THE RATE-CONSTANT OF PHOTOINHIBITION IN-VITRO IS INDEPENDENT OF THE ANTENNA SIZE OF PHOTOSYSTEM-II BUT DEPENDS ON TEMPERATURE

Photoinhibition of Photosystem (PS) II was studied in thylakoid membra nes, inside-out and rightside-out thylakoid vesicles derived from appr essed and non-appressed membrane regions, respectively, in detergent f ractionated PS II membranes and in oxygen evolving PS II core complexe s. The preparations were illuminated without added electron accepters, and care was taken to keep the oxidizing side of PS II in a functiona l condition during the experiments. The first-order rate constant of p hotoinhibition, measured under given photon flux density, was similar in all preparations derived from appressed thylakoid regions and indep endent of the antenna size. This antenna size independence indicates t hat under photoinhibitory conditions in vitro, when most PS II traps a re closed, the probability of finding an exciton in the reaction centr e is not much larger than the probability of finding it in the antenna . Spillover of excitation energy from PS II to PS I may be an importan t factor protecting the PS II beta of stroma thylakoids from photoinhi bition. Photoinhibition in vitro is a first-order reaction even at low temperatures where the degradation of the D1 protein is slow, which d emonstrates that the photoinhibited PS II centres do not protect the r emaining active ones from photoinhibition, at least not in vitro. The activation energy of photoinhibition in pumpkin thylakoids, as measure d between 6 and 25 degrees C, was 15 kJ/mol; the rate constant of phot oinhibition in pumpkin thylakoids increased both below 6 and above 25 degrees C.