# Profiling Taste and Aroma Compound Metabolism during Apricot Fruit Development and Ripening
## 3 Discussion
No significant changes in NI and AI enzymes were observed during the process, which showed that invertases is not important for sugar accumulation in apricot fruit, which is similar to the results found in apple [24], but different from peach in which NI is a clear candidate for mediating several aspects of sugar metabolism [8]. Even trace sorbitol was detected in apricot throughout the development and ripening period. A significant increase in glucose and fructose were found, which corresponded with the increase in SO and SDH enzymes activities. Therefore, it is possible that sorbitol is converted to glucose and fructose via the two enzymes, which suggests that the accumulation of these sugars mainly comes from sorbitol catalysis. However, no significant increases in GK and FK enzymes activities were observed in apricot fruit, which suggested that little glucose and fructose were converted into other sugars. These results and the data shown in the present work suggest that SS, SO and SDH are under tight developmental control and that they might play important roles in sugar accumulation in apricot fruit. Though there were no significant differences found between peels and pulps, as the sweetest sugar, fructose ratio was significantly higher in pulps than that in peels, which was consistent with that fact that the sweetness taste was stronger in pulps than that in peels. At the same time, the tissue difference may be due to high SDH enzyme activity. For cultivars, the sucrose content in DX, HY and BX were significantly higher than that in YC and LT, but the glucose content was significantly lower. On the whole, LT presented significantly higher fructose, glucose, sucrose and total sugars than other cultivars, which supports the evidence that LT is sweeter than other cultivars, and its high sugar content accounts for the high SPS, SSca and SDH enzymes activities. In this study, almost all identified organic acids increased at first and then fell throughout fruit development and ripening process. Quinate, malate and citrate were the most abundant organic acids, similar to peach fruit [25]. At early stages, quinate and malate are the major organic acids, they occupied more than 85.18% of total organic acid, whereas the ratio was changed with the rapid increase of citrate at the maturation stage. The quinate, malate and citrate become the major organic acids at the ripening stage, these three organic acids accounted for over 95% of total organic acid. As the major organic acids in apricot, citrate, quinate and malate increased at fruit development stage and then fell slightly at the ripening stage, which is similar with the trend that found in peach maturation stage [20]. Quinate and malate declined at fruit maturation, however, the citrate decreased at the ripening stage, which resulted in high ratio of citrate to total organic acid. Citric and malic acids were found to be the major organic acids in apple. A large decrease in citric acid and a small reduction in malic acid were responsible for the loss of acidity [22], which lead to the result that malic acid is the main organic acid in mature apple. For citrus, quinic acid was the major organic acid whatever the fruit variety during the first 50 days of development, afterward, citric acid predominated in acidic varieties, while in acidless, malic acid exceeded it [26]. The major organic acids found in date plum fruit were citric and malic acids, which increased through the immature stages to maturity, and then decreased in the overripe fruit [27]. QH was the last enzyme responsible for quinate synthesis, the significant decrease of enzyme activity and quinate were concomitantly observed, which suggested that QH was the key enzyme for quinate accumulation. During fruit development, apricot undergoes a continuous accumulation of organic acids, which are used as respiratory substrates [28]. MS is the key enzyme for malate formation, while NADP-ME and NAD-ME are two key enzymes for malate oxidation by mitochondria [5].