Between 1 and 5% of the 14C recovered from pumpkin leaves within 15–60min after pulse-labelling with 14CO2 was in the lipids. The specific radioactivity of the phospholipids was higher than that of the glycolipids. Phosphatidylcholine had five times the specific radioactivity of monogalactosyl diglyceride, and the specific radioactivity of neither galactolipid changed significantly between 1 and 48h after labelling. It therefore seemed unlikely that the galactose moieties of the galactolipids were involved in the transport of assimilated compounds across the chloroplast membrane. Within 60min of the application of [1-14C]acetate to the surfaces of mature, intact pumpkin leaves 70% of the recovered 14C was in the lipid fraction. Of the separated glycerolipids, phosphatidylcholine had by far the highest specific radioactivity at the shorter time-intervals, and the glycolipids again had the lowest specific radioactivities. Phosphatidylcholine was the only lipid to show a significant turnover of radiocarbon as judged by the decrease in specific radioactivity with time. From a comparison of the changes with time of the labelling of fatty acid fractions from phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol and monogalactosyl diglyceride, it is suggested that the primary site of linolenic acid biosynthesis in leaf cells is within the phosphatidylcholine molecule.
