Is the A-1 Pigment in Photosystem I Part of P700? A (P700+-P700) FTIR Difference Spectroscopy Study of A-1 Mutants.

The involvement of the second pair of chlorophylls, termed A-1A and A-1B, in light-induced electron transfer in photosystem I (PSI) is currently debated. Asparagines at PsaA600 and PsaB582 are involved in coordinating the A-1B and A-1A pigments, respectively. Here we have mutated these asparagine residues to methionine in two single mutants and a double mutant in PSI from Synechocystis sp. PCC 6803, which we term NA600M, NB582M, and NA600M/NB582M mutants. (P700+-P700) FTIR difference spectra (DS) at 293 K were obtained for the wild-type and the three mutant PSI samples. The wild-type and mutant FTIR DS differ considerably. This difference indicates that the observed changes in the (P700+-P700) FTIR DS cannot be due to only the PA and PB pigments of P700. Comparison of the wild-type and mutant FTIR DS allows the assignment of different features to both A-1 pigments in the FTIR DS for wild-type PSI and assesses how these features shift upon cation formation and upon mutation. While the exact role the A-1 pigments play in the species we call P700 is unclear, we demonstrate that the vibrational modes of the A-1A and A-1B pigments are modified upon P700+ formation. Previously, we showed that the A-1 pigments contribute to P700 in green algae. In this manuscript, we demonstrate that this is also the case in cyanobacterial PSI. The nature of the mutation-induced changes in algal and cyanobacterial PSI is similar and can be considered within the same framework, suggesting a universality in the nature of P700 in different photosynthetic organisms.

Functional consequences of modification of the photosystem I/photosystem II ratio in the cyanobacterium Synechocystis sp. PCC 6803.

The stoichiometry of photosystem II (PSII) and photosystem I (PSI) varies between photoautotrophic organisms. The cyanobacterium Synechocystis sp. PCC 6803 maintains two- to fivefold more PSI than PSII reaction center complexes, and we sought to modify this stoichiometry by changing the promoter region of the psaAB operon. We thus generated mutants with varied psaAB expression, ranging from ~3% to almost 200% of the wild-type transcript level, but all showing a reduction in PSI levels, relative to wild type, suggesting a role of the psaAB promoter region in translational regulation. Mutants with 25%-70% of wild-type PSI levels were photoautotrophic, with whole-chain oxygen evolution rates on a per-cell basis comparable to that of wild type. In contrast, mutant strains with <10% of the wild-type level of PSI were obligate photoheterotrophs. Variable fluorescence yields of all mutants were much higher than those of wild type, indicating that the PSI content is localized differently than in wild type, with less transfer of PSII-absorbed energy to PSI. Strains with less PSI saturate at a higher light intensity, enhancing productivity at higher light intensities. This is similar to what is found in mutants with reduced antennae. With 3-(3,4-dichlorophenyl)-1,1-dimethylurea present, P700+ re-reduction kinetics in the mutants were slower than in wild type, consistent with the notion that there is less cyclic electron transport if less PSI is present. Overall, strains with a reduction in PSI content displayed surprisingly vigorous growth and linear electron transport. IMPORTANCE: Consequences of reduction in photosystem I content were investigated in the cyanobacterium Synechocystis sp. PCC 6803 where photosystem I far exceeds the number of photosystem II complexes. Strains with less photosystem I displayed less cyclic electron transport, grew more slowly at lower light intensity and needed more light for saturation but were surprisingly normal in their whole-chain electron transport rates, implying that a significant fraction of photosystem I is dispensable for linear electron transport in cyanobacteria. These strains with reduced photosystem I levels may have biotechnological relevance as they grow well at higher light intensities.

Accounting for photosystem I photoinhibition sheds new light on seasonal acclimation strategies of boreal conifers.

The photosynthetic acclimation of boreal evergreen conifers is controlled by regulatory and photoprotective mechanisms that allow conifers to cope with extreme environmental changes. However, the underlying dynamics of photosystem II (PSII) and photosystem I (PSI) remain unresolved. Here, we investigated the dynamics of PSII and PSI during the spring recovery of photosynthesis in Pinus sylvestris and Picea abies using a combination of chlorophyll a fluorescence, P700 difference absorbance measurements, and quantification of key thylakoid protein abundances. In particular, we derived a new set of PSI quantum yield equations, correcting for the effects of PSI photoinhibition. Using the corrected equations, we found that the seasonal dynamics of PSII and PSI photochemical yields remained largely in balance, despite substantial seasonal changes in the stoichiometry of PSII and PSI core complexes driven by PSI photoinhibition. Similarly, the previously reported seasonal up-regulation of cyclic electron flow was no longer evident, after accounting for PSI photoinhibition. Overall, our results emphasize the importance of considering the dynamics of PSII and PSI to elucidate the seasonal acclimation of photosynthesis in overwintering evergreens. Beyond the scope of conifers, our corrected PSI quantum yields expand the toolkit for future studies aimed at elucidating the dynamic regulation of PSI.

APX2 Is an Ascorbate Peroxidase-Related Protein that Regulates the Levels of Plastocyanin in Chlamydomonas.

The function of ascorbate peroxidase-related (APX-R) proteins, present in all green photosynthetic eukaryotes, remains unclear. This study focuses on APX-R from Chlamydomonas reinhardtii, namely, ascorbate peroxidase 2 (APX2). We showed that apx2 mutants exhibited a faster oxidation of the photosystem I primary electron donor, P700, upon sudden light increase and a slower re-reduction rate compared to the wild type, pointing to a limitation of plastocyanin. Spectroscopic, proteomic and immunoblot analyses confirmed that the phenotype was a result of lower levels of plastocyanin in the apx2 mutants. The redox state of P700 did not differ between wild type and apx2 mutants when the loss of function in plastocyanin was nutritionally complemented by growing apx2 mutants under copper deficiency. In this case, cytochrome c6 functionally replaces plastocyanin, confirming that lower levels of plastocyanin were the primary defect caused by the absence of APX2. Overall, the results presented here shed light on an unexpected regulation of plastocyanin level under copper-replete conditions, induced by APX2 in Chlamydomonas.

Roles of ApcD and orange carotenoid protein in photoinduction of electron transport upon dark-light transition in the Synechocystis PCC 6803 mutant deficient in flavodiiron protein Flv1.

Flavodiiron proteins Flv1/Flv3 accept electrons from photosystem (PS) I. In this work we investigated light adaptation mechanisms of Flv1-deficient mutant of Synechocystis PCC 6803, incapable to form the Flv1/Flv3 heterodimer. First seconds of dark-light transition were studied by parallel measurements of light-induced changes in chlorophyll fluorescence, P700 redox transformations, fluorescence emission at 77 K, and OCP-dependent fluorescence quenching. During the period of Calvin cycle activation upon dark-light transition, the linear electron transport (LET) in wild type is supported by the Flv1/Flv3 heterodimer, whereas in Δflv1 mutant activation of LET upon illumination is preceded by cyclic electron flow that maintains State 2. The State 2-State 1 transition and Orange Carotenoid Protein (OCP)-dependent non-photochemical quenching occur independently of each other, begin in about 10 s after the illumination of the cells and are accompanied by a short-term re-reduction of the PSI reaction center (P700+). ApcD is important for the State 2-State 1 transition in the Δflv1 mutant, but S-M rise in chlorophyll fluorescence was not completely inhibited in Δflv1/ΔapcD mutant. LET in Δflv1 mutant starts earlier than the S-M rise in chlorophyll fluorescence, and the oxidation of plastoquinol (PQH2) pool promotes the activation of PSII, transient re-reduction of P700+ and transition to State 1. An attempt to induce state transition in the wild type under high intensity light using methyl viologen, highly oxidizing P700 and PQH2, was unsuccessful, showing that oxidation of intersystem electron-transport carriers might be insufficient for the induction of State 2-State 1 transition in wild type of Synechocystis under high light.

Nanosecond time-resolved infrared spectroscopy for the study of electron transfer in photosystem I.

Microsecond time-resolved step-scan FTIR difference spectroscopy was used to study photosystem I (PSI) from Thermosynechococcus vestitus BP-1 (T. vestitus, formerly known as T. elongatus) at 77 K. In addition, photoaccumulated (P700+-P700) FTIR difference spectra were obtained at both 77 and 293 K. The FTIR difference spectra are presented here for the first time. To extend upon these FTIR studies nanosecond time-resolved infrared difference spectroscopy was also used to study PSI from T. vestitus at 296 K. Nanosecond infrared spectroscopy has never been used to study PSI samples at physiological temperatures, and here it is shown that such an approach has great value as it allows a direct probe of electron transfer down both branches in PSI. In PSI at 296 K, the infrared flash-induced absorption changes indicate electron transfer down the B- and A-branches is characterized by time constants of 33 and 364 ns, respectively, in good agreement with visible spectroscopy studies. These time constants are associated with forward electron transfer from A1- to FX on the B- and A-branches, respectively. At several infrared wavelengths flash-induced absorption changes at 296 K recover in tens to hundreds of milliseconds. The dominant decay phase is characterized by a lifetime of 128 ms. These millisecond changes are assigned to radical pair recombination reactions, with the changes being associated primarily with P700+ rereduction. This conclusion follows from the observation that the millisecond infrared spectrum is very similar to the photoaccumulated (P700+-P700) FTIR difference spectrum.

The Key Role of Cyclic Electron Flow in the Recovery of Photosynthesis in the Photobiont during Rehydration of the Lichen Cladonia stellaris.

Lichens are poikilohydric organisms and an important part of the ecosystem. They show high desiccation tolerance, but the mechanism of dehydration resistance still needs to be studied. The photosynthesis recovery of the photobiont in rehydrated lichen Cladonia stellaris after 11-year desiccation was investigated by simultaneously monitoring both photosystem I and II (PSI and PSII) activities. The responses of the photochemical efficiency and relative electron transport rate (rETR) of PSI and PSII, and the quantum yield of the cyclic electron flow (CEF) were measured using a Dual-PAM-100 system. PSI recovered rapidly, but PSII hardly recovered in C. stellaris during rehydration. The maximal photochemical efficiency of PSII (Fv/Fm) was generally very low and reached about just 0.4 during the rehydration. These results indicated that PSII had restored little and was largely inactivated during rehydration. The quantum yield of PSI recovered quickly to almost 0.9 within 4 h and remained constant at nearly 1 thereafter. The results showed that the activation of the CEF in the early stages of rehydration helped the rapid recovery of PSI. The quantum yield of the CEF made up a considerable fraction of the quantum yield of PSI during rehydration. A regulated excess energy dissipation mechanism and non-photochemical quenching (NPQ) also recovered. However, the small extent of the recovery of the NPQ was not enough to dissipate the excess energy during rehydration, which may be responsible for the weak activity of PSII during rehydration. The results indicated that both CEF and NPQ were essential during the rehydration of the photobiont in C. stellaris. The methods used in the measurements of chlorophyll a fluorescence and P700+ absorbance changes in this study provided a speedy and simple way to detect the physiological characteristics of the photobionts of lichen during rehydration. This work improves our understanding of the mechanism behind lichen's desiccation tolerance.

Exposed anthocyanic leaves of Prunus cerasifera are special shade leaves with high resistance to blue light but low resistance to red light against photoinhibition of photosynthesis.

BACKGROUND AND AIMS: The photoprotective role of foliar anthocyanins has long been ambiguous: exacerbating, being indifferent to or ameliorating the photoinhibition of photosynthesis. The photoinhibitory light spectrum and failure to separate photo-resistance from repair, as well as the different methods used to quantify the photo-susceptibility of the photosystems, could lead to such a discrepancy. METHODS: We selected two congeneric deciduous shrubs, Prunus cerasifera with anthocyanic leaves and Prunus triloba with green leaves, grown under identical growth conditions in an open field. The photo-susceptibilities of photosystem II (PSII) and photosystem I (PSI) to red light and blue light, in the presence of lincomycin (to block the repair), of exposed leaves were quantified by a non-intrusive P700+ signal from PSI. Leaf absorption, pigments, gas exchange and Chl a fluorescence were also measured. KEY RESULTS: The content of anthocyanins in red leaves (P. cerasifera) was >13 times greater than that in green leaves (P. triloba). With no difference in maximum quantum efficiency of PSII photochemistry (Fv/Fm) and apparent CO2 quantum yield (AQY) in red light, anthocyanic leaves (P. cerasifera) showed some shade-acclimated suites, including lower Chl a/b ratio, lower photosynthesis rate, lower stomatal conductance and lower PSII/PSI ratio (on an arbitrary scale), compared with green leaves (P. triloba). In the absence of repair of PSII, anthocyanic leaves (P. cerasifera) showed a rate coefficient of PSII photoinactivation (ki) that was 1.8 times higher than that of green leaves (P. triloba) under red light, but significantly lower (-18 %) under blue light. PSI of both types of leaves was not photoinactivated under blue or red light. CONCLUSIONS: In the absence of repair, anthocyanic leaves exhibited an exacerbation of PSII photoinactivation under red light and a mitigation under blue light, which can partially reconcile the existing controversy in terms of the photoprotection by anthocyanins. Overall, the results demonstrate that appropriate methodology applied to test the photoprotection hypothesis of anthocyanins is critical.

Sensitivity of the Photosynthetic Apparatus in Maize and Sorghum under Different Drought Levels.

Drought is one of the main environmental stress factors affecting plant growth and yield. The impact of different PEG concentrations on the photosynthetic performance of maize (Zea mays L. Mayflower) and sorghum (Sorghum bicolor L. Foehn) was investigated. The activity of the photosynthetic apparatus was assessed using chlorophyll fluorescence (PAM and JIP test) and photooxidation of P700. The data revealed that water deficiency decreased the photochemical quenching (qP), the ratio of photochemical to nonphotochemical processes (Fv/Fo), the effective quantum yield of the photochemical energy conversion in PSII (ΦPSII), the rate of the electron transport (ETR), and the performance indexes PItotal and PIABS, as the impact was stronger in sorghum than in maize and depended on drought level. The PSI photochemistry (P700 photooxidation) in sorghum was inhibited after the application of all studied drought levels, while in maize, it was registered only after treatment with higher PEG concentrations (30% and 40%). Enhanced regulated energy losses (ΦNPQ) and activation of the state transition under drought were also observed in maize, while in sorghum, an increase mainly in nonregulated energy losses (ΦNO). A decrease in pigment content and relative water content and an increase in membrane damage were also registered after PEG treatment. The experimental results showed better drought tolerance of maize than sorghum. This study provides new information about the role of regulated energy losses and state transition for the protection of the photosynthetic apparatus under drought and might be a practical approach to the determination of the drought tolerance of plants.

Protective Effects of Sodium Nitroprusside on Photosynthetic Performance of Sorghum bicolor L. under Salt Stress.

In this study, the impacts of the foliar application of different sodium nitroprusside (SNP, as a donor of nitric oxide) concentrations (0-300 µM) on two sorghum varieties (Sorghum bicolor L. Albanus and Sorghum bicolor L. Shamal) under salt stress (150 mM NaCl) were investigated. The data revealed that salinity leads to an increase in oxidative stress markers and damage of the membrane integrity, accompanied by a decrease in the chlorophyll content, the open photosystem II (PSII) centers, and the performance indexes (PI ABS and PI total), as well as having an influence on the electron flux reducing photosystem I (PSI) end acceptors (REo/RC). Spraying with SNP alleviated the NaCl toxicity on the photosynthetic functions; the protection was concentration-dependent, and greater in Shamal than in Albanus, i.e., variety specific. Furthermore, the experimental results revealed that the degree of SNP protection under salt stress also depends on the endogenous nitric oxide (NO) amount in leaves, the number of active reaction centers per PSII antenna chlorophylls, the enhanced electron flux reducing end acceptors at the acceptor side of PSI, as well as the stimulation of the cyclic electron transport around PSI. The results showed better protection in both varieties of sorghum for SNP concentrations up to 150 µM, which corresponds to about a 50% increase in the endogenous NO leaf content in comparison to the control plants. Our study provides valuable insight into the molecular mechanisms underlying SNP-induced salt tolerance in sorghum varieties and might be a practical approach to correcting salt intolerance.

Lower air humidity reduced both the plant growth and activities of photosystems I and II under prolonged heat stress.

The warming is global problem. In natural environments, heat stress is usually accompanied by drought. Under drought conditions, water content decreases in both soil and air; yet,the effect of lower air humidity remains obscure. We supplied maize and barley plants with an unlimited source of water for the root uptake and studied the effect of relative air humidity under heat stress. Young plants were subjected for 48 h to several degrees of heat stress: moderate (37 °C), genuine (42 °C), and nearly lethal (46 °C). The conditions of lower air humidity decreased the photochemical activities of photosystem I and photosystem II. The small effect was revealed in the control (24 °C). Elevating temperature to 37 °C and 42 °C increased the relative activities of both photosystems; the photosystem II was activated more. Probably, this is why the effect of air humidity disappeared at 37 °C; the small inhibiting effect was observed at 42 °C. At 46 °C, lower air humidity substantially magnified the inhibitory effect of heat. As a result, the maximal and relative activities of both photosystems decreased in maize and barley; the photosystem II was inhibited more. Under the conditions of 46 °C at lower air humidity, the plant growth was greatly reduced. Maize plants increased water uptake by roots and survived; barley plants were unable to increase water uptake and died. Therefore, air humidity is an important component of environmental heat stress influencing activities of photosystem I and photosystem II and thereby plant growth and viability under severe stress conditions.

The Binding Ability of Mercury (Hg) to Photosystem I and II Explained the Difference in Its Toxicity on the Two Photosystems of Chlorella pyrenoidosa.

Mercury (Hg) poses high toxicity to organisms including algae. Studies showed that the growth and photosynthesis of green algae such as Chlorella are vulnerable to Hg stress. However, the differences between the activities and tolerance of photosystem I and II (PSI and PSII) of green microalgae under Hg exposure are still little known. Responses of quantum yields and electron transport rates (ETRs) of PSI and PSII of Chlorella pyrenoidosa to 0.05−1 mg/L Hg2+ were simultaneously measured for the first time by using the Dual-PAM-100 system. The photosystems were isolated to analyze the characteristics of toxicity of Hg during the binding process. The inhibition of Hg2+ on growth and photosystems was found. PSII was more seriously affected by Hg2+ than PSI. After Hg2+ exposure, the photochemical quantum yield of PSII [Y(II)] decreased with the increase in non-photochemical fluorescence quenching [Y(NO) and Y(NPQ)]. The toxic effects of Hg on the photochemical quantum yield and ETR in PSI were lower than those of PSII. The stimulation of cyclic electron yield (CEF) was essential for the stability and protection of PSI under Hg stress and played an important role in the induction of non-photochemical quenching (NPQ). The results showed a strong combination ability of Hg ions and photosystem particles. The number of the binding sites (n) of Hg on PSII was more than that of PSI, which may explain the different toxicity of Hg on PSII and PSI.

Determining photosynthetic control, a probe for the balance between electron transport and Calvin-Benson cycle activity, with the DUAL-KLAS-NIR.

Photosynthetic Control is defined as the control imposed on photosynthetic electron transport by the lumen-pH-sensitive re-oxidation of plastoquinol (PQH2) by cytochrome b6f. Photosynthetic Control leads at higher actinic light intensities to an electron transport chain with a (relatively) reduced photosystem (PS) II and PQ pool and a (relatively) oxidized PS I. Making Light Curves of more than 33 plant species with the recently introduced DUAL-KLAS-NIR (Chl a fluorescence + the redox states of plastocyanin (PC), P700, and ferredoxin (Fd)) the light intensity-dependent induction of Photosynthetic Control was probed and characterized. It was observed that PC became completely oxidized at light intensities ≤ 400 µmol photons m-2 s-1 (at lower light intensities in shade than in sun leaves). The relationship between qP and P700(red) was used to determine the extent of Photosynthetic Control. Instead of measuring the whole Light Curve, it was shown that a single moderate light intensity can be used to characterize the status of a leaf relative to that of other leaves. It was further found that in some shade-acclimated leaves Fd becomes again more oxidized at high light intensities indicating that electron transfer from the PQ pool to P700 cannot keep up with the outflow of electrons on the acceptor side of PS I. It was observed as well that for NPQ-induction a lower light intensity (less acidified lumen) was needed than for the induction of Photosynthetic Control. The measurements were also used to make a comparison between the parameters qP and qL, a comparison suggesting that qP was the more relevant parameter.

Identification of Twelve Different Mineral Deficiencies in Hydroponically Grown Sunflower Plants on the Basis of Short Measurements of the Fluorescence and P700 Oxidation/Reduction Kinetics.

The photosynthetic electron transport chain is mineral rich. Specific mineral deficiencies can modify the electron transport chain specifically. Here, it is shown that on the basis of 2 short Chl fluorescence and P700+ measurements (approx. 1 s each), it is possible to discriminate between 10 out of 12 different mineral deficiencies: B, Ca, Cu, Fe, K, Mg, Mn, Mo, N, P, S, and Zn. B- and Mo-deficient plants require somewhat longer measurements to detect the feedback inhibition they induce. Eight out of twelve deficiencies mainly affect PS I and NIR measurements are, therefore, very important for this analysis. In Cu- and P-deficient plants, electron flow from the plastoquinone pool to PS I, is affected. In the case of Cu-deficiency due to the loss of plastocyanin and in the case of P-deficiency probably due to a fast and strong generation of Photosynthetic Control. For several Ca-, K-, and Zn-deficient plant species, higher levels of reactive oxygen species have been measured in the literature. Here, it is shown that this not only leads to a loss of Pm (maximum P700 redox change) reflecting a lower PS I content, but also to much faster P700+ re-reduction kinetics during the I2-P (~30-200 ms) fluorescence rise phase. The different mineral deficiencies affect the relation between the I2-P and P700+ kinetics in different ways and this is used to discuss the nature of the relationship between these two parameters.

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.

The noninvasive monitoring of the redox status of photosynthetic electron transport chains in Hibiscus rosa-sinensis and Tradescantia leaves.

We present an approach to the noninvasive determination of the electron capacity of the intersystem pool of electron carriers in chloroplasts in situ. As apt experimental models, we used the leaves of Hibiscus rosa-sinensis and Tradescantia species. Electron paramagnetic resonance and optical response of P700 (the primary electron donor in Photosystem I) were applied to measuring electron transport in chloroplasts. Electron capacities of the intersystem electron transport chain (ETC) were determined from redox transients of P700 upon chromatic transitions (white light → far-red light). During the induction period, we observed the nonmonotonic changes in the number of electron equivalents in the intersystem ETC per P700 (parameter Q). In Hibiscus rosa-sinensis, the light-induced rise of Q from ≈2.5 (in the dark) to Q ≈ 12 was followed by its decrease to Q ≈ 6. The data obtained are discussed in the context of pH-dependent regulation of electron transport in chloroplasts, which provides the well-balanced operation of the intersystem ETC. The decay of Q is explained by the attenuation of Photosystem II activity due to the lumen acidification and the acceleration of plastoquinol re-oxidation as a result of the Calvin-Benson cycle activation. Our computer model of electron and proton transport coupled to ATP synthesis in chloroplasts was used to analyze the up and down feedbacks responsible for pH-dependent regulation of electron transport in chloroplasts. The procedures introduced here may be important for subsequent works aimed at defining the plastoquinone participation in regulation of photosynthetic processes in chloroplasts in situ.

The ability of P700 oxidation in photosystem I reflects chilling stress tolerance in cucumber.

Low temperature inhibits photosynthesis and negatively affects plant growth. Cucumber (Cucumis sativus L.) is a chilling-sensitive plant, and its greenhouse production requires considerable energy during the winter. Therefore, a useful stress marker for selecting chilling-tolerant cucumber cultivars is desirable. In this study, we evaluated chilling-stress damage in different cucumber cultivars by measuring photosynthetic parameters. The majority of cultivars showed decreases in the quantum yield of photosystem (PS) II [Fv/Fm and Y(II)] and the quantity of active PS I (Pm) after chilling stress. In contrast, Y(ND)-the ratio of the oxidized state of PSI reaction center chlorophyll P700 (P700+)-differed among cultivars and was perfectly inversely correlated with Y(NA)-the ratio of the non-photooxidizable P700. It has been known that P700+ accumulates under stress conditions and protects plants to suppress the generation of reactive oxygen species. In fact, cultivars unable to induce Y(ND) after chilling stress showed growth retardation with reductions in chlorophyll content and leaf area. Therefore, Y(ND) can be a useful marker to evaluate chilling-stress tolerance in cucumber.

Assessment of the Photosynthetic Apparatus Functions by Chlorophyll Fluorescence and P700 Absorbance in C3 and C4 Plants under Physiological Conditions and under Salt Stress.

Functions of the photosynthetic apparatus of C3 (Pisum sativum L.) and C4 (Zea mays L.) plants under physiological conditions and after treatment with different NaCl concentrations (0-200 mM) were investigated using chlorophyll a fluorescence (pulse-amplitude-modulated (PAM) and JIP test) and P700 photooxidation measurement. Data revealed lower density of the photosynthetic structures (RC/CSo), larger relative size of the plastoquinone (PQ) pool (N) and higher electron transport capacity and photosynthetic rate (parameter RFd) in C4 than in C3 plants. Furthermore, the differences were observed between the two studied species in the parameters characterizing the possibility of reduction in the photosystem (PSI) end acceptors (REo/RC, REo/CSo and δRo). Data revealed that NaCl treatment caused a decrease in the density of the photosynthetic structures and relative size of the PQ pool as well as decrease in the electron transport to the PSI end electron acceptors and the probability of their reduction as well as an increase in the thermal dissipation. The effects were stronger in pea than in maize. The enhanced energy losses after high salt treatment in maize were mainly from the increase in the regulated energy losses (ΦNPQ), while in pea from the increase in non-regulated energy losses (ΦNO). The reduction in the electron transport from QA to the PSI end electron acceptors influenced PSI activity. Analysis of the P700 photooxidation and its decay kinetics revealed an influence of two PSI populations in pea after treatment with 150 mM and 200 mM NaCl, while in maize the negligible changes were registered only at 200 mM NaCl. The experimental results clearly show less salt tolerance of pea than maize.

Primary donor triplet states of Photosystem I and II studied by Q-band pulse ENDOR spectroscopy.

The photoexcited triplet state of the "primary donors" in the two photosystems of oxygenic photosynthesis has been investigated by means of electron-nuclear double resonance (ENDOR) at Q-band (34 GHz). The data obtained represent the first set of 1H hyperfine coupling tensors of the 3P700 triplet state in PSI and expand the existing data set for 3P680. We achieved an extensive assignment of the observed electron-nuclear hyperfine coupling constants (hfcs) corresponding to the methine α-protons and the methyl group ß-protons of the chlorophyll (Chl) macrocycle. The data clearly confirm that in both photosystems the primary donor triplet is located on one specific monomeric Chl at cryogenic temperature. In comparison to previous transient ENDOR and pulse ENDOR experiments at standard X-band (9-10 GHz), the pulse Q-band ENDOR spectra demonstrate both improved signal-to-noise ratio and increased resolution. The observed ENDOR spectra for 3P700 and 3P680 differ in terms of the intensity loss of lines from specific methyl group protons, which is explained by hindered methyl group rotation produced by binding site effects. Contact analysis of the methyl groups in the PSI crystal structure in combination with the ENDOR analysis of 3P700 suggests that the triplet is located on the Chl a' (PA) in PSI. The results also provide additional evidence for the localization of 3P680 on the accessory ChlD1 in PSII.