Profiling of Volatile Compounds and Associated Gene Expression and Enzyme Activity during Fruit Development in Two Cucumber Cultivars

Changes in volatile content, as well as associated gene expression and enzyme activity in developing cucumber fruits were investigated in two Cucumis sativus L. lines (No. 26 and No. 14) that differ significantly in fruit flavor. Total volatile, six-carbon (C6) aldehyde, linolenic and linoleic acid content were higher during the early stages, whereas the nine-carbon (C9) aldehyde content was higher during the latter stages in both lines. Expression of C. sativus hydroperoxide lyase (CsHPL) mirrored 13-hydroperoxide lyase (13-HPL) enzyme activity in variety No. 26, whereas CsHPL expression was correlated with 9-hydroperoxide lyase (9-HPL) enzyme activity in cultivar No. 14. 13-HPL activity decreased significantly, while LOX (lipoxygenase) and 9-HPL activity increased along with fruit ripening in both lines, which accounted for the higher C6 and C9 aldehyde content at 0-6 day post anthesis (dpa) and 9-12 dpa, respectively. Volatile compounds from fruits at five developmental stages were analyzed by principal component analysis (PCA), and heatmaps of volatile content, gene expression and enzyme activity were constructed.