Photosynthetic Oxygen Evolution Rate Research Articles

Effects of strontium on photosynthetic characteristics of oilseed rape seedlings

The threat of environmental strontium pollution led to an increased interest to elucidating the mechanisms of this metal toxicity in the organism. To investigate strontium effects on vital photosynthesis characteristics, three-leaf stage oilseed rape seedlings (Brassica napus L., cv. Mianyou No.15), raised in the hydroponic culture, were provided with a nutrient solution containing 0, 10, 20, and 40 mM SrCl2. Strontium uptake and distribution in oilseed rape plants and its effect on various aspects of photosynthesis were investigated after 0, 7, 14, and 21 days of strontium treatment. Oilseed rape seedlings demonstrated a strong ability of strontium accumulation. Strontium absorbed by roots was primarily transferred to leaves and accumulated there. The leaf photosynthetic oxygen evolution rate, chlorophyll content, and Rubisco (EC 4.1.3.9) and phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) activities declined progressively with increasing concentration of applied strontium and also with increasing the duration of exposure time. These results indicate that strontium accumulated in leaves damaged various processes of photosynthesis, such as energy absorption, energy transfer, and photosynthetic carbon assimilation.

Characterization of LexA-mediated Transcriptional Enhancement of Bidirectional Hydrogenase in Synechocystis sp. PCC 6803 upon Exposure to Gamma Rays

Influence of gamma rays on the cyanobacterium Synechocystis sp. PCC 6803 cells was investigated in terms of a bidirectional hydrogenase, which is encoded by hoxEFUYH genes and responsible for biohydrogen production. Irradiated cells revealed a substantial change in stoichiometry of photosystems at one day after gamma irradiation at different doses. However, as evaluated by the maximal rate of photosynthetic oxygen evolution, maximal photochemical efficiency of photosystem II, and chlorophyll content, net photosynthesis or photosynthetic capacity was not significantly different between the control and irradiated cells. Instead, transcription of hoxE, hoxH, or lexA, which encodes a subunit of bidirectional hydrogenase or the only transcriptional activator, LexA, for hox genes, was commonly enhanced in the irradiated cells. This transcriptional enhancement was more conspicuously observed immediately after gamma irradiation. In contrast, hydrogenase activities were found to somewhat lower in the irradiated cells. Therefore, we propose that transcription of hox genes should be enhanced by gamma irradiation in a LexA-mediated and possibly photosynthesis-independent manner and that this enhancement might not induce a subsequent increase in hydrogenase activities, probably due to the presence of post-transcriptional and/or post-translational regulatory mechanisms.

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.

The relationship between calcification and photosynthesis in the coccolithophorid Pleurochrysis carterae

Coccolithophorids are one of the dominant groups of marine phytoplankton. They are found in large numbers throughout the surface euphotic zone of the ocean, and are able to form large-scale blooms that persist for long periods of time. Coccolithophorid cells are covered by species-specific calcium carbonate crystals of various structures. In the process of calcification in coccolithophorids, Ca 2+ is absorbed into cells from the culture medium, and a coccolith unit is formed inside the cell. Then, the coccolith unit extrudes to the cell surface where it is constructed into crystal layers. The formation of these crystals is regulated by cellular metabolism under different environmental conditions. The carbon biogeochemical cycle in the coccolithophorids involves both photosynthetic and calcification processes, which not only play an important role in population dynamics, but also in the global carbon cycle and climate change. However, one important question remains, namely, whether the relationship between photosynthesis and calcification is species-dependent. Previous studies have yielded controversial results, even in the same species. In this paper, we selected Pleurochrysis carterae, a coccolithophore species that frequently blooms in coastal areas, to study the relationship between calcification and photosynthesis. First, we studied population growth in a batch culture over several days. For batch cultures, P. carterae was inoculated into a 10 L bioreactor at an initial cell density of approximately 5 × 10 4 cells mL −1. The culture conditions were optimal for cell growth. Dissolved oxygen (DO) was detected during all the culture period, and the rate of photosynthetic oxygen evolution was calculated according the DO changes during the 12-h illumination period. Algal samples (10 mL) were collected during the population growth phases. The calcium carbonate content on the cell surface was determined each day by chemical titration. Next, we studied the relationship between photosynthesis and calcification at the cellular level by observing patterns of recalcification during a 12-h period. In this study, non-calcified cells were obtained by decalcifying calcified cells collected during the exponential growth period in MES-NaOH buffer solution (pH 5.5). The non-calcified cells were inoculated into culture media containing different concentrations of Ca 2+ (0, 5, 20, 40, 50, or 100 mg L −1). The rate of recalcification was determined by microscopic analyses in which the number of recalcified cells per 100 cells was counted at 0, 3, 6, 9, and 12 h of culture. Ca 2+ absorbed into the cell was detected by measuring the fluorescence intensity of Fluo-3/AM labeled Ca 2+. The rate of photosynthetic oxygen evolution in the non-calcified cell cultures was detected by measuring the changes in dissolved oxygen during the 12-h illumination period. The results showed that during the population growth period, the rate of photosynthetic oxygen evolution was inversely related to the calcium carbonate content per cell. When the amount of calcium carbonate on the cell surface increased, the relative photosynthetic ability (the rate of photosynthetic oxygen evolution) decreased, and vice versa. Both recalcification rates and photosynthetic oxygen evolution were affected by the extracellular calcium concentration. Non-calcified cells showed different recalcification abilities at different extracellular Ca 2+ concentrations. The recalcification rate of non-calcified cells was positively correlated with the extracellular calcium concentration when [Ca 2+] in the medium ranged from 0 to 100 mg L −1. However, photosynthetic oxygen evolution was suppressed at higher cell calcification rates, especially when extracellular [Ca 2+] was 50–100 mg L −1. Our analyses of the population growth process and the cell recalcification process confirmed that photosynthesis is inversely related to calcification in P. carterae.

Responses of photosynthetic apparatus of the halotolerant microalga Dunalliella maritima to hyperosmotic salt shock

Chlorophyll fluorescence induction curves were used as a means to assess the functional condition of the photosynthetic apparatus in cells of the halotolerant green microalga Dunaliella maritima (Massjuk) (division Chlorophyta) exposed to hyperosmotic salt shock of various intensities. The shock was caused by the transfer of algal cells grown in the medium with 0.5 M NaCl to the media with elevated NaCl concentrations (1.0, 1.5, and 2.0 M). Parameters of chlorophyll fluorescence (F 0, F m, F 0′, F t′) were measured by means of a specialized pulse-amplitude-modulation fluorometer PAM 2100. In addition, the rate of photosynthetic oxygen evolution as well as the intracellular Na+ and glycerol content (the main osmolyte in this microalga) were determined. The hyperosmotic salt shock was found to elevate the intracellular Na+ content and reduce the functional activity of PSII in D. maritima. The suppression of PSII activity was evident from the decrease in the maximal quantum yield of photochemical energy conversion in PSII, the decreased rate of linear electron transport, the increased reduction of the primary acceptor QA, and the suppression of photosynthetic O2 evolution. The functional activity of PSII recovered gradually along with restoration of osmotic and ionic balance in algal cells. It is proposed that PSI ensures energy supply during cell responses of D. maritima to hyperosmotic salt shock.

The response of a natural phytoplankton community from the Godavari River Estuary to increasing CO2 concentration during the pre-monsoon period

This paper reports for the first time upon the effects of increasing CO2 concentrations on a natural phytoplankton assemblage in a tropical estuary (the Godavari River Estuary in India). Two short-term (5-day) bottle experiments were conducted (with and without nutrient addition) during the pre-monsoon season when the partial pressure of CO2 in the surface water is quite low. The results reveal that the concentrations of total chlorophyll, the phytoplankton growth rate, the concentrations of particulate organic matter, the photosynthetic oxygen evolution rates, and the total bacterial count were higher under elevated CO2 treatments, as compared to ambient conditions (control). δ13C of particulate organic matter (POM) varied inversely with respect to CO2, indicating a clear signature of higher CO2 influx under the elevated CO2 levels. Whereas, δ13CPOM in the controls indicated the existence of an active bicarbonate transport system under limited CO2 supply. A considerable change in phytoplankton community structure was noticed, with marker pigment analysis by HPLC revealing that cyanobacteria were dominant over diatoms as CO2 concentrations increased. A mass balance calculation indicated that insufficient nutrients (N, P and Si) might have inhibited diatom growth compared to cyanobacteria, regardless of increased CO2 supply. The present study suggests that CO2 concentration and nutrient supply could have significant effects on phytoplankton physiology and community composition for natural phytoplankton communities in this region. However, this work was conducted during a non-discharge period (nutrient-limited conditions) and the responses of phytoplankton to increasing CO2 might not necessarily be the same during other seasons with high physicochemical variability. Further investigation is therefore needed.

Desiccation of the resurrection plant Haberlea rhodopensis at high temperature

Haberlea rhodopensis plants, growing under low irradiance in their natural habitat, were desiccated to air-dry state at a similar light intensity (about 30 μmol m(-2) s(-1)) under optimal (23/20°C, day/night) or high (38/30°C) temperature. Dehydration of plants at high temperature increased the rate of water loss threefold and had a more detrimental effect than either drought or high temperature alone. Water deficit decreased the photochemical activity of PSII and PSI and the rate of photosynthetic oxygen evolution, and these effects were stronger when desiccation was carried out at 38°C. Some reduction in the amount of the main PSI and PSII proteins was observed especially in severely desiccated Haberlea leaves. The results clearly showed that desiccation of the homoiochlorophyllous poikilohydric plant Haberlea rhodopensis at high temperature had more damaging effects than desiccation at optimal temperature and in addition recovery was slower. Increased thermal energy dissipation together with higher proline and carotenoid content in the course of desiccation at 38°C compared to desiccation at 23°C probably helped in overcoming the stress.

Photosynthetic biofilms in pure culture harness solar energy in a mediatorless bio-photovoltaic cell (BPV) system

Microbial fuel cells are an emerging technology for converting organic substrates into electrical power. Recent research has shown that biofilms of some bacterial species are capable of self-mediated extracellular electron transfer. The prospect of exploiting this trait in photoautotrophic microbes that do not require an organic substrate has important implications for the future development of renewable solar energy technologies. Here we report on light-driven electrical power generated with biofilms grown from photosynthetic fresh water or marine species without the addition of an artificial electron-shuttling mediator. Green alga (Chlorella vulgaris, Dunaliella tertiolecta) or cyanobacteria (Synechocystis sp. PCC 6803, Synechococcus sp. WH 5701) strains were grown directly on a transparent, conductive anode (indium tin oxide-coated polyethylene terephthalate) and power generation under light and dark conditions was evaluated using a single-chamber bio-photovoltaic cell (BPV) system. Increased power outputs were observed for all strains upon illumination, with the largest light effect observed for Synechococcus (maximum 10.3 mW m−2 total power output recorded under 10 W m−2 white light). Further experiments conducted with Synechococcus and C. vulgaris showed that photosynthetic oxygen evolution rates were consistent with BPV power outputs under different light regimes (red, green and blue light), indicating a direct link between power output and photosynthetic activity. Biofilm power generation in these BPV devices was self-sustained for several weeks and was highly sensitive to ambient light levels. When connected in series, four BPVs (each 0.011 m2) generated enough power to run a commercial digital clock.

A new filter that accurately mimics the solar UV‐B spectrum using standard UV lamps: the photochemical properties, stabilization and use of the urate anion liquid filter

The physiological effects unique to solar ultraviolet (UV)-B exposure (280-315 nm) are difficult to accurately replicate in the laboratory. This study evaluates the effectiveness of the sodium urate anion in a liquid filter that yields a spectrum nearly indistinguishable from the solar UV-B spectrum while filtering the emissions of widely used UV-B lamps. The photochemical properties and stability of this filter are examined and weighed against a typical spectrum of ground-level solar UV-B radiation. To test the effectiveness of this filter, light-saturated photosynthetic oxygen evolution rates were measured following exposure to UV-B filtered either by this urate filter or the widely used cellulose acetate (CA) filter. The ubiquitous marine Chlorophyte alga Dunaliella tertiolecta was tested under identical UV-B flux densities coupled with ecologically realistic fluxes of UV-A and visible radiation for 6 and 12 h exposures. These results indicate that the urate-filtered UV-B radiation yields minor photosynthetic inhibition when compared with exposures lacking in UV-B. This is in agreement with published experiments using solar radiation. In sharp contrast, radiation filtered by CA filters produced large inhibition of photosynthesis.

Physiological regulation of high transpiration efficiency in winter wheat under drought conditions

Pot experiments were conducted to study the variation and physiological regulation of transpiration efficiency (TE) of four winter wheat (Triticum aestivum L.) varieties that are widely grown in different ecological regions in North China. Plants were grown under two soil moisture regimes, normal and drought stress. The results showed that under drought stress condition, both TE at plant level and TE at leaf level (TEl) increased significantly. The transpiration rate (Tr) was reduced more strongly than leaf net CO<sub>2</sub> assimilation rate (Pn). The decline of Tr was mainly affected by stomatal conductance and the decline of Pn was affected by non-stomatal factors, which was confirmed by the decline in net photosynthetic oxygen evolution rate. The leaf soluble sugar content and proline content were significantly increased under drought stress. The stomatal density was increased and the stomatal length was reduced. These results led us to make the following conclusions: (1) Under drought stress, the increase in TEl appears to be regulated in two ways: via the stomata by regulating Tr, and independent of the stomata through regulation of Pn; regulation via the stomata was more sensitive; (2) Osmotic adjustment was closely correlated to the non-stomatal regulation, and stomatal aperture was closely correlated to the stomatal way.

Artificial tripartite symbiosis involving a green alga (Chlamydomonas), a bacterium (Azotobacter) and a fungus (Alternaria): Morphological and physiological characterization

A long-living artificial tripartite symbiosis involving a green alga (Chlamydomonas), a bacterium (Azotobacter) and a fungus (Alternaria) was established on carbon- and nitrogen-free medium. The basis of the interdependence is the complementation of photosynthetic CO2 assimilation and atmospheric nitrogen fixation. Green color of the colonies indicated that the algal cells had enough nitrogen to synthesize chlorophylls. The chlorophyll content was nearly 40% of the control cells. The relatively high rate of photosynthetic oxygen evolution proved that nitrogen was effectively used for building up a well functioning photosynthetic apparatus. This was supported by the analysis of photosystems and ultrastructural investigations. In comparison with degreened algae cultured on nitrogen-free medium, the chloroplasts in the symbiont algal cells contained a well-developed, stacked thylakoid membrane system without extreme starch or lipid accumulation. The occurrence of the fungus in the association greatly increased the chlorophyll content. Far fewer types of amino acids were excreted by the tripartite cultures than by pure cultures. Cystathionine, which is a common intermediate in the sulfur-containing amino acid metabolism, was produced in high quantities by the tripartite symbiosis. This can mostly be attributed to the activity of the fungus.

Hydrogen production by Chlamydomonas reinhardtii revisited: Rubisco as a biotechnological target

Hydrogen production by C. reinhardtii seems a promising alternative as a source of non-polluting biofuel. Hydrogen is generated as a result of combining free protons and electrons (supplied by ferredoxin) through the activity of an oxygen-sensitive hydrogenase. Thus, substantial hydrogen production is only observed in the light under anaerobic conditions. These require a reduced rate of photosynthetic oxygen evolution which is usually achieved by impairing photosystem II through sulphur starvation. Several approaches have been conducted to enhance and extend hydrogen production by addressing problems such as the mechanism of hydrogenase inhibition by oxygen, the stressing impact on the cells of the culture conditions, the use of starch as an alternate source of electrons under reduced photosynthetic activity, and the need of maintaining a balance between oxygen evolution and consumption. The photosynthetic enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) appears as suitable objective for biotechnological optimization of hydrogen production because of its relevance controlling the hydrogenase main competitor electron sink (the Calvin-Benson cycle), as well as starch accumulation and photorespiratory oxygen consumption. Possible strategies for increasing hydrogen generation based on alteration of Rubisco properties and/or catabolism through site-directed mutagenesis are discussed.

Distribution and functional role of carbonic anhydrase Cah3 associated with thylakoid membranes in the chloroplast and pyrenoid of Chlamydomonas reinhardtii

Localization of lumenal carbonic anhydrase Cah3 in thylakoid membranes of Chlamydomonas reinhardtii was studied using wild-type algae and photosynthetic mutants with different composition of chlorophyll-protein complexes in the photosystems. In addition, the photosynthetic characteristics of wild-type C. reinhardtii and cia3 mutants lacking the activity of carbonic anhydrase Cah3 were examined. Western blot analysis revealed the lack of cross reaction with antibodies to Cah3 in the mutant lacking the photosystem II (PSII) reaction center, in contrast to the mutant deficient in light-harvesting complex of PSII. These data show that the lumenal Cah3 is associated with polypeptides on the donor side of PSII reaction center. Using immunoelectron microscopy and antibodies to Cah3 from C. reinhardtii, we showed for the first time that the major part of thylakoid Cah3 is localized in the pyrenoid where the bulk of Rubisco is located. The rate of photosynthetic oxygen evolution and PSII photochemical efficiency were lower in C. reinhardtii cia3 mutant than in the wild type, especially in the cells grown at limiting CO2 concentrations. These observations show that Cah3 takes part in CO2-concentrating mechanism of the chloroplast. The results support our hypothesis [1, 2] that the carboxylation reaction in microalgae proceeds in the pyrenoid, a specific Rubisco-containing part of the chloroplast, which acquires CO2 from the lumen of intrapyrenoid thylakoids. We discuss significance of the pyrenoid as an autonomous metabolic microcompartment, in which Cah3 plays a key role in the production and concentration of CO2 for Rubisco. These functions may promote the photosynthetic efficiency owing to the effective CO2 supply for the Calvin cycle.

EFFECT OF LIGHTING,TEMPERATURE AND pH ON PHOTOSYNTHETIC CHARACTERS OF <I>HAEMATOCOCCUS PLUVIALIS</I> CG-11

The effects of light intensity, temperature and pH on photosynthetic characters of Haematococcus pluvialis CG-11 were studied by measuring photosynthetic O2 evolution and fluorescence kinetics. The results showed that the saturation light intensity was 109.1 μmol/m2·s of H. pluvialis CG-11, with the maximum photosynthetic oxygen evolution rate of 75.9 μmol O2/mg Chla·h; the range of suitable growth temperature was between 25℃ to 30℃, the maximum photosynthetic rate was obtained at 25℃; the photosynthetic efficiency kept quite high at the pH range of 7.5—8.0,with the maximum photosynthetic oxygen evolution rate of 75.5 μmol O2/mg Chla·h at pH value of 7.5. Under experimental conditions of pH, a similar tendency manifested in the chlorophyll fluorescence kinetics parameters for H. pluvialis, at the pH range of 7.5—8.0, the Fv/ Fm, Fv/ F0, ΦPSⅡand ETR increased following the rising of pH, with their maximum values at pH 7.5, Fv/ Fm and Fv/ F0 decreased significantly over pH 7.5, while the decreasing trend didn’t change obviously in ΦPSⅡ and ETR.

Methyl Jasmonate Counteract UV‐B Stress in Barley Seedlings

AbstractThe role of exogenously applied phytohormone methyl jasmonate (MeJA) in counteracting the ultraviolet B (UV‐B) stress in barley seedlings was investigated. Barley seedlings (Hordeum vulgare L., cv. Alfa) 4 days old were supplied with 5 × 10−5 m MeJA through the roots for 3 days and then exposed for 2 days for 5 h per day to UV‐B (312 nm, biological effectiveness of UV‐B radiation 28.8 kJ m−2day−1). The rate of 14CO2 fixation, PSI and PSII activities and chlorophyll content decreased, but flavonoids, H2O2, malondialdehyde, proline and UV‐B induced compounds increased after UV‐B treatment. The rate of photosynthetic oxygen evolution was more strongly inhibited by UV‐B‐irradiation than PSI and PSII efficiency. MeJA itself increased the content of free proline, which acts as a stress protector due to its radical scavenging ability. Increased superoxide dismutase, catalase and peroxidase (POX) activities in the leaves and in the roots and the POX isoforms induction revealed the MeJA involvement in plant tolerance to oxidative stress caused by UV‐B irradiation. It was shown that pre‐treatment with MeJA counteracted UV‐B stress. Therefore, it was suggested that MeJA could acts as a mediator in plant defense responses to UV‐B irradiation by enhancing the activity of antioxidant system and free radical scavenging capability of plant cells.

Effects of Irradiance and Temperature on the Photosynthesis and Vegetative Propagation of Caulerpa serrulata

The photosynthetic oxygen evolution of Caulerpa serrulata was determined with oxygen electrodes. The effects of light and temperature on the growth and regeneration of fragmented C. serrulata thalli were analyzed. The regenerating rate and establishment of different sizes and portions of C. serrulata were studied. The results showed that the light saturation point of C. serrulata was 200 micromol photons/m(2) per s and the optimum growth temperature was 25-30 degrees C. Under these conditions, the maximum photosynthetic oxygen evolution rate was 15.1 +/- 0.29 mg O(2)/mg Chl a/h, the growth rate and elongation rate reached the highest values, 4.67 +/- 0.09 mg FW/d and 0.78 +/- 0.01 mm/d, respectively. The fragmented C. serrulata thalli was regenerated at 20-35 degrees C and survived at 15 degrees C and 200 micromol photons/m(2) per s. A different survival rate was detected according to fragment size. All of these results indicated that C. serrulata was a candidate to become an invasive species if introduced into a new place. Therefore, we should pay more attention to C. serrulata for its potential threat to marine ecosystem when it is sold for aquarium use.

Characterization of a sodium-regulated glutaminase from cyanobacterium Synechocystis sp. PCC 6803

Glutaminase is widely distributed among microorganisms and mammals with important functions. Little is known regarding the biochemical properties and functions of the deamidating enzyme glutaminase in cyanobacteria. In this study a putative glutaminase encoded by gene slr2079 in Synechocystis sp. PCC 6803 was investigated. The slr2079 was expressed as histidine-tagged fusion protein in Escherichia coli. The purified protein possessed glutaminase activity, validating the functional assignment of the genomic annotation. The apparent K (m) value of the recombinant protein for glutamine was 26.6 +/- 0.9 mmol/L, which was comparable to that for some of other microbial glutaminases. Analysis of the purified protein revealed a two-fold increase in catalytic activity in the presence of 1 mol/L Na(+). Moreover, the K (m) value was decreased to 12.2 +/- 1.9 mmol/L in the presence of Na(+). These data demonstrate that the recombinant protein Slr2079 is a glutaminase which is regulated by Na(+) through increasing its affinity for substrate glutamine. The slr2079 gene was successfully disrupted in Synechocystis by targeted mutagenesis and the Deltaslr2079 mutant strain was analyzed. No differences in cell growth and oxygen evolution rate were observed between Deltaslr2079 and the wild type under standard growth conditions, demonstrating slr2079 is not essential in Synechocystis. Under high salt stress condition, however, Deltaslr2079 cells grew 1.25-fold faster than wild-type cells. Moreover, the photosynthetic oxygen evolution rate of Deltaslr2079 cells was higher than that of the wild-type. To further characterize this phenotype, a number of salt stress-related genes were analyzed by semi-quantitative RT-PCR. Expression of gdhB and prc was enhanced and expression of desD and guaA was repressed in Deltaslr2079 compared to the wild type. In addition, expression of two key enzymes of ammonium assimilation in cyanobacteria, glutamine synthetase (GS) and glutamate synthase (GOGAT) was examined by semi-quantitative RT-PCR. Expression of GOGAT was enhanced in Deltaslr2079 compared to the wild type while GS expression was unchanged. The results indicate that slr2079 functions in the salt stress response by regulating the expression of salt stress related genes and might not play a major role in glutamine breakdown in Synechocystis.

Excess irradiance causes early symptoms of senescence during leaf expansion in photoautotrophically in vitro grown tobacco plants

Photosynthetic parameters, growth, and pigment contents were determined during expansion of the fourth leaf of in vitro photoautotrophically cultured Nicotiana tabacum L. plants at three irradiances [photosynthetically active radiation (400-700 nm): low, LI 60 µmol m-2 s-1; middle, MI 180 µmol m-2 s-1; and high, HI 270 µmol m-2 s-1]. During leaf expansion, several symptoms usually accompanying leaf senescence appeared very early in HI and then in MI plants. Symptoms of senescence in developing leaves were: decreasing chlorophyll (Chl) a+b content and Chl a/b ratio, decreasing both maximum (FV/FM) and actual (ΦPS2) photochemical efficiency of photosystem 2, and increasing non-photochemical quenching. Nevertheless, net photosynthetic oxygen evolution rate (P N) did not decrease consistently with decrease in Chl content, but exhibited a typical ontogenetic course with gradual increase. P N reached its maximum before full leaf expansion and then tended to decline. Thus excess irradiance during in vitro cultivation did not cause early start of leaf senescence, but impaired photosynthetic performance and Chl content in leaves and changed their typical ontogenetic course.

An Activated Glutamate Residue Identified in Photosystem II at the Interface between the Manganese-stabilizing Subunit and the D2 Polypeptide

Photosystem II (PSII) catalyzes the oxidation of water during oxygenic photosynthesis. PSII is composed both of intrinsic subunits, such as D1, D2, and CP47, and extrinsic subunits, such as the manganese-stabilizing subunit (MSP). Previous work has shown that amines covalently bind to amino acid residues in the CP47, D1, and D2 subunits of plant and cyanobacterial PSII, and that these covalent reactions are prevented by the addition of chloride in plant preparations depleted of the 18- and 24-kDa extrinsic subunits. It has been proposed that these reactive groups are carbonyl-containing, post-translationally modified amino acid side chains (Ouellette, A. J. A., Anderson, L. B., and Barry, B. A. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 2204-2209 and Anderson, L. B., Ouellette, A. J. A., and Barry, B. A. (2000) J. Biol. Chem. 275, 4920-4927). To identify the amino acid binding site in the spinach D2 subunit, we have employed a biotin-amine labeling reagent, which can be used in conjunction with avidin affinity chromatography to purify biotinylated peptides from the PSII complex. Multidimensional chromato-graphic separation and multistage mass spectrometry localizes a novel post-translational modification in the D2 subunit to glutamate 303. We propose that this glutamate is activated for amine reaction by post-translational modification. Because the modified glutamate is located at a contact site between the D2 and manganese-stabilizing subunits, we suggest that the modification is important in vivo in stabilizing the interaction between these two PSII subunits. Consistent with this conclusion, mutations at the modified glutamate alter the steady-state rate of photosynthetic oxygen evolution.

Osmoregulation in Dunaliella, Part I: Effects of osmotic stress on photosynthesis, dark respiration and glycerol metabolism in Dunaliella tertiolecta and its salt-sensitive mutant (HL 25/8)

The photosynthetic oxygen evolution increased by about 30% over control when Dunaliella tertiolecta and its salt-sensitive mutant (HL 25/8) were stressed by raising NaCl concentration from 0.17 to 0.4 M, however, during the dilution stress the photosynthetic oxygen evolution was progressively decreased with increasing dilution (decreasing the salinity). The photosynthetic oxygen evolution is affected by the water potential of the medium rather than by the ionic strength. Mitochondrial dark respiration was unaffected by salt stress, however, it was increased by about 50% (parent strain) and 35% (the mutant) upon dilution as if reduced pyridine nucleotide generated during glycerol dissimilation reaction were continuously oxidized by the mitochondria. The salt stress-induced changes in photosynthetic 14CO 2 fixation were consistent with the observed rates of photosynthetic oxygen evolution. The mutant strain showed about one-half the capability for photosynthesis, and glycerol synthesis compared to the parent strain. However, the proportion of photosynthetically newly fixed carbon during salt stress in glycerol was similar in both strains. The glycerol dissimilation capabilities of both strains were also similar. It is suggested that the salt sensitivity of the mutant is probably due to its reduced dissolved inorganic carbon transport, photosynthetic and starch metabolism capabilities to provide carbon for glycerol synthesis in the time frame of adaptation process.