From the Red Drop to the Z-scheme of photosynthesis

The US-American biophysicist Robert Emerson and his collaborator, the physicist C. Charlton M. Lewis, were investigating the efficiency of photosynthesis in 1943. In the course of this work, they observed, at low light intensities, an unexpected sharp decline of photosynthetic efficiency in the far red, that is, from 685 nm towards the infrared region of the spectrum. This was the phenomenon that later became known as the 'Red Drop' of photosynthesis efficiency. In 1955, Emerson came back to his former line of enquiry, and he hit upon an even stranger effect. It turned out that the Red Drop of photosynthetic efficiency disappeared if a supplementary light beam of shorter wavelengths was provided. The important idea that emerged from Emerson's findings was that photosynthesis might involve two different light reactions, prompted by light of different wavelengths. In conclusions arrived at from a range of different directions, the surprising Enhancement Effect that Emerson had hit upon in 1957 was rendered perfectly explicable.