[Quantum yield and variable behavior of the two photosystems of the photosynthetic apparatus during the life cycle of Scenedesmus obliquus in synchronous cultures].

Cultures of Scenedesmus obliquus strain D3 synchronized in a light-dark regime of 14:10 hours were used for studies of the photosynthetic activity during the life cycle of the organism.The quantum yield measured throughout the life cycle follows closely the photosynthetic capacity, with a maximum at the 8th and a minimum at the 16th hour in cells whose life cycle was initiated with the beginning of the light period.The amount of photosystem II activity (measured as p-benzoquinone Hill-reaction) and the action spectra demonstrate the same decrease from the 8th to the 16th hour as quantum yield does. However, the reaction of photosystem I seems to be the same throughout the life cycle, when measured as photoreduction or as a part of the light induced absorption change at 520 nm.With support from the observation that the Emerson-enhancement effect is highest at the 8th and lowest at the 16th hour of the life cycle, it was concluded that the two photosystems work with highest activity and in pace at the 8th hour. At the 16th hour the activity of photosystem I remains constant. Since cyclic photophosphorylation is higher at the 16th than at the 8th hour, it is suggested that the capacity of photosystem I, which is freed from photosystem II by a decreased electron flow, is used for additional cyclic photophosphorylation.