Effect of Light upon the Behaviour of Citric Acid in Leaves of Bryophyllum Calycinum Salisb.

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Abstract
An examination of the effect upon the citric acid content of leaves of B. calycinum, picked at daybreak, of exposure to sunlight and to artificial light under various conditions has shown that citric acid may at first increase in concentration although subsequently it decreases. The diurnal behavior of citric acid is thus not identical with that of malic acid. It is suggested that citric acid is formed in this species by reactions to which malic acid makes a major contribution, and that these reactions are, at least to some extent, reversible, the direction of the reactions being controlled to a large extent by the relative proportions of the 2 substances present. The concentration of citric acid is thus responsive to the diurnal rise and fall of the concentration of malic acid. This view of the situation accounts not only for the diurnal fluctuations of citric acid, but also for the temporary increase of citric acid shortly after daybreak while the relative concentration of malic acid is high. An explanation of the accumulation of isocitric acid in high concentrations in Crassulacean plants remains to be obtained.