Aufnahme und Umsatz exportierter Photosyntheseprodukte durch die Chloroplasten während der Assimilation von CO2

Abstract

Chloroplasts were permitted to reduce 14 CO 2 or 14 C-labeled phosphoglycerate in the light. During a subsequent tracer-chase with 12 CO 2 14 C continued to be incorporated especially into a polyglucan (starch), glycolate, pentose monophosphates and fructose diphosphate. Label was lost mainly from dihydroxyacetone phosphate, which had previously been excreted by the chloroplasts. Ribulose diphosphate still contained a significant amount of 14 C even after prolonged photosynthesis in the presence of 12 CO 2 . The loss of label from other intermediates of the Calvin cycle was even less apparent than from ribulose diphosphate. The tracer distribution data reflect the slow export of some sugar phosphates from the chloroplasts during photosynthesis. They further show that chloroplasts are able to take up dihydroxyacetone phosphate from the medium against the flux of newly synthesized photosynthetic products and incorporate it via the carbon cycle mainly into starch. In contrast to dihydroxyacetone phosphate, little uptake and reassimilation of phosphoglycerate and of pentosemonophosphates appeared to occur during the reduction of CO 2 . The relative metabolic inactivity of external phosphoglycerate during photosynthesis is of interest in relation to current assumptions on the mechanism of photosynthesis in C 4 -plants. It affects the interpretation of tracer chase experiments which have led to the postulate that in such plants carbon must flow through dicarboxylates before it enters phosphoglycerate.