Kinetics of prolonged photoinhibition revisited: photoinhibited Photosystem II centres do not protect the active ones against loss of oxygen evolution.

Photoinhibition of Photosystem II (PSII) in lincomycin-treated leaves begins as a first-order reaction, but fluorescence measurements have suggested that after prolonged illumination, the number of active PSII centres stabilizes to 15-20% of control. The stabilization has been interpreted to indicate that photoinhibited PSII centres protect the remaining active centres against photoinhibition (Lee, Hong and Chow, Planta 212:332-342, 2001). In an attempt to study the mechanism of this protection, we measured the reaction kinetics of photoinhibition in lincomycin-treated pumpkin (Cucurbita pepo L.) and pepper (Capsicum annuum L.) leaves in vivo. The light-saturated rate of PSII oxygen evolution, assayed from thylakoids and isolated from the treated leaves, was used as a direct measure of the number of remaining active PSII centres, and the fluorescence parameters F (V)/F (M) and (F (V)/F (M))/F (0) (=1/F (0) - 1/F (M)) were measured for comparison. To our surprise, no stabilization of PSII activity was observed and photoinhibition followed first-order kinetics until PSII activity had virtually declined to zero. A series of in vitro experiments was carried out to see whether stabilization of PSII activity occurs if a particular combination of light intensity and wavelength range is applied, or if a specific PSII preparation is used as experimental material. The results of the in vitro experiments confirmed the in vivo result about persistent first-order kinetics. We conclude that photoinhibited PSII centres offer no measurable protection against photoinhibition.