## 2 Results and Discussion
### Relationship between C6 and C9 Aldehydes and Gene Expressions in the 9930 Line
The color scale for fold-change values is shown on the right. Values of gene expression folds are means of three biological replicates with three technical replicates. Values of short-chain volatiles are means of three biological replicates. Figure 7: Changes in content of short-chain volatiles in six inbred cucumber leaves at 5 day after N-deficient treatments. Bars present means \(\pm\) standard deviations. + and \(-\) mean N-sufficient and deficient treatments, respectively. ** mean significant difference from control at \(p\) < 0.01. Values are means of three biological replicates. (RSLVs) such as (_E_)-2-hexenal can act as signal chemicals that strongly induce the gene expression of abiotic-related transcription factors. (_E_)-2-hexenal could up-regulate genes that respond to salt, oxidation, osmotic drought, cold, and wounding [21]. In the present study, RSLVs might involve in the induction of the expression of down-stream transcription factors during nitrogen starvation. The Expression Patterns of Cucumber LOX Genes in Response to N-Deficient Treatments in Six Inbred Cucumber Lines  
LOX proteins and their metabolites have been demonstrated to involve in production of short-chain volatile compounds [11] and plant defense responses during various stresses [24; 25]. To study the effect of nitrogen deficiency on the biosynthesis of volatiles in cucumber seedlings, _LOX_ gene expression profiles in cucumber leaves were analyzed by qPCR. Of the 12 _LOX_ genes, six _CsLOXs_ (_CsLOX1, 2, 4, 8, 9,_ and _10_) were predicted to be type-1 _LOXs_, which are involved in the biosynthesis of C9 aldehydes. The other six _LOXs_ (_CsLOX16, 17, 19, 20, 22,_ and _23_) were predicted to be type-2, which are involved in the biosynthesis of C6 aldehydes [23]. In our study, compared to N-sufficient treatments in six inbred cucumber lines, 157 of 276 sample points of _CsLOXs_ expressed higher levels during N-deficient treatments (Table S2), including 68 type-1 _CsLOXs_ and 89 type-2 _CsLOXs_ sample points. Two types of _CsLOXs_ displayed a clear difference in response to N-deficiency treatments, and type-2 _CsLOXs_ had more up-regulated expression patterns. Of the six inbred lines, different expression patterns of _CsLOXs_ were observed during N-deficient treatments. For A23, _CsLOX9_ had higher up-regulated patterns; for A74 and A91, _CsLOX19, 20_, and _22_ displayed higher up-regulation. For A38, _CsLOX10_ displayed up-regulation (Table S2). HCA was employed to explore potential correlations among 12 _CsLOXs_, and, to stratify samples based on trait, [PERSON]'s linkages in Euclidian distance were used. HCA data resulted in two distinct sample clusters (Figure 8). Cluster I included 6 _CsLOXs_ (_CsLOX8, 10, 19, 20,_ 22,_ and _23_) which displayed more up-regulated expression patterns during N-deficient treatments. The six other _CsLOXs_ belonged to Cluster II and displayed more down-regulated patterns. Figure 8: Hierarchical cluster dendrogram and heatmap of _CsLOXs_ expression folds in six inbred cucumber leaves during N-deficient treatments. The heat map has been generated based on the fold-change values of _CsLOXs_ expression level in the treated samples when compared with its unstressed control samples. The color scale for fold-change values is shown at the right. Values are means of three biological replicates with three technical replicates. A previous study showed that _LOXs_ were involved in the response to abiotic stress and plant growth regulators [11]. Our results from the expression of _CsLOXs_ demonstrated that type-1 and type-2 _CsLOXs_ had different response patterns to short term N-deficiency. In most sample points during N-deficient treatments, type-1 _CsLOXs_ were down-regulated and type-2 _CsLOXs_ were up-regulated.