ATP induces rapid quenching of fluorescence yield in light-activated class D chloroplasts, distinguishable from the slower quenching caused by protein phosphorylation. The mechanism of the rapid quenching is shown to be photochemical, by application of the saturation pulse method with a modulated measuring system [(1986) Photosynth. Res. 10, 51-62]. The effect is most pronounced at relatively low light intensities (optimum 2 81282-6/asset/equation/feb20014579387812826-math-si1.gif?v=1&s=29c7c06663d8ae088825fc39040f2a54ef4aabb2)
) without addition of electron acceptors. The properties of the ATP-induced quenching with respect to ionophores, uncouplers and ATPase inhibitors suggest involvement of the reversible, light-activated thylakoid ATPase. As a working hypothesis, it is proposed that part of PS II reaction centers, which are stroma-exposed and in close proximity to CF0-CF1, are modified by ATPase activity. Transformation of cytochrome b-559 high-potential form to low-potential form by membrane acidification is discussed as a possible mechanism.
