Organic acids are abundant soluble constituents of the flesh of many ripe fruits, and are responsible for the sourness of fruits and contribute to their flavour. The most abundant organic acids in the flesh of many fruits are the Krebs cycle acids malic and citric, and the bulk of these is located in the vacuoles of the parenchyma cells that make up the flesh. During development a characteristic feature of many fruits is that the concentration (mg g-1 fresh weight, FW) of these Krebs cycle acids increases until the start of ripening and then decreases. During the ripening of some fruits such as grapes and some soft fruits the amount of malate/citrate per fruit (mg fruit-1) also decreases, and this shows that stored organic acids are dissimilated/metabolised. In contrast, in some other fruits such as strawberries and cherries, although the concentration of these compounds (mg g-1 FW) decrease there is an increase in the amount per fruit (mg-1 fruit). This arises because of the large increase in fruit fresh weight that occurs at this stage of development, which is brought about largely by the expansion of the large vacuole of the parenchyma cells of the flesh. Hence the decrease of the concentration (mg g-1 FW) of Krebs cycle acids, in some fruits, is a dilution effect. This is brought about by the import of large amounts of water into the vacuole and its subsequent expansion. During the ripening, many enzymes show a decrease in abundance when expressed on per gram FW basis. Also this decrease in enzyme abundance on per gram of FW is often just a dilution effect. There are a number of fates for the Krebs cycle acids in fruits in which they are dissimilated during ripening. In ripening grapes, a large proportion of stored malate is utilised in respiration, and this process involves its complete oxidation to CO2 by the Krebs cycle. There does not appear to be a correlation between the net dissimilation of Krebs cycle acids in the flesh of fruits during ripening and the occurrence of a respiratory climacteric. In some fruits another fate for dissimilated Krebs cycle acids is gluconeogenesis. Organic acids can also be converted to ethanol, and this occurs in both the flesh of grapes and citrus. A number of factors can affect the content of these Krebs cycle acids in fruits during their development and ripening. These include ambient temperature, light, cultivar, rootstock, mineral nutrition, water availability and fruit load (source-sink relationships). However, how these factors interact with the fruit metabolism to determine the content of these compounds is in most cases uncertain. In the flesh of fruits, the Krebs cycle acids, being at the core of central metabolism, have an important metabolic role, and in many they are also likely to play a role in making the fruits unpalatable until their seeds have developed. Moreover, they also appear to be involved in a pH regulatory mechanism in fruit that plays a key role in the avoidance of pH perturbations brought about by a high internal concentration of CO2.
Keywords: malate, citrate, malic enzyme, phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate carboxykinase (PEPCK)