Studies on the slow fluorescence decline in isolated chloroplasts

A comparison between plant photosystem I and photosystem II architecture and functioning

Oxygenic photosynthesis is indispensable both for the development and maintenance of life on earth by converting light energy into chemical energy and by producing molecular oxygen and consuming carbon dioxide. This latter process has been responsible for reducing the CO2 from its very high levels in the primitive atmosphere to the present low levels and thus reducing global temperatures to levels conducive to the development of life. Photosystem I and photosystem II are the two multi-protein complexes that contain the pigments necessary to harvest photons and use light energy to catalyse the primary photosynthetic endergonic reactions producing high energy compounds. Both photosystems are highly organised membrane supercomplexes composed of a core complex, containing the reaction centre where electron transport is initiated, and of a peripheral antenna system, which is important for light harvesting and photosynthetic activity regulation. If on the one hand both the chemical reactions catalysed by the two photosystems and their detailed structure are different, on the other hand they share many similarities. In this review we discuss and compare various aspects of the organisation, functioning and regulation of plant photosystems by comparing them for similarities and differences as obtained by structural, biochemical and spectroscopic investigations.

Photosynthesis research in Italy: a review

This historical review was compiled and edited by Giorgio Forti, whereas the other authors of the different sections are listed alphabetically after his name, below the title of the paper; they are also listed in the individual sections. This review deals with the research on photosynthesis performed in several Italian laboratories during the last 50 years; it includes research done, in collaboration, at several international laboratories, particularly USA, UK, Switzerland, Hungary, Germany, France, Finland, Denmark, and Austria. Wherever pertinent, references are provided, especially to other historical papers in Govindjee et al. [Govindjee, Beatty JT, Gest H, Allen JF (eds) (2005) Discoveries in Photosynthesis. Springer, Dordrecht]. This paper covers the physical and chemical events starting with the absorption of a quantum of light by a pigment molecule to the conversion of the radiation energy into the stable chemical forms of the reducing power and of ATP. It describes the work done on the structure, function and regulation of the photosynthetic apparatus in higher plants, unicellular algae and in photosynthetic bacteria. Phenomena such as photoinhibition and the protection from it are also included. Research in biophysics of photosynthesis in Padova (Italy) is discussed by G.M. Giacometti and G. Giacometti (2006).

Inhibition of Photosystem 2 primary photochemistry by photogenerated protons

Photosystem 2 photochemical efficiency, measured as the rate of Qa reduction, was observed to be inhibited by preillumination with single turnover flashes, whilst Fo and Fm were not affected. Such inhibition was reversed by the uncoupler nigericin or by incubating the thylakoids in the dark for ca. 2 min after the preillumination. The presence of ATP in micromolar concentrations increased the time of dark recovery from the inhibition. The inhibition of fluorescence rise was not changed when 70% of the excitation energy available in the antenna was quenched by dinitrobenzene. Quantitative analysis of the observed fluorescence induction indicates that this phenomenon is due to the inhibition of the photochemical reaction itself. Uncouplers such NH4Cl were unable to reverse the inhibition and only a few flashes of saturating intensity (10 or less) were required for the onset of it. This suggests that protons localised in domains rather than a pH gradient between the thylakoid lumen bulk solution and the external one are involved in this regulation of PS 2 efficiency.

The effect of high-energy-state excitation quenching on maximum and dark level chlorophyll fluorescence yield

The quenching of variable fluorescence yield (qN) and the quenching of dark level fluorescence yield (q0) directly atributable to high-energy-state fluorescence quenching (qE) was studied to distinguish between energy dissipation in the antenna and light harvesting complexes (antenna quenching) and energy dissipation at the reaction centres (reaction centre quenching). A consistent relationship was obtained between qN and q0 in barley leaves, the green alga Dunaliella C9AA and in pea thylakoids with 2,3,5,6-tetramethyl-p-phenylene diamine (DAD) as mediator of cyclic electron flow around PS 1. This correlated well with the relationship obtained using m-dinitrobenzene (DNB), a chemical model for antenna quenching, to quench fluorescence in Dunaliella C9AA or pea thylakoids. The results also correlated reasonably well with theoretical predictions by the Butler model for antenna quenching, but did not correlate with the predictions for reaction centre quenching. It is postulated that qE quenching therefore occures in the antenna and light harvesting complexes, and that the small deviation from the Butler prediction is due to PS 2 heterogeneity.

Light-dependent quenching of chlorophyll fluorescence in pea chloroplasts induced by adenosine 5'-triphosphate

Addition of ATP to chloroplasts causes a reversible 25-30% decrease in chlorophyll fluorescence. This quenching is light-dependent, uncoupler insensitive but inhibited by DCMU and electron acceptors and has a half-time of 3 minutes. Electron donors to Photosystem I can not overcome the inhibitory effect of DCMU, suggesting that light activation depends on the reduced state of plastoquinone. Fluorescence emission spectra recorded at -196 degrees C indicate that ATP treatment increases the amount of excitation energy transferred to Photosystem I. Examination of fluorescence induction curves indicate that ATP treatment decreases both the initial (F0) and variable (Fv) fluorescence such that the ratio of Fv to the maximum (Fm) yield is unchanged. The initial sigmoidal phase of induction is slowed down by ATP treatment and is quenched 3-fold more than the exponential slow phase, the rate of which is unchanged. A plot of Fv against area above the induction curve was identical plus or minus ATP. Thus ATP treatment can alter quantal distribution between Photosystems II and I without altering Photosystem II-Photosystem II interaction. The effect of ATP strongly resembles in its properties the phosphorylation of the light-harvesting complex by a light activated, ATP-dependent protein kinase found in chloroplast membranes and could be the basis of physiological mechanisms which contribute to slow fluorescence quenching in vivo and regulate excitation energy distribution between Photosystem I and II. It is suggested that the sensor for this regulation is the redox state of plastoquinone.

A quantitative study of the slow decline of chlorophyll a fluorescence in isolated chloroplasts

A detailed study of the photo-induced decline in chlorophyll a fluorescence intensity (Kautsky phenomenon) in coupled isolated chloroplasts from a high level (P) to a low stationary level (S) is presented. 1. A linear relationship between P leads to S quenching and intrathylakoid H+ concentration was found. When the light-induced proton gradient was abolished by uncoupling, the fluorescence emission at room temperature was lowered proportionally to increased H+ concentration in the medium. 2. Fluorescence spectra at -196 degrees C of samples frozen at the P and S states showed no significant differences in the Photosystem I/Photosystem II ratio of fluorescence emission. Furthermore, freezing to -196 degrees C reversed the P leads to S quenching. This indicates that the P leads to S quenching is not related to an increase of spillover of excitation energy from Photosystem II to Photosystem I. 3. When Mg2+ was added to thylakoids suspended in a medium free of divalent cations, the inhibition of spillover required lower Mg2+ concentrations (half saturation at 0.6 mM). Increased proton concentration in the medium also inhibited spillover. 4. The results are interpreted in terms of two sites of Mg2+ and H+ effects on excitation deactivation in Photosystem II. One site is located on the outer face of the thylakoid membrane; action of both Mg2+ and H+ at this side diminishes spillover. The second site is located on the inner face of the membrane; as Mg2+ is displaced there by protons, a non-photochemical quenching of Photosystem II fluorescence is induced, which is manifested by the P leads to S decline.

Requirement of the light-harvesting pigment.protein complex for magnesium ion regulation of excitation energy distribution in chloroplasts

Cation regulation of excitation energy distribution was examined in chloroplasts isolated from (a) pea seedlings, grown in intermittent illumination, which contain no light-harvesting complex, (b) a barley mutant which is deficient in the major polypeptide component of the light-harvesting complex, and (c) a soybean mutant which contains a reduced amount of light-harvesting complex. It was found that: (1) Mg2+-induced increase in Photosystem II fluorescence at room temperature is small in the chloroplasts of the soybean mutant, smaller in the barley mutant, and almost absent in the light-harvesting complex-less chloroplasts of pea as compared to their respective controls. (2) Mg2+-induced increase in the F685/F730 emission peak ratio at 77 K is not detected in the isolated chloroplasts of the intermittent light-grown pea and the barley mutant. (3) Pre-illumination induced State 1-State 2 and adaptation in vivo is absent in the barley mutant and is less pronounced in the soybean mutant as compared to their respective controls. (4) Increase of slow fluorescence decay upon addition of Mg2+ observed in control chloroplasts was not detected in chloroplasts of intermittent-light grown peas. These results confirm earlier conclusions (Armond, P.A., Arntzen, C.J., Briantais, J.M. and Vernotte, C. (1976) Arch. Biochem. Biophys. 175, 54--63; Davis, D.J., Armond, P.A., Gross, E.L. and Arntzen, C.J. (1976) Arch. Biochem. Biophys. 175, 64--70) that light-harvesting complex is required for the Mg2+-induced regulation of the excitation energy distribution between Photosystems I and II. The characteristic P-S decay and I-D dip of the in vivo fluorescence inductions (Kautsky effect) were not significantly altered in the light-harvesting complex-less and the light-harvesting complex-deficient chloroplasts as compared to their respective controls. These results indicate that light-harvesting complex is not obligatorily required to observe the P-S decay or the I-D dip.

Effects of uncouplers on Mg(2+)-dependent fluorescence quenching in isolated chloroplasts

Uncoupling concentrations (about 1 μmol l(-1)) of desaspidin or carbonyl cyanide-4-trifluoromethoxyphenyl hydrazone reverse the slow light-induced, Mg(2+)-dependent quenching of fluorescence of chlorophyll a in isolated (intact and broken) spinach chloroplasts. Likewise, uncoupling inhibits the light-induced increase of the Mg(2+) concentration in the stroma of intact chloroplasts, as determined with Eriochrome Blue SE. Addition of higher amounts of the uncouplers to the chloroplasts leads to a slow, light-dependent fluorescence lowering which appears to be promoted by high light intensities and is not reversed in the dark. The reversal of the fluorescence quenching by uncoupling is interpreted to reflect exchange of protons for Mg(2+) ions at negative sites of the inner thylakoid face, caused by the collapse of the proton gradient across the membrane. The secondary fluorescence lowering caused by high levels of the uncouplers and high light intensities is suggested to be related to an inhibition of non-cyclic photosynthetic electron transport.

Slow fluorescence quenching of type A chloroplasts. Resolution into two components

The divalent-cation-specific ionophore A23187 is used to define two components of the slow fluorescence quenching of type a spinach chloroplasts: ionophore-reversible and ionophore-resistant quenching. Ionophore-reversible quenching predominates at relatively low light intensities and approaches saturation as light levels are increased. It is sensitive to uncouplers and to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and is dark reversible. At high light intensities the bulk (greater than 80%) of slow fluorescence quenching is ionophore-resistant. Ionophore-resistant quenching is stimulated by carbonyl cyanide m-chlorophenyl hydrazone (CCCP) at pH 7.6 and by both CCCP and methylamine at pH 9.0. It is insensitive to DCMU and is not reversed in subsequent darkness. Taken together, the two components account for all quenching observed in Type A chloroplasts. Ionophore-reversible quenching is identified with the Mg2+-mediated fluorescence quenching described by Krause (Biochim. Biophys. Acta (1974) 333, 301-313) and by Barber and Telfer (in Membrane Transport in Plants (Dainty, J., AND Zimmermann, U., eds.), pp. 281-288, Springer-Verlag, Berlin, 1974). Ionophore-resistant quenching, a first-order process requiring high light, resembles the quenching reported by Jennings et al. (Biochim. Biophys. Acta (1976) 423, 264-274). The resolution of the fluorescence quenching phenomenon into two distinct components reconciles the apparently contradictory observations of these earlier investigations.