The effect of cations and membrane permeability modifying agents on the dark kinetics of the photoelectric response in isolated chloroplasts

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Potassium retention in leaf mesophyll as an element of salinity tissue tolerance in halophytes

Soil salinity remains a major threat to global food security, and the progress in crop breeding for salinity stress tolerance may be achieved only by pyramiding key traits mediating plant adaptive responses to high amounts of dissolved salts in the rhizosphere. This task may be facilitated by studying natural variation in salinity tolerance among plant species and, specifically, exploring mechanisms of salinity tolerance in halophytes. The aim of this work was to establish the causal link between mesophyll ion transport activity and plant salt tolerance in a range of evolutionary contrasting halophyte and glycophyte species. Plants were grown under saline conditions in a glasshouse, followed by assessing their growth and photosynthetic performance. In a parallel set of experiments, net K⁺ and H⁺ transport across leaf mesophyll and their modulation by light were studied in control and salt-treated mesophyll segments using vibrating non-invasive ion selective microelectrode (the MIFE) technique. The reported results show that mesophyll cells in glycophyte species loses 2-6 fold more K⁺ compared with their halophyte counterparts. This decline was reflected in a reduced maximum photochemical efficiency of photosystem II, chlorophyll content and growth observed in the glasshouse experiments. In addition to reduced K⁺ efflux, the more tolerant species also exhibited reduced H⁺ efflux, which is interpreted as an energy-saving strategy allowing more resources to be redirected towards plant growth. It is concluded that the ability of mesophyll to retain K⁺ without a need to activate plasma membrane H⁺-ATPase is an essential component of salinity tolerance in halophytes and halophytic crop plants.

Salinity effects on chloroplast PSII performance in glycophytes and halophytes

The effects of NaCl stress and K+ nutrition on photosynthetic parameters of isolated chloroplasts were investigated using PAM fluorescence. Intact mesophyll cells were able to maintain optimal photosynthetic performance when exposed to salinity for more than 24h whereas isolated chloroplasts showed declines in both the relative electron transport rate (rETR) and the maximal photochemical efficiency of PSII (Fv/Fm) within the first hour of treatment. The rETR was much more sensitive to salt stress compared with Fv/Fm, with 40% inhibition of rETR observed at apoplastic NaCl concentration as low as 20mM. In isolated chloroplasts, absolute K+ concentrations were more essential for the maintenance of the optimal photochemical performance (Fv/Fm values) rather than sodium concentrations per se. Chloroplasts from halophyte species of quinoa (Chenopodium quinoa Willd.) and pigface (Carpobrotus rosii (Haw.) Schwantes) showed less than 18% decline in Fv/Fm under salinity, whereas the Fv/Fm decline in chloroplasts from glycophyte pea (Pisum sativum L.) and bean (Vicia faba L.) species was much stronger (31 and 47% respectively). Vanadate (a P-type ATPase inhibitor) significantly reduced Fv/Fm in both control and salinity treated chloroplasts (by 7 and 25% respectively), whereas no significant effects of gadolinium (blocker of non-selective cation channels) were observed in salt-treated chloroplasts. Tetraethyl ammonium (TEA) (K+ channel inhibitor) and amiloride (inhibitor of the Na+/H+ antiporter) increased the Fv/Fm of salinity treated chloroplasts by 16 and 17% respectively. These results suggest that chloroplasts' ability to regulate ion transport across the envelope and thylakoid membranes play a critical role in leaf photosynthetic performance under salinity.

Protonmotive force and photophosphorylation in single swollen thylakoid vesicles

Swollen vesicles generally 40 micron in diameter were prepared from spinach chloroplasts. These vesicles appear to originate from thylakoids. The present study reports results obtained with individual vesicles using micromanipulative procedures. The electric potential across the membrane was measured with microelectrodes and the pH of the internal space was calculated from the fluorescence of the pH indicator pyranine. The individual vesicles photophosphorylate as measured with luciferin-luciferase. Impalement with microelectrodes did not affect the ability of individual vesicles to photophosphorylate. However, there was no significant membrane potential either with continuous illumination or light flashes. In contrast, we found a delta pH of 3.7 under photophosphorylative conditions and the incubation with the appropriate buffers blocked photophosphorylation presumably by preventing formation of a pH gradient. We propose that, in these vesicles, the membrane potential plays no role in photophosphorylation, whereas a pH gradient is obligatory.

The residual protonmotive force in mitochondria after an oxygen pulse

Both from irreversible thermodynamics and from mass-action kinetics it can be derived that upon anaerobiosis in an oxygen-pulse experiment the protonmotive force across a mitochondrial membrane undergoes a sudden drop. Under representative conditions the protonmotive force after the drop (the residual protonmotive force) is shown to be less than 3 kJ . mol-1 as opposed to steady-state values for the protonmotive force of 19 kJ . mol-1. It is concluded that correction for proton leakage in pulse experiments by back extrapolation underestimate proton leakage. Consequently the observed H+/O stoichiometries must underestimate the true H+/O ratios.

The concept of chemical capacitance, A critique

The concept of chemical capacitance as introduced by Hong and Mauzerall (Proc. Natl. Acad. Sci. U.S.A. 1974. 71:1564) is critically reexamined. This novel capacitance was introduced to explain the time-course of flash-induced photocurrents observed in lipid bilayer membranes containing porphyrins. According to Hong and Mauzerall, the chemical capacitance results from a combination of three fundamental capacitances: the geometric membrane capacitance and the two interfacial double layer capacitances. The concept of chemical capacitance is questioned for the following reasons: (i) The system analysis is insufficiently determinate. (ii) The measured chemical capacitance is approximately 0.16% of that predicted by the theory. (iii) The fact that only 20% of the membrane area is illuminated was not considered in the analysis. The latter point offers an alternative explanation of the capacitance in question: this capacitance may reflect that fraction of the total membrane capacitance that is photochemically active. If so, the concept of chemical capacitance lacks general significance.

Effect of ionophores A23187 and nigericin on the light-induced redistribution of Mg2+, K+ and H+ across the thylakoid membrane

Passive redistributions of Mg2+ and K+ ions across the thylakoid membranes, occurring in association with the light-driven electrogenic influx of hydrogen ions have been examined in suspensions of broken spinach chloroplasts under a variety of conditions. (i) In accord with results of Hind el al. (Proc. Natl. Acad. Sci. U.S. (1974) 71, 1484), it was found that at a low K/Mg concentration ratio in the medium, the K-efflux is negligibly small, whereas a substantial Mg-efflux is observed. The converse is true when the K/Mg concentration ratio in the medium is high. (ii) In the presence of A23187, which was found to cause approximately a 60% inhibition of the light-induced pH-gradient, a significant influx of Mg2+ was observed in the light at a high K/Mg concentration ratio. Conversely the Mg-influx was small in the presence of A23187 when the K/Mg concentration ratio in the medium was low. Under these conditions, the Mg-influx was considerably increased upon the addition of valinomycin. A23187 was found not to affect the K-efflux in the light. (iii) The light-induced K-influx observed in the presence of nigericin also was found to be dependent on the concentration ratio of the monovalent and divalent cation. Its magnitude increased upon an increase in the K/Mg ratio. The results are interpreted in terms of a simplified model in which the total passive efflux of cations, driven by the potential set by the electrogenic proton pump, is considered to be a constant fraction of the proton influx. According to this, an increase in the flux of an ion species, induced either by raising its concentration, or by increasing its permeability through the membrane, will cause a decrease in the flux of the other cations. The relevance of the results is discussed with respect to conclusions about the involvement and relative magnitudes of the passive K and Mg effluxes across the thylakoid membrane during energization of intact chloroplasts and chloroplasts in situ.

II. Electrical measurements in the nanosecond range of the charge separation from chloroplasts spread at a heptane-water interface. Application of a novel capacitive electrode

Spinach chloroplasts are spread at a heptane-water interface. Applying a novel capacitative electrode introduced in the preceding paper (Trissl, H.-W. (1980) Biochim. Biophys. Acta 595, 82-95) the changes of the interface potential induced by single laser flashes are investigated. The following results are obtained: (1) The chloroplasts spread at the interface form a thin layer with asymmetrical orientation. The structural state of this layer is discussed. (2) The photovoltage from the interfacial layer shows similar characteristics as the field-indicating absorption change of chloroplast suspensions, the latter reflecting the photosynthetic primary charge separation: (a) Both can be abolished by addition of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea. (b) About one half of the signals can be reactivated by addition of N-methyl-phenazonium methosulfate. (c) Both signals saturate at low flash light intensities. (d) Both signals can be abolished by background illumination of comparable intensities. (e) Both signals are independent of the ionic strength. (3) The half-rise time of the photovoltage is determined to be less than 3 ns. It is suggested from these results that the photovoltage from the interfacial layer reflects the primary charge separation process in photosynthesis, i.e. the latter is accomplished also within less than 3 ns.