###### Abstract
PAR in _Rosa_ has been reported to reduce benzaldehyde to BA with only half the activity as compared to its role in the synthesis of 2-PE [27]. The metabolism of FSs in plants involves secondary pathways and is regulated by various factors, including environmental conditions (light, temperature etc. ), as well as physiological states [28; 29; 30]. Soluble proteins, indispensable intracellular biomolecules, play a pivotal role in catalyzing biochemical reactions, providing structural support, facilitating transport, and participating in signaling, among other functions [31]. They are integral to maintaining normal biological functions and metabolic activities in plants. The content of soluble proteins significantly contributes to aroma production in plants, as exemplified in _Camellia sinensis_[32]. Metabolic pathways in plant cells utilize various raw materials, where soluble sugars (e.g., glucose and fructose) serve as energy and carbon sources. Organic acids, amino acids, and other small molecule compounds participate in enzyme-catalyzed reactions to synthesize complex organic compounds like lignin [33]. The carbon to nitrogen ratio (C/N) reflects the plant's utilization of carbon and nitrogen. Some studies suggest that elevated nitrogen levels in the soil can enhance the synthesis of FSs to some extent [34], particularly for compounds derived from phenylalanine [35]. According to the carbon balance hypothesis, higher C/N ratios induce the metabolism of carbon-based secondary compounds, including phenolic compounds [36; 37]. It was demonstrated in a meta-analysis that phenolic reductions in simulated nitrogen (N) deposition were positively correlated with the C/N ratio and soluble sugar concentrations in plants [38]. This may result in a decrease in aroma compound production in flowers, affecting the richness and quality of FSs. Therefore, maintaining an appropriate C/N ratio is crucial for the production and quality of FSs in plants. Researchers are actively exploring ways to enhance the production of volatile compounds in flowers by modulating the physiological characteristics of plants, aiming to improve both the quality and quantity of floral scents. For instance, it has been found that the volatile content of \(\beta\)-ocimene and linalool in _Lilium_ 'Siberia' increased in the presence of exogenous glucose and fructose [39]. Crabapples (_Malus_ spp.) are deciduous small trees, belonging to the genus _Malus_ in the Rosaceae family, characterized by a fruit diameter of less than or equal to 5 cm [40]. Varieties such as \(M\). 'Praire Rose' and \(M\). 'Lollipop' showcase diverse flower shapes, boasting significant ornamental value. These cultivars making them popular choices for a wide range of garden landscaping applications [41]. In plant breeding, an excessive focus on flower shape and color has led to the gradual loss of aroma in many varieties. Recent years have witnessed a growing interest in research on aroma components during flower development in plants [42; 43; 44]. However, most of the current crabapple FS research focuses on the characterization of the aroma components, and the effects of the differences in enzyme activities and physiological characteristics between crabapple varieties on the mechanism of their aroma formation remain unclear. To investigate the mechanism of the aroma component formation during crabapple development, we selected \(M\). 'Praire Rose' and \(M\). 'Lollipop' as materials, which exhibit significant differences in 2-PE and BA. Our study involved the determination of relevant metabolic enzyme concentrations, the content of soluble sugar, soluble protein, carbon (C) and N, as well as the C/N ratio. The objective was to elucidate the distinctions in the formation of characteristic benzene volatile compounds (BA and 2-PE). This study endeavors to provide new perspectives on the study of FS in plants, and establish a theoretical basis for scent breeding, scent biosynthesis, and physiological ecology in crabapple.