The Metabolism of the Organic Acids of Tobacco Leaves XIX. EFFECT OF CULTURE OF EXCISED LEAVES IN SOLUTIONS OF POTASSIUM GLUTAMATE*

The metabolism of glutamic acid in the animal body and in a number of microorganisms has been studied estensively in recent years, but there are few records of its behavior when administered to higher plants. Naylor and Tolbert (1) cultured seedling leaves of barley (Ziordeum vulgare) on solutions of glutamic acidCl4 for periods up to 3 hours and examined extracts of the tissues by paper chromatographic methods. Major products during culture in darkness were glutamine, malic acid, and aspartic acid. W7hen oxygen was excluded, the major product was y-aminobutyric acid, and they suggested that this substance is an important intermediate on the metabolic pathway from glutamic acid to succinic acid and other members of the tricarboxylic acid cycle. Somewhat similar experiments were carried out by Champigny (2) with cuttings of Kalunchod diagremontiana. After culture for 6 hours on solutions of glutamic acid-3,4-Cl4 in light, y-aminobutyric acid and pyruvic acid were the most highly labeled components, but all members of the tricarboxylic acid cycle as well as a few amino acids were also appreciably radioactive. After culture in darkness, malic acid became highly labeled as would be anticipated from the well known behavior of this substance in crassulacean plants. The recent work of Graham and Walker (3), who examined the metabolism of a number of radioactive organic acids, including glutamic acid, in leaves of mung bean (Phaseolus mungo) seedlings both in light and in darkness, is discussed more fully below. The only experiments in which the tobacco plant (Nicotiana tabacum) has been used appear to be those of Leete (4) and of Lamberts and Byerrum (5), who administered glutamic acid-ZCl4 through the roots to young growing plants. The nicotine subsequently isolated in both laboratories contained labeled carbon in positions 2 and 5 of the pyrrolidine ring, but less than 0.01 y0 of the radioactive carbon was so used. Wu, Griffith, and Byerrum (6) have recently emphasized the importance of glutamic acid and its semialdehyde as intermediates in the synthesis of nicotine in the tobacco plant. In view of the limited amount of information available on the behavior of glutamic acid in higher plants, it has seemed desirable to examine the effect of administering this substance to tobacco leaves. The experiments have shown that after culturing the leaves in darkness for 48 hours on a 0.2 M solution of glutamate, the glutamic acid taken up is nearly completely metabolized;