The pH‐dependence of the photosystem II fluorescence: Cooperative transition to a quenching state

Abstract

AbstractThylakoid membranes were isolated from various cultures of the green alga Scenedesmus obliquus which differed in the extent of the observable‐high‐energy‐state quenching of chlorophyll fluorescence (qE). In uncoupled thylakoid membranes the pH‐dependence of the fluorescence parameters FM and F0 exhibits two distinct phases: (1) for pH‐values ranging from 6.5 to 5.7, acidification results in a pronounced decrease of FM and F0; (2) for pH‐values below 5.7, acidification results in a slight decrease of FM and a significant increase of F0. The extent of the first phase of F and F quenching corresponds to the extent of qE in intact cells; the variability of qE of intact cells is not explainable by a pK‐shift. Simulations on basis of the reversible radical pair model using pH‐dependent molecular rate constants indicate the occurrence of a cooperative transition to a state with an enhanced rate of non‐radiative decay of excited chlorophyll‐singlet states (Hill coefficient n1 ≥ 4, pK1 ≈︁ 6.1 for all cultures). For pH‐values below 5.7, a pH effect on Photosystem II electron transfer reactions is involved (n2 ≈︁ 1, pK2 ≥ 4.5).