## 2 Results and Discussion
### Changes of C6 and C9 Aldehyde Content during N-Deficient Treatment  
C6 and C9 aldehydes are known to contribute to the flavor and aroma of plants and resistance to pathogens and are powerful inducers of abiotic stress-related genes [8,10,23]. C6 aldehydes provide a more \"green grass\" flavor, while C9 aldehydes enhance the unique \"cucumber-like\" flavor [22]. In this study, C6 and C9 aldehydes constitute the majority of the cucumber leaf volatiles (Figure 3.); three C6 aldehydes were detected in cucumber leaves including hexanal, (_E_)-2-hexanal, and (_E_,_E_)-2,4-hexadienal. The proportion of these C6 compounds to total volatiles accounted for 77.96% to 91.11% in N-sufficient treatments and 86.05% to 94.94% in N-deficient treatments. Most of the relative composition of C6 aldehydes in N-deficient treatments is higher than those that were N-deficient, except on 7 day. The same trends of the C6 aldehyde ratio \"low-high-low-high-low\" pattern were observed during N-sufficient and N-deficient treatments; this might be caused by difference of environment. Of the three C6 aldehydes, (_E_)-2-hexanal had the highest relative proportion, which accounted for 69.05% to 84.05%. Six C9 aldehydes were detected in cucumber leaves, including (_E_,_Z_)-2,6-nonadienal, (_E_,_E_)-2,6-nonadienal, (_Z_,_Z_)-3,6-nonadienal, (_Z_)-6-nonanal, nonanal, and (_E_)-2-nonenal. The relative proportion of C9 aldehydes accounted for 4.04% to 15.87% during N-sufficient treatments and for 2.84% to 8.42% during N-deficient treatments. Among the six C9 aldehydes, (_E_,_Z_)-2,6-nonadienal had the highest proportion and accounted for 2.06% to 12.06% in all sample points. At four days after N-deficient treatments, the C9 relative composition was higher than cultivated in N-sufficient treatments, while the other six sampling points displayed lower values than controls. During  
Figure 3: Relative composition (%) of volatiles from 9930 leaves of N-sufficient and deficient treatments. N+ and N\(-\) mean nitrogen sufficient and nitrogen deficient treatments, respectively. Values are means of three replicates. N-deficient treatments, the relative composition of C6 and C9 aldehydes were varied, the relative composition of C6 aldehydes was improved, and the amount of C9 aldehydes was reduced for most sample points. Compared to N-sufficient treatments, content of (_E,E_)-2,_A_-hexadienal displayed significant improvement at 2 and 3 day, hexanal and (_E_)-2-hexanal were also significantly improved at 5 day (Figure 4). The amount of total C6 aldehydes ranged from 1696.69 to 3679.16 ng/g FW during the N-sufficient treatments and from 358.87 to 5255.86 ng/g FW during N-deficient treatments. The amount of (_E_)-2-hexanal reached a maximum value of 4989.15 ng/g FW on Day 5. During N-deficient treatments, the amount of (_E_)-2-hexanal and hexanal displayed a decrease compared to N-sufficient treatments, with the exception of Day 5, on which a significant increase was displayed. The content of (_E,E_)-2,4-hexadienal displayed a different pattern during N-deficient treatment, increasing from Day 1 to Day 3 and decreasing from Day 4 to Day 7. During N-deficient treatments, the amount of C9 aldehydes also displayed significant changes compared to the control. As shown in Figure 5, the total amount of C9 aldehydes in N-deficient treatments was significantly reduced at Days 1, 3, 6, and 7. During the N-sufficient and N-deficient treatments, the highest amounts of C9 aldehydes were exhibited at Days 3 and 4 with 345.11 and 175.68 ng/g FW, respectively. (_E,Z_)-2,6-nonadienal was the main component of C9 aldehydes, accounting for 70.13% to 88.74% of the total C9 aldehydes. A higher ratio of (_E,Z_)-2,6-nonadienal between N-sufficient and N-deficient treatments was observed on Days 4 and 5, but no significant difference was observed.