Partitioning of Noncyclic Photosynthetic Electron Transport to O(2)-Dependent Dissipative Processes as Probed by Fluorescence and CO(2) Exchange

The partitioning of noncyclic photosynthetic electron transport between net fixation of CO₂ and collective O₂-dependent, dissipative processes such as photorespiration has been examined in intact leaf tissue from Nicotiana tabacum. The method involves simultaneous application of CO₂ exchange and pulse modulated fluorescence measurements. As either irradiance or CO₂ concentration is varied at 1% O₂ (i.e. absence of significant O₂-dependent electron flow), the quantum efficiency of PSII electron transport (_se) with CO₂ as the terminal acceptor is a linear function of the ratio of photochemical:nonphotochemical fluorescence quenching coefficients (i.e. q_Q:q_NP). When the ambient O₂ concentration is raised to 20.5% or 42% the q_Q:q_NP is assumed to predict the quantum efficiency of total noncyclic electron transport (′_se). A factor which represents the proportion of electron flow diverted to the aforementioned dissipative processes is calculated as (′_se − _se)/′_se where _se is now the observed quantum efficiency of electron transport in support of net fixation of CO₂. Examination of changes in electron allocation with CO₂ and O₂ concentration and irradiance at 25°C provides a test of the applicability of the Rubisco model to photosynthesis in vivo.     

Aspartate Kinase from Some Higher Plants and Algae

Regulation of aspartate kinase activity was examined in some higher plant seedlings (Helianthus annuus, Raphanus sativus, Cucutnis sativus, Sinapis alba), a green (Chlorellapyrenoidosa), and a blue-green alga (Anacystis nidulans). In Cucumis sativus a concerted feedback regulation by L-lysine and L-threonine was indicated. In all the other species examined, aspartate kinase was inhibited by both L-lysine and L-threonine and it depended upon the species which of them was the strongest inhibitor. Only slight effects of L-leucine, L-isoIeucine and L-valine were observed, except in the Sinapis alba enzyme.     

Dynamics of Xanthophyll-Cycle Activity in Different Antenna Subcomplexes in the Photosynthetic Membranes of Higher Plants (The Relationship between Zeaxanthin Conversion and Nonphotochemical Fluorescence Quenching)

The generation of nonphotochemical quenching of chlorophyll fluorescence (qN) in the antenna of photosystem II (PSII) is accompanied by the de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin. The function of zeaxanthin in two mechanisms of qN, energy-dependent quenching (qE) and photoinhibitory quenching (qI), was investigated by measuring the de-epoxidation state in the antenna subcomplexes of PSII during the generation and relaxation of qN under varying conditions. Three different antenna subcomplexes were separated by isoelectric focusing: Lhcb1/2/3, Lhcb5/6, and the Lhcb4/PSII core. Under all conditions, the highest de-epoxidation state was detected in Lhcb1/2/3 and Lhcb5/6. The kinetics of de-epoxidation in these complexes were found to be similar to the formation of qE. The Lhcb4/PSII core showed the most pronounced differences in the de-epoxidation state when illumination with low and high light intensities was compared, correlating roughly with the differences in qI. Furthermore, the epoxidation kinetics in the Lhcb4/PSII core showed the most pronounced differences of all subcomplexes when comparing the epoxidation after either moderate or very strong photoinhibitory preillumination. Our data support the suggestion that zeaxanthin formation/epoxidation in Lhcb1-3 and Lhcb5/6 may be related to qE, and in Lhcb4 (and/or PSII core) to qI.     

N2O Evolution by Green Algae

Evidence is presented here that axenic cultures of Chlorella, Scenedesmus, Coelastrum, and Chlorococcum spp. evolve N₂O when grown on NO₂⁻, showing that the Chlorophyceae are a source of N₂O in aquatic systems.     

Biosynthesis of Carotenoids in the Chloroplasts of Algae and Higher Plants

Physiological, biochemical, and genetic aspects of carotenoid biosynthesis in the chloroplast membranes of green algae and higher plants are discussed starting from the earliest stages of biosynthesis of key C₅-isoprene units. The latter are synthesized either from acetate (C₂) to mevalonic acid (C₆) or from glucose (C₆) by forming glyceraldehyde 3-phosphate (C₃) and pyruvate decarboxylation product (C₂) through intermediate compounds to isopentenyl diphosphate (C₅). In all organisms, the further carotenoid synthesis from isopentenyl diphosphate and its isomer dimethylallyl diphosphate (C₅) proceeds through their transformation into geranyl diphosphate (C₁₀), farnesyl diphosphate (C₁₅), geranylgeranyl diphosphate (C₂₀) and phytoene (C₄₀). Phytoene desaturation (dehydrogenation) to carotene, neurosporene, and lycopene, and all steps of their cyclization to , and carotenes are discussed in detail. The synthesis of xanthophylls in chloroplasts is presented as the sequential formation of hydroxy-, epoxy- and oxo- groups. Genetic control of biosynthesis, as well as the localization and functional role of carotenoids in the chloroplast membranes of plants and algae are briefly discussed.     

藻类产生及清除过氧化氢的研究

过氧化氢的生物生成是天然水体中H2O2的来源之一。从藻类产生及分解过氧化氢的途径,影响过氧化氢产量的主要因素,如藻的种类、细胞的渗透性、藻的生长阶段、藻浓度和光照条件等几方面对这一领域的研究作了综述。     

高等植物环式电子传递的生理作用

环式电子传递做为一种可供选择的电子传递途径之一,近几年被证实它对于许多高等植物的生长是必需的.环式电子传递通过促进跨类囊体膜质子梯度的建立一方面激发ATP合成酶合成ATP,另一方面加强了光系统Ⅱ处的热耗散,稳定了放氧复合体,从而保护光系统Ⅱ免受光抑制.同时,它还可以缓解光系统Ⅰ处电子受体的过度还原,减少超氧阴离子在光系统Ⅰ处的合成,防止光系统Ⅰ受到光抑制.本文简要地综述了环式电子传递的途径、其参与ATP合成的作用、对光系统Ⅱ和光系统Ⅰ光保护作用及其对环境胁迫的响应和调节,并对环式电子传递的研究提出了展望.     

植物ABA-H2O2介导的气孔关闭

本文综述了脱落酸作为根源信号物质经由木质部被传递到叶片,经重新分配再与脱落酸受体结合,然后刺激气孔开放因子,调节烟酰胺腺嘌呤二核苷酸磷酸氧化酶等关键酶活性产生过氧化氢,过氧化氢可使胞质碱化并刺激钙离子通道使钙离子内流,活化阴离子通道使阴离子外流,最终导致气孔关闭的一系列过程。该过程涉及到的因子包括：脱落酸受体、气孔开放因子、磷脂酰环己六醇、分裂原激活蛋白激酶、烟酰胺腺嘌呤二核苷酸磷酸氧化酶、Ca^（2＋）、pH、一氧化氮等。     

Histochemical Demonstration and Localization of H2O2 in Organs of Higher Plants by Tissue Printing on Nitrocellulose Paper

A sensitive tissue-print assay for the detection and histological localization of H2O2 in freshly cut organ sections was developed by impregnating nitrocellulose paper with a mixture of Kl and soluble starch. H2O2 transferred from the cut surface of the section to the dried paper forms I2, which can be visualized by the intensely colored I2-starch complex. The detection limit of the assay is in the range of 0.1 to 0.2 mmol L-1 H2O2. Due to the rapid immobilization of H2O2 in the paper, very clear prints of the tissue distribution of H2O2 can be obtained with a spatial resolution on the level of single cells. The application of this rapid and simple assay is explored in five experimental examples demonstrating that the in vivo level of H2O2 varies strikingly in different tissues and can be regulated by developmental factors such as hormones, light, and wounding. The results show that: (a) In the hypocotyl of soybean (Glycine max L.) seedlings the apoplastic H2O2 level increases strongly from top to base, accompanied by characteristic changes in its histological distribution. (b) In the epicotyl of pea (Pisum sativum L.) seedlings the induction of lateral expansion by ethylene is correlated with a depletion of H2O2 in the cell walls of the expanding tissues. (c) In the hypocotyl of bean (Phaseolus vulgaris L.) seedlings H2O2 is primarily localized in a ring of parenchymatic tissue between xylem and cortex next to lignifying cells but not in the lignifying cells themselves. (d) In the hypocotyl of sunflower (Helianthus annuus L.) and cucumber (Cucumis sativus L.) seedlings the light-mediated inhibition of elongation growth is correlated with a strong increase in H2O2 in the epidermis and in the vascular bundles. (e) Potato (Solanum tuberosum L.) tubers show high levels of H2O2 only in the outer cell layers but are able to accumulate H2O2 in the inner tissue upon wounding.     

Quenching Analysis of Chlorophyll Fluorescence by the Saturation Pulse Method: Particular Aspects Relating to the Study of Eukaryotic Algae and Cyanobacteria

The principles of chlorophyll fluorescence quenching analysisby the saturation pulse method are outlined with emphasis onparticular aspects encountered in the study of eukaryotic algaeand cyanobacteria. Major differences of these photosyntheticorganisms with respect to higher plant leaves, for which quenchinganalysis originally was developed, are very rapid inductionof O₂-dependent electron flow, close interaction between photosyntheticand respiratory metabolism, dark reduction of the plastoquinonepool and pronounced state transitions of energy distributionbetween the two photosystems. It is shown that the use of 25–50ms pulses of saturating light for determination of maximal fluorescenceyield is advantageous, in contrast to the 0.5–2 s pulsescommonly used with higher plants. The shorter pulses are lessinvasive with respect to the induction of energising electronflow which can induce non-photochemical quenching and statechanges. In particular, short saturation pulses are essentialto study true dark changes of fluorescence yield. As an example,the induction of pronounced quenching of maximal fluorescencein Chlamydomonas by dark-anaerobic incubation is demonstrated.Analysis of the rapid rise kinetics upon onset of saturatinglight reveals two major phases, O-I₁ and I₁-I₂, with distinctlydifferent properties. Arguments are put forward that an assessmentof maximal fluorescence yield with single turnover saturatingflashes is problematic, as there is a type of photochemicalquenching, the elimination of which during the I₁-I₂ phase requiresmultiple turnovers at photosystem II. Furthermore, the variablefluorescence represented by the two phases is affected differentlyby non-photochemical quenching. It is shown that dark-anaerobicquenching in Chlamydomonas as well as state 2 quenching in Synechocystisare correlated with a preferential suppression of the I₁-I₂phase. Experiments with Synechocystis are presented which demonstratethe potential of saturation pulse quenching analysis for thestudy of reversible state changes. The mutant M55 of Synechocystis6803 appears to be "locked" in state 1. ¹Present address: Lehrstuhl Botanik I, Mittlerer Dallenbergweg64, D-97082 Würzburg, Germany     

Simultaneous Measurements of Steady State Chlorophyll a Fluorescence and CO(2) Assimilation in Leaves: The Relationship between Fluorescence and Photosynthesis in C(3) and C(4) Plants

Rates of CO₂ assimilation and steady state chlorophyll a fluorescence were measured simultaneously at different intercellular partial pressures of CO₂ in attached cotton (Gossypium hirsutum L. cv Deltapine 16) leaves at 25°C. Electron transport activity for CO₂ assimilation plus photorespiration was calculated for these experiments. Under light saturating (1750 microeinsteins per square meter per second) and light limiting (700 microeinsteins per square meter per second) conditions there was a good correlation between fluorescence and the calculated electron transport activity at 19 and 200 millibars O₂, and between fluorescence and rates of CO₂ assimilation at 19 millibars but not 200 millibars O₂. The values of fluorescence measured at about 220 microbars intercellular CO₂ were not greatly affected by increasing O₂ from 19 to 800 millibars. Fluorescence increased with light intensity at any one intercellular CO₂ partial pressure. But the values obtained for fluorescence, expressed as a ratio of the maximum fluorescence obtained in DCMU-treated tissue, over the same range of CO₂ partial pressure at 500 microeinsteins per square meter per second were similar to those obtained at 1000 and 2000 microeinsteins per square meter per second. There were two phases in the observed correlation between fluorescence and calculated electron transport activity: an initial inverse relationship at low CO₂ partial pressures which reversed to a positive correlation at higher values of CO₂ partial pressures. Similar results were observed in the C₃ species Helianthus annuus L., Phaseolus vulgaris L., and Brassica chinensis. In all C₄ species (Zea mays L., Sorghum bicolor L., Panicum maximum Jacq., Amaranthus edulis Speg., and Echinochloa frumentacea [Roxb.] Link) examined changes in fluorescence were directly correlated with changes in CO₂ assimilation rates. The nature and the extent to which Q (primary quencher) and high-energy state (q_E) quenching function in determining the steady state fluorescence obtained during photosynthesis in leaves is discussed.     

Cd-Binding Complexes from the Root Tissues of Various Higher Plants Cultivated in Cd2+-Containing Medium

Stone parsley, soybean, sunflower, sweet potato, potato, andadlay cultivated in a Cd²⁺-containing medium had Cd-bindingcomplexes with molecular weights of about 4,000 in the roottissues. The complexes were similar to the complex previouslyfound in water hyacinth roots in their absorption and CD spectraand their amino acid compositions. The results indicate thewidespread existence of complexes similar to fission yeast Cd-BPlin roots of various plants. (Received June 30, 1986; Accepted December 18, 1986)     

The Effect of Temperature on the Occurrence of O(2) and CO(2) Insensitive Photosynthesis in Field Grown Plants

The sensitivity of photosynthesis to O₂ and CO₂ was measured in leaves from field grown plants of six species (Phaseolus vulgaris, Capsicum annuum, Lycopersicon esculentum, Scrophularia desertorum, Cardaria draba, and Populus fremontii) from 5°C to 35°C using gas-exchange techniques. In all species but Phaseolus, photosynthesis was insensitive to O₂ in normal air below a species dependent temperature. CO₂ insensitivity occurred under the same conditions that resulted in O₂ insensitivity. A complete loss of O₂ sensitivity occurred up to 22°C in Lycopersicon but only up to 6°C in Scrophularia. In Lycopersicon and Populus, O₂ and CO₂ insensitivity occurred under conditions regularly encountered during the cooler portions of the day. Because O₂ insensitivity is an indicator of feedback limited photosynthesis, these results indicate that feedback limitations can play a role in determining the diurnal carbon gain in the field. At higher partial pressures of CO₂ the temperature at which O₂ insensitivity occurred was higher, indicating that feedback limitations in the field will become more important as the CO₂ concentration in the atmosphere increases.     

Fluorescence Quenching and Gas Exchange in a Water Stressed C(3) Plant, Digitalis lanata

A leaf cuvette has been adapted for use with a pulse-modulation fluorometer and an open gas exchange system. Leaf water potential (ψ) was decreased by withholding watering from Digitalis lanata EHRH. plants. At different stages of water deficiency the photochemical (q_Q) and nonphotochemical (q_E) fluorescence quenching was determined during the transition between darkness and light-induced steady state photosynthesis of the attached leaves. In addition, the steady state CO₂ and H₂O gas exchange was recorded. Following a decrease of leaf water potential with increasing water deficiency, the transition of photochemical quenching was almost unaffected, whereas nonphotochemical quenching increased. This is indicative of an enhanced thylakoid membrane energization during the transition and is interpreted as a partial inhibition of either the ATP generating or the ATP consuming reaction sequences. Complete reversion of the stress induced changes was achieved within 6 hours after rewatering. In contrast to the variations during transition, the final steady state values of q_Q and q_E remained unchanged over the entire stress range from −0.7 to −2.5 megapascals. From these results we conclude that, once established, electron transport via photosystem II and the transmembrane proton gradient remain unaffected by water stress. These data are indicative of a protective mechanism against photoinhibition during stress, when net CO₂ uptake is limited.     

Compartment-dependent management of H(2)O(2) by peroxisomes

Peroxisomes (PO) are essential and ubiquitous single-membrane-bound organelles whose ultrastructure is characterized by a matrix and often a crystalloid core. A unique feature is their capacity to generate and degrade H(2)O(2) via several oxidases and catalase, respectively. Handling of H(2)O(2) within PO is poorly understood and, in contrast to mitochondria, they are not regarded as a default H(2)O(2) source. Using an ultrasensitive luminometric H(2)O(2) assay, we show in real time that H(2)O(2) handling by matrix-localized catalase depends on the localization of H(2)O(2) generation in- and outside the PO. Thus, intact PO are inefficient at degrading external but also internal H(2)O(2) that is generated by the core-localized urate oxidase (UOX). Our findings suggest that, in addition to the PO membrane, the matrix forms a significant diffusion barrier for H(2)O(2). In contrast, matrix-generated H(2)O(2) is efficiently degraded. We further show that the tubular structures in crystalloid cores of UOX are associated with and perpendicularly oriented toward the PO membrane. Studies on metabolically active liver slices demonstrate that UOX directly releases H(2)O(2) into the cytoplasm, with the 5-nm primary tubules in crystalloid cores serving as exhaust conduits. Apparently, PO are inefficient detoxifiers of external H(2)O(2) but rather can become an obligatory source of H(2)O(2)--an important signaling molecule and a potential toxin.