Mehler Reaction Research Articles

A novel insight into the mode of action of glufosinate: how reactive oxygen species are formed.

Glufosinate targets glutamine synthetase (GS), but its fast herbicidal action is triggered by reactive oxygen species (ROS). The relationship between GS inhibition and ROS accumulation was investigated in Amaranthus palmeri. Glufosinate's fast action is light-dependent with no visual symptoms or ROS formation in the dark. Inhibition of GS leads to accumulation of ammonia and metabolites of the photorespiration pathway, such as glycolate and glyoxylate, as well as depletion of other intermediates such as glycine, serine, hydroxypyruvate, and glycerate. Exogenous supply of glycolate to glufosinate-treated plants enhanced herbicidal activity and dramatically increased hydrogen peroxide accumulation (possibly from peroxisomal glycolate oxidase activity). Glufosinate affected the balance between ROS generation and scavenging. The activity of superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase increased after glufosinate treatment in an attempt to quench the nascent ROS burst. Low doses of atrazine and dinoseb were used to investigate the sources of ROS by manipulating photosynthetic electron transport and oxygen (O2) evolution. ROS formation depended on electron flow and O2 evolution in photosystem II (PSII). Inhibition of GS disrupted photorespiration, carbon assimilation, and linear electron flow in the light reactions. Consequently, the antioxidant machinery and the water-water cycle are overwhelmed in the presence of light and glufosinate. The O2 generated by the splitting of water in PSII becomes the acceptor of electrons, generating ROS. The cascade of events leads to lipid peroxidation and forms the basis for the fast action of glufosinate.

Dissecting the interaction of photosynthetic electron transfer with mitochondrial signalling and hypoxic response in the Arabidopsis rcd1 mutant.

The Arabidopsis mutant rcd1 is tolerant to methyl viologen (MV). MV enhances the Mehler reaction, i.e. electron transfer from Photosystem I (PSI) to O2, generating reactive oxygen species (ROS) in the chloroplast. To study the MV tolerance of rcd1, we first addressed chloroplast thiol redox enzymes potentially implicated in ROS scavenging. NADPH-thioredoxin oxidoreductase type C (NTRC) was more reduced in rcd1. NTRC contributed to the photosynthetic and metabolic phenotypes of rcd1, but did not determine its MV tolerance. We next tested rcd1 for alterations in the Mehler reaction. In rcd1, but not in the wild type, the PSI-to-MV electron transfer was abolished by hypoxic atmosphere. A characteristic feature of rcd1 is constitutive expression of mitochondrial dysfunction stimulon (MDS) genes that affect mitochondrial respiration. Similarly to rcd1, in other MDS-overexpressing plants hypoxia also inhibited the PSI-to-MV electron transfer. One possible explanation is that the MDS gene products may affect the Mehler reaction by altering the availability of O2. In green tissues, this putative effect is masked by photosynthetic O2 evolution. However, O2 evolution was rapidly suppressed in MV-treated plants. Transcriptomic meta-analysis indicated that MDS gene expression is linked to hypoxic response not only under MV, but also in standard growth conditions.This article is part of the theme issue ‘Retrograde signalling from endosymbiotic organelles’.

Minimizing an Electron Flow to Molecular Oxygen in Photosynthetic Electron Transfer Chain: An Evolutionary View.

Recruitment of H2O as the final donor of electrons for light-governed reactions in photosynthesis has been an utmost breakthrough, bursting the evolution of life and leading to the accumulation of O2 molecules in the atmosphere. O2 molecule has a great potential to accept electrons from the components of the photosynthetic electron transfer chain (PETC) (so-called the Mehler reaction). Here we overview the Mehler reaction mechanisms, specifying the changes in the structure of the PETC of oxygenic phototrophs that probably had occurred as the result of evolutionary pressure to minimize the electron flow to O2. These changes are warranted by the fact that the efficient electron flow to O2 would decrease the quantum yield of photosynthesis. Moreover, the reduction of O2 leads to the formation of reactive oxygen species (ROS), namely, the superoxide anion radical and hydrogen peroxide, which cause oxidative stress to plant cells if they are accumulated at a significant amount. From another side, hydrogen peroxide acts as a signaling molecule. We particularly zoom in into the role of photosystem I (PSI) and the plastoquinone (PQ) pool in the Mehler reaction.

Dark biological superoxide production as a significant flux and sink of marine dissolved oxygen.

The balance between sources and sinks of molecular oxygen in the oceans has greatly impacted the composition of Earth's atmosphere since the evolution of oxygenic photosynthesis, thereby exerting key influence on Earth's climate and the redox state of (sub)surface Earth. The canonical source and sink terms of the marine oxygen budget include photosynthesis, respiration, photorespiration, the Mehler reaction, and other smaller terms. However, recent advances in understanding cryptic oxygen cycling, namely the ubiquitous one-electron reduction of O2 to superoxide by microorganisms outside the cell, remains unexplored as a potential player in global oxygen dynamics. Here we show that dark extracellular superoxide production by marine microbes represents a previously unconsidered global oxygen flux and sink comparable in magnitude to other key terms. We estimate that extracellular superoxide production represents a gross oxygen sink comprising about a third of marine gross oxygen production, and a net oxygen sink amounting to 15 to 50% of that. We further demonstrate that this total marine dark extracellular superoxide flux is consistent with concentrations of superoxide in marine environments. These findings underscore prolific marine sources of reactive oxygen species and a complex and dynamic oxygen cycle in which oxygen consumption and corresponding carbon oxidation are not necessarily confined to cell membranes or exclusively related to respiration. This revised model of the marine oxygen cycle will ultimately allow for greater reconciliation among estimates of primary production and respiration and a greater mechanistic understanding of redox cycling in the ocean.

Photorespiration is the major alternative electron sink under high light in alpine evergreen sclerophyllous Rhododendron species

Owing to the high leaf mass per area, alpine evergreen sclerophyllous Rhododendron have low values of mesophyll conductance (gm). The resulting low chloroplast CO2 concentration aggravates photorespiration, which requires a higher ATP/NADPH ratio. However, the significance of photorespiration and underlying mechanisms of energy balance in these species are little known. In this study, eight alpine evergreen sclerophyllous Rhododendron species grown in a common garden were tested for their gm, electron flow to photorespiration, and energy balancing. Under saturating light, gm was the most limiting factor for net photosynthesis (AN) in all species, and the species differences in AN were primarily driven by gm rather than stomatal conductance. The total electron flow through photosystem II (ETRII) nearly equaled the electron transport required for Rubisco carboxylation and oxygenation. Furthermore, blocking electron flow to photosystem I with appropriate inhibitors showed that electron flow to plastic terminal oxidase was not observed. As a result, these studied species showed little alternative electron flow mediated by water-water cycle or plastic terminal oxidase. By comparison, the ratio of electron transport consumed by photorespiration to ETRII (JPR/ETRII), ranging from 43%∼55%, was negatively correlated to AN and gm. Furthermore, the increased ATP production required by enhanced photorespiration was regulated by cyclic electron flow around photosystem I. These results indicate that photorespiration is the major electron sink for dissipation of excess excitation energy in the alpine evergreen sclerophyllous Rhododendron species. The coordination of gm, photorespiration and cyclic electron flow is important for sustaining leaf photosynthesis.

Photosynthetic regulation under fluctuating light in young and mature leaves of the CAM plant Bryophyllum pinnatum

Photosystem I (PSI) is the potential target of photodamage under fluctuating light in angiosperms. However, the response of PSI to fluctuating light in young leaves has not yet been clarified. Furthermore, the photosynthetic regulation under fluctuating light in crassulacean acid metabolism (CAM) plants is little known. In this study, we measured PSI redox state and the electrochromic shift signal in the mature and young leaves of a CAM species Bryophyllum pinnatum. The mature leaves showed stronger capacity for photo-reduction of O2 mediated by the alternative electron flow (probably the water-water cycle) when compared with the young leaves. After an increase in light intensity, both the mature and young leaves showed insufficient proton gradient (ΔpH) across the thylakoid membranes within the first seconds. Meanwhile, PSI was highly oxidized in the mature leaves but was in a more reduced state in the young leaves. Furthermore, young leaves were more susceptible to PSI photoinhibition under fluctuating light. Therefore, in the mature leaves, the alternative electron flow significantly optimized the PSI redox state under fluctuating light at relatively low ΔpH. By comparison, in the young leaves, PSI redox state was largely determined by the buildup of ΔpH. Therefore, the major photoprotective mechanism responsible for safeguarding PSI under fluctuating light can be influenced by leaf developmental stages.

The role of water-water cycle in regulating the redox state of photosystem I under fluctuating light

The regulation of photosystem I (PSI) redox state under fluctuating light was investigated for four species using P700 measurement and electrochromic shift analysis. Species included the angiosperms Camellia japonica, Bletilla striata and Arabidopsis thaliana and the fern Cyrtomium fortunei. For the first seconds after transition from low to high light, all species showed relatively low levels of the proton gradient (ΔpH) across the thylakoid membranes. At this moment, PSI was highly oxidized in C. japonica and C. fortunei but was over-reduced in B. striata and A. thaliana. In B. striata and A. thaliana, the redox state of PSI was largely dependent on ΔpH. In contrast, the rapid oxidation of P700 in C. japonica was relatively independent of ΔpH, but was mainly dependent on the outflow of electrons to O2 via the water-water cycle. In the fern C. fortunei, PSI redox state was rapidly regulated by the fast photo-reduction of O2 rather than the ΔpH. These results indicate that mechanisms regulating PSI redox state under fluctuating light differ greatly between species. We propose that the water-water cycle is an important mechanism regulating the PSI redox state in angiosperms.

The regulation of P700 is an important photoprotective mechanism to NaCl-salinity in Jatropha curcas.

Salinity commonly affects photosynthesis and crop production worldwide. Salt stress disrupts the fine balance between photosynthetic electron transport and the Calvin cycle reactions, leading to over-reduction and excess energy within the thylakoids. The excess energy triggers reactive oxygen species (ROS) overproduction that causes photoinhibition in both photosystems (PS) I and II. However, the role of PSI photoinhibition and its physiological mechanisms for photoprotection have not yet been fully elucidated. In the present study, we analyzed the effects of 15 consecutive days of 100 mM NaCl in Jatropha curcas plants, primarily focusing on the photosynthetic electron flow at PSI level. We found that J. curcas plants have important photoprotective mechanisms to cope with the harmful effects of salinity. We show that maintaining P700 in an oxidized state is an important photoprotector mechanism, avoiding ROS burst in J. curcas exposed to salinity. In addition, upon photoinhibition of PSI, the highly reduced electron transport chain triggers a significant increase in H2 O2 content which can lead to the production of hydroxyl radical by Mehler reactions in chloroplast, thereby increasing PSI photoinhibition.

Modeling of Electron and Proton Transport in Chloroplast Membranes with Regard to Thioredoxin-Dependent Activation of the Calvin–Benson Cycle and ATP Synthase

In this work, the analysis of electron and proton transport in chloroplasts of higher plants has been carried out on the basis of a mathematical model, which takes into account the pH-dependent regulation of electron transfer and thioredoxin-dependent activation of the Calvin–Benson cycle (CBC) enzymes and the ATP synthase. The impact of reduced thioredoxin on the kinetics of electron transport, pH changes in the intrathylakoid space and ATP production has been simulated. Comparison of the computed and experimental data on the kinetics of P700 photooxidation has shown that the consideration of thioredoxin-dependent activation of the CBC and the ATP synthase provides an adequate description of the multiphase kinetics of P 700 + induction. The dynamics of electron flow through PSI and the partitioning of electron fluxes on the acceptor site of PSI has been simulated. The model predicts that at the initial stage of the induction period the alternative pathways, cyclic electron transport around PSI and electron flow to O2 (the Mehler reaction), play a significant role in photosynthetic electron transport chain, but their contribution attenuates upon the activation of the CBC reactions.

The artificial humic substance HS1500 does not inhibit photosynthesis of the green alga Desmodesmus armatus in vivo but interacts with the photosynthetic apparatus of isolated spinach thylakoids in vitro.

Humic substances (HSs) can influence the growth and composition of freshwater phytoplankton assemblage. Since HSs contain many phenolic and quinonic moieties and cause growth reductions in eco-physiological field experiments, HSs are considered photosystem II herbicides. To test this specific mode of action in vivo and in vitro, respectively, we used intact cells of the green alga Desmodesmus armatus, as well as thylakoids isolated from spinach (Spinacia oleracea) as a model system for the green algal chloroplast. Photosynthetic electron transport was measured as oxygen evolution and variable chlorophyll fluorescence. The in vivo effect of the artificial humic substance HS1500 on algae consisted of no impact on photosynthesis-irradiance curves of intact green algae compared to untreated controls. In contrast, addition of HS1500 to isolated thylakoids resulted in light-induced oxygen consumption (Mehler reaction) as an in vitro effect. Fluorescence induction kinetics of HS-treated thylakoids revealed a large static quenching effect of HS1500, but no inhibitory effect on electron transport. For the case of intact algal cells, we conclude that the highly hydrophilic and rather large molecules of HS1500 are not taken up in effective quantities and, therefore, cannot interfere with photosynthesis. The in vitro tests show that HS1500 has no inhibitory effect on photosystem II but operates as a weak, oxygen-consuming Hill acceptor at photosystem I. Hence, the results indicate that eco-physiological field experiments should focus more strongly on effects of HSs on extracellular features, such as reducing and red-shifting the underwater light field or influencing nutrient availability by cation exchange within the plankton network.

OsPRX2 contributes to stomatal closure and improves potassium deficiency tolerance in rice

Peroxiredoxins (Prxs) which are thiol-based peroxidases have been implicated in the toxic reduction and intracellular concentration regulation of hydrogen peroxide. In Arabidopsis thaliana At2-CysPrxB (At5g06290) has been demonstrated to be essential in maintaining the water-water cycle for proper H2O2 scavenging. Although the mechanisms of 2-Cys Prxs have been extensively studied in Arabidopsis thaliana, the function of 2-Cys Prxs in rice is unclear. In this study, a rice homologue gene of At2-CysPrxB, OsPRX2 was investigated aiming to characterize the effect of 2-Cys Prxs on the K+-deficiency tolerance in rice. We found that OsPRX2 was localized in the chloroplast. Overexpressed OsPRX2 causes the stomatal closing and K+-deficiency tolerance increasing, while knockout of OsPRX2 lead to serious defects in leaves phenotype and the stomatal opening under the K+-deficiency tolerance. Detection of K+ accumulation, antioxidant activity of transgenic plants under the starvation of potassium, further confirmed that OsPRX2 is a potential target for engineering plants with improved potassium deficiency tolerance.

Electron Transfer through the Acceptor Side of Photosystem I: Interaction with Exogenous Acceptors and Molecular Oxygen.

This review considers the state-of-the-art on mechanisms and alternative pathways of electron transfer in photosynthetic electron transport chains of chloroplasts and cyanobacteria. The mechanisms of electron transport control between photosystems (PS) I and II and the Calvin-Benson cycle are considered. The redistribution of electron fluxes between the noncyclic, cyclic, and pseudocyclic pathways plays an important role in the regulation of photosynthesis. Mathematical modeling of light-induced electron transport processes is considered. Particular attention is given to the electron transfer reactions on the acceptor side of PS I and to interactions of PS I with exogenous acceptors, including molecular oxygen. A kinetic model of PS I and its interaction with exogenous electron acceptors has been developed. This model is based on experimental kinetics of charge recombination in isolated PS I. Kinetic and thermodynamic parameters of the electron transfer reactions in PS I are scrutinized. The free energies of electron transfer between quinone acceptors A1A/A1B in the symmetric redox cofactor branches of PS I and iron-sulfur clusters FX, FA, and FB have been estimated. The second-order rate constants of electron transfer from PS I to external acceptors have been determined. The data suggest that byproduct formation of superoxide radical in PS I due to the reduction of molecular oxygen in the A1 site (Mehler reaction) can exceed 0.3% of the total electron flux in PS I.

Involvement of the chloroplast plastoquinone pool in the Mehler reaction.

Light-dependent oxygen reduction in the photosynthetic electron transfer chain, i.e. the Mehler reaction, has been studied using isolated pea thylakoids. The role of the plastoquinone pool in the Mehler reaction was investigated in the presence of dinitrophenyl ether of 2-iodo-4-nitrothymol (DNP-INT), the inhibitor of plastohydroquinone oxidation by cytochrome b6/f complex. Oxygen reduction rate in the presence of DNP-INT was higher than in the absence of the inhibitor in low light at pH 6.5 and 7.6, showing that the capacity of the plastoquinone pool to reduce molecular oxygen in this case exceeded that of the entire electron transfer chain. In the presence of DNP-INT, appearance of superoxide anion radicals outside thylakoid membrane represented approximately 60% of the total superoxide anion radicals produced. The remaining 40% of the produced superoxide anion radicals was suggested to be trapped by plastohydroquinone molecules within thylakoid membrane, leading to the formation of hydrogen peroxide (H2 O2 ). To validate the reaction of superoxide anion radical with plastohydroquinone, xanthine/xanthine oxidase system was integrated with thylakoid membrane in order to generate superoxide anion radical in close vicinity of plastohydroquinone. Addition of xanthine/xanthine oxidase to the thylakoid suspension resulted in a decrease in the reduction level of the plastoquinone pool in the light. The obtained data provide additional clarification of the aspects that the plastoquinone pool is involved in both reduction of oxygen to superoxide anion radicals and reduction of superoxide anion radicals to H2 O2 . Significance of the plastoquinone pool involvement in the Mehler reaction for the acclimation of plants to light conditions is discussed.

Opposite domination of cyclic and pseudocyclic electron flows in short-illuminated dark-adapted leaves of angiosperms and gymnosperms.

The present work was aimed to explain the recently reported higher O2-dependent electron flow capacity in gymnosperms than in angiosperms and to search for other differences in the electron transport processes by simultaneous characterization of the relative capacities of pseudocyclic (direct or Flavodiiron proteins (Flv)-mediated O2-reduction, Mehler(-like) reactions) and cyclic electron flows around photosystem I (CEF-PSI). To this end, a comparative multicomponent analysis was performed on the fluorescence decay curves of dark-adapted leaves after illumination with a 1-s saturating light pulse. In both gymnosperms and angiosperms, two or three exponential decay components were resolved: fast (t 1/21 ~ 170-260ms), middle (~1.0-2.3s), and slow (>4.2s). The sensitivity of the decay parameters (amplitudes A1-3, halftimes t 1/2 1-3) to the alternative electron flows was assessed using Arabidopsis pgr5 and ndhM mutants, defective in CEF-PSI, Synechocystis sp. PCC 6803 Δflv1 mutant, defective in Flv-mediated O2-photoreduction, different O2 concentrations, and methyl viologen treatment. A1 reflected the part of electrons involved in linear and O2-photoreduction pathways after PSI. The middle component appeared in pgr5 (but not in ndhM), in gymnosperms under low O2, and in Δflv1, and reflected limitations at the PSI acceptor side. The slow component was sensitive to CEF-PSI. The comparison of decay parameters provided evidence that Flv mediate O2-photoreduction in gymnosperms, which explains their higher O2-dependent electron flow capacity. The concomitant quantification of relative electrons branching in O2-photoreduction and CEF-PSI pathways under the applied non-steady-state photosynthetic conditions reveals that CEF-PSI capacity significantly exceeds that of O2-photoreduction in angiosperms while the opposite occurs in gymnosperms.

Photo-protective mechanisms in reed canary grass to alleviate photo-inhibition of PSII on the Qinghai-Tibet Plateau

Due to its characteristic of high biomass yield potential, there is considerable interest in cultivating Phalaris arundinacea L. cv. ‘chuancaoyin No.3' (reed canary grass) on the Qinghai-Tibet Plateau where there is an abundance of alpine steppe meadow and a potential large market for animal husbandry. In this study, we 1) investigate whether reed canary grass exhibits superior productive capacity to Elymus nutans ‘Aba' (E. nutans), ordinary common pasture, during the long warm days of summer at high-altitude; and 2) compare the cold tolerance between reed canary grass and E. nutans, including photosynthesis, photo-inhibition, and photo-protection. The results suggest that reed canary grass exhibits higher photosynthetic capacity compared to E. nutans at latitudes of the cool temperate zone. Meanwhile, cold-induced photo-inhibition and photo-damage at high altitudes in reed canary grass were due to both stomatal and non-stomatal limitation, and the enhancement in photo-respiration, thermal dissipation, and Mehler reaction are important processes to minimize the negative effects of high elevation and a cold environment.

The Synergistic Responses of Different Photoprotective Pathways in Dwarf Bamboo (Fargesia rufa) to Drought and Subsequent Rewatering.

Dwarf bamboo-dominated forests are often subjected to temporary periods of drought due to rising air temperature and decreasing rainfall. Nevertheless, the relationship among CO2 assimilation, photoprotective pathways and metabolism of reactive oxygen species (ROS) remains unexplored in bamboo species. Changes in leaf gas exchange, chlorophyll fluorescence, energy partitioning, antioxidative system and compounds related to ROS metabolism in Fargesia rufa plants subjected to drought and subsequent rewatering were analyzed. Drought resulted in a reversible inhibition of photochemistry, particularly net CO2 assimilation, and lipid peroxidation due to ROS accumulation. Meanwhile, photoprotective pathways, including the water–water cycle (especially for moderate drought), and adjustment in antenna pigments, thermal dissipation and antioxidative defense capacity at organelle levels (especially for severe drought), were up-regulated at the stress phase. Conversely, photorespiration was down-regulated after drought stress. As a result, rewatering restored most of the photochemical activity under drought, especially moderate drought. Moreover, thermal dissipation under severe drought was still operated for avoiding high ROS levels after rewatering. Therefore, the synergistic function of these photoprotective pathways except photorespiration can protect the photosynthetic apparatus from oxidative damage in response to varying intensities of drought stress when CO2 assimilation is restricted. This is helpful for the gradual recovery of photosynthetic capacity after rewatering. Thus, F. rufa plants can withstand drought and is capable of survival in such environment.Highlights:1. The effects of drought and subsequent rewatering on Fargesia rufa were studied.2. Drought resulted in a reversible inhibition of photochemistry.3. Photoprotective pathways except photorespiration were up-regulated at the drought phase.4. Rewatering rapidly restored photochemical activity, especially under moderate drought.5. Fargesia rufa plant is capable of resisting and surviving drought environment.

Kinetic modeling of electron transfer reactions in photosystem I complexes of various structures with substituted quinone acceptors.

The reduction kinetics of the photo-oxidized primary electron donor P700 in photosystem I (PS I) complexes from cyanobacteria Synechocystis sp. PCC 6803 were analyzed within the kinetic model, which considers electron transfer (ET) reactions between P700, secondary quinone acceptor A1, iron-sulfur clusters and external electron donor and acceptors-methylviologen (MV), 2,3-dichloro-naphthoquinone (Cl2NQ) and oxygen. PS I complexes containing various quinones in the A1-binding site (phylloquinone PhQ, plastoquinone-9 PQ and Cl2NQ) as well as F X-core complexes, depleted of terminal iron-sulfur F A/F B clusters, were studied. The acceleration of charge recombination in F X-core complexes by PhQ/PQ substitution indicates that backward ET from the iron-sulfur clusters involves quinone in the A1-binding site. The kinetic parameters of ET reactions were obtained by global fitting of the P700+ reduction with the kinetic model. The free energy gap ΔG 0 between F X and F A/F B clusters was estimated as -130meV. The driving force of ET from A1 to F X was determined as -50 and -220meV for PhQ in the A and B cofactor branches, respectively. For PQ in A1A-site, this reaction was found to be endergonic (ΔG 0 = +75meV). The interaction of PS I with external acceptors was quantitatively described in terms of Michaelis-Menten kinetics. The second-order rate constants of ET from F A/F B, F X and Cl2NQ in the A1-site of PS I to external acceptors were estimated. The side production of superoxide radical in the A1-site by oxygen reduction via the Mehler reaction might comprise ≥0.3% of the total electron flow in PS I.

Solar light-driven photocatalytic destruction of cyanobacteria by F-Ce-TiO2/expanded perlite floating composites

A series of efficient solar light-driven F-Ce-TiO2/expanded perlite (EP) floating photocatalysts were successfully synthesized using a facile sol-gel method and applied as algaecides in eutrophic waters. The as-synthesized photocatalysts were characterized by means of XRD, FESEM/EDX, TEM, N2 adsorption/desorption, XPS, UV–vis DRS and PL spectrum. The results show that F-Ce-TiO2/EP has a honeycomb-like structure with F-Ce co-doped TiO2 particles distributed on EP surface. Calcination temperature can greatly affect the doping status, specific surface area and photoelectric properties of F-Ce-TiO2/EP. Among the synthesized photocatalysts, those calcined at 450°C (i.e., F-Ce-TiO2/EP450) exhibited the highest photocatalytic activity. The photocatalytic destruction process could be described by the modified Chick–Watson kinetic model and Hom kinetic model. F-Ce-TiO2/EP450 can achieve the algal cells removal rate of 98.1% within 9h. The algal cell walls obviously damaged in this process. Following the damages, the electron transfer in water-water cycle was also inhibited, thereby accelerating the death of algal cells with the accumulation of the reactive oxygen species (ROS). In addition, microcystin-LR (MC-LR) and metabolic products proved to be effectively removed by photocatalytic oxidation during the algal destruction process. After three consecutive cycles, the removal rate of algal cells can still achieve 88.7% by F-Ce-TiO2/EP450, indicating the F-Ce-TiO2/EP photocatalyst has good reusability and stability.

Plant Responses to Salt Stress: Adaptive Mechanisms

This review deals with the adaptive mechanisms that plants can implement to cope with the challenge of salt stress. Plants tolerant to NaCl implement a series of adaptations to acclimate to salinity, including morphological, physiological and biochemical changes. These changes include increases in the root/canopy ratio and in the chlorophyll content in addition to changes in the leaf anatomy that ultimately lead to preventing leaf ion toxicity, thus maintaining the water status in order to limit water loss and protect the photosynthesis process. Furthermore, we deal with the effect of salt stress on photosynthesis and chlorophyll fluorescence and some of the mechanisms thought to protect the photosynthetic machinery, including the xanthophyll cycle, photorespiration pathway, and water-water cycle. Finally, we also provide an updated discussion on salt-induced oxidative stress at the subcellular level and its effect on the antioxidant machinery in both salt-tolerant and salt-sensitive plants. The aim is to extend our understanding of how salinity may affect the physiological characteristics of plants.

The water-water cycle is a major electron sink in Camellia species when CO2 assimilation is restricted

The water-water cycle (WWC) is thought to dissipate excess excitation energy and balance the ATP/NADPH energy budget under some conditions. However, the importance of the WWC in photosynthetic regulation remains controversy. We observed that three Camellia cultivars exhibited high rates of photosynthetic electron flow under high light when photosynthesis was restricted. We thus tested the hypothesis that the WWC is a major electron sink in the three Camellia cultivars when CO2 assimilation is restricted. Light response curves indicated that the WWC was strongly increased with photorespiration and was positively correlated with extra ATP supplied from other flexible mechanisms excluding linear electron flow, implying that the WWC is an important alternative electron sink to balance ATP/NADPH energy demand for sustaining photorespiration in Camellia cultivars. Interestingly, when photosynthesis was depressed by the decreases in stomatal and mesophyll conductance, the rates of photosynthetic electron flow through photosystem II declined slightly and the rates of WWC was enhanced. Furthermore, the increased electron flow of WWC was positively correlated with the ratio of Rubisco oxygenation to carboxylation, supporting the involvement of alternative electron flow in balancing the ATP/NADPH energy budget. We propose that the WWC is a crucial electron sink to regulate ATP/NADPH energy budget and dissipate excess energy excitation in Camellia species when CO2 assimilation is restricted.