OXYGEN RELEASE TIME IN LEAF-DISKS AND THYLAKOIDS OF PEAS AND PHOTOSYSTEM-II MEMBRANE-FRAGMENTS OF SPINACH

The half-time of O2 release in Chlorella, thylakoids and Photosystem II membranes has generally been found to be 0.9 to 3.0 ms, but recently Plijter et al. (Biochim. Biophys. Acta 935 (1988) 299-311) reported it to be between 30 to 130 ms. This work presents a new method, using a Clark electrode, for measuring O2 release times in leaf discs, thylakoids and Photosystem II membranes. The sample is illuminated with saturating pulses of light of 3 μs to 200 ms duration given at 5 Hz. Presuming the Kok model of four charge transfers per O2 evolved, the O2 release time could be calculated from the time needed to observe the evolution of one O2 less 4-times the turnover time of the charge transfer reaction. Upper bounds for the half times of O2 release were found to be 6, 11, and 5 ms in thylakoids, leaf discs, and Photosystem II membrane fragments, respectively. In thylakoids, using a bare platinum electrode operated under weak bias conditions, the half-time of O2 signal rise is 2.7 ms. The rate of rise of this O2 signal could be slowed by treating the thylakoids with nitrate or formate. The turnover time of the oxygen evolution system, the minimum time between evolution of oxygen molecules, of Photosystem II that is necessary to support light saturated photosynthesis was measured to be 16, 28 and 22 ms in thylakoids, leaf discs and Photosystem II membrane fragments, respectively. The rate limitation for photosynthesis is not the O2 release time but is the rate of charge transfer through or beyond the plastoquinone pool. © 1990.