## 3 Results and Analysis
### Effect of OIINS Irrigation on Characteristic Aroma Compounds of Tomato Fruit  
Table 4 presents the characteristic aromatic compounds, with their odor descriptions and relative content, that have been suggested to be important tomato aroma contributors [12; 29; 30]. Nine characteristic aromatic compounds were identified. Among these, cis-3-hexenal, trans-2-hexenal, trans-2-heptenal, methyl salicylate, 1-penten-3-one, and 3-methyl-1-butanol were detected in all treatments. Hexanal was only detected in the NNNN and NNNO treatments; 3-methylbutanol in the ONNN, NNNO, and NNNO treatments; and 2-isobutylthiazolo only in the ONNN treatment. In the NNNNN- and OIINS-related treatments, the number of characteristic effect compounds was seven, eight, six, seven, and eight, and the relative content was 77.52%, 79.59%, 77.96%, 81.49%, and 82.52%, respectively. The OIINS-related treatments increased the total relative content of these compounds. cis-3-hexenal, trans-2-hexenal, trans-2-heptenal, and hexanal have green odors. cis-3-hexenal, which is associated with bitterness [7], had significantly higher content in the OIINS-related treatments (51.49%, 43.82%, 49.49%, and 29.1%) than in the NNNNN treatment (20.99%). The content of trans-2-hexenal significantly decreased in the ONNN treatment, whereas hexanal, with a sweet taste [20], had significantly lower content in the NNNN treatment than in the NNNN treatment. Methyl salicylate has a wintergreen odor, with the highest and lowest content in the NNNO and NNNON treatments, respectively. Furthermore, 1-penten-3-one and 6-methyl-5-hepten-2-one have fruits and floral odors. Moreover, 6-methyl-5-hepten-2-one has been linked to the tomato flavor, overall satisfaction, and taste decay. Meanwhile, 3-methyl-1-butanol has an earthy and musty odor. The relative content of these three compounds showed significant variations across the different treatments. Therefore, OIINS irrigation affected the type and relative content of the characteristic aroma compounds, with variations depending on the stage of fruit development. Figure 4: Relative content of chemical classes of tomato fruit volatile compounds under different treatments. Effect of OIINS Irrigation on Soluble Solids, Soluble Sugars, and Titratable Acidity of Tomato Fruits  
The sugar and acid content of tomato fruits, closely related to their sweetness and acidity, are common indicators of the fruit flavor quality. As shown in Figure 5, the soluble sugar content of the tomato fruits was reduced by OIINS irrigation. Furthermore, the NNON and NNNO treatments reached a significant level, with no clear differences. The ONNN treatment had the lowest organic acid content, followed by the NONN treatment, both reaching a significant level, with no significant differences among the other three treatments (\(p<0.05\)). The soluble solid content was ranked as NNNN \(>\) NNNON \(>\) NNNO \(>\) NONN \(>\) ONNN, with a significant difference observed between the NNNNN- and OIINS-related treatments. The fruit sugar-acid ratio was significantly higher in the ONNN treatment than in the other treatments, followed by the NONN treatment. The NNNO and NNNO treatments had significantly lower values than the NNNN treatment, with no significant differences.