The Polyphasic Rise of Chlorophyll Fluorescence upon Onset of Strong Continuous Illumination: I. Saturation Characteristics and Partial Control by the Photosystem II Acceptor Side

Abstract

Applying a rapid modulation system for measurement of chlorophyll fluorescence yield (U. Schreiber. Photosynth. Res. 9. 261-272 (1986)) the induction kinetics upon onset of strong actinic illumination previously studied by Delosme (Biochim, Biophys, Acta 143, 108-128 (1967)) are reinvestigated. With increasing actinic intensity the fluorescence rise is changed from the typical O-I-P characteristic to a more complex rise curve with two intermediary levels I 1 and I 2 both of which show saturation at high intensity. The typical kinetics at saturating light intensity (O-I 1-D-I 2-P) are observed in a variety of plant species. The properties of the kinetics with respect to light intensity, temperature, electron acceptors and PS II inhibitors suggest that the O-I 1 phase is controlled by photochemical charge separation (photochemical phase), while the I 1-D-I 2-P transients are limited by dark reactions (thermal phases). Dichlorophenyl-dimethylurea (DCMU) eliminates the thermal phases by raising I, to the original I level. While in principal the previous findings by Delosme are confirmed, there, is the new aspect of two distinct components in the thermal part of the rise curve, which display different properties. Electron acceptors suppress only the I 2-P phase, which appears to parallel the reduction of the plastoquinonc pool, which is a fluorescence quencher when oxidized. While the DCMU effect suggests quenching control during I 1-I 2 by reoxidation of PS II acceptors, this suggestion is contradicted by the observed saturation of I 1 with light intensity and at low temperatures. The relevance of these results with respect to quenching analysis of chlorophyll fluorescence by the saturation pulse method is discussed.