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 membranes, inside-out and rightside-out thylakoid vesicles derived from appressed and non-appressed membrane regions, respectively, in detergent fractionated PS II membranes and in oxygen evolving PS II core complexes. The preparations were illuminated without added electron accepters, and care was taken to keep the oxidizing side of PS II in a functional condition during the experiments. The first-order rate constant of photoinhibition, measured under given photon flux density, was similar in all preparations derived from appressed thylakoid regions and independent of the antenna size. This antenna size independence indicates that under photoinhibitory conditions in vitro, when most PS II traps are closed, the probability of finding an exciton in the reaction centre 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 important factor protecting the PS II beta of stroma thylakoids from photoinhibition. Photoinhibition in vitro is a first-order reaction even at low temperatures where the degradation of the D1 protein is slow, which demonstrates that the photoinhibited PS II centres do not protect the remaining active ones from photoinhibition, at least not in vitro. The activation energy of photoinhibition in pumpkin thylakoids, as measured between 6 and 25 degrees C, was 15 kJ/mol; the rate constant of photoinhibition in pumpkin thylakoids increased both below 6 and above 25 degrees C.