## 4 Discussion  
More than 400 volatile compounds have been identified in tomatoes and categorized into aldehydes, ketones, alcohols, esters, and hydrocarbons based on their chemical structures [11,29]. In this study, 66 volatile compounds were detected in the tomato fruits across the five treatments, of which 22 were common volatile compounds, accounting for 53.66-64.71% in quantity and 69.85-88.56% in relative content. Irrigation with OIINS increased the content of common volatile compounds, aldehydes, and characteristic aroma compounds. This indicates that the genotype plays a key role in determining the volatile compound composition in tomato fruits, and OIINS irrigation significantly impacts the types and content of volatile compounds. Previous studies have shown that organic fertilizers are more effective than chemical fertilizers in enhancing the variety and quantity of volatile compounds in fruits [16,19,20,32]. In this study, the quantity of volatile compounds and aldehydes decreased significantly under the OIINS-related treatments, in contrast to the results of previous studies. This difference may be attributed to the type of organic fertilizer, application method, nutrient characteristics, and crop residue. Tomato fruit development can be classified into two stages: early and mature. The early fruit development stage includes the flowering-fruit setting and fruit expanding stages, characterized by cell division and enlargement, which determine the fruit size and structure, whereas the maturity stage involves fruit color turning and the formation of metabolic products, which play a key role in quality development [33]. In this experiment, under the ONNN, NONN, and NNON treatments with OIINS irrigation, the second spike occurred in the early fruit development stage, whereas, under the NNNO treatment with OIINS irrigation, they were in the fruit maturity stage. During the early fruit developmental stage with OIINS irrigation, there was higher accumulation of trans-3-hexenal in the fruit, whereas hexanal was not detected. The synthesis of methyl salicate and anti-2-hexenal (ONNN) was affected. At the fruit maturity stage with OIINS irrigation, there was higher accumulation of anti-2-hexenal and methyl salicate in the fruits, with a slight increase and decrease in the relative content of trans-3-hexenal and hexanal, respectively. The total relative content of the fruit characteristic aroma compounds was in the following order: NNNO > NNONN > ONNN > NONNN > NNNNN. This suggests that OIINS irrigation has a stage-specific impact on the volatile compounds in fruits. Irrigation during the early fruit development stage significantly influenced the trans-3-hexenal and hexanal levels, whereas irrigation during the fruit maturity stage promoted the accumulation of characteristic aromatic compounds in the tomato fruits. Trans-3-hexenal and hexanal are key volatile compounds in tomato fruits, formed from linoleic acid and linolenic acid precursors, respectively, and catalyzed by lipoxygenase C (TomloxC) and 13-hydroperoxide lyase (13-HPL) [31,34]. Their synthesis initiates post-cell division. Linoleic acid is the primary substrate for LOX enzyme activity in tomato fruits, followed by linolenic acid [35]. Figure 5: Soluble sugars, titratable acidity, soluble solids, and sugar–acid ratio in tomato fruits supplied with different nutrient solutions. (**A**) Soluble sugars; (**B**) titratable acidity; (**C**) soluble solids; (**D**) sugar–acid ratio. Vertical bars represent the standard error of the mean (\(n\) = 3). Different letters indicate significant differences between the treatments (Duncan’s test, \(p\) < 0.05). [PERSON] et al. [20] suggested that organic liquid fertilizers might affect the content of volatile compounds in tomato fruits by regulating the substrate concentration and enzyme activity. In this study, the relative content of trans-3-hexenal in the ONNN, NONN, and NNNO treatments was more than double that in the NNNNN treatment, with no detection of hexanal.