Betaine Synthesis from Radioactive Precursors in Attached, Water-stressed Barley Leaves

In wilted barley leaves, betaine accumulates at about 200 nanomoles per 10 centimeters leaf per day. Results with (14)C-labeled precursors were qualitatively and quantitatively consistent with de novo synthesis of this betaine from serine via ethanolamine, choline, and betaine aldehyde and indicated that water stress may increase the activities of all steps in this pathway except the last.Doses (1 micromole) of each (14)C-labeled precursor were supplied as droplets to the tips of attached, 10-centimeter, second-leaf blades of turgid and wilted plants, and the incorporation of (14)C into betaine was followed. From the rates of betaine labeling, estimates were made of the potential capacities (nanomoles per 10 centimeters leaf per day) for the methylation and oxidation steps. Labeling of betaine from [(14)C]choline, [(14)C]ethanolamine, and [(14)C]serine was about 7- to 10-fold greater in leaves wilted for 2 days than in turgid leaves, whereas label from [(14)C]betaine aldehyde appeared in betaine at about the same rate in both turgid and wilted leaves. In leaves wilted for 2 days, the potential capacities for converting [(14)C]ethanolamine, [(14)C]choline, and [(14)C]betaine aldehyde to betaine all approached or exceeded the rate of betaine accumulation (about 200 nanomoles per 10 centimeters leaf per day); in turgid leaves, only the potential for converting betaine aldehyde to betaine exceeded this rate. The rate of conversion of [(14)C]ethanolamine to betaine increased 4-fold after 6 to 10 hours of wilting, which was soon enough to account for the onset of betaine accumulation.