## 4 Discussion  
In this study, we systematically analyzed the underlying mechanisms responsible for differences in aroma formation among crabapple flowers. Employing a combination of HS-SPME-GC/MS and UPLC-MS/MS technologies, and focusing on both VOCs and AAs, we constructed an OPLS-DA-based model that enabled the precise classification of aroma phenotypes. Crabapple cultivars were categorized into three primary aroma types--fruit-almond, fruit-sweet, and mixed--based on their volatile compound profiles. HS-SPME-GC/MS, renowned for its high sensitivity and resolution, is extensively utilized for capturing and analyzing trace floral VOCs [33; 34]. Utilizing this technique, we detected a total of 51 volatile compounds in crabapple flowers, encompassing six classes of metabolites, including monoterpenoids, phenylpropanoids, and fatty acid derivatives. This diversity indicates a rich chemical foundation for the scent of crabapple flowers. Compared to some highly fragrant Rosaceous plants, such as roses [35], the aroma intensity of crabapple flowers was significantly lower, likely due to a lesser proportion of high-odor-impact components in their VOC profiles. Notable differences in VOC composition were observed among the cultivars: 'Royalty' exhibited the highest diversity, with 27 identified compounds, while 'Sparkler' and 'Indian Magic' each had the fewest, with 17 compounds. This variability suggests a considerable degree of diversity in the secondary metabolic networks among crabapple cultivars. Notably, a sulfur-containing metabolite, timonacic, was consistently detected across all crabapple samples. Despite its relatively low content among the VOCs, timonacic was found in 100% of the samples. This consistency suggests that timonacic may be derived from a conserved sulfur metabolic pathway in crabapple species, pointing to new avenues for future research on its ecological adaptations and potential functions [36]. Previous studies have indicated that timonacic possesses strong antioxidant and anti-tumor properties [37], suggesting its involvement in the environmental stress responses of crabapples and its value in research for development as a potential functional ingredient [38]. This discovery provides new insights into the relationship between characteristic metabolites and plant adaptability, as well as the multifunctional utilization of natural product resources. The odor-active compounds are pivotal in determining the scent profile of a plant, and the approach based on odor thresholds is a fundamental strategy for identifying these compounds. This approach finds extensive application in the realms of food aroma research and the study of floral scents [39]. In the current study, employing a screening criterion of a relative concentration greater than or equal to 0.1 mg/L, a total of 25 characteristic VOCs were identified, which accounted for 99.11% of the total VOC content. Through a systematic analysis using OAVs, 19 key aroma-active compounds were identified (OAV > 1). Among these, benzyl alcohol, benzaldehyde, and ethyl benzoate, with OAVs surpassing 100, collectively contributed to 98% of the total aroma intensity and were recognized as the primary contributors to the characteristic scent of crabapple flowers. Notably, some components, though present in minor quantities, demonstrated significant OAVs [40], such as ethyl cinnamate (OAV = 25.53) and ethyl caprylate (OAV = 10.40), indicating that a \"low concentration-high contribution\" mechanism plays a critical role in the aroma formation of crabapple flowers, particularly in imparting sweet notes [41]. Previous research has shown that the characteristic aroma components of the chrysanthem included isocyclictal, eucalyptus alcohol, \(\alpha\)-pinene, \(\beta\)-farnesene, and carophyllene [42], while Rhododend fortunei was predominantly characterized by eucalypt and \(\beta\)-myrcene [43]. In contrast, the aroma profile of spp. is markedly different, being primarily dominated by phenylpropanoidderived compounds such as ethyl benzoate and benzaldehyde.