Metabolism of citric and malic acids during ripening of tomato fruit

Tomato fruit at various stages of ripeness were injected with 14 C‐labelled citric and malic acids. The 14 CO 2 evolved by individual fruit was determined over a 72 h period and the fruit then analysed for the incorporation of 14 C into sugars, amino acids and individual non‐volatile organic acids. In mature green tomato fruit citric and malic acids were found to be metabolised to a comparable extent. Appreciable interconversion occurred together with oxidation to CO 2 and some labelling of glutamic acid and glutamine. In red fruit, citric acid was not as readily oxidised to CO 2 and little conversion to malic acid was observed. On the other hand, malic acid was oxidised to CO 2 and converted to citric acid as rapidly as in green fruit with, in addition, appreciable conversion to glutamic acid. These results are discussed in relation to the changes in concentration of the endogenous acids which take place during the ripening of tomato fruit.