Stomatal penetration by aqueous solutions--an update involving leaf surface particles

The recent visualization of stomatal nanoparticle uptake ended a 40-yr-old paradigm. Assuming clean, hydrophobic leaf surfaces, the paradigm considered stomatal liquid water transport to be impossible as a result of water surface tension. However, real leaves are not clean, and deposited aerosols may change hydrophobicity and water surface tension. Droplets containing NaCl, NaClO(3), (NH(4))(2) SO(4), glyphosate, an organosilicone surfactant or various combinations thereof were evaporated on stomatous abaxial and astomatous adaxial surfaces of apple (Malus domestica) leaves. The effects on photosynthesis, necrosis and biomass were determined. Observed using an environmental scanning electron microscope, NaCl and NaClO(3) crystals on hydrophobic tomato (Solanum lycopersicum) cuticles underwent several humidity cycles, causing repeated deliquescence and efflorescence of the salts. All physiological parameters were more strongly affected by abaxial than adaxial treatments. Spatial expansion and dendritic crystallization of the salts occurred and cuticular hydrophobicity was decreased more rapidly by NaClO(3) than NaCl. The results confirmed the stomatal uptake of aqueous solutions. Humidity fluctuations promote the spatial expansion of salts into the stomata. The ion-specific effects point to the Hofmeister series: chaotropic ions reduce surface tension, probably contributing to the defoliant action of NaClO(3), whereas the salt spray tolerance of coastal plants is probably linked to the kosmotropic nature of chloride ions.

Analysis of gene sequences indicates that quantity not quality of chloroplast small HSPs improves thermotolerance in C4 and CAM plants

Chloroplast-localized small heat-shock proteins (Cp-sHSP) protect Photosystem II and thylakoid membranes during heat and other stresses, and Cp-sHSP production levels are related to plant thermotolerance. However, to date, a paucity of Cp-sHSP sequences from C4 or CAM species, or from other extremely heat-tolerant species, has precluded an examination to determine if Cp-sHSP genes or proteins might differ among plants with photosynthetic pathways or between heat-sensitive and heat-tolerant species. To investigate this, we isolated and characterized novel Cp-sHSP genes in four plant species: two moderately heat-tolerant C4 species, Spartina alterniflora (monocot) and Amaranthus retroflexus (eudicot), and two very heat-tolerant CAM species, Agave americana (monocot) and Ferocactus wislizenii (eudicot) (respective genes: SasHSP27.12, ArsHSP26.43, AasHSP26.85 and FwsHSP27.52) by PCR-based genome walking and cDNA RACE. Analysis of these Cp-sHSPs has confirmed the presence of conserved domains common to previously examined species. As expected, the transit peptide was found to be the most variable part of these proteins. Promoter analysis of these genes revealed differences in CAM versus C3 and C4 species that were independent of a general difference between monocots and eudicots observed for the entire protein. Heat-induced gene and protein expression indicated that Cp-sHSP protein levels were correlated with thermotolerance of photosynthetic electron transport, and that in most cases protein and transcript levels were correlated. Thus, available evidence indicates little variation in the amino acid sequence of Cp-sHSP mature proteins between heat-sensitive and -tolerant species, but that variation in Cp-sHSP protein production is related to heat tolerance or photosynthetic pathway (CAM vs. C3 and C4) and is driven by promoter differences. Key message We isolated and characterized four novel Cp-sHSP genes with promoters from wild plants, analysis has shown qualitative and quantitative interspecific variations in Cp-sHSPs of C3, C4, and CAM plant thermotolerance.

Photosystem II function and dynamics in three widely used Arabidopsis thaliana accessions

Columbia-0 (Col-0), Wassilewskija-4 (Ws-4), and Landsberg erecta-0 (Ler-0) are used as background lines for many public Arabidopsis mutant collections, and for investigation in laboratory conditions of plant processes, including photosynthesis and response to high-intensity light (HL). The photosystem II (PSII) complex is sensitive to HL and requires repair to sustain its function. PSII repair is a multistep process controlled by numerous factors, including protein phosphorylation and thylakoid membrane stacking. Here we have characterized the function and dynamics of PSII complex under growth-light and HL conditions. Ws-4 displayed 30% more thylakoid lipids per chlorophyll and 40% less chlorophyll per carotenoid than Col-0 and Ler-0. There were no large differences in thylakoid stacking, photoprotection and relative levels of photosynthetic complexes among the three accessions. An increased efficiency of PSII closure was found in Ws-4 following illumination with saturation flashes or continuous light. Phosphorylation of the PSII D1/D2 proteins was reduced by 50% in Ws-4 as compared to Col-0 and Ler-0. An increase in abundance of the responsible STN8 kinase in response to HL treatment was found in all three accessions, but Ws-4 displayed 50% lower levels than Col-0 and Ler-0. Despite this, the HL treatment caused in Ws-4 the lagest extent of PSII inactivation, disassembly, D1 protein degradation, and the largest decrease in the size of stacked thylakoids. The dilution of chlorophyll-protein complexes with additional lipids and carotenoids in Ws-4 may represent a mechanism to facilitate lateral protein traffic in the membrane, thus compensating for the lack of a full complement of STN8 kinase. Nevertheless, additional PSII damage occurs in Ws-4, which exceeds the D1 protein synthesis capacity, thus leading to enhanced photoinhibition. Our findings are valuable for selection of appropriate background line for PSII characterization in Arabidopsis mutants, and also provide the first insights into natural variation of PSII protein phosphorylation.

Differences in ascorbate and glutathione levels as indicators of resistance and susceptibility in Eucalyptus trees infected with Phytophthora cinnamomi

In this study we investigated the role that ascorbate (AA) and glutathione (GSH) play in the plant pathogen interaction of susceptible Eucalyptus sieberi L. A. Johnson and resistant Eucalyptus sideroxylon Woolls with Phytophthora cinnamomi Rands root infection. In a glasshouse study, seedlings were grown in soil-free plant boxes to facilitate the inoculation of the root systems by a P. cinnamomi zoospore solution. Ascorbate and GSH concentrations were measured in infected roots and leaves, along with leaf gas exchange, chlorophyll fluorescence and carbohydrate concentrations over a time course up to 312 h (13 days) post-inoculation (pi). At the early stages of infection (from 24 h pi), significant decreases in AA and GSH concentrations were observed in the infected roots and leaves of the susceptible E. sieberi seedlings. At the later stage of infection (312 h pi), the earlier AA decreases in the leaves of infected plants had become significant increases. In contrast, late, significant AA increases in the absence of any GSH changes were observed in the infected roots of the resistant E. sideroxylon seedlings. In E. sideroxylon leaves, a significant GSH increase occurred at 24 h pi; however, by 312 h pi the earlier increase had become a significant decrease, while no changes occurred in AA. In E. sieberi, photosynthesis (A), stomatal conductance (g(s)) and PSII quantum efficiency (Φ(PSII)) were reduced by ~60, 80 and 30%, respectively, in infected plants and remained significantly lower than uninfected controls for the duration of the experiment. Significant reductions in these parameters did not occur until later (120 h pi for g(s) and 312 h pi for A and Φ(PSII)), and to a lesser extent in the resistant species. Non-structural carbohydrate analysis of roots and leaves indicate that carbohydrate metabolism and resource flow between shoots and roots may have been altered at later infection stages. This study suggests that reduced antioxidant capacity, leaf physiological function and carbohydrate metabolism are associated with susceptibility in E. sieberi to P. cinnamomi infection, while AA increases and new root formation were associated with resistance in E. sideroxylon.

Reconstitution, spectroscopy, and redox properties of the photosynthetic recombinant cytochrome b(559) from higher plants

A study of the in vitro reconstitution of sugar beet cytochrome b(559) of the photosystem II is described. Both α and β cytochrome subunits were first cloned and expressed in Escherichia coli. In vitro reconstitution of this cytochrome was carried out with partially purified recombinant subunits from inclusion bodies. Reconstitution with commercial heme of both (αα) and (ββ) homodimers and (αβ) heterodimer was possible, the latter being more efficient. The absorption spectra of these reconstituted samples were similar to that of the native heterodimer cytochrome b(559) form. As shown by electron paramagnetic resonance and potentiometry, most of the reconstituted cytochrome corresponded to a low spin form with a midpoint redox potential +36 mV, similar to that from the native purified cytochrome b(559). Furthermore, during the expression of sugar beet and Synechocystis sp. PCC 6803 cytochrome b(559) subunits, part of the protein subunits were incorporated into the host bacterial inner membrane, but only in the case of the β subunit from the cyanobacterium the formation of a cytochrome b(559)-like structure with the bacterial endogenous heme was observed. The reason for that surprising result is unknown. This in vivo formed (ββ) homodimer cytochrome b(559)-like structure showed similar absorption and electron paramagnetic resonance spectral properties as the native purified cytochrome b(559). A higher midpoint redox potential (+126 mV) was detected in the in vivo formed protein compared to the in vitro reconstituted form, most likely due to a more hydrophobic environment imposed by the lipid membrane surrounding the heme.

The photoprotective protein PsbS exerts control over CO(2) assimilation rate in fluctuating light in rice

A direct impact of chloroplastic protective energy dissipation (qE) on photosynthetic CO(2) assimilation has not been shown directly in plants in the absence of photoinhibition. To test this empirically we transformed rice to possess higher (overexpressors, OE) and lower (RNA interference, RNAi) levels of expression of the regulatory psbS gene and analysed CO(2) assimilation in transformants in a fluctuating measurement light regime. Western blots showed a several-fold difference in levels of PsbS protein between RNAi and OE plants with the wild type (WT) being intermediate. At a growth light intensity of 600 μmol m(-2) sec(-1) , the carboxylation capacity, electron transport capacity and dark adapted F(v)/F(m) (ratio of variable to maximum fluorescence) were inhibited in RNAi plants compared with WT and OE. The PsbS content had a significant impact on qE (measured here as non-photochemical quenching, NPQ) but the strongest effect was observed transiently, immediately following the application of light. This capacity for qE was several-fold lower in RNAi plants and significantly higher in OE plants during the first 10 min of illumination. At steady state the differences were reduced: notably at 500 μmol m(-2) sec(-1) all plants had the same NPQ values regardless of PsbS content. During a series of light-dark transitions the induction of CO(2) assimilation was inhibited in OE plants, reducing integrated photosynthesis during the light period. We conclude that the accumulation of PsbS and the resultant qE exerts control over photosynthesis in fluctuating light, showing that optimization of photoprotective processes is necessary for maximum photosynthetic productivity even in the absence of photoinhibitory stress.

Chloroplast movement behavior varies widely among species and does not correlate with high light stress tolerance

It is well known that chloroplasts move in response to changes in blue light intensity in order to optimize light interception, however, little is known about interspecific variation and the relative importance of this mechanism for the high light stress tolerance of plants. We characterized chloroplast movement behavior as changes in light transmission through a leaf in a variety of species ranging from ferns to monocots and eudicots and found a wide spectrum of responses. Most species exhibited a distinct accumulation response compared to the dark positioning, and all species showed a distinct avoidance response. The speed with which transmission values changed during the avoidance response was consistently faster than that during the accumulation response and speeds varied greatly between species. Plants thriving in higher growth light intensities showed greater degrees of accumulation responses and faster changes in transmission than those that prefer lower light intensities. In some species, the chloroplasts on both the adaxial and abaxial leaf surfaces changed their positioning in response to light, while in other species only the chloroplasts on one leaf side responded. No correlation was found between high light stress tolerance and the speed or degree of transmission changes, indicating that plants can compensate for slow and limited transmission changes using other photoprotective mechanisms.

Antisense reductions in the PsbO protein of photosystem II leads to decreased quantum yield but similar maximal photosynthetic rates

Photosystem (PS) II is the multisubunit complex which uses light energy to split water, providing the reducing equivalents needed for photosynthesis. The complex is susceptible to damage from environmental stresses such as excess excitation energy and high temperature. This research investigated the in vivo photosynthetic consequences of impairments to PSII in Arabidopsis thaliana (ecotype Columbia) expressing an antisense construct to the PsbO proteins of PSII. Transgenic lines were obtained with between 25 and 60% of wild-type (WT) total PsbO protein content, with the PsbO1 isoform being more strongly reduced than PsbO2. These changes coincided with a decrease in functional PSII content. Low PsbO (less than 50% WT) plants grew more slowly and had lower chlorophyll content per leaf area. There was no change in content per unit area of cytochrome b6f, ATP synthase, or Rubisco, whereas PSI decreased in proportion to the reduction in chlorophyll content. The irradiance response of photosynthetic oxygen evolution showed that low PsbO plants had a reduced quantum yield, but matched the oxygen evolution rates of WT plants at saturating irradiance. It is suggested that these plants had a smaller pool of PSII centres, which are inefficiently connected to antenna pigments resulting in reduced photochemical efficiency.

Mathematical review of the energy transduction stoichiometries of C(4) leaf photosynthesis under limiting light

A generalized model for electron (e(-) ) transport limited C(4) photosynthesis of NAD-malic enzyme and NADP-malic enzyme subtypes is presented. The model is used to review the thylakoid stoichiometries in vivo under strictly limiting light conditions, using published data on photosynthetic quantum yield and on photochemical efficiencies of photosystems (PS). Model review showed that cyclic e(-) transport (CET), rather than direct O(2) photoreduction, most likely contributed significantly to the production of extra ATP required for the C(4) cycle. Estimated CET, and non-cyclic e(-) transport supporting processes like nitrogen reduction, accounted for ca. 45 and 7% of total photosystem I (PSI) e(-) fluxes, respectively. The factor for excitation partitioning to photosystem II (PSII) was ca. 0.4. Further model analysis, in terms of the balanced NADPH: ATP ratio required for metabolism, indicated that: (1) the Q-cycle is obligatory; (2) the proton: ATP ratio is 4; and (3) the efficiency of proton pumping per e(-) transferred through the cytochrome b(6) /f complex is the same for CET and non-cyclic pathways. The analysis also gave an approach to theoretically assess CO(2) leakiness from bundle-sheath cells, and projected a leakiness of 0.07-0.16. Compared with C(3) photosynthesis, the most striking C(4) stoichiometry is its high fraction of CET.

Chloroplasts of Arabidopsis are the source and a primary target of a plant-specific programmed cell death signaling pathway

Enhanced levels of singlet oxygen ((1)O(2)) in chloroplasts trigger programmed cell death. The impact of (1)O(2) production in chloroplasts was monitored first in the conditional fluorescent (flu) mutant of Arabidopsis thaliana that accumulates (1)O(2) upon a dark/light shift. The onset of (1)O(2) production is rapidly followed by a loss of chloroplast integrity that precedes the rupture of the central vacuole and the final collapse of the cell. Inactivation of the two plastid proteins EXECUTER (EX1) and EX2 in the flu mutant abrogates these responses, indicating that disintegration of chloroplasts is due to EX-dependent signaling rather than (1)O(2) directly. In flu seedlings, (1)O(2)-mediated cell death signaling operates as a default pathway that results in seedlings committing suicide. By contrast, EX-dependent signaling in the wild type induces the formation of microlesions without decreasing the viability of seedlings. (1)O(2)-mediated and EX-dependent loss of plastid integrity and cell death in these plants occurs only in cells containing fully developed chloroplasts. Our findings support an as yet unreported signaling role of (1)O(2) in the wild type exposed to mild light stress that invokes photoinhibition of photosystem II without causing photooxidative damage of the plant.

The CRYPTOCHROME1-dependent response to excess light is mediated through the transcriptional activators ZINC FINGER PROTEIN EXPRESSED IN INFLORESCENCE MERISTEM LIKE1 and ZML2 in Arabidopsis

Exposure of plants to light intensities that exceed the electron utilization capacity of the chloroplast has a dramatic impact on nuclear gene expression. The photoreceptor Cryptochrome 1 (cry1) is essential to the induction of genes encoding photoprotective components in Arabidopsis thaliana. Bioinformatic analysis of the cry1 regulon revealed the putative cis-element CryR1 (GnTCKAG), and here we demonstrate an interaction between CryR1 and the zinc finger GATA-type transcription factors ZINC FINGER PROTEIN EXPRESSED IN INFLORESCENCE MERISTEM LIKE1 (ZML1) and ZML2. The ZML proteins specifically bind to the CryR1 cis-element as demonstrated in vitro and in vivo, and TCTAG was shown to constitute the core sequence required for ZML2 binding. In addition, ZML2 activated transcription of the yellow fluorescent protein reporter gene driven by the CryR1 cis-element in Arabidopsis leaf protoplasts. T-DNA insertion lines for ZML2 and its homolog ZML1 demonstrated misregulation of several cry1-dependent genes in response to excess light. Furthermore, the zml1 and zml2 T-DNA insertion lines displayed a high irradiance-sensitive phenotype with significant photoinactivation of photosystem II (PSII), indicated by reduced maximum quantum efficiency of PSII, and severe photobleaching. Thus, we identified the ZML2 and ZML1 GATA transcription factors as two essential components of the cry1-mediated photoprotective response.

The enhancement of cyclic electron flow around photosystem I improves the recovery of severely desiccated Porphyra yezoensis (Bangiales, Rhodophyta)

Porphyra yezoensis, a representative species of intertidal macro-algae, is able to withstand periodic desiccation at low tide but is submerged in seawater at high tide. In this study, changes in photosynthetic electron flow in P. yezoensis during desiccation and re-hydration were investigated. The results suggested that the cyclic electron flow around photosystem I (PSI) increased significantly during desiccation, continued to operate at times of severe desiccation, and showed greater tolerance to desiccation than the electron flow around PSII. In addition, PSI activity in desiccated blades recovered faster than PSII activity during re-hydration. Even though linear electron flow was suppressed by DCMU [3-(3',4'-dichlorophenyl)-1,1-dimethylurea], cyclic electron flow could still be restored. This process was insensitive to antimycin A and could be suppressed by dibromothymoquinone (DBMIB). The prolonged dark treatment of blades reduced the speed in which the cyclic electron flow around PSI recovered, suggesting that stromal reductants, including NAD(P)H, played an important role in the donation of electrons to PSI and were the main cause of the rapid recovery of cyclic electron flow in desiccated blades during re-hydration. These results suggested that cyclic electron flow in P. yezoensis played a significant physiological role during desiccation and re-hydration and may be one of the most important factors allowing P. yezoensis blades to adapt to intertidal environments.

The tolerance of Jatropha curcas seedlings to NaCl: an ecophysiological analysis

Jatropha curcas L. is a biodiesel crop that is resistant to drought stress. However, the salt tolerance of this plant has not yet been studied. To address this question, J. curcas seedlings were grown in a fertilised substrate to evaluate the effects of salinity stress on growth, leaf water relation and organic solutes, leaf and root mineral concentrations, chlorophyll fluorescence parameters, and carbohydrate concentration. The experiment consisted of six treatments with different concentrations of NaCl in the irrigation water: 0 (control), 30, 60, 90, 120 and 150 mM. The total biomass exhibited a salt-induced decrease in the 60 mM or higher NaCl concentrations. The Cl(r) concentration was higher than the Na(+) concentration in all of the plant tissues. The water potential and relative water content of the leaves were not affected by any of the salt treatments. However, salinity induced a decline in the leaf K(+) concentration, together with a significant enhancement in the leaf P, S, Fe, Zn, Mn and Cu levels. The net assimilation of CO₂ also decreased with the salt treatment, due in part to non-stomatal limitation from the increase in C(a)/C(i) and a decrease in the maximum quantum efficiency (F(v)/F(m)) of photosystem II and soil plant analysis development (SPAD) units. This work suggests that J. curcas seedlings exhibit a moderate tolerance to salinity, as the plants were able to tolerate up to 4 dS m(-1) (EC water irrigation; 30 mM NaCl). The negative influences of salinity in this crop are mainly due to Cl(r) and/or Na(+) toxicity and to a nutritional imbalance caused by an increase in the Na(+)/K(+) ratio. The osmotic effect of salinity in this species is negligible, perhaps due to its strong control of leaf transpiration, which reduces water loss.

Inhibitory effects of hypo-osmotic stress on extracellular carbonic anhydrase and photosynthetic efficiency of green alga Dunaliella salina possibly through reactive oxygen species formation

In this study, Dunaliella salina (D. salina) maintained in 30‰ salinity for more than two years was exposed to the salinities of 5‰, 10‰, 20‰, 30‰ (control) in order to investigate oxidative burst and it's possible connection with extracellular carbonic anhydrase (CA) under hypo-osmotic stress (low salinity). The results indicated that intracellular ROS contents increased significantly when cells were exposed to salinity of 5 and 10‰, and the increase also occurred at 20‰ salinity. The activity of extracellular CA and its gene (P60) expression decreased significantly when cells were exposed to salinity of 5-20‰. Data from H₂O₂ treatments hinted that ROS production was possibly one of the factors affecting CA, including enzyme activity and gene expression levels. Significant inhibition of effective quantum efficiency of PSII and photosynthetic oxygen evolution rate were observed with the increase of ROS production and decline of CA activities. Taken together, hypo-osmotic stresses could induce ROS production in D. salina, and CA enzyme activities and expression levels were consequently inhibited. As a result, algal photosynthesis and oxygen evolution were inhibited.

Photosynthetic performance, lipid production and biomass composition in response to nitrogen limitation in marine microalgae

Increasing energy prices demand a renewable, carbon neutral, transport fuel that is environmentally and commercially sustainable. The interest in the production of microalgae as biofuels is increasing due to their high oil content, rapid biomass production and small foot print. In this research, marine microalgae Dunaliella tertiolecta (Chlorophyceae) and Thalassiosira pseudonana (Bacillariophyceae) were incubated in nitrogen (N)-replete medium, and then transferred to N-free medium for 15 and 11 days, respectively. Fluorescence induction and relaxation (FIRe) fluorometry and Fourier transform infrared spectroscopy (FTIR) were used to monitor the photosynthetic performance, lipid production and metabolic responses to changing N availability. Growth rates of D. tertiolecta and T. pseudonana were 0.84 ± 0.16 d(-1) and 1.21 ± 0.09 d(-), respectively in N-replete medium. Upon transfer to N-free medium. The growth rates of T. pseudonana declined rapidly, while D. tertiolecta continued to grow for 5 days in N-free medium before growth declined slowly. The maximum quantum yield of photochemistry (F(v)/F(m)) remained high initially for D. tertiolecta but decreased immediately after transfer to N-free media for T. pseudonana. The functional absorption cross section for PSII (σ(PSII)) increased, the time constant for Q(A) reoxidation (τ(Qa)) and connectivity factor (p) decreased in parallel to the nutritional status of the microalgae. The relative protein and lipid content varied in response to N limitation, but carbohydrates did not change. Based on FTIR, D. tertiolecta and T. pseudonana produced 20-26% lipid when most stressed. The combination of photosynthetic efficiency and biomass composition monitoring provided evidence that metabolic strategies to changing nutrient status are species-specific.

Optimization of variable fluorescence measurements of phytoplankton communities with cyanobacteria

Excitation-emission fluorescence matrices of phytoplankton communities were simulated from laboratory-grown algae and cyanobacteria cultures, to define the optical configurations of theoretical fluorometers that either minimize or maximize the representation of these phytoplankton groups in community variable fluorescence measurements. Excitation sources that match the photosystem II (PSII) action spectrum of cyanobacteria do not necessarily lead to equal representation of cyanobacteria in community fluorescence. In communities with an equal share of algae and cyanobacteria, inducible PSII fluorescence in algae can be retrieved from community fluorescence under blue excitation (450-470 nm) with high accuracy (R (2) = 1.00). The highest correlation between community and cyanobacterial variable fluorescence is obtained under orange-red excitation in the 590-650 nm range (R (2) = 0.54). Gaussian band decomposition reveals that in the presence of cyanobacteria, the emission detection slit must be narrow (up to 10 nm) and centred on PSII chlorophyll-a emission (~683 nm) to avoid severe dampening of the signal by weakly variable phycobilisomal fluorescence and non-variable photosystem I fluorescence. When these optimizations of the optical configuration of the fluorometer are followed, both cyanobacterial and algal cultures in nutrient replete exponential growth exhibit values of the maximum quantum yield of charge separation in PSII in the range of 0.65-0.7.

Small CAB-like proteins prevent formation of singlet oxygen in the damaged photosystem II complex of the cyanobacterium Synechocystis sp. PCC 6803

The cyanobacterial small CAB-like proteins (SCPs) are single-helix membrane proteins mostly associated with the photosystem II (PSII) complex that accumulate under stress conditions. Their function is still ambiguous although they are assumed to regulate chlorophyll (Chl) biosynthesis and/or to protect PSII against oxidative damage. In this study, the effect of SCPs on the PSII-specific light-induced damage and generation of singlet oxygen ((1)O(2)) was assessed in the strains of the cyanobacterium Synechocystis sp. PCC 6803 lacking PSI (PSI-less strain) or lacking PSI together with all SCPs (PSI-less/scpABCDE(-) strain). The light-induced oxidative modifications of the PSII D1 protein reflected by a mobility shift of the D1 protein and by generation of a D1-cytochrome b-559 adduct were more pronounced in the PSI-less/scpABCDE(-) strain. This increased protein oxidation correlated with a faster formation of (1)O(2) as detected by the green fluorescence of Singlet Oxygen Sensor Green assessed by a laser confocal scanning microscopy and by electron paramagnetic resonance spin-trapping technique using 2, 2, 6, 6-tetramethyl-4-piperidone (TEMPD) as a spin trap. In contrast, the formation of hydroxyl radicals was similar in both strains. Our results show that SCPs prevent (1)O(2) formation during PSII damage, most probably by the binding of free Chl released from the damaged PSII complexes.

Physiological and proteomic responses of cotton (Gossypium herbaceum L.) to drought stress

Cotton genotype RAHS 187 was analyzed for changes in physiology, biochemistry and proteome due to drought stress. The deleterious effect of drought in cotton plants was mainly targeted towards photosynthesis. The gas-exchange parameters of net photosynthesis (A), stomatal conductance (g(s)) and transpiration (E) showed a decreasing trend as the drought intensity increased. The fluorescence parameters of, effective quantum yield of PSII (Φ(PSII)), and electron transport rates (ETR), also showed a declining trend. As the intensity of drought increased, both H(2)O(2) and MDA levels increased indicating oxidative stress. Anthocyanin levels were increased by more than four folds in the droughted plants. Two-dimensional gel electrophoresis detected more than 550 protein spots. Significantly expressed proteins were analyzed by peptide mass fingerprinting (PMF) using MALDI-TOF-TOF. The number of up-regulated spots was found to be 16 while 6 spots were down-regulated. The reasonable implications in drought response of the identified proteins vis-à-vis physiological changes are discussed. Results provide some additional information that can lead to a better understanding of the molecular basis of drought-sensitivity in cotton plants.

Assessing the effects of polychlorinated biphenyls (Aroclor 1254) on a scleractinian coral (Stylophora pistillata) at organism, physiological, and molecular levels

Polychlorinated biphenyls (PCBs) are a group of widespread contaminants, and accumulation of PCBs has been observed in corals in the field. However, the toxic effects of PCBs on corals have not been investigated. In this study, we tested short and long term toxicity of Aroclor 1254, a commercial PCB mixture, on the scleractinian coral Stylophora pistillata. Coral nubbins were incubated in either control seawater or seawater dosed with PCBs (approximately 300ng/L) for 96h. The effect of PCB exposure on coral gene expression at 4h post exposure was tested with the suppression subtractive hybridization (SSH) and quantitative PCR methods. Photosystem II activity of the zooxanthellae was measured at 96h. After the exposure, nubbins were moved into clean seawater and their survival and growth were observed for another 50 days. All nubbins survived during the exposure and the following 50-d recovery period. Photosystem II activity and coral growth were not affected by PCB exposure in this study. Fifty-four clones were sequenced for gene expression analysis, and 15% of these sequences were identified, including genes involved in general stress response, peptide metabolism, cellular receptor, cytoskeleton organization, membrane trafficking, and oxidative stress response. However, the quantitative PCR did not show significant difference in the five selected genes. In conclusion, acute exposure of S. pistillata to Aroclor 1254 at 300ng/L did not affect coral survival, photosynthesis or growth but may alter the expression of certain genes involved in various important cellular functions. The nubbin technique proved to be an efficient approach to simultaneously characterize the impact of PCBs on the corals at multiple biological levels.

[Effects of high- and low temperature stress on the leaf PSII functions and physiological characteristics of tree peony (Paeonia suffruticosa cv. 'Roufurong')]

Taking the detached leaves of tree peony (Paeonia suffruticosa cv. 'Roufurong') as test materials, this paper studied the effects of high temperature (40 degrees C) and low temperature (15 degrees C) stresses on the PS II functions and physiological characteristics of peony leaves under strong light intensity (1400 micromol x m(-2) x s(-1)), with 25 degrees C as the control. With the increasing time of high- and low temperature stress, the maximal photochemical efficiency (Fv/Fm), actual quantum yield of photosystem II (Phi(PS II)) , and efficiency of excitation capture of open PS II center (Fv'/Fm') all decreased continuously. After recovered in the dark for 4 hours, the Fv/Fm in treatments 15 degrees C and 25 degrees C quickly recovered, but that in treatment 40 degrees C only recovered to 75.5% of non-treatment, even if the leaves were treated in the dark for 15 hours. At 40 degrees C, the balance of excited energy between PS I and PS II under strong light intensity was perturbed seriously. Treatment 40 degrees C inhibited the superoxide dismutase (SOD) activity, enhanced the production of O2-, H2O2, and MDA, and reduced the contents of chlorophyll and soluble protein. This study revealed that strong light combined with high temperature impaired the photosynthetic apparatus of the tree peony irreversibly, whereas strong light plus low temperature had weaker impact.

Characterization of PSI recovery after chilling-induced photoinhibition in cucumber (Cucumis sativus L.) leaves

By simultaneously analyzing the chlorophyll a fluorescence transient and light absorbance at 820 nm as well as chlorophyll fluorescence quenching, we investigated the effects of different photon flux densities (0, 15, 200 μmol m(-2) s(-1)) with or without 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) on the repair process of cucumber (Cucumis sativus L.) leaves after treatment with low temperature (6°C) combined with moderate photon flux density (200 μmol m(-2 )s(-1)) for 6 h. Both the maximal photochemical efficiency of Photosystem II (PSII) (F (v)/F (m)) and the content of active P700 (ΔI/I (o)) significantly decreased after chilling treatment under 200 μmol m(-2 )s(-1) light. After the leaves were transferred to 25°C, F (v)/F (m) recovered quickly under both 200 and 15 μmol m(-2 )s(-1) light. ΔI/I (o) recovered quickly under 15 μmol m(-2) s(-1) light, but the recovery rate of ΔI/I (o) was slower than that of F (v)/F (m). The cyclic electron transport was inhibited by chilling-light treatment obviously. The recovery of ΔI/I (o) was severely suppressed by 200 μmol m(-2) s(-1) light, whereas a pretreatment with DCMU effectively relieved this suppression. The cyclic electron transport around PSI recovered in a similar way as the active P700 content did, and the recovery of them was both accelerated by pretreatment with DCMU. The results indicate that limiting electron transport from PSII to PSI protected PSI from further photoinhibition, accelerating the recovery of PSI. Under a given photon flux density, faster recovery of PSII compared to PSI was detrimental to the recovery of PSI or even to the whole photosystem.

Photosynthetic alterations of pea leaves infected systemically by pea enation mosaic virus: A coordinated decrease in efficiencies of CO(2) assimilation and photosystem II photochemistry

We have investigated photosynthetic changes of fully expanded pea leaves infected systemically by pea enation mosaic virus (PEMV) that often attacks legumes particularly in northern temperate regions. A typical compatible virus-host interaction was monitored during 40 post-inoculation days (dpi). An initial PEMV-induced decrease in photosynthetic CO(2) assimilation was detected at 15 dpi, when the virus appeared in the measured leaves. This decrease was not induced by stomata closure and corresponded with a decrease in the efficiency of photosystem II photochemistry (Φ(PSII)). Despite of a slight impairment of oxygen evolution at this stage, PSII function was not primarily responsible for the decrease in Φ(PSII). Chlorophyll fluorescence imaging revealed that Φ(PSII) started to decrease from the leaf tip to the base. More pronounced symptoms of PEMV disease appeared at later stages, when a typical mosaic and enations appeared in the infected leaves and oxidative damage of cell membranes was detected. From 30 dpi, a degradation of photosynthetic pigments accelerated, stomata were closing and corresponding pronounced decline in CO(2) assimilation was observed. A concomitant photoprotective responses, i.e. an increase in non-photochemical quenching and accumulation of de-epoxidized xanthophylls, were also detected. Interestingly, alternative electron sinks in chloroplasts were not stimulated by PEMV infection, which is in contradiction to earlier reports dealing with virus-induced plant stresses. The presented results show that the PEMV-induced alterations in mature pea leaves accelerated leaf senescence during which a decrease in Φ(PSII) took place in coordinated manner with an inhibition of CO(2) assimilation.

Characterization of PSI recovery after chilling-induced photoinhibition in cucumber (Cucumis sativus L.) leaves

By simultaneously analyzing the chlorophyll a fluorescence transient and light absorbance at 820 nm as well as chlorophyll fluorescence quenching, we investigated the effects of different photon flux densities (0, 15, 200 μmol m(-2) s(-1)) with or without 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) on the repair process of cucumber (Cucumis sativus L.) leaves after treatment with low temperature (6°C) combined with moderate photon flux density (200 μmol m(-2 )s(-1)) for 6 h. Both the maximal photochemical efficiency of Photosystem II (PSII) (F (v)/F (m)) and the content of active P700 (ΔI/I (o)) significantly decreased after chilling treatment under 200 μmol m(-2 )s(-1) light. After the leaves were transferred to 25°C, F (v)/F (m) recovered quickly under both 200 and 15 μmol m(-2 )s(-1) light. ΔI/I (o) recovered quickly under 15 μmol m(-2) s(-1) light, but the recovery rate of ΔI/I (o) was slower than that of F (v)/F (m). The cyclic electron transport was inhibited by chilling-light treatment obviously. The recovery of ΔI/I (o) was severely suppressed by 200 μmol m(-2) s(-1) light, whereas a pretreatment with DCMU effectively relieved this suppression. The cyclic electron transport around PSI recovered in a similar way as the active P700 content did, and the recovery of them was both accelerated by pretreatment with DCMU. The results indicate that limiting electron transport from PSII to PSI protected PSI from further photoinhibition, accelerating the recovery of PSI. Under a given photon flux density, faster recovery of PSII compared to PSI was detrimental to the recovery of PSI or even to the whole photosystem.

[Allelopathic influence of Myriophyllum spicatum on the photosynthetic efficiency of Microcystis aeruginosa]

The allelopathic influence of Myriophyllum spicatum on chlorophyll content and chlorophyll fluorescence parameters of Microcystis aeruginosa was studied in coexistence condition. Chlorophyll fluorescence parameters included q(N) (non-photochemical quenching), Y II (effective quantum yield), F(v)/F(m) (maximum quantum yield), F'(v)/F'(m) (effective quantum yield of photosystem II photochemistry) and ETR (electron transport rate). During the three days under coexistence condition, chlorophyll content and chlorophyll fluorescence parameters of M. aeruginosa were affected by M. spicatum and presented different sensitivities. Chlorophyll content of M. aeruginosa was significantly inhibited by 20.80% on the second day at 10.0 g/L of M. spicatum (P < 0.05). However, chlorophyll fluorescence parameters of M. aeruginosa decreased earlier and rapider than chlorophyll content. On the first day, q(N) and Y II of M. aeruginosa were significantly inhibited by 15.59% and 13.00% at 5.0 g/L of M. spicatum (P < 0.05), and F(v)/F(m) and F'(v) /F'(m) were declined by 15.87% and 12.07% at 10.0 g/L (P < 0.05), respectively. On the third day, ETR and three parameters based on ETR were affected at all levels of M. spicatum (P < 0.05). The inhibition effects on the photosynthetic activity of M. aeruginosa might be considered as one of the target sites of M. spicatum and chlorophyll fluorescence parameters were more sensitive parameters than chlorophyll content, especially q(n).

[Photosystem II characteristics of nine Gramineae species in southern Taklamakan Desert]

Taking the Gramineae species Elytrigia intermedia, Avena sativa, Bromus inermis, Elymus sibiricus, Leymus tianschanicus, Elymus dahuricus, Festuca elata, Agropyron cristatum, and Puccinellia distans at the edge of Cele Oasis in southern Taklimakan Desert as test objects, this paper monitored their fast chlorophyll fluorescence kinetics after 20 minutes adaptation in darkness, compared their photosystem II (PS II) characteristics, and analyzed their adaptability to the local environment. Among the nine Gramineae species, L. tianschanicus and E. dahuricus had markedly higher values of maximum fluorescence yield (F(m)), maximum photochemical efficiency of PS II (F(v)/F(m)), and active reaction centers per cross-section (RC/CS0), but lower values of minimum fluorescence yield (F0), absorption flux per reaction center (ABC/RC), maximal trapping flux per reaction center (TR0/RC), flux of dissipated excitation energy per reaction center (DI0/RC), and initial slope of fluorescence intensity (M0), as compared to F. elata, A. cristatum, and P. distans, whereas E. intermedia, A. sativa, B. inermis, and E. sibiricus had a medium level of the values. These results suggested that all the test pasture species were suffered from the severe environmental conditions of Cele Oasis to some extent, as indicated by the inactivation of PS II reaction center and the depression of electron transport chain. L. tianschanicus and E. dahuricus were least impacted, while F. elata, A. cristatum, and P. distans were most impacted.