## 1 Introduction  
The apple has a complex aroma composed of a wide variety of volatile compounds including alcohols, aldehydes, and esters, which are the basic components involved in fruit flavor formation [1]. Over 300 types of volatile components were identified in apple fruit; however, only 20-30 volatiles are directly related to the formation of the characteristic apple aroma [2, 3]. Esters, including butyl acetate, butyl butyrate, butyl caproate, hexyl butyrate, hexyl propanoate, and hexyl 2-methylbutyrate, account for more than 50% of total aromatic compounds in most apple cultivars [3, 4]. Notably, hexyl butyrate was also reported as a major contributor to apple sweetness, which is known as an odor-induced enhancement of taste perception [5]. The dual role of aromatic components emphasizes their great significance to the overall sensory quality of apple fruit. However, the production of aromatic volatiles in apple fruit was largely suppressed during cold or controlled atmosphere (CA) storage, especially for long-term periods [4, 6, 7]. Most importantly, the negative impact of commercial storage conditions on aroma emission in fruit persists throughout the retail chain, resulting in poor consumer acceptability [8].  
Oxygen partial pressure is considered an important factor for the CA storage of most apple cultivars, being applied as follows: 2.0-5.0 kPa for conventional CA, 1.5-2.0 kPa for low oxygen, and 0.8-1.2 kPa for ultra-low oxygen [9]. Generally, compared with single cold storage, the application of low-oxygen partial pressure could result in better fruit quality, especially in skin color and flesh firmness, thereby preventing physiological alterations [4]. Notably, the storage of apples under low oxygen may effectively inhibit the production of straight-chain esters due to the reduced availability of corresponding acid and alcohol precursors [1; 2; 6]. Although the GD apple has a strong tolerance in terms of its physiological response to low-oxygen partial pressure, its emission of straight-chain esters can still be suppressed under hypoxic conditions during long-term storage in a CA [10; 11; 12]. [PERSON] et al. hypothesized that the suppression of aroma production in GD apples might be related to retarded synthesis or degradation of fatty acids [10]. However, the underlying response mechanism of aroma production to low oxygen in GD apples is still unclear.  
Additionally, the existence of the oxygen gradient in plant tissue is a common phenomenon: the area with low-oxygen perfusion expands when surrounding oxygen availability decreases [13]. In apple fruit, the local oxygen gradient in the tissue was found to be higher than the surrounding environment [14]. The hypoxia caused by the oxygen gradient may cause negative metabolic adaptation in apple tissues [15]. Therefore, the oxygen gradient might, to some extent, be an influencing factor in the spatiotemporal pattern of aroma biosynthesis in fruit. However, limited data about the spatial response of aroma biosynthesis to low oxygen in GD apple tissues are available so far.  
Since the early 20 th century, the Golden Delicious apple was cultivated around the world, and its high yield and sweet flavor make it a favorite amongst apple growers and customers. The study aimed to investigate the spatial effect of low oxygen on the emission of volatile compounds in GD apples under CA storage and for their subsequent shelf life, and the mechanism underlying the impact of low-oxygen storage on the pathways involved in aroma biosynthesis of GD apples. This work provides insights into the tissue-specific recovery capability of aroma biosynthesis in GD apples after storage under low-oxygen conditions.