Photochemical and photoelectrochemical quenching of chlorophyll fluorescence in photosystem II.

This paper deals with kinetics and properties of variable fluorescence in leaves and thylakoids upon excitation with low intensity multi-turnover actinic light pulses corresponding with an excitation rate of about 10 Hz. These show a relatively small and amply documented rise in the sub-s time range towards the plateau level F(pl) followed by a delayed and S-shaped rise towards a steady state level F(m) which is between three and four fold the initial dark fluorescence level F(o). Properties of this retarded slow rise are i) rate of dark recovery is (1-6 s)(-1), ii) suppression by low concentration of protonophores, iii) responsiveness to complementary single turnover flash excitation with transient amplitude towards a level F(m) which is between five and six fold the initial dark fluorescence level F(o) and iv) in harmony with and quantitatively interpretable in terms of a release of photoelectrochemical quenching controlled by the trans-thylakoid proton pump powered by the light-driven Q cycle. Data show evidence for a sizeable fluorescence increase upon release of (photo) electrochemical quenching, defined as qPE. Release of qPE occurs independent of photochemical quenching defined here as qPP even under conditions at which qPP = 1. The term photochemical quenching, hitherto symbolized by qP, will require a new definition, because it incorporates in its present form a sizeable photoelectrochemical component. The same is likely to be true for definition and use of qN as an indicator of non photochemical quenching.