## 3 Discussion  
Aroma is an important quality indicator of fruit flavor that can significantly influence the consumer choices [29]. Many factors can affect the aroma composition of apple fruit, among which cultivar is the most important [6]. In this study, volatile compounds of two famous apple varieties 'Granny Smith' and 'Jonagold' were determined using HS-SPME with GC-MS. The types of volatile compounds in the Jonagold apple were more than that in the Granny Smith apple. Among these volatiles, esters were the predominant compounds in the Jonagold apple, accounting for more than 60% of the total volatiles, which was consistent with previous report that esters were the most abundant group that contribute mainly to the sweet and fruity odors of apple fruits [11; 30]. Moreover, hexyl acetate, confers a sweet and fruity odor with floral notes, was the most plentiful ester in the Jonagold apple, which was in accordance with the results of previous studies [31]. The Granny Smith apple is known to have a very low volatile emission [1]. In the present study, despite the fact that the total content of volatile compounds in the Granny Smith apple was significantly lower than that in the Jonagold apple, Granny Smith possessed more types of alcohols and aldehyde, occupied over 90% of the total volatiles. For the Granny Smith apple, 1-hexanol was the most abundant compound among alcohols and (_E_)-2-hexenal was the major aldehyde. Additionally, a range of volatiles such as \(\alpha\)-farnesene and estragole have been detected in the Granny Smith apple, but accumulated more in Jonagold apples. The aroma volatiles of various apple varieties were different in class and quantity, and gave a characteristic sensorial perception for each type of apple fruit [32]. Previous studies have classified Jonagold into the 'ester type' apple based on its high ester production [5; 33]. According to this classification, the Granny Smith studied here would be the 'aldehyde type' apple because the aldehydes in its fruit represent more than 85% of the total aromatic components. The differences in composition and concentration of volatile compounds between Granny Smith and Jonagold apples may determine diverse aroma profiles of the fruits. In order to reveal the molecular mechanisms underlying the synthesis of volatile compounds, transcriptome sequencing of the Granny Smith and Jonagold apples was performed using the Illumina Novaseq 6000. It has been reported that at least four pathways are responsible for the production of volatile compounds in apples [6]: Straight-chain aldehydes, alcohols and esters are synthesized from fatty acid metabolism pathway [13]; Branched-chain aldehydes, alcohols and esters are derived from isoleucine pathway [34]; Sesquiterpenes are synthesized via the mevalonate pathway and phenylpropenes are synthesized from the phenylpropanoid pathway [12; 35]. In this study, a total of 94 DEGs were detected in these four pathways, indicating the dramatical differences in volatile synthesis between the Granny Smith and Jonagold apples. For the formation of aldehydes, alcohols and esters in apples, \(\beta\)-oxidation and the lipoxygenase pathway are the two essential enzymatic systems in the catabolism of fatty acids [36; 37]. The \(\beta\)-oxidation of long-chain fatty acids produces shorter acids such as acetic, butanoic and hexanoic acids, which can be reduced to their corresponding alcohols before being esterified with acyl-CoA by the AAT enzyme [9]. The lipoxygenase pathway contains four vital enzymes, inclusive of LOX, HPL, ADH and AAT [36]. Lipoxygenase (LOX) is a dioxygenase that catalyzes the oxygenation of polyunsaturated fatty acids. In 'Royal Gala' and 'Prima' apples, the genes _MdLOX1a_ and _MdLOX5e_ were found to be involved in the production of volatiles [38]. In our study, the twelve _MdLOX_ genes showed significantly higher expression levels in the Jonagold apple than that in the Granny Smith apple.