Induction Kinetics of Photosystem I-Activated P700 Oxidation in Plant Leaves and Their Dependence on Pre-Energization

Absorbance changes ¿A 810 were measured in pea (Pisum sativum L., cv. Premium) leaves to track redox transients of chlorophyll P700 during and after irradiation with far red (FR) light under various preillumination conditions in the absence and presence of inhibitors and protonophorous uncoupler of photosynthetic electron transport. It was shown that cyclic electron transport (CET) in chloroplasts of pea leaves operates at its highest rate after preillumination of leaves with white light and is strongly suppressed after preillumination with FR light. The FR light-induced suppression was partly released during prolonged dark adaptation. Upon FR illumination of dark-adapted leaves, the induction of CET was observed, during which CET activity increased to the peak from the low level and then decreased gradually. The kinetics of P700 oxidation induced by FR light of various intensities in leaves preilluminated with white light were fit to empirical sigmoid curves containing two variables. In leaves treated with a protonophore FCCP, the amplitude of FR light-induced changes ¿A 810 was strongly suppressed, indicating that the rate of CET is controlled by the pH gradient across the thylakoid membran

Photoactivation of electrogenic activity in chloroplasts and its relation to photoinduced swelling of thylakoids

In patch-clamp experiments on isolated chloroplasts of Peperomia metallica Lind. et Rodig. (Piperaceae), the replacement of 50 mM KCl in a medium with 50 mM NH4Cl strongly influenced the parameters of photocurrent known to reflect the generation of electric potential in thylakoids. The addition of NH+ 4 to the medium modified the induction curves of the photocurrent as well as the currents induced by single-turnover flashes in preilluminated chloroplasts. Under the action of a prolonged light pulse (∼ l s), the steady-state current was much higher in the ammonium-containing medium than in the presence of K+. Preillumination of a dark-adapted chloroplast with a l-s light pulse suppressed the current induced by a single-turnover flash (6 μs) in the presence of K+ but caused an elevation (by 50-150%) of the flash-induced current and shortening of its relaxation time in the presence of NH+ 4. The origin of different induction kinetics for the photocurrent in K+ and NH+ 4 media is partly clear, because ammonium prevents generation of the pH gradient and, subsequently, eliminates the ΔpH-dependent suppression of the electron transport rate. However, this does not explain the origin of NH+ 4-dependent photostimulation of the current generated by single-turnover flashes. This phenomenon arises from the thylakoid swelling caused by the accumulation of NH+ 4 in the lumen and from the respective changes in the network resistances. The network element most sensitive to thylakoid swelling is the lateral resistance of the lumen: it decreases upon enlargement of the cross-section area. Stimulation of the flash-induced current by preillumination in the presence of NH+ 4 was accompanied by accelerated relaxation of the current, indicating that the phenomena observed are caused by the reduction of network resistance involved in the discharge of the membrane capacity. Thus, the light-induced structural changes in the thylakoid system have a marked effect on the currents measured with the patch-clamp technique

Spatio-temporal patterns of photosystem II activity and plasma-membrane proton flows in Chara corallina cells exposed to overall and local illumination

Pulse-amplitude modulated microfluorometry and an extracellular pH microprobe were used to examine light-induced spatial heterogeneity of photosynthetic and H+-transporting activities in cells of Chara corallina Klein ex Willd. Subcellular domains featuring different PSII photochemical activities were found to conform to alternate alkaline and acid zones produced near the cell surface, with peaks of PSII activity correlating with the position of acid zones. Buffers eliminated pH variations near the cell surface but did not destroy the variations in PSII photochemical yield (DeltaF/F-m'). When a dark-adapted cell was exposed to actinic light, the PSII effective yield decreased within 5-15 min in the alkaline regions but rose after the initial decline in the acid regions. The light-induced decrease in DeltaF/F-m' in the alkaline regions occurred prior to or synchronously with the steep rise in local pH. The kinetics of DeltaF/F-m', F-m', and F observed in alkaline regions under overall illumination of Chara cells were replaced by those typical of acid regions, when the illumination area size was restricted to 1.5-2 mm. The data show that photoinduced patterns in photosynthetic activity are not predetermined by the particular structural organization of alkaline and acid cell regions but are subject to dynamic changes

The outward component of photoinduced current in chloroplasts of peperomia metallica

The photoinduced currents in whole chloroplasts of Peperomia metallica were studied using suction electrodes and single-turnover flashes. The kinetic profile of the photocurrent contained a minor outward component (rise time, 100 μs). Local application (from the inside of the pipette) of a photosystem 2 inhibitor, DCMU, rapidly suppressed the outward current; conversely, addition of DCMU to the outer medium produced a transient stimulation of the outward component. Permeabilization of the tip-located membrane fragments with Triton X-100 eliminated the outward current, but had no significant influence on the inward current. The data suggest that the outward current originated in the tip-located nonruptured portions of the thylakoid membrane. Different involvement of two photosystems in the generation of the outward current indicates that granal thylakoids enriched with photosystem 2 are less susceptible to the rupture in the pipette tip as compared with stromal thylakoids

Off-Axis Ions Extraction Simulation in Tubular Electron String Ion Source (TESIS)

Tubular Electron String Ion Source (TESIS) – improved version of Electron String Ion Source (ESIS) aiming for nearly 200-fold increase in ion yield – is under development at JINR. TESIS advantages over ESIS are discussed and their characteristics are compared. Basic scheme of TESIS operation is presented. One of the crucial processes in TESIS – off-axis ion extraction – requires special confilguration of output electrodes providing steep potential gradient. Numerical simulations of ion extraction trajectories for two different electrode confilgurations are compared and the optimal one is discussed. The obtained results will be useful for TESIS design and construction