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  • 1
    Electronic Resource
    Electronic Resource
    Intact chloroplasts display pH 5 optimum of O2-reduction in the absence of methyl viologen: Indirect evidence for a regulatory role of superoxide protonation (1993)
    Springer
    Photosynthesis research 37 (1993), S. 69-80 
    Details
    ISSN: 1573-5079
    Keywords: superoxide protonation ; Mehler reaction ; ascorbate peroxidase ; chlorophyll fluorescence ; quenching analysis ; cyclic PS I
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The pH-dependence of light-driven O2-reduction in intact spinach chloroplasts is studied by means of chlorophyll fluorescence quenching analysis and polarographic O2-uptake measurements. Most experiments are carried out in presence of KCN, which blocks activities of Calvin cycle, ascorbate peroxidase and superoxide dismutase. pH is varied by equilibration with external buffers in presence of nigericin. Vastly different pH-optima for O2-dependent electron flow are observed in the presence and absence of the redox catalyst methyl viologen. Both fluorescence quenching analysis and O2-uptake reveal a distinct pH 5 optimum of O2-reduction in the absence of methyl viologen. In the presence of this catalyst, O2-reduction is favoured in the alkaline region, with an optimum around pH 8, similar to other types of Hill reaction. It is suggested that in the absence of methyl viologen the extent of irreversibility of O2-reduction is determined by the rate of superoxide protonation. This implies that O2-reduction takes place within the aprotic phase of the thylakoid membrane and that superoxide-reoxidation via oxidized PS I donors competes with protonation. Superoxide protonation is proposed to occur at the internal surface of the thylakoid membrane. There is no competition between superoxide reoxidation and protonation when in the presence of methyl viologen the site of O2-reduction is shifted into the protic stroma phase. In confirmation of this interpretation, fluorescence measurements in the absence of KCN reveal, that non-catalysed O2-dependent electron flow is unique in beingstimulated by the transthylakoidal pH-gradient. On the basis of these findings a major regulatory role of O2-dependent electron flow under excess light conditions is postulated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02185440
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  • 2
    Electronic Resource
    Electronic Resource
    An active Mehler-peroxidase reaction sequence can prevent cyclic PS I electron transport in the presence of dioxygen in intact spinach chloroplasts (1994)
    Springer
    Photosynthesis research 41 (1994), S. 429-437 
    Details
    ISSN: 1573-5079
    Keywords: photosynthesis ; ascorbate peroxidase ; Mehler reaction ; cyclic PS I ; chlorophyll fluorescence ; 9-aminoacridine fluorescence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Simultaneous measurements of 9-aminoacridine (9-AA) fluorescence quenching, O2-uptake and chlorophyll fluorescence of intact spinach chloroplasts were carried out to assess the relationship between the transthylakoidal ΔpH and linear electron flux passing through Photosystem II. Three different types of O2-dependent electron flow were investigated: (1) Catalysed by methyl viologen; (2) in the absence of a catalyst and presence of an active ascorbate peroxidase (Mehler-peroxidase reaction); (3) in the absence of a catalyst and with the ascorbate peroxidase being inhibited by KCN (Mehler reaction). The aim of this study was to assess the relative contribution of ΔpH-formation which is not associated with electron flow through Photosystem II and, which should reflect Photosystem I cyclic flow under the different conditions. The relationship between the extent of 9-AA fluorescence quenching and O2-uptake rate was found to be almost linear when methyl viologen was present. In the absence of methyl viologen (Mehler reaction) an increase of 9-AA fluorescence quenching to a value of 20% at low light intensities was associated with considerably less O2-uptake than in the presence of methyl viologen, indicating the involvement of cyclic flow. These findings are in agreement with a preceding study of Kobayashi and Heber (1994). However, when no KCN was added, such that the complete Mehler-peroxidase reaction sequence was operative, the relationship between 9-AA fluorescence quenching and the flux through PS II, as measured via the chlorophyll fluorescence parameter ΔF/Fm′ × PAR, was identical to that observed in the presence of methyl viologen. Under the assumption that methyl viologen prevents cyclic flow, it is concluded that there is no significant contribution of cyclic electron flow to ΔpH-generation in intact spinach chloroplasts.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02183045
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  • 3
    Electronic Resource
    Electronic Resource
    On the relationship between chlorophyll fluorescence quenching and the quantum yield of electron transport in isolated thylakoids (1994)
    Springer
    Photosynthesis research 40 (1994), S. 93-106 
    Details
    ISSN: 1573-5079
    Keywords: PS II quantum yield ; fluorescence quenching analysis ; methyl viologen reduction ; ferricyanide reduction ; inactive centers
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The relationship between the empirical fluorescence index ΔF/Fm′ and the quantum yield of linear electron flow, Φs, was investigated in isolated spinach thylakoids. Conditions were optimised for reliable determination of ΔF/Fm′ and Φs with methyl viologen or ferricyanide as electron acceptors under coupled and uncoupled conditions. Ascorbate in combination with methyl viologen was found to stimulate light-induced O2-uptake which is not reflected in ΔF/Fm′ and interpreted to reflect superoxide reduction by ascorbate. In the absence of ascorbate, the plot of ΔF/Fm′ vs. Φs was mostly linear, except for the range of high quantum yields, i.e. at rather low photon flux densities. With ferricyanide as acceptor, use of relatively low concentrations (0.1–0.3 mM) was essential for correct Fm′-determinations, particularly under uncoupled conditions. Under coupled and uncoupled conditions the same basic relationship between ΔF/Fm′ and Φs was observed, irrespective of Φs being decreased by increasing light intensity or by DCMU-addition. The plots obtained with methyl viologen and ferricyanide as acceptors were almost identical and similar to corresponding plots reported previously by other researchers for intact leaves. It is concluded that the index ΔF/Fm′ can be used with isolated chloroplasts for characterisation of such types of electron flow which are difficult to assess otherwise, as e.g. O2 dependent flux. The origin of the ‘non-linear’ part of the relationship is discussed. An involvement of ‘inactive’ PS II centers with separate units and inefficient QA-QB electron transfer is considered likely.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00019048
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