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1Author    Michel Havaux3, Murielle EylettersbRequires cookie*
 Title    Is the in vivo Photosystem I Function Resistant to Photoinhibition? An Answer from Photoacoustic and Far-Red Absorbance Measurements in Intact Leaves  
 Abstract    Preillumination o f intact pea leaves with a strong blue-green light o f 400 W m "2 markedly inhibited both photoacoustically monitored 0 2-evolution activity and PS II photochemistry as estimated from chlorophyll fluorescence measurements. The aim o f the present work was to examine, with the help o f the photoacoustic technique, whether this high-light treatment dete­ riorated the in vivo PS I function too. High-frequency photoacoustic measurements indicated that photochemical conversion o f far-red light energy in PS I was preserved (and even tran­ siently stimulated) whereas photochemical energy storage monitored in light exciting both PS I and PS II was markedly diminished. Low-frequency photoacoustic measurements of the Emerson enhancement showed a spectacular change in the PS II/PS I activity balance in favor o f PS I. It was also observed that the linear portion o f the saturation curve of the far-red light effect in the Emerson enhancement was not changed by the light treatment. Those results lead to the conclusion that, in contrast to PS II, the in vivo PS I photofunctioning was resistant to strong light stress, thus confirming previous suggestions derived from in vitro studies. Estima­ tion of the redox state of the PS I reaction center by leaf absorbance measurements at ca. 820 nm suggested that, under steady illum ination, a considerably larger fraction o f PS I cen­ ters were in the closed state in high-light pretreated leaves as compared to control leaves, pre­ sumably allowing passive adjustment o f the macroscopic quantum yield o f PS I photochemis­ try to the strongly reduced photochemical efficiency o f photoinhibited PS II. 
  Reference    Z. Naturforsch. 46c, 1038—1044 (1991); received M arch 25/June 21 1991 
  Published    1991 
  Keywords    Photosystem I, Photoacoustics, Photoinhibition 
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 TEI-XML for    default:Reihe_C/46/ZNC-1991-46c-1038.pdf 
 Identifier    ZNC-1991-46c-1038 
 Volume    46 
2Author    Maya Velitchkova, A. Ntoaneta Popova, Tzvetelina MarkovaRequires cookie*
 Title    Effect of Membrane Fluidity on Photoinhibition of Isolated Thylakoids Membranes at Room and Low Temperature  
 Abstract    The relationship betw een thylakoid membrane fluidity and the process of photoinhibition at room and low (4 °C) temperature was investigated. Two different membrane perturbing agents -cholesterol and benzylalcohol were applied to manipulate the fluidity of isolated pea thylakoids. The photochemical activity of photosystem I (PSI) and photosystem II (PSII), polarographically determ ined, were measured at high light intensity for different time of illumination at both temperatures. The exposure of cholesterol-and benzylalcohol-treated thylakoid membranes to high light intensities resulted in inhibition of both studied photo­ chemical activities, being more pronounced for PSII compared to PSI. Time dependencies of inhibition o f PSI and PSII electron transport rates for untreated and membranes with altered fluidity were determ ined at 20 °C and 4 °C. The effect is more pronounced for PSII activity during low-temperature photoinhibition. The data are discussed in terms of the determining role of physico-chemical properties of thylakoid membranes for the response o f photosyn­ thetic apparatus to light stress. 
  Reference    Z. Naturforsch. 56c, 369—3 (2001); received January 3/February 1 2001 
  Published    2001 
  Keywords    Membrane Fluidity, Photoinhibition, Thylakoid Membranes 
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 TEI-XML for    default:Reihe_C/56/ZNC-2001-56c-0369.pdf 
 Identifier    ZNC-2001-56c-0369 
 Volume    56 
3Author    G. H. Einrich Krause, Henrik LaaschRequires cookie*
 Title    Energy-Dependent Chlorophyll Fluorescence Quenching in Chloroplasts Correlated with Quantum Yield of Photosynthesis  
 Abstract    Chlorophyll a fluorescence quenching was studied in intact, C 0 2 fixing chloroplasts isolated from spinach. Energy-dependent quenching (qE), which is correlated with the light-induced pro­ ton gradient across the thylakoid membrane presumably reflects an increase in the rate-constant o f thermal dissipation o f excitation energy in the photosynthetic pigm ent system . The extent o f qE was found to be linearly related to the decrease o f quantum yield o f photosynthesis. We suggest that this relationship indicates a dynamic property o f the membrane to adjust thermal dissipation o f absorbed light energy to the energy requirem ent o f photosynthesis. 
  Reference    Z. Naturforsch. 42c, 581 (1987); received January 19 1987 
  Published    1987 
  Keywords    Chlorophyll Fluorescence Quenching, Chloroplast, Photoinhibition, Photosynthesis, Thermal Energy Dissipation 
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 TEI-XML for    default:Reihe_C/42/ZNC-1987-42c-0581.pdf 
 Identifier    ZNC-1987-42c-0581 
 Volume    42 
4Author    Imre Vass, Narendranath Mohanty, Sándor DemeterRequires cookie*
 Title    Photoinhibition of Electron Transport Activity of Photosystem II in Isolated Thylakoids Studied by Thermoluminescence and Delayed Luminescence  
 Abstract    The effect of photoinhibition on the primary (Oa) and secondary (Ob) quinone acceptors of photosystem II was investigated in isolated spinach thylakoids by the methods of thermolumines-cence and delayed luminescence. The amplitudes of the Q (at about 2 °C) and B (at about 30 °C) thermoluminescence bands which are associated with the recombination of the S;OA and S2QB charge pairs, respectively, exhibited parallel decay courses during photoinhibitory treatment. Similarly, the amplitudes of the flash-induced delayed luminescence components ascribed to the recombination of S 2 0A and S 2 OB charge pairs and having half life-times of about 3 s and 30 s, respectively, declined in parallel with the amplitudes of the corresponding Q and B thermo-luminescence bands. The course of inhibition of thermoluminescence and delayed luminescence intensity was parallel with that of the rate of oxygen evolution. The peak positions of the B and Q thermoluminescence bands as well as the half life-times of the corresponding delayed lumines-cence components were not affected by photoinhibition. These results indicate that in isolated thylakoids neither the amount nor the stability of the reduced OB acceptor is preferentially decreased by photoinhibition. We conclude that either the primary target of photodamage is located before the Ob binding site in the reaction center of photosystem II or QA and OB undergo simultaneous damage. 
  Reference    Z. Naturforsch. 43c, 871—876 (1988); received August 12 1988 
  Published    1988 
  Keywords    Photosynthesis, Photoinhibition, Photosystem II, Thermoluminescence, Delayed Luminescence 
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 TEI-XML for    default:Reihe_C/43/ZNC-1988-43c-0871.pdf 
 Identifier    ZNC-1988-43c-0871 
 Volume    43 
5Author    Wim Vermaas, Jeroen Charité, Gaozhong ShenRequires cookie*
 Title    Q a Binding to D2 Contributes to the Functional and Structural Integrity of Photosystem II  
 Abstract    Two D 2 mutants were created with a site-directed mutation near the presumable binding site of Q a. In one of the mutants, in which Trp-253, the aromatic residue potentially involved in facilitating electron transport from pheophytin to Q A and/or in binding of Q A, had been replaced by Leu, PS II was undetectable in thylakoids. This mutant is an obligate photohetero-troph. In another mutant the Gly-215 residue, located next to the His residue that is pro­ posed to bind Q a and Fe2+, was mutated to Trp. This mutation leads to a rapid inactivation of oxygen evolution capacity in the light, and to a virtual elimination of the potential to grow photoautotrophically, but does not greatly affect the number of photosystem II reaction cen­ ters on a chlorophyll basis. We propose that proper binding of Q A to the photosystem II reac­ tion center complex is a prerequisite for stability of the photosystem II complex. Impairment of Q a binding leads to rapid inactivation of photosystem II, which may be followed by a struc­ tural disintegration of the complex. 
  Reference    Z. Naturforsch. 45c, 359—365 (1990); received November 3 1989 
  Published    1990 
  Keywords    Photoinhibition, Plastoquinone, Photosynthesis, Site-Directed Mutagenesis, Cyanobacteria 
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 TEI-XML for    default:Reihe_C/45/ZNC-1990-45c-0359.pdf 
 Identifier    ZNC-1990-45c-0359 
 Volume    45 
6Author    MarcelA K Jansen, ChristoM. Alan, Yoseph Shaaltiel, Jonathan GresselRequires cookie*
 Title    Mode of Evolved Photooxidant Resistance to Herbicides and Xenobiotics  
 Abstract    A few species have evolved resistance to paraquat after repeated selection. As paraquat still inhibited N A D P reduction, we hypothesized that resistance might be due to (a) detoxification o f the paraquat-generated active oxygen species and (b) that resistant plants would have some cross resistance to other xenobiotic oxidants as well as to photoinhibition, which we subse­ quently demonstrated. The levels o f plastid isozymes o f the oxygen detoxification pathway: (CuZn) superoxide dismutase, ascorbate peroxidase and glutathione reductase were genetical­ ly higher in the resistant than in the sensitive biotype o f C onyza bonariensis through the F2 generation. Resistance was suppressed by chelators o f copper and/or zinc. Intact chloroplasts from resistant plants had less membrane damage with and without paraquat, than those from sensitive plants. Resistant C onyza plants recover from paraquat inhibition o f photosynthesis in 3 -4 h in high light, whereas sensitive plants died. Both resistant and sensitive plants recov­ ered from paraquat in 3 -4 h in low light intensities. Paraquat-resistant Conyza plants were cross-tolerant to S 0 2, atrazine, acifluorfen and to photoinhibition. Drought-tolerant maize inbreds were cross-tolerant to paraquat, SO, and acifluorfen (compared to sensitive lines) and they also possessed higher levels o f (Cu/Zn) superoxide dismutase and glutathione reductase. The tolerance to oxidant stresses in C onyza and maize increases with plant age, suggesting that the shift to resistance is a constitutive, earlier expression o f the genes normally expressed later in development. 
  Reference    Z. Naturforsch. 45c, 463 (1990); received November 9 1989 
  Published    1990 
  Keywords    Paraquat, Acifluorfen, Atrazine, Photoinhibition, Superoxide Dismutase 
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 TEI-XML for    default:Reihe_C/45/ZNC-1990-45c-0463.pdf 
 Identifier    ZNC-1990-45c-0463 
 Volume    45 
7Author    Jack FarineauRequires cookie*
 Title    In Pea Thylakoids, a Special Thermoluminescence Band Was Observed at Low Temperature after Photoinhibitory Treatments Performed under Aerobic Conditions, either in vivo (in Leaf) or in vitro  
 Abstract    Characteristics o f thermoluminescence (TL) glow curves were studied in thylakoids isolated from pea leaves after an exposure to very high light in the TL device. The inhibition o f p h oto­ synthesis was detected both as decreases o f oxygen evolution rates and o f variable fluores­ cence. In another more classical experiment, leaves were exposed to high light at low tempera­ ture (5 °C) for 4 h and recovery o f normal PS II characteristics were followed during a 20 h time period under low light at 20 °C. TL study was performed on thylakoids isolated from these leaves. Charging o f bands was performed by an illumination at low temperature (-8 0 °C). Illumination at low temperature (-8 0 °C) induces 2 types o f TL bands called Z variable (Zv) band peaking at about —70 °C and a classical B band peaking at 35 °C. B band is ascribed to recombination in pairs S2/3QB", whereas the origin o f Zv bands remains unclear. M odified Zv bands, with flat shape and presenting a large part resistant to ethanol, were evidenced after som e minutes o f dark adaptation, both in light-exposed thylakoids and in thylakoids isolated from photoinhibited leaves. This ethanol resistant part in Zv band was only induced by p h oto­ inhibitory treatment performed under aerobic conditions (2 1 % 0 2 in media), both in vitro and in vivo; thus, the appearance o f the special Zv band would be related to an oxidative process accompanying photoinhibition. B bands presented reduced sizes and a shift o f peak maximum o f about 5 °C after some minutes o f dark adaptation. In vivo, the recovery o f a high PS II activ­ ity and o f B bands displaying normal characteristics occurs parallely to reappearance o f Zv bands with normal shape and with a vestigial ethanol resistant band. 
  Reference    Z. Naturforsch. 47c, 875—8 (1992); received July 27/A ugust 27 1992 
  Published    1992 
  Keywords    Pea, Photoinhibition, Photosystem II, Thermoluminescence, Thylakoid, Zv Band 
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 TEI-XML for    default:Reihe_C/47/ZNC-1992-47c-0875.pdf 
 Identifier    ZNC-1992-47c-0875 
 Volume    47 
8Author    V. Ictor, B. Curw, G. Ert, S. Chan Sk Er, O. Scar, J. De, V. Os, J. S. Van RensenRequires cookie*
 Title    Comparison of Photosynthetic Activities in Triazine-Resistant and Susceptible Biotypes of Chenopodium album  
 Abstract    Triazine-resistant and susceptible Chenopodium album plants were grown at low and at high light irradiances. A t the lower light irradiance the dry m atter production of the resistant and the susceptible plants were almost similar. At the higher irradiance the resistant biotype had a significantly lower production. Fluorescence studies showed that the photochemical yield and the photosystem II electron transport rate were lower in the resistant biotype. It could be demonstrated in intact leaves that the lower productivity of the resistant biotype is caused by a higher sensitivity to photoinhibition. However, when studying effects of photoinhibition on electron flow and photophosphorylation in isolated thylakoids o f the two biotypes, no signifi­ cant differences between resistant and susceptible plant materials were observed. It is suggest­ ed that the difference between resistant and susceptible biotypes connected with processes pro­ tective against photoinhibition in intact leaves, are lost during the isolation of thylakoids. 
  Reference    Z. Naturforsch. 48c, 278 (1993); received November 26 1992 
  Published    1993 
  Keywords    Chloroplasts, Triazine-Resistance, Photosystem II, Photoinhibition, Fluorescence 
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 TEI-XML for    default:Reihe_C/48/ZNC-1993-48c-0278.pdf 
 Identifier    ZNC-1993-48c-0278 
 Volume    48 
9Author    Kiriakos Kotzabasis3, D. Ieter, D. Örnem AnnbRequires cookie*
 Title    Differential Changes in the Photosynthetic Pigments and Polyamine Content during Photoadaptation and Photoinhibition in the Unicellular Green Alga Scenedesmus obliquus  
 Abstract    In the unicellular green alga Scenedesmus obliquus the level o f photoinhibition and the recovery of the cells after reversal to the initial light conditions in relation to the pre-photo-adaptation of the culture to low, medium and high light intensity was determined. The changes in the photosynthetic pigment content and in the intracellular polyam ine concentra­ tion allowed to distinguish between photoadaptation and photoinhibition. In particular, the level of chlorophylls, xanthophylls and carotenoids decreased inversely proportional to the light intensity applied during photoadaptation, whereas their concentrations remained con­ stant during photoinhibition. The violaxanthin/zeaxanthin and the loroxanthin/lutein cycle work only under photoinhibitory conditions, but not under photoadaptive premises. Changes in the level of these carotenoids in relation to the changes in the photosynthetic apparatus during photoadaptation are discussed. In addition, it was found that the intracellular polya­ mine level increased only under stress conditions, i. e. during photoinhibition, and decreased during recovery of the cells after reversal to the initial light conditions. The increase of the putrescine level during photoinhibition is inversely proportional to the light intensity used for pre-adaptation. This rise of the polyamine level in the cells photoadapted to high light conditions is an additional indication for the finding that photoadaptation and photoinhibi­ tion are different phenomena which are clearly distinguishable from each other. Finally, the changes of the chlorophyll, violaxanthin, zeaxanthin, loroxanthin, lutein and polyamine levels under photoadaptation in high light intensity (50 W m '2) in relation to the range of photo­ adaptation in Scenedesmus obliquus are discussed. 
  Reference    Z. Naturforsch. 53c, 833—8 (1998); received May 8/June 15 1998 
  Published    1998 
  Keywords    Polyamines, Photosynthetic Apparatus, Photoinhibition, Photoadaptation, Scenedesmus obliquus 
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 TEI-XML for    default:Reihe_C/53/ZNC-1998-53c-0833.pdf 
 Identifier    ZNC-1998-53c-0833 
 Volume    53 
10Author    Matthias Kuhn, Andreas Thiel, Peter BögerRequires cookie*
 Title    The 9-kDa Phosphoprotein Involved in Photoinhibition  
 Abstract    Photosystem-II particles exhibit strong photoinhibition. Short-term illumination of photosys-tem-II particles with high-intensity light (5000 piE/m 2 x s) leads to a typical change of the protein pattern on SDS-PAGE. Two proteins are mainly affected, namely the well-described 32-kDa herbicide-binding protein which probably is degraded [1] and, first published here, the 9-kDa phosphoprotein, whose function in the PS-II complex is still unknown. This protein is not de-graded, but seems to be linked to other polypeptides of the PS-II complex. During light treatment new bands of 23, 41, 50 and 54 kDa appear in the protein pattern of SDS-PAGE. A monospecific antiserum was produced against the 9-kDa phosphoprotein to investigate its fate. After light treatment the antibodies reacted with new proteins of higher molecular weights, most pronounced with a 23-kDa and a 41-kDa peptide. 
  Reference    Z. Naturforsch. 43c, 413—417 (1988); received February 15 1988 
  Published    1988 
  Keywords    9-kDa Phosphoprotein, Photosystem-II Particles, Photoinhibition, Protein Phosphorylation, Antibody 
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 TEI-XML for    default:Reihe_C/43/ZNC-1988-43c-0413.pdf 
 Identifier    ZNC-1988-43c-0413 
 Volume    43 
11Author    U. H. Eber, J. Viil, S. Neimanis, T. Mimura, K.-J DietzRequires cookie*
 Title    Photoinhibitory Damage to Chloroplasts under Phosphate Deficiency and Alleviation of Deficiency and Damage by Photorespiratory Reactions  
 Abstract    D edicated to P rofessor A chim Trebst on the occasion o f his 60th birthday Effects of Pi deficiency on photosynthesis ot isolated spinach chloroplasts were examined. The following observations were made: (1) Chloroplasts isolated in Pi-free media evolved oxygen in the light in the absence of added Pi until acid-extractable P, in the chloroplasts had decreased to 1 to 2.5 m M . This Pj was unavailable for photophosphorylation as shown by the inability of the chloroplasts to respond by oxygen evolution to the addition of PGA. In the state of Prdeficiency, stromal ATP to A DP ratios were in the light close to or below ratios observed in the dark. In the presence of 2 mM PGA, addition of 20 (j.m Pi was insufficient to increase ATP to ADP ratios, but sufficient for appreciable oxygen evolution. (2) More Pi was available for oxygen evolution of phosphate-deficient chloroplasts at low levels of C 0 2 than at high levels. This was due mainly to the suppression of oxygenation of RuBP by high C 0 2 levels which prevented formation of phosphoglycolate and the subsequent release of Pi into the chloroplast stroma. (3) More oxygen was produced by phosphate-deficient chloroplasts at a low light intensity than at a high light intensity. This was due to increased availability of endogenous Pi under low light and to photoinhibition of the chloroplasts by high light. The main product of photosynthesis of phosphate-deficient chloroplasts in the presence of a high bicarbonate concentration was starch, and the main soluble product was PGA. (4) After phosphate-deficient chloroplasts had ceased to evolve oxygen in the light, they be­ came photosensitive. Part of the loss of the capacity for oxygen evolution is attributed to leakage of PGA, but the main reason for loss of function is photoinactivation of electron transport. Both photosystems of the electron transport chain were damaged by light. (5) Protection against photoinactivation was provided by coupled electron transport. Photo­ inactivation of phosphate-deficient chloroplasts was less extensive in the presence of low C 0 2 concentrations which permitted oxygenation of RuBP than at high CO: concentrations. Electron transport to C 0 2 and other physiological electron acceptors and to the herbicide methylviologen was also protective. However, electron transport to oxygen in the Mehler reaction failed to provide appreciable protection against high light intensities, because oxygen reduction is slow and reactive oxygen species produced in the light contribute to photoinactivation. 
  Reference    Z. Naturforsch. 44c, 524 (1989); received February 3 1989 
  Published    1989 
  Keywords    Photosynthesis, Photoinhibition, Photorespiration, Electron Transport, Phosphate Deficiency, Phosphate Homeostasis 
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 TEI-XML for    default:Reihe_C/44/ZNC-1989-44c-0524.pdf 
 Identifier    ZNC-1989-44c-0524 
 Volume    44 
12Author    J. SymRequires cookie*
 Title    The Role of D 1* in Light-Induced D 1 Protein Turnover in Leaves  
 Abstract    ea, H arald R . B olh à r-N ord en kam pfb, and C hrista C ritch ley3 Light-induced degradation o f the D 1 protein of photosystem II (PS II) was determined by radioactive pulse-chase labelling experiments in intact leaves o f Schefflera polybotrya. PS II photochemical efficiency was monitored by measuring chlorophyll fluorescence. A significant and consistent decline in the F J F m ratio was taken to indicate photoinhibition. The formation and degradation o f a modified form o f the D 1 protein, D 1*, was different under photoinhibi-tory or non-photoinhibitory light conditions. At photoinhibitory irradiance greater amounts o f D 1 * were formed relative to D 1, and the degradation of D 1* was slower when compared with non-photoinhibitory irradiance. The formation and degradation o f D 1* were therefore shown to be at least partly light intensity dependent. Higher light intensities appeared to slow D 1* degradation, which suggests a modification in PS II turnover properties. 
  Reference    Z. Naturforsch. 48c, 246 (1993); received December 12 1992 
  Published    1993 
  Keywords    Photosynthesis, Photosystem II, Photoinhibition, D 1 Degradation, Chlorophyll Fluorescence 
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 Identifier    ZNC-1993-48c-0246 
 Volume    48 
13Author    E. B. Racht, A. TrebstRequires cookie*
 Title    Hypothesis on the Control of D 1 Protein Turnover by Nuclear Coded Proteins in Chlamydomonas reinhardtii  
 Abstract    A hypothesis is presented on the events in the degradation of the D 1 protein of photosys­ tem II in the light. It proposes the existence of a nuclear encoded cleavage system that is turning over and which is m odulated by its phosphorylation state. A new experimental ap­ proach is presented in which the D 1 protein degradation under photoinhibitory light is tested in Chlam ydom onas reinhardtii grown under phosphate deficiency and pretreated with cyclo-heximide. Under these conditions the degradation of the D 1 protein is delayed whereas in C hlam y­ dom onas reinhardtii grown in full medium the D 1 protein is rapidly disappearing in high light upon addition o f chloramphenicol (CAP) or lincomycin for inhibiting the resynthesis of the D 1 protein . Cycloheximide (C H I) has little effect on photoinhibition in such control cells. In cells grown, however, for 20 h in phosphate deficiency -such that there is not yet loss of photosynthesis capacity -pretreatment with cycloheximide or canavanine in low light the degradation of the D 1 protein even in 6 h high light is prevented to an appreciable extent. Further addition of CAP or lincomycin has only a small effect. [14C]leucine incorporation was used to show that there is no resynthesis and that the presence of the D 1 protein is due to a delay of degradation. The results are interpreted to show that excess high light which converts the D 1 protein into a potentially, degradable m ode is not sufficient for degradation of the D 1 protein. A cleavage system is needed as well. It is postulated that under phosphate deficiency and pre­ treatment with CHI or canavanine a nuclear coded cleavage system for the D 1 protein is depleted, i.e. the cleavage system for the rapidly turning over D 1 is also turning over. It is shown that under phosphate deficiency an alkaline phosphatase activity in the chloro­ plast and the thylakoid membrane o f Chlam ydom onas reinhardtii is increased. It is proposed that the ratio of kinase/phosphatase converts an active, stable phosphorylated cleavage system into a labile unphosphorylated and turning over state. 
  Reference    Z. Naturforsch. 49c, 439 (1994); received January 31/May 13 1994 
  Published    1994 
  Keywords    Chlamydomonas, D 1 Protein, Photoinhibition, Photosystem II, Phosphate Deficiency 
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 TEI-XML for    default:Reihe_C/49/ZNC-1994-49c-0439.pdf 
 Identifier    ZNC-1994-49c-0439 
 Volume    49 
14Author    Carina Barth, G.Heinrich KrauseRequires cookie*
 Title    Inhibition of Photosystems I and II in Chilling-Sensitive and Chilling-Tolerant Plants under Light and Low-Temperature Stress  
 Abstract    The responses of photosystems (PS) I and II to light stress at 4 °C and 20 °C were studied in leaf discs from three chilling-sensitive plant species, Cucumis sativus, Cucurbita maxima and Nicotiana tabacum, and in the chilling-tolerant Spinacia oleracea. The chilling-sensitive plants were grown at 24 °C under 8 0 -1 2 0 j.imol photons m-2 s-1 (Cucumis and Cucurbita) or 30 [imol photons m -2 s_1 (Nicotiana). Spinacia was cultivated outdoors during winter and early spring. The P700 absorbance change around 820 nm served as a relative measure of PSI activity. The potential efficiency of PSII was determined in dark-adapted leaf discs by means of the ratio of variable to maximum chlorophyll (Chi) a fluorescence emission (F v/ F m). In Cucurbita, Nicotiana and Spinacia, PSI was not or only slightly inhibited by 2 h illumination with 200 [imol m-2 s-1 at 4 °C or with 2000 ^imol m-2 s-1 at 20 °C. In leaves of Cucurbita and Nicotiana, exposure to 2000 j.imol photons m -2 s_1 at 4 °C resulted in a decline in PSI activity and potential PSII efficiency approximately to the same extent (about 50% within 2 h). In contrast, in Cucumis, both moderate and high light at low temperature caused a PSI inhibition that proceeded considerably faster than the decline in PSII efficiency. Such preferential photoinhibition of PSI was not observed in the other three species tested. In Spinacia, a lower susceptibility of PSI and PSII to photoinhibition at 4 °C was associated with a faster de-epoxidation kinetics of violaxanthin, as compared to the three chilling-sensi-tive species. In addition, leaves of Spinacia were characterized by a significantly larger pool of xanthophyll-cycle pigments and a higher content of ß-carotene based on Chi a+b. When leaves of Cucurbita were preincubated with methylviologen, which catalyzes formation of superoxide anion radicals at the acceptor side of PSI, the decline in potential PSII efficiency under 2000 jimol photons m -2 s_1 at 20 °C and 4 °C was strongly enhanced, whereas the P700 signal was less affected. Our data demonstrate that in the species tested, PSI may be inhibited in vivo besides PSII under light stress, but preferential photoinhibition of PSI is not a general phenomenon in chilling-sensitive plants. At low temperatures, a reduced function of the xanthophyll cycle and of the antioxidative scavenging system might account for enhanced PSI and PSII inhibition in vivo. 
  Reference    Z. Naturforsch. 54c, 645 (1999); received November 8 1998/January 30 1999 
  Published    1999 
  Keywords    Active Oxygen Species, Chlorophyll Fluorescence, P700 Absorbance Change, Photoinhibition, Xanthophyll Cycle 
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 TEI-XML for    default:Reihe_C/54/ZNC-1999-54c-0645.pdf 
 Identifier    ZNC-1999-54c-0645 
 Volume    54 
15Author    Susana Shochata, Noam Adira, Alma Gala, Yorinao Inoueb, Laurence Metsc, Itzhak OhadaRequires cookie*
 Title    Photoinactivation of Photosystem II and Degradation of the D 1 Protein are Reduced in a Cytochrome b j f -Less Mutant of Chlamydomonas reinhardtii  
 Abstract    The effect of unoccupancy of the Q B site by plastoquinone on the photoinactivation of reac­ tion center II in a Cyt b jf-less mutant of Chlamydomonas reinhardtii, Bb, was investigated. In these cells the oxidation of plastoquinol generated by electron flow via RC II to plastoquinone and thus the turnover of PQH2/PQ via the Q B site are drastically reduced. Reaction center II of the mutant cells was resistant to photoinactivation relative to the control cells as demonstrated by measurements of light-induced destabilization of S2-QB charge recombination, rise in in­ trinsic fluorescence and loss of variable fluorescence. These parameters relate to functions in­ volving the reaction center II D 1 protein. The light-induced degradation of D 1 in the mutant cells was also considerably reduced, with a ;l/ 2 value of 7 h as compared, under similar condi­ tions, to about 1.5 h for the control cells. These results indicate that the photoinactivation of RC II and turnover of the D 1 protein are related and require occupancy of the Q B site by PQ and its light-driven reduction. 
  Reference    Z. Naturforsch. 45c, 395—401 (1990); received November 24 1989 
  Published    1990 
  Keywords    Cytochrome b j f, Chlamydomonas, D 1 Turnover, Q B, Thermoluminescence, Photoinhibition 
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 TEI-XML for    default:Reihe_C/45/ZNC-1990-45c-0395.pdf 
 Identifier    ZNC-1990-45c-0395 
 Volume    45