PSI Cyclic Electron Flow Research Articles

The Effect of Foliar Selenium (Se) Treatment on Growth, Photosynthesis, and Oxidative-Nitrosative Signalling of Stevia rebaudiana Leaves

Selenium (Se) enrichment of Stevia rebaudiana Bertoni can serve a dual purpose, on the one hand to increase plant biomass and stress tolerance and on the other hand to produce Se fortified plant-based food. Foliar Se spraying (0, 6, 8, 10 mg/L selenate, 14 days) of Stevia plantlets resulted in slightly decreased stevioside and rebaudioside A concentrations, and it also caused significant increment in stem elongation, leaf number, and Se content, suggesting that foliar Se supplementation can be used as a biofortifying approach. Furthermore, Se slightly limited photosynthetic CO2 assimilation (AN, gsw, Ci/Ca), but exerted no significant effect on chlorophyll, carotenoid contents and on parameters associated with photosystem II (PSII) activity (FV/FM, F0, Y(NO)), indicating that Se causes no photodamage in PSII. Further results indicate that Se is able to activate PSI-cyclic electron flow independent protection mechanisms of the photosynthetic apparatus of Stevia plants. The applied Se activated superoxide dismutase (SOD) isoenzymes (MnSOD1, FeSOD1, FeSOD2, Cu/ZnSOD1, Cu/ZnSOD2) and down-regulated NADPH oxidase suggesting the Se-induced limitation of superoxide anion levels and consequent oxidative signalling in Stevia leaves. Additionally, the decrease in S-nitrosoglutathione reductase protein abundance and the intensification of protein tyrosine nitration indicate Se-triggered nitrosative signalling. Collectively, these results suggest that Se supplementation alters Stevia shoot morphology without significantly affecting biomass yield and photosynthesis, but increasing Se content and performing antioxidant effects, which indicates that foliar application of Se may be a promising method in Stevia cultivation.

Chlamydomonas sp. UWO 241 Exhibits High Cyclic Electron Flow and Rewired Metabolism under High Salinity.

The Antarctic green alga Chlamydomonas sp. UWO 241 (UWO 241) is adapted to permanent low temperatures, hypersalinity, and extreme shade. One of the most striking phenotypes of UWO 241 is an altered PSI organization and constitutive PSI cyclic electron flow (CEF). To date, little attention has been paid to CEF during long-term stress acclimation, and the consequences of sustained CEF in UWO 241 are not known. In this study, we combined photobiology, proteomics, and metabolomics to understand the underlying role of sustained CEF in high-salinity stress acclimation. High salt-grown UWO 241 exhibited increased thylakoid proton motive flux and an increased capacity for nonphotochemical quenching. Under high salt, a significant proportion of the up-regulated enzymes were associated with the Calvin-Benson-Bassham cycle, carbon storage metabolism, and protein translation. Two key enzymes of the shikimate pathway, 3-deoxy-d-arabinoheptulosonate 7-phosphate synthase and chorismate synthase, were also up-regulated, as well as indole-3-glycerol phosphate synthase, an enzyme involved in the biosynthesis of l-Trp and indole acetic acid. In addition, several compatible solutes (glycerol, Pro, and Suc) accumulated to high levels in high salt-grown UWO 241 cultures. We suggest that UWO 241 maintains constitutively high CEF through the associated PSI-cytochrome b 6 f supercomplex to support robust growth and strong photosynthetic capacity under a constant growth regime of low temperatures and high salinity.

Cold-Adapted Protein Kinases and Thylakoid Remodeling Impact Energy Distribution in an Antarctic Psychrophile.

The Antarctic psychrophile Chlamydomonas sp. UWO241 evolved in a permanently ice-covered lake whose aquatic environment is characterized not only by constant low temperature and high salt but also by low light during the austral summer coupled with 6 months of complete darkness during the austral winter. Since the UWO241 genome indicated the presence of Stt7 and Stl1 protein kinases, we examined protein phosphorylation and the state transition phenomenon in this psychrophile. Light-dependent [γ-33P]ATP labeling of thylakoid membranes from Chlamydomonas sp. UWO241 exhibited a distinct low temperature-dependent phosphorylation pattern compared to Chlamydomonas reinhardtii despite comparable levels of the Stt7 protein kinase. The sequence and structure of the UWO241 Stt7 kinase domain exhibits substantial alterations, which we suggest predisposes it to be more active at low temperature. Comparative purification of PSII and PSI combined with digitonin fractionation of thylakoid membranes indicated that UWO241 altered its thylakoid membrane architecture and reorganized the distribution of PSI and PSII units between granal and stromal lamellae. Although UWO241 grown at low salt and low temperature exhibited comparable thylakoid membrane appression to that of C. reinhardtii at its optimal growth condition, UWO241 grown under its natural condition of high salt resulted in swelling of the thylakoid lumen. This was associated with an upregulation of PSI cyclic electron flow by 50% compared to growth at low salt. Due to the unique 77K fluorescence emission spectra of intact UWO241 cells, deconvolution was necessary to detect enhancement in energy distribution between PSII and PSI, which was sensitive to the redox state of the plastoquinone pool and to the NaCl concentrations of the growth medium. We conclude that a reorganization of PSII and PSI in UWO241 results in a unique state transition phenomenon that is associated with altered protein phosphorylation and enhanced PSI cyclic electron flow. These data are discussed with respect to a possible PSII-PSI energy spillover mechanism that regulates photosystem energy partitioning and quenching.

NDH-PSI Supercomplex Assembly Precedes Full Assembly of the NDH Complex in Chloroplast

The chloroplast NADH dehydrogenase-like (NDH) complex is structurally similar to respiratory complex I and mediates PSI cyclic electron flow. In Arabidopsis (Arabidopsis thaliana), chloroplast NDH is composed of at least 29 subunits and associates with two copies of PSI to form the NDH-PSI supercomplex. Here, we found that CHLORORESPIRATORY REDUCTION3 (CRR3) is an assembly factor required for the accumulation of subcomplex B (SubB) of chloroplast NDH. In Suc density gradient centrifugation, CRR3 was detected in three protein complexes. Accumulation of the largest peak III complex was impaired in mutants defective in the SubB subunits PnsB2-PnsB5. The oligomeric form of CRR3 likely functions to assemble the core of SubB to form the peak III complex as an assembly intermediate. A defect in the PnsL3 subunit increased the level of the peak III complex, suggesting that CRR3 was released from the assembly intermediate after PnsL3 binding. Unlike PnsB2-PnsB5 and PnsL3, PnsB1 was not absolutely necessary for stabilizing SubB. PnsB1 is likely incorporated into the intermediate at the final step during SubB assembly. Lhca6 is a linker protein mediating NDH-PSI supercomplex formation, and its site of contact with NDH was suggested to be SubB. In the lhca6 mutant, accumulation of the peak III complex was impaired, suggesting that SubB interacted with Lhca6 during the step of SubB assembly. The process of supercomplex formation was triggered before the completion of the NDH assembly. Consistent with its predicted function, CRR3 accumulated in young leaves, where the NDH complex was assembled.

Photosynthetic electron transport and specific photoprotective responses in wheat leaves under drought stress

The photosynthetic responses of wheat (Triticum aestivum L.) leaves to different levels of drought stress were analyzed in potted plants cultivated in growth chamber under moderate light. Low-to-medium drought stress was induced by limiting irrigation, maintaining 20% of soil water holding capacity for 14days followed by 3days without water supply to induce severe stress. Measurements of CO2 exchange and photosystem II (PSII) yield (by chlorophyll fluorescence) were followed by simultaneous measurements of yield of PSI (by P700 absorbance changes) and that of PSII. Drought stress gradually decreased PSII electron transport, but the capacity for nonphotochemical quenching increased more slowly until there was a large decrease in leaf relative water content (where the photosynthetic rate had decreased by half or more). We identified a substantial part of PSII electron transport, which was not used by carbon assimilation or by photorespiration, which clearly indicates activities of alternative electron sinks. Decreasing the fraction of light absorbed by PSII and increasing the fraction absorbed by PSI with increasing drought stress (rather than assuming equal absorption by the two photosystems) support a proposed function of PSI cyclic electron flow to generate a proton-motive force to activate nonphotochemical dissipation of energy, and it is consistent with the observed accumulation of oxidized P700 which causes a decrease in PSI electron acceptors. Our results support the roles of alternative electron sinks (either from PSII or PSI) and cyclic electron flow in photoprotection of PSII and PSI in drought stress conditions. In future studies on plant stress, analyses of the partitioning of absorbed energy between photosystems are needed for interpreting flux through linear electron flow, PSI cyclic electron flow, along with alternative electron sinks.

Photosynthetic electron transport and proton flux under moderate heat stress

Moderate heat stress has been reported to increase PSI cyclic electron flow (CEF). We subjected leaves of Arabidopsis (Arabidopsis thaliana) mutants disrupted in the regulation of one or the other pathway of CEF flow-crr2 (chlororespiratory reduction, deficient in regulation of chloroplast NAD(P)H dehydrogenase-dependent CEF) and pgr5 (proton gradient regulation, proposed to have reduced efficiency of antimycin-A-sensitive-CEF regulation) to moderate heat stress. Light-adapted leaves were switched from 23 to 40 degrees C in 2 min. Gas exchange, chlorophyll fluorescence, the electrochromic shift (ECS), and P700 were measured. Photosynthesis of crr2 and pgr5 was more sensitive to heat and had less ability to recover than the genetic background gl. The proton conductance in light was increased by heat and it was twice as much in pgr5, which had much smaller light-induced proton motive force. We confirmed that P700 becomes more reduced at high temperature and show that, in contrast, the proportion of PSII open centers (with Q (A) oxidized) increases. The two mutants had much slower P700(+) reduction rate during and after heat than gl. The proportion of light absorbed by PSI versus PSII was increased in gl and crr2 during and after heat treatment, but not in pgr5. We propose that heat alters the redox balance away from PSII and toward PSI and that the regulation of CEF helps photosynthesis tolerate heat stress.

Novel Nuclear-encoded Subunits of the Chloroplast NAD(P)H Dehydrogenase Complex

The NAD(P)H dehydrogenase (NDH) complex functions in photosystem I cyclic electron transfer in higher plant chloroplasts and is crucial for plant responses to environmental stress. Chloroplast NDH complex is a close relative to cyanobacterial NDH-1L complex, and all fifteen subunits so far identified in NDH-1L have homologs in the chloroplast NDH complex. Here we report on the identification of two nuclear-encoded proteins NDH48 and NDH45 in higher plant chloroplasts and show their intimate association with the NDH complex. These two membrane proteins are shown to interact with each other and with the NDH complex enriched in stroma thylakoids. Moreover, the deficiency of either the NDH45 protein or the NDH48 protein in respective mutant plants leads to severe defects in both the accumulation and the function of the NDH complex. The NDH48 and NDH45 proteins are not components of the hydrophilic connecting domain of the NDH complex but are strongly attached to the hydrophobic membrane domain. We conclude that NDH48 and NDH45 are novel nuclear-encoded subunits of the chloroplast NDH complex and crucial both for the stable structure and function of the NDH complex.

A T-DNA insertion mutant of AtHMA1 gene encoding a Cu transporting ATPase in Arabidopsis thaliana has a defect in the water–water cycle of photosynthesis

The water–water cycle is the electron flow through scavenging enzymes for the reactive species of oxygen in chloroplasts, and is proposed to play a role in alternative electron sink in photosynthesis. Here we showed that the water–water cycle is impaired in the T-DNA insertion mutant of AtHMA1 gene encoding a Cu transporting ATPase in chloroplasts. Chlorophyll fluorescence under steady state was not affected in hma1, indicating that photosynthetic electron transport under normal condition was not impaired. Under electron acceptor limited conditions, however, hma1 showed distinguished phenotype in chlorophyll fluorescence characteristics. The most severe phenotype of hma1 could be observed in high (0.1%) CO 2 concentrations, indicating that hma1 has the defect other than photorespiration. The transient increase of chlorophyll fluorescence upon the cessation of the actinic light as well as the NPQ induction of chlorophyll fluorescence revealed that the two pathways of cyclic electron flow around PSI, NDH-pathway and FQR-pathway, are both intact in hma1. Based on the NPQ induction under 0% oxygen condition, we conclude that the water–water cycle is impaired in hma1, presumably due to the decreased level of Cu/Zn SOD in the mutant. Under high CO 2 condition, hma1 exhibited slightly higher NPQ induction than wild type plants, while this increase of NPQ in hma1 was suppressed when hma1 was crossed with crr2 having a defect in NDH-mediated PSI cyclic electron flow. We propose that the water–water cycle and NDH-mediated pathways might be regulated compensationally with each other especially when photorespiration is suppressed.

A Novel Nuclear-Encoded Protein, NDH-Dependent Cyclic Electron Flow 5, is Essential for the Accumulation of Chloroplast NAD(P)H Dehydrogenase Complexes

The chloroplast NAD(P)H dehydrogenase (NDH) complex, which reduces plastoquinones in thylakoid membranes, is involved in PSI cyclic electron flow and chlororespiration. In addition to land plants, the NDH complex is conserved in cyanobacteria. In this study, we identified a novel NDH-related gene of Arabidopsis, NDH-dependent cyclic electron flow 5 (NDF5, At1g55370). Post-illumination increases in chlorophyll fluorescence were absent in ndf5 mutant plants, which indicated that NDF5 is essential for NDH activity. Sequence analysis did not reveal any known functional motifs in NDF5, but there was some homology in amino acid sequence between NDF5 and NDF2, a known NDH subunit. NDF5 and NDF2 homologs were present in higher plants, but not cyanobacteria. A single homolog, which had similarity to both NDF5 and NDF2, was identified in the moss Physcomitrella patens. Immunoblot analysis showed that NDF5 localizes to membrane fractions of chloroplasts. The stability of NdhH, a subunit of the NDH complex, as well as NDF5 and NDF2, was decreased in ndf5, ndf2 and double ndf2/ndf5 mutants, resulting in a loss of NDH activity in these mutants. These results indicated that both NDF5 and NDF2 have essential functions in the stabilization of the NDH complex. We propose that NDF5 and NDF2 were acquired by land plants during evolution, and that in higher plants both NDF5 and NDF2 are critical to regulate NDH activity and each other's protein stability, as well as the stability of additional NDH subunits.

Rates and Roles of Cyclic and Alternative Electron Flow in Potato Leaves

Measurements of 810 nm transmittance changes in leaves, simultaneously with Chl fluorescence, CO(2) uptake and O(2) evolution, were carried out on potato (Solanum tuberosum L.) leaves with altered expression of plastidic NADP-dependent malate dehydrogenase. Electron transport rates were calculated: J(C) from the CO(2) uptake rate considering ribulose-1,5-bisphosphate (RuBP) carboxylation and oxygenation, J(O) from the O(2) evolution rate, J(F) from Chl fluorescence parameters and J(I) from the post-illumination re-reduction speed of PSI donors. In the absence of external O(2), J(O) equaled (1.005 +/- 0.003) J(C), independent of the transgenic treatment, light intensity and CO(2) concentration. This showed that nitrite and oxaloacetate reduction rates were very slow. The Mehler-type O(2) reduction was evaluated from the rate of electron accumulation at PSI after the O(2) concentration was decreased from 210 to 20 mmol mol(-1), and resulted in <1% of the linear flow. J(F) and J(I) did not differ from J(C) while photosynthesis was light-limited, but considerably exceeded J(C) at saturating light. Then, typically, J(F) = 1.2 J(C) and J(I) = 1.3 J(C), and J(F) -J(C) and J(I) -J(C) depended little on CO(2) and O(2) concentrations. The results showed that the alternative and cyclic electron flow necessary to compensate variations in the ATP/NADPH ratio were only a few percent of the linear flow. The data do not support the requirement of 14H(+)/3ATP by the chloroplast ATP synthase. We suggest that the fast PSI cyclic electron flow J(I) - J(C), as well as the fast J(F) - J(C) are energy-dissipating cycles around PSI and PSII at light saturation.

Cyanobacterial NDH‐1 complexes: multiplicity in function and subunit composition

In cyanobacteria, the NAD(P)H:quinone oxidoreductase (NDH-1) is involved in a variety of functions like respiration, cyclic electron flow around PSI and CO(2) uptake. Several types of NDH-1 complexes, which differ in structure and are responsible for these functions, exist in cyanobacterial membranes. This minireview is based on data obtained by reverse genetics and proteomics studies and focuses on the structural and functional differences of the two types of cyanobacterial NDH-1 complexes: NDH-1L, important for respiration and PSI cyclic electron flow, and NDH-1MS, the low-CO(2) inducible complex participating in CO(2) uptake. The NDH-1 complexes in cyanobacteria share a common NDH-1M 'core' complex and differ in the composition of the distal membrane domain composed of specific NdhD and NdhF proteins, which in complexes involved in CO(2) uptake is further associated with the hydrophilic carbon uptake (CUP) domain. At present, however, very important questions concerning the nature of catalytically active subunits that constitute the electron input device (like NADH dehydrogenase module of the eubacterial 'model' NDH-1 analogs), the substrate specificity and reaction mechanisms of cyanobacterial complexes remain unanswered and are shortly discussed here.

The role of chloroplast NAD(P)H dehydrogenase in protection of tobacco plant against heat stress

After incubation at 42 degrees C for more than 48 h, brown damages occurred on the stems of tobacco (Nicotiana tabacum L.) ndhC-ndhK-ndhJ deletion mutant (deltandhCKJ), followed by wilt of the leaves, while less the phenotype was found in its wild type (WT). Analysis of the kinetics of post-illumination rise in chlorophyll fluorescence indicated that the PSI cyclic electron flow and the chlororespiration mediated by NAD(P)H dehydrogenase (NDH) was significantly enhanced in WT under the high temperature. After leaf disks were treated with methyl viologen (MV), photosynthetic apparatus of deltandhCKJ exhibited more severe photo-oxidative damage, even bleaching of chlorophyll. Analysis of P700 oxidation and reduction showed that the NDH mediated cyclic electron flow probably functioned as an electron competitor with Mehler reaction, to reduce the accumulation of reactive oxygen species (ROS). When leaf disks were heat stressed at 42 degrees C for 6 h, the photochemical activity declined more markedly in deltandhCKJ than in WT, accompanied with more evident decrease in the amount of soluble Rubisco activase. In addition, the slow phase of millisecond-delayed light emission (ms-DLE) of chlorophyll fluorescence indicated that NDH was involved in the building-up of transthylakoid proton gradient (deltapH), while the consumption of deltapH was highly inhibited in deltandhCKJ after heat stress. Based on the results, we supposed that the cyclic electron flow mediated by NDH could be stimulated under the heat stressed conditions, to divert excess electrons via chlororespiration pathway, and sustain CO2 assimilation by providing extra deltapH, thus reducing the photooxidative damage.

Recovery of photosynthetic systems during rewetting is quite rapid in a terrestrial cyanobacterium, Nostoc commune.

Recovery processes of photosynthetic systems during rewetting were studied in detail in a terrestrial, highly drought-tolerant cyanobacterium, Nostoc commune. With absorption of water, the weight of N. commune colony increased in three phases with half-increase times of about 1 min, 2 h and 9 h. Fluorescence intensities of phycobiliproteins and photosystem (PS) I complexes were recovered almost completely within 1 min, suggesting that their functional forms were restored very quickly. Energy transfer from allophycocyanin to the core-membrane linker peptide (L(CM)) was recovered within 1 min, but not that from L(CM) to PSII. PSI activity and cyclic electron flow around PSI recovered within 2 min, while the PSII activity recovered in two phases after a time lag of about 5 min, with half times of about 20 min and 2 h. Photosynthetic CO(2) fixation was restored almost in parallel with the first recovery phase of the PSII reaction center activity. Although the amount of absorbed water became more than 20 times the initial dry weight of the N. commune colony in the presence of sufficient water, about twice the initial dry weight was enough for recovery and maintenance of the PSII activity.

Photo-induction of an NADPH dehydrogenase which functions as a mediator of electron transport to the intersystem chain in the cyanobacterium Synechocystis PCC6803.

Illumination of the dark-incubated cells of Synechocystis PCC6803 caused recovery of both respiratory activity of oxygen uptake and PS I-cyclic electron flow, which was monitored by the dark reduction of P700(+) in the presence of DCMU after a 50 ms pulse light (MT) under background far-red light, but the effects were much smaller in those of the mutant M55, which has an ndh-B defective gene. Activity of an NADPH-NBT oxidoreductase with a higher molecular mass (around 380 kDa), which was only found in wild type but not in M55, became evident after the dark-incubated cells were exposed to the light. Immuno-blotting analysis indicated that the NADPH-NBT oxidoreductase contains the NdhB subunit of NDH. The expression of NdhB decreased in dark-incubated cells and increased upon transfer of the cells back to light. These results indicate that an NADPH-specific NDH participates in the light-regulated cyclic electron transport around Photosystem I as well as in respiratory electron transport to the intersystem chain in Synechocystis 6803.

Manipulation of the imbalance for linear electron flow activities between photosystems I and II of photosynthesis by cyclic electron flow cofactors

Imbalance for linear electron flow in light 2 (e.g. λ < 690 nm) in favor of PS II, may be regulated by a mechanism which changes the ratio between competing linear and cyclic electron flows. In order to examine this possibility, artificial PS I-cyclic electron flow cofactors were added to broken chloroplasts, which performed linear whole electron transport in light 2 excitation with methylviologen as an electron acceptor. A significant increase of the apparent photosystems imbalance for linear electron transport activities in favor of PS II, monitored by modulated fluorimetry, occurred upon addition of oxidized or reduced (with excess ascorbate) forms of PMS and TMPD as well as with the reduced form of DAD. It is suggested that these effects arise from a competition with PS II on electron donation to PS I, and establishment of a steady-state where cyclic electron flow competes with the linear one. Most convincing was the observation of a similar increase of this imbalance in favor of PS II, reproduced in leaf discs treated with PMS, performing in this case a more direct measurement of oxygen evolution by photoacoustic spectroscopy. The wavelength dependence of the imbalance for linear electron transport activities was checked in chloroplasts for the case of PMS, and had maxima at wavelengths where absorption of chlorophyll b predominates. These observations suggest that the final photoactivity distribution between the two photosystems for the linear electron flow (affected by the cofactor) depends on the basal photosystems imbalance of quarta absorbed in each photosystem due to the pigment distribution pattern between PS I and PS II, as influenced by the cationic level and the abolition of membrane energization. The relevance to a possible regulation mechanism in-vivo, which controls the ratio between cyclic to non-cyclic electron flows, is discussed.

Manipulation of the imbalance for linear electron flow activities between photosystems I and II of photosynthesis by cyclic electron flow cofactors

Imbalance for linear electron flow in light 2 (e.g. λ < 690 nm) in favor of PS II, may be regulated by a mechanism which changes the ratio between competing linear and cyclic electron flows. In order to examine this possibility, artificial PS I-cyclic electron flow cofactors were added to broken chloroplasts, which performed linear whole electron transport in light 2 excitation with methylviologen as an electron acceptor. A significant increase of the apparent photosystems imbalance for linear electron transport activities in favor of PS II, monitored by modulated fluorimetry, occurred upon addition of oxidized or reduced (with excess ascorbate) forms of PMS and TMPD as well as with the reduced form of DAD. It is suggested that these effects arise from a competition with PS II on electron donation to PS I, and establishment of a steady-state where cyclic electron flow competes with the linear one. Most convincing was the observation of a similar increase of this imbalance in favor of PS II, reproduced in leaf discs treated with PMS, performing in this case a more direct measurement of oxygen evolution by photoacoustic spectroscopy. The wavelength dependence of the imbalance for linear electron transport activities was checked in chloroplasts for the case of PMS, and had maxima at wavelengths where absorption of chlorophyll b predominates. These observations suggest that the final photoactivity distribution between the two photosystems for the linear electron flow (affected by the cofactor) depends on the basal photosystems imbalance of quarta absorbed in each photosystem due to the pigment distribution pattern between PS I and PS II, as influenced by the cationic level and the abolition of membrane energization. The relevance to a possible regulation mechanism in-vivo, which controls the ratio between cyclic to non-cyclic electron flows, is discussed.

Manipulation of the imbalance for linear electron flow activities between photosystems I and II of photosynthesis by cyclic electron flow cofactors

Imbalance for linear electron flow in light 2 (e.g. λ < 690 nm) in favor of PS II, may be regulated by a mechanism which changes the ratio between competing linear and cyclic electron flows. In order to examine this possibility, artificial PS I-cyclic electron flow cofactors were added to broken chloroplasts, which performed linear whole electron transport in light 2 excitation with methylviologen as an electron acceptor. A significant increase of the apparent photosystems imbalance for linear electron transport activities in favor of PS II, monitored by modulated fluorimetry, occurred upon addition of oxidized or reduced (with excess ascorbate) forms of PMS and TMPD as well as with the reduced form of DAD. It is suggested that these effects arise from a competition with PS II on electron donation to PS I, and establishment of a steady-state where cyclic electron flow competes with the linear one. Most convincing was the observation of a similar increase of this imbalance in favor of PS II, reproduced in leaf discs treated with PMS, performing in this case a more direct measurement of oxygen evolution by photoacoustic spectroscopy. The wavelength dependence of the imbalance for linear electron transport activities was checked in chloroplasts for the case of PMS, and had maxima at wavelengths where absorption of chlorophyll b predominates. These observations suggest that the final photoactivity distribution between the two photosystems for the linear electron flow (affected by the cofactor) depends on the basal photosystems imbalance of quarta absorbed in each photosystem due to the pigment distribution pattern between PS I and PS II, as influenced by the cationic level and the abolition of membrane energization. The relevance to a possible regulation mechanism in-vivo, which controls the ratio between cyclic to non-cyclic electron flows, is discussed.

Photoacoustic Measurements in Vivo of Energy Storage by Cyclic Electron Flow in Algae and Higher Plants

Energy storage by cyclic electron flow through photosystem I (PSI) was measured in vivo using the photoacoustic technique. A wide variety of photosynthetic organisms were considered and all showed measurable energy storage by PSI-cyclic electron flow except for higher plants using the C-3 carbon fixation pathway. The capacity for energy storage by PSI-cyclic electron flow alone was found to be small in comparison to that of linear and cyclic electron flows combined but may be significant, nonetheless, under conditions when photosystem II is damaged, particularly in cyanobacteria. Light-induced dynamics of energy storage by PSI-cyclic electron flow were evident, demonstrating regulation under changing environmental conditions.

Light potentiation of photosynthetic oxygen evolution inhibition by water soluble chemical modifiers

Oxygen evolution was inhibited after reacting chloroplast membranes with four different water soluble protein modification reagents. Photosystem II photochemistry was not affected, whown by unimpaired oxidation of an alternate PSII donor, diphenyl carbazide. Concomitant with oxygen evolution inhibition by the diazonium reagent, there was a four-fold increase in covalent binding of the compound to the membranes, suggesting an electron transport dependent conformational change is involved in the effect. PSI cyclic electron flow with DCMU present did not potentiate the oxygen evolution inhibition nor the increased diazo coupling, indicating that the effect is not simply a manifestation of the same energized state driven by cyclic electron flow. Since the effects are due to non-membrane penetrating reagents, we conclude that a protein component associated with oxygen evolution is localized at the external surface of grana membranes.