LIGHT DISTRIBUTION AND ELECTRON DONATION IN THE Z SCHEME*

Abstract

Abstract— Several recent reports indicate that the photosynthetic machinery possesses a device whereby the relative distribution of absorbed light energy into the two reaction centers can be varied in response to the environment in which the photosynthesizing cells or chloro‐plasts are placed. Conditions reported which affect this distribution include the electron acceptor or electron donor system employed, the concentration of magnesium ions, and the preillumination history of the preparation. These observations are described and discussed in terms of their relevance to the interpretation of quantum yield and enhancement data.Several laboratories have presented evidence recently that chloroplasts, after undergoing a pretreatment which destroys their capacity to evolve oxygen, can still transfer electrons via both photosystems, provided an appropriate electron donor is added. It is shown that electron donors like manganous ions and ascorbate can compete effectively with water as electron donors to Photosystem II, in untreated chloroplasts.