A concept of three light reactions in photosynthesis by green plants.

It has generally been accepted that plant photosynthesis involves two light reactions, one that proceeds best in short-wavelength light and is identified with oxygen evolution (System II) and another that proceeds best in long-wavelength light and is identified with a cycle electron flow (System I). This paper presents a concept of three light reactions in photosynthesis, based on new evidence that System II comprises two rather than one short-wavelength light reaction. These appear to operate in series and to be connected by an electron transport chain peculiar to System II. Parallel to System II is the long-wavelength light reaction of System I.