THE PH-DEPENDENCE OF THE PHOTOSYSTEM-II FLUORESCENCE - COOPERATIVE TRANSITION TO A QUENCHING STATE

Thylakoid membranes were isolated from various cultures of the green a lga Scenedesmus obliquus which differed in the extent of the observabl e high-energy-state quenching of chlorophyll fluorescence (qE). In unc oupled thylakoid membranes the pH-dependence of the fluorescence param eters F-M and F-0 exhibits two distinct phases: (1) for pH-values rang ing from 6.5 to 5.7, acidification results in a pronounced decrease of F-M and F-0; (2) for pH-values below 5.7, acidification results in a slight decrease of F-M and a significant increase of F-0. The extent o f the first phase of F-M- and F-0-quenching corresponds to the extent of qE in intact cells; the variability of qE of intact cells is not ex plainable by a pK-shift. Simulations on basis of the reversible radica l pair model using pH-dependent molecular rate constants indicate the occurrence of a cooperative transition to a state with an enhanced rat e of non-radiative decay of excited chlorophyll-singlet states (Hill c oefficient n(1) greater than or equal to 4, pK(1) approximate to 6.1 f ar all cultures). For pH-values below 5.7, a pH effect on Photosystem II electron transfer reactions is involved (n(2) approximate to 1, pK( 2) approximate to 4.5).