Distribution of Metabolites between Chloroplast and Cytoplasm during the Induction Phase of Photosynthesis in Leaf Protoplasts

Abstract

A method for rapid separation of the chloroplast and cytoplasmic fractions from isolated leaf protoplasts of wheat (Triticum aestivum cv. Sappo) and spinach (Spinacia oleracea cv. United States Hybrid 424) was used to determine the distribution of 14C-labeled products during photosynthesis. In the dark, CO2 fixation was only 1-2% of that in the light and the products were mainly in the cytoplasmic fraction suggesting fixation by phosphoenolpyruvate carboxylase. Label appeared rapidly in the chloroplast fraction following illumination, but the amount leveled off after 4-5 min reflecting the buildup of intermediates to steady state levels. There was only a slight lag before label appeared in the cytoplasmic fraction and it continued to increase at a constant rate reflecting synthesis of neutral products. In the light, the percentage of label in the chloroplast fraction decreased rapidly in the 1st minute of illumination and was only 10-20% in the steady-state. The chloroplast phosphate transporter apparently promotes a rapid transfer of sugar phosphates from the chloroplast to the cytoplasm, even during the induction phase of photosynthesis.