Active Reaction Centers Research Articles

Effects of salinity on photosynthesis in maize probed by prompt fluorescence, delayed fluorescence and P700 signals

Salinity affects the photochemical activity of photosystems (PSs) in plants. However, it is not clear which components in the photosynthetic processes are sensitive to salt stress. In this study, simultaneous measurements of prompt chlorophyll a fluorescence (PF), delayed chlorophyll a fluorescence (DF) and modulated 820nm reflection (MR) were employed to investigate the effect of salt stress on the entire photosynthetic electron chain in maize leaf tissues, including the PSII donor side, electron transfer between PSII and PSI, and the PSI acceptor side. For the PF transients, salt stress induced a pronounced K-band; a positive L-band; a significant reduction in PIABS, RC/CSO, TRO/ABS, and ETO/TRO; and a significant increase in ABS/RC and REO/ETO. Analysis of the normalized MR kinetics showed that the re-reduction kinetics of P700+ and PC+ became slower and occurred at later times under salt treatment. For the DF signals, a decrease in the amplitude of the DF induction curve and a change in the shape of both the induction curve and the decay curve were observed under salt stress. These results suggest that salt stress decreased the number of active PSII reaction centers, impaired the connectivity between independent PSII units, destroyed the oxygen-evolving complex, and limited electron transport beyond the primary quinone acceptor QA−. In contrast, the photochemical activity of PSI was largely unscathed. The results obtained from measuring three simultaneous signals were in good agreement.

Photosynthetic Efficiency as Bioindicator of Environmental Pressure in A. halleri.

In earlier ecophysiological studies that were conducted on Arabidopsis halleri plants, scientists focused on the mechanisms of Cd and Zn hyperaccumulation but did not take into consideration the environmental factors that can significantly affect the physiological responses of plants in situ. In this study, we investigated A. halleri that was growing on two nonmetalliferous and three metalliferous sites, which were characterized by different environmental conditions. We compared these populations in order to find differences within the metallicolous and nonmetallicolous groups that have not yet been investigated. The concentrations of several elements in the plant and soil samples also were investigated. To our knowledge, the concentration and fluorescence of chlorophyll were measured for A. halleri in situ for the first time. Our study confirmed the hyperaccumulation of Cd and Zn for each metallicolous population. For the metallicolous populations, the inhibition of parameters that describe the efficiency of the photosynthetic apparatus with increasing accumulations of heavy metals in the shoots also was observed. It was found that the nonmetallicolous plant populations from the summit of Ciemniak Mountain had larger antenna dimensions and chlorophyll content but a lower percentage of active reaction centers. To our knowledge, in this study, the internal high physiological diversity within the populations that inhabit metalliferous and nonmetalliferous sites is presented for the first time.

Photochemistry of thylakoid membranes in two pea cultivars with different leaf colouration

Photochemical features of thylakoid membranes and chlorophyll content were investigated in two pea (Pisum sativum L.) cultivars with different leaf colouration: ‘Assas’ (red leaves) and ‘Arvika’ (green leaves). In vivo measured chlorophyll fluorescence (OJIP) was used to evaluate overall photosynthetic efficiency (expressed as performance index, PIABS) as well as for advanced analysis of main photochemical processes per active reaction centre (RC) of photosystem II (PSII). To evaluate the response of PSII driven linear electron transport (relETR) and non-photochemical quenching (NPQ), the plants were challenged with short-term high light (~2000 µmol m−2 s−1). In spite of prevailing red colour, leaves of ‘Assas’ had identical concentrations of chlorophylls a and b as green ones. The OJIP transients showed that red leaves grown in field (500–800 µmol m−2 s−1) had decreased PIABS and electron transport per RC beyond primary acceptor (QA−) compared to green leaves. After high light exposure red leaves revealed impaired relETR accompanied with increased NPQ values. Anthocyanins located in epidermal cells affected neither chlorophylls concentrations nor the light capture features of PSII. Despite equal concentrations of chlorophylls and PSII photochemistry further than QA− in both leaf types, red leaves reduced overall photosynthetic efficiency due to impaired relETR in thylakoid membranes.

Adaptive strategies of Zostera japonica photosynthetic electron transport in response to thermal stress

Seawater warming is emerging as a result of increasing global temperature. In this study, Zostera japonica (Ascherson and Graebner) collected from the intertidal zones of Changdao (37°N, 120°E) were used to investigate the responses of photosynthetic electron transport to thermal exposure in April 2016. As seawater temperature rose from 20 to 32 °C, increases of the relative variable fluorescence at the K-step to the amplitude of F J–F o (W k) and the maximum photochemical efficiency (F v/F m) indicated inhibition of the oxygen-evolving complex (OEC) and photosystem II (PSII) reaction centers, respectively. When exposed to 32 °C for 4 h, both OEC and PSII reaction centers suffered irreversible damage, confirming that 32 °C is the upper critical seawater temperature for Z. japonica photosynthetic electron transport. As a thermal inhibition indicator, PIabs exhibited a time-dependent linear decrease, with irreversible damage of the PSII reaction center occurring once the value of PIabs dropped below 10.60. Based on results of the photosynthetic performance, thermal response strategies were summarized as: (1) an enhancement in the efficiency of the active PSII reaction centers; (2) an increase in the activity of the PSII electron acceptor side; (3) an enhancement in the activities of both PSI and the cyclic electron transport around PSI; (4) the alternation between PSII and PSI. Such adaptive strategies may balance the redox state of electron transport and regulate the distribution of excitation energy between the two photosystems, thereby protecting Z. japonica from ocean warming.

Light absorption and impacts of low salinities on photosynthetic behaviour in the epiphytic alga Neosiphonia savatieri (Rhodomelaceae, Rhodophyta)

Filamentous epiphyte outbreaks, a serious problem in the commercial cultivation of Kappaphycus/Eucheuma, are often triggered by large changes in abiotic factors. However, the physiological characteristics of epiphytes are poorly understood. In this study, the main pigment contents of Neosiphonia savatieri and its healthy host strains, including green thalli of Kappaphycus striatum (G-KS), brown and green thalli of Kappaphycus alvarezii (B-KA, G-KA), were quantified. The light absorption capacities of lipid- and water-soluble pigments of these seaweeds were also measured. Photosynthetic parameters, including pigment content and fast chlorophyll a fluorescence kinetics, were further studied in N. savatieri exposed to different salinities (16–34 psu) for 10 h in an outdoor experiment. The phycobiliprotein (PBP), Chl-a, and carotenoid contents of N. savatieri were comparatively higher than the hosts, with the values of 2.50, 0.29, and 0.12 mg g−1 (fw), respectively. The mean extinction coefficients of lipid- and water-soluble pigments in N. savatieri were both higher than G-KS, B-KA, and G-KA in the 400–700 nm bands. Salinity of 28 psu increased the pigment contents of the epiphyte, and 16–22 psu did not have a negative impact. Polyphasic fluorescence transients revealed that salinities at 22–28 psu were beneficial to photosynthetic electron transport, mainly attributed to the enhancement of energy absorption per active PSII reaction centre. Given these observations, N. savatieri has a higher light-harvesting capacity and better tolerance to low salinities. These two physiological properties made N. savatieri well suited to the low light and hypo-saline conditions that are frequently triggered by fluctuations in abiotic factors.

Changes of Photosynthetic Behaviors and Photoprotection during Cell Transformation and Astaxanthin Accumulation in Haematococcus pluvialis Grown Outdoors in Tubular Photobioreactors.

The cell transformation from green motile cells to non-motile cells and astaxanthin accumulation can be induced in the green alga Haematococcus pluvialis cultured outdoors. In the initial 3 d of incubation (cell transformation phase), light absorption and photosynthetic electron transport became more efficient. After five days of incubation (astaxanthin accumulation phase), the light absorption per active reaction center (ABS/RC) increased, but the efficiency of electron transport (ψo) and the quantum yield of electron transport (φEo) decreased with increased time, indicating that the capacity of photosynthetic energy utilization decreased significantly during astaxanthin accumulation, leading to an imbalance between photosynthetic light absorption and energy utilization. It would inevitably aggravate photoinhibition under high light, e.g., at midday. However, the level of photoinhibition in H. pluvialis decreased as the incubation time increased, which is reflected by the fact that Fv/Fm determined at midday decreased significantly in the initial 3 d of incubation, but was affected very little after seven days of incubation, compared with that determined at predawn. This might be because the non-photochemical quenching, plastid terminal oxidase, photosystem I cyclic electron transport, defensive enzymes and the accumulated astaxanthin can protect cells against photoinhibition.

Effects of extreme soil water stress on photosynthetic efficiency and water consumption characteristics of Tamarix chinensis in China’s Yellow River Delta

Soil moisture is a major limiting factor for plant growth on shell ridge islands in the Yellow River Delta. However, it is difficult to carry out situ experiment to study dominant plant photosynthesis physiological on the shell ridge islands under extreme soil water stress. To evaluate the adaptability of plants to light and moisture variations under extreme soil moisture conditions present on these islands, we measured photosynthetic gas exchange process, chlorophyll fluorescence, and stem sap flow variables for 3-year-old trees of Tamarix chinensis Lour, a restoration species on these islands, subjected to three types of soil water levels: waterlogging stress (WS), alternating dry–wet (WD), and severe drought stress (SS) to inform decisions on its planting and management on shell ridge islands. Gas exchange, chlorophyll fluorescence, and stem sap flow in T. chinensis were then measured. Net photosynthetic rate (PN), transpiration rate (E), and water use efficiency (WUE) were similar under WS and alternating dry–wet conditions, but their mean E and WUE differed significantly (P 0.05) under WS and dry–wet conditions; however, under extreme drought stress, compared with the dry–wet conditions, LCP was higher, LSP was lower, and AQY and PNmax were both at the lowest level. Therefore, drought stress weakened light adaptability of leaves, and the efficiency of light transformation was poorer. (3) Maximum photochemical efficiency (Fv/Fm) and the actual photochemical efficiency (ΦPSII) were similar under waterlogged stress and dry–wet conditions, indicating a similar healthy photosynthetic apparatus and photosynthetic reaction center activity, respectively. Under SS, Fv/Fm was 0.631, and the coefficient of non-photochemical quenching (NPQ) was 0.814, which indicated that while the photosynthetic mechanism was damaged, the absorbed light energy was mainly dissipated in the form of heat, and the potential photosynthetic productivity was significantly reduced. The daily cumulants of sap flow of T. chinensis under dry–wet alternation and severe drought stress were 22.25 and 63.97% higher, respectively, than under waterlogging stress. Daily changes in sap flow velocity for T. chinensis differed under the three soil water levels. Stem sap flow was weak at night under severe drought stress. Under dry–wet alternation, daytime average stem sap flow velocity was the highest, and night stem flow accounted for 10.26% of the day cumulants, while under waterlogged stress, the average nightly stem flow velocity was the highest, accounting for 31.82% of the day cumulants. These results provide important information for regional vegetation restoration and ecological reconstruction.

Quantitating active photosystem II reaction center content from fluorescence induction transients

AbstractPhotosystem II (PSII) is a pigment‐protein complex that photochemically extracts electrons from water, generating the reductant that supports biological productivity in all biomes. Estimating the content of active PSII reaction centers in a liquid sample is a key input for estimating aquatic photosynthesis rates, as well as for analyzing phytoplankton stress responses. Established procedures for PSII content quantification based on oxygen evolution are slow, imprecise and require dense cell suspensions, and are thus inapplicable to many laboratory or field studies. A new approach uses baseline chlorophyll fluorescence emission divided by the effective absorbance cross section for PSII photochemistry, with both variables derivable from single turnover fluorescence induction protocols. This approach has not been widely tested and is potentially subject to variation in samples suffering progressive photoinactivation or induction of non‐photochemical quenching under variable light. We evaluated the validity of this approach for a marine picocyanobacteria, low and high light Prochlorococcus ecotypes, arctic and temperate prasinophyte green alga and two centric diatoms, generating 209 paired determinations from a range of growth and treatment conditions. We successfully calibrated the fluorescence derived estimator for PSII reaction center content, and demonstrate a modification that corrects for the short term influence of photoinactivation. The modified parameter shows little response to induction of non‐photochemical quenching. In doing so we show the potential and limitations of an estimator of active PSII reaction center content that is sufficiently robust to support rapid, time‐resolved autonomous measures of primary productivity from lakes and oceans.

Complete inactivation of photosynthetic activity during desiccation and rapid recovery by rehydration in the aerial microalga Trentepohlia jolithus.

Aerial microalgae are more exposed to harsh and rapidly changing environmental conditions, including desiccation and radiation. Under high light, aerial algae in the desiccated state would be highly subject to photodamage. Therefore, aerial algae need effective protective mechanisms to dissipate excess excitation energy. In this study, the changes in photosynthetic behaviors during desiccation and after rehydration in Trentepohlia jolithus were confirmed using chlorophyll a fluorescence (OJIP) transient, allowing determination of the photoprotection mechanisms of this aerial alga. The filaments of T.jolithus cells at 25% relative air humidity (RH) are significantly shrunken compared with those at 100% and 87% RH, decreasing the surface area for light absorption. At 25% RH, the shape and intensity of the OJIP transient disappeared, but recovered rapidly to the level at 100% RH after 5s of rehydration. Compared with 100% RH, the maximum quantum yield of PSII (φPo ), phenomenological energy fluxes for absorption (ABS/CSm) and active PSII reaction centers (RCs) at 25% RH decreased significantly, the specific energy fluxes for absorption (ABS/RC) increased significantly, but the specific energy fluxes for trapping (TRo/RC) at 25% RH did not change. These parameters at 25% RH recovered rapidly to the level at 100% RH after 5s of rehydration. These results suggest that the efficiency of PSII light absorption and activities of PSII RCs were reversibly down-regulated in desiccated T.jolithus, which may be a special adaptive mechanism for the survivability of aerial microalgae in habitats with rapidly changing water availability.

Responses of photosynthesis, nitrogen and proline metabolism to salinity stress in Solanum lycopersicum under different levels of nitrogen supplementation

In the present study, effect of different levels of nitrogen (N0, deprived; N25, sub-optimum; N75, optimum and N150, supra-optimum) in Solanum lycopersicum L. seedlings under NaCl (NaCl1, 0.3 g kg−1 sand and NaCl2, 0.5 g kg−1sand) stress was investigated. Biomass accumulation, pigments, K+ concentration, nitrate and nitrite contents were declined by NaCl in dose dependent manner. As compared to control (N75 without NaCl), fresh weight declined by 4% and 11%, and dry weight by 7 and 13% when seedlings were grown under N75+NaCl1 and N75+NaCl2 combinations, respectively. Furthermore, fluorescence parameters (JIP-test): the size and number of active reaction centres of photosynthetic apparatus (Fv/F0), efficiency of water splitting complex (F0/Fv), quantum yield of primary photochemistry (φP0 or Phi_P0), yield of electron transport per trapped excitation (Ψ0 or Psi_0), the quantum yield of electron transport (φE0), and performance index of PS II (PIABS) and parameters related to energy fluxes per reaction centre (ABS/RC, TR0/RC, ET0/RC and DI0/RC) were also affected by NaCl. However, toxic effect of NaCl on photosystem II photochemistry was ameliorated by N. The lower dose (NaCl1) of NaCl exerts damaging effect on oxidation side of PS II, while higher dose (NaCl2) damages PS II reaction centre and its reduction side. Moreover, control seedlings (N75 without NaCl) when exposed to NaCl1 and NaCl2 exhibited a significant enhancement in respiration rate by 6 and 16%, Na+ accumulation by 111 and 169% in shoot, and 141 and 223% in root and ammonium contents by 19 and 34% respectively. Nitrate and ammonium assimilating enzymes such as nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS) and glutamate synthase (GOGAT) were adversely affected by NaCl stress while glutamate dehydrogenase (GDH) showed reverse trend. N addition caused further enhancement in free proline, and activity of Δ1-pyrroline-5-carboxylate synthetase (P5CS), while activity of proline dehydrogenase (ProDH) decreased. The results indicate that different levels of N significantly modulated NaCl-induced damaging effects in tomato seedlings. Furthermore, the results suggest that after N addition Na+, nitrite, nitrate, ammonium contents, nitrogen metabolic enzymes, proline content, and activity of P5CS are favourably regulated, which might be associated with mitigation of NaCl stress and effect was more pronounced with supra-optimum level of N (N150).

Gas exchange, photosystem II photochemistry, and the antioxidant system of longan plant (Dimocarpus longan Lour.) leaves in response to lead (Pb) stress

Longan is one of the most important subtropical fruit trees and a famous special product in south China. Increased fruit demand brings longan cultivation to Pb-affected regions. Seedlings of longan (cv. Wulongling) in pots with sands were irrigated daily for 30 d with a freshly prepared nutrient solution containing different concentrations of Pb(NO3)2 (0, 100 , 200 , 400 , 600 , 800 and 1000 mg L) to determine physiological and biochemical responses of longan seedlings to various levels of lead (Pb). The results indicated that Pb stress substantially inhibited the growth of longan plants and markedly declined in their dry biomass. However, when the plants were grown at 100 mg L Pb, the growth and dry biomass of the plants showed no significant difference from control. In addition, the chlorophyll a fluorescence and gas exchange parameters were correlated with the growth and yield response. Pb treatments increased the minimum fluorescence (Fo) and caused a decrease in maximum fluorescence (Fm), variable fluorescence (Fv), the maximum quantum efficiency of PSII photochemistry (Fv/Fm), trapped energy flux per cross section (CS) at t=0 (TRo/CSo), electron transport flux per CS at t=0 (ETo/CSo), dissipated energy flux per CS at t=0(DIo/CSo), and the amount of active PSII reaction centers (RCs) per CS at t = 0 (RC/CSo). Furthermore, Pb stress led to decreases in the protein contents, the activity of peroxidase (POD, EC 1.11.1.7) and the accumulation of proline and malondialdehyde (MDA), and enhanced superoxide dismutase activity (SOD, EC 1.15.1.1), whereas catalase (CAT, EC 1.11.1.6) and ascorbate peroxidase (APX,EC 1.11.1.11) were enhanced at low Pb levels and decreased under high Pb stress. Nonetheless, these changes were closely related to the severity of the Pb stress.

Combining gas exchange and chlorophyll a fluorescence measurements to analyze the photosynthetic activity of drip-irrigated cotton under different soil water deficits

Gas exchange and chlorophyll a fluorescence were measured to study the effects of soil water deficit (75, 60 and 45% of field capacity, FC) on the photosynthetic activity of drip-irrigated cotton under field conditions. At light intensities above 1 200 μmol m−2 s−1, leaf net photosynthetic rate (Pn) at 60 and 45% FC was 0.75 and 0.45 times respectively than that of 75% FC. The chlorophyll content, leaf water potential and yield decreased as soil water deficit decreased. Fiber length was significantly lower at 45% FC than at 75% FC. The actual quantum yield of the photosystem II (PSII) primary photochemistry and the photochemical quenching were significantly greater at 60% FC than at 75% FC. The electron transport rate and non-photochemical quenching at 45% FC were 0.91 and 1.29 times than those at 75% FC, respectively. The amplitudes of the K- and L-bands were higher at 45% FC than at 60% FC. As soil water content decreased, active PSII reaction centers per chlorophyll decreased, functional PSII antenna size increased, and energetic connectivity between PSII units decreased. Electron flow from plastoquinol to the PSI end electron acceptors was significantly lower at 45% FC than at 75% FC. Similar to the effect on leaf Pn, water deficit reduced the performance index (PIABS, total) in the dark-adapted state. These results suggest that (i) the effect of mild water deficit on photosystem activity was mainly related to processes between plastoquinol and the PSI end electron acceptors, (ii) PSI end electron acceptors were only affected at moderate water deficit, and (iii) PIABS, total can reliably indicate the effect of water deficit on the energy supply for cotton metabolism.

Photosynthetic characteristics and nitrogen distribution of large-spike wheat in Northwest China

The leaf photosynthesis and nitrogen (N) translocation in three large-spike lines and control cultivar (Xi'nong 979) of winter wheat (Triticum aestivum L.) were studied in 2010–2011 and 2011–2012. The objectives of this study were to investigate the differences in the physiological characteristics of large-spike lines and control cultivar and identify the limiting factors that play a role in improving the yield of breeding materials. The average yield, grain number per spike, kernel weight per spike, and 1000-kernel weight of the large-spike lines were 16.0, 26.8, 42.6, and 15.4%, respectively, significantly higher than those of control. The average photosynthetic rates (Pn) were not significant between the large-spike lines and control cultivar during the active growth period. The average PSII maximum energy conversion efficiency (Fv/Fm), PSII actual quantum efficiency (ΦPS??), photochemical quenching coefficient (qP), PSII reaction center activity (Fv‘/Fv‘) and water-use efficiency (WUE) of the large-spike lines were 1.0, 5.1, 3.6, 0.8, and 43.4%, respectively, higher than those of the control during the active growth stages. The N distribution proportions in different tissues were ranked in the order of grains<culms+sheathes<ra-chis+glumes<flag leaves<penultimate leaves<remain leaves. This study suggested that utilization of the large-spike wheat might be a promising approach to obtain higher grain yield in Northwest China.

Investigation of deleterious effects of chromium phytotoxicity and photosynthesis in wheat plant

Increasing human and industrial activities lead to heavy metal pollution. Heavy metal chromium (Cr) is considered to be a serious environmental contaminant for the biota. Phytotoxic effects of Cr were studied in wheat plants. Growth parameters were largely inhibited as a result of disturbances in the plant cell metabolism in response to Cr toxicity. Chromium toxicity led to decline in a number of active reaction centres of PSII, rate of electron transport, and change in PSII heterogeneity. Chromium did not cause any change in heterogeneity of the reducing side. A significant change in antenna size heterogeneity of PSII occurred in response to Cr toxicity. Chromium seems to have extensive effects on the light harvesting complex of PSII.

Chlorophyll fluorescence parameters allow the rapid detection and differentiation of plant responses in three different wheat pathosystems.

The present study was undertaken to identify chlorophyll fluorescence (CF) parameters that can quantify changes in PSII associated with plant responses in three different wheat pathosystems of foliar, stem-base and ear diseases. The pathosystems included powdery mildew caused by Blumeria graminis, eyespot caused by Oculimacula yallundae or Oculimacula acuformis and Fusarium head blight (FHB) caused by Fusarium culmorum, F. avenaceum or F. langsethiae. Fast CF transients (OJIP) were analysed with the JIP-test to determine changes in PSII photochemistry. Measurements on asymptomatic leaves showed that electron transport related parameters (ETo/RC, ψo and ϕEo) were important to identify varietal differences in resistance to powdery mildew during early stages of infection. The same parameters also allowed differentiation between F. langsethiae and other Fusarium spp. Where infections were caused by the necrotrophic pathogens, Oculimacula spp., F. culmorum or F. avenaceum, changes related to maximum efficiency of PSII photochemistry (Fv'/Fm') as well as flux of dissipated (DIo/RC), trapped (TRo/RC), or absorbed (ABS/RC) energy per active reaction centers were significant in detecting biotic stress and the effectiveness of fungicide treatment for disease control. Our results demonstrated that Fv'/Fm' correlated significantly with visual disease and pathogen DNA of different wheat pathosystems. OJIP was shown as a sensitive technique that can be explored as diagnostic tool in future crop disease management and varietal breeding programs.

To rebound or dissociate? This is the mechanistic question in C-H hydroxylation by heme and nonheme metal-oxo complexes.

Enzymatic reactions that involve C-H bond activation of alkanes by high-valent iron-oxo species can be explained by the rebound mechanism (RM). Hydroxylation reactions of alkane substrates effected by the reactive compound I (Cpd I) species of cytochrome P450 enzymes are good examples. There was initially little doubt that the rebound paradigm could be carried over in the same form to the arena of synthetic nonheme high-valent iron-oxo or other metal-oxo complexes. However, the active reaction centres of these synthetic systems are not well-caged, in contrast to the active sites of enzymes; therefore, the relative importance of the radical dissociation pathway can become prominent. Indeed, accumulating experimental and theoretical evidence shows that introduction of the non-rebound mechanism (non-RM) is necessary to rationalise the different reactivity patterns observed for synthetic nonheme complexes. In this tutorial review, we discuss several specific examples involving the non-RM while making frequent comparisons to the RM, mainly from the perspective of computational chemistry. We also provide a technical guide to DFT calculations of RM and non-RM and to the interpretations of computational outcomes.

Controlled fabrication and enhanced visible-light photocatalytic hydrogen production of Au@CdS/MIL-101 heterostructure

A novel and highly efficient three-component Au@CdS/MIL-101 heterostructure was successfully synthesized. The MIL-101(Cr) with large surface area was introduced as a matrix for the well-dispersed growth of Au nanoparticles, and the CdS was selectively coated on the Au nanoparticles. Under visible light irradiation, the Au@CdS/MIL-101 heterostructure presents superior hydrogen evolution rate over the pure CdS, CdS/MIL-101 and Au/MIL-101 composites. The Au@CdS/MIL-101 heterostructure exhibits an unusual H2 production rate of 250μmolh−1/10mg, which is 2.6 times higher than that of pure CdS. The performance enhancement of Au@CdS/MIL-101 heterostructure can be attributed to the following reasons: (i) the large surface area of MIL-101(Cr) can effectively disperse the Au and CdS nanoparticles, resulting in more active adsorption sites and reaction centers. (ii) the strong surface plasmon resonance absorption of Au could accelerate the charge transfer and extend the light response spectrum of CdS. This three-component Au@CdS/MIL-101 heterostructure combining the large surface area of MOF and the surface plasmon resonance of Au into a single structure may provide a potential way to design highly efficient and solar-energy-harvesting photocatalysts.

Photodamage to the oxygen evolving complex of photosystem II by visible light.

Light damages photosynthetic machinery, primarily photosystem II (PSII), and it results in photoinhibition. A new photodamage model, the two-step photodamage model, suggests that photodamage to PSII initially occurs at the oxygen evolving complex (OEC) by light energy absorbed by manganese and that the PSII reaction center is subsequently damaged by light energy absorbed by photosynthetic pigments due to the limitation of electrons to the PSII reaction center. However, it is still uncertain whether this model is applicable to photodamage to PSII under visible light as manganese absorbs visible light only weakly. In the present study, we identified the initial site of photodamage to PSII upon illumination of visible light using PSII membrane fragments isolated from spinach leaves. When PSII samples were exposed to visible light in the presence of an exogenous electron acceptor, both PSII total activity and the PSII reaction centre activity declined due to photodamage. The supplemental addition of an electron donor to the PSII reaction centre alleviated the decline of the reaction centre activity but not the PSII total activity upon the light exposure. Our results demonstrate that visible light damages OEC prior to photodamage to the PSII reaction center, consistent with two-step photodamage model.

Chlorophyll a fluorescence analysis of high-yield rice (Oryza sativa L.) LYPJ during leaf senescence

Photosystem II (PSII) photochemistry was examined by chlorophyll (Chl) a fluorescence analysis in high-yield rice LYPJ flag leaves during senescence. Parameters deduced from the JIP-test showed that inhibition of the donor side of PSII was greater than that of the acceptor side in hybrid rice LYPJ. The natural senescence process was accompanied by the increased inactivation of oxygen-evolving complex (OEC) and a lower total number of active reaction centers per absorption. It indicated that the inhibition of electron transport caused by natural senescence might be caused partly by uncoupling of the OEC and/or inactivation of PSII reaction centers. Chl fluorescence parameters analyzed in this study suggested that energy dissipation was enhanced in order to protect senescent leaves from photodamage. Nevertheless, considerably reduced PSI electron transport activity was observed at the later senescence. Thus, natural senescence inhibited OEC-PSII electron transport, but also significantly limited the PSII-PSI electron flow.

Carbon Nanohorn-Derived Graphene Nanotubes as a Platinum-Free Fuel Cell Cathode.

Current low-temperature fuel cell research mainly focuses on the development of efficient nonprecious electrocatalysts for the reduction of dioxygen molecule due to the reasons like exorbitant cost and scarcity of the current state-of-the-art Pt-based catalysts. As a potential alternative to such costly electrocatalysts, we report here the preparation of an efficient graphene nanotube based oxygen reduction electrocatalyst which has been derived from single walled nanohorns, comprising a thin layer of graphene nanotubes and encapsulated iron oxide nanoparticles (FeGNT). FeGNT shows a surface area of 750 m(2)/g, which is the highest ever reported among the metal encapsulated nanotubes. Moreover, the graphene protected iron oxide nanoparticles assist the system to attain efficient distribution of Fe-Nx and quaternary nitrogen based active reaction centers, which provides better activity and stability toward the oxygen reduction reaction (ORR) in acidic as well as alkaline conditions. Single cell performance of a proton exchange membrane fuel cell by using FeGNT as the cathode catalyst delivered a maximum power density of 200 mW cm(-2) with Nafion as the proton exchange membrane at 60 °C. The facile synthesis strategy with iron oxide encapsulated graphitic carbon morphology opens up a new horizon of hope toward developing Pt-free fuel cells and metal-air batteries along with its applicability in other energy conversion and storage devices.