Linear Electron Transport Rate Research Articles

Maximum and effective PSII yields in the cortex of the main stem of young Prunus cerasus trees: effects of seasons and exposure

Seasonal changes in chlorophyll fluorescence parameters of corticular chlorenchyma in the main trunk of Prunus cerasus were followed in the field under ambient temperature and light conditions during bright days. Concomitantly, measurements of periderm light transmittance also allowed the calculation of linear electron transport rates along PSII. Pre-dawn PSII photochemical efficiency was high during late spring, summer and early autumn, but low during winter in the North-facing, permanently shaded, side and extremely low in the South-facing, exposed side. Corresponding mid-day PSII effective yield and linear electron transport rates peaked at late spring and early summer with the exposed side always displaying lower values for effective yield, but higher values for electron transport rate. However, corticular electron transport rates were more than sixfold lower compared to leaves. Non-photochemical quenching was higher in the exposed side throughout the year while peak values appeared at early autumn. Although a photoinhibitory damage during winter can be claimed, we may note that Mediterranean winter temperatures are mild, while the light reaching the trunk photosynthetic tissues is very low (maximum at 30 and 280 μmol m−2 s−1 in the shaded and the exposed side, respectively) to be considered as photoinhibitory. Based on recent findings for the retention of PSI activity and a concomitant inhibition of PSII under low temperatures in leaves, together with an adequate cyclic electron flow found in bark chlorenchyma, we suggest a temperature-dependent adaptive adjustment in the relative rates of PSI over PSII activity, possibly linked to seasonally changing needs for metabolic energy supply.

Are saturating pulses indeed saturating? Evidence for considerable PSII yield underestimation in leaves adapted to high levels of natural light

The "saturating pulse" method of in vivo Chl fluorescence measurement has been widely used by physiologists and especially ecophysiologists, as it allows a simple, rapid and non-invasive assessment of PSII function and the allocation of absorbed energy into photochemical and non-photochemical processes. It is based on the accurate determination of the so-called Fm('), i.e. the fluorescence signal emitted when a "saturating" light pulse closes all PSII centers. In this methodological investigation, we examined whether the saturating pulse intensities required to obtain maximal fluorescence yields differ between leaves of various species receiving varying actinic light irradiances. It was shown that, in leaves adapted to comparatively high (yet realistic) levels of natural irradiances, the saturating pulses usually applied are not able to close all PSII reaction centers. As a result, there is a high risk of considerable Fm(') underestimation. Accordingly, the derived values of effective PSII yields and linear electron transport rates (ETR) are also underestimated, even at the highest saturation pulse levels afforded by commercial instruments. Since the extent of underestimation increases with actinic irradiance, the ETR versus light curves are considerably distorted. The possible reasons for the apparent inability of "saturating" pulses to close all PSII centers at high actinic light and the practical implications, especially in field work, are discussed.

Evidence for active cyclic electron flow in twig chlorenchyma in the presence of an extremely deficient linear electron transport activity

Fast and slow chlorophyll fluorescence induction curves at high and low actinic visible light, post-illumination changes in fluorescence yield and reflectance changes at 820 nm induced by far-red light were used to characterize the state of PSII and PSI and their electron transport capabilities in chlorophyllous twig cortices of Eleagnus angustifolius L., while corresponding leaves served as controls. Twigs displayed low dark-adapted PSII photochemical efficiencies and particularly low linear electron transport rates when illuminated. In addition, their PSII population was characterized by a high proportion of inactive, non-Q(B)-reducing centers and an incomplete quenching of fluorescence during the slow induction phase. It is suggested that PSII in twigs is an inefficient electron donor to PSI and/or the reductive pentose phosphate cycle. Yet, in spite of this apparent PSII deficiency, pools of intermediate electron carriers and potential PSI activity were more than sufficient to support the observed linear electron transport rates. Moreover, the rate of PSI reduction upon far-red/dark transitions and the magnitude of fluorescence yield increase upon white light/dark transitions were compatible with an efficient electron flow to PSI from stromal donors in the absence of PSII activity. We conclude that corticular chlorenchyma may be actively engaged in cyclic at the expense of a linear electron flow and discuss the possible physiological significance of this finding in conjunction with the particular microenvironmental conditions encountered within twigs.

Changes in the photosynthetic apparatus during fusarium wilt of tomato

Development of fusarium wilt was studied in 4-to 6-month-old tomato plants (Lycopersicon esculentum L., cv. Kunera). It was shown that the development of this disease could follow two patterns. When the wilt developed slowly (type I disease), the mycelium of Fusarium oxysporum fungus partly blocked the xylem and grew extensively within parenchyma. When the wilt developed fast (type II syndrome), the occlusion of both xylem and phloem was observed; the xylem sap circulation was suppressed and, consequently, tomato plant tissues were dehydrated. The development of type I and type II diseases led to suppression of photosynthetic activity in plants. In the case of slow wilt (type I), both light and dark stages of photosynthesis were damaged. This was evident from the decrease in the effectiveness of light harvesting and charge separations in the reaction centers of photosystem II (PSII), suppression of electron transport at the acceptor side of PSII, and the decrease in activity of Rubisco. In the case of fast wilt (type II), the Rubisco activity did not change, and photochemical activity of chloroplasts was suppressed to a smaller degree than during type I fusarium wilt. The decrease in the rate of linear electron transport in tomato leaves was mostly due to inhibition of electron flow at the acceptor side of PSII. The data obtained suggest that photosynthetic activity in tomato plants is suppressed by different mechanisms depending on the developmental pattern of fusarium wilt.

PsbS genotype in relation to coordinated function of PS II and PS I in Arabidopsis leaves

Application of multiple probes to systems that carry specific mutations provides a powerful means for studying how known regulators of light utilization interact in vivo. Two lines of Arabidopsis thaliana were studied, each carrying a unique lesion in the nuclear psbS gene encoding a 22-kDa pigment-binding protein (PS II-S) essential for full expression of photoprotective, rapid-phase, nonphotochemical quenching of chlorophyll fluorescence (NPQ). The PS II-S protein is absent in line npq4-1 due to deletion of psbS. Line npq4-9 expresses normal levels of PS II-S but carries a single amino acid substitution that lowers NPQ capacity by about 50%. A prior report [Peterson RB and Havir EA (2001) Planta 214: 142-152] described an altered pattern of redox states of the acceptor side of Photosystem II (PS II) and donor side of Photosystem I (PS I) for npq4-9 suggesting that interphotosystem electron transport may be restricted by a higher transthylakoid DeltapH in this line. In vivo steady state fluorescence and absorbance measurements (820 nm) confirmed these earlier observations for line npq4-9 but not for npq4-1. Thus, the prior results cannot be correlated simply to a loss of NPQ capacity. Likewise, the kinetics of the 820-nm absorbance change did not indicate a substantial effect of psbS genotype on electron flow from plastoquinol to PS I. A simple model is proposed to relate linear electron transport rate (measured gasometrically) to a parameter (based on fluorescence) that provides a relative measure of the density of excitation available for photochemistry in PS II. Surprisingly, analyses using this model suggested that the in vivo midpoint potential of the primary quinone acceptor in PS II (Q(A)) is lowered in both psbS mutant lines. This heretofore-unsuspected role for PS II-S is discussed with regard to: (1) numerous prior reports indicating plasticity of the redox potential of Q(A) and (2) the basis for the contrasting regulation of quantum yields of PS I and II in npq4-1 and npq4-9.

Effects of soil temperature and elevated atmospheric CO2 concentration on gas exchange, in vivo carboxylation and chlorophyll fluorescence in jack pine and white birch seedlings

One-year-old jack pine (Pinus banksiana Lamb.) and current-year white birch (Betula papyrifera Marsh.) seedlings were grown in ambient (360 ppm) or twice ambient (720 ppm) atmospheric CO2 concentration ([CO2]) and at three soil temperatures (Tsoil = 7, 17 and 27 degrees C initially, increased to 10, 20 and 30 degrees C two months later, respectively) in a greenhouse for 4 months. In situ foliar gas exchange, in vivo carboxylation characteristics and chlorophyll fluorescence were measured after 2.5 and 4 months of treatment. Low Tsoil suppressed net photosynthetic rate (Pn), stomatal conductance (g(s)) and transpiration rate (E) in jack pine in both CO2 treatments and g(s) and E in white birch in ambient [CO2], but enhanced instantaneous water-use efficiency (IWUE) in both species after 2.5 months of treatment. Treatment effects on g(s) and E remained significant throughout the 4-month study. Low Tsoil reduced maximal carboxylation rate (Vcmax) and PAR-saturated electron transport rate (Jmax) in jack pine in elevated [CO2] after 2.5 months of treatment, but not after 4 months of treatment. Low Tsoil increased actual photochemical efficiency of photosystem II (PSII) in the light (DeltaF/Fm') in jack pine, but decreased DeltaF/Fm' in white birch after 4 months of treatment. In response to low Tsoil, photosynthetic linear electron transport to carboxylation (Jc) decreased in jack pine after 2.5 months and in white birch after 4 months of treatment. Low Tsoil increased the ratio of the photosynthetic linear electron transport to oxygenation (Jo) to the total photosynthetic linear electron transport rate through PSII (Jo/J(T)) in both species after 2.5 months of treatment, but the effects became statistically insignificant in white birch after 4 months of treatment. High Tsoil decreased foliar N concentration in white birch. Elevated [CO2] increased Pn, IWUE and Jc but decreased Jo/J(T) in both species at both measurement times except Jc in white birch after 2.5 months of treatment. Elevated [CO2] also decreased g(s) and E in white birch at high Tsoil, Vcmax in both species and triose phosphate utilization in white birch at low Tsoil after 4 months of treatment, and DeltaF/Fm' in white birch after 2.5 months of treatment. Elevated [CO2] also increased foliar N concentration in both species. Low Tsoil caused no permanent damage to PSII in either species, but jack pine responded and acclimated to low Tsoil more quickly than white birch. Photosynthetic down-regulation and a decrease in photosynthetic electron transport to photorespiration occurred in both species in response to elevated [CO2].

Field-grown cotton plants with elevated activity of chloroplastic glutathione reductase exhibit no significant alteration of diurnal or seasonal patterns of excitation energy partitioning and CO 2 fixation

Transgenic cotton plants with elevated activity of chloroplast-targeted glutathione reductase (GR+) were grown in field plots over two seasons in order to compare their photosynthetic performance with that of wildtype plants. We hypothesised that transgenic plants would show enhanced protection of the photosynthetic apparatus against photoinhibition, primarily through an increase in electron transport associated with the role of the chloroplastic antioxidant system as an alternative electron sink. Diurnal measurements of chlorophyll a fluorescence from cotyledons, stem leaves, and leaves subtending developing fruits (bolls) were used to estimate the rate of linear electron transport ( J e) and the rates of reversible, regulated (NPD REG) and photoinhibitory (NPD PI) non-photochemical energy dissipation (NPD) at several times (June, July, September, October) during each growing season. GR+ cotyledons exhibited greater J e than wildtype cotyledons during the middle of a day in early June, while NPD PI was the same for both genotypes. Throughout most days on which measurements were conducted, no genotypic differences in J e and NPD were observed for stem leaves. Only during the late morning of one day in early October did leaves subtending bolls of GR+ plants exhibit greater J e compared to that for wildtype plants. As leaves subtending bolls of both genotypes aged, J e and CO 2 assimilation declined, while NPD increased. Maximum NPD PI and minimum F v/ F m remained essentially the same for all measurement days. However, the maximum NPD REG, the greatest contributor to NPD, increased with leaf age. We conclude that the rapid rise in leaf temperature during most mornings created conditions in which elevated GR activity conferred no advantage. Also, as light energy absorption became excessive in late morning, cotton leaves exhibited a strong capacity for regulated, non-photochemical energy dissipation that may have served as the major photoprotective mechanism.

Changes in stem lignins (monomer composition and crosslinking) and peroxidase are related with the maintenance of leaf photosynthetic integrity during Verticillium wilt in Capsicum annuum.

• Verticillium dahliae is a soilborne pathogen that causes vascular wilt in pepper (Capsicum annuum var. annuum). Here we study to what extent changes in the lignification response of peppers condition tolerance of wilt. • For this, the quantum yield (ΦPSII ), the linear electron transport rate (ETR), and the lignification response (monomer composition and crosslinking) were studied in three C. annuum cultivars differing in degree of tolerance. • The results showed that in tolerant cultivars (Padrón and Yolo Wonder), both ΦPSII and ETR showed significantly higher levels at saturating photosynthetically active radiation values. This was not, however, the case for cv. Luesia, which showed a significant decrease in ΦPSII , ETR and nonphotochemical quenching values, suggesting that photochemical processes are strongly damaged in this cultivar as a consequence of the disease. The analysis of stem lignins in tolerant cultivars revealed that they were mainly composed of p-hydroxyphenyl units, while levels of β-O-4-linked coniferyl and sinapyl alcohols were significantly lower. • It is concluded that through the observed changes in stem lignins (monomer composition and crosslinking) peppers retard, since they maintain leaf photosynthetic integrity, but do not stop (since wilt symptoms are not avoided) V. dahliae fungal hyphae penetration.

Probing corticular photosynthesis through in vivo chlorophyll fluorescence measurements: evidence that high internal CO2 levels suppress electron flow and increase the risk of photoinhibition

Twigs of many woody plants possess chlorenchyma under a well-developed periderm which lacks stomata and impedes both gas diffusion and light penetration. The so-called corticular photosynthesis, occurring in the shade and under extremely high CO(2) concentrations, was probed in this study through in vivo chlorophyll fluorescence measurements. Field comparisons between twigs and corresponding leaves in five species indicated that both the dark- and light-adapted PSII photochemical efficiencies are considerably lower in twigs at all incident photon fluence rates, in spite of the significant attenuation of solar radiation by the periderm. Light saturation curves for linear electron transport rates (corrected according to the actual light intensities reaching twig chlorenchyma) were compatible with a shade-acclimated photosynthetic machinery, showing very low maximum electron transport rates (at approximately 5% of the corresponding leaf values) and threshold irradiances for light saturation. However, removing periderms from twig segments (i.e. relieving the twig interior form the high CO(2) partial pressures) considerably improved the light-adapted (but not the dark-adapted) PSII photochemical efficiency, allowing the assumption that the high internal CO(2) levels may interfere with the smooth functioning of photosynthesis. Indeed, laboratory experiments with twig segments equilibrated under various CO(2) levels (0.036-20%), resulted in a progressive decrease of light-adapted PSII photochemical yield, with the values obtained at 20% CO(2) being similar to those obtained with intact twigs in the field. Further experiments indicated that high CO(2) combined with high light suppressed the development of a photoprotective non-photochemical quenching through a reduction of its fast relaxing component, accompanied by a higher risk of photoinhibition. It is suggested that high internal CO(2) concentrations in twigs impede photosynthesis possibly through acidification of protoplasm and impairment of the pH-dependent high energy state quenching followed by reduction in the efficiency of heat dissipation.

Photosynthesizing in the rain: beneficial effects of twig wetting on corticular photosynthesis through changes in the periderm optical properties

Summary Field observations with young twigs of woody species during rain indicated that their normally grayish color turns darker or even greenish upon wetting. The change persists as long as rain is falling and recovery to the normal grayish tint upon re-drying occurs within minutes. These naked eye observations were confirmed through spectral reflectance measurements on twig surfaces of seven species, invariably showing considerable reduction in reflectance, particularly at the spectral regions of maximum absorbance by photosynthetic pigments, when wet. Since isolated periderms were not permeable to water and wetting of isolated chlorenchymatous strips did not appreciably affect their reflectance, we argued that wetting affects the optical properties of periderms per se , possibly leading to corresponding changes in corticular photosynthesis. Indeed, light transmittance of isolated periderms from Quercus coccifera was increased by ca. 45% due to a corresponding decrease in reflectance. As a result, wetting of intact twigs resulted in higher photosynthetic linear electron transport rates at low but lower at high incident photon fluence rates, the later indicating arrival at potentially photoinhibitory light levels. Since light levels are low during rain, a beneficial effect on corticular photosynthesis is inferred. The above changes were fully reversible during repeated brief wet/dry cycles. Yet, when wetting was artificially prolonged at high light, chronic photoinhibition was evident. It is suggested that the rapid and reversible changes in periderm optical properties upon wetting and drying are of dual adaptive significance allowing more corticular photosynthesis during rain and minimizing the risk of photoinhibition after sudden sunlight.

Blue- and green-light signals for gamete release in the brown alga, Silvetia compressa.

The intertidal brown alga Silvetia compressa releases gametes from receptacles (the reproductive tissue) rapidly upon a dark transfer (following a photosynthesis-dependent period in the light, termed potentiation). In this study, the wavelength-dependence of this process was investigated. During the potentiation period in white light (WL), gametes are not released. However, gametes were released during potentiation in blue light (BL), or in low red light/blue light (RL/BL) ratios, but not in RL alone, high RL/BL ratios, or in broadband blue-green light (B-GL) (presence of BL, but absence of RL). RL was as effective as WL for potentiation, i.e., both lead to gamete release following transfer to darkness. Rates of linear photosynthetic electron transport were similar in RL and BL. Gamete release in BL was inhibited by equal amounts of additional narrow-waveband light between the green and red regions of the spectrum, with light-induced gamete release restricted between <491 nm and 509 nm. Very little light-induced gamete release occurred between 530 nm and 650 nm. It is proposed that a BL-responsive photoreceptor is responsible for light-induced gamete release. Transfer of WL-potentiated receptacles to GL near 530 nm resulted in significant de-potentiation and reduced gamete release during a subsequent dark transfer. This effect was not seen at 509 nm or 560 nm and revealed the presence of a second photoreceptor system repressing or counteracting potentiation in the light. We propose that the restriction of gamete release to periods when irradiance is blue-shifted may constitute a depth-sensing mechanism for this intertidal alga, allowing controlled release of gametes at high tide and/or less turbid periods, thus minimizing gamete dilution, and promoting fertilization success.

Induction of CO2-gas exchange and electron transport: comparison of dynamic and steady-state responses in Fagus sylvatica leaves

Photosynthetic induction was followed in a juvenile understorey beech (Fagus sylvatica L.) in a shaded habitat which was temporarily exposed to direct sunlight passing through a gap in the canopy. Simultaneous in situ measurements of leaf gas exchange and chlorophyll fluorescence were carried out and a steady-state light response curve of photosynthesis was recorded. The measured dynamic carbon gain was compared to the predicted carbon gain, calculated from the steady-state light response curve and the ambient photon flux density (PPFD) incident during the sunfleck event. Integration over the first 20 min, during which induction took place, resulted in a deviation of 27% if any induction effects are disregarded. The carbon gain was overestimated by 11% when the carbon gain was integrated over the whole measuring period of about 1 h including a 40-min period of full induction. In situ gas exchange measurements under constant saturating light, following dark adaptation, revealed an induction error of 21%. About 20 min was required to reach the final steady-state level of photosynthesis. The data show that the prediction of the carbon gain by steady-state models leads to a distinct overestimation of the CO2 uptake, irrespective of whether the induction state rises concurrently with the incident radiation or saturating light induces photosynthesis. From chlorophyll fluorescence and absorptance data, rates of linear electron transport (ETR) were calculated. Uninduced leaves exposed to saturating light of constant PPFD show an initial fast down-regulation of ETR, due to excessive light, and a subsequent increase in electron transport attributable to the increasing energy demand during induction of carbon fixation. No difference in the ETR to PPFD relationship between data sets sampled during induction and in the fully induced state was found when both the incident light and the induction state of carbon fixation increased concomitantly. The proportion of electrons fed into alternative pathways besides carbon fixation was higher during induction as compared to the fully induced state. Thus, electrons were used for carbon fixation with a higher efficiency when full induction was reached.

Chemical probes for water-oxidation: synthetic manganese complexes in photoactivation of water splitting complex and as exogenous electron donors to photosystem II.

Photoactivation of the water splitting enzyme was performed with 13 different synthetic manganese complexes and characterized by oxygen evolution yield, thermoluminescence and chlorophyll fluorescence induction kinetics. The efficiency of different compounds in photoactivation correlated with the rate of linear electron transport in the presence of these compounds. The organic ligands, associated with the manganese ions, do not prevent the photoactivation of the water splitting complex (WOC). Photoactivation with different manganese complexes depended on the number of the Mn-ions in the complex, their valence state and the nature of their donor atoms. The most efficient restorations were achieved by using tetrameric complexes having a dimer+dimer structure, complexes containing Mn(II) ions, and having 4-6 oxygen and 0-2 nitrogen atoms as donor atoms. Further, the effectiveness of photoactivation depended largely on the structure of the complexes. Our data support the notion that WOC in intact thylakoids requires the cooperation and well determined arrangement of all four manganese ions, and argue against the hypothesis that two manganese ions are sufficient for water splitting. Photoactivation by some complexes led to anomalous flash-oxygen patterns, which are explained by a modified/perturbed water splitting complex.

Sink-source transition in tobacco leaves visualized using chlorophyll fluorescence imaging.

• The sink-source transition of developing Nicotiana tabacum (tobacco) leaves was studied here using chlorophyll fluorescence imaging. • In accordance with leaf development, the quantum efficiency of PSII, showed a steep gradient across the leaf with increasing values towards the tip. • The linear electron transport rate (ETR) saturated at higher CO2 concentrations in the younger, than in the mature, part of the leaf, probably due to a lower Rubisco activity or a higher CO2 diffusion resistance. • The induction of ETR at CO2 concentrations near the compensation point after long-term dark adaptation of the young leaf, showed distinct responses; ETR rose rapidly in the basal but more slowly in the apical regions. There was a correlation between fast induction and carbohydrate import, as measured by 14 C-translocation. In the basal regions, larger pools of metabolic intermediates are expected due to imported carbohydrates. These might be used in the Calvin cycle directly after dark-light transition providing the electron acceptors for the faster induction of ETR. Additionally, a higher mitochondrial respiration can provide CO2 for the Calvin cycle in these regions.

Analysis of PSI and PSII driven electron transportduring steady-state photosynthesis in the cyanobacteriumSynechococcus PCC7942

Photosynthetic electron transport is required for the transport of CO2 and HCO3- in Synechococcus PCC 7942. This transport supports the CO2 concentrating mechanism in cyanobacteria. The roles that linear and cyclic electron flow play in the energization of these processes is unclear. This study describes a system for measuring the quantum yields of photochemistry in both photosystems and, thus, the extent of electron flow, during steady-state cyanobacterial photosynthesis. These measurements were made in a custom-built cuvette coupled by a membrane inlet to a mass spectrometer enabling the concomitant measurement of photosynthetic gas exchange. In wild-type cells, the relationship between f PSI and f PSII was found to be linear between the CO2 compensation point and to approximately ¼ Vmax. The ratio of f PSI/f PSII was always greater than 1 and increased as net photosynthesis approached zero. This was suggestive of significant cyclic electron transport around PSI which became more pronounced as the rate of Ci supply decreased. Whether CO2 or HCO3- was used to drive photosynthesis the relationship between f PSI and f PSII was the same indicative of similar rates of linear and cyclic electron transport. Since we found no greater engagement of f PSI in the presence of enhanced CO2 supply we could not provide evidence for a model of CO2 transport coupled exclusively to cyclic electron transport. A double mutant (ndhD3-/ndhD4-) of Synechococcus PCC7942 in which CO2 transport is absent showed an alteration in electron transport properties relative to wild-type cells.

159 The Relationship between Actual Photosystem II Efficiency and Quantum Yield for CO2 Assimilation is Not Affected by Nitrogen Content in Apple Leaves

Bench-grafted Fuji/M26 apple (Malus domestica Borkh) trees were fertigated with different concentrations of nitrogen by using a modified Hoagland's solution for 45 days. CO2 assimilation and actual photosystem II (PSII) efficiency in response to incident photon flux density (PFD) were measured simultaneously in recent fully expanded leaves under low O2 (2%) and saturated CO2 (1300 ppm) conditions. A single curvilinear relationship was found between true quantum yield for CO2 assimilation and actual PSII 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 CO2/mol quanta, 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 linear electron transport associated with rubisco and partitioning of electron flow between CO2 assimilation and photorespiration in different N leaves in response to intercellular CO2 concentration (Ci) under normal O2 conditions. Both the rate of linear electron flow, and the rate to CO2 or O2 increased with increasing leaf N at any given Ci, but the percentage of linear electron flow to CO2 assimilation remained the same regardless of leaf N content. As Ci increased, the percentage of linear electron flow to CO2 assimilation increased. In conclusion, the relationship between actual PSII efficiency and quantum yield for CO2 assimilation and the partitioning of electron flow between CO2 assimilation and photorespiration are not affected by N content in apple leaves.

Kinetic evidence for the PsaE-dependent transient ternary complex photosystem I/Ferredoxin/Ferredoxin:NADP(+) reductase in a cyanobacterium.

A mutant of Synechocystis PCC 6803, deficient in psaE, assembles photosystem I reaction centers without the PsaE subunit. Under conditions of acceptor-side rate-limited photoreduction assays in vitro (with 15 microM plastocyanin included), using 100 nM ferredoxin:NADP(+) reductase (FNR) and either Synechocystis flavodoxin or spinach ferredoxin, lower rates of NADP(+) photoreduction were measured when PsaE-deficient membranes were used, as compared to the wild type. This effect of the psaE mutation proved to be due to a decrease of the apparent affinity of the photoreduction assay system for the reductase. In the psaE mutant, the relative petH (encoding FNR) expression level was found to be significantly increased, providing a possible explanation for the lack of a phenotype (i.e., a decrease in growth rate) that was expected from the lower rate of linear electron transport in the mutant. A kinetic model was constructed in order to simulate the electron transfer from reduced plastocyanin to NADP(+), and test for possible causes for the observed change in affinity for FNR. The numerical simulations predict that the altered reduction kinetics of ferredoxin, determined for the psaE mutant [Barth, P., et al., (1998) Biochemistry 37, 16233-16241], do not significantly influence the rate of linear electron transport to NADP(+). Rather, a change in the dissociation constant of ferredoxin for FNR does affect the saturation profile for FNR. We therefore propose that the PsaE-dependent transient ternary complex PSI/ferredoxin/FNR is formed during linear electron transport. Using the yeast two-hybrid system, however, no direct interaction could be demonstrated in vivo between FNR and PsaE fusion proteins.

Comparative Study of the C3/CAM Intermediate Species Clusia parviflora Saldanha et Engi. and the Obligate CAM Species Clusia hilariana Schlecht. Growing Sympatrically Exposed and Shaded in the Coastal Restinga of Brazil

Abstract: The C3/CAM intermediate species, C/usia parviflora Saldanha et EngI., and the obligate CAM species Clusia hilariana Schlecht., occur sympatrically in the coastal sand dune vegetation of the Restinga of Brazil. Their photosynthetic activity at an exposed and at a shaded site was compared by measuring gas exchange (porometry), chlorophyll a fluorescence parameters, organic acid levels (malic and citric) and carbon isotope ratios. At the shaded site, low photosynthetic photon flux densities (PPFD) strongly restricted photosynthetic activity. However, C parviflora could readily make use of light flecks. At the exposed site, C. parviflora was much less affected by photoinhibition than C. hilariana. The CAM species showed higher apparent rates of linear photosynthetic electron transport (ETR) and higher effective quantum yield of PSII (ΔF/F'm) than did C. parviflora during high insolation in the middle of the day, i.e., the time of Phase Ill of CAM. Nevertheless, it suffered much more severe acute photoinhibition that was not reversible after 20 min of darkening during this time, and even some chronic photoinhibition not reversible overnight. Comparative studies of sympatric physiotypes with different modes of photosynthesis of a given leaf morphotype, as available in the genus Cksia, challenge some CAM dogmas, e.g., CAM may not always be superior at exposed sites and may not always provide better photoprotection at high PPFD. However, the idea that C3/CAM plasticity allows occupation of a wider range of habitats is supported.

588 Photosystem II Efficiency and CO2 Assimilation in Response to Light and CO2 in Leaves of Deciduous Tree Fruit

Photosystem II (PSII) efficiency and CO2 assimilation in response to photon flux density (PFD) and intercellular CO2 concentration (Ci) were monitored simultaneously in leaves of apple, pear, apricot, and cherry with a combined system for measuring chlorophyll fluorescence and gas exchange. When photorespiration was minimized by low O2 (2%) and saturated CO2 (1300 ppm), a linear relationship was found between PSII efficiency and the quantum yield for CO2 assimilation with altering PFD, indicating CO2 assimilation in this case is closely linked to PSII activity. As PFD increased from 80 to 1900 μmol·m–2·s–1 under ambient CO2 (350 ppm) and O2 (21%) conditions, PSII efficiency decreased by increased nonphotochemical quenching and decreased concentration of open PSII reaction centers. The rate of linear electron transport showed a similar response to PFD as CO2 assimilation. As Ci increased from 50 to 1000 ppm under saturating PFD (1000 μmol·m–2·s–1) and ambient O2, PSII efficiency was increased initially by decreased nonphotochemical quenching and increased concentration of open PSII reaction centers and then leveled off with further a rise in Ci. CO2 assimilation reached a plateau at a higher Ci than PSII efficiency because increasing Ci diverted electron flow from O2 reduction to CO2 assimilation by depressing photorespiration. It is concluded that PSII efficiency is regulated by both nonphotochemical quenching and concentration of open PSII reaction centers in response to light and CO2 to meet the requirement for photosynthetic electron transport.

Oxygen and electron flow in C 4 photosynthesis: Mehler reaction, photorespiration and CO 2 concentration in the bundle sheath

The photosynthetic linear electron transport rate in excess of that used for CO2 reduction was evaluated in Sorghum bicolor Moench. [NADP-malic enzyme (ME)-type C4 plant], Amaranthus cruentus L. (NAD-ME-type C4 plant) and Helianthus annuus L. (C3 plant) leaves at different CO2 and O2 concentrations. The electron transport rate (JF) was calculated from fluorescence using the light partitioning factor (relative PSII cross-section) determined under conditions where excess electron transport was assumed to be negligible: low light intensities, 500 μmol CO2 · mol−1 and 2% O2. Under high light intensities there was a large excess of JF/4 at 10–100% O2 in the C3 plant due to photorespiration, but very little in sorghum and somewhat more in amaranth, showing that photorespiration is suppressed, more in the NADP-ME- and less in the NAD-ME-type species. It is concluded that when C4 photosynthesis is limited by supply of atmospheric CO2 to the C4 cycle, the C3 cycle becomes limited by regeneration of ribulose 1,5-bisphosphate (RuBP) which in turn limits RuBP oxygenase activity and photorespiration. The rate of excess electron transport over that consumed for CO2 fixation in C4 plants was very sensitive to the presence of O2 in the gas phase, rapidly increasing between 0.01 and 0.1% O2, and at 2% O2 it was about two-thirds of that at 21% O2. This shows the importance of the Mehler O2 reduction as an electron sink, compared with photorespiration in C4 plants. However, the rate of the Mehler reaction is still too low to fully account for the extra ATP which is needed in C4 photosynthesis.