The discovery of the two photosynthetic systems: a personal account

A personal account is given of research leading to the discovery of the two photosystems in oxygenic photosynthesis. The following topics are discussed: transfer of excitation energy to (bacterio)chlorophyll and evidence for two pigment systems, the introduction of absorption difference spectroscopy, the discovery of reaction centers, cytochromes and other intermediates as photosynthetic electron or hydrogen transfer agents. Also discussed are chromatic transients and enhancement effects, phenomena indicating the existence of several photochemical but not necessarily major photosynthetic reactions. Various hypotheses or speculations by Eugene Rabinowitch, Robert Emerson, and Robin Hill are critically discussed. The discovery is described of the opposing effects of excitation at two wavelengths on the redox reactions of cytochrome f and other redox components, giving evidence for the existence of an electron transfer chain containing two major photochemical reactions in series, photoreactions 1 and 2, driven by two pigment systems. Finally it is argued that, after the successful introduction of absorption difference spectroscopy, the discovery of the two photosystems would probably have occurred within a decade, even in the absence of previous suggestive experiments or speculations.