## 3 Results
### Classification of Crabapple Cultivars Based on Fragrance Profiles  
Utilizing the OAVs of major volatile compounds and the HCA applied to these OAVs (as depicted in Figure 5A,B), the fragrance phenotypes of 18 crabapple cultivars were categorized into three primary scent types: fruit-almond, fruit-sweet, and mixed. Below, we describe the key chemical markers and representative sensory attributes associated with each aroma type. 1. Fruit-Almond Type: This classification includes the cultivars 'Tamlin', 'Harvest Gold', 'Donald Wyman', 'Prairifire', 'Royal Raindrops', 'Indian Magic', 'Profusion', 'Red Jewel', and 'Adams'. The predominant aromatic driver in these cultivars is benzaldehyde, which constitutes over 80% of the total OAV. Benzaldehyde imparts an almond-like scent, stemming from its molecular structure that combines a benzene ring with a formyl substituent. This compound is prevalent in plants of the Rosaceae family and is typically produced through the deamination of Phe via the shikimate pathway [20]. The scent profile of this group aligns closely with that of the tropical water lily _Nymphhaa'_ Eldorado' at full bloom, which is also dominated by benzaldehyde [29]. 2. Fruit-Sweet Type: This group comprises 'Kelsey', 'Radiant', and 'Purple Prince'. Their fragrance is primarily characterized by ethyl benzoate, accounting for over 65% of the total OAV. Ethyl benzoate delivers a distinct fruity sweet aroma. Biologically, the synthesis of ethyl benzoate is generally facilitated by BAHD acyltransferases, such as LoAAT1, which utilize benzoyl-CoA and ethanol as substrates to form this ester aroma molecule [30]. 3. Mixed Type: The cultivars 'Purple Classic', 'Flame', 'Toringo', Tang-2', 'Sparkler', 'Spring Snow', and 'Royalty' fall under the mixed type category. The fragrance of these cultivars is defined by a co-dominance of benzaldehyde, ethyl benzoate, and benzyl alcohol, with their contributions to the OAV being relatively balanced. This results in a complex and layered aroma profile. These cultivars are often referred to as  
Figure 5: HCA and radar chart of volatile characteristics 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**) HCA dendrogram of the 18 crabapple cultivars. Red, yellow, and blue represent the fruit-almond, fruit-sweet, and mixed aroma types, respectively. (**B**) Radar chart of the 18 crabapple cultivars. having a \"mixed\" fragrance type [31], displaying diverse and challenging-to-classify sensory characteristics during evaluations. To explore variations in aroma characteristics among three distinct aroma types: fruit-almond, fruit-sweet, and mixed, OPLS-DA was applied to the VOC composition data derived from 18 cultivars. The analysis employed the relative concentrations of 11 key aroma compounds (VIP > 1) as predictor variables, with aroma type serving as the response variable. The model demonstrated robust goodness-of-fit and predictive accuracy, evidenced by an R\({}^{2}\)Y of 0.608 and a Q\({}^{2}\) of 0.423 (Figure 6A). To ensure model reliability, 200 permutation tests were conducted. The results, as depicted in Figure 6B, showed that the Q\({}^{2}\) regression line's intercept on the vertical axis was below zero, confirming that the model was not subject to overfitting and thus validating its utility for discriminating among aroma types. The OPLS-DA score plot (Figure 6A) delineated a clear demarcation among the three groups of aroma types within the principal component space, highlighting systematic differences in VOC profiles across the cultivars. Cultivars characterized by the fruit-almond aroma type generally occupied the positive side of the first principal component (t [1] > 0). This clustering pattern indicates their association with higher benzaldehyde content.