The metabolism of the organic acids of tobacco leaves. VII. Effect of culture of excised leaves in solutions of (+)-tartrate.

Oxalic acid has long been regarded as an end-product of oxidative metabolic reactions in leaves. It is widely distributed in plants although the relative quantity present is not commonly great; nevertheless, in certain genera characterized by highly acid saps, such as Oxalis, Rumex, Begonia, and Rheum, oxalic acid may be the organic acid component present in greatest proportion. In tobacco, a species characterized by a high relative proportion of malic acid in the leaves, oxalic acid usually accounts for about 12 per cent of the total organic acids and for about 2 per cent of the organic solids; in the leaves employed for the present experiments, these figures were, respectively, 9.7 and 1.8 per cent. When tobacco leaves are excised and subjected to culture in darkness with their bases in water, malic acid diminishes in concentration and citric acid increases. Evidence has been presented in earlier papers (l-3) which suggests that this is an expression of a normal series of metabolic reactions whereby approximately 2 moles of malic acid undergo chemical changes that result in the formation of 1 mole of citric acid. It has also been shown that, if organic acids are taken up by the tissues as potassium salts dissolved in the culture solution, marked effects upon the course and extent of this reaction can be demonstrated. Members of the Krebs tricarboxylic acid cycle are especially effective. Malic acid has been found to stimulate the reaction while citric acid appears to reverse it. It seemed desirable, therefore, to. examine the effect of oxalic acid, the third most important organic acid component of tobacco leaves, to see whether any influence is exerted by this substance upon the metabolic reactions of the acids of the leaf cells.