Mechanism of the light-dependent accumulation of starch in chloroplasts of Acetabularia, and its regulation.

Abstract

The decrease of photosynthetic activity under red light conditions which is not caused by a photosynthetically inactive spectral region can be correlated to the amount of thylakoids in the chloroplasts. The increase of photosynthetic activity under blue light conditions during the induction period after a lag-phase of about 8 hours is correlated to a membranede novo synthesis in the chloroplasts. In parallel to this behaviour a decrease of starch accumulated under red light conditions has been observed. Starch accumulation during the red light treatment is a consequence of reduced cytoplasmic activities which at least are governed by nucleus activity. Investigations on nuclei of red and blue light treated cells have shown that the structure of nucleoli from cells kept under red light conditions is altered. This behaviour indicates a reduced transcription activity of the nucleus. 30 days pre-dark treated cells which were subsequently transferred into red light for 21 days showed a higher content of starch (about 44%) as compared to the red light control cells. Light induced changes in the permeability of the chloroplast membranes are therefore excluded. The amount of starch in enucleated blue light cells in comparison with pre-dark red light treated cells favours the assumption of a nucleus-chloroplast interrelationship on the level of “dissimilation”. Accumulation of photosynthesis products under conditions of reduced or blocked nuclear activity coincides with this assumption. Dissimilation in this sense is discussed. Possible feed-back reactions between starch synthesis and starch accumulation are discussed on the level of a reduced volume activity of UDPG-pyrophosphorylase under red light conditions.