## 4 Discussion
These aromatic substances are widely distributed in plants of the Rosaceae family, such as 1 ilac [44] and peny [45], and are considered key aroma compounds that contribute to sweet and almond-like notes. This observation provides valuable insights into the comparative study of aroma evolution across different plant families and genera. The analysis revealed that certain cultivars exhibited unique olfactory traits. For instance, 'Purple Classic' was characterized by a laverder-like scent, largely due to the dominance of geraniol, whose biosynthetic pathway might be closely associated with the phylogenetic conservation of the species [28]. Building on these findings, this study developed an aroma classification system based on the sensory attributes of VOCs. The 25 core VOCs were categorized into six aroma groups: floral, almond-fruity, sweet-fruity, fatty, woody, and spicy. A weighted aroma model was constructed using contributions from OAVs as weighting factors. Ultimately, the 18 cultivars were classified into three main aroma types: almond-fruity, sweet-fruity-floral, and multi-component blended. During the process of secondary metabolism in plants, AAs function as crucial precursor molecules [46]. They are transformed into various volatile aromatic compounds, including aldehydes, alcohols, and esters, through reactions such as decarboxylation, deamination, and transamination. The pivotal role of AAs in the biosynthesis of flavor compounds is well-documented in food chemistry research [47]. In this study, we employed UPLC-MS/MS to systematically quantify free AAs in the flowers of 18 crabapple cultivars. Our findings revealed a pronounced enrichment of Glu, Asn, Asp, Ser, and Ala, which collectively accounted for 83.6% of the total AA content. These AAs are engaged in key physiological processes such as nitrogen transport, carbon flux regulation, and precursor supply, which are essential for the metabolic underpinnings of floral scent production [48]. Flowers, being metabolically active tissues with robust glycolytic activity, are likely to enhance the synthesis of specific AAs [49]. Notably, Glu serves as the primary donor of amino groups in plants, participating in various transamination processes and the regulation of nitrogen balance [50]. Both Asn and Asp not only act as forms of nitrogen storage and transport but also serve as precursors for aromatic AAs like Phe and Tyr, which are integral to the phenylpropanol metabolic pathways [51]. Additionally, Ser, formed from glycolytic intermediates, can be converted into Cys and participate in the synthesis of thiol-based aroma compounds [52]. The consistent detection of the sulfur-containing compound timonacic across all samples suggests that its biosynthesis may be intimately linked to Ser enrichment in sulfur metabolism. A correlation analysis between VOCs and AAs revealed significant positive correlations between Phe and benzaldehyde, Leu and ethyl benzoate, and Cit with fatty acid-derived VOCs. This analysis suggests that certain AAs may influence the differential accumulation of characteristic aroma compounds through the regulation of substrate flux, enzyme activities, or metabolic network orientation [53]. Collectively, these AAs not only act as precursors for the formation of crabapple floral scents but also offer potential targets for metabolic regulation and targeted aroma breeding. In analyzing the aroma type differences among crabapple flowers, a chemometric network approach was utilized, employing a multiple-validation system. Initially, separate OPLS-DA models were constructed based on VOCs data (R\({}^{2}\)Y = 0.608, Q\({}^{2}\) = 423) and AA profiles (R\({}^{2}\)Y = 0.631, Q\({}^{2}\) = 0.520) from 18 crabapple cultivars, which demonstrated limited predictive capabilities when using either VOCs or AAs alone for aroma classification. Subsequently, an innovative combined VOC-AA model was developed, achieving a significant improvement in predictive performance (R\({}^{2}\)Y = 0.942, Q\({}^{2}\) = 0.732).