## 3 Results and Discussion
### Effect of Nitrogen Fertilizer Reduction on Volatile Metabolomics of Suijing 18
#### 3.2.4. Differential Volatile Metabolite Screening  
The reduced application of nitrogen fertilizer induced changes in 16 volatile metabolites in Suijing 18, including 3,4-dimethylcycloocta-1,5-diene, 4,6,8-trimethyl-1-nonene, (z)-3,7-dimethylocta-2,6-dienal, 3,4-dimethyl-1,2-cyclopentaned, 2-cyclopentaneethanol, ethyl 3-(2-furyl)propionate, hexyl acetate, and allyl benzoate (Table 4). The 16 key compounds were selected on the basis of VIP values >1 derived from the OPLS-DA analysis, which indicated that these compounds contribute significantly to differentiation between treatments. The differences in the contents of the 16 aroma components with VIP > 1 in A1, A2, and A3 were examined in the different treatments of Suijing 18. Significant differences were found in the contents of (z)-3,7-dimethylocta-2,6-dienal, 1-methoxycyclohexene, allyl benzoate, and other compounds in the different treatments (Figure 5). Among them, (z)-3,7-dimethylocta-2,6-dienal had a sweet and distinct lemon aroma, 1-methoxycyclohexene had a grassy and spicy aroma, and allyl benzoate had sweet, floral, and cherry flavors. Nitrogen reduction may alter sugar and ester metabolism, leading to increased sweetness and floral aromas [33]. Therefore, with a decrease in the amount of nitrogen fertilizer applied, the faint sweet flavor of Suijing 18 became more prominent.  
Figure 4. Effect of reduced nitrogen fertilizer on the metabolic variation of Suijing 18: (**a**) OPLS-DA score plot; (**b**) permutation testing of OPLS-DA in A1 vs. A2 vs. A3 group.  
Figure 5: Plot of VIP values of differential volatile metabolites from different nitrogen fertilizer treatment groups.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Class**} & \multirow{2}{*}{**Index**} & \multirow{2}{*}{**Compounds**} & \multirow{2}{*}{**VIP**} & \multirow{2}{*}{**_p-Value_**} & \multicolumn{3}{c}{**FC**} \\ \cline{5-8}  & & & & **A2 vs. A1** & **A3 vs. A1** & **A3 vs. A2** \\ \hline Hydrocarbons & XMM0771 & \
\begin{tabular}{c} 3,4-Dimethylcycloocta-1,5- \\ diene \\ \end{tabular} & 1.45 & 0.032 & 2.78 & 2.42 & 0.87 \\ \hline Hydrocarbons & XMM0121 & 4,6,8-Timethyl-1-nonene & 1.80 & 0.018 & 1.03 & 1.24 & 1.20 \\ \hline Terpenoids & KMW0432 & (z)-3,7-Dimethylocta-2,6-dienal & 1.98 & 0.008 & 1.14 & 1.43 & 1.26 \\ \hline Terpenoids & KMW0420 & Trans-piperitol & 1.11 & 0.049 & 1.39 & 1.23 & 0.89 \\ \hline Ketone & KMW0260 &
\begin{tabular}{c} 3,4-Dimethyl-1,2- \\ cyclopentaned \\ \end{tabular} & 1.08 & 0.034 & 0.97 & 1.04 & 1.08 \\ \hline Alcohol & WMW0103 & 2-Cyclopentaneethanol & 1.21 & 0.006 & 1.41 & 1.23 & 0.87 \\ \hline Heterocyclic compound & D81 & Ethyl3-(2-furyl)propionate & 1.49 & 0.034 & 1.02 & 0.87 & 0.85 \\ \hline Others & XMM1440 & Methacrylic anhydride & 1.02 & 0.044 & 1.48 & 1.24 & 0.84 \\ \hline Ester & KMW0204 & Hexyl acetate & 1.07 & 0.021 & 1.83 & 1.41 & 0.77 \\ \hline Aromatics & WMW0227 & 1,4-Dichlorobenzene & 1.87 & 0.014 & 1.04 & 1.24 & 1.19 \\ \hline Aldehyde & D173 & 4-Methyl-benzeneacetaldehyde & 1.15 & 0.048 & 1.27 & 1.16 & 0.92 \\ \hline Ester & XMM0776 & Allyl benzoate & 1.91 & 0.021 & 1.16 & 1.26 & 1.08 \\ \hline Ketone & XMM0039 & 3-Methylcycloheptanone & 1.73 & 0.041 & 1.02 & 1.15 & 1.12 \\ \hline Aromatics & KMW0205 & 1-Methoxycyclohex & 1.96 & 0.020 & 1.13 & 1.26 & 1.11 \\ \hline Aromatics & w08\(\times\)082 & 1-Methyllnphthalene & 1.39 & 0.031 & 1.02 & 0.92 & 0.91 \\ \hline Aromatics & XMM0662\(\times\)082 & 2-Methyllnphthalene & 1.39 & 0.031 & 1.02 & 0.92 & 0.91 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Differences in volatile metabolites between the three treatments.