Studies on the Nature of the Primary Reactions of Photosystem II in Photosynthesis II. The Modification of the Functional Integrity of the Photochemical Active Centers of System II by a-Bromo-a-benzyl-Malodinitril and Accompanying Effects on Chloroplasts

The effect of a-bromo-a-benzylmalodinitril (BBMD) on the oxygen evolution and on the ab­ sorption changes at 515 nm and 704 nm has been investigated in spinach chloroplasts. It has been found: 1. Under repetitive flash excitation conditions, where the back reaction around system II is practically excluded for kinetical reasons, BBMD does not restore the 515 nm absorption change in DCMU poisoned chloroplasts. 2. Under single flash excitation conditions, where the back reaction around system II becomes prominent in the presence of DCMU, BBMD moderately inhibits this back reaction. The deleterious effect is pronounced by preillumination with short flashes during the BBMD incubation period of the chloroplasts in the absence of DCMU. 3. Incubation of the chloroplasts with BBMD leads to an activity loss of oxygen evolution which increases with the time td between the repetitive short excitation flashes and with the dark incu­ bation time. Preillumination during the incubation period with BBMD significantly enhances the effect. 4. In the absence of artificial electron acceptors BBMD suppresses in DCMU poisoned chloro­ plasts the 704 nm absorption change reflecting an internal cyclic electron flow around system I. On the other hand the linear electron transport at system I mediated by DCIP plus ascorbate as electron donor couple and benzylviologen as electron acceptor is not disturbed by BBMD. 5. BBMD incubation of chloroplasts accelerates the decay rate of the field indicating 515 nm absorption change. Based on these experimental findings the conclusion has been drawn, that — in contrast to the assumption of Brandon and Elgersma (Biochim. Biophys. Acta 292, 753 — 762 [1973]) — BBMD does not accept electrons from the primary electron acceptor X 320 of system II in DCMU poisoned chloroplasts. BBMD rather acts as a system I electron acceptor. Furthermore, BBMD exerts deleterious effects on the center of photosystem II, accompanied by a weak ADRY-effect on the water-splitting enzyme system Y. As a tentative explanation of the BBMD-action on system II it is assumed that BBMD transforms the photochemical centers of system II into dissipative energy sinks.