CYCLIC ELECTRON FLOW AROUND PHOTOSYSTEM-II IN-VIVO

The oxygen Rash yield (Y-o2) and photochemical yield of PS II (Phi(PS II)) were simultaneously detected in intact chlorella cells on a bare platinum oxygen rate electrode. The two yields were measured as a func tion of background irradiance in the steady-state and following a tran sition from light to darkness. During steady-state illumination at mod erate irradiance levels, Y-O2 and Phi(PS) (II) followed each other, su ggesting a close coupling between the oxidation of water and QA reduct ion (Falkowski et al. (1988) Biochim. Biophys. Acta 933: 432-443). Fol lowing a light-to-dark transition, however, the relationship between Q A reduction and the fraction of PS II reaction centers capable of evol ving O-2 became temporarily uncoupled. Phi(PS) (II) recovered to the p reillumination levels within 5-10 s, while the Y-O2 required up to 60 s to recover under aerobic conditions. The recovery of Y-O2 was indepe ndent of the redox state of Q(A), but was accompanied by a 30% increas e in the functional absorption cross-section of PS II (sigma(PS II)) T he hysteresis between Y-O2 and the reduction of Q(A) during the light- to-dark transition was dependent upon the reduction level of the plast oquinone pool and does not appear to be due to a direct radiative char ge back-reaction, but rather is a consequence of a transient cyclic el ectron flow around PS II. The cycle is engaged in vivo only when the p lastoquinone pool is reduced. Hence, the plastoquinone pool can act as a clutch that disconnects the oxygen evolution from photochemical cha rge separation in PS II.