Compared thermoluminescence characteristics of pea thylakoids studied in vitro and in situ (in leaves). The effect of photoinhibitory treatments

The role of transmembrane electrochemical potential and phosphorylation of PS II proteins in temperature induced light emission from ATP-treated lettuce thylakoids

Changes in characteristics of flash-induced thermoluminescence (TL) glow curves in thylakoids of lettuce following incubation of the organelles with ATP, under illumination or in the dark, were investigated. TL bands were induced by 1 or 2 flashes fired at -10°C or 1°C in thylakoids: TL curves in control thylakoids which were dark-adapted or submitted to an illumination without ATP, can be deconvoluted as the sum of one single B band and minor contributions. In thylakoids incubated for 90 s with 0.5 mM ATP, either under light or in the dark (after a 90 s preillumination), bands presented complex shapes; after deconvolution, they appeared composed of a B band with a low Ea (activation energy): 0.6 e.v. as compared to 0.75 in control, and a supplementary band peaking at about 10°C. The band at low temperature was suppressed by low concentrations (10-20 nM) of valinomycin, nigericin or FCCP as well as by 10 mM ammonium chloride, leaving B bands with the same characteristics as in control material. Finally with higher nigericin concentrations, the bands became single B bands with high Ea (0.9 e.v.). These characteristics would define 3 different energized states (in the form of a transmembrane electrochemical potential) for thylakoids based upon the presence of the 10°C band and the value of the activation energy for the B band component. The presence of a large 10°C band was also correlated to the existence of a larger transmembrane pH gradient, in the dark, after an ATP-treatment, than in controls. The 10°C band was specifically suppressed by the action of low concentrations of alkaline phosphatase with minor changes in characteristics of the remaining B band suggesting that phosphorylation of PS II proteins is also involved in the appearance of this low temperature band. The main mechanism at the origin of the low temperature band would be a destabilization of S2/3QB (-) charge pairs in energized membranes.

Effects of dark- and light-induced proton gradients in thylakoids on the Q and B thermoluminescence bands

Thermoluminescence experiments have been carried out to study the effect of a transmembrane proton gradient on the recombination properties of the S2 and S3 states of the oxygen evolving complex with QA (-) and QB (-), the reduced electron acceptors of Photosystem II. We first determined the properties of the S2QA (-) (Q band), S2QB (-) and S3QB (-) (B bands) recombinations in the pH range 5.5 to 9.0, using uncoupled thylakoids. The, a proton gradient was created in the dark, using the ATP-hydrolase function of ATPases, in coupled unfrozen thylakoids. A shift towards low temperature of both Q and B bands was observed to increase with the magnitude of the proton gradient measured by the fluorescence quenching of 9-aminoacridine. This downshift was larger for S3QB (-) than for S2QB (-) and it was suppressed by nigericin, but not by valinomycin. Similar results were obtained when a proton gradient was formed by photosystem I photochemistry. When Photosystem II electron transfer was induced by a flash sequence, the reduction of the plastoquinone pool also contributed to the downshift in the absence of an electron acceptor. In leaves submitted to a flash sequence above 0°C, a downshift was also observed, which was supressed by nigericin infiltration. Thus, thermoluminescence provides direct evidence on the enhancing effect of lumen acidification on the S3→S2 and S2→S1 reverse-transitions. Both reduction of the plastoquinone pool and lumen acidification induce a shift of the Q and B bands to lower temperature, with a predominance of lumen acidification in non-freezing, moderate light conditions.