Inhibition by ammonium chloride of the oxygen yield of photosynthesis

Flash O2 yield experiments are described in spinach chloroplasts and Chlorella after addition of NH4Cl. (1) The damping of the sequence is increased by NH4Cl. (2) The turnover times are accelerated but the reaction during which O2 is released (S3bv leads to S4 leads to S0) is slowed. (3) We observed between the end of the turnover kinetic and the beginning of deactivation a latency time tl during which the S2 and S3 states are perfectly stable. In the presence of NH4Cl, this latency time for S2 is shortened, diminishing from 1 s to less than 12 ms; whereas it is lengthened for S3 (up to 4 s, 5 s). (4) After this latency time, two phases are clearly distinguished in the S2 deactivation: the beginning of the deactivation is abrupt, varying like the square root of the time, i.e. as is characteristic of a diffusive process. During the second phase, S2 is stabilized. Our experiment shows that S2 deactivation is monitored by the release of some product F after the latency time from one particular locus, so that nearby Photosystem II reaction centers are rapidly deactivated by diffusion of the product, whereas centers far from this locus are very slowly deactivated. These results are qualitatively complementary to the luminescence experiments of Velthuys (Biochim. Biophys, Acta (1975) 396, 392-401) except for the latency time which is invisible in the Velthuys results. We propose a modified model in which the binding of NH3 on states S2 and S3 occurs during deactivation.