Quantification of the electrochemical proton gradient and activation of ATP synthase in leaves.

We have developed a new method to quantify the transmembrane electrochemical proton gradient present in chloroplasts of dark-adapted leaves. When a leaf is illuminated by a short pulse of intense light, we observed that the light-induced membrane potential changes, measured by the difference of absorption (520 nm-546 nm), reach a maximum value (approximately 190 mV) determined by ion leaks that occur above a threshold level of the electrochemical proton gradient. After the light-pulse, the decay of the membrane potential follows a multiphasic kinetics.

On the advantages of using green light to study fluorescence yield changes in leaves.

In photosynthetic chains, the kinetics of fluorescence yield depends on the photochemical rates at the level of both Photosystem I and II and thus on the absorption cross section of the photosynthetic units as well as on the coupling between light harvesting complexes and photosynthetic traps. A new set-up is described which, at variance with the commonly used set-ups, uses of a weakly absorbed light source (light-emitting diodes with maximum output at 520 nm) to excite the photosynthetic electron chain and probe the resulting fluorescence yield changes and their time course. This approach optimizes the homogeneity of the exciting light throughout the leaf and we show that this homogeneity narrows the distribution of the photochemical rates.

Cyclic electron flow in C3 plants.

This paper summarized our present view on the mechanism of cyclic electron flow in C3 plants. We propose that cyclic and linear pathways are in competition for the reoxidation of the soluble primary PSI acceptor, Ferredoxin (Fd), that freely diffuses in the stromal compartment. In the linear mode, Fd binds ferredoxin-NADP-reductase and electrons are transferred to NADP+ and then to the Benson and Calvin cycle.

Excitation transfer between photosynthetic units: the 1964 experiment.

We review here the background and the experiments that led to the concept of excitation energy transfer among photosystem (PS) II units. On the basis of a kinetic analysis of oxygen evolution and chlorophyll a fluorescence yield, the authors showed, in 1964, that the PS II photochemical reaction involved in the formation of oxygen is not a first-order process. We concluded that excitation energy localized in a 'photosynthetic unit' including a reduced primary acceptor is transferred with a high probability to neighboring PS II units.

Quantification of cyclic and linear flows in plants.

A method was developed to quantify the fraction of photosystem I (PSI) centers that operate according to the cyclic or linear mode, respectively. P(700) and plastocyanin oxidation were analyzed under a weak far-red excitation (approximately eight photons per s(-1) per PSI) that induces P(700) oxidation in approximately 20 s and approximately 3 s in dark-adapted and preilluminated leaves, respectively. This finding implies that, in dark-adapted leaves, most of the electrons formed on the stromal side of PSI are transferred back to PSI through an efficient cyclic chain, whereas in preilluminated leaves, electrons are transferred to NADP and then to the Benson-Calvin cycle.

Fast oxidation of the primary electron acceptor under anaerobic conditions requires the organization of the photosynthetic chain of Rhodobacter sphaeroides in supercomplexes.

The kinetics of reoxidation of the primary acceptor Q(a) has been followed by measuring the changes in the fluorescence yield induced by a series of saturating flashes in intact cells of Rhodobacter sphaeroides in anaerobic conditions. At 0 degrees C, about half of Q(a)(-) is reoxidized in about 200 ms while reoxidation of the remaining fraction is completed in several seconds to minutes. The fast phase is associated with the transfer of ubiquinone formed at site Q(o) of the cytochrome bc(1) complex while the slowest phase is associated with the diffusion of ubiquinone present in the membrane prior to the flash excitation.

Cyclic electron flow under saturating excitation of dark-adapted Arabidopsis leaves.

The rate of cyclic electron flow measured in dark-adapted leaves under aerobic conditions submitted to a saturating illumination has been performed by the analysis of the transmembrane potential changes induced by a light to dark transfer. Using a new highly sensitive spectrophotometric technique, a rate of the cyclic flow of approximately 130 s(-1) has been measured in the presence or absence of 3-(3,4-dichloro-phenyl)-1,1-dimethylurea (DCMU). This value is approximately 1.

Cyclic electron transfer in plant leaf.

The turnover of linear and cyclic electron flows has been determined in fragments of dark-adapted spinach leaf by measuring the kinetics of fluorescence yield and of the transmembrane electrical potential changes under saturating illumination. When Photosystem (PS) II is inhibited, a cyclic electron flow around PSI operates transiently at a rate close to the maximum turnover of photosynthesis. When PSII is active, the cyclic flow operates with a similar rate during the first seconds of illumination.