Photosynthetic evolution of oxygen by flashes of light

Abstract

An attempt is made to reconcile the apparently contradictory observations in flash photosynthesis using short flashes ( ca. 10 −5 sec.) and long flashes ( ca. 10 −3 sec.). It is shown that the introduction of a new period of ca. 10 −4 sec. not only obviates these previous difficulties, but also is not in contradiction with any existing observations. In addition, a mechanism for the primary photochemical process, suggested by one of us (J. F.) and discussed in detail elsewhere, is presented. Using reversible phosphorescence quenching as a means of measuring very small concentrations of oxygen, we have also studied the evolution of oxygen under anaerobic conditions by single flashes of light. The biological material was the algae Scenedesmus obliquus in the presence of either carbon dioxide or the Hill reagent quinone. It has been observed that while a single, intense, half-millisecond flash evolves oxygen in the Hill reaction, it does not evolve oxygen in photosynthesis. A flash 50 times as long and of much lower intensity results in the evolution of oxygen in both systems. Further, in the Hill reaction, the yield from an intense half-millisecond flash is increased by a previous similar flash preceding it by one full second. This observation suggests that certain photoproducts are capable of surviving longer than the generally assumed 10 −2 sec. Various other observations on the effect of overlapping short and long flashes, and some plausible explanations, are described.