Photosynthesis Research (v.110, #2)

This report describes a new method to measure the chloroplastic lumenal proton pool in leaves (tobacco and sunflower). The method is based on measurement of CO2 outbursts from leaves caused by the shift in the CO2 + H2O ↔ HCO3  + H+ equilibrium in the chloroplast stroma as protons return from the lumen after darkening. Protons did not accumulate in the lumen to a significant extent when photosynthesis was light-limited, but a large pool of >100 μmol H+ m−2 accumulated in the lumen as photosynthesis became light-saturated. During thylakoid energization in the light, large amounts of protons are moved from binding sites in the stroma to binding sites in the lumen. The transthylakoidal difference in the chemical potential of free protons (ΔpH) is largely based on the difference in the chemical potential of bound protons in the lumenal and stromal compartments (pK). Over the course of the dark-light induction of photosynthesis protons accumulate in the lumen during reduction of 3-phosphoglycerate. The accumulation of electrons in reduced compounds of the stroma and cytosol is the natural cause for accumulation of a stoichiometric pool of lumenal protons during this transient event.
Keywords: Leaves; Photosynthesis; Thylakoid; Protons

Carbonic anhydrase activity in Arabidopsis thaliana thylakoid membrane and fragments enriched with PSI or PSII by Lyudmila K. Ignatova; Natalia N. Rudenko; Vilen A. Mudrik; Tat’yana P. Fedorchuk; Boris N. Ivanov (89-98).
The procedure of isolating the thylakoids and the thylakoid membrane fragments enriched with either photosystem I or photosystem II (PSI- and PSII-membranes) from Arabidopsis thaliana leaves was developed. It differed from the one used with pea and spinach in durations of detergent treatment and centrifugation, and in concentrations of detergent and Mg2+ in the media. Both the thylakoid and the fragments preserved carbonic anhydrase (CA) activities. Using nondenaturing electrophoresis followed by detection of CA activity in the gel stained with bromo thymol blue, one low molecular mass carrier of CA activity was found in the PSI-membranes, and two carriers, a low molecular mass one and a high molecular mass one, were found in the PSII-membranes. The proteins in the PSII-membranes differed in their sensitivity to acetazolamide (AA), a specific CA inhibitor. AA at 5 × 10−7 M inhibited the CA activity of the high molecular mass protein but stimulated the activity of the low molecular mass carrier in the PSII-membranes. At the same concentration, AA moderately inhibited, by 30%, the CA activity of PSI-membranes. CA activity of the PSII-membranes was almost completely suppressed by the lipophilic CA inhibitor, ethoxyzolamide at 10−9 M, whereas CA activity of the PSI-membranes was inhibited by this inhibitor even at 5 × 10−7 M just the same as for AA. The observed distribution of CA activity in the thylakoid membranes from A. thaliana was close to the one found in the membranes of pea, evidencing the general pattern of CA activity in the thylakoid membranes of C3-plants.
Keywords: Carbonic anhydrase; Arabidopsis thaliana ; Thylakoids; Photosystem I; Photosystem II; Acetazolamide; Ethoxyzolamide

Oxygen evolution per single-turnover flash (STF) or multiple-turnover pulse (MTP) was measured with a zirconium O2 analyzer from sunflower leaves at 22°C. STF were generated by Xe arc lamp, MTP by red LED light of up to 18000 μmol quanta m−2 s−1. Ambient O2 concentration was 10–30 ppm, STF and MTP were superimposed on far-red background light in order to oxidize plastoquinone (PQ) and randomize S-states. Electron (e) flow was calculated as 4 times O2 evolution. Q A → Q B electron transport was investigated firing double STF with a delay of 0 to 2 ms between the two. Total O2 evolution per two flashes equaled to that from a single flash when the delay was zero and doubled when the delay exceeded 2 ms. This trend was fitted with two exponentials with time constants of 0.25 and 0.95 ms, equal amplitudes. Illumination with MTP of increasing length resulted in increasing O2 evolution per pulse, which was differentiated with an aim to find the time course of O2 evolution with sub-millisecond resolution. At the highest pulse intensity of 2.9 photons ms−1 per PSII, 3 e initially accumulated inside PSII and the catalytic rate of PQ reduction was determined from the throughput rate of the fourth and fifth e. A light response curve for the reduction of completely oxidized PQ was a rectangular hyperbola with the initial slope of 1.2 PSII quanta per e and V m of 0.6 e ms−1 per PSII. When PQ was gradually reduced during longer MTP, V m decreased proportionally with the fraction of oxidized PQ. It is suggested that the linear kinetics with respect to PQ are apparent, caused by strong product inhibition due to about equal binding constants of PQ and PQH2 to the Q B site. The strong product inhibition is an appropriate mechanism for down-regulation of PSII electron transport in accordance with rate of PQH2 oxidation by cytochrome b6f.
Keywords: Leaves; Photosystem II; Oxygen evolution; Electron transport

The photosystem II (PSII) manganese-stabilizing protein (PsbO) is known to be the essential PSII extrinsic subunit for stabilization and retention of the Mn and Cl cofactors in the oxygen evolving complex (OEC) of PSII, but its function relative to Ca2+ is less clear. To obtain a better insight into the relationship, if any, between PsbO and Ca2+ binding in the OEC, samples with altered PsbO-PSII binding properties were probed for their potential to promote the ability of Ca2+ to protect the Mn cluster against dark-inhibition by an exogenous artificial reductant, N,N-dimethylhydroxylamine. In the absence of the PsbP and PsbQ extrinsic subunits, Ca2+ and its surrogates (Sr2+, Cd2+) shield Mn atoms from inhibitory reduction (Kuntzleman et al., Phys Chem Chem Phys 6:4897, 2004). The results presented here show that PsbO exhibits a positive effect on Ca2+ binding in the OEC by facilitating the ability of the metal to prevent inhibition of activity by the reductant. The data presented here suggest that PsbO may have a role in the formation of the OEC-associated Ca2+ binding site by promoting the equilibrium between bound and free Ca2+ that favors the bound metal.
Keywords: Calcium; Manganese reduction; Manganese-stabilizing protein; Mutation; Photosystem II

Highly time-resolved photoacclimation patterns of the chlorophyte microalga Dunaliella tertiolecta during exposure to an off–on–off (block) light pattern of saturating photon flux, and to a regime of consecutive increasing light intensities are presented. Non-photochemical quenching (NPQ) mechanisms unexpectedly responded with an initial decrease during dark–light transitions. NPQ values started to rise after light exposure of approximately 4 min. State-transitions, measured as a change of PSII:PSI fluorescence emission at 77 K, did not contribute to early NPQ oscillations. Addition of the uncoupler CCCP, however, caused a rapid increase in fluorescence and showed the significance of qE for NPQ. Partitioning of the quantum efficiencies showed that constitutive NPQ was (a) higher than qE-driven NPQ and (b) responded to light treatment within seconds, suggesting an active role of constitutive NPQ in variable energy dissipation, although it is thought to contribute statically to NPQ. The PSII connectivity parameter p correlated well with F′, F m ′ and NPQ during the early phase of the dark–light transients in sub-saturating light, suggesting a plastic energy distribution pattern within energetically connected PSII centres. In consecutive increasing photon flux experiments, correlations were weaker during the second light increment. Changes in connectivity can present an early photoresponse that are reflected in fluorescence signals and NPQ and might be responsive to the short-term acclimation state, and/or to the actinic photon flux.
Keywords: Connectivity; Constitutive NPQ; FRRF; Light acclimation; NPQ; qE; Photosynthesis; Chlorophyll a fluorescence

Using photographs taken at the conference site, we provide a perspective on (i) the awards that were given to four young investigators at the 2011 Gordon Research Conference on Photosynthesis, and (ii) the ambiance at this conference, held at Davidson College, North Carolina, during June 12–17, 2011.
Keywords: Aaron M. Collins; Nicholas J. Cox; Joshua K. Endow; Yan Lu

Erratum to: Electrochromism: a useful probe to study algal photosynthesis by Benjamin Bailleul; Pierre Cardol; Cécile Breyton; Giovanni Finazzi (151-152).