The Source of Carbon for Proline Synthesis in Osmotically Stressed Artichoke Tuber Slices

When thin slices of tubers of Helianthus tuberosus L. are plasmolysed in mannitol solutions, the initially large pool of free arginine decreases within 24 h and then, during the next 72 h, a several- fold increase in the concentration of free proline occurs. Radioactivity from DL-ornithine, L-arginine and L-glutamate was shown to label proline in plasmolysed discs of tubers. The percentage conversion to proline of each of the precursors was 20 to 50 times greater in discs plasmolysed for 24 h than in freshly plasmolysed discs. By comparing the transfer of radioactivity from L-[U-14C]arginine and L-[5-3H]arginine to proline, metabolism of arginine directly to proline without passage through a glutamate pool was established. It is concluded from this and other evidence that pathways from arginine and glutamate to proline operate independently. Calculations based on the specific activity of precursors, the pool sizes of free arginine and glutamate, and the transfer rates of radioactivity to proline indicate that arginine is quantitatively the more important precursor for proline synthesis in this tissue.