membrane potential for this condition shows that accumulation of protons does not lead to (1) a reversible increase of membrane capacitance and (2) a reversible suppression of PSI-dependent electrogenesis. Reversible inactivation of PSII by a low pH in the thylakoid lumen is therefore proposed to be the cause for the temporary suppression of the flash-induced electrical potential. The flash-induced PSII-dependent membrane potential, as measured after major oxidation of P700 in far-red background light, was indeed found to be suppressed at low assay pH (pH 5) in isolated spinach ( Spinacia oleracea ) chloroplasts.     

Oxygenic photoreduction of methyl viologen and nicotinamide adenine dinucleotide phosphate without the involvement of photosystem I during plastid development

Studies on the appearance of various electron transport functions were followed during greening of etiolated cucumber cotyledons. Appearance of dichlorodimethoxy-p-benzoquinone, dimethyl quinone, tetramethyl-p-phenylenediamine, dichlorophenol indophenol and ferricyanide Hill reactions were observed after 8h of greening. However, photoreduction of methyl viologen (MV) and nicotinamide adenine dinucleotide phosphate (NADP) was observed from 2h of greening. Variable fluorescence, which is a direct indication of water-splitting function, was observed from 2h of greening in cotyledons, thylakoid membranes and photosystem II (PSII) particles. The decrease in variable fluorescence in the presence of MV (due to rapid reoxidation of Q-) observed from early stages of greening confirmed the photoreduction of MV by PSII. The early development of water-splitting function was further confirmed by the abolition of variable fluorescence in thylakoid membranes and PSII particles by heat treatment and concomittant loss of light dependent oxygen uptake in the presence of MV in heat treated chloroplasts. However, the photoreduction of MV and NADP was insensitive to intersystem electron transport inhibitors, dichlorophenyl dimethylurea or dibromomethyl isopropyl-p-benzoquinone till 8h of greening. Though the oxidation of intersystem electron carrier cytochrome f was observed from early stages of greening, the reduction of cytochrome f was not observed till 8h of greening. All these observations confirm that during early stages of greening MV and NADP are photoreduced by PSII without the involvement of intersystem electron carriers or the collaboration of PSI. Since these observations are at variance with the currently prevalent concept (Z-Scheme) of the photosynthetic generation of reducing power, which requires definite collaboration of the two photosystems, an alternate electron flow pathway is proposed.     

The effect of cactus spines on light interception and Photosystem II for three sympatric species of Opuntia from the Mojave Desert

Cactus spines reduce herbivory, direct water toward roots and reduce the impacts of high- and low-temperature extremes. Yet, shading of stems by spines reduces incident photosynthetic photon flux density (PFD), photosynthesis and growth. This study compared spinescence, PFD interception, stem temperature, Photosystem II (PSII) photochemistry and xanthophyll pigment composition for three species of cacti from the Mojave Desert, CA. The species vary in spinescence: Opuntia basilaris , which has no central or radial spines; Opuntia erinacea , which is densely covered with spines; and Opuntia phaeacantha , which has an intermediate coverage of spines. The role of spines was tested by removing spines from stems of O. erinacea . PFD interception was similar for both O. basilaris and O. phaeacantha , and about three times that for densely spined O. erinacea ; removal of spines increased incident PFD three-fold. There were no effects of spines on stem temperatures. Steady-state light-response curves of chlorophyll a fluorescence from PSII indicated that ΦPSII, photochemical quenching (qP) and electron flux within PSII were lower, and non-photochemical quenching was higher, for O. erinacea in comparison to the other two species with less spines. After 2 months, qP was higher and electron flux lower, and xanthophyll pigment pool size was higher, for stems from which spines had been removed compared with intact stems. These three species allocate different amounts of biomass to spines, resulting in species-specific PFD interception, PSII photochemistry and xanthophyll pigment pool size, which may help maintain rates of photosynthesis during the hot, dry Mojave Desert summer.     

Coregulation of electron transport and Benson-Calvin cycle activity in isolated spinach chloroplasts: studies on glycerate 3-phosphate reduction

Glycerate 3-phosphate-dependent O2 evolution was measured in intact chloroplasts in the absence of CO2. At all concentrations of added glycerate 3-phosphate oxygen evolution ceased before stoichiometric amounts of oxygen were evolved. The inhibition of glycerate 3-phosphate-dependent-O2 evolution increased with increasing concentrations of substrate added. A similar response was observed in chloroplasts treated with KCN which inhibits ribulose-1,5-bisphosphate carboxylase-oxygenase. Oxygen uptake via the oxygenase activity of this enzyme is therefore not the cause of the discrepancy in stoichiometry of oxygen release in this system. The addition of NaHCO3 to chloroplasts in which oxygen evolution was inhibited by glycerate 3-phosphate caused an immediate sustained rate of oxygen evolution in the absence of KCN but not with KCN present. Simultaneous measurements of chlorophyll a fluorescence showed that qQ remained oxidized, although net O2 evolution had ceased. As O2 evolution decreased, qE and delta pH increased. Upon the addition of the NaHCO3, QA became more oxidized while delta pH and qE were decreased, suggesting that the inhibition of electron transport at high glycerate 3-phosphate concentrations was mediated by photosynthetic control via delta pH. However, the levels of ATP, ADP, ribulose 1,5-bisphosphate, and Pi concentrations and ATP/ADP ratio. The stromal glycerate 3-phosphate content declined upon illumination until O2 evolution ceased. At this time a constant stromal glycerate 3-phosphate concentration of 8-10 mM was maintained while net import of glycerate 3-phosphate into the stroma had virtually ceased. The stromal triosephosphate content remained at a constant low level throughout but the glycerate 3-phosphate level increased slightly after addition of NaHCO3. The data provided by the measurements of thylakoid reactions and stromal metabolites suggest that photosynthetic electron transport is tightly coupled to the requirements of the stroma for ATP and NADPH. Glycerate 3-phosphate reduction requires much less ATP than the operation of the complete Benson-Calvin cycle since the stoichiometry of ATP and NADPH utilization is reduced to 1:1. We conclude that thylakoid electron flow is not sufficiently flexible to maintain NADPH and ATP production in the ratio of 1:1. This situation will favor overenergization of the thylakoid membrane, increased leakiness of protons, increased electron drainage to O2, and result in progressive inhibition of noncyclic electron flow.     

Correlation between spatial (3D) structure of pea and bean thylakoid membranes and arrangement of chlorophyll-protein complexes

ABSTRACT: BACKGROUND: The thylakoid system in plant chloroplasts is organized into two distinct domains: granaarranged in stacks of appressed membranes and non-appressed membranes consisting ofstroma thylakoids and margins of granal stacks. It is argued that the reason for thedevelopment of appressed membranes in plants is that their photosynthetic apparatus need tocope with and survive ever-changing environmental conditions. It is not known however,why different plant species have different arrangements of grana within their chloroplasts. Itis important to elucidate whether a different arrangement and distribution of appressed andnon-appressed thylakoids in chloroplasts are linked with different qualitative and/orquantitative organization of chlorophyll-protein (CP) complexes in the thylakoid membranesand whether this arrangement influences the photosynthetic efficiency. RESULTS: Our results from TEM and in situ CLSM strongly indicate the existence of differentarrangements of pea and bean thylakoid membranes. In pea, larger appressed thylakoids areregularly arranged within chloroplasts as uniformly distributed red fluorescent bodies, whileirregular appressed thylakoid membranes within bean chloroplasts correspond to smaller andless distinguished fluorescent areas in CLSM images. 3D models of pea chloroplasts show adistinct spatial separation of stacked thylakoids from stromal spaces whereas spatial divisionof stroma and thylakoid areas in bean chloroplasts are more complex. Structural differencesinfluenced the PSII photochemistry, however without significant changes in photosyntheticefficiency. Qualitative and quantitative analysis of chlorophyll-protein complexes as well asspectroscopic investigations indicated a similar proportion between PSI and PSII corecomplexes in pea and bean thylakoids, but higher abundance of LHCII antenna in pea ones.Furthermore, distinct differences in size and arrangements of LHCII-PSII and LHCI-PSIsupercomplexes between species are suggested. CONCLUSIONS: Based on proteomic and spectroscopic investigations we postulate that the differences in thechloroplast structure between the analyzed species are a consequence of quantitativeproportions between the individual CP complexes and its arrangement inside membranes.Such a structure of membranes induced the formation of large stacked domains in pea, orsmaller heterogeneous regions in bean thylakoids. Presented 3D models of chloroplasts showed that stacked areas are noticeably irregular with variable thickness, merging with eachother and not always parallel to each other.     

Photoinhibition, 77K chlorophyll fluorescence quenching and phosphorylation of the light-harvesting chlorophyll-protein complex of photosystem II in soybean leaves

When the capacity of leaves for orderly dissipation of excitation energy in photosynthesis is exceeded, one mechanism by which the excess energy appears to be dissipated is through a nonradiative decay process. This process is observed as a reversible quenching of chlorophyll fluorescence emission (77K) from both photosystem II and photosystem I which persists in darkness (Demmig and Björkman 1987, Planta 171, 171–184). Fluorescence quenching was induced in soybean (Glycine max (L.) Merr.) leaves by two methods: 1) changing the composition of the gas surrounding the leaf from normal air to 2% O₂, 0% CO₂ at a low, constant photon flux density (PFD=photon fluence rate), and 2) increasing the PFD in the presence of normal air. In either case the quenching was fully reversible after return to the original condition (low PFD, normal air). The half-time of the relaxation of the quenching was in the order of 30 min. Both treatments resulted in reversible dephosphorylation of the light-harvesting chlorophyll-protein complex of photosystem II (LHC-II). Treatment under photoinhibitory conditions (high PFD plus chloramphenicol) also caused dephosphorylation of LHC-II. Therefore, phosphorylation of LHC-II cannot account for the observed fluorescence quenching. In addition, our results indicate that in vivo a factor other than the redox state of the plastoquinone pool controls LHC-II phosphorylation. This factor may be pH, the pH gradient across the thylakoid membranes.Abbreviations and symbols CAP chloramphenicol - F_o, F_M, F_v instantaneous, maximumr variable fluorescence emission - LHC-II light-haryesting chlorophyll-protein complex of PSII - kDa kilodalton - pH pH gradient across the thylakoid membrane - PFD photon flux density (photon fluence rate) - PQ plastoquinone - PSI, PSII photosystem I, II - Q acceptor of PSII C.I.W.-D.P.B. Publication No. 926     

Release of Photosynthetic Protein Catabolites by Blebbing from Thylakoids

Thylakoid proteins and their catabolites have been detected in lipid-protein particles isolated from the stroma of intact chloroplasts obtained from primary leaves of 2-week-old bean seedlings (Phaseolus vulgaris L. cv Kinghorn). The lipid-protein particles bear morphological resemblance to plastoglobuli seen in the chloroplasts of senescing leaves, but they are much smaller. They range from 10 to 320 nm in radius, are uniformly stained in thin sections visualized by transmission electron microscopy, and are discernible in the stroma of chloroplasts in corresponding thin-sectioned leaf tissue. The lipid-protein particles contain thylakoid lipids and are enriched in free fatty acids. Specifically, the free-to-esterified fatty acid ratio is about 1:1 in the particles compared to only 1:18 for corresponding thylakoid membranes. Western blot analyses indicate that these particles also contain thylakoid proteins and, in some cases, catabolites of these proteins including the CF1 [beta] and [gamma] subunits of ATPase, cytochrome f, and the 31- and 33-kD proteins of PSII. Lipid-protein particles with similar properties were generated in vitro from isolated, light-stressed thylakoids. Collectively, these data suggest that blebbing of lipid-protein particles may be a means of removing potentially destabilizing macromolecular catabolites from thylakoid membrane bilayers.     

Structural characterization of a complex of photosystem I and light-harvesting complex II of Arabidopsis thaliana

Chloroplasts are central to the provision of energy for green plants. Their photosynthetic membrane consists of two major complexes converting sunlight: photosystem I (PSI) and photosystem II (PSII). The energy flow toward both photosystems is regulated by light-harvesting complex II (LHCII), which after phosphorylation can move from PSII to PSI in the so-called state 1 to state 2 transition and can move back to PSII after dephosphorylation. To investigate the changes of PSI and PSII during state transitions, we studied the structures and frequencies of all major membrane complexes from Arabidopsis thaliana chloroplasts at conditions favoring either state 1 or state 2. We solubilized thylakoid membranes with digitonin and analyzed the complete set of complexes immediately after solubilization by electron microscopy and image analysis. Classification indicated the presence of a PSI-LHCII supercomplex consisting of one PSI-LHCI complex and one LHCII trimer, which was more abundant in state 2 conditions. The presence of LHCII was confirmed by excitation spectra of the PSI emission of membranes in state 1 or state 2. The PSI-LHCII complex could be averaged with a resolution of 16 A, showing that LHCII has a specific binding site at the PSI-A, -H, -L, and -K subunits.     

The function of chloroplastic NAD(P)H dehydrogenase in tobacco during chilling stress under low irradiance

The function of chloroplastic NAD(P)H dehydrogenase (NDH) was examined by comparing a tobacco transformant (DeltandhB) in which the ndhB gene had been disrupted with its wild type, upon exposure to chilling temperature (4 degrees C) under low irradiance (100 micro mol m(-2) s(-1) PFD). During the chilling stress, the maximum photochemical efficiency of PSII (F(v)/F(m)) decreased markedly in both the wild type and DeltandhB. However, both F(v)/F(m) and P700(+), as well as the PSII-driven electron transport rate (ETR), in DeltandhB were lower than that in the wild type, implying that NDH-dependent cyclic electron flow around PSI functioned to protect the photosynthetic apparatus from chilling stress under low irradiance. Under the stress, non-photochemical quenching (NPQ), particularly the fast relaxing NPQ component (qf) and the de-epoxidized ratio of the xanthophyll cycle pigments, (A+Z)/(V+A+Z), were distinguishable in DeltandhB from those in the wild type. The lower NPQ in DeltandhB might be related to an inefficient proton gradient across thylakoid membranes (DeltapH) because of lacking an NDH-dependent cyclic electron flow around PSI at chilling temperature under low irradiance.     

Determining the limitations and regulation of photosynthetic energy transduction in leaves

The light-dependent production of ATP and reductants by the photosynthetic apparatus in vivo involves a series of electron and proton transfers. Consideration is given as to how electron fluxes through photosystem I (PSI), using absorption spectroscopy, and through photosystem II (PSII), using chlorophyll fluorescence analyses, can be estimated in vivo. Measurements of light-induced electrochromic shifts using absorption spectroscopy provide a means of analyzing the proton fluxes across the thylakoid membranes in vivo. Regulation of these electron and proton fluxes is required for the thylakoids to meet the fluctuating metabolic demands of the cell. Chloroplasts exhibit a wide and flexible range of mechanisms to regulate electron and proton fluxes that enable chloroplasts to match light use for ATP and reductant production with the prevailing metabolic requirements. Non-invasive probing of electron fluxes through PSI and PSII, and proton fluxes across the thylakoid membranes can provide insights into the operation of such regulatory processes in vivo.     

The relationship between photosystem II efficiency and quantum yield for CO(2) assimilation is not affected by nitrogen content in apple leaves

Bench-grafted Fuji/M.26 apple (Malus domestica Borkh.) trees were fertigated with different concentrations of nitrogen by using a modified Hoagland's solution for 45 d. CO(2) assimilation and photosystem II (PSII) quantum efficiency in response to incident photon flux density (PFD) were measured simultaneously in recent fully expanded leaves under low O(2) (2%) and saturated CO(2) (1300 micromol mol(-1)) conditions. A single curvilinear relationship was found between true quantum yield for CO(2) assimilation and PSII quantum efficiency for leaves with a wide range of leaf N content. The relationship was linear up to a quantum yield of approximately 0.05 mol CO(2) mol(-1) quanta. It then became curvilinear with a further rise in quantum yield in response to decreasing PFD. This relationship was subsequently used as a calibration curve to assess the rate of non-cyclic electron transport associated with Rubisco and the partitioning of electron flow between CO(2) assimilation and photorespiration in different N leaves in response to intercellular CO(2) concentration (C(i)) under normal O(2) conditions. Both the rate of non-cyclic electron flow and the rate of electron flow to CO(2) or O(2) increased with increasing leaf N at any given C(i). The percentage of non-cyclic electron flow to CO(2) assimilation, however, remained the same regardless of leaf N content. As C(i) increased, the percentage of non-cyclic electron flow to CO(2) assimilation increased. In conclusion, the relationship between PSII quantum efficiency and quantum yield for CO(2) assimilation and the partitioning of electron flow between CO(2) assimilation and photorespiration are not affected by N content in apple leaves.     

The effects of external electric field on the electron transport system of spinach chloroplasts

Since the thylakoid membranes of an active chloroplast are constantly exposed to the electric fields generated by the electron transport system inside the membranes, we have studied the effects of pretreating chloroplasts of spinach ( Spinacia oleracea L.) leaves with an external AC (alternating current) electric field on their electron transport system. It was found that a few minutes electric field pretreatment (333 V cm^-1 across chloroplast samples), especially at low frequency, irreversibly inhibited the activity of photosystem II (PSII), but under certain conditions, stimulated that of photosystem I (PSI). From the measurements of fluorescence from PSII, we ascribe the inhibition to a lesion close to its reaction center P680, leading to increased dissipation of excitation energy to heat. The effect on PSI was investigated by the reduction of its reaction center, P700 by various artificial donors. We suggest that the stimulative effect can be attributed to a positive shift of the surface charge density of thylakoid membranes that brings about an increase in the accessibility of exogenous electronegative donors.