Labelling Studies in Vivo on the Metabolism of the Acyl and Glycerol Moieties of the Glycerolipids in the Developing Maize Leaf

1. When [2-3H]glycerol was supplied to developing maize-leaf laminae, label entered 3-sn-phosphatidycholine at a linear rate essentially from zero time, whereas other lipids were labelled at accelerating rates. On transfer of laminae from [3H]glycerol to unlabelled glycerol, radioactivity was rapidly lost from 3-sn-phosphatidylcholine and accumulated in other lipids, principally monogalactosyl diacyglycerol. 2. Degradation of these lipids showed that 3H was present only in the glycerol moiety of the lipids. 3. In double-labelling pulse-chase experiments with [14C]acetate, which labelled essentially only fatty acids and [3H]glycerol similar amounts of 14C and 3H radioactivity were lost from 3-sn-phosphatidylcholine and accumulated by monogalactosyl diacylglycerol. 4. The different molecular species of both lipids isolated from laminae during a double-labelled pulse-chase study were separated by argentation t.l.c., and the changes in the amount of radioactivity and the 14C/3H ratio in different species were compared. The greatest loss of radioactivity during the period in unlabelled substrates occurred from the 3-sn-phosphatidylcholine species containing oleate and from the dilinoleate species, and radioactivity accumulated by monogalactosyl diacyglycerol was mainly in the dilinolenate species. However, despite the considerable change in the radioactivity in these species during the chase, the 14C/3H ratio in each of them remained relatively unchanged. 5. It is proposed that 3-sn-phosphatidylcholine in the developing leaf may serve as a donor or linoleate-containing diacyl-glycerols which are incorporated into other lipids, principally monogalactosyl diacylglycerol.