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Abstract
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.
Highlights
Cucumber (Cucumis sativus Linn.) is a widely consumed vegetable with a fresh and distinct flavor[1], and fruit quality is important for satisfying consumer demand [2]
Was correlated with 9-hydroperoxide lyase (9-HPL) enzyme activity. 13-hydroperoxide lyase (13-HPL) enzyme activity decreased significantly with fruit ripening in both lines, while LOX and 9-HPL activities increased significantly, which accounted for the higher C6 aldehyde content at 0–6 d dpa, and the higher C9 aldehyde content at 9–12 dpa
Two cultures grouped together within cluster 1 (Fig. 17), and the first subgroup containing 26–6 and 14–6 shared some characteristics with cluster II. These results suggest that the total volatile, C6 aldehyde, linolenic and linoleic acid content, and the (E)-2-nonenal / (E, Z)-2,6-nonadienal ratio were all higher during the stages of fruit development
Summary
Cucumber (Cucumis sativus Linn.) is a widely consumed vegetable with a fresh and distinct flavor[1], and fruit quality is important for satisfying consumer demand [2]. The typical cucumber flavor results from the enzymatic action of LOX on linolenic and linoleic acids, which introduces molecular oxygen at C13 or C9, forming 13-hydroperoxylinolenic acid (13-HPOT) or 9-hydroperoxylinolenic acid (9-HPOT). HPL cleaves 13-hydroperoxide (13-HPO) and 9HPO to produce the C6 and C9 aldehydes that are responsible for the cucumber flavor [3]. These aldehydes can be reduced to the corresponding C6 alcohols by alcohol dehydrogenase (ADH). Studies have reported that only the oxylipin metabolic pathway contributes to aldehyde and alcohol content, and flavor [4].
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