Oxygen Evolving Complex Activity Research Articles

High-nitrogen fertilizer alleviated adverse effects of drought stress on the growth and photosynthetic characteristics of Hosta ‘Guacamole’

BackgroundSeveral plants are facing drought stress due to climate change in recent years. In this study, we aimed to explore the effect of varying watering frequency on the growth and photosynthetic characteristics of Hosta ‘Guacamole’. Moreover, we investigated the effect of high-nitrogen and -potassium fertilizers on alleviating the impacts of drought stress on the morphology, photosynthetic characteristics, chlorophyll fluorescence, fast chlorophyll a fluorescence transient, JIP-test parameters, and enzymatic and non-enzymatic scavenging system for reactive oxygen species (ROS) in this species.ResultsLeaf senescence, decreased chlorophyll contents, limited leaf area, and reduced photosynthetic characteristics and oxygen-evolving complex (OEC) activity were observed in Hosta ‘Guacamole’ under drought stress. However, high-nitrogen fertilizer (30-10-10) could efficiently alleviate and prevent the adverse effects of drought stress. High-nitrogen fertilizer significantly increased chlorophyll contents, which was higher by 106% than drought stress. Additionally, high-nitrogen fertilizer significantly improved net photosynthetic rate and water use efficiency, which were higher by 467% and 2900% than those under drought stress. It attributes that high-nitrogen fertilizer could reduce transpiration rate of leaf cells and stomatal opening size in drought stress. On the other hand, high-nitrogen fertilizer enhanced actual photochemical efficiency of PS II and photochemical quenching coefficient, and actual photochemical efficiency of PS II significantly higher by 177% than that under drought stress. Furthermore, high-nitrogen fertilizer significantly activated OEC and ascorbate peroxidase activities, and enhanced the performance of photosystem II and photosynthetic capacity compared with high-potassium fertilizers (15-10-30).ConclusionsHigh-nitrogen fertilizer (30-10-10) could efficiently alleviate the adverse effects of drought stress in Hosta ‘Guacamole’ via enhancing OEC activity and photosynthetic performance and stimulating enzymatic ROS scavenging system.

Insights into melatonin-induced photosynthetic electron transport under low-temperature stress in cucumber.

In this study, the differences in chlorophyll fluorescence transient (OJIP) and modulated 820 nm reflection (MR820) of cucumber leaves were probed to demonstrate an insight into the precise influence of melatonin (MT) on cucumber photosystems under low temperature stress. We pre-treated cucumber seedlings with different levels of MT (0, 25, 50, 100, 200, and 400 μmol · L-1) before imposing low temperature stress (10 °C/6 °C). The results indicated that moderate concentrations of MT had a positive effect on the growth of low temperature-stressed cucumber seedlings. Under low temperature stress conditions, 100 μmol · L-1 (MT 100) improved the performance of the active photosystem II (PSII) reaction centers (PIabs), the oxygen evolving complex activity (OEC centers) and electron transport between PSII and PSI, mainly by decreasing the L-band, K-band, and G-band, but showed differences with different duration of low temperature stress. In addition, these indicators related to quantum yield and energy flux of PSII regulated by MT indicated that MT (MT 100) effectively protected the electron transport and energy distribution in the photosystem. According to the results of WO-I ≥ 1 and MR820 signals, MT also affected PSI activity. MT 100 decreased the minimal value of MR/MRO and the oxidation rate of plastocyanin (PC) and PSI reaction center (P700) (Vox ), while increased △MRslow/MRO and deoxidation rates of PC+ and P700 + (Vred ). The loss of the slow phase of MT 200 and MT 400-treated plants in the MR820 kinetics was due to the complete prevention of electron movement from PSII to re-reduce the PC+ and P700 +. These results suggest that appropriate MT concentration (100 μmol · L-1) can improve the photosynthetic performance of PS II and electron transport from primary quinone electron acceptor (QA) to secondary quinone electron acceptor (QB), promote the balance of energy distribution, strengthen the connectivity of PSI and PSII, improve the electron flow of PSII via QA to PC+ and P700 + from reaching PSI by regulating multiple sites of electron transport chain in photosynthesis, and increase the pool size and reduction rates of PSI in low temperature-stressed cucumber plants, All these modifications by MT 100 treatment promoted the photosynthetic electron transfer smoothly, and further restored the cucumber plant growth under low temperature stress. Therefore, we conclude that spraying MT at an appropriate concentration is beneficial for protecting the photosynthetic electron transport chain, while spraying high concentrations of MT has a negative effect on regulating the low temperature tolerance in cucumber.

A biochemical, physiological and molecular evaluation of how the herbicide 2, 4-dichlorophenoxyacetic acid intercedes photosynthesis and diazotrophy in the cyanobacterium Nostoc muscorum Meg 1.

Among the non-target microorganisms residing in crop fields that are potentially vulnerable to herbicides are cyanobacteria. They contribute to the maintenance of soil quality and fertility and hence are considered to be an important component of soil microflora. Consequently, the present study was aimed to check the influence of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) on some major parameters of carbon (CO2) and nitrogen (N2) fixations of a cyanobacterium Nostoc muscorum Meg 1 isolated from a rice field in Cherrapunji, Meghalaya, India. These include various photosynthetic pigments, the oxygen-evolving complex activity of the PSII, the protein contents of RuBisCO, D1 protein, isocitrate dehydrogenase (IDH), nitrogenase and glutamine synthetase (GS) enzymes, the heterocyst percentage, nitrogenase and GS enzyme activities, and production of total proteins and carbohydrates in the cyanobacterium in a varying range of 50 to 125 ppm doses of 2,4-D. The mRNA levels of several proteins were also analyzed. Besides carotenoid concentration that enhanced at 50 ppm, all other parameters were compromised by 2,4-D in a dose-dependent manner resulting in a reduction in photosynthetic and N2-fixing activities. The negative effect on N2-fixation was partly due to compromised IDH activity. RT-PCR analysis further showed that these negative effects were initiated at transcription levels as mRNA contents of all enzymes studied were found compromised under 2,4-D exposure. The scanning and transmission electron microscopy further revealed herbicide induced adverse changes in the morphology and ultrastructure of the organism. The significance of the work lies in its detailed analysis of the effect of 2,4-D at biochemical, physiological, and molecular levels.

Ultraviolet radiation underlies metabolic energy reprograming in Coffea arabica and Coffea canephora genotypes

Depletion of the stratospheric ozone layer enhances UV exposure to living organisms. We hypothesized that the adaptative physiological and bioenergetic responses of modern genotypes of Coffea arabica cv. Catuaí Amarelo IAC 62, and C. canephora (Conilon LB1) to actual UV levels would generate additional costs to the detriment of biomass formation and partitioning. Coffee plants were cultivated for six months in a mini greenhouse under either near ambient (UVam) or reduced (UVre) UV conditions, after which leaf gas exchanges, chlorophyll a fluorescence parameters and proton pump phosphohydrolase activities were analyzed. Growth under UVam did not affect the photochemical efficiency and net CO2 assimilation in both Coffea species, but reduced stomatal conductance (gs) and increased intrinsic water use efficiency (iWUE), maximum photosynthetic O2 evolution, and apparent quantum yield. Coffee plants under UVam showed decreased P-type H+-ATPase activity, whilst H+-PPase activity increased, preserving metabolic energy. Coffee biomass accumulation decreases under UVam were more pronounced in C. canephora, which generally invested more in the root system than in shoot biomass as compared to C. arabica. Reduced biomass in C. canephora under UVam susuggested restrained C-sequestration in plant biomass, which might be partly associated with gs reduction, along with a equilibrium between the ATP- and PPi-driven electrochemical H+ gradients, providing a mechanism for energy buffering and cellular robustness. The stability of fluorescence parameters and net CO2 assimilation rate under UVam, reduced gs and increased iWUE, oxygen-evolving complex activity, and elevated SPAD values, supported photomorphogenetic responses acclimation to actual UV in two modern coffee genotypes, with detrimental impact on shoot biomass, especially in C. canephora.

Cassava common mosaic virus infection causes alterations in chloroplast ultrastructure, function, and carbohydrate metabolism of cassava plants

AbstractCassava common mosaic virus (CsCMV) is a potexvirus that causes systemic infections in cassava plants, leading to chlorotic mosaic and producing significant yield losses. To date, the physiological alterations and the mechanism underlying biotic stress during the cassava–CsCMV compatible interaction remains unknown. In this study, we found that CsCMV infection adversely modified chloroplast structure and had functional effects on chloroplasts in source leaves during the course of viral infection. Extrusion of the chloroplast membrane with amoeboid‐shaped appearance and disorganized grana stacks were observed in infected mesophyll cells. These alterations were associated with up to 35% reduction of relative chlorophyll content, and a decline of CO2 fixation (13.5% and 24.2% at 90 and 210 days after planting, respectively). The effects of CsCMV infection on the performance index on absorption basis dropped up to 37%. The analysis of chlorophyll a fluorescence showed a progressive loss of both oxygen evolving complex activity and “connectivity” within the tripartite system (core antenna‐LHCII‐reaction centre). Here, we report the latter phenomenon for the first time in a viral infection. The oxidative stress process was observed in CsCMV‐infected plants (20.8% reduction of antioxidant capacity with respect to noninfected plants). Other effects of the pathogen included reduction of starch and maltose content in source leaves, and a significant increase (24.7%) of the sucrose:starch ratio, which indicates an altered pattern of carbon allocation. Our results suggest that CsCMV induces chloroplast distortion associated with progressive chloroplast function loss and diversion of carbon flux in source leaf tissue, leading to the loss of cassava tuber yield.

Close relationship between the state of the oxygen evolving complex and rice cold stress tolerance

The results of the present work suggested a relationship between the growth stability and functional/structural parameters associated to the primary photochemistry and oxygen evolving complex (OEC) in tolerant rice plants under suboptimal low temperatures (SLT) stress. This was concluded from the absence of changes in net photosynthetic rate and in fraction of reaction centers to reduce quinone A, and very small changes in P680 efficiency to trap and donate electrons to quinone A and in fraction of active OEC in tolerant plants under cold stress but not in sensitive plants. The SLT stress also induced OEC activity limitations in both genotypes, but in a greater extent in sensitive plants. However, an assay using an artificial electron donor to replace OEC indicated that the P680+ capacity to accept electrons was not altered in both genotypes under SLT stress from the beginning of the stress treatment, suggesting that the OEC structure stability is related to rice SLT tolerance to sustain the photosynthesis. This hypothesis was also supported by the fact that tolerant plants but not sensitive plants did not alter the gene expression and protein content of PsbP under SLT stress, an OEC subunit with a role in stabilizing of OEC structure.

Can Ceylon Leadwort (Plumbago zeylanica L.) Acclimate to Lead Toxicity?-Studies of Photosynthetic Apparatus Efficiency.

Ceylon leadwort (Plumbago zeylanica) is ornamental plant known for its pharmacological properties arising from the abundant production of various secondary metabolites. It often grows in lead polluted areas. The aim of presented study was to evaluate the survival strategy of P. zeylanica to lead toxicity via photosynthetic apparatus acclimatization. Shoots of P. zeylanica were cultivated on media with different Pb concentrations (0.0, 0.05, and 0.1 g Pb∙l−1). After a four-week culture, the efficiency of the photosynthetic apparatus of plants was evaluated by Chl a fluorescence measurement, photosynthetic pigment, and Lhcb1, PsbA, PsbO, and RuBisCo protein accumulation, antioxidant enzymes activity, and chloroplast ultrastructure observation. Plants from lower Pb concentration revealed no changes in photosynthetic pigments content and light-harvesting complex (LHCII) size, as well as no limitation on the donor side of Photosystem II Reaction Centre (PSII RC). However, the activity and content of antioxidant enzymes indicated a high risk of limitation on the acceptor side of Photosystem I. In turn, plants from 0.1 g Pb∙l−1 showed a significant decrease in pigments content, LHCII size, the amount of active PSII RC, oxygen-evolving complex activity, and significant remodeling of chloroplast ultrastructure indicated limitation of PSII RC donor side. Obtained results indicate that P. zeylanica plants acclimate to lead toxicity by Pb accumulation in roots and, depending on Pb concentration, by adjusting their photosynthetic apparatus via the activation of alternative (cyclic and pseudocyclic) electron transport pathways.

Photosynthetic and growth responses of green and purple basil plants under different spectral compositions.

Light spectrum of growing environment is a determinant factor for plant growth and photosynthesis. Plants under different light spectra exhibit different growth and photosynthetic behaviors. To unravel the effects of light spectra on plant growth, photosynthetic pigments and electron transport chain reactions, purple and green basil varieties were grown under five different light spectra including white (W: 400-730nm), blue (B: 400-500nm), red (R: 600-700nm) and two combinations of R and B lights (R50B50 and R70B30), with same PPFD (photosynthetic photon flux density). Almost all values for shoot and root growth traits were higher in purple variety and were improved by combinational R and B lights (especially under R70B30), while they were negatively influenced by B monochromatic light when compared to growth traits of W-grown plants. Highest concentration of photosynthetic pigments was detected in R70B30. Biophysical properties of photosynthetic electron transport chain showed higher florescence intensity at all steps of OJIP kinetics in plants grown under R light in both varieties. Oxygen evolving complex activity (Fv/Fo) and PSII maximum quantum efficiency (Fv/Fm) in R-grown plants were lower than plants grown under other light spectra. Values for parameters related to specific energy fluxes per reaction center (ABS/RC, TRo/RC, ETo/RC and DIo/RC) were increased under R light (especially for purple variety). Performance index was significantly decreased under R light in both varieties. In conclusion, light spectra other than RB combination, induced various limitations on pigmentations, efficiency of electron transport and growth of basil plants and the responses were cultivar specific.

Effects of High Light and Chilling Stress on Photosystem II Efficiency of Aloe vera L. Plants Probing by Chlorophyll a Fluorescence Measurements

In this study, Aloe vera, as a CAM plant, was used to study photosystem II (PSII) activity in response to chilling (2 °C) and high light (HL, 1200 µmol m−2 s−1) stresses. Using chlorophyll (Chl) a fluorescence transient (OJIP) measurements, we indicated that the photosynthetic apparatus of Aloe vera was damaged at chilling, as evaluated from decrease in maximal efficiency of PSII photochemistry (F v/F m) and in performance index (PIABS) coupled with lower values of photosynthetic electron transport chain components including the electron transport flux (φE o) and the inferred oxygen evolving complex activity (F v/F o). Performance index exhibited negative correlation with hydrogen peroxide (H2O2) content, which indicated that the sensitivity of PSII photochemistry exposed to chilling stress was associated with higher H2O2 and malondialdehyde accumulation. In contrary, Aloe vera leaves accumulated Chl b and carotenoids to prevent photoinhibition of PSII under excess light. Interestingly, we found that F v/F m parameter remained unchanged under different light intensities, whereas the HL-treated plants had higher PIABS values than control plants, suggesting that this parameter is more sensitive to different light intensities than commonly used parameter F v/F m.

High salicylic acid concentration alters the electron flow associated with photosystem II in barley

In this study, the effects of exogenously applied salicylic acid (0.5 and 5 mM SA) on the rates of photosystem II (PSII) activity was analysed in 4-week-old barley (Hordeum vulgare‘Bahman’ ) seedlings using chlorophyll (Chl) a fluorescence transient (OJIP) measurements. No evident changes in Chl and carotenoid contents as well as chlorophyll fluorescence transient curves were observed in either of the studied concentrations after 24 h of SA application. After 5 d, low SA concentration (0.5 mM) increased PSII activity, Chl b and carotenoid contents in barley seedlings. In contrary, 5 days after 5 mM SA treatment, the maximal quantum efficiency of PSII (Fv/Fm) and the Performance Index (PIABS), as an indicator of PSII structure and functioning, were significantly decreased. This lower Fv/Fm and PIABS coupled with lower levels of Chl b and carotenoids, and lower values of photosynthetic electron transport chain components including the electron transport flux (φEo) and the inferred oxygen evolving complex activity (Fv/Fo). By monitoring the chlorophyll a fluorescence rise kinetics, from the initial “O” level to the “P” (the peak) level, a dramatic increase in “OJ” phase was detected, which coincides with an increased photo-reduction of QA as a result of blockage of electron flow. This study provided the evidence that the high concentration of SA induced damage to different sites of the PSII.

Physiological, photochemical and ionic responses of sunflower seedlings to exogenous selenium supply under salt stress

Since the specific ionic and photochemical mechanisms of Se-mediated adaptation to salt stress is not yet clear, the present study was performed to address this issue. Sunflower (Helianthus annuus L.) plants were grown under controlled conditions, and soils of Se treatment were fertilized with selenium (5 mg kg−1 as Na2SeO4) before filling the pots. Four weeks after sowing, salinity treatment using NaCl salt was started as EC = 15 dS m−1. Salt stress significantly decreased seedling growth, whereas application of Se alleviated the detrimental effect of NaCl stress. Salt treatment decreased maximum quantum yield of photochemistry (about 12%), inferred oxygen evolving complex activity (F v/F o) (about 39%), and calculated Performance Index (PIABS) (about 75%) comparing with control plants; however, Se ameliorated these effects. Consequently, the photosynthetic apparatus of the Se-supplied seedlings exhibited enhanced resistance to salt stress, compared to those under non-Se condition. Salt stress increased Na content and reduced K concentration, whereas shoot and root K were both significantly increased in seedlings pretreated with Se. Under saline conditions, treatment with Se enhanced the glutathione peroxidase activity, and also resulted in a significant reduction of malondialdehyde contents. These results suggest that Se decreases salt-induced damages through stimulating the antioxidant activities, which is associated with the improvement of the K/Na ratio needed for normal photochemical functioning, resulting in better plant growth under salt stress.

Environmental pH and the Requirement for the Extrinsic Proteins of Photosystem II in the Function of Cyanobacterial Photosynthesis.

In one of the final stages of cyanobacterial Photosystem II (PS II) assembly, binding of up to four extrinsic proteins to PS II stabilizes the oxygen-evolving complex (OEC). Growth of cyanobacterial mutants deficient in certain combinations of these thylakoid-lumen-associated polypeptides is sensitive to changes in environmental pH, despite the physical separation of the membrane-embedded PS II complex from the external environment. In this perspective we discuss the effect of environmental pH on OEC function and photoautotrophic growth in cyanobacteria with reference to pH-sensitive PS II mutants lacking extrinsic proteins. We consider the possibilities that, compared to pH 10.0, pH 7.5 increases susceptibility to PS II-generated reactive oxygen species (ROS) causing photoinhibition and reducing PS II assembly in some mutants, and that perturbations to channels in the lumenal regions of PS II might alter the accessibility of water to the active site as well as egress of oxygen and protons to the thylakoid lumen. Reduced levels of PS II in these mutants, and reduced OEC activity arising from the disruption of substrate/product channels, could reduce the trans-thylakoid pH gradient (ΔpH), leading to the impairment of photosynthesis. Growth of some PS II mutants at pH 7.5 can be rescued by elevating CO2 levels, suggesting that the pH-sensitive phenotype might primarily be an indirect result of back-pressure in the electron transport chain that results in heightened production of ROS by the impaired photosystem.

Evidence for the role of cyclic electron flow in photoprotection for oxygen-evolving complex

Cyclic electron flow (CEF) alleviates PSII photo-inhibition under high light by at least two different mechanisms: one is liked to thermal energy dissipation (qE) and the other one is independent of qE. However, the latter mechanism is unclear. Because the photodamage to PSII primarily occurred at the oxygen-evolving complex (OEC), and the stability of OEC is dependent on proton gradient across thylakoid membrane (ΔpH), we hypothesize that the CEF-dependent generation of ΔpH can alleviate photodamage to OEC. To test this hypothesis, we determined the effects of antimycin A (AA), methyl viologen (MV), chloramphenicol (CM), nigericin (Nig) on PSII activity and the stability of OEC for leaves of a light-demanding tropical tree species Erythrophleum guineense by the analysis of OKJIP chlorophyll a fluorescence transient. After high light treatment, the stronger decrease in Fv/Fm in the AA-, CM-, MV-, and Nig-treated samples was accompanied with larger photo damage of OEC. The AA-treated samples significantly showed lower CEF activity than the H2O-treated samples. Although the AA-treated leaves significantly showed stronger PSII photo-inhibition and photo-damage of OEC compared to the H2O-treated leaves, the value of non-photochemical quenching did not differ between them. Therefore, CEF activity was partly inhibited in the AA-treated samples, and the stronger PSII photo-inhibition in the AA-treated leaves was independent of qE. Taking together, we propose a hypothesis that CEF-dependent generation of ΔpH under high light plays an important role in photoprotection for the OEC activity.

Insusceptibility of oxygen-evolving complex to high light in Betula platyphylla.

High mountain plants growing at high altitude have to regularly cope with high light and high UV radiation that can lead to photodamage of oxygen-evolving complex (OEC). However, the underlying mechanism of photoprotection for OEC in high mountain plants is unclear. Sun leaves of Betula platyphylla were used to examine whether cyclic electron flow (CEF) around photosystem I (PSI) plays an important role in photoprotection for OEC. Our results indicated that the value of ETRI/ETRII ratio significantly increased under high light. With increasing light intensity, non-photochemical quenching (NPQ) gradually increased, and the fraction of P700 that is oxidized in a given state gradually increased. These results indicated that CEF was significantly activated under high light. After treatment with a high light of 1600 μmol photons m(-2) s(-1) for 8 h, the OEC activity did not decline, but the maximum quantum yield of PSII (F v /F m ) ratio significantly decreased. These results suggested that CEF-dependent generation of proton gradient across thylakoid membrane protected OEC activity against high light. Furthermore, the stability of PSI activity during exposure to high light suggested that the high CEF activity in B. platyphylla played an important role in photoprotection for PSI activity.

The effect of high UV-B dosage on apple fruit photosystems at different fruit maturity stages

Apple fruits (Malus domestica Borkh) of the cultivars Granny Smith, Fuji, Cripp's Pink, Braeburn, Golden Delicious and Topred were harvested at three stages during fruit growth. Previously sun-exposed peels of the apple fruits were exposed to a high ultraviolet radiation-B (UV-B) dosage for 150min at each stage. In a second experiment mature ‘Granny Smith’, ‘Fuji’ and ‘Cripps’ Pink’ fruits previously sun or shade exposed were also exposed to the UV-B stress. The effect of UV-B stress on fruits photosystem components was assessed by measuring the change in maximum light use efficiency and light reflection of fruit peels. UV-B induced pigment changes were analysed for ‘Braeburn’, ‘Fuji’ and ‘Cripps’ Pink’. The UV-B stress did not cause photoinhibition to any of the cultivars during fruit growth. However, UV-B stress did cause photoinhibition to previously shaded mature ‘Granny Smith’ and ‘Fuji’ fruits. Previously shaded ‘Cripps’ Pink’ fruits were conversely as insensitive to UV-B stress as the previously sun exposed fruits. ‘Braeburn’ showed no major pigment response to UV-B stress throughout the season. However, in ‘Fuji’ and ‘Cripps’ Pink’ fruits, total phenolic content increased at mid-season and maturity, while decreasing at the juvenile stage. All cultivars appear to have a stronger light reflection response to UV-B stress at the juvenile stage than later in the season. Photosystem II (PS II) units (as indicated by the Fm values) and the oxygen evolving complex activity (as indicated by the Fv values) in all the cultivars decreased with fruit maturity. Shaded ‘Cripps’ Pink’ fruits seemed to use the xanthophyll cycle as a photoprotective mechanism after UV-B stress. Photosynthetic systems in sun-exposed, therefore acclimatised, apple fruit peel are possibly not sensitive to UV-B stress in isolation. The fruits are probably well screened against UV light. Conversely, shaded peel may be less adapted and therefore more sensitive to high UV-B exposure. The light reflection response to UV-B stress at the juvenile stage could be due to the reduced phenolic content after stress and the presence of more PS II units at this stage compared to the mature stage.

Effects of salt stress on photosystem II efficiency and CO 2 assimilation of two Syrian barley landraces

The photosynthetic activity of two Syrian barley landraces, Arabi (A.) Aswad and A. Abiad, grown under 120 mM NaCl, was studied, using gas exchange and chlorophyll (Chl) a fluorescence transient (OJIP) measurements. Salt treatment of barley seedlings decreased both the rates of photosynthesis and photosystem II (PSII) activity, as evaluated from chlorophyll fluorescence data. However, the noted decrease was dependent on the duration of the salt treatment and the barley cultivar. Several parameters (e.g., light absorption flux per cross section of leaf; time to reach maximum chlorophyll a fluorescence intensity; plastoquinone pool size; yield of heat loss; rate of reaction center closure; and the so-called Performance Index), calculated and inferred from Chl fluorescence measurements, and related to PSII activity, were affected after 24 h of salt application, but these changes were much more pronounced after 7 days of salt treatment. Similar changes were found for measured gas exchange parameters: CO 2 uptake (photosynthetic) rate and stomatal conductance. The photosynthetic apparatus of the cultivar variety (c.v.) Arabi Aswad was found to be much more tolerant to salt treatment, compared with c.v. Arabi Abiad. After 7 days of salt treatment, the latter showed a very high value of the initial (minimal) fluorescence ( F o) and then essentially almost flat fluorescence transient curve; this result may be due to several causes that include structural changes as well as changes in the rate constants of different dissipative processes. The parameters that were most affected, by salt treatment, were: the time needed to reach the maximal chlorophyll fluorescence ( F m), and the inferred oxygen evolving complex activity ( F v/ F o, where F v, is F m − F o), and the calculated Performance Index (PI ABS) that depends on the efficiency and the yield of energy transfer and primary photochemistry. We suggest that the early reactions of the photosynthetic apparatus of barley plants could play a key role in their tolerance to salt stress. Further, we found that the first stage of salinity effect on photosynthesis of barley plants is related to stomatal conductance limitation rather than to PSII activity reduction. Thus, on the basis of our results on the two barley landraces, we recommend the use of a combination of gas exchange measurements along with the analysis of the OJIP fluorescence transient for the detection of salt stress-induced changes in plants.

H2O2-induced inhibition of photosynthetic O2 evolution by Anabaena variabilis cells.

Hydrogen peroxide inhibits photosynthetic O2 evolution. It has been shown that H2O2 destroys the function of the oxygen-evolving complex (OEC) in some chloroplast and Photosystem (PS) II preparations causing release of manganese from the OEC. In other preparations, H2O2 did not cause or caused only insignificant release of manganese. In this work, we tested the effect of H2O2 on the photosynthetic electron transfer and the state of OEC manganese in a native system (intact cells of the cyanobacterium Anabaena variabilis). According to EPR spectroscopy data, H2O2 caused an increase in the level of photooxidation of P700, the reaction centers of PS I, and decreased the rate of their subsequent reduction in the dark by a factor larger than four. Combined effect of H2O2, CN-, and EDTA caused more than eight- to ninefold suppression of the dark reduction of P700+. EPR spectroscopy revealed that the content of free (or loosely bound) Mn2+ in washed cyanobacterial cells was ~20% of the total manganese pool. This content remained unchanged upon the addition of CN- and increased to 25-30% after addition of H2O2. The content of the total manganese decreased to 35% after the treatment of the cells with EDTA. The level of the H2O2-induced release of manganese increased after the treatment of the cells with EDTA. Incubation of cells with H2O2 for 2 h had no effect on the absorption spectra of the photosynthetic pigments. More prolonged incubation with H2O2 (20 h) brought about degradation of phycobilins and chlorophyll a and lysis of cells. Thus, H2O2 causes extraction of manganese from cyanobacterial cells, inhibits the OEC activity and photosynthetic electron transfer, and leads to the destruction of the photosynthetic apparatus. H2O2 is unable to serve as a physiological electron donor in photosynthesis.

Heat-induced Multiple Effects on PSII in Wheat Plants

Summary The effects of heat stress on the various functional aspects of photosystem II (PSII) were investigated by analysis of in vivo chlorophyll fluorescence in wheat ( Tritium aestivum L.) leaves exposed in the dark to a wide range of elevated temperatures (25–45°C) for 10 min. The results revealed that the effects of heat stress on PSII were characterised by two distinct domains of temperatures: moderately elevated temperatures (30–37.5°C) and severely elevated temperatures (higher than 37.5°C). In moderately elevated temperatures, no changes in the maximal efficiency of PSII photochemistry (F 4 /F m ) and photochemical quenching (qp) were observed. The decrease in the quantum yield of PSII electron transport (psn) and the efficiency of excitation energy capture by open PSII reaction centers (Fv'/Fm') was reversible and was due to a significant increase in non-photochemical quenching (qN). In severely elevated temperatures, the further decrease in psn and Fv'/Fm' was irreversible and was associated with a decrease in FJFm, which was a result of the decrease in the oxygen-evolving complex activity and of an inhibition of electron transport at the acceptor side of PSII. Our results suggest that heat stress displayed multiple effects on PSII. The following sequential events leading to an inhibition of PSII electron transport in the descending order of sensitivity to heat stress can be proposed: excitation energy capture by open PSII reaction centers ~ the oxygenevolving complex ofPSII ~ the acceptor side of PSII.

The Second Report

<h3>Abstract</h3> <i>Cassava common mosaic virus</i> (CsCMV) is a potexvirus that causes systemic infections in cassava plants, leading to chlorotic mosaic and producing significant yield losses. To date, the physiological alterations and the mechanism underlying biotic stress during the cassava-CsCMV compatible interaction remains unknown. In this study, we found that CsCMV infection adversely modified chloroplast structure and had functional effects on chloroplasts in source leaves during the course of viral infection. Extrusion of the chloroplast membrane with amoeboid-shaped appearance was observed in infected mesophyll cells. These alterations were associated with lower relative chlorophyll content, and reduced PSII efficiency and CO₂ fixation. Moreover, an oxidative stress process was observed in CsCMV-infected plants. Strong declines in the maximum quantum yield of primary photochemistry (F_v/F_m) were observed in infected plants. Furthermore, the analysis of Chlorophyll-a fluorescence (ChlF) evidenced a progressive loss of both oxygen evolving complex activity and “connectivity” within the tripartite system (core antenna-LHCII-Reaction Centre). Other effects of the pathogen included reduction of starch and maltose content in source leaves, and a significant increase of the sucrose/starch ratio, which indicates alteration pattern of carbon. Our results suggest that CsCMV induces chloroplast distortion associated with progressive chloroplast function loss and diversion of carbon flux in source leaf tissue, which should be key in inducing yield losses of infected crops. <h3>Main conclusion</h3> CsCMV infection adversely modified chloroplast structure and had functional effects on chloroplasts during the course of viral infection, associated with metabolic adjustment in cassava source leaves, which would partly explain cassava root yield losses.