Light-Induced Changes In Fluorescence Yield Research Articles

Oxygen dependence of photosynthetic electron transport in a bacteriochlorophyll-containing rhizobium

Bacteriochlorophyll-containing rhizobia, which form nitrogen-fixing nodules on the stems and roots of the legume Aeschynomene, grow photosynthetically only in the presence of oxygen or auxiliary electron acceptors. We show that, in whole cells of the Rhizobium strain BTAi 1, a single-turnover excitation flash photooxidized c-type cytochrome under aerobic but not anaerobic conditions. Light-induced fluorescence yield changes show that under anaerobic conditions, the primary acceptor quinone, Q A, is predominantly in the reduced state and so unable to accept electrons. Thus, as is the case for the aerobic photosynthetic bacterium Roseobacter denitrificans, over-reduction of Q A likely prohibits photosynthesis under anaerobic conditions.

Light-Harvesting Function in the Diatom Phaeodactylum tricornutum

The distribution of excitation energy between photosystems I and II (PSI and PSII) was investigated in the marine diatom Phaeodactylum tricornutum (Bohlin) using light-induced changes in fluorescence yield and rate of modulated O(2) evolution. The intensity dependence of the fast fluorescence rise in dark adapted cells (+/-DCMU) suggests that light absorbed by the major antenna complex was not delivered preferentially to PSII but is more equally distributed between the photosystems. Reversible, slow fluorescence yield changes measured in the absence of DCMU were correlated with decreased initial fluorescence and rate constants for PSII photochemistry, increased variable fluorescence, alteration of the fluorescence excitation and emission spectra, and could be effected by either 510 nm (PSII) or 704 nm (PSI) light. Slow, reversible fluorescence yield changes were also observed in the presence of DCMU, but were characterized by a loss of both initial and variable fluorescence and could not be induced by PSI light. The absence of slow changes in the yield of fluorescence and rate of modulated O(2) evolution, following addition or removal of PSI background light to modulated PSII excitation, does not support regulation of excitation energy density in PSI at the expense of PSII. The results suggest that adjustments are made at the level of excitation energy transfer to the PSII reaction center which prevent prolonged loss of photosynthetic capacity. Energy distribution is regulated by ionic distributions independently of the plastoquinone pool redox state. These differences in light-harvesting function are probably a response to the aquatic light field and may account for the success of diatoms in low and variable light environments.

On the function of the fluorescence quenchers in chloroplasts and their relation to the primary electron acceptor of photosystem II

Redox titrations of the fluorescence quenching components in chloroplasts indicate the presence of two components, one with E m 7.6 = + 25 m V and the second with E m 7.6 = -270 m V. These midpoint potentials are almost the same as those of two Photosystem II components previously shown to contribute to the chloroplast electrogenic reaction measured at 518 nm ( R. Malkin, 1978, FEBS Lett. 87, 329–333). Comparison of light-induced fluorescence yield changes with those obtained by redox titration suggests that both fluorescence quenchers are photoreduced. A direct demonstration of the photoreduction of the low-potential fluorescence quencher was observed in experiments at defined redox potentials. Fluorescence induction curves measured at low light intensity in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) also showed a contribution from both fluorescence quenchers. An additional electron acceptor, other than the two fluorescence quenchers, was also identified in the acceptor complex. These results are discussed in terms of several electron acceptors functioning in the Photosystem II reaction center complex, and the possible function of these acceptors is considered.

Does the rate of cooling affect fluorescence properties of chloroplasts at −196 °C?

The question addressed in the title was examined by measuring fluorescence emission spectra and light-induced fluorescence-yield changes of chloroplasts which had been frozen to −196 °C rapidly, as very thin samples adsorbed into substrates which were plunged directly into liquid nitrogen, or slowly by the cooling action of liquid nitrogen through the wall of the cuvette. Contrary to previous reports, we found that the rate of cooling had no influence on the shape of the emission spectrum, the extent of the variable fluorescence or the fraction of the absorbed quanta which are delivered initially to Photosystem I.

Energy transfer in the photochemical apparatus of flashed bean leaves

Fluorescence and energy transfer properties of bean leaves greened by brief, repetitive xenon flashes were studied at −196 °C. The bleaching of P-700 has no influence on the yield of fluorescence at any wavelength of emission. The light-induced fluorescence yield changes which are observed in both the 690 and 730 nm emission bands in the low temperature fluorescence spectra are due to changes in the state of the Photosystem II reaction centers. The fluorescence yield changes in the 730 nm band are attributed to energy transfer from Photosystem II to Photosystem I. Such energy transfer was also confirmed by measurements of the rate of photooxidation of P-700 at −196 °C in leaves in which the Photosystem II reaction centers were either all open or all closed. It is concluded that energy transfer from Photosystem II to Photosystem I occurs in the flashed bean leaves which lack the light-harvesting chlorophyll a b protein.

Absence of Photosystem 2 in heterocysts of the blue-green alga Anabaena

Heterocysts of the filamentous blue-green alga Anabaena cylindrica have a high concentration of the System 1 reaction center P700 and are able to photooxidize a cytochrome. They have a low yield of chlorophyll a fluorescence, do not show light-induced changes in fluorescence yield, show a very low intensity of delayed light emission and do not show Hill-reaction activity. It is concluded that heterocysts contain Photosystem 1 only.

Destruction of C-550 by ultraviolet radiation

Low-temperature absorption spectra of spinach chloroplasts measured after different periods of ultraviolet radiation show that C-550 is destroyed by the ultraviolent treatment. Cytochrome b 559 is oxidized and denatured so that it is no longer reducible by ascorbate. The light-induced fluorescence yield changes induced at low temperature diminish as C-550 is destroyed.

The effects of lipase on spinach and Chlamydomonas chloroplasts

Light-induced fluorescence yield changes and low-temperature changes were measured in spinach and Chlamydomonas reinhardi chloroplasts before and after treatment with pancreatic lipase. Lipase treatment destroyed the C-550 absorption band and eliminated the fluorescence of variable yield. In spinach chloroplasts the invariant fluorescence yield, after lipase treatment, was low whereas in Chlamydomonas chloroplasts it was high. Lipase treatment modified the cytochrome b 559 in spinach chloroplasts so that it was no longer ascorbate reducible but was dithionite reducible. In Chlamydomonas chloroplasts some of the cytochrome b 559 was destroyed by lipase treatment. After lipase treatment, cytochrome b 559 could be reduced by Photosystem I activity if ascorbate was present as an electron donor and benzyl viologen present as an electron acceptor for Photosystem I. Lipase also eliminated the EPR Signal II.

Potentiometric titration of the fluorescence yield of spinach chloroplasts

The fluorescence yield of spinach chloroplasts was measured as a function of oxidation-reduction potential under anaerobic conditions at pH 6.0, 7.0, and 8.0. The potentiometric-titration curve showed two fluorescence quenching processes, both quenching in the oxidized state. The complete titration curve could be reasonably fit with a Nernst equation for the sum of two one-electron-transfer reactions with different midpoint potentials. At pH 7.0 the two transitions had midpoint potentials of about −35 mV and −270 mV and both showed a pH dependence of about −60 mV per pH unit. The hypothetical quencher, Q, assumed to be responsible for light-induced fluorescence-yield changes in green photosynthetic systems could be ascribed to the more positive quenching component.

Development of photosynthetic system 1 and 2 in a greening leaf.

Abstract Dark-grown leaves were examined at early stages of greening to determine when photosynthetic electron transport, as indicated by light-induced fluorescence-yield changes, first appeared. The pigment content of the developing leaves was also followed. For the first 1.5 h of illumination there were no light-induced fluorescence-yield changes and the chlorophyll appeared as a single symmetrical absorption band at about 670 nm. After 2 h of illumination the fluorescence-yield changed slightly in response to actinic irradiation, chlorophyll b was detected, the chlorophyll a began to differentiate into chlorophyll a -670 and chlorophyll a -680, and C -705 appeared to be forming. All of these changes increased markedly on further illumination. It was concluded that the pigment development signified the organization of photosynthetic Systems 1 and 2 and that photosynthetic electron transport began as soon as the pigments of Systems 1 and 2 were present.