PSII Reaction Center Research Articles

Structural insight into light harvesting for photosystem II in green algae.

Green algae and plants rely on light-harvesting complex II (LHCII) to collect photon energy for oxygenic photosynthesis. In Chlamydomonas reinhardtii, LHCII molecules associate with photosystem II (PSII) to form various supercomplexes, including the C2S2M2L2 type, which is the largest PSII-LHCII supercomplex in algae and plants that is presently known. Here, we report high-resolution cryo-electron microscopy (cryo-EM) maps and structural models of the C2S2M2L2 and C2S2 supercomplexes from C. reinhardtii. The C2S2 supercomplex contains an LhcbM1-LhcbM2/7-LhcbM3 heterotrimer in the strongly associated LHCII, and the LhcbM1 subunit assembles with CP43 through two interfacial galactolipid molecules. The loosely and moderately associated LHCII trimers interact closely with the minor antenna complex CP29 to form an intricate subcomplex bound to CP47 in the C2S2M2L2 supercomplex. A notable direct pathway is established for energy transfer from the loosely associated LHCII to the PSII reaction centre, as well as several indirect routes. Structure-based computational analysis on the excitation energy transfer within the two supercomplexes provides detailed mechanistic insights into the light-harvesting process in green algae.

Two Quenchers Formed During Photodamage of Phostosystem II and The Role of One Quencher in Preemptive Photoprotection

The quenching of chlorophyll fluorescence caused by photodamage of Photosystem II (qI) is a well recognized phenomenon, where the nature and physiological role of which are still debatable. Paradoxically, photodamage to the reaction centre of Photosystem II is supposed to be alleviated by excitation quenching mechanisms which manifest as fluorescence quenchers. Here we investigated the time course of PSII photodamage in vivo and in vitro and that of picosecond time-resolved chlorophyll fluorescence (quencher formation). Two long-lived fluorescence quenching processes during photodamage were observed and were formed at different speeds. The slow-developing quenching process exhibited a time course similar to that of the accumulation of photodamaged PSII, while the fast-developing process took place faster than the light-induced PSII damage. We attribute the slow process to the accumulation of photodamaged PSII and the fast process to an independent quenching mechanism that precedes PSII photodamage and that alleviates the inactivation of the PSII reaction centre.

Identification of assembly precursors to photosystems emitting fluorescence at 683 nm and 687 nm by cryogenic fluorescence microspectroscopy

Photosystem I (PSI) and photosystem II (PSII) play key roles in photoinduced electron-transfer reaction in oxygenic photosynthesis. Assemblies of these PSs can be initiated by illumination of the etiolated seedlings (greening). The study aimed to identify specific fluorescence spectral components relevant to PSI and PSII assembly intermediates emerging in greening seedlings of Zea mays, a typical C4 plant. The different PSII contents between the bundle sheath (BS) and mesophyll (M) cells were utilized to spectrally isolate the precursors to PSI and PSII. The greening Zea mays leaf thin sections were observed with the cryogenic microscope combined with a spectrometer. With the aid of the singular-value decomposition analysis, we could identify four independent fluorescent species, SAS677, SAS685, SAS683, and SAS687, named after their fluorescence peak wavelengths. SAS677 and SAS685 are the dominant components after the 30-minute greening, and the distributions of these components showed no clear differences between M and BS cells, indicating immature cell differentiation in this developing stage. On the other hand, the 1-hour greening resulted in reduced distributions of SAS683 in BS cells leading us to assign this species to PSII precursors. The 2-hour greening induced the enrichment of SAS687 in BS cells suggesting its PSI relevance. Similarity in the peak wavelengths of SAS683 and the reported reaction center of PSII implied their connection. SAS687 showed an intense sub-band at around 740 nm, which can be assigned to the emission from the red chlorophylls specific to the mature PSI.

Sigma factor 1 in chloroplast gene transcription and photosynthetic light acclimation.

Sigma factors are dissociable subunits of bacterial RNA polymerase that ensure efficient transcription initiation from gene promoters. Owing to their prokaryotic origin, chloroplasts possess a typical bacterial RNA polymerase together with its sigma factor subunit. The higher plant Arabidopsis thaliana contain as many as six sigma factors for the hundred or so of its chloroplast genes. The role of this relatively large number of transcription initiation factors for the miniature chloroplast genome, however, is not fully understood. Using two Arabidopsis T-DNA insertion mutants, we show that sigma factor 1 (SIG1) initiates transcription of a specific subset of chloroplast genes. We further show that the photosynthetic control of PSI reaction center gene transcription requires complementary regulation of the nuclear SIG1 gene at the transcriptional level. This SIG1 gene regulation is dependent on both a plastid redox signal and a light signal transduced by the phytochrome photoreceptor.

Effect of high temperature on the balance between photosynthetic light absorption and energy utilization in Chlorella pyrenoidosa (Chlorophyceae)

Chlorella pyrenoidosa (Chlorophyceae) is widely cultured for production of health food and animal feed. In outdoors, mass cultivation of C. pyrenoidosa often suffers from high temperature. A better understanding of the effects of high temperature on photosynthesis and photoprotection can help optimize the productivity of C. pyrenoidosa cultures. In this study, we investigated effect of high temperature (35, 38, or 41°C) on the balance between photosynthetic light absorption and energy utilization of C. pyrenoidosa. In contrast to 30°C, higher temperature of 35 or 38°C did not inhibit the growth of C. pyrenoidosa. Treatment in 35°C maintained the balance. Moreover, the PSI acceptor side in 38°C was over-reduced and PSII reaction centers were over-excited under strong light, which destroyed the balance and generated active oxygen species (AOS). However, the activated antioxidant enzymes might remove completely the over-production of AOS, thereby protect C. pyrenoidosa cells from photodamage. It shows that this C. pyrenoidosa strain could tolerate as high as 38°C. Furthermore, treatment in 41°C resulted in more lack of the balance than that in 38°C. However, the activities of antioxidant enzymes stopped increasing in 41°C, and were not strong enough to remove the excess AOS. Therefore, treatment in 41°C could decrease the growth of C. pyrenoidosa. In addition, strong and longtime light exposure would cause serious photodamage to C. pyrenoidosa cells.

High throughput procedure utilising chlorophyll fluorescence imaging to phenotype dynamic photosynthesis and photoprotection in leaves under controlled gaseous conditions

BackgroundAs yields of major crops such as wheat (T. aestivum) have begun to plateau in recent years, there is growing pressure to efficiently phenotype large populations for traits associated with genetic advancement in yield. Photosynthesis encompasses a range of steady state and dynamic traits that are key targets for raising Radiation Use Efficiency (RUE), biomass production and grain yield in crops. Traditional methodologies to assess the full range of responses of photosynthesis, such a leaf gas exchange, are slow and limited to one leaf (or part of a leaf) per instrument. Due to constraints imposed by time, equipment and plant size, photosynthetic data is often collected at one or two phenological stages and in response to limited environmental conditions.ResultsHere we describe a high throughput procedure utilising chlorophyll fluorescence imaging to phenotype dynamic photosynthesis and photoprotection in excised leaves under controlled gaseous conditions. When measured throughout the day, no significant differences (P > 0.081) were observed between the responses of excised and intact leaves. Using excised leaves, the response of three cultivars of T. aestivum to a user—defined dynamic lighting regime was examined. Cultivar specific differences were observed for maximum PSII efficiency (Fv′/Fm′—P < 0.01) and PSII operating efficiency (Fq′/Fm′—P = 0.04) under both low and high light. In addition, the rate of induction and relaxation of non-photochemical quenching (NPQ) was also cultivar specific. A specialised imaging chamber was designed and built in-house to maintain gaseous conditions around excised leaf sections. The purpose of this is to manipulate electron sinks such as photorespiration. The stability of carbon dioxide (CO2) and oxygen (O2) was monitored inside the chambers and found to be within ± 4.5% and ± 1% of the mean respectively. To test the chamber, T. aestivum ‘Pavon76’ leaf sections were measured under at 20 and 200 mmol mol−1 O2 and ambient [CO2] during a light response curve. The Fv′/Fm′was significantly higher (P < 0.05) under low [O2] for the majority of light intensities while values of NPQ and the proportion of open PSII reaction centers (qP) were significantly lower under > 130 μmol m−2 s−1 photosynthetic photon flux density (PPFD).ConclusionsHere we demonstrate the development of a high-throughput (> 500 samples day−1) method for phenotyping photosynthetic and photo-protective parameters in a dynamic light environment. The technique exploits chlorophyll fluorescence imaging in a specifically designed chamber, enabling controlled gaseous environment around leaf sections. In addition, we have demonstrated that leaf sections do not different from intact plant material even > 3 h after sampling, thus enabling transportation of material of interest from the field to this laboratory based platform. The methodologies described here allow rapid, custom screening of field material for variation in photosynthetic processes.

Growth and photosynthetic performance of Chlamydopodium fusiforme cells cultivated in BG11 and Bristol media

Nitrogen is an essential nutrient for all life forms and its availability is periodically limited in marine and freshwater ecosystems. To optimize the growth of the freshwater microalga Chlamydopodium fusiforme and to investigate the cell acclimation process to different amounts of nitrogen, we grew cells in Bristol and BG11, under fixed conditions of 10 h:14 h light/dark cycle. Cultures grown in BG11 exhibited higher biomass accumulation, reaching a cell dry weight of 1.94 ± 0.1 g L−1, while in Bristol it reached 1.58 ± 0.1 g L−1. In cells grown in BG11, chlorophyll content increased continuously up to 79.6 mg L−1 (9th day), while in Bristol it was much lower (29.5 mg L−1). This fact caused a strong difference in their spectrally averaged optical cross-section normalized to chlorophyll a (a*), and consequently in their photosynthetic performance (oxygen evolution and electron transport rates). Carotenoid content followed a similar pattern. It was interesting to note that despite this great difference in chlorophyll concentration, both photosynthesis rate and cell dry weight were not affected proportionally. The chlorophyll a fluorescence measurements indicated a stable effective quantum yield of photosystem II in both culture conditions. During the dark phase of the cycle we observed in both cultures a remarkable reduction of both electron transfer and photosynthesis rates, while maximum quantum yield of PSII remained stable (0.74 ± 0.01). This indicated that the reduction in both photosynthesis rate and maximum electron transport rate (ETRmax) observed during the dark phase were not due to damage of PSII reaction centers but rather to reduced flow of electron subsequent to PSII. Recovery of the fully photosynthetic capacity required about 1 h of illumination.

Response of photosynthetic active radiation interception, dry matter accumulation, and grain yield to tillage in two winter wheat genotypes

ABSTRACT To examine the effects of winter wheat genotypes on dry matter (DM) accumulation and grain yield (GY) under no-tillage conditions in North China Plain (NCP), a field experiment was conducted using the genotypes Tainong 18 (F) and Jimai 22 (J). Two tillage systems were tested, conventional tillage (CT) and no-tillage (NT) during the 2015/2016 and 2016/2017 winter wheat growing seasons. Genotypes and tillage systems were compared regarding DM accumulation, GY, leaf area index (LAI), photosynthetic active radiation (PAR) interception, and flag leaf fluorescence parameters. LAI, PAR interceptions, and flag leaf fluorescence parameters were significantly higher under CT than under NT conditions. This suggests that the efficiency of light harvesting by the PSII reaction center of leaves can increase due to CT. DM accumulation was significantly higher under CT than under NT. Spike DM accumulation was higher in F than in J in the heading and the filling stages. In both growing seasons, GY was significantly higher under CT than under NT, GY of NTF was significantly higher than that of NTJ, which was due to a significantly higher number of kernels per spike. This indicates that genotype F can compensate for a low GY due to NT.

Effects of directional planting on light environment and leaf photosynthesis of summer maize population

To improve light environment, photosynthetic capacity, and thus the yield of maize, the effects of directional planting on light distribution in canopy and photosynthetic characteristics of ear leaves, as well as the performance of PSII that closely related with photosynthetic characteristics and reflected by the rapid chlorophyll fluorescence kinetic curves were examined in Zhengdan 958 maize variety. The results showed that the orientation of leaves remarkably affected photosynthetically active radiation (PAR) interception of ear leaves, with PAR interception of ear leaves in southward treatment being 271.8% higher than that under northward treatment. The orientation of leaves affec-ted photosynthetic light use efficiency of ear leaves under high and low light conditions. The southward treatment increased net photosynthetic rate (Pn) under saturated light in ear leaves, indicating that the use efficiency to high light was enhanced in leaves of southward treatment. In contrast, the northward treatment increased the apparent quantum yield (α) of ear leaves, indicating leaves in southward treatment adapted the light-limited environment. During the early stage after anthesis, the performance of PSII electron donor side and electron acceptor side was significantly improved, and thus enhanced the performance of PSII reaction center (PIABS) and fluorescence photochemical quenching coefficient (Ψo) in ear leaves of southward treatment. The increase of quantum yield of electron transfer (φEo) indicated the enhancement of transfer performance of electrons from photosystem 2 (PSII) to photosystem 1 (PSI) in leaves of southward treatment. The photosynthetic performance of ear leaves showed a trend of southward > eastward > westward > northward during the early stage after anthesis. Forty days after anthesis, the use efficiency to high light decreased in ear leaves of southward treatment, but the ear leaves of southward treatment showed high use efficiency to low light, which changed the trend of photosynthetic performance of ear leaves to northward > westward > eastward > southward. In summary, northward and eastward treatments improved the light distribution in canopy, the PAR interception of ear leaves, the capacity of photosynthesis and dry matter production, and consequently increased the yield of summer maize.

A New Light on Photosystem II Maintenance in Oxygenic Photosynthesis.

Life on earth is sustained by oxygenic photosynthesis, a process that converts solar energy, carbon dioxide, and water into chemical energy and biomass. Sunlight is essential for growth and productivity of photosynthetic organisms. However, exposure to an excessive amount of light adversely affects fitness due to photooxidative damage to the photosynthetic machinery, primarily to the reaction center of the oxygen-evolving photosystem II (PSII). Photosynthetic organisms have evolved diverse photoprotective and adaptive strategies to avoid, alleviate, and repair PSII damage caused by high-irradiance or fluctuating light. Rapid and harmless dissipation of excess absorbed light within antenna as heat, which is measured by chlorophyll fluorescence as non-photochemical quenching (NPQ), constitutes one of the most efficient protective strategies. In parallel, an elaborate repair system represents another efficient strategy to maintain PSII reaction centers in active states. This article reviews both the reaction center-based strategy for robust repair of photodamaged PSII and the antenna-based strategy for swift control of PSII light-harvesting (NPQ). We discuss evolutionarily and mechanistically diverse strategies used by photosynthetic organisms to maintain PSII function for growth and productivity under static high-irradiance light or fluctuating light environments. Knowledge of mechanisms underlying PSII maintenance would facilitate bioengineering photosynthesis to enhance agricultural productivity and sustainability to feed a growing world population amidst climate change.

Effects of temperature on the photosynthetic performance in mature thalli of the red alga Gelidium amansii (Gelidiaceae)

Gelidium amansii, a commercial marine red alga, yields agar of excellent quality, but thus far its cultivation has been unsuccessful. Effects of temperature, especially high temperature, on the photosynthetic performance of this alga are poorly understood. The responses of net O2 evolution and pigment content in mature thalli exposed to various temperatures of 14, 18, 22, 26, and 30 °C for 2 days were studied. Fast Chl-a fluorescence kinetics and RuBPCase activity of plants exposed to 30 °C for 6, 12, 24, and 48 h were also determined. The results showed that 14–26 °C favored the net O2 evolution of G. amansii, which ranged from 0.50 to 1.53 μmol O2∙g−1∙min−1. High temperature (30 °C) decreases the contents of Chl-a and carotenoids. Phycobiliproteins increased in the thalli treated for 12 h and longer at 30 °C, suggesting that phycobiliproteins might play an important role in alleviation of oxidative damage caused by high temperature. In addition, the carbon assimilation process, light harvesting pigments, PSII reaction centers, and PSII acceptor side of G. amansii were damaged under 30 °C at 12 h; however, the PSII acceptor side was relatively stable during the initial 24 h treatment. At 30 °C both PSII photochemistry and carbon assimilation were negatively affected and impacted the photosynthetic response of G. amansii. RuBPCase activity showed a stronger positive correlation with photosynthetic oxygen release than the photochemistry. The results of this study confirm that the optimal temperature for G. amansii photosynthesis is between 22 °C to 26 °C and high temperatures of 30 °C and above limit G. amansii growth in near-shore waters.

Cationic penetrating antioxidants switch off Mn cluster of photosystem II in situ.

Mitochondria-targeted antioxidants (also known as 'Skulachev Ions' electrophoretically accumulated by mitochondria) exert anti-ageing and ROS-protecting effects well documented in animal and human cells. However, their effects on chloroplast in photosynthetic cells and corresponding mechanisms are scarcely known. For the first time, we describe a dramatic quenching effect of (10-(6-plastoquinonyl)decyl triphenylphosphonium (SkQ1) on chlorophyll fluorescence, apparently mediated by redox interaction of SkQ1 with Mn cluster in Photosystem II (PSII) of chlorophyte microalga Chlorella vulgaris and disabling the oxygen-evolving complex (OEC). Microalgal cells displayed a vigorous uptake of SkQ1 which internal concentration built up to a very high level. Using optical and EPR spectroscopy, as well as electron donors and in silico molecular simulation techniques, we found that SkQ1 molecule can interact with Mn atoms of the OEC in PSII. This stops water splitting giving rise to potent quencher(s), e.g. oxidized reaction centre of PSII. Other components of the photosynthetic apparatus proved to be mostly intact. This effect of the Skulachev ions might help to develop in vivo models of photosynthetic cells with impaired OEC function but essentially intact otherwise. The observed phenomenon suggests that SkQ1 can be applied to study stress-induced damages to OEC in photosynthetic organisms.

Photoinhibition of Photosystem I Provides Oxidative Protection During Imbalanced Photosynthetic Electron Transport in Arabidopsis thaliana.

Photosynthesis involves the conversion of sunlight energy into stored chemical energy, which is achieved through electron transport along a series of redox reactions. Excess photosynthetic electron transport might be dangerous due to the risk of molecular oxygen reduction, generating reactive oxygen species (ROS) over-accumulation. Avoiding excess ROS production requires the rate of electron transport to be coordinated with the capacity of electron acceptors in the chloroplast stroma. Imbalance between the donor and acceptor sides of photosystem I (PSI) can lead to inactivation, which is called PSI photoinhibition. We used a light-inducible PSI photoinhibition system in Arabidopsis thaliana to resolve the time dynamics of inhibition and to investigate its impact on ROS production and turnover. The oxidation state of the PSI reaction center and rates of CO2 fixation both indicated strong and rapid PSI photoinhibition upon donor side/acceptor side imbalance, while the rate of inhibition eased during prolonged imbalance. PSI photoinhibition was not associated with any major changes in ROS accumulation or antioxidant activity; however, a lower level of lipid oxidation correlated with lower abundance of chloroplast lipoxygenase in PSI-inhibited leaves. The results of this study suggest that rapid activation of PSI photoinhibition under severe photosynthetic imbalance protects the chloroplast from over-reduction and excess ROS formation.

Sensing β-carotene oxidation in photosystem II to master plant stress tolerance.

Stressful environmental conditions lead to the production of reactive oxygen species in the chloroplasts, due to limited photosynthesis and enhanced excitation pressure on the photosystems. Among these reactive species, singlet oxygen (1 O2 ), which is generated at the level of the PSII reaction center, is very reactive, readily oxidizing macromolecules in its immediate surroundings, and it has been identified as the principal cause of photooxidative damage in plant leaves. The two β-carotene molecules present in the PSII reaction center are prime targets of 1 O2 oxidation, leading to the formation of various oxidized derivatives. Plants have evolved sensing mechanisms for those PSII-generated metabolites, which regulate gene expression, putting in place defense mechanisms and alleviating the effects of PSII-damaging conditions. A new picture is thus emerging which places PSII as a sensor and transducer in plant stress resilience through its capacity to generate signaling metabolites under excess light energy. This review summarizes new advances in the characterization of the apocarotenoids involved in the PSII-mediated stress response and of the pathways elicited by these molecules, among which is the xenobiotic detoxification.

Photosynthetic characteristics and chlorophyll a fluorescence transient in Lonicera japonica under drought stress

Chlorophyll a fluorescence transient can reveal the degree of thermal damage to plant photosynthetic organs and provide much physiological information about PSII fleetly. However, it is not clear how it reveals photosynthetic mechanisms of plants under decreasing soil moisture. In this paper, the photosynthesis characteristics and photosystem II (PSII) activities in leaves of Lonicera japonica under 11-level soil moisture were explored by gas-exchange analysis and JIP test. Both parameters of net photosynthetic rate (Pn) and water use efficiency (WUE) exhibited an increasing–decreasing trend with the decreasing relative soil water content (RSWC). At 29.7% ≤ RSWC≤ 79.6%, the main reason for Pn decrease was stomatal limitation; at 36.4% ≤ RSWC≤ 55.1%, both Pn and WUE were higher than the average. When RSWC was below 29.7%, Pn, Tr, and WUE significantly decreased. Meanwhile, a JIP-test analysis revealed a distinct K peak with decreases in the quantum yield and energy distribution ratio (ψo or φEo) and increases in the K phase (Wk) and J phase (Vj) at RSWC≤ 29.7%, indicating that severe drought (RSWC≤ 29.7%) damaged the leaf oxygen-evolving complex, caused the accumulation of Q A − in PSII electron acceptors, and, therefore, hindered the progression of the PSII electron transport chain. In addition, at RSWC≤ 29.7%, the absorption flux per CS (ABS/CSo) increased, while the phenomenological energy fluxes for TRo/CSo, ETo/CSo, RC/CSo decreased, indicating that the PSII reaction center was damaged and excessive light energy was accumulated. Therefore, the main reason for the decrease of photosynthetic efficiency was non-stomatal limitation at RSWC≤ 29.7%, and the maximum moisture deficit for the growth of Lonicera japonica is RSWC= 29.7%.

Impaired PSII Proteostasis Promotes Retrograde Signaling via Salicylic Acid.

Photodamage of the PSII reaction center (RC) is an inevitable process in an oxygen-rich environment. The damaged PSII RC proteins (Dam-PSII) undergo degradation via the thylakoid membrane-bound FtsH metalloprotease, followed by posttranslational assembly of PSII. While the effect of Dam-PSII on gene regulation is described for cyanobacteria, its role in land plants is largely unknown. In this study, we reveal an intriguing retrograde signaling pathway by using the Arabidopsis (Arabidopsis thaliana) yellow variegated2-9 mutant, which expresses a mutated FtsH2 (FtsH2G267D) metalloprotease, specifically impairing its substrate-unfolding activity. This lesion leads to the perturbation of PSII protein homeostasis (proteostasis) and the accumulation of Dam-PSII. Subsequently, this results in an up-regulation of salicylic acid (SA)-responsive genes, which is abrogated by inactivation of either an SA transporter in the chloroplast envelope membrane or extraplastidic SA signaling components as well as by removal of SA. These results suggest that the stress hormone SA, which is mainly synthesized via the chloroplast isochorismate pathway in response to the impaired PSII proteostasis, mediates the retrograde signaling. These findings reinforce the emerging view of chloroplast function toward plant stress responses and suggest SA as a potential plastid factor mediating retrograde signaling.

Yield losses and electron transport rate as indicators of thermal stress in Fucus serratus (Ochrophyta)

Global warming is driving shifts in the geographical distribution of seaweeds, such as the contraction of the Southern limit of cold-temperate Atlantic macroalgae, including the foundation intertidal fucoid Fucus serratus. However, the ecophysiological mechanisms explaining the decline of seaweeds is not fully understood. In vivo chlorophyll a fluorescence is used as an estimator of photosynthetic activity and physiological state of seaweeds. Collected at its southern distribution limit, Fucus serratus was cultured for two weeks in different temperatures (from 8 to 30 °C with increase of 2 °C). Growth rate, reproductive stage and fluorescence parameters were measured as maximal quantum yield (Fv/Fm), Electron transport rate (ETR), non-photochemical quenching (NPQ) and the yield losses (YNO and YNPQ).At high temperature, the photosynthetic responses mimic the pattern of shade adapted plants, i.e., increase of photosynthetic efficiency (αETR) and decrease of irradiance for saturation of photosynthetic rate (EkETR). At low irradiances (<66 μmol m−2 s−1) and mainly at extreme temperatures (high and low), yield loss is dominated by YNO, suggesting heat dissipation due to closure of PSII reaction centers. However, at high irradiances (1150–1500 μmol m−2 s−1) and in a wide range of temperatures (8–28 °C), yield loss is mainly due to Y(NPQ), related to the energy dissipation as heat throughout regulated photoprotective NPQ mechanisms such as the xanthophyll cycle. The optimal temperature for growth was 14–20 °C as ETRmax, whereas Fv/Fm maintains high values in a wider range of temperatures compared to ETRmax and Yield losses, indicating that Fv/Fm is not a good parameter to discriminate the effect of temperature on algal production. We discuss the use of ETR and yield losses to relate photosynthesis with growth, survival or reproduction under stress conditions. Our study shows that warming can increase photoprotection through yield loss and decrease the photosynthetic activity in macroalgae.

Extra-plastidial degradation of chlorophyll and photosystem I in tobacco leaves involving 'senescence-associated vacuoles'.

Chlorophyll (Chl) loss is the main visible symptom of senescence in leaves. The initial steps of Chl degradation operate within the chloroplast, but the observation that 'senescence-associated vacuoles' (SAVs) contain Chl raises the question of whether SAVs might also contribute to Chl breakdown. Previous confocal microscope observations (Martínez etal., 2008) showed many SAVs containing Chl. Isolated SAVs contained Chla and b (with a Chla/b ratio close to 5) and lower levels of chlorophyllidea. Pheophytina and pheophorbidea were formed after the incubation of SAVs at 30°C in darkness, suggesting the presence of Chl-degrading activities in SAVs. Chl in SAVs was bound to a number of 'green bands'. In the most abundant green band of SAVs, Western blot analysis showed the presence of photosystem I (PSI) Chl-binding proteins, including the PsaA protein of the PSI reaction center and the apoproteins of the light-harvesting complexes (Lhca1-4). This was confirmed by: (i) measurements of 77-K fluorescence emission spectra showing a single emission peak at around 730nm in SAVs; (ii) mass spectrometry of the most prominent green band with the slowest electrophoretic mobility; and (iii) immunofluorescence detection of PsaA in SAVs observed through confocal microscopy. Incubation of SAVs at 30°C in darkness caused a steady decrease in PsaA levels. Overall, these results indicate that SAVs may be involved in the degradation of PSI proteins and their associated chlorophylls during the senescence of leaves.

Response of leaf photosynthetic characteristics of Syringa oblata and Syringa reticulata var. mandshurica to chilling stress

Syringa species not only have good ornamental properties but also play an important role in the landscaping and environmental purification of cities. To investigate the chilling stress resistance of Syringa oblata Lindl. and Syringa reticulata var. mandshurica and provide theoretical grounds for the practical cultivation of Syringa species, in vitro leaves were used to study photosynthetic gas exchange parameters and chlorophyll fluorescence parameters. After nine hours of chilling, decreasing rates of net photosynthesis, stomatal conductance, and transpiration in S. reticulata var. mandshurica leaves were significantly greater than that of the S. oblata, while intercellular CO2 concentrations in S. oblata leaves were higher than those in S. reticulata var. mandshurica. The quantum yield of PSII reaction center (ФPSII) declined in S. reticulata and light capture efficiency (Fv′/Fm′) was stable. However, reduction percentages of Fv′/Fm′, ФPSII, and Fv/Fm in S. oblata were significant higher than those of S. reticulata var. mandshurica. After nine hours of chilling, the relative variable fluorescence of VJ and VI of S. oblata increased and the increasing rate of VJ was greater than VI. In contrast, the change of VJ and VI in S. reticulata var. mandshurica leaves was relatively small. This suggests that chilling primarily damaged the electron transport process of QA to QB at the receptor site of the PSII reaction center. Photosynthetic capacity of S.oblata was more sensitive to chilling stress compared to S. reticulate var. mandshurica, which the limitations were mainly due to non-stomatal factors such as the decrease in electron transport efficiency, activity in the PSII reaction center, and the destruction of the photodamage defense system.

Quantitative assessment of the high-light tolerance in plants with an impaired photosystem II donor side

Photoinhibition is the light-induced down-regulation of photosynthetic efficiency, the primary target of which is photosystem II (PSII). Currently, there is no clear consensus on the exact mechanism of this process. However, it is clear that inhibition can occur through limitations on both the acceptor- and donor side of PSII. The former mechanism is caused by electron transport limitations at the PSII acceptor side. Whilst, the latter mechanism relies on the disruption of the oxygen-evolving complex. Both of these mechanisms damage the PSII reaction centre (RC). Using a novel chlorophyll fluorescence methodology, RC photoinactivation can be sensitively measured and quantified alongside photoprotection in vivo This is achieved through estimation of the redox state of Q A, using the parameter of photochemical quenching in the dark (qPd). This study shows that through the use of PSII donor-side inhibitors, such as UV-B and Cd2+, there is a steeper gradient of photoinactivation in the systems with a weakened donor side, independent of the level of NPQ attained. This is coupled with a concomitant decline in the light tolerance of PSII. The native light tolerance is partially restored upon the use of 1,5-diphenylcarbazide (DPC), a PSII electron donor, allowing for the balance between the inhibitory pathways to be sensitively quantified. Thus, this study confirms that the impact of donor-side inhibition can be detected alongside acceptor-side photoinhibition using the qPd parameter and confirms qPd as a valid, sensitive and unambiguous parameter to sensitively quantify the onset of photoinhibition through both acceptor- or donor-side mechanisms.