STUDIES IN THE METABOLISM OF CRASSULACEAN PLANTS: DIURNAL VARIATION OF ORGANIC ACIDS AND STARCH IN EXCISED LEAVES OF BRYOPHYLLUM CALYCINUM

Abstract

Excised leaves of B. calycinum cultured in water under greenhouse conditions for 3 days undergo successive alterations in organic acid and starch content in mutually opposite directions in a manner which suggests a close relationship between the metabolism of these substances. The observations thus accord with the views of Bennet-Clark. The extent and speed of the reactions appear to be influenced by the weather conditions, being appreciably greater on a bright sunny day than on a partially cloudy day. Malic acid is the organic acid which undergoes the greatest fluctuations in concn., but citric acid shares in them to a moderate extent; iso-citric acid, however, does not appear to vary. No significant alteration in organic solids from sample to sample was observed. It may be inferred that the reactions which involve exchanges of gas with the atmosphere were such that there was no detectable net change in wt. Preliminary calculations of the relative molar quantities of the substances concerned suggest that approx. one molar proportion of glucose (mostly derived from starch) is transformed to one molar proportion of malic and citric acids (malic acid the main component) during the night and that the t reverse reaction occurs during the day. It is possible that the enzymatic mechanisms involved are analogous to those descr. by the Krebs tricarboxylic acid cycle, but the direct part taken by citric acid and the failure of isocitric acid to share at all cannot easily be accounted for by the hypothesis that the Krebs cycle, in the form widely accepted as the explanation of carbohydrate oxidation in animal tissues, exactly defines the mechanism of diurnal variation of acidity in B. calycinum leaves.