Low temperature delays development of photosystem II activity in winter rye leaves

The ability of leaves to acclimate photosynthetically to low temperature was examined during leaf development in winter rye plants ( Secale cereale L. cv. Puma) grown at 20°C or at 6°C. All leaves grown at 6°C exhibit increased chlorophyll (Chl) levels per leaf area, higher rates of uncoupled, light-saturated photosystem I (PSI) electron transport, and slower increases in photosystem II (PSII) electron transport capacity, when compared with 20°C leaves. The stoiehiometry of PSI and PSII was estimated for each leaf age class by quantifying Chl in elcctrophorctic separations of Chl-protein complexes. The ratio of PSII/PSI electron transport in 20°C leaves is highly correlated with the ratio of core Chl a -proteins associated with PSII (CPa) to those associated with PSI (CP1). In contrast, PSII/PSI electron transport in 6°C leaves is not as well correlated with CPa/CP1 and is related, in part, to the amount and organization of light-harvesting Chl a/b -proteins associated with PSII. CPa/CP1 increases slowly in 6°C leaves, although the ratio of CPa/CP1 in mature 20°C and 6°C leaves is not different. The results suggest that increased PSI activity at low temperature is not related to an increase in the relative proportion of PSI and may reflect, instead, a regulatory change. Photosynthetic acclimation to low environmental temperature involves increased PSI activity in mature leaves shifted to 6°C. In leaves grown entirely at 6°C, however, acclimation includes both increased PSI activity and modifications in the rate of accumlation of PSII and in the organization of LHCII.     

Dissection of respiratory and cyclic electron transport in Synechocystis sp. PCC 6803

Cyclic electron transport (CET) is an attractive hypothesis for regulating photosynthetic electron transport and producing the additional ATP in oxygenic phototrophs. The concept of CET has been established in the last decades, and it is proposed to function in the progenitor of oxygenic photosynthesis, cyanobacteria. The in vivo activity of CET is frequently evaluated either from the redox state of the reaction center chlorophyll in photosystem (PS) I, P700, in the absence of PSII activity or by comparing PSI and PSII activities through the P700 redox state and chlorophyll fluorescence, respectively. The evaluation of CET activity, however, is complicated especially in cyanobacteria, where CET shares the intersystem chain, including plastoquinone, cytochrome b₆/f complex, plastocyanin, and cytochrome c₆, with photosynthetic linear electron transport (LET) and respiratory electron transport (RET). Here we sought to distinguish the in vivo electron transport rates in RET and CET in the cyanobacterium Synechocystis sp. PCC 6803. The reduction rate of oxidized P700 (P700⁺) decreased to less than 10% when PSII was inhibited, indicating that PSII is the dominant electron source to PSI but P700⁺ is also reduced by electrons derived from other sources. The oxidative pentose phosphate (OPP) pathway functions as the dominant electron source for RET, which was found to be inhibited by glycolaldehyde (GA). In the condition where the OPP pathway and respiratory terminal oxidases were inhibited by GA and KCN, the P700⁺ reduction rate was less than 1% of that without any inhibitors. This study indicate that the electron transport to PSI when PSII is inhibited is dominantly derived from the OPP pathway in Synechocystis sp. PCC 6803.

     

Regulation of NAD(P)H dehydrogenase-dependent cyclic electron transport around PSI by NaHSO₃ at low concentrations in tobacco chloroplasts

Although bisulfite at low concentrations (L-NaHSO?) has been found to increase the cyclic electron transport around PSI (CET), its regulative mechanism remains unknown. In this work, the role of L-NaHSO? (0.1-500 μM) in NAD(P)H dehydrogenase-dependent CET (the NDH pathway) was investigated. After treatment of tobacco leaves with L-NaHSO?, the NDH pathway, as reflected by a transient post-illumination increase in Chl fluorescence, the dark reduction of P700+ after far-red light and the amount of NDH, was increased after the light-dark-light transition, but was slightly lowered under continuous light. Meanwhile, the linear electron transport (LET) was accelerated by L-NaHSO? under both the light regimes. Experiments in thylakoids further demonstrated that both LET, monitored by light-dependent oxygen uptake, and CET, as determined from the NADPH-dependent oxygen uptake and dark reduction of P700+, were enhanced by L-NaHSO? and the enhancements were abolished by superoxide dismutase. Furthermore, L-NaHSO?-induced CET was partially impaired in thylakoids of the ΔndhCKJ mutant, while L-NaHSO?-induced LET was not affected. Based on these results, we propose that the photooxidation of L-NaHSO? initiated by superoxide anions in PSI regulates NDH pathway to maintain efficient photosynthesis.     

Mitochondrial alternative oxidase pathway protects plants against photoinhibition by alleviating inhibition of the repair of photodamaged PSII through preventing formation of reactive oxygen species in Rumex K-1 leaves

The purpose of this study was to explore how the mitochondrial AOX (alternative oxidase) pathway alleviates photoinhibition in Rumex K-1 leaves. Inhibition of the AOX pathway decreased the initial activity of NADP-malate dehydrogenase (EC 1.1.1.82, NADP-MDH) and the pool size of photosynthetic end electron acceptors, resulting in an over-reduction of the photosystem I (PSI) acceptor side. The over-reduction of the PSI acceptor side further inhibited electron transport from the photosystem II (PSII) reaction centers to the PSII acceptor side as indicated by an increase in V(J) (the relative variable fluorescence at J-step), causing an imbalance between photosynthetic light absorption and energy utilization per active reaction center (RC) under high light, which led to the over-excitation of the PSII reaction centers. The over-reduction of the PSI acceptor side and the over-excitation of the PSII reaction centers enhanced the accumulation of reactive oxygen species (ROS), which inhibited the repair of the photodamaged PSII. However, the inhibition of the AOX pathway did not change the level of photoinhibition under high light in the presence of the chloroplast D1 protein synthesis inhibitor chloramphenicol, indicating that the inhibition of the AOX pathway did not accelerate the photodamage to PSII directly. All these results suggest that the AOX pathway plays an important role in the protection of plants against photoinhibition by minimizing the inhibition of the repair of the photodamaged PSII through preventing the over-production of ROS.     

Competition between linear and cyclic electron flow in plants deficient in Photosystem I

Photosynthetic electron transport can involve either a linear flow from water to NADP, via Photosystems (PS) II and I or a cyclic flow just involving PSI. Little is known about factors regulating the relative flow through each of these pathways. We have examined photosynthetic electron transport through each system in plants of Arabidopsis thaliana in which either the PSI-D1 or PSI-E1 subunits of PSI have been knocked out. In both cases, this results in an imbalance in the turnover of PSI and PSII, such that PSII electron transport is limited by PSI turnover. Phosphorylation of light-harvesting complex II (LHCII) and its migration to PSI is enhanced but only partially reversible and not sufficient to balance photosystem turnover. In spite of this, cyclic electron flow is able to compete efficiently with PSI across a range of conditions. In dark-adapted leaves, the efficiency of cyclic relative to linear flow induced by far-red light is increased, implying that the limiting step of cyclic flow lies in the re-injection of electrons into the electron transport chain. Illumination of leaves with white light resulted in transient induction of a significant non-photochemical quenching in knockout plants which is probably high energy state quenching induced by cyclic electron flow. At high light and at low CO(2), non-photochemical quenching was greater in the knockout plants than in the wildtype. Comparison of PSI and PSII turnover under such conditions suggested that this is generated by cyclic electron flow around PSI. We conclude that, when the concentration of PSI is limiting, cyclic electron flow is still able to compete effectively with linear flow to maintain a high DeltapH to regulate photosynthesis.     

Alternative pathways of photosystem I-dependent electron transport in two genetically different potato cultivars in vitro

Alternative pathways of electron transport involving photosystem I (PSI) only were studied in leaves of potato plants (Solanum tuberosum L., cv. Desiree), modified by yeast invertase gene, controlled by tuber-specific class I patatin B33 promoter with proteinase II signal peptide for apoplastic localization of the enzyme. Nontransformed (wild-type) potato cultivar Desiree was used as a source of control plants. Phototrophic cultures grown in vitro on the sucrose-free Murashige and Skoog medium, as well as plants grown on the medium with 4% sucrose were examined. Various PSI-dependent alternative pathways of electron transport were discriminated by quantitative analysis of kinetic curves of dark reduction of P700⁺, the primary electron donor of PSI, oxidized by far-red light known to excite selectively PSI. In potato plants with two different genotypes, four exponentially decaying kinetic components were found, which suggests the existence of multiple alternative routes for electron input to PSI. Inhibitor analysis (with diuron and antimycin A) allowed identification of each route. A minor ultra-fast component originated from weak residual excitation of PSII by far-red light and represented electron flow from PSII to PSI. Ferredoxin-dependent cyclic electron flow around PSI accounted for the middle component, and two slower components were assigned to donation of electrons to PSI from reductants localized in the chloroplast stroma. The rates of all components were somewhat higher in leaves of the transformed plants than in the wild-type plants. However, relative contributions of separate components to the kinetics of dark P700⁺ reduction in leaves of both potato genotypes were similar. Growing plants on the medium with sucrose dramatically increased the amplitude of absorbance change at 830 nm in the transformed (but not in wild type) plants, which indicated a drastic increase in P700 concentration in their leaves.     

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

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

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 14 days followed by 3 days 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.     

Competition between linear and cyclic electron flow in plants deficient in Photosystem I

Photosynthetic electron transport can involve either a linear flow from water to NADP, via Photosystems (PS) II and I or a cyclic flow just involving PSI. Little is known about factors regulating the relative flow through each of these pathways. We have examined photosynthetic electron transport through each system in plants of Arabidopsis thaliana in which either the PSI-D1 or PSI-E1 subunits of PSI have been knocked out. In both cases, this results in an imbalance in the turnover of PSI and PSII, such that PSII electron transport is limited by PSI turnover. Phosphorylation of light-harvesting complex II (LHCII) and its migration to PSI is enhanced but only partially reversible and not sufficient to balance photosystem turnover. In spite of this, cyclic electron flow is able to compete efficiently with PSI across a range of conditions. In dark-adapted leaves, the efficiency of cyclic relative to linear flow induced by far-red light is increased, implying that the limiting step of cyclic flow lies in the re-injection of electrons into the electron transport chain. Illumination of leaves with white light resulted in transient induction of a significant non-photochemical quenching in knockout plants which is probably high energy state quenching induced by cyclic electron flow. At high light and at low CO₂, non-photochemical quenching was greater in the knockout plants than in the wildtype. Comparison of PSI and PSII turnover under such conditions suggested that this is generated by cyclic electron flow around PSI. We conclude that, when the concentration of PSI is limiting, cyclic electron flow is still able to compete effectively with linear flow to maintain a high ΔpH to regulate photosynthesis.     

Heterogeneity of photosystem II reaction centers as influenced by heat treatment of barley leaves

Three functionally distinct populations of PSII reaction centers differing in the ability to keep the primary acceptors in a reduced state and to transfer electrons to PSI were estimated using chlorophyll fluorescence measurements in primary barley leaves exposed to elevated temperatures in the range of 37–51°C. The capacity of the PSII reaction centers to perform at least one light-induced charge separation was not affected by a 5-min heat treatment at temperatures up to 51°C. The first population containing Q_B-non-reducing centers corresponded to 15–20% of the total PSII activity up to 45°C. In a second population, PSII reaction centers maintained Q_A reduction under light in the presence of oxygen, but not in the presence of a strong artificial PSI electron acceptor, methyl viologen. In a third population that gradually increases from zero at 37°C to about 60% at 45°C, the PSII centers were not able to keep Q_A in the reduced state even in the presence of oxygen as the sole electron acceptor. Three electron transport pathways, the pseudocyclic one involving both PSII and PSI, the NAD(P)H-dependent pathway mediated by PSI alone after the loss of activity in some PSII centers, and the PSI-driven ferredoxin-dependent route enhanced by weakly efficient PSII centers that are able to provide only catalytic amounts of electrons, are suggested to create a proton gradient in chloroplasts of heat-stressed leaves thus protecting PSII reaction centers from photodamage.     

Mechanisms and functions of photosystem I-related alternative electron transport pathways in chloroplasts

This review analyzes various alternative pathways of chloroplast electron transport mediated by photoreactions of photosystem I (PSI) and unrelated to activity of photosystem II (PSII). The mechanisms and functional significance of the alternative pathways are considered. These pathways are complexly organized and comprise ferredoxin-dependent electron recycling around PSI, as well as electron donation to noncyclic chain in the region between PSII and PSI from reduced substances localized in the chloroplast stroma. For each of the alternative pathways, the origin of corresponding enzymes and their compartmentalization in the complex membrane system of the chloroplast are discussed. It is shown that operation of alternative electron transport pathways contributes to energy transduction and cell defense function, facilitates the absorption of inorganic carbon, and is significant for chloroplast respiration. Multiple mechanisms for regulation of alternative pathways have been revealed. It is concluded that PSI-related alternative electron transport pathways constitute an integral part of entire system of photosynthetic electron transport, this system being principally responsible for energy supply of phototrophic cells and whole plants.     

The role of electron transport in determining the temperature dependence of the photosynthetic rate in spinach leaves grown at contrasting temperatures

The temperature response of the uncoupled whole-chain electron transport rate (ETR) in thylakoid membranes differs depending on the growth temperature. However, the steps that limit whole-chain ETR are still unclear and the question of whether the temperature dependence of whole-chain ETR reflects that of the photosynthetic rate remains unresolved. Here, we determined the whole-chain, PSI and PSII ETR in thylakoid membranes isolated from spinach leaves grown at 30 degrees C [high temperature (HT)] and 15 degrees C [low temperature (LT)]. We measured temperature dependencies of the light-saturated photosynthetic rate at 360 microl l(-1) CO2 (A360) in HT and LT leaves. Both of the temperature dependences of whole-chain ETR and of A360 were different depending on the growth temperature. Whole-chain ETR was less than the rates of PSI ETR and PSII ETR in the broad temperature range, indicating that the process was limited by diffusion processes between the PSI and PSII. However, at high temperatures, whole-chain ETR appeared to be limited by not only the diffusion processes but also PSII ETR. The C3 photosynthesis model was used to evaluate the limitations of A360 by whole-chain ETR (Pr) and ribulose bisphosphate carboxylation (Pc). In HT leaves, A360 was co-limited by Pc and Pr at low temperatures, whereas at high temperatures, A360 was limited by Pc. On the other hand, in LT leaves, A360 was solely limited by Pc over the entire temperature range. The optimum temperature for A360 was determined by Pc in both HT and LT leaves. Thus, this study showed that, at low temperatures, the limiting step of A360 was different depending on the growth temperature, but was limited by Pc at high temperatures regardless of the growth temperatures.     

Net light-induced oxygen evolution in photosystem I deletion mutants of the cyanobacterium Synechocystis sp. PCC 6803

Oxygenic photosynthesis in cyanobacteria, algae, and plants requires photosystem II (PSII) to extract electrons from H(2)O and depends on photosystem I (PSI) to reduce NADP(+). Here we demonstrate that mixotrophically-grown mutants of the cyanobacterium Synechocystis sp. PCC 6803 that lack PSI (ΔPSI) are capable of net light-induced O(2) evolution in vivo. The net light-induced O(2) evolution requires glucose and can be sustained for more than 30min. Utilizing electron transport inhibitors and chlorophyll a fluorescence measurements, we show that in these mutants PSII is the source of the light-induced O(2) evolution, and that the plastoquinone pool is reduced by PSII and subsequently oxidized by an unidentified electron acceptor that does not involve the plastoquinol oxidase site of the cytochrome b(6)f complex. Moreover, both O(2) evolution and chlorophyll a fluorescence kinetics of the ΔPSI mutants are highly sensitive to KCN, indicating the involvement of a KCN-sensitive enzyme(s). Experiments using (14)C-labeled bicarbonate show that the ΔPSI mutants assimilate more CO(2) in the light compared to the dark. However, the rate of the light-minus-dark CO(2) assimilation accounts for just over half of the net light-induced O(2) evolution rate, indicating the involvement of unidentified terminal electron acceptors. Based on these results we suggest that O(2) evolution in ΔPSI cells can be sustained by an alternative electron transport pathway that results in CO(2) assimilation and that includes PSII, the platoquinone pool, and a KCN-sensitive enzyme.     

Photosynthetic acclimation of Tradescantia albiflora to growth irradiance: Lack of adjustment of light-harvesting components and its consequences

The photosynthetic acclimation of Tradescantia albiflora (Kunth), a trailing ground species naturally occurring in the deep shade of rainforests, was studied in relation to growth irradiance (glasshouse; direct light and 1 to 4 layers of shade cloth, giving 100 to 1.4% relative growth irradiance). Contrary to other irradiance studies of higher plants grown in natural habitats or controlled light environments, the chlorophyll a/b ratios of Tradescantia leaves were low (∼2.2) and constant. Acclimation to growth irradiance caused no changes in the relative amounts of specific Chl-proteins or the numbers of photosystem I (PSI) and PSII reaction centres on a chlorophyll basis, indicating that the light-harvesting antenna sizes of PSII and PSI, as well as the photosystem stoichiometry, were independent of growth irradiance. However, the amount of cytochrome f and ATP synthase on a chlorophyll basis increased with increasing the relative growth irradiance from 1.4 to 35%, showing acclimation of electron transport and photophosphorylation capacity. The photosynthetic capacity and ribulose 1, 5-bisphosphate carboxylase (EC 4.1.1.39) activity also increased with increase of the growth irradiance to 35%. Beyond that, the inflexible PSII/PSI stoichiometry and shade-type photosystem II/light-harvesting units in Tradescaniia are a disadvantage for long-term exposure to high irradiance since the leaves are more prone to photoinhibition.     

Cold stress effects on PSI photochemistry in Zea mays: differential increase of FQR-dependent cyclic electron flow and functional implications

Cold-induced inhibition of CO(2) assimilation in maize (Zea mays L.) is associated with a persistent depression of the photochemical efficiency of PSII. However, very limited information is available on PSI photochemistry and PSI-dependent electron flow in cold-stressed maize. The extent of the absorbance change (ΔA(820)) used for in vivo quantitative estimation of photooxidizable P700(+) indicated a 32% lower steady-state oxidation level of the PSI reaction center P700 (P700(+)) in cold-stressed compared with control maize leaves. This was accompanied by a 2-fold faster re-reduction rate of P700(+) in the dark, indicating a higher capacity for cyclic electron flow (CEF) around PSI in cold-stressed maize leaves. Furthermore, the increased PSI-dependent CEF(s) was associated with a much higher stromal electron pool size and 56% lower capacity for state transitions compared with control plants. To examine NADP(H) dehydrogenase (NDH)- and ferredoxin:plastoquinone oxidoreductase (FQR)-dependent CEF in vivo, the post-illumination transient increase of F(o)' was measured in the presence of electron transport inhibitors. The results indicate that under optimal growth conditions the relatively low CEF in the maize mesophyll cells is mostly due to the NDH-dependent pathway. However, the increased CEF in cold-stressed plants appears to originate from the up-regulated FQR pathway. The physiological role of PSI down-regulation, the increased capacity for CEF and the shift of preferred CEF mode in modulating the photosynthetic electron fluxes and distribution of excitation light energy in maize plants under cold stress conditions are discussed.     

Maximum and effective PSII yields in the cortex of the main stem of young <Emphasis Type="Italic">Prunus cerasus</Emphasis> 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.     

Concerning a dual function of coupled cyclic electron transport in leaves

Coupled cyclic electron transport is assigned a role in the protection of leaves against photoinhibition in addition to its role in ATP synthesis. In leaves, as in reconstituted thylakoid systems, cyclic electron transport requires “poising,” i.e. availability of electrons at the reducing side of photosystem I (PSI) and the presence of some oxidized plastoquinone between photosystem II (PSII) and PSI. Under self-regulatory poising conditions that are established when carbon dioxide limits photosynthesis at high light intensities, and particularly when stomata are partially or fully closed as a result of water stress, coupled cyclic electron transport controls linear electron transport by helping to establish a proton gradient large enough to decrease PSII activity and electron flow to PSI. This brings electron donation by PSII, and electron consumption by available electron acceptors, into a balance in which PSI becomes more oxidized than it is during fast carbon assimilation. Avoidance of overreduction of the electron transport chain is a prerequisite for the efficient protection of the photosynthetic apparatus against photoinactivation.     

CO2 response of cyclic electron flow around PSI (CEF-PSI) in tobacco leaves--relative electron fluxes through PSI and PSII determine the magnitude of non-photochemical quenching (NPQ) of Chl fluorescence

We hypothesized that cyclic electron flow around photosystem I (CEF-PSI) participates in the induction of non-photochemical quenching (NPQ) of chlorophyll (Chl) fluorescence when the rate of photosynthetic linear electron flow (LEF) is electron-acceptor limited. To test this hypothesis, the relationships among photosynthesis rate, electron fluxes through both PSI and PSII [Je(PSI) and Je(PSII)] and Chl fluorescence parameters were analyzed simultaneously in intact leaves of tobacco plants at several light intensities and partial pressures of ambient CO2 (Ca). At low light intensities, decreasing Ca lowered the photosynthesis rate, but Je(PSI) and Je(PSII) remained constant. Je(PSI) was larger than Je(PSII), indicating the existence of CEF-PSI. Increasing the light intensity enhanced photosynthesis and both Je(PSI) and Je (PSII). Je(PSI)/Je(PSII) also increased at high light and at high light and low Ca combined, showing a strong, positive relationship with NPQ of Chl fluorescence. These results indicated that CEF-PSI contributed to the dissipation of photon energy in excess of that consumed by photosynthesis by driving NPQ of Chl fluorescence. The main physiological function of CEF-PSI in photosynthesis of higher plants is discussed.     

Chlorophyll a fluorescence of typical desert plant Alhagi sparsifolia Shap. at two light levels

Alhagi sparsifolia Shap. is exposed to a high-irradiance environment as the main vegetation found in the forelands of the Taklamakan Desert. We investigated chlorophyll a fluorescence emission of A. sparsifolia seedlings grown under ambient (HL) and shade (LL) conditions. Our results indicated that the fluorescence intensity in the leaves was significantly higher for LL-grown plants than that under HL. High values of the maximum quantum yield of PSII for primary photochemistry (φP_o) and the quantum yield that an electron moves further than Q_A - (φE_o) in the plants under LL conditions suggested that the electron flow from Q_A - (primary quinone electron acceptors of PSII) to Q_B (secondary quinone acceptor of PSII) or Q_B - was enhanced at LL compared to natural HL conditions. The efficiency/probability with which an electron from the intersystem electron carriers was transferred to reduce end electron acceptors at the PSI acceptor side and the quantum yield for the reduction of end electron acceptors at the PSI acceptor side were opposite to φP_o, and φE_o. Thus, we concluded that the electron transport on the donor side of PSII was blocked under LL conditions, while acceptor side was inhibited at the HL conditions. The PSII activity of electron transport in the plants grown in shade was enhanced, while the energy transport from PSII to PSI was blocked compared to the plants grown at HL conditions. Furthermore, PSII activity under HL was seriously affected in midday, while the plants grown in shade enhanced their energy transport.     

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.