above, thereby providing an even more sensitive technique than quantum yield of PSII for assessing high-temperature tolerance. Among the cultivars tested, the membrane fluidity of Acala Maxxa and ST213 was not significantly affected by increas - ing temperatures. Membrane leakage in the other cultivars increased in a linear pattern with increas- ing high temperature. This method of screening for high-temperature tolerance has been used in soybean (Glycine max L.; Martineau et al., 1979), cowpeas (Vigna unguiculata L.; Ismail and Hall, 1999), wheat (Triticum aestivum L.; Shanahan et al., 1990), holly (Ilex opaca Aiton; Ruter, 1993), turfgrass species (Wallner et al., 1982), and cotton (Rahman et al., 2003). Data from our studies support the findings of Rahman et al. (2003) that membrane leakage of cotton cultivars respond differently across increas- ing temperature regimes allowing screening for high-temperature tolerance. In addition, our studies corroborate these findings with two additional meth- ods, photosynthesis and quantum yield of PSII, for evaluating temperature tolerance.
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Microorganisms confined to annual sea ice in the Southern Ocean are exposed to highly variable oxygen and carbonate chemistry dynamics because of the seasonal increase in biomass and limited exchange with the underlying water column. For sea-ice algae, physio- logical stress is likely to be exacerbated when the ice melts; however, variation in carbonate speciation has rarely been monitored during this important state-transition. Using pulse amplitude modulated fluorometry (Imaging-PAM, Walz), we documented in situ changes in the maximum quantum yield of photosystem II ( F v / F m ) of sea-ice algae melting out into
Photosystem II (PSII) herbicides have been detected in nearshore tropical waters such as those of the Great Barrier Reef and may add to the pressure posed by runoff containing sed- iments and nutrients to threatened seagrass habitats. There is a growing number of studies into the potential effects of herbicides on seagrass, generally using large experimental set- ups with potted plants. Here we describe the successful development of an acute 12-well plate phytotoxicity assay for the PSII herbicide Diuron using isolated Halophila ovalis leaves. Fluorescence images demonstrated Diuron affected the entire leaf surface evenly and responses were not influenced by isolating leaves from the plant. The optimum expo- sure duration was 24 h, by which time the inhibition of effective quantum yield of PSII ( Δ F/F m ’ ) was highest and no deterioration of photosystems was evident in control leaves.
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by Diuron was between 3.7 and 7.7 hours for the four species. Although this inhibition is comparable to the 2 to 4 hours observed for coral symbionts , the response of microalgae is faster still, often reaching maximum inhibition within 20 min of exposure [24,53]. In agricultural weeds, PSII herbicides are taken up by the roots and transported through the vascular system to PSII in the leaves. The same mechanism may occur in seagrass, although Schwarzschild et al.  demonstrated low sensitivity of the seagrass Zostera marina exposed to Atrazine through the root-rhizome complex, concluding that these herbicides are more likely to be rapidly transported directly across the semi-permeable cell walls of leaves. Hexazinone was the slowest-acting PSII herbicide tested; taking four-times longer to reach 90% maximum inhibition compared with Diuron and was more than 6-fold slower than Atrazine and Tebuthiuron. A similar result was observed for the gradual effect of Hexazinone (2-3 hours rather than minutes for Diuron) on diatoms and green algae [24,25]. The reason for Table 4. Herbicide concentrations that inhibit effective quantum yield in seagrass after 72 h.
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Although the interaction between genotypes and water levels when averaged across the two water levels was found to be non-significant, there were detecta- ble differences in the performance of genotypes under drought stress. Drought stress consistently and significantly reduced the efficiency of photosystem II, Chlorophyll content of the tested genotypes (Table 4) though the effect varied in both improved and local genotypes. The reduction in efficiency of photosystem II was supported by an increase in non-photochemical quenching (NPQ) as shown by a strong negative correlation (r = −0.76, P ≤ 0.001), suggesting that a greater portion of the energy was thermally dissipated (Figure 1 and Table 5).  reported that an increase in NPQ protects the plant against photo damage and this reduces the quantum yield of photosystem II. NPQ plays a key role in the protection of PSII from photodamage. NPQ is considered as an indicator of excess excitation energy . The maximum quantum efficiency of photosystem II provides a measure of the rate of linear electron transport, hence, an indica- tion of overall photosynthetic capacity .
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Corals can obtain up to 100% of their daily carbon requirements from photosynthesis of their endosymbiotic zooxanthellae (e.g. Grottoli et al., 2006; Muscatine et al., 1981). Under normal conditions, light is absorbed by antenna pigments of the photosynthetic apparatus in zooxanthellae chloroplasts. Excitation energy is transferred to the reaction centers of photosystem II (PS II) and down the photosynthetic electron transport chain where the primary photochemical reactions of the cell produce reducing power and adenosine triphosphate (ATP) (Krause and Weis, 1991). Photoinhibition occurs when photosynthetic electron transport decreases and absorption of excitation energy increases (Osmond, 1994; Smith et al., 2005). In zooxanthellae, excess excitation energy can produce reactive oxygen species (ROS), ultimately affecting the quantum yield of PS II fluorescence ( F v / F m ; F v , variable
T o compensate for changes in light intensity or spectral quality, plants have developed several short-term and long- term mechanisms to regulate the amount of light that is captured by each photosystem (1). One important long-term adaptation strategy of plant organisms involves the complex expression regulation of various nuclear-encoded light harvesting complex (Lhcb) genes (1). All levels of LHCII gene expression are targeted by regulation mechanisms (2–5) which rely on a com- plex retrograde and anterograde communication between plas- tid, nucleus, and cytosol (6). The cytosolic translation repressor NAB1, which was identified in a Chlamydomonas reinhardtii light acclimation mutant (4), is the center of interest within this work. NAB1 harbors 2 RNA-binding motifs and 1 of these motifs, located at the N terminus, belongs to the highly conserved family of CSD (cold shock domain) domains. Proteins containing a CSD motif are referred to as Y-box proteins and eukaryotic members of this large family generally contain a second auxiliary RNA-binding domain, which modulates the RNA affinity of the protein but can be dispensable for selective RNA recognition (7). In the case of NAB1, the CSD motif is combined with a C-terminal RRM (RNA recognition motif) domain, which was demonstrated not to be essential for selective RNA recognition (4). It was shown that NAB1 binds to the mRNA of LHCBM (major light-harvesting complex of photosystem II) genes, thereby preventing translation via sequestration of the message in translationally silent messenger ribonucleoprotein complexes
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membrane preparations that are enriched in photosystem II activity and are routinely used to evaluate cofactor requirements. Consistent with the previous determinations of the 18 O exchange behavior in thylakoids, the initial 18 O exchange measurements of native PSII membranes at m/e = 34 (which is sensitive to the 16 O 18 O product) show that the ‘fast’ and ‘slowly’ exchanging substrate-waters are bound to the catalytic site in the S 3 state, immediately prior to O 2 release. Although the slowly exchanging water is
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The biogenesis of photosynthetic complexes is assisted by a growing number of trans-acting factors in both chloroplasts and cyanobacteria. We have previously shown that the periplasmic PratA factor from Synechocystis sp. PCC 6803 (Synechocystis 6803) is required for adequate C-terminal processing of the PsbA (D1) subunit of photosystem II (PSII) supporting the idea that the early steps of PSII assembly occur at the plasma mem- brane. Here we report on the molecular analysis of the interac- tion between PratA and the D1 protein. Both yeast two-hybrid and glutathione S-transferase pulldown assays revealed that PratA binds to the soluble forms of both mature and precursor D1 C-terminal regions. In agreement with that finding, the binding region was mapped to amino acid positions 314 –328 of D1 by applying a peptide-scanning approach. Approximately 10 –20% of the soluble PratA factor was found to be associated with membranes in a D1-dependent manner. Sucrose density gradient centrifugations allowed the identification of a specific membrane subfraction that contains both PratA and D1 and which might represent a transfer and/or connecting region between plasma and thylakoid membrane. Imaging data obtained with enhanced cyan fluorescent protein-labeled D1 protein in wild-type and pratA mutant backgrounds further supported this notion.
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The analysis of fast ChlF transient was applied in numerous studies in crop plants, e.g. to as- sess the environmental effects in wheat, such as drought (Živčák et al. 2008a), high temperature (Brestič et al. 2012), light stress (Kalaji et al. 2012, Živčák et al. 2014). The JIP-test analyses were applied several times also in studies dealing with nitrogen deficiency in plants and the effect of poor nitrogen supply on PS II is recently well described (Lu et al. 2001, Redillas et al. 2011, Li et al. 2012, etc.). Although in many of published works the rapid ChlF is denoted as a useful tool for assessing the physiological effects of nitrogen deficiency on plants, there is still a lack of data on the usefulness of the method in assessment of plant photosynthetic performance in crop trials with different nitrogen supply. In our study we present the results obtained in three growth stages of winter wheat exposed either to full or partial nitrogen deficiency compared to plants grown in normal nitrogen supply as well as in the excess of nitrogen. We focused on practical aspects and we tried to suggest the useful way to utilize the rapid chlorophyll a fluorescence method for de- tecting nitrogen deficiency or monitoring effects of nitrogen nutrition on wheat photosynthetic performance.
Barley (Hordeum vulgare L.) originates in the Eastern Mediterranean region where plants experi- ence many abiotic stresses in the ﬁeld. Its production has become more intense and complex in recent years, and thus crop managers need to better under- stand factors eﬀecting the yield of this plant. This will consist of trials aimed at estimating responses of barley to different unfavourable conditions.
Photosynthesis is a fundamental natural process that en- ables the conversion of solar energy into storable chemical energy under the release of molecular oxygen derived from water. The essential processes of photosynthetic water splitting [1,2] take place in a membrane-bound protein assembly denoted as Photosystem II (PS II). The structure of the PS II core at 3.8 ˚ A resolution  is shown in Fig. 1. The PS II core comprises two different types of pigment-protein complexes with distinct functional designation: i) the light-harvesting complexes CP43 and CP47 and ii) the reaction center (RC) represented by the central heterodimer D1/D2, where a light-triggered charge separation initiates the photochemical processes in PS II [1,2].
quantum yield values than those of pure CdSe QDs  may be due to the fact that magnetic nanoparticles facilitate the electron hole recombination and enhance the emission from the quantum dots. It is known that the emission from quantum dots is due to the electron hole recombination from the dark exciton state, which is a forbidden state. The electron has to flip its spin in order to recombine with the hole. For this reason, the life time of the quantum dots becomes long and the Stockes shift is large. The presence of magnetic particles attached to the CdSe enhances the coupling between the hole and the electron. This means the emission is enhanced greatly upon loading CdSe QDs on Fe 3 O 4 nanocrystals.
Isotope-exchange experiments using 18 O labelled water 15 show that in Sr-substituted PS II, the exchange rate for the slowly exchanging water increases by a factor of B4 (in all S states). This increase is totally consistent with the Sr–W3 bond being approx. 0.2 Å longer, and thus more weakly bound, than in the Ca structure. Furthermore, while our modelling strongly suggests that W4 is deprotonated and bound to Sr in the form of a hydroxide, it also reveals that W4 undergoes H-bonding with W3 and Tyr161. This H-bonding interaction may provide insight to understanding the reduction, by a factor of 3–10, in the oxygen- evolving activity of Sr-substituted PS II compared to Ca-based PS II, 16 as it is likely to impact on the ability of W3 to form an O–O bond with the other substrate water, O5, in the higher S states, and consequently, the oxygen-evolving activity of the WOC. Fig. 3 Calculated WOC structure showing H-bonding network connecting Sr with Tyr161, His190 and Gln165. Inclusion of these residues, and the associated H-bonding network, yields good agreement with the crystallographic Sr–W3 and Sr–W4 distances (see Table 1). For clarity, only metal atoms, oxo bridges, and water–ligand O atoms are shown explicitly. 17
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light’ treatments that alternated between high and low conditions every 5 days. In the vari- able treatments, the shade-tolerant coral Pachyseris speciosa displayed cycles of rapid declines in maximum quantum yield during high-light and subsequent recoveries during low-light, showing photoacclimation at a time scale of 3–5 days. In contrast, the shallow- water coral Acropora millepora showed slow (>20 days) photoacclimation, and minimal changes in photosynthetic yields despite contrasting light exposure. However, growth (change in buoyant weight) in A. millepora was significantly slower under variable light, and even more so under low-light conditions, compared with high-light conditions. The responses of yields in P. speciosa match their preference for low-light environments, but suggest a vulnerability to even short periods of high-light exposure. In contrast, A. millepora had better tolerance of high-light conditions, however its slow photoacclimatory responses limit its growth under low and variable conditions. The study shows contrasting photoaccli- matory responses in variable light environments, which is important to identify and under- stand as many coastal and midshelf reefs are becoming increasingly more turbid, and may experience higher variability in light availability.
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3.3. Chlorophyll a Fluorescence Transient Measurements in the Presence of Triterpenes To corroborate the interaction site of triterpenes at PSII, freshly lysed chloroplasts were incubated for 5 min in the dark at room temperature at 200 µM of the compounds, 10 µM DCMU, and 0.8 M Tris, which were used as positive controls (Figure 6). The thylakoids control showed a polyphasic fluorescence curve with OJIP se- quence of transients similar to that previously described for plants, green algae, and cyanobacteria ; the ad- dition of 10 µM of the herbicide DCMU induces a fast rise of the fluorescence yield during the first 2 ms of il- lumination; it transforms the regular OJIP sequence into an OJ curve (Figure 6) . When the thylakoids are treated with Tris, a well-known donor side of PS II inhibitor  (Figure 6), the fluorescence induction curve change and the maximum fluorescence yield was reduced and the K-band appears, a rapid rise to a maximum (at 300 µs) followed by a decreased fluorescence yield to level close to F 0 (Table 2). All other steps, J and I are ab-
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46. Porsbring, T., Backhaus, T., Johansson, P., Kuylenstierna, M. & Blanck, H. Mixture toxicity from photosystem II inhibitors on microalgal community succession is predictable by concentration addition. Environ. Toxicol. Chem. 29, 2806–2813 (2010). 47. Berenbaum, M. C. The expected effect of a combination of agents: the general solution. J. Theor. Biol. 114, 413–431 (1985). 48. Moss, A., Brodie, J. & Furnas, M. Water quality guidelines for the Great Barrier Reef World Heritage Area: a basis for development
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Oxygenic photo synthetic organisms adjust to changes in the abundance and spectrum o f available light. They achieve this by moving their antenna systems (Section 1.4.1) between PSII and PSI. The movement o f these mobile LHCs are termed state transitions. State transitions are performed to maintain a balance in the electron transport between the two complexes (recently reviewed by Haldrup et al, 2001; Wollman 2001), and also as a photoprotection mechanism minimising the potential for damage o f the PSII complex in high light conditions (Schroda et a l, 1999). The redox state o f the plastoquinone pool is generally considered to be a regulatory point. When the two photosystems are harvesting equal amounts o f light energy the electrons transported from PSII to PSI are balanced. If the available light favours one photosystem over the other this balance is disrupted. State transitions function to redistribute the available excitation energy to the complex that it favours less. State 1 occurs when the available Ught preferentially excites PSI. In this situation the antenna system is bound to PSII. State 2 is induced by illumination that predominantly excites PSII. In this situation PSI cannot keep up with the electrons from PSII which accumulate between the two photosystems. Under these conditions the light antenna detach from PSII and associate with PSI. Therefore the light-limited photosystem receives more energy.
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and F685 is makes only a small contribution to the fluorescence spectrum—at the lowest temperatures a small contribution from the highly delocalised B state of CP43 can be identified at ~684 nm by its strong negative CPL. It is likely that although the lowest state of CP43 is at <685 nm in many centres, the majority of these are able to transfer to the reaction centre even at 0 K, making the yield of F685 small compared to that of F695 or F689. At higher temperatures, (pseudo-‐) thermal equilibration of the excited states means that fewer excitations are trapped at the two lower sites and in many complexes the equilibration includes the photochemically-‐active RC, reducing the overall yield dramatically. F685 is the dominant emission band in thermally equilibrated complexes (above ~100 K), simply because the combined oscillator strength of chlorophylls emitting at this energy greatly outweighs those at lower energies (Figure 6.7).
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The variability in symbiont photosynthetic activity between species, experimental conditions and temperature treatments was broadly consistent with the observed differences in PS1, PS2 and Rubisco protein contents. In this study, temperature stress caused a decrease in non-photochemical quenching capacity of PS2 for coral symbionts, whereas incubation under high light intensity increased Y(NPQ). Maximum photochemical yield was lower in colonies grown at high light intensity but high light levels also caused up-regulation of photoprotective mechanisms. Congruent with these changes, the protein assays showed that for both species RbcL (Rubisco) content was highest following incubation under high light. Moreover, for Turbinaria, content of PsbA (PS2) was lower under the high-light treatment, consistent with a down- regulation or impairment of PS2 function. These results support the general consensus that PS2 is the primary site of photodamage when light intensity is excessive . Overall, our results regarding the function of PS2 are largely consistent with fluorometry-based studies of coral photosynthesis and photoprotection [20,51,52]. The novelty of our study lies in simultaneous assessment of PS1 and PS2 function and quantification of the content of key protein components of the photosynthetic machinery.
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