Electron Transport In Thylakoids Research Articles

Fluorescence kinetic parameters and cyclic electron transport in guard cell chloroplasts of chlorophyll-deficient leaf tissues from variegated weeping fig (Ficus benjamina L.)

Residual chlorophyll in chlorophyll-deficient (albino) areas of variegated leaves of Ficus benjamina originates from guard cell chloroplasts. Photosynthetic features of green and albino sectors of F. benjamina were studied by imaging the distribution of the fluorescence decrease ratio Rfd within a leaf calculated from maximum (Fm) and steady-state leaf chlorophyll fluorescence (Fs) at 690 and 740 nm. Local areas of albino sectors demonstrated an abnormally high Rfd(740)/Rfd(690) ratio. Fluorescence transients excited in albino sectors at red (640 and 690 nm) wavelengths showed an abrupt decrease of the Rfd values (0.4 and 0.1, correspondingly) as compared with those excited at blue wavelengths (1.7-2.4). This "Red Drop" was not observed for green sectors. Normal and chlorophyll-deficient leaf sectors of F. benjamina were also tested for linear and cyclic electron transport in thylakoids. The tests have been performed studying fluorescence at a steady-state phase with CO(2)-excess impulse feeding, photoacoustic signal generated by pulse light source at wavelengths selectively exciting PSI, fluorescence kinetics under anaerobiosis and fluorescence changes observed by dual-wavelength excitation method. The data obtained for albino sectors strongly suggest the possibility of a cyclic electron transport simultaneously occurring in guard cell thylakoids around photosystems I and II under blue light, whereas linear electron transport is absent or insufficient.

Impact of UV-B irradiation on photosynthetic performance and chloroplast membrane components in Oryza sativa L.

The impact of UV-B radiation on photosynthetic related parameters was studied in Oryza sativa L. cv. Safari plants, after an UV-B irradiation performed 1 h per day for 7 days (between 8 and 14 days after germination) with a ten narrow-band ( λ 311 nm) that resulted in a total biological effective UV-B (UVB BE) of 2.975 kJ m −2 day −1 and a total of 20.825 kJ m −2. Gas exchange measurements were severely affected, showing reductions higher than 80% in net photosynthesis ( P n), stomatal conductance and photosynthetic capacity ( A max), 1 day after the end of the 7-days UV-B treatment. Similarly, several fluorescence parameters ( F o, F v/ F m, F v ′ / F m ′ , ϕ e, q P and q E) and thylakoid electron transport (involving both photosystems) were also severely reduced. Concomitantly, a decline of xanthophylls, carotenes, Chl a, Chl ( a + b) and Chl ( a/ b) values was accompanied by the increase of the lipoperoxidation level in chloroplast membranes, altogether reflecting a loss of protection against oxidative stress. Seven days after of the end of UV-B treatment, most fluorescence parameters recovered, but in P n, A max, thylakoid electron transport rates, Chl a and lipid classes, as well as the level of lipoperoxidation, the impacts were even stronger than immediately after the end of stress, denoting a clear loss of performance of photosynthetic structures. However, only a moderate impact on total lipids was observed, accompanied by some changes in the relative weight of the major chloroplast membrane lipid classes, with emphasis on the decrease of MGDG and the increase of phospholipids. That suggested an ability to de novo lipid synthesis allowing qualitative changes in the lipid matrix. Notably, the leaves developed after the end of UV-B irradiation showed a much lower impact, with significantly decreased values only in P n and g s, rises in several fluorescence parameters, thylakoid electron transport, photosynthetic pigments (xanthophylls and chls) and DEPS, while lipid classes presented values close to control. The results showed a global impact of UV-B in the photosynthetic structures and performance in irradiated leaves, but revealed also a low impairment extent in the leaves entirely developed after the end of the irradiation, reflecting a remarkable recovery of the plant after the end of stress, what could constitute an advantage under occasional UV-B exposure events in this vital worldwide staple food crop.

ArabidopsisSTN7 Kinase Provides a Link between Short- and Long-Term Photosynthetic Acclimation

Flowering plants control energy allocation to their photosystems in response to light quality changes. This includes the phosphorylation and migration of light-harvesting complex II (LHCII) proteins (state transitions or short-term response) as well as long-term alterations in thylakoid composition (long-term response or LTR). Both responses require the thylakoid protein kinase STN7. Here, we show that the signaling pathways triggering state transitions and LTR diverge at, or immediately downstream from, STN7. Both responses require STN7 activity that can be regulated according to the plastoquinone pool redox state. However, LTR signaling does not involve LHCII phosphorylation or any other state transition step. State transitions appear to play a prominent role in flowering plants, and the ability to perform state transitions becomes critical for photosynthesis in Arabidopsis thaliana mutants that are impaired in thylakoid electron transport but retain a functional LTR. Our data imply that STN7-dependent phosphorylation of an as yet unknown thylakoid protein triggers LTR signaling events, whereby an involvement of the TSP9 protein in the signaling pathway could be excluded. The LTR signaling events then ultimately regulate in chloroplasts the expression of photosynthesis-related genes on the transcript level, whereas expression of nuclear-encoded proteins is regulated at multiple levels, as indicated by transcript and protein profiling in LTR mutants.

The Response of Leaf Photosynthesis and Dry Matter Accumulation to Nitrogen Supply in an Older and a Newer Maize Hybrid

Nitrogen use efficiency is higher in newer than in older maize (Zea mays L.) hybrids, but the physiological mechanisms underlying differences in N‐use efficiency are unknown. The objective of this study was to quantify differences between an older and a newer maize hybrid in their response to N availability throughout the life cycle at both the leaf and the whole‐plant level. An older and a newer maize hybrid were grown in a field hydroponic system located near Guelph, ON, in 2005 at a high and a low N level. Leaf carbon exchange rate (CER), chlorophyll index, and the thylakoid electron transport rate (ETR) were measured weekly from 2 wk presilking to 8 wk postsilking. Plant‐component dry matter and N content were determined from 1 wk presilking to maturity. At the leaf level, leaf CER declined during the grain‐filling period, and the decline was greater under low than high N availability. The decline in leaf CER during the grain‐filling period was less in the newer than in the older hybrid under both high and low N availability, and differences in leaf CER were associated most strongly with a reduction in leaf CER per unit absorbed photosynthetic photon flux density. At the whole‐plant level, reduction in grain yield in low vs. high N was greater in the older than in the newer hybrid. The hybrid × N interaction for grain yield was attributable predominantly to a greater decline in the proportion of dry matter allocated to the grain in the older hybrid.

The lipid composition of the unicellular green alga Chlamydomonas reinhardtii and the diatom Cyclotella meneghiniana investigated by MALDI-TOF MS and TLC

The lipid composition of algae is crucial for numerous structural and physiological aspects, e.g. the integrity of the photosynthetic complexes and the functionality of membrane-embedded processes as the photosynthetic electron transport in thylakoids or the mitochondrial respiration. In this paper the lipid composition of the organic extracts of the green alga Chlamydomonas reinhardtii and the diatom Cyclotella meneghiniana are compared by using matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) in combination with thin-layer chromatography (TLC). The combined methods enable quantitative evaluation of the individual lipid classes as well as the determination of the relative acyl compositions. It will be shown that both algae differ in (a) the lipid classes, (b) the relative contribution of the individual lipid classes and (c) the acyl compositions. Differences in the acyl composition concern particularly the mono- and digalactosyl diacylglycerols. Glycerol-trimethylhomoserine and phosphatidylethanolamine are exclusively detected in the C. reinhardtii extracts, whereas phosphatidylcholine is a characteristic lipid of C. meneghiniana. Furthermore, the proportion of the acidic lipids sulfoquinovosyl-diacylglycerol and phosphatidylglycerol is significantly higher in the diatom than in C. reinhardtii.

Kinetic study of the plastoquinone pool availability correlated with H 2O 2 release in seawater and antioxidant responses in the red alga Kappaphycus alvarezii exposed to single or combined high light, chilling and chemical stresses

Under biotic/abiotic stresses, the red alga Kappaphycus alvarezii reportedly releases massive amounts of H 2O 2 into the surrounding seawater. As an essential redox signal, the role of chloroplast-originated H 2O 2 in the orchestration of overall antioxidant responses in algal species has thus been questioned. This work purported to study the kinetic decay profiles of the redox-sensitive plastoquinone pool correlated to H 2O 2 release in seawater, parameters of oxidative lesions and antioxidant enzyme activities in the red alga Kappaphycus alvarezii under the single or combined effects of high light, low temperature, and sub-lethal doses of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), which are inhibitors of the thylakoid electron transport system. Within 24 h, high light and chilling stresses distinctly affected the availability of the PQ pool for photosynthesis, following Gaussian and exponential kinetic profiles, respectively, whereas combined stimuli were mostly reflected in exponential decays. No significant correlation was found in a comparison of the PQ pool levels after 24 h with either catalase (CAT) or ascorbate peroxidase (APX) activities, although the H 2O 2 concentration in seawater ( R = 0.673), total superoxide dismutase activity ( R = 0.689), and particularly indexes of protein ( R = 0.869) and lipid oxidation ( R = 0.864), were moderately correlated. These data suggest that the release of H 2O 2 from plastids into seawater possibly impaired efficient and immediate responses of pivotal H 2O 2-scavenging activities of CAT and APX in the red alga K. alvarezii, culminating in short-term exacerbated levels of protein and lipid oxidation. These facts provided a molecular basis for the recognized limited resistance of the red alga K. alvarezii under unfavorable conditions, especially under chilling stress.

Synthesis and Inhibitory Effect on Photosynthetic Electron Transport of 1,3,5-Triazinylcarboxylic Acid Derivatives

This study relates to the modification of 2-benzylamino-4-methyl-6-trifluoromethyl-1,3,5-triazine. New 1,3,5-triazine compounds with an electron-withdrawing carboxyl group, e.g. ester group, substituted for the trifluoromethyl group, were synthesized and assayed for activity to inhibit photosynthetic electron transport (PET) in thylakoids of spinach as well as both atrazine-resistant and wild-type Chenopodium album. Among the compounds with an alkylamino group, 2-ethoxycarbonyl-4-isopropylamino-6-methyl-1,3,5-triazine was the most potent PET-inhibitor, exhibiting a pI50 of 6.11. The inhibitory activity was generally more potent with 2-alkoxycarbonyl-4-(branched alkyl)amino-6-methyl-1,3,5-triazines than amino-analogues with straight chain alkyl groups or unsaturated alkyl groups. © Presticide Science Society of Japan

Estimating photosynthetic electron transport via chlorophyll fluorometry without Photosystem II light saturation

Estimates of thylakoid electron transport rates (J(e)) from chlorophyll fluorometry are often used in combination with leaf gas exchange measurements to provide detailed information about photosynthetic activity of leaves in situ. Estimating J(e) requires accurate determination of the quantum efficiency of Photosystem II (Phi(P)), which in turn requires momentary light saturation of the Photosystem II light harvesting complex to induce the maximum fluorescence signal (F(M)'). In practice, full saturation is often difficult to achieve, especially when incident photosynthetic photon flux density (Q) is high and energy is effectively dissipated by non-photochemical quenching. In the present work, a method for estimating the true F(M)' under high Q was developed, using multiple light pulses of varying intensity (Q'). The form of the expected relationship between the apparent F(M)' and Q' was derived from theoretical considerations. This allowed the true F(M)' at infinite Q' to be estimated from linear regression. Using a commercially available leaf gas exchange/ chlorophyll fluorescence measurement system, J(e) was compared to gross photosynthetic CO(2) assimilation (A(G)) under conditions where the relationship between J(e) and A(G) was expected to be linear. Both in C(4) leaves (Zea mays) in ambient air and also in C(3) leaves (Gossypium hirsutum) under non-photorespiratory conditions the apparent ratio between J(e) and A(G) declined at high Q when Phi(P) was calculated from F(M)' measured simply using the highest available saturating pulse intensity. When F(M)' was determined using the multiple pulse / linear regression technique, the expected relationship between J(e) and A(G) at high Q was restored, indicating that the Phi(P) estimate was improved. This method of determining F(M)' should prove useful for verifying when saturating pulse intensities are sufficient, and for accurately determining Phi(P) when they are not.

Reduction of the thylakoid electron transport chain by stromal reductants--evidence for activation of cyclic electron transport upon dark adaptation or under drought.

The reduction of P700(+), the primary electron donor of photosystem I (PSI), following a saturating flash of white light in the presence of the photosystem II (PSII) inhibitor 3-(3.4-dichlorophenyl)-1,1-dimethylurea (DCMU), was examined in barley plants exposed to a variety of conditions. The decay kinetic fitted to a double exponential decay curve, implying the presence of two distinct pools of PSI. A fast component, with a rate constant for decay of around 0.03-0.04 ms(-1) was observed to be sensitive to the duration of illumination. This rate constant was slower than, but comparable to, that observed in non-inhibited samples (i.e. where linear flow was active). It was substantially faster than values typically reported for experiments where PSII activity is inhibited. The magnitude of this component rose in leaves that were dark-adapted or exposed to drought. This component was assigned to PSI centres involved in cyclic electron transport. The remaining slowly decaying P700(+) population (rate constant of around 0.001-0.002 ms(-1)) was assigned to centres normally involved in linear electron transport (but inhibited here because of the presence of DCMU), or inactivated centres involved in the cyclic pathway. Processes that might regulate the relative flux through cyclic electron transport are discussed.

The Changes in Some Photosynthetic Characteristics of Transgenic Tobacco Plants, Resistant to Bacteria Pseudomonas Syringae pv. Tabaci

ABSTRACTThe changes in the photosynthetic reactions of tobacco plants, infected by Pseudomonas syringae pv. tabaci were investigated. The protection against pathogen were studied in sensitive to wildfire disease tobacco plants—Nevrokop 1146 (N1146) and pigment deficient “aurea-like mutant”, A1146. The responses were compared with a transgenic resistant line of Nevrokop 1146 - (T1146) developed by Batchvarova et al, (1998) and carrying the ttr (tabtoxin resistance) gene. The changes in thylakoid electron transport, especially on PS2, were evaluated by induction curves of variable and the delayed fluorescence. Correlation with some changes in enzymatic processes related to fast and slow protection against pathogen was also investigated. More expressed effects of infection on photosynthesis in N1146 and A1146 than in T1146 were probably due to the direct tabtoxin action on both chlorophyll a and b. The hypersensitive reaction was expressed during the initial stages of the bacterial infection as evaluated by enhanced peroxidase activity. The increase in reduced sugars and α—amylase activity was related to development of later stages of protection against pathogen. The investigations showed that the changes in the photosynthetic apparatus could not only be correlated to the tabtoxin action and thence limited photorespiration. The resistant T1146 did not develop typical disease symptoms because of the limited tabtoxin action but pathogen induced changes in photosynthesis were obtained. The differences in the effects in N1146 and A1146 were probably due to their possible different chloroplast structure.

Thiol regulation of the thylakoid electron transport chain--a missing link in the regulation of photosynthesis?

Avoidance of over-reduction of the chloroplast ferredoxin pool is of paramount importance for plants in avoiding oxidative stress. The redox state of this pool can be controlled through regulation of the thylakoid electron transport chain. A model is presented for regulation of this chain via a thiol reduction mechanism, possibly involving a thioredoxin. It is shown in isolated thylakoids that electron transport is inhibited by the thiol reducing agent dithiothreitol. The kinetics of this reduction are rapid and readily reversible. The midpoint redox potential is -365 mV at pH 7.7, with a pH dependency of about -90 mV/pH. At physiological pH values, this places the potential of the species titrated between that of ferredoxin and NADPH and thus in the right potential range to be regulating the redox poise of the ferredoxin pool. This is also close to the potential of NADPH-malate dehydrogenase, an enzyme known to be regulated by thioredoxin. Regulation of electron transport by thioredoxin provides a mechanistic link between the regulation of photosynthesis and gene expression by sugars and the redox regulation of gene expression mediated through the plastoquinone pool.

ABA-induced Increase in Thermostability of Photosynthetic Electron Transport of Thylakoids in Leaves of Cucumber (Cucumis sativus L.).

The effect of ABA on thermostability of the photosynthetic apparatus in leaves of cucumber (Cucumis sativus L.) was investigated. Cucumber plants grown in soil at 25°C (day and night) were administered foliar sprays of 1 mM ABA when the third leaves were fully expanded. Compared to untreated control leaves, ABA-treated leaves had faster rates of photosynthetic O2 evolution and a higher photosystem (PS) II chlorophyll fluorescence yield following exposure to 45°C in the dark for 10 min. These effects of ABA were attenuated over time after ABA treatment. Heating the isolated thylakoids from treated and control leaves at 40 °C in the dark for 5 min revealed that heat-induced loss of PS II- and PS I-mediated electron transport rates was alleviated by the ABA treatment. The PS II reaction center was not heat-damaged, regardless of the source of thylakoids. On the other hand, a reduction in the loss of proteins and manganese during heat treatment occurred in thylakoids from ABA-treated leaves compared to those from control leaves. This reduction indicates that ABA increased the thermostability of the O2 evolving complex. Direct treatment of isolated thylakoids with ABA was ineffective in preventing the heat inactivation of the photosystems. Thus, it seems that high foliar levels of ABA induce a mechanism associated with thermostability of the O2 evolving complex. However, these effects of ABA on isolated thylakoids were smaller than were those on intact leaves, indicating that ABA can also induce thermostability in other components of the photosynthetic apparatus besides the O2 evolving complex.

Enhanced susceptibility of photosynthesis to low-temperature photoinhibition due to interruption of chill-induced increase of S-adenosylmethionine decarboxylase activity in leaves of spinach (Spinacia oleracea L.).

The possible involvement of polyamines in the chilling tolerance of spinach (Spinacia oleracea L.) was investigated focusing on photosynthesis. During chilling at 8/5C (day/night) for 6 d, S-adenosylmethionine decarboxylase (SAMDC) activity increased significantly in leaves in parallel with the increase in putrescine and spermidine (Spd) content in leaves and chloroplasts. Treatment of leaves with methylglyoxal-bis(guanylhydrazone) (MGBG), an SAMDC inhibitor, resulted in the deterioration of plant growth and photosynthesis under chilling conditions, which was reversed by the concomitant treatment with Spd through the roots. Plants treated with MGBG showed lower photochemical efficiency of PSII than either the control or plants treated with MGBG plus Spd during chilling and even after transfer to warm conditions, suggesting an increase of photoinhibition due to low Spd in chloroplasts. Indeed, MGBG-treated plants had much lower activities of thylakoid electron transport and enzymes in carbon metabolism as well as higher degrees of lipid peroxidation of thylakoid membranes compared to the control. These results indicate that the enhanced activity of SAMDC with a consequential rise of Spd in chloroplasts is crucial for the cold acclimation of the photosynthetic apparatus in spinach leaves.

Chromatic variation of the abundance of PSII complexes observed with the red alga Prophyridium cruentum.

Chromatic regulation of photosystem stoichiometry in cyanophytes, green algae and probably vascular plants is achieved by regulation of the abundance of PSI in response to thylakoid electron transport state at least under our experimental conditions [cf. Fujita (1997) Photosyn. Res. 53: 83]. However, variation of not only PSI but also PSII, in reverse of each other, is characteristic of the stoichiometry regulation in red algae and some of marine cyanophytes. Our previous study with the red alga Porphyridium cruentum has revealed that PSII is inactivated by 50% upon a light shift from the light absorbed by Chl a, PSI light, to that mainly absorbed by phycobilisomes (PBS), PSII light [Fujita (1999) Plant Cell Physiol. 40: 924]. To evaluate the contribution of the photoinactivation to the chromatic variation of PSII, variation of the abundance of PSI, PSII and PBS, together with the fluorescence parameter and the activity of PSII, was followed after a light shift from PSI light to PSII light. Upon a light shift to PSII light, PSII, determined as Cyt b(559) per PBS, decreased rapidly, following the photoinactivation, down to the level a half of that before the light shift, and remained constant. Since the increase in PBS was not significant during this period, a rapid decrease of PSII/PBS led us to tentatively conclude that the degradation of PSII is a main cause for variation of the abundance of PSII. Photoinactivation of PSII, and also decrease in Cyt b(559), was accelerated, but only slightly, by the addition of chloramphenicol (CAP) at a moderate concentration while CAP at the same concentration significantly suppressed the increment of PSI determined as P700. A selective effect of CAP supports the above conclusion.

Avoiding alternative electron sinks - regulating the thylakoid electron transport chain to avoid oxidative stress.

Avoiding alternative electron sinks - regulating the thylakoid electron transport chain to avoid oxidative stress.

Effect of lichen metabolites on thylakoid electron transport and photophosphorylation in isolated spinach chloroplasts.

Investigation of the lichen Parmotrema tinctorum led to the isolation of several known compounds. Among them, lecanorin (1), methyl-beta-orcinol carboxylate, methyl orsellinate, orcinol, and methyl haematommate (3) caused significant inhibition of the radicle growth and germination of seedlings of Amaranthus hypochondriacus and Echinochloa crusgalli. In addition, lecanorin (1) and gyrophoric acid (2) significantly inhibited the light-dependent synthesis of ATP and uncoupled electron transfer on the reducing side of photosystem II in freshly lysed, illuminated spinach chloroplasts. The targets of 1 and 2 were located at the water-splitting enzyme level and in one of the redox enzymes in the range of electron transport from P(680) to Q(A), respectively.

Using chlorophyll fluorometry to compare photosynthetic performance of commercial maize ( Zea mays L.) hybrids in the field

The main objective of the present work was to determine if chlorophyll fluorescence techniques could be used under field conditions to detect differences in leaf photosynthetic rates among commercial maize hybrids known to differ in agronomic performance. Chlorophyll fluorometry was used to estimate thylakoid electron transport rates ( J e) in leaves of three commercial maize hybrids on 13 days between 48 days after sowing and first frost. The maximum quantum efficiency of thylakoid electron transport at low PPFD ( φ), and the thylakoid electron transport rate when PPFD absorbed by leaves was equal to 1200 μmol m −2 s −1 ( J e1200) were estimated using a curve-fitting approach. Canopy absorptance of incident PPFD ( α c) and leaf absorptance of incident PPFD ( α l) were measured at regular intervals. Significant fluctuations in both φ and J e1200 were observed over the season, and these appeared to be associated with different leaf temperatures on different measuring days. Measured values of J e1200 declined in a linear fashion over the season, but the decline was more rapid in an older hybrid than in two more modern hybrids. All three hybrids could be differentiated from one another on the basis of J e1200 measurements in the last part of the growing season. The two newer hybrids did not differ in their mean crop growth rates during this part of the season, however, despite the observed difference in J e1200. Regression analyses revealed a strong relationship between mean crop growth rates and α c J e1200, supporting the idea that chlorophyll fluorescence can be used to assess photosynthetic performance of maize under field conditions.

TAKs, Thylakoid Membrane Protein Kinases Associated with Energy Transduction

The phosphorylation of proteins within the eukaryotic photosynthetic membrane is thought to regulate a number of photosynthetic processes in land plants and algae. Both light quality and intensity influence protein kinase activity via the levels of reductants produced by the thylakoid electron transport chain. We have isolated a family of proteins called TAKs, Arabidopsis thylakoid membrane threonine kinases that phosphorylate the light harvesting complex proteins. TAK activity is enhanced by reductant and is associated with the photosynthetic reaction center II and the cytochrome b6f complex. TAKs are encoded by a gene family that has striking similarity to transforming growth factor beta receptors of metazoans. Thus thylakoid protein phosphorylation may be regulated by a cascade of reductant-controlled membrane-bound protein kinases.

Proton to electron stoichiometry in electron transport of spinach thylakoids

According to the concept of the Q-cycle, the H +/e − ratio of the electron transport chain of thylakoids can be raised from 2 to 3 by means of the rereduction of plastoquinone across the cytochrome b 6 f complex. In order to investigate the H +/e − ratio we compared stationary rates of electron transport and proton translocation in spinach thylakoids both in the presence of the artificial electron acceptor ferricyanide and in the presence of the natural acceptor system ferredoxin+NADP. The results may be summarised as follows: (1) a variability of the H +/e − ratio occurs with either acceptor. H +/e − ratios of 3 (or even higher in the case of the natural acceptor system, see below) are decreased towards 2 if strong light intensity and low membrane permeability are employed. Mechanistically this could be explained by proton channels connecting the plastoquinol binding site alternatively to the lumenal or stromal side of the cytochrome b 6 f complex, giving rise to a proton slip reaction at high transmembrane ΔpH. In this slip reaction protons are deposited on the stromal instead of the lumenal side. In addition to the pH effect there seems to be a contribution of the redox state of the plastoquinone pool to the control of proton translocation; switching over to stromal proton deposition is favoured when the reduced state of plastoquinone becomes dominant. (2) In the presence of NADP a competition of both NADP and oxygen for the electrons supplied by photosystem I takes place, inducing a general increase of the H +/e − ratios above the values obtained with ferricyanide. The implications with respect to the adjustment of a proper ATP/NADPH ratio for CO 2 reduction are discussed.

Personal recollections of 40 years in photosynthesis research

The major theme of my work in photosynthesis has been electron transport in green plant thylakoids. In particular, we investigated the properties and the role of the NADP-reducing flavoprotein and its possible function in cyclic electron transport, the regulation by protons of electron transport, and the redox system of ascorbate and monodehydroascorbate (the ascorbate free radical). The function of this system in providing ATP in the stoichiometric amount needed for carbon assimilation, and the regulation of the alternative transfer of electrons to NADP and to the ascorbate free radical were among the achievements of my collaborators and myself. Specifically, the early conviction that cyclic phosphorylation was essential part in photosynthesis was shattered as far as higher plants are concerned, and replaced by a modified Mehler reaction providing additional ATP to run the Calvin cycle. The situation seems to be different in unicellular green algae, where quantitatively much l arger changes of the relative size of Photosystem (PS) I and PS II antennae during the so-called ‘state’ transitions have been reported, and these seem to be associated with a high activity of cyclic electron transport in state 2. Beyond the science, the friendly interactions with so many persons around the world sharing my interest in photosynthesis and in other aspects of human life have been most rewarding.