## 3 Results
### Correlation Analysis  
To systematically explore the potential synergistic relationships between AAs and volatile aroma compounds, a correlation heatmap was constructed. This heatmap included 11 aroma compounds identified with VIP scores greater than 1, and 21 AAs, as depicted in Figure 8A. This visual representation facilitated an understanding of the interrelationships among these compounds. The analysis revealed significant correlations between specific metabolites. For instance, disioamyl demonstrated an extremely significant positive correlation with Cit (\(p<0.001\)). Similarly, benzaldehyde was significantly positively correlated with Leu (\(p<0.01\)) and showed positive correlations with Phe, Lys, and His (\(p<0.05\)). Nonadecane exhibited positive correlations with Gln and Asn (\(p<0.05\)). Ethyl benzoate displayed positive correlations with both Phe and Leu (\(p<0.05\)). Additionally, ethyl palmitate was positively correlated with Asn (\(p<0.05\)) but negatively correlated with Lys and Pro (\(p<0.05\)). Conversely, (E)-3-hexen-1-ol showed a significant negative correlation with Asp (\(p<0.01\)). These findings imply that certain AAs may act either as substrates or as regulatory factors in the biosynthesis of characteristic aroma compounds, thus influencing the formation of distinct aroma profiles. Figure 7: Relative contents of the amino acids and OPLS-DA analysis in 18 crabapple cultivars. Numbers 1–18 represent the following cultivars: 1—Tamlin’, 2—Purple Classic’, 3—Harvest Gold’, 4—Flame’, 5—Kelsey’, 6—Tang-2’, 7—Donald Wyman’, 8—Prairifire’, 9—‘Sparkler’, 10—Royal Raindrops’, 11—Radiant’, 12—Purple Prince’, 13—Indian Magic’, 14—Profusion’, 15—Red Jewel’, 16—’Spring Snow’, 17—’Adams’, 18—Royalty’. (**A**) Relative contents of the amino acids in the 18 crabapple cultivars. (**B**) Proponents of the 21 amino acids in the 18 crabapple cultivars. (**C**) OPLS-DA score plot based on the relative amino acid contents of the 18 crabapple cultivars. Red, yellow, and blue represent the fruit-almond, fruit-sweet, and mixed aroma types, respectively. (**D**) Model cross-validation results. The differences among the three aroma types--fruit-almond, fruit-sweet, and mixed--were further examined through the integration of AA and aroma compound profiles using OPLS-DA-based multivariate modeling (Figure 8B). In this model, aroma compounds with VIP values greater than 1 and the relative content of various AAs served as independent variables, while aroma type was the dependent variable. The integrated model exhibited excellent fit and predictive capabilities (R\({}^{2}\)Y = 0.942, Q\({}^{2}\) = 0.732). The OPLS-DA score plot demonstrated a clear separation of the three aroma types within the principal component space, illustrating classification boundaries that were notably superior to those of single-variable models. The stability of the model was further confirmed by 200 permutation tests (Figure 8C), which indicated that the Q\({}^{2}\) regression line of the permutation model intersected the vertical axis at a negative value, thereby confirming the absence of overfitting and validating the model's reliability for discriminating aroma types in crabapple. The incorporation of AA variables did not substantially alter the distribution boundaries among the three aroma types within the principal component space; however, it did enhance the intra-group clustering of samples. These results suggest that AAs may play a role in differentiating aroma types by enhancing metabolic stability. The findings  
Figure 8: Correlation analysis of 18 crabapple cultivars. Numbers 1–18 represent the following cultivars: 1—Tamlin’, 2—Purple Classic’, 3—Harvest Gold’, 4—Flame’, 5—Kelsey’, 6—Tang-2’, 7—Donald Wyman’, 8—Praififire’, 9—Sparkler’, 10—Royal Raindrops’, 11—Radiant’, 12—Purple Prince’, 13—Indian Magic’, 14—Profusion’, 15—Red Jewel’, 16—Spring Snow’, 17—Adams’, 18—Royalty’. (**A**) Heatmap showing the correlations between relative contents of aroma compounds and amino acids.