Ether-Soluble Organic Acids of Mature Valencia Orange Leaves

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Abstract
The total organic acids were extracted from mature Valencia orange leaves with absolute ether and the water soln. of the extract was subsequently analyzed for total acid and for citric, malic, and oxalic acids. The undetd. acid fraction in the ether extract amounted to a mean of 17.76% of the total acids, indicating the presence of one or more unknown organic acids in the leaves. The mean cones. of citric, malic, and oxalic acids were 12.28, 30.57, and 39.38%, respectively, of the total acids. An analysis of the water extract of the leaf samples showed that while most of the citric and malic acids were water-soluble, most of the oxalic acid was insoluble in the water extract. Some of the Ca and all the K proved to be water-soluble. More than half of the total Ca is combined in the leaves in an insoluble form. The insoluble oxalates are correlated with the insoluble Ca. The increase in insoluble oxalates with the growth and age of the leaves is similar to the increase in total Ca. If the sum of water-soluble Ca and that combined with the oxalic acid is subtracted from the total Ca, there remains a portion of insoluble Ca to combine with other substances (Ca pectate, etc.) of the leaves. The sum of the water-soluble acids and the total oxalic acid is nearly equal to the total acids of the leaves. The water-soluble organic acids and the soluble cations (Ca and K) form the free acid-salt relationships that contribute to the buffer capacity of the leaf sap. The leaves contained nearly 3 times as much malic as citric acid, and the juice of the pulp contained 8 times as much citric as malic acid. To reverse this ratio during transfer from leaves to vesicles of the pulp, it is postulated that a conversion of malic acid to citric must take place, or citric acid must be synthesized in the juice vesicles. The possible synthesis of the organic acids in the vesicles of citrus fruits is discussed.