Analysis of the changes of electron transfer and heterogeneity of photosystem II in Deg1-reduced Arabidopsis plants

Deg1 protease functions in protease and chaperone of PSII complex components, but few works were performed to study the effects of Deg1 on electron transport activities on the donor and acceptor side of PSII and its correlation with the photoprotection of PSII during photoinhibition. Therefore, we performed systematic and comprehensive investigations of electron transfers on the donor and acceptor sides of photosystem II (PSII) in the Deg1-reduced transgenic lines deg1-2 and deg1-4. Both the maximal quantum efficiency of PSII photochemistry (F_v/F_m) and the actual PSII efficiency (Φ_PSII) decreased significantly in the transgenic plants. Increases in nonphotochemical quenching (NPQ) and the dissipated energy flux per reaction center (DI₀/RC) were also shown in the transgenic plants. Along with the decreased D1, CP47, and CP43 content, these results suggested photoinhibition under growth light conditions in transgenic plants. Decreased Deg1 caused inhibition of electron transfer on the PSII reducing side, leading to a decline in the number of Q_B-reducing centers and accumulation of Q_B-nonreducing centers. The Tm of the Q band shifted from 5.7 °C in the wild-type plant to 10.4 °C and 14.2 °C in the deg1-2 and deg1-4 plants, respectively, indicating an increase in the stability of S₂Q_A^¯ in transgenic plants. PSIIα in the transgenic plants largely reduced, while PSIIβ and PSIIγ increased with the decline in the Deg1 levels in transgenic plants suggesting PSIIα centers gradually converted into PSIIβ and PSIIγ centers in the transgenic plants. Besides, the connectivity of PSIIα and PSIIβ was downregulated in transgenic plants. Our results reveal that downregulation of Deg1 protein levels induced photoinhibition in transgenic plants, leading to loss of PSII activities on both the donor and acceptor sides in transgenic plants. These results give a new insight into the regulation role of Deg1 in PSII electron transport.

     

Effects of high temperature on the chlorophyll a fluorescence of Alhagi sparsifolia at the southern Taklamakan Desert

Climate change is expected to result in an increase in the frequency and magnitude of extreme weather events. Alhagi sparsifolia is an important factor for wind prevention and sand fixation in the forelands of the Taklamakan Desert. The effects of high temperature on desert plants remain widely unknown. In this work, chlorophyll a fluorescence induction kinetics were investigated at different time stresses of 5, 20, 40, and 60 min at temperature gradients of 38–44 °C at 2 °C intervals. A pronounced K-step was found, and the values of the maximum quantum yield for primary photochemistry, the quantum yield of electron transport, the density of reaction centers and the performance index on absorption basis were lowest after 60 min at 44 °C, thus indicating that the oxygen-evolving complex was damaged, the inactivated reaction centers increased, and the activity of the photosystem II (PSII) reaction center in leaves was seriously limited. Therefore, we suggest that under normal temperature (below 42 °C), the PSII of A. sparsifolia would be unaffected. When such temperature is maintained for 40 min, the activity of PSII would be limited, and when retained for 60 min, PSII may be severely damaged.     

Biosynthesis of Photosystem II Reaction Centers, Antenna and Plastoquinone Pool in Greening Cells of Cyanidium caldarium Mutant III-C

Dark-grown etiolated cells of Cyanidium caldarium mutant III-C lacking ≥99% of their normal chlorophyll content and inactive for photosynthesis were greened in continuous light. Measurements of oxygen evolution and fluorescence kinetics indicate that during greening: (a) the photosystem II (PSII) antenna containing between 30 and 40 chlorophyll a per center undergoes little change in size from 5% of the centers synthesized per cell to fully active cells; (b) energy transfer between PSII centers appears very early in the greening process; (c) the plastoquinone pool size per PSII center (about 14 equivalents) does not vary during greening and has already attained full size after synthesis of only 13% of the full complement of centers.     

PHOTOPHYSIOLOGICAL RESPONSES OF FRAGILARIOPSIS CYLINDRUS (BACILLARIOPHYCEAE) TO NITROGEN DEPLETION AT TWO TEMPERATURES1

The photosynthetic efficiency and photoprotective capacity of the sea‐ice diatom, Fragilariopsis cylindrus (Grunow) W. Krieg., grown in a matrix of nitrogen repletion and depletion at two different temperatures (?1°C and +6°C) was investigated. Temperature showed no significant effect on photosynthetic efficiency or photoprotection in F. cylindrus. Cultures under nitrogen depletion showed enhanced photoprotective capacity with an increase in nonphotochemical quenching (NPQ) when compared with nitrogen‐replete cultures. This phenomenon was achieved at no apparent cost to the photosynthetic efficiency of PSII (F_V/F_M). Nitrogen depletion yielded a partially reduced electron transport chain in which maximum fluorescence (F_M) could only be obtained by adding 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU). reoxidation curves showed the presence of Q_B nonreducing PSII centers under nitrogen depletion. Fast induction curves (FICs) and electron transport rates (ETRs) revealed slowing of the electrons transferred from the primary (Q_A) to the secondary (Q_B) quinone electron acceptors of PSII. The data presented show that nitrogen depletion in F. cylindrus leads to the formation of Q_B nonreducing PSII centers within the photosystem. On a physiological level, the formation of Q_B nonreducing PSII centers in F. cylindrus provides the cell with protection against photoinhibition by facilitating the rapid induction of NPQ. This strategy provides an important ecological advantage, especially during the Antarctic spring, maintaining photosynthetic efficiency under high light and nutrient‐limiting conditions.     

Photoinhibition and light-dependent turnover of the D1 reaction-centre polypeptide of photosystem II are enhanced by mineral-stress conditions

The function of photosystem II (PSII) and the turnover of its D1 reaction-center protein were studied in spinach (Spinacia oleracea L.) plants set under mineral stress. The mineral deficiencies were induced either by supplying the plants with an acidic nutrient solution or by strongly reducing the supply of magnesium alone or together with sulfur. After exposure for 8–10 weeks to the different media, the plants were characterized by a loss of chlorophyll and an increase in starch content, indicating a disturbance in the allocation of assimilates. Depending on the severity of the mineral deficiencies the plants lost their ability to adapt even to moderate iradiances of 400 mol photons·m^–2·s^–1 and became photoinhibited, as indicated by the decrease in F_v/F_m (the ratio of yield of variable fluorescence to yield of maximal fluorescence when all reaction centers are closed). The loss of PSII function was induced by changes on the acceptor side of PSII. Fast fluorescence decay showed a loss of PSII centers with bound Q_B, the secondary quinone acceptor of PSII, and a fast reoxidation kinetic of q _a ^- , the primary quinone acceptor of PSII, in the photoinactivated plants. No appreciable change could be observed in the amount of PSII centers with unbound Q_B and in Q_B-nonreducing PSII centers. Immunological studies showed that the contents of the D1 and D2 proteins of the PSII reaction center and of the 33-kDa protein of the water-splitting complex were diminished in the photoinhibited plants, and the occurrance of a new polypetide of 14 kDa that reacted with an antibody against the C-termius of the D1 protein. As shown by pulse-labelling experiments with [¹⁴C]leucine both degradation and synthesis of the D1 protein were enhanced in the mineral-deficient plants when compared to non-deficient plants. A stimulation of D1-protein turnover was also observed in pH 3-grown plants, which were not inhibited at growth-light conditions. Obviously, stimulation of D1-protein turnover prevented photoinhibition in these plants. However, in the Mg- and Mg/S-deficient plants even a further stimulation of D1-protein turnover could not counteract the increased rate of photoinactivation.Abbreviations amp_(f,m,s) amplitude of the fast, (medium and slow) exponential component of fluorescence decay - F_m yield of maximum fluorescenc when all reaction centers are closed - F_o yield of intrinsic fluorescence at open PSII reaction centers in the dark - F_v yield of variable fluorescence, (difference between F_m and F_o) - LHC light-harvesting complex - PFD photon flux density - Q_A primary quinone acceptor of PSII - Q_B secondary quinone acceptor of PSII Dedicated to Professor Dr. Dres. hc. Achim Trebst on the occasion of his 65th birthdayThis work was supported by grants from the BMFT and the Ministerium für Umwelt, Raumordnung and Landwirtschaft, Nordrhein-Westfalen. The authors thank H. Wietoska and M. Bronzel for skilful technical assistance.     

Benzoxazolin-2(3H)-one (BOA) induced changes in leaf water relations,photosynthesis and carbon isotope discrimination in Lactuca sativa

The effects are reported here of Benzoxazolin-2(3H)-one (BOA), an allelopathic compound, on plant water relations, growth, components of chlorophyll fluorescence, and carbon isotope discrimination in lettuce (Lactuca sativa L.). Lettuce seedlings were grown in 1:1 Hoagland solution in perlite culture medium in environmentally controlled glasshouse. After 30 days, BOA was applied at concentration of 0.1, 0.5, 1.0 and 1.5 mM and distilled water (control). BOA, in the range (0.1–1.5 mM), decreased the shoot length, root length, leaf and root fresh weight. Within this concentration range, BOA significantly reduced relative water content while leaf osmotic potential remained unaltered. Stress response of lettuce was evaluated on the basis of six days of treatment with 1.5 mM BOA by analyzing several chlorophyll fluorescence parameters determined under dark-adapted and steady state conditions. There was no change in initial fluorescence (F_o) in response to BOA treatment while maximum chlorophyll fluorescence (F_m) was significantly reduced. BOA treatment significantly reduced variable fluorescence (F_v) on first, second, third, fourth, fifth and sixth day. Quantum efficiency of open PSII reaction centers (F_v/F_m) in the dark-adapted state was significantly reduced in response to BOA treatment. Quantum yield of photosystem II (ΦPSII) electron transport was significantly reduced because of decrease in the efficiency of excitation energy trapping of PSII reaction centers. Maximum fluorescence in light-adapted leaves (F′_m) was significantly decreased but there was no change in initial fluorescence in light-adapted state (F′_o) in response to 1.5 mM BOA treatment. BOA application significantly reduced photochemical fluorescence quenching (qP) indicating that the balance between excitation rate and electron transfer rate has changed leading to a more reduced state of PSII reaction centers. Non photochemical quenching (NPQ) was also significantly reduced by BOA treatment on third, fourth and fifth day. BOA had dominant effect on C isotope ratios (δ¹³C) that was significantly less negative (?26.93) at 1.0 mM concentration as compared to control (?27.61). Carbon isotope discrimination (Δ¹³C) values were significantly less (19.45) as compared to control (20.17) at 1.0 mM. BOA also affect ratio of intercellular to air CO₂ concentration (ci/ca) that was significantly less (0.66) as compared to control (0.69) when treated with 1.0 mM BOA. Protein content of lettuce leaf tissue decreased under BOA treatment at 1.5 mM concentration as compared to control.     

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.

     

Effects of elevated temperature on photosynthesis in desert plant <Emphasis Type="Italic">Alhagi sparsifolia</Emphasis> S

Most plants growing in temperate desert zone exhibit brief temperature-induced inhibition of photosynthesis at midday in the summer. Heat stress has been suggested to restrain the photosynthesis of desert plants like Alhagi sparsifolia S. It is therefore possible that high midday temperatures damage photosynthetic tissues, leading to the observed inhibition of photosynthesis. In this study, we investigated the mechanisms underlying heat-induced inhibition of photosynthesis in A. sparsifolia, a dominant species found at the transition zone between oasis and sandy desert on the southern fringe of the Taklamakan desert. The chlorophyll (Chl) a fluorescence induction kinetics and CO₂ response curves were used to analyze the thermodynamic characters of both photosystem II (PSII) and Rubisco after leaves were exposed to heat stress. When the leaves were heated to temperatures below 43°C, the initial fluorescence of the dark-adapted state (F_o), and the maximum photochemical efficiency of PSII (F_v/F_m), the number of active reaction centers per cross section (RCs) and the leaf vitality index (PI) increased or declined moderately. These responses were reversed, however, upon cooling. Moreover, the energy allocation in PSII remained stable. The gradual appearance of a K point in the fluorescence curve at 48°C indicated that higher temperatures strongly impaired PSII and caused irreversible damage. As the leaf temperature increased, the activity of Rubisco first increased to a maximum at 34°C and then decreased as the temperature rose higher. Under high-temperature stress, cell began to accumulate oxidative species, including ammoniacal nitrogen, hydrogen peroxide (H₂O₂), and superoxide (O₂ ^·−), suggesting that disruption of photosynthesis may result from oxidative damage to photosynthetic proteins and thylakoid membranes. Under heat stress, the biosynthesis of nonenzyme radical scavenging carotenoids (Cars) increased. We suggest that although elevated temperature affects the heat-sensitive components comprising of PSII and Rubisco, under moderately high temperature the decrease in photosynthesis is mostly due to inactivation of dark reactions.     

Differential response of chloride binding sites to elevated temperature: a comparative study in spinach thylakoids and PSII-enriched membranes

A study of heat effects was performed in thylakoids and photosystem II (PSII)-enriched membranes isolated from spinach in relation to Cl⁻-induced activation of PSII catalyzed oxygen evolution and the retention of Cl⁻ in the PSII complex. For this, Cl⁻-sufficient membranes and low-Cl⁻ membranes were used. The presence of Cl⁻ in the reaction medium did accelerate oxygen evolution, which remained unaffected by heat treatment up to 40°C in PSII membranes and up to 42.5°C in thylakoids. Heat resistance of Cl⁻-induced activation of oxygen evolution was found to be independent of the presence of ‘bound Cl⁻’ in the preparations. However, the functional stability of the PSII complex during heat treatment showed a marked dependence on the presence of bound Cl⁻ in PSII. Electron paramagnetic resonance study of manganese (Mn) release per reaction center/Y_D⁺ showed that there was little loss of Mn²⁺ up to 42°C in our preparations, although the PSII activity was significantly lowered. These observations together with data from steady state chlorophyll a fluorescence imply that the site of action of Cl⁻ causing direct activation of oxygen evolution was different from the site of primary heat damage. A differential response of chloride binding sites to heat stress was observed. The high-affinity (tightly bound, slow exchanging) site of chloride is affected earlier (∼37°C) while low-affinity (loosely bound, fast exchanging) site gets affected at higher temperatures (42.5°C in thylakoids and 40°C in the case of PSII-enriched membranes). Prasanna Mohanty is an INSA Honorary Scientist and Professor on Courtesy, DAVV, Indore.     

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。     

Chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence induction kinetics in leaves predicted from a model describing each discrete step of excitation energy and electron transfer associated with Photosystem II

Chlorophyll a fluorescence induction (FI) is widely used as a probe for studying photosynthesis. On illumination, fluorescence emission rises from an initial level O to a maximum P through transient steps, termed J and I. FI kinetics reflect the overall performance of photosystem II (PSII). Although FI kinetics are commonly and easily measured, there is a lack of consensus as to what controls the characteristic series of transients, partially because most of the current models of FI focus on subsets of reactions of PSII, but not the whole. Here we present a model of fluorescence induction, which includes all discrete energy and electron transfer steps in and around PSII, avoiding any assumptions about what is critical to obtaining O J I P kinetics. This model successfully simulates the observed kinetics of fluorescence induction including O J I P transients. The fluorescence emission in this model was calculated directly from the amount of excited singlet-state chlorophyll in the core and peripheral antennae of PSII. Electron and energy transfer were simulated by a series of linked differential equations. A variable step numerical integration procedure (ode15s) from MATLAB provided a computationally efficient method of solving these linked equations. This in silico representation of the complete molecular system provides an experimental workbench for testing hypotheses as to the underlying mechanism controlling the O J I P kinetics and fluorescence emission at these points. Simulations based on this model showed that J corresponds to the peak concentrations of Q _A ⁻ Q_B (Q_A and Q_B are the first and second quinone electron acceptor of PSII respectively) and Q _A ⁻ Q _B ⁻ and I to the first shoulder in the increase in concentration of Q _A ⁻ Q _B ²⁻ . The P peak coincides with maximum concentrations of both Q _A ⁻ Q _B ²⁻ and PQH₂. In addition, simulations using this model suggest that different ratios of the peripheral antenna and core antenna lead to differences in fluorescence emission at O without affecting fluorescence emission at J, I and P. An increase in the concentration of Q_B-nonreducing PSII centers leads to higher fluorescence emission at O and correspondingly decreases the variable to maximum fluorescence ratio (F _v/F _m).     

樟树幼苗机械损伤叶片对挥发性有机化合物及叶绿素荧光参数的影响

为探讨植物在机械损伤后C₆-C₁₀醛类化合物的释放机理, 及C₆-C₁₀醛类化合物对叶片光系统II (PSII)的影响, 以樟树(Cinnamomum camphora)为材料, 采用动态顶空气体循环法和热脱附/气相色谱/质谱联用技术(TDS-GC-MS), 对樟树幼苗叶片损伤后释放的C₆-C₁₀醛类化合物进行采集与分析, 并同步测定了脂氧合酶活性和损伤叶片的叶绿素荧光动力学参数。结果表明: 樟树幼苗叶片损伤后, 其挥发性有机化合物中己醛、庚醛、辛醛、壬醛和癸醛的释放量比损伤前分别增加了2.47、0.99、1.34、0.91和28.38倍(p < 0.01); 同时新增4种醛类化合物, 分别是: 2-己烯醛、2,4-己二烯醛、(E)-2-辛烯醛和(E)-2-壬烯醛。脂氧合酶活性比损伤前增加1.2倍(p < 0.01)。PSII单位反应中心复合体吸收的能量和被核心捕获的能量分别比损伤前下降12.8%和9.8% (p < 0.01)。单位面积反应中心的数量、电子传递量子产额、捕获激子能导致电子传递效率和叶片性能指数分别比损伤前增加23.3%、24.4%、22.6%和82.7% (p < 0.01)。损伤24 h后, 醛类化合物的种类、释放量、脂氧合酶活性及叶片叶绿素荧光动力学参数基本恢复到损伤前水平。说明机械损伤使PSII供体侧受损、脂氧合酶活性升高, 致使C₆-C₁₀醛类化合物大量释放, 樟树幼苗通过增加单位面积反应中心的数量来提高光合效率应对胁迫。     

Photoinhibition in outdoor Spirulina platensis cultures assessed by polyphasic chlorophyll fluorescence transients

Photoinhibition in outdoor cultures of Spirulina platensis was studied by measuring the polyphasic rise of chlorophyll fluorescence transients, which provide information on the primary photochemistry of PSII. The maximum efficiency of PSII photochemustry (F_v/F_m) declined in response to daily increasing irradiance and recovered as daily irradiance decreased. The greatest inhibition (15%) in F_v/F_m was observed at 12:00 hr which responded to the highest irradiance. The absorption flux, the trapping flux, and the electron transport flux per PSII reaction center increased in response to daily increasing irradiance and decreased as irradiance decreased. The daily change in the concentration of PSII reaction centers followed the same pattern as F_v/F_m. However, no significant changes in the probability of electron transport beyond Q_A (Ψ_o) were observed during the day. The results suggest that the decrease in F_v/F_m induced by photoinhibition in outdoor Spirulina cultures was a result of the inactivation of PSII reaction centers. The results also suggest that the measurement of polyphasic fluorescence transients is a powerful tool to study the mechanism of photoinhibition in outdoor Spirulina cultures and to screen strains for photoinhibition tolerance. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photosynthetic performance and resistance to photoinhibition of Zea mays L. leaves grown at sub-optimal temperature

The performance of the photosynthetic apparatus was examined in the third leaves of Zea mays L. seedlings grown at near-optimal (25 °C) or at suboptimal (15 °C) temperature by measuring chlorophyll (ChI) a fluorescence parameters and oxygen evolution in different temperature and light conditions. In leaf tissue grown at 25 and 15 °C, the quantum yield of PSII electron transport (ψ_PSII) and the rate of O₂ evolution decreased with decreasing temperature (from 25 to 4 °C) at a photon flux density of 125 μmol m^?2 s^?1. In leaves grown at 25 °C, the decrease of ψ_PSII correlated with a decrease of photochemical ChI fluorescence quenching (q_p), whereas in leaves crown at 15 °C q_p was largely insensitive to the temperature decrease. Compared with leaves grown at 25 °C, leaves grown at 15 °C were also able to maintain a higher fraction of oxidized to reduced Q_A (greater q_p) at high photon flux densities (up to 2000 μmol m^?2 s^?1), particularly when the measurements were performed at high temperature (25 °C). With decreasing temperature and/or increasing light intensity, leaves grown at 15 °C exhibited a substantial quenching of the dark level of fluorescence F₀ (q₀) whereas this type of quenching was virtually absent in leaves grown at 25 °C. Furthermore, leaves grown at 15 °C were able to recover faster from photo inhibition of photosynthesis after a photoinhibitory treatment (1200 μmol m^?2 s^?1 at 25, 15 or 6 °C for 8 h) than leaves grown at 25 °C. The results suggest that, in spite of having a low photosynthetic capacity, Z. mays leaves grown at sub optimal temperature possess efficient mechanisms of energy dissipation which enable them to cope better with photoinhibition than leaves grown at near-optimal temperature. It is suggested that the resistance of Z. mays leaves grown at 15 °C to photoinhibition is related to the higher content of carotenoids of the xanthophyll cycle (violaxanthin + antheraxanthin + zeaxanthin) measured in these leaves than in leaves grown at 25 °C.     

Seasonal Changes in Primary Photosynthetic Events during Low Temperature Adaptation of <Emphasis Type="Italic">Pinus sylvestris</Emphasis> in Central Yakutia

The methods of chlorophyll fluorescence induction and HPLC were used to study the influence of autumnal temperature decrease on photochemical electron-transport activity of photosystem II (PSII), nonphotochemical quenching of excessive excitation energy, and the composition of pigments in the firstyear needles of Pinus sуlvestris L. trees grown naturally in Central Yakutia. In the period from the beginning of September to October 10, the chlorophyll content was reduced by half, while the Chl a/b ratio increased from 2.9 to 4.3–4.5, indicating the degradation of peripheral antenna complexes. The decrease in average daily temperature to 4.9–6.4°C led to a transient increase in the quantum yield of nonphotochemical quenching (ΔpH-dependent parameter φNPQ). These changes were accompanied by a slow accumulation of unregulated zeaxanthin fraction insensitive to illumination conditions. The further decrease in average daily temperature to near-zero levels was paralleled by a sharp increase in zeaxanthin content, while the pH-dependent quenching was replaced with the constitutive quenching (parameter φ_f,D) because of the supposed structural reorganization of PSII. These processes were accompanied by a fast decrease in PSII functional activity, which was mostly due to the impairment of plastoquinone photochemical reduction. Freezing temperatures (from–3.6 to–12.1) destructed the oxygen-evolving complex in PSII and completely inactivated the PSII reaction centers. It is concluded that the largest changes in the condition of photosynthetic apparatus occur at a near-zero temperature range and proceed until complete inactivation of PSII under the action of freezing temperatures.     

Impairment of the photosynthetic apparatus by oxidative stress induced by photosensitization reaction of protoporphyrin IX

Treatment with the herbicide acifluorfen-sodium (AF-Na), an inhibitor of protoporphyrinogen oxidase, caused an accumulation of protoporphyrin IX (Proto IX) , light-induced necrotic spots on the cucumber cotyledon within 12-24 h, and photobleaching after 48-72 h of light exposure. Proto IX-sensitized and singlet oxygen (¹O₂)-mediated oxidative stress caused by AF-Na treatment impaired photosystem I (PSI), photosystem II (PSII) and whole chain electron transport reactions. As compared to controls, the F_v/F_m (variable to maximal chlorophyll a fluorescence) ratio of treated samples was reduced. The PSII electron donor NH₂OH failed to restore the F_v/F_m ratio suggesting that the reduction of F_v/F_m reflects the loss of reaction center functions. This explanation is further supported by the practically near-similar loss of PSI and PSII activities. As revealed from the light saturation curve (rate of oxygen evolution as a function of light intensity), the reduction of PSII activity was both due to the reduction in the quantum yield at limiting light intensities and impairment of light-saturated electron transport. In treated cotyledons both the Q (due to recombination of Q_A⁻ with S₂) and B (due to recombination of Q_B⁻ with S₂/S₃) band of thermoluminescence decreased by 50% suggesting a loss of active PSII reaction centers. In both the control and treated samples, the thermoluminescence yield of B band exhibited a periodicity of 4 suggesting normal functioning of the S states in centers that were still active. The low temperature (77 K) fluorescence emission spectra revealed that the F₆₉₅ band (that originates in CP-47) increased probably due to reduced energy transfer from the CP47 to the reaction center. These demonstrated an overall damage to the PSI and PSII reaction centers by ¹O₂ produced in response to photosensitization reaction of protoporphyrin IX in AF-Na-treated cucumber seedlings.     

Thermoluminescence and fluorescence study of changes in Photosystem II photochemistry in desiccating barley leaves

An effect of desiccation (a decrease of relative water content from 97% to 10% within 35 h) on Photosystem II was studied in barley leaf segments (Hordeum vulgare L. cv. Akcent) using chlorophyll a fluorescence and thermoluminescence (TL). The O-J-I-P fluorescence induction curve revealed a decrease of F_P and a slight shift of the J step to a shorter time with no change in its height. The analysis of the fluorescence decline after a saturating light flash revealed an increased portion of slow exponential components with increasing desiccation. The TL bands obtained after excitation by continuous light were situated at about –27°C (Z_v band – recombination of P680⁺Q_A ⁻), –14 °C (A band – S₃Q_A ⁻), +12 °C (B band – S_2/3Q_B ⁻) and +45 °C (C band – TyrD⁺Q_A ⁻). The bands related to the S-states of oxygen evolving complex (A and B) were reduced by desiccation and shifted to higher and lower temperatures, respectively. In accordance with this, the band observed at about +27 °C (S₂Q_B ⁻) after excitation by 1 flash fired at –10 °C and band at about +20 °C (S_2/3Q_B ⁻) after 2 flashes decreased with increasing water deficit and shifted to lower temperatures. A new band around 5 °C appeared in both regimes of TL excitation for a relative water content of under 42% and was attributed to the Q band (S₂Q_A ⁻). It is suggested that under desiccation, an inhibition of the formation of S₂- and S₃-states in OEC occurred simultaneously with a lowering of electron transport on the acceptor side of PS II. The temperature down-shift of the TL bands obtained after the flash excitation was induced at the initial phases of water stress, indicating a decrease of the activation energy for the S_2/3Q_B ⁻recombination. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effects of low temperature acclimation and photoinhibitory treatments on Photosystem 2 studied by thermoluminescence and fluorescence decay kinetics

The effects of low temperature acclimation and photoinhibitory treatment on Photosystem 2 (PS 2) have been studied by thermoluminescence and chlorophyll fluorescence decay kinetics after a single turnover saturating flash. A comparison of unhardened and hardened leaves showed that, in the hardened case, a decrease in overall and B-band thermoluminescence emissions occurred, indicating the presence of fewer active PS 2 reaction centers. A modification in the form of the B-band emission was also observed and is attributed to a decrease in the apparent activation energy of recombination in the hardened leaves. The acclimated leaves also produced slower Q_A ^– reoxidation kinetics as judged from the chlorophyll fluorescence decay kinetics. This change was mainly seen in an increased lifetime of the slow reoxidation component with only a small increase in its amplitude. Similar changes in both thermoluminescence and fluorescence decay kinetics were observed when unhardened leaves were given a high light photoinhibitory treatment at 4°C, whereas the hardened leaves were affected to a much lesser extent by a similar treatment. These results suggest that the acclimated plants undergo photoinhibition at 4°C even at low light intensities and that a subsequent high light treatment produces only a small additive photoinhibitory effect. Furthermore, it can be seen that photoinhibition eventually gives rise to PS 2 reaction centers which are no longer functional and which do not produce thermoluminescence or variable chlorophyll fluorescence.Abbreviations D1 The 32 kDa protein of Photosystem 2 reaction center - F_m maximum chlorophyll fluorescence yield - F₀ minimal chlorophyll fluorescence yield obtained when all PS 2 centers are open - F_i intermediate fluorescence level corresponding to PS 2 centers which are loosely or not connected to plastoquinone (non-B centers) - F_v maximum variable chlorophyll fluorescence yield (F_v=F_m–F₀) - PS 2 Photosystem 2 - Q_A and Q_B respectively, primary and secondary quinonic acceptors of PS 2 - S1, S2 and S3 respectively, the one, two and three positively charged states of the oxygen evolving system - Z secondary donor of PS 2     

Multiple effects of salinity on photosynthesis of the protist Euglena gracilis

The effect of NaCl in the culture medium on growth, photosynthesis and cell content of chlorophyll, K⁺, Na⁺, Ca²⁺ and Mg²⁺ in Euglena gracilis was studied. O₂ production, quantum yield of photosystem II (PSII), the non-photochemical quenching of chlorophyll fluorescence (q_N) and the chlorophyll alb ratio all diminished by 0.2 M NaCl. Respiration and chlorophyll a and b increased, whereas the photochemical quenching (q_p) of chlorophyll fluorescence was not affected by 0.2 M NaCl. Salt stress also induced an increase in cell volume and in K⁺ and Na⁺ concentrations, but decreased the concentrations of Ca²⁺ and Mg²⁺. Except for a protective effect on O₂ production, additional Ca²⁺ in the culture medium did not attenuate the salt effect on the parameters measured. The addition of HCO₃^? restored the PSII quantum yield of O₂ production in cells grown in high salt. Salt stress promoted a decrease in the apparent rate of quinone A (Q_A) reduction and an apparent obstruction of Q_B reduction, which were not prevented by excess HCO₃^?; the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) did not increase chlorophyll fluorescence in salt-grown cells. These results indicate that photosynthesis in Euglena grown under salt stress exhibits: (1) diminution of the HCO₃^? dependent water-splitting activity of PSII; (2) inhibition of the electron transfer at the quinone pool level; (3) probable increase in thylakoid stacking (as indicated by the effect on the chlorophyll alb ratio); and (4) dissipation of the H⁺ gradient across the thylakoid membranes (as indicated by the decrease of q_N).     

Recovery of photosystem I and II activities during re-hydration of lichen<Emphasis Type="Italic"> Hypogymnia physodes</Emphasis> thalli

Photochemical efficiencies of photosystem I (PSI) and photosystem II (PSII) were studied in dry thalli of the lichen Hypogymnia physodes and during their re-hydration. In dry thalli, PSII reaction centers are photochemically inactive, as evidenced by the absence of variable chlorophyll (Chl) fluorescence, whereas the primary electron donor of PSI, P700, exhibits irreversible oxidation under continuous light. Upon application of multiple- and, particularly, single-turnover pulses in dry lichen, P700 oxidation partially reversed, which indicated recombination between P700⁺ and the reduced acceptor F_X of PSI. Re-wetting of air-dried H. physodes initiated the gradual restoration of reversible light-induced redox reactions in both PSII and PSI, but the recovery was faster in PSI. Two slow components of P700⁺ reduction occurred after irradiation of partially and completely hydrated thalli with strong white light. In contrast, no slow component was found in the kinetics of re-oxidation of Q_A^–, the reduced primary acceptor of PSII, after exposure of such thalli to white light. This finding indicated the inability of PSII in H. physodes to provide the reduction of the plastoquinone pool to significant levels. It is concluded that slow alternative electron transport routes may contribute to the energetics of photosynthesis to a larger extent in H. physodes than in higher plants.Abbreviations A₀ and A₁ Primary acceptor chlorophyll and secondary electron acceptor phylloquinone - Chl a Chlorophyll a - F_m Maximal level of chlorophyll fluorescence when all PSII centers are closed - F_o Minimal level of fluorescence when all PSII centers are open after dark adaptation - FR Far-red - F_v Variable fluorescence (=F_m–F_o) - F_X, F_A, and F_B Iron–sulfur centers - MT pulse Multiple-turnover pulse - PS Photosystem - P700 Reaction center chlorophyll of PSI - Q_A Primary quinone acceptor of PSII - Q_B Secondary quinone acceptor of PSII - ST pulse Single-turnover pulse