UNCOUPLER TITRATION OF ENERGY-DEPENDENT CHLOROPHYLL FLUORESCENCE QUENCHING AND PHOTOSYSTEM-II PHOTOCHEMICAL YIELD IN INTACT PEA-CHLOROPLASTS

An investigation into how the photochemical yield of Photosystem II (F{cyrillic}p = electron transport rate (light intensity · coefficient for photochemical quenching of chlorophyll fluorescence)) and energy-dependent quenching of chlorophyll fluorescence (qE) depend upon the transthylakoid pH gradient (ΔpH) was carried out using intact pea chloroplasts. Room-temperature chlorophyll fluorescence, 9-aminoacridine fluorescence, and oxygen uptake were measured simultaneously during titrations against nigericin, in the presence of methyl viologen. Photochemical chlorophyll fluorescence quenching (qQ) and qE were measured by the technique of DCMU addition. Titrations revealed a consistent dependence of both F{cyrillic}p and qE upon ΔpH, estimated by 9-aminoacridine fluorescence quenching (q9-aa), at three widely differing light intensities. However, whereas a threshold value of q9-aa was required in order to observe qE, F{cyrillic}p was seen to decrease at much lower values of q9-aa. The relationship between φ{symbol}p and q9-aa during titrations against nigericin was affected only marginally by the presence of antimycin A concentrations which markedly inhibited qE. A plot of φ{symbol}p against qE gave a relationship inconsistent with the two-state model of Weis and Berry (Biochim. Biophys. Acta 894 (1987) 198-208). The data support the existence of another mechanism, such as an electron cycle around Photosystem II, which can operate to reduce the efficiency of linear electron transport through Photosystem II. © 1990.