Response of the Photosynthetic Apparatus in Dunaliella salina (Green Algae) to Irradiance Stress

The response of the photosynthetic apparatus in the green alga Dunaliella salina, to irradiance stress was investigated. Cells were grown under physiological conditions at 500 millimoles per square meter per second (control) and under irradiance-stress conditions at 1700 millimoles per square meter per second incident intensity (high light, HL). In control cells, the light-harvesting antenna of photosystem I (PSI) contained 210 chlorophyll a/b molecules. It was reduced to 105 chlorophyll a/b in HL-grown cells. In control cells, the dominant form of photosystem II (PSII) was PSII_α(about 63% of the total PSII) containing >250 chlorophyll a/b molecules. The smaller antenna size PSII_β centers (about 37% of PSII) contained 135 ± 10 chlorophyll a/b molecules. In sharp contrast, the dominant form of PSII in HL-grown cells accounted for about 95% of all PSII centers and had an antenna size of only about 60 chlorophyll a molecules. This newly identified PSII unit is termed PSII_γ. The HL-grown cells showed a substantially elevated PSII/PSI stoichiometry ratio in their thylakoid membranes (PSII/PSI = 3.0/1.0) compared to that of control cells (PSII/PSI = 1.4/1.0). The steady state irradiance stress created a chronic photoinhibition condition in which D. salina thylakoids accumulate an excess of photochemically inactive PSII units. These PSII units contain both the reaction center proteins and the core chlorophyll-protein antenna complex but cannot perform a photochemical charge separation. The results are discussed in terms of regulatory mechanism(s) in the plant cell whose function is to alleviate the adverse effect of irradiance stress.     

Effects of growth irradiance levels on the ratio of reaction centers in two species of marine phytoplankton

Cells of two species of single-celled marine algae, the diatom Skeletonema costatum (Greve), Cleve, and the chlorophyte Dunaliella tertiolecta Butcher, were cultured in white light of high (500-600 microeinsteins per square meter per second) and low (30 microeinsteins per square meter per second) intensity. For both algal species, cells grown at low light levels contained more chlorophyll a and had a lower ratio of chlorophyll a to chlorophylls b or c than did cells grown at high light levels. When photosynthetic unit sizes were measured on the basis of either oxygen flash yields or P₇₀₀ photooxidation, different results were obtained with the different species. In the chlorophyte, the cellular content of photosystem I (PSI) and photosystem II (PSII) reaction centers increased in tandem as chlorophyll a content increased so that photosynthetic unit sizes changed only slightly and the ratio PSI:PSII reaction centers remained constant at about 1.1. In the diatom, as the chlorophyll content of the cells increased, the number of PSI reaction centers decreased and the number of PSII reaction centers increased so that the ratio of PSI:PSII reaction centers decreased from about unity to 0.44. In neither organism did photosynthetic capacity correlate with changes in cellular content of PSI or PSII reaction centers. The results are discussed in relationship to the physical and biological significance of the photosynthetic unit concept.     

Functional and Structural Organization of Chlorophyll in the Developing Photosynthetic Membranes of Euglena gracilis Z: II. CHARACTERISTICS OF THE LATE FORMATION OF ACTIVE PHOTOSYSTEM II REACTION CENTERS DURING FIRST STAGES OF GREENING

During light-induced greening of dark-grown, nondividing Euglena gracilis Z, there is a delay of about 10 hours in the formation of active photosystem II (PSII) reaction centers compared to chlorophyll synthesis. Experiments with greening under different light intensities rule out the possibility that this delay results from a late induction of active PSII reaction center formation when a definite amount of chlorophyll is attained in the early greened cells. Experiments on greening after preillumination show that this delay does not originate in a long, light-induced formation of specific synthesizing machinery for reaction center components. Experiments with greening in the presence of streptomycin show that, when this inhibitor of protein synthesis by chloroplastic ribosomes is added to dark-grown, preilluminated cells or to cells already greened for 24 hours, the formation of active PSII reaction centers is inhibited after a time which depends on the light intensity used for greening. Under very low light intensity (150 lux), the addition of streptomycin to 24-hour greened cells does not prevent further development of functional chloroplasts. These observations lead to the conclusion that streptomycin-insensitive chloro-plast development occurs due to syntheses of cytoplasmic origin and of light-induced pools of components synthesized early by chloroplastic ribo-somes. Conformational changes requiring time may allow the insertion of components necessary for the reorganization of PSII reaction centers in the developing thylakoid after synthesis. This hypothesis accounts for the observed delay in PSII reaction center formation compared to chlorophyll synthesis.     

Functional and Structural Organization of Chlorophyll in the Developing Photosynthetic Membranes of Euglena gracilis Z: III. SEQUENCE OF EVENTS LEADING TO THE FORMATION OF FUNCTIONAL PHOTOSYSTEM II PHOTOSYNTHETIC UNITS

Greening cells of Euglena were transferred back to darkness at different stages of chloroplast development in the presence or absence of specific inhibitors of protein synthesis. The analysis of chloroplast components showed that: (a) cycloheximide or streptomycin does not significantly inhibit the formation in darkness of active photosystem II (PSII) reaction centers if added after the lag phase for chloroplast development; (b) a limited number of active reaction centers are formed in the dark, sufficient to increase PSII reaction center to chlorophyll ratios to values close to those found in fully greened cells; (c) these dark-formed reaction centers appear to be inserted in already constituted and complete light-harvesting antennae. These results complement previous ones and lead us to propose a model for a sequential formation of PSII photosynthetic units during greening of Euglena, whereby conformational changes requiring time would allow already synthesized components of PSII reaction centers to be inserted or reorganized as active photochemical complexes in association with previously formed light-harvesting antennae.     

EFFECTS OF LONG TERM EXPOSURE TO LOW TEMPERATURE ON THE PHOTOSYNTHETIC APPARATUS OF DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE)1

The effects of long term exposure to suboptimal growth temperature on the photosynthetic apparatus of Dunaliella tertiolecta Butcher were investigated using carbon fixation rate versus irradiance curves and the variable fluorescence induction method. Carbon fixation rates per unite chlorophyll a at saturating (p^B_m) and subsaturating (α^B) irradiances were 55% and 39% lower, respectively, at 12° C than at 20° C. Chlorophyll a quotas and the spectrally averaged in vivo absorption cross section normalized to chlorophyll a (a^*) were not significantly different at these two temperatures. Analysis of the fluorescence kinetics revealed 1) no significant variations of the amount of PSII photoactive reaction centers per unit chlorophyll a, 2) a 14% decrease of the PSII quantum yield(+) and 3) a 29% decrease of the energy transfer efficiency between the light harvesting chlorophyll a pigment bed and the PSII reaction centers. The decrease in energy transfer efficiency between the antennae and the PSII reaction centers at 12° C was interpreted as a mechanism to avoid photoinhibition.     

Effect of Light Quality on the Composition, Function, and Structure of Photosynthetic Thylakoid Membranes of Asplenium australasicum (Sm.) Hook

The effect of light quality on the composition, function and structure of the thylakoid membranes, as well as on the photosynthetic rates of intact fronds from Asplenium australasicum, a shade plant, grown in blue, white, or red light of equal intensity (50 microeinsteins per square meter per second) was investigated. When compared with those isolated from plants grown in white and blue light, thylakoids from plants grown in red light have higher chlorophyll a/chlorophyll b ratios and lower amounts of light-harvesting chlorophyll a/b-protein complexes than those grown in blue light. On a chlorophyll basis, there were higher levels of PSII reaction centers, cytochrome f and coupling factor activity in thylakoids from red light-grown ferns, but lower levels of PSI reaction centers and plastoquinone. The red light-grown ferns had a higher PSII/PSI reaction center ratio of 4.1 compared to 2.1 in blue light-grown ferns, and a larger apparent PSI unit size and a lower PSII unit size. The CO₂ assimilation rates in fronds from red light-grown ferns were lower on a unit area or fresh weight basis, but higher on a chlorophyll basis, reflecting the higher levels of electron carriers and electron transport in the thylakoids.

The structure of thylakoids isolated from plants grown under the three light treatments was similar, with no significant differences in the number of thylakoids per granal stack or the ratio of appressed membrane length/nonappressed membrane length. The large freeze-fracture particles had the same size in the red-, blue-, and white-grown ferns, but there were some differences in their density. Light quality is an important factor in the regulation of the composition and function of thylakoid membranes, but the effects depend upon the plant species.

     

Dynamics of Photosystem II and Its Light Harvesting System in Response to Light Changes in the Halotolerant Alga Dunaliella salina

A photosystem two (PSII) core complex consisting of five major polypeptides (47, 40, 32, 30, and 10 kilodaltons) and a light harvesting chlorophyll a/b complex (LHC-2) have been isolated from the halotolerant alga Dunaliella salina. The chlorophyll and polypeptide composition of both complexes were compared in illuminated and dark-adapted cultures. Dark adaptation is accompanied by a decrease in the chlorophyll a to chlorophyll b (Chl a/Chl b) ratio of intact thylakoids without any change in total chlorophyll. These changes occur with a half-time of 3 hours and are reversed upon reillumination. Analyses of PSII enriched membrane fragments suggest that the decrease in the Chl a/Chl b is due partly to an increase in the Chl b content of LHC-2 and partly to changes in the relative levels of the two complexes. Apparently during dark adaptation there is: (a) a net synthesis of chlorophyll b, (b) removal of PSII core complexes resulting in a 2-fold drop in the PSII cores to LHC-2 chlorophyll ratio. These changes should dramatically increase the light harvesting capacity of the remaining PSII reaction centers. Presumably this adjustment of antenna size and composition is a physiological mechanism necessary for responding to shade conditions. Also detected, using ³²P, are light-induced phosphorylation of the LHC-2 (consistent with the ability to undergo State transitions) and of the 40 and 30 kilodalton subunits of the PSII core complex. These observations indicate that additional mechanisms may also exist to help optimize the interception of quanta during rapid changes in illumination conditions.     

Reorganization of the Photosystem II Unit in Developing Thylakoids of Higher Plants after Transfer to Darkness : Changes in Chlorophyll b, Light-Harvesting Chlorophyll Protein Content, and Grana Stacking

A light-dependent reversible grana stacking-unstacking process, paralleled by a reorganization of thylakoid components, has been noticed in greening etiolated bean (Phaseolus vulgaris, var. red kidney) leaves upon transfer to darkness. The reorganization, based on biochemical and biophysical criteria, involves mainly the photosystem II (PSII) unit components: upon transfer to darkness, the light-harvesting chlorophyll protein (LHCP), its 25 kilodalton polypeptide and chlorophyll b are decreased, while the CPa and its 42 kilodalton polypeptide are increased and new PSII units of smaller size are formed. This reorganization of components occurs only in thylakoids still in the process of development and not in those present in steady state conditions.

It is proposed that this process does not reflect the turnover of the LHCP component per se, but a regulatory process operating during development, by which the ratio of light-harvesting to PSII reaction center components, determined by the environmental conditions, controls the photosynthetic rate.

     

INFLUENCE OF ULTRAVIOLET RADIATION ON GROWTH AND PHOTOSYNTHESIS OF TWO COLD OCEAN DIATOMS1

The influence of chronic exposure to UV-B and UV-A radiation on growth and photosynthesis of two polar marine diatoms (Pseudonitzschia seriata and Nitzschia sp.) was investigated in cultures exposed to moderate photon fluences for 3–7 days. Population growth rates were diminished 50% by UV-B. Fluorescence induction kinetics of photo-system II (PSII) revealed that UV-B caused lower F_v/F_m ratios and half-rise times, indicating damage to the reaction center of PSII and to related elements of the photosynthetic electron transport chain. Carbon assimilation rates per cell and per chlorophyll a were nonetheless highest for UV-B—exposed populations, which also had the highest chlorophyll a content per cell. The UV-B—exposed cells were, however, more vulnerable to visible light-induced photoinhibition. Exposure to UV-A in the absence of UV-B had little effect on growth, fluorescence induction of PSII, or chlorophyll a contents but did have some inhibitory effects on carbon assimilation per chlorophyll a and per cell. The increased photosynthetic capacity of UV-B-exposed cells suggested some ability to compensate for damage to the photosynthetic apparatus.     

Isolation and characterization of oxygen-evolving thylakoid membranes and Photosystem II particles from a marine diatom Chaetoceros gracilis

Thylakoid membranes retaining high oxygen-evolving activity (about 250 μmol O₂/mg Chl/h) were prepared from a marine centric diatom, Chaetoceros gracilis, after disruption of the cells by freeze-thawing. We also succeeded in purification of Photosystem II (PSII) particles by differential centrifugation of the thylakoid membranes after treatment with 1% Triton X-100. The diatom PSII particles showed an oxygen-evolving activity of 850 and 1045 μmol O₂/mg Chl/h in the absence and presence of CaCl₂, respectively. The PSII particles contained fucoxanthin chlorophyll a/c-binding proteins in addition to main intrinsic proteins of CP47, CP43, D2, D1, cytochrome b559, and the antenna size was estimated to be 229 Chl a per 2 molecules of pheophytin. Five extrinsic proteins were stoichiometrically released from the diatom PSII particles by alkaline Tris-treatment. Among these five extrinsic proteins, four proteins were red algal-type extrinsic proteins, namely, PsbO, PsbQ', PsbV and PsbU, whereas the other one was a novel, hypothetical protein. This is the first report on isolation and characterization of diatom PSII particles that are highly active in oxygen evolution and retain the full set of extrinsic proteins including an unknown protein.     

Adaptation to quantum flux by the emerson photosynthetic unit

The size of the Emerson photosynthetic unit was measured in Chlorella pyrenoidosa strain no. 252 grown at light intensities between 50 and 1000 foot candles. The Emerson photosynthetic unit changed from a minimum size of 1970 molecules chlorophyll a + b/O₂ per flash in cells grown at 1000 foot candles to a maximum size of 3150 molecules chlorophyll a + b/O₂ per flash for cells grown at 50 foot candles. The size changes were interpreted as a partial adaptation where the trapping center antenna responded to changes in incident light intensity. Light-induced changes in chlorophyll content and size of the Emerson photosynthetic unit were directly related.     

Antenna Sizes of Photosystem I and Photosystem II in Chlorophyll b-Deficient Mutants of Rice. Evidence for an Antenna Function of Photosystem II Centers That are Inactive in Electron Transport

Light-harvesting capacities of photosystem I (PSI) and photosystemII (PSII) in a wild-type and three chlorophyll b-deficient mutantstrains of rice were determined by measuring the initial slopeof light-response curve of PSI and PSII electron transport andkinetics of light-induced redox changes of P-700 and Q_A, respectively.The light-harvesting capacity of PSI determined by the two methodswas only moderately reduced by chlorophyll b-deficiency. Analysisof the fluorescence induction that monitors time course of Q_Aphotoreduction showed that both relative abundance and antennasize of PSII_a decrease with increasing deficiency of chlorophyllb and there is only PSII_rß in chlorina 2 which totallylacks chlorophyll b. The numbers of antenna chlorophyll moleculesassociated with the mutant PSII centers were, therefore, threeto five times smaller than that of PSII_a in the wild type rice.Rates of PSII electron transport determined on the basis ofPSII centers in the three mutants were 60–70% of thatin the normal plant at all photon flux densities examined, indicatingthat substantial portions of the mutant PSII centers are inactivein electron transport. The initial slopes of light-responsecurves of PSII electron transport revealed that the functionalantenna sizes of the active populations of PSII centers in themutants correspond to about half that of PSII in the wild typerice. Thus, the numbers of chlorophyll molecules that serveas antenna of the oxygen-evolving PSII centers in the mutantsare significantly larger than those that are actually associatedwith each PSII center. It is proposed that the inactive PSIIserves as an antenna of the active PSII in the three chlorophyllb-deficient mutants of rice. In spite of the reduced antennasize of PSII, therefore, the total light-harvesting capacityof PSII approximately matches that of PSI in the mutants. (Received July 29, 1994; Accepted February 7, 1996)     

The psbO mutant of Chlamydomonas reinhardtii is capable of assembling stable, photochemically active reaction center of photosystem II

The functional status of photosystem II (PSII) complex in the dark-grown PsbO-deficient mutant of green alga Chlamydomonas reinhardtii was studied. It was found that ΔpsbO mutant cells of C. reinhardtii grown under heterotrophic conditions (dark + acetate) were capable of assembling stable, photochemically-competent reaction centers of PSII (as confirmed by immunological analysis of D1 protein level, pigments content and photoinduced changes of PSII chlorophyll fluorescence yield), while O₂-evolution activity was not revealed. The ratio F _v/F _m for the dark-grown ΔpsbO mutant C. reinhardtii was 0.37 and that for the dark-grown wild type cells was 0.56. Analysis of chlorophyll fluorescence induction curve indicated that the absence of oxygen-evolving activity could be due to some defects in the organization of the PSII catalytic manganese cluster. Decrease of the rate of the electron donation from water-oxidizing complex to the PSII reaction center as well as the appearance of an additional transient fluorescence peak during the dark relaxation of F _v testify to the damages to the PSII donor side. The data obtained suggest that the dark-grown PsbO-deficient cells of C. reinhardtii are able to form stable, photochemically active PSII reaction center, unable to oxidize water due to probable defects in the assembly of the manganese cluster.     

Fluorescence Properties Indicate that Photosystem II Reaction Centers and Light-Harvesting Complex Are Modified by Low Temperature Growth in Winter Rye

Thylakoids isolated from winter rye (Secale cereale L. cv Puma) grown at 20°C (nonhardened rye, RNH) or 5°C (cold-hardened rye, RH) were characterized using chlorophyll (Chl) fluorescence. Low temperature fluorescence emission spectra of RH thylakoids contained emission bands at 680 and 695 nanometers not present in RNH thylakoids which were interpreted as changes in the association of light-harvesting Chl a/b proteins and photosystem II (PSII) reaction centers. RH thylakoids also exhibited a decrease in the emission ratio of 742/685 nanometers relative to RNH thylakoids.

Room temperature fluorescence induction revealed that a larger proportion of Chl in RH thylakoids was inactive in transferring energy to PSII reaction centers when compared with RNH thylakoids. Fluorescence induction kinetics at 20°C indicated that RNH and RH thylakoids contained the same proportions of fast (α) and slow (β) components of the biphasic induction curve. In RH thylakoids, however, the rate constant for α components increased and the rate constant for β components decreased relative to RNH thylakoids. Thus, energy was transferred more quickly within a PSII reaction center complex in RH thylakoids. In addition, PSII reaction centers in RH thylakoids were less connected, thus reducing energy transfers between reaction center complexes. We concluded that both PSII reaction centers and light-harvesting Chl a/b proteins had been modified during development of rye chloroplasts at 5°C.

     

Examination of chlorophyll fluorescence in sulfur-deprived cells of <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>

Pulse amplitude modulation fluorimetry was used to assess chlorophyll fluorescence parameters in Chlamydomonas reinhardtii cells during sulfur deprivation. A significant (fourfold) increase in the chlorophyll fluorescence yield (parameters F ₀ and F _m) normalized to the chlorophyll concentration was shown for deprived cells. The chlorophyll content did not change during the deprivation experiments. An analysis of nonphotochemical quenching of chlorophyll fluorescence indicated a considerable modification of the energy deactivation pathways in photosystem II (PSII) of sulfur-deprived cells. For example, starved cells exhibited a less pronounced pH-dependent quenching of excited states and a higher thermal dissipation of excess light energy in the reaction centers of PSII. It was also shown that the photosynthetic apparatus of starved cells is primarily in state 2 and that back transition to state 1 is suppressed. However, these changes cannot cause the discovered elevation of chlorophyll fluorescence intensity (F ₀ and F _m) in the cells under sulfur limitation. The observed increase in the chlorophyll fluorescence intensity under sulfur deprivation may be due to partial dissociation of peripheral light-harvesting complexes from the reaction centers of PSII or a malfunction of the dissipative cycle in PSII, involving cytochrome b ₅₅₉.     

Photosystem electron-transport capacity and light-harvesting antenna size in maize chloroplasts

Spectrophotometric and kinetic measurements were applied to yield photosystem (PS) stoichiometries and the functional antenna size of PSI, PSII_α, and PSII_β in Zea mays chloroplasts in situ. Concentrations of PSII and PSI reaction centers were determined from the amplitude of the light-induced absorbance change at 320 and 700 nm, which reflect the photoreduction of the primary electron acceptor Q of PSII and the photooxidation of the reaction center P700 of PSI, respectively. Determination of the functional chlorophyll antenna size (N) for each photosystem was obtained from the measurement of the rate of light absorption by the respective reaction center. Under the experimental conditions employed, the rate of light absorption by each reaction center was directly proportional to the number of light-harvesting chlorophyll molecules associated with the respective photosystem. We determined N_P700 = 195, N_α = 230, N_β = 50 for the number of chlorophyll molecules in the light-harvesting antenna of PSI, PSII_α, and PSII_β, respectively. The above values were used to estimate the PSII/PSI electron-transport capacity ratio (C) in maize chloroplasts. In mesophyll chloroplasts C > 1.4, indicating that, under green actinic excitation when Chl a and Chl b molecules absorb nearly equal amounts of excitation, PSII has a capacity to turn over electrons faster than PSI. In bundle sheath chloroplasts C < 1, suggesting that such chloroplasts are not optimally poised for linear electron transport and reductant generation.     

RESPONSE OF THE PHOTOSYNTHETIC APPARATUS OF PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE) TO NITRATE,PHOSPHATE, OR IRON STARVATION1

The effects of nitrate, phosphate, and iron starvation and resupply on photosynthetic pigments, selected photosynthetic proteins, and photosystem II (PSII) photochemistry were examined in the diatom Phaeodactylum tricornutum Bohlin (CCMP 1327). Although cell chlorophyll a (chl a) content decreased in nutrient-starved cells, the ratios of light-harvesting accessory pigments (chl c and fucoxanthin) to chl a were unaffected by nutrient starvation. The chl a-specific light absorpition coefficient (a*) and the functional absorption cross-section of PSII (σ) increased during nutrient starvation, consistent with reduction of intracellular self-shading (i.e. a reduction of the “package effect”) as cells became chlorotic. The light-harvesting complex proteins remained a constant proportion of total cell protein during nutrient starvation, indicating that chlorosis mirrored a general reduction in cell protein content. The ratio of the xanthophylls cycle pigments diatoxanthin and diadinoxanthin to chl a increased during nutrient starvation. These pigments are thought to play a photo-protective role by increasing dissipation of excitation energy in the pigment bed upstream from the reaction centers. Despite the increase in diatoxanthin and diadinoxanthin, the efficiency of PSII photochemistry, as measured by the ration of variable to maximum fluorescence (F_v/F_m) of dark-adapted cells, declined markedly under nitrate and iron starvation and moderately under phosphate starvation. Parallel to changes in F_v/F_m were decreases in abundance of the reaction center protein D1 consistent with damage of PSII reaction centers in nutrient-starved cells. The relative abundance of the carboxylating enzyme, ribulose bisphosphate carboxylase/oxygenase (RUBISCO), decreased in response to nitrate and iron starvation but not phosphate starvation. Most marked was the decline in the abundance of the small subunit of RUBISCO in nitrate-starved cells. The changes in pigment content and fluorescence characteristics were typically reversed within 24 h of resupply of the limiting nutrient.     

两种生境条件下6种牧草叶绿素含量及荧光参数的比较

昆仑山前山牧场海拔较高, 策勒绿洲海拔相对较低, 两者生境差异较大。以昆仑山前山牧场和策勒绿洲边缘两种不同生境条件下生长的6种牧草: 冰草(Agropyron cristatum)、无芒雀麦(Bromus inermis)、矮生高羊茅(Festuca elata)、披碱草(Elymus dahuricus )、红豆草(Onobrychis pulchella)及和田大叶(Medicago sativa var. luxurians)为试验材料, 研究了不同生境条件下牧草叶片叶绿素含量及叶绿素荧光动力学参数的变化情况。结果显示: (1)在两种生境条件下, 昆仑山前山牧场生境生长的牧草叶绿素a、叶绿素b、总叶绿素的含量明显较高, 生长在策勒绿洲生境的牧草品种叶绿素a/b值较高; (2)昆仑山前山牧场生境牧草最大荧光、光系统II (PSII)最大光化学效率、PSII潜在活性和单位面积反应中心的数量的值明显高于策勒绿洲生境品种, 而初始荧光、单位反应中心吸收的光能、单位反应中心捕获的能量、单位反应中心耗散的能量、荧光诱导曲线初始斜率值则低于策勒绿洲生境品种。因此, 两种生境下环境因子发生了改变, 对牧草产生综合的胁迫作用; 策勒绿洲生境明显对牧草生长产生了抑制, 策勒绿洲生境牧草的色素含量降低以及PSII的机构遭到损坏, 导致反应中心一部分失活或裂解, 剩余有活性的反应中心的效率增加, 昆仑山生境则相对比较适宜牧草生长; 两种生境不同牧草叶绿素含量和叶绿素荧光参数的变化幅度不同。     

Activation of a Reserve Pool of Photosystem II in Chlamydomonas reinhardtii Counteracts Photoinhibition

The effect of strong irradiance (2000 micromole photons per square meter per second) on PSII heterogeneity in intact cells of Chlamydomonas reinhardtii was investigated. Low light (LL, 15 micromole photons per square meter per second) grown C. reinhardtii are photoinhibited upon exposure to strong irradiance, and the loss of photosynthetic functioning is due to damage to PSII. Under physiological growth conditions, PSII is distributed into two pools. The large antenna size (PSII_α) centers account for about 70% of all PSII in the thylakoid membrane and are responsible for plastoquinone reduction (Q_b-reducing centers). The smaller antenna (PSII_β) account for the remainder of PSII and exist in a state not yet able to photoreduce plastoquinone (Q_b-nonreducing centers). The exposure of C. reinhardtii cells to 60 minutes of strong irradiance disabled about half of the primary charge separation between P680 and pheophytin. The PSII_β content remained the same or slightly increased during strong-irradiance treatment, whereas the photochemical activity of PSII_α decreased by 80%. Analysis of fluorescence induction transients displayed by intact cells indicated that strong irradiance led to a conversion of PSII_β from a Q_b-nonreducing to a Q_b-reducing state. Parallel measurements of the rate of oxygen evolution revealed that photosynthetic electron transport was maintained at high rates, despite the loss of activity by a majority of PSII_α. The results suggest that PSII_β in C. reinhardtii may serve as a reserve pool of PSII that augments photosynthetic electron-transport rates during exposure to strong irradiance and partially compensates for the adverse effect of photoinhibition on PSII_α.     

Identification and characterization of a cytochrome b559 Synechocystis 6803 mutant spontaneously generated from DCMU-inhibited photoheterotrophical growth conditions

We identified a spontaneously generated mutant from Synechocystis sp. PCC6803 wild-type cells grown in BG-11 agar plates containing 5 mM Glu and 10 μM DCMU. This mutant carries an R7L mutation on the α-subunit of cyt b559 in photosystem II (PSII). In the recent 2.9 Å PSII crystal structural model, the side chain of this arginine residue is in close contact with the heme propionates of cyt b559. We called this mutant WR7Lα cyt b559. This mutant grew at about the same rate as wild-type cells under photoautotrophical conditions but grew faster than wild-type cells under photoheterotrophical conditions. In addition, 77 K fluorescence and 295 K chlorophyll a fluorescence spectral results indicated that the energy delivery from phycobilisomes to PSII reaction centers was partially inhibited or uncoupled in this mutant. Moreover, WR7Lα cyt b559 mutant cells were more susceptible to photoinhibition than wild-type cells under high light conditions. Furthermore, our EPR results indicated that in a significant fraction of mutant reaction centers, the R7Lα cyt b559 mutation induced the displacement of one of the axial histidine ligands to the heme of cyt b559. On the basis of these results, we propose that the Arg7Leu mutation on the α-subunit of cyt b559 alters the interaction between the APC core complex and PSII reaction centers, which reduces energy delivery from the antenna to the reaction center and thus protects mutant cells from DCMU-induced photo-oxidative stress.