The Chlorophyll <Emphasis Type="Italic">a</Emphasis> Fluorescence Characteristic in Different Types of Leaf Senescence in <Emphasis Type="Italic">Alhagi sparsifolia</Emphasis>

Senescence is both a highly controlled and a strictly regulated process that is gene dependent. To study the PSII reaction in different types of leaf senescence processes, stem girdling was performed on Alhagi sparsifolia to investigate the leaf status in the control, natural senescence, and girdling-induced senescence leaves. The results showed that during senescence, leaf soluble sugar content, starch content, and the energy absorbed by the unit reaction center (ABS/RC) increased; whereas leaf photosynthetic rate, photosynthetic pigment content, maximum photochemical efficiency (φ _Po), and energy used by the acceptor site in electron transfer (ET_o/RC) decreased. The result of the present research implied that stem girdling significantly accelerated leaf senescence, which was due to the accumulation of carbohydrate. Natural senescence is a highly controlled process, which is an ordered process played by genes, whereas girdling-induced senescence is a disordered one. In addition, natural senescence slightly inhibits the acceptor site of PSII but did not damage the donor site of PSII. Conversely, girdling-induced senescence not only damaged the donor site of PSII (for example, oxygen-evolving complex), but also significantly inhibited the acceptor site of PSII. Moreover, both types of senescence led to an increase in the energy absorbed by the unit reaction center (ABS/RC), which subsequently resulted in an increasing excitation pressure in the reaction center (DI_o/RC), as well as additional saved carotenoid for absorbing residual light energy and quenching reactive oxygen species during senescence.     

The evolution of Photosystem II: insights into the past and future

This article attempts to address the molecular origin of Photosystem II (PSII), the central component in oxygenic photosynthesis. It discusses the possible evolution of the relevant cofactors needed for splitting water into molecular O₂ with respect to the following functional domains in PSII: the reaction center (RC), the oxygen evolving complex (OEC), and the manganese stabilizing protein (MSP). Possible ancestral sources of the relevant cofactors are considered, as are scenarios of how these components may have been brought together to produce the intermediate steps in the evolution of PSII. Most importantly, the driving forces that maintained these intermediates for continued adaptation are considered. We then apply our understanding of the evolution of PSII to the bioengineering of a water oxidizing catalyst for utilization of solar energy.     

Excitonic connectivity between photosystem II units: what is it, and how to measure it?

In photosynthetic organisms, light energy is absorbed by a complex network of chromophores embedded in light-harvesting antenna complexes. In photosystem II (PSII), the excitation energy from the antenna is transferred very efficiently to an active reaction center (RC) (i.e., with oxidized primary quinone acceptor Q _A), where the photochemistry begins, leading to O₂ evolution, and reduction of plastoquinones. A very small part of the excitation energy is dissipated as fluorescence and heat. Measurements on chlorophyll (Chl) fluorescence and oxygen have shown that a nonlinear (hyperbolic) relationship exists between the fluorescence yield (Φ _F ) (or the oxygen emission yield, $ \Phi _{{{\text{O}}_{2} }} $ ) and the fraction of closed PSII RCs (i.e., with reduced Q _A). This nonlinearity is assumed to be related to the transfer of the excitation energy from a closed PSII RC to an open (active) PSII RC, a process called PSII excitonic connectivity by Joliot and Joliot (CR Acad Sci Paris 258: 4622–4625, 1964). Different theoretical approaches of the PSII excitonic connectivity, and experimental methods used to measure it, are discussed in this review. In addition, we present alternative explanations of the observed sigmoidicity of the fluorescence induction and oxygen evolution curves.     

Chlorophyll a fluorescence kinetics of mung bean (Vigna radiata L.) grown under artificial continuous light

Continuous light can be used as a tool to understand the diurnal rhythm of plants and it can also be used to increase the plant production. In the present research, we aimed to investigate the photosynthetic performance of V. radiata under continuous light as compared with the plants grown under normal light duration. Chlorophyll a fluorescence transient (OJIP test) technique was used to understand the effect on various stages of photosynthesis and their consequences under continuous light condition. Various Chl a Fluorescence kinetic parameters such as Specific energy fluxes (per Q_A-reducing PSII reaction center (RC)) (ABS /RC; TR₀/RC; ET₀/RC; DI₀/RC), phenomenological fluxes, leaf model, (ABS/CSm; TR/CSm; ETo/CSm), Quantum yields and efficiencies (φPo; φEo; Ψo) and Performance index (PI_abs) was extracted and analysed in our investigation. Conclusively, our study has revealed that continuous light alters the photosynthetic performance of V. radiata at a different point but also improve plant productivity.     

Subunit composition of CP43-less photosystem II complexes of Synechocystis sp. PCC 6803: implications for the assembly and repair of photosystem II

Photosystem II (PSII) mutants are useful experimental tools to trap potential intermediates involved in the assembly of the oxygen-evolving PSII complex. Here, we focus on the subunit composition of the RC47 assembly complex that accumulates in a psbC null mutant of the cyanobacterium Synechocystis sp. PCC 6803 unable to make the CP43 apopolypeptide. By using native gel electrophoresis, we showed that RC47 is heterogeneous and mainly found as a monomer of 220 kDa. RC47 complexes co-purify with small Cab-like proteins (ScpC and/or ScpD) and with Psb28 and its homologue Psb28-2. Analysis of isolated His-tagged RC47 indicated the presence of D1, D2, the CP47 apopolypeptide, plus nine of the 13 low-molecular-mass (LMM) subunits found in the PSII holoenzyme, including PsbL, PsbM and PsbT, which lie at the interface between the two momomers in the dimeric holoenzyme. Not detected were the LMM subunits (PsbK, PsbZ, Psb30 and PsbJ) located in the vicinity of CP43 in the holoenzyme. The photochemical activity of isolated RC47-His complexes, including the rate of reduction of P680⁺, was similar to that of PSII complexes lacking the Mn₄CaO₅ cluster. The implications of our results for the assembly and repair of PSII in vivo are discussed.     

Combined effects of girdling and leaf removal on fluorescence characteristic of Alhagi sparsifolia leaf senescence

Plant senescence is largely influenced by carbohydrate content. In order to investigate the impact of carbohydrate content on leaf senescence and photosystem II (PSII) during the senescence process, phloem girdling (PG), leaf removal (LR) and a combination of phloem girdling and leaf removal (GR) were performed on Alhagi sparsifolia (Fabaceae) at the end of the growing season. The results showed that during senescence, leaf soluble sugar content, starch content, the energy absorbed by the unit reaction centre (ABS/RC) increased; whereas, leaf photosynthetic rate, photosynthetic pigment content, maximum photochemical efficiency (φ_Po) and energy used by the acceptor site in electron transfer (ET_o/RC) decreased. The degree of change was PG> GR> CK (control)> LR. The results of the present work implied that phloem girdling (PG) significantly accelerated leaf senescence, and that single leaf removal (LR) slightly delayed leaf senescence; although leaf removal significantly delayed the senescence process on the girdled leaf (GR). Natural or delayed senescence only slightly inhibited the acceptor site of PSII and did not damage the donor site of PSII. On the other hand, induced senescence not only damaged the donor site of PSII (e.g. oxygen‐evolving complex), but also significantly inhibited the acceptor site of PSII. In addition, leaf senescence led to an increase in the energy absorbed by the unit reaction centre (ABS/RC), which subsequently resulted in increasing excitation pressure in the reaction centre (DI_o/RC), as well as additional saved Car for absorbing residual light energy and quenching reactive oxygen species during senescence.     

‘Birth defects’ of photosystem II make it highly susceptible to photodamage during chloroplast biogenesis

High solar flux is known to diminish photosynthetic growth rates, reducing biomass productivity and lowering disease tolerance. Photosystem II (PSII) of plants is susceptible to photodamage (also known as photoinactivation) in strong light, resulting in severe loss of water oxidation capacity and destruction of the water‐oxidizing complex (WOC). The repair of damaged PSIIs comes at a high energy cost and requires de novo biosynthesis of damaged PSII subunits, reassembly of the WOC inorganic cofactors and membrane remodeling. Employing membrane‐inlet mass spectrometry and O₂‐polarography under flashing light conditions, we demonstrate that newly synthesized PSII complexes are far more susceptible to photodamage than are mature PSII complexes. We examined these ‘PSII birth defects’ in barley seedlings and plastids (etiochloroplasts and chloroplasts) isolated at various times during de‐etiolation as chloroplast development begins and matures in synchronization with thylakoid membrane biogenesis and grana membrane formation. We show that the degree of PSII photodamage decreases simultaneously with biogenesis of the PSII turnover efficiency measured by O₂‐polarography, and with grana membrane stacking, as determined by electron microscopy. Our data from fluorescence, Q_B‐inhibitor binding, and thermoluminescence studies indicate that the decline of the high‐light susceptibility of PSII to photodamage is coincident with appearance of electron transfer capability Q_A^? → Q_B during de‐etiolation. This rate depends in turn on the downstream clearing of electrons upon buildup of the complete linear electron transfer chain and the formation of stacked grana membranes capable of longer‐range energy transfer.     

PnF3H,a flavanone 3-hydroxylase from the Antarctic moss <Emphasis Type="Italic">Pohlia nutans</Emphasis>, confers tolerance to salt stress and ABA treatment in transgenic <Emphasis Type="Italic">Arabidopsis</Emphasis>

In the condition of prolonged drought stress during the reproductive stage, we addressed the photosynthetic performance in flag leaves of the high-yield hybrid rice (Oryza sativa L.) LYPJ. The chlorophyll a fluorescence transient dynamics analysis indicated a timely and constant responsive pattern involving in both PSI and PSII. For PSII functionality, uncoupling of oxygen evolving complex at the donor side and inhibition of electron transport from Q_A to Q_B at the accepter side were both accounted for the decrease of quantum yield of primary photochemistry at early stage (before 21 days after the onset of drought stress). Likewise, increased size of functional antenna may be primarily responsible for early reaction centers inactivation in drought stressed plants, but transformation to non-Q_A-reducing centers for the later. The consequent redundant excitation energy was predominantly eliminated by the increasing thermal dissipation. Advanced accumulation of drought stress (from 21 to 35 days) showed preferential impact on the donor side of PSII and significant loss of RC/CS₀ was induced during this period. In brief, up-regulation of thermal dissipation and possible cyclic electron transport, as well as down-regulation of activated reaction centers and linear electron transport was crucial for rebalance the energy distribution between the two photosystems from deviant stoichiometry resulting from the uncoupling of oxygen evolving complex.     

Growth,fluorescence, photosynthetic O<Subscript>2</Subscript> production and pigment content of salt adapted cultures of <Emphasis Type="Italic">Arthrospira</Emphasis> (<Emphasis Type="Italic">Spirulina</Emphasis>) <Emphasis Type="Italic">platensis</Emphasis>

The effect of salt concentration (NaCl) on growth, fluorescence, photosynthetic activities and pigment content of the cyanobacterium Arthrospira platensis has been investigated over 15 days. It has been observed that high NaCl concentration induces an increase of the growth, photosynthetic efficiency (α), phycobilin/chlorophyll ratio and a slight decrease of dark respiration and compensation points. Moreover, high NaCl concentration enhances photosystem II (PSII) activity compared to photosystem I (PSI). Results show that the phycobilin-PSII energy transfer compared to the chlorophyll-PSII (F_695,600/F_695,440) increases. However, data obtained about the maximal efficiency of PSII photochemistry are controversial. Indeed, the Fv/Fm ratio decreases in salt adapted cultures, while at the same time the trapping flux per PSII reaction center (TR₀/RC) and the probability of electron transport beyond Q_A (₀) remain unchanged at the level of the donor and the acceptor sites of PSII. This effect can be attributed to the interference of phycobilin fluorescence with Chl a when performing polyphasic transient measurements.     

Enhanced NPQ affects long-term acclimation in the spring ephemeral Berteroa incana

The spring ephemeral Berteroa incana is a familial relative of Arabidopsis thaliana and thrives in a diverse range of terrestrial ecosystems. Within this study, the novel chlorophyll fluorescence parameter of photochemical quenching in the dark (qPd) was used to measure the redox state of the primary quinone electron acceptor (Q_A) in order to estimate the openness of photosystem II (PSII) reaction centres (RC). From this, the early onset of photoinactivation can be sensitively quantified alongside the light tolerance of PSII and the photoprotective efficiency of nonphotochemical quenching (NPQ). This study shows that, with regards to A. thaliana, NPQ is enhanced in B. incana in both low-light (LL) and high-light (HL) acclimation states. Moreover, light tolerance is increased by up to 500%, the rate of photoinactivation is heavily diminished, and the ability to recover from light stress is enhanced in B. incana, relative to A. thaliana. This is due to faster synthesis of zeaxanthin and a larger xanthophyll cycle (XC) pool available for deepoxidation. Moreover, preferential energy transfer via CP47 around the RC further enhances efficient photoprotection. As a result, a high functional cross-section of photosystem II is maintained and is not downregulated when B. incana is acclimated to HL. A greater capacity for protective NPQ allows B. incana to maintain an enhanced light-harvesting capability when acclimated to a range of light conditions. This enhancement of flexible short-term protection saves the metabolic cost of long-term acclimatory changes.     

Psb28 Protein Is Involved in the Biogenesis of the Photosystem II Inner Antenna CP47 (PsbB) in the Cyanobacterium Synechocystis sp. PCC 6803

The role of the Psb28 protein in the structure and function of the photosystem II (PSII) complex has been studied in the cyanobacterium Synechocystis sp. PCC 6803. The protein was localized in the membrane fraction and, whereas most of the protein was detected as an unassembled protein, a small portion was found in the PSII core complex lacking the CP43 antenna (RC47). The association of Psb28 with RC47 was further confirmed by preferential isolation of RC47 from the strain containing a histidine-tagged derivative of Psb28 using nickel-affinity chromatography. However, the affinity-purified fraction also contained a small amount of the unassembled PSII inner antenna CP47 bound to Psb28-histidine, indicating a structural relationship between Psb28 and CP47. A psb28 deletion mutant exhibited slower autotrophic growth than wild type, although the absence of Psb28 did not affect the functional properties of PSII. The mutant showed accelerated turnover of the D1 protein, faster PSII repair, and a decrease in the cellular content of PSI. Radioactive labeling revealed a limitation in the synthesis of both CP47 and the PSI subunits PsaA/PsaB in the absence of Psb28. The mutant cells contained a high level of magnesium protoporphyrin IX methylester, a decreased level of protochlorophyllide, and released large quantities of protoporphyrin IX into the medium, indicating inhibition of chlorophyll (Chl) biosynthesis at the cyclization step yielding the isocyclic ring E. Overall, our results show the importance of Psb28 for synthesis of Chls and/or apoproteins of Chl-binding proteins CP47 and PsaA/PsaB.PSII is a multisubunit pigment-protein complex of plants, algae, and cyanobacteria, which is responsible for oxidation of water and reduction of plastoquinone during oxygenic photosynthesis (Barber, 2006). In the heart of the complex, there are two similar membrane-spanning proteins, D1 and D2, that bind the cofactors involved in primary charge separation (Nanba and Satoh, 1987) and subsequent electron transfer within PSII (for review, see Barber, 2006). Peripherally to the D1-D2 heterodimer, there are two chlorophyll (Chl)-binding inner antenna proteins, CP47 and CP43, that deliver energy to the reaction center (RC), driving electron transfer. In addition, CP43 also provides important ligands to the Mn₄Ca cluster, the site of water oxidation (Ferreira et al., 2004; Loll et al., 2005). These four large proteins are surrounded by a number of smaller membrane polypeptides (for review, see Shi and Schröder, 2004). One of them, the so-called PsbW, was originally detected in the isolated RC complex from spinach (Spinacia oleracea; Irrgang et al., 1995; Lorković et al., 1995). The mature protein with a predicted one-transmembrane α-helix in the central hydrophobic region seems to have (unlike most of PSII membrane proteins) the N terminus oriented into the lumen in close vicinity to the extrinsic, nuclear-encoded 33-kD PsbO protein. Cross-linking experiments also indicated a close association of PsbW with D1, D2, and the α-subunit of cytochrome (cyt) b-559 in the isolated RC complex (Irrgang et al., 1995; Lorković et al., 1995). At variance with these results, Rokka et al. (2005) located PsbW predominantly in PSII-light-harvesting complex II (LHCII) supercomplexes and only minor amounts were found in PSII core dimers and monomers. In transgenic plants of Arabidopsis (Arabidopsis thaliana) lacking the PsbW protein, the stability of the dimeric PSII was diminished and the PSII-LHCII supercomplexes could not be detected. It has been suggested that PsbW functions as a linker for LHCII binding to the PSII complex (Shi et al., 2000). Because LHCII is absent in cyanobacteria, it was intelligible that the PsbW was not detected in these oxygenic autotrophs. Nevertheless, N-terminal sequencing and mass spectrometric analyses of protein subunits in the purified His-tagged PSII from Synechocystis sp. PCC 6803 (Synechocystis 6803) revealed the presence of an unknown protein with 16% sequence identity to PsbW from Arabidopsis (Kashino et al., 2002). This protein was designated as Psb28 (also Psb13 or ycf79). Its amino acid sequence suggests that it is a rather hydrophilic protein without a transmembrane helix and is larger than PsbW (about 13 kD). In the recent crystal structures of the cyanobacterial PSII (Ferreira et al., 2004; Loll et al., 2005), this protein was not identified and it remains an issue of contention whether the protein is a true PSII subunit, a transiently associated assembly factor, or just an impurity of the preparation. The relatively low content of this protein in the isolated preparation suggested that the two latter possibilities are more probable. Very recently, the protein has been detected as a component of PSII complexes in Synechocystis depleted of phosphatidylglycerol (Sakurai et al., 2007). It has been proposed that the protein may play a regulatory role during the assembly of PSII. A gene encoding a similar soluble protein has also been found in the genome of Arabidopsis and the protein was designated PsbW-like.Here, we present a detailed analysis of the role of Psb28 in the structure and function of PSII in Synechocystis 6803. The results showed that Psb28 is not a component of the fully assembled dimeric PSII core complex, but it is preferentially bound to PSII assembly intermediates containing the inner antenna CP47. The results support the role of the protein in biogenesis of certain Chl-binding proteins via regulating synthesis of their apoproteins or Chls.     

Promotion of Energy Transfer and Oxygen Evolution in Spinach Photosystem II by Nano-anatase TiO<Subscript>2</Subscript>

Being a proven photocatalyst, nano-anatase is capable of undergoing electron transfer reactions under light. In previous studies we had proven that nano-anatase improved photosynthesis and greatly promoted spinach growth. The mechanisms by which nano-anatase promotes energy transfer and the conversion efficiency of the process are still not clearly understood. In the present paper, we report the results obtained with the photosystem II (PSII) isolated from spinach and treated by nano-anatase TiO₂ and studied the effect of nano-anatase TiO₂ on energy transfer in PSII by spectroscopy and on oxygen evolution. The results showed that nano-anatase TiO₂ treatment at a suitable concentration could significantly change PSII microenvironment and increase absorbance for visible light, improve energy transfer among amino acids within PSII protein complex, and accelerate energy transport from tyrosine residue to chlorophyll a. The photochemical activity of PSII (fluorescence quantum yield) and its oxygen-evolving rate were enhanced by nano-anatase TiO₂. This is viewed as evidence that nano-anatase TiO₂ can promote energy transfer and oxygen evolution in PSII of spinach.     

The Regulation of Photosynthetic Electron Transport during Nutrient Deprivation in Chlamydomonas reinhardtii

The light-saturated rate of photosynthetic O₂ evolution in Chlamydomonas reinhardtii declined by approximately 75% on a per-cell basis after 4 d of P starvation or 1 d of S starvation. Quantitation of the partial reactions of photosynthetic electron transport demonstrated that the light-saturated rate of photosystem (PS) I activity was unaffected by P or S limitation, whereas light-saturated PSII activity was reduced by more than 50%. This decline in PSII activity correlated with a decline in both the maximal quantum efficiency of PSII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers (PSII centers capable of performing a charge separation but unable to reduce the plastoquinone pool). In addition to a decline in the light-saturated rate of O₂ evolution, there was reduced efficiency of excitation energy transfer to the reaction centers of PSII (because of dissipation of absorbed light energy as heat and because of a transition to state 2). These findings establish a common suite of alterations in photosynthetic electron transport that results in decreased linear electron flow when C. reinhardtii is limited for either P or S. It was interesting that the decline in the maximum quantum efficiency of PSII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers were regulated specifically during S-limited growth by the SacI gene product, which was previously shown to be critical for the acclimation of C. reinhardtii to S limitation (J.P. Davies, F.H. Yildiz, and A.R. Grossman [1996] EMBO J 15: 2150–2159).     

Effect of Antenna-Depletion in Photosystem II on Excitation Energy Transfer in Arabidopsis thaliana

The role of individual photosynthetic antenna complexes of Photosystem II (PSII) both in membrane organization and excitation energy transfer have been investigated. Thylakoid membranes from wild-type Arabidopsis thaliana, and three mutants lacking light-harvesting complexes CP24, CP26, or CP29, respectively, were studied by picosecond-fluorescence spectroscopy. By using different excitation/detection wavelength combinations it was possible for the first time, to our knowledge, to separate PSI and PSII fluorescence kinetics. The sub-100 ps component, previously ascribed entirely to PSI, turns out to be due partly to PSII. Moreover, the migration time of excitations from antenna to PSII reaction center (RC) was determined for the first time, to our knowledge, for thylakoid membranes. It is four times longer than for PSII-only membranes, due to additional antenna complexes, which are less well connected to the RC. The results in the absence of CP26 are very similar to those of wild-type, demonstrating that the PSII organization is not disturbed. However, the kinetics in the absence of CP29 and, especially, of CP24 show that a large fraction of the light-harvesting complexes becomes badly connected to the RCs. Interestingly, the excited-state lifetimes of the disconnected light-harvesting complexes seem to be substantially quenched.     

Fluorescence kinetics of PSII crystals containing Ca2 + or Sr2 + in the oxygen evolving complex

Photosystem II (PSII) is the pigment–protein complex which converts sunlight energy into chemical energy by catalysing the process of light-driven oxidation of water into reducing equivalents in the form of protons and electrons. Three-dimensional structures from x-ray crystallography have been used extensively to model these processes. However, the crystal structures are not necessarily identical to those of the solubilised complexes. Here we compared picosecond fluorescence of solubilised and crystallised PSII core particles isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus. The fluorescence of the crystals is sensitive to the presence of artificial electron acceptors (K₃Fe(CN)₃) and electron transport inhibitors (DCMU). In PSII with reaction centres in the open state, the picosecond fluorescence of PSII crystals and solubilised PSII is indistinguishable. Additionally we compared picosecond fluorescence of native PSII with PSII in which Ca² in the oxygen evolving complex (OEC) is biosynthetically replaced by Sr^2 +. With the Sr^2 + replaced OEC the average fluorescence decay slows down slightly (81 ps to 85 ps), and reaction centres are less readily closed, indicating that both energy transfer/trapping and electron transfer are affected by the replacement.     

Effects of drought stress on fluorescence characteristics of photosystem II in leaves of Plectranthus scutellarioides

Drought stress has multiple effects on the photosynthetic apparatus. Herein, we aimed to study the effect of drought stress on fluorescence characteristics of PSII in leaves of Plectranthus scutellarioides and explore potentially underlying mechanisms. Plants of P. scutellarioides were grown in a greenhouse and subjected to drought (DS, drought-stressed) or daily irrigation (control group). Leaf chlorophyll (Chl) index and induction kinetics curves of Chl a fluorescence and the JIP-test were used to evaluate effects of drought lasting for 20 d. Our results showed that both the leaf and soil relative water content decreased with increasing treatment duration. The leaf Chl index was reduced to half in the DS plants compared with the control group after 20 d. The minimal fluorescence in the DS plants was higher than that in the control plants after 10 d of the treatment. Maximum photochemical efficiency and lateral reactivity decreased with increasing treatment duration in the DS plants. With the continuing treatment, values of absorption flux per reaction center (RC), trapped energy flux per RC, dissipated energy flux per RC, and electron transport flux per RC increased in the earlier stage in the DS plants, while obviously decreased at the later stage of the treatment. In conclusion, drought stress inhibited the electron transport and reduced PSII photochemical activity in leaves of P. scutellarioides.     

Ecophysiological response of native and invasive <Emphasis Type="Italic">Spartina</Emphasis> species to extreme temperature events in Mediterranean marshes

The recent IPCC WG2 5th Assessment Report (IPCC 2014), notes an increase in the frequency and duration of extreme climatic events, especially for the Mediterranean region. Together with climate change, the invasion of natural communities by non-indigenous species (NIS) constitutes a serious threat to biodiversity. One of these NIS is the American Spartina patens, now present in Western European marshes. The present study aims to understand the biochemical and photochemical responses of S. patens compared with S. maritima under extreme temperature events. Under normal and extreme heat conditions, S. patens had a higher photosynthetic efficiency (α), compared with cold wave events, where the native S. maritima was far more efficient. This reduced photosynthetic efficiency was mostly due to a decrease in the connectivity between photosystem II (PSII) antennae. This was accompanied by severe damage to the oxygen-evolving complex of PSII. On the other hand, S. patens oxygen evolving complexes (OECs) seem to be temperature insensitive. The light absorption capacity was maintained due to a higher net rate of reaction centre (RC) closure as a counteractive measure of the reduced number of RC, especially in S. maritima. The loss of connectivity between PSII antennae and damage in OECs under heat stress leads to a severe reduction in the maximum yield for photochemistry enhanced by the low probability of each absorbed quanta to produce electronic work. However, while S. maritima presents high energy losses under heat stress, S. patens developed efficient quenching mechanisms under thermal stress, through auroxanthin. In S. patens, cold wave-treated individuals also displayed a very active line of enzymatic defences for reactive oxygen species scavenging. In fact, only cold treated individuals of this species presented higher activities of anti-oxidant enzymes, revealing some degree of adaptation to this new environment. In contrast, in S. maritima the exposure to extreme heat periods led, in most cases, to a decrease in the enzymatic defences, leaving the cell prone to oxidative damage. In summary, S. patens appears to have a higher fitness for the incoming climatic scenarios, being more tolerant to heat stress, while S. maritima will have its photobiological fitness decreased. This will impose a shift in the salt marsh biodiversity, favouring the non-indigenous S. patens expansion.     

Determination of the excitation migration time in Photosystem II: Consequences for the membrane organization and charge separation parameters

The fluorescence decay kinetics of Photosystem II (PSII) membranes from spinach with open reaction centers (RCs), were compared after exciting at 420 and 484 nm. These wavelengths lead to preferential excitation of chlorophyll (Chl) a and Chl b, respectively, which causes different initial excited-state populations in the inner and outer antenna system. The non-exponential fluorescence decay appears to be 4.3 ± 1.8 ps slower upon 484 nm excitation for preparations that contain on average 2.45 LHCII (light-harvesting complex II) trimers per reaction center. Using a recently introduced coarse-grained model it can be concluded that the average migration time of an electronic excitation towards the RC contributes ~ 23% to the overall average trapping time. The migration time appears to be approximately two times faster than expected based on previous ultrafast transient absorption and fluorescence measurements. It is concluded that excitation energy transfer in PSII follows specific energy transfer pathways that require an optimized organization of the antenna complexes with respect to each other. Within the context of the coarse-grained model it can be calculated that the rate of primary charge separation of the RC is (5.5 ± 0.4 ps)^− 1, the rate of secondary charge separation is (137 ± 5 ps)^− 1 and the drop in free energy upon primary charge separation is 826 ± 30 cm^− 1. These parameters are in rather good agreement with recently published results on isolated core complexes [Y. Miloslavina, M. Szczepaniak, M.G. Muller, J. Sander, M. Nowaczyk, M. Rögner, A.R. Holzwarth, Charge separation kinetics in intact Photosystem II core particles is trap-limited. A picosecond fluorescence study, Biochemistry 45 (2006) 2436-2442].