## 4 Discussion  
Deliberate storage under low-oxygen partial pressure in CA was widely applied to reduce the respiration rate and ethylene biosynthesis in fruits, which would consequently delay ripening, and thereby prolong fruit storage life, especially for apples and pears [22; 23]. As expected, GD apples stored under 2 kPaO\({}_{2}\) had reduced ethylene production and respiration rates; the progression of softening was also retarded (Figure 1). Notably, the residual effect of storage under hypoxic conditions on aroma emission depended upon aroma components and fruit tissues, and was associated with the aroma biosynthetic pathways that directly influenced the recovery ability of aroma during the shelf-life period (Figures 2-4). The flesh tissue of GD apples demonstrated a high recovery capability of ester biosynthesis, which was confirmed by the relatively high levels of expression of \(MdLOXIa\) and \(MdATIs\) during the shelf-life period (Figures 2 and 4). In terms of fruit quality, the results presented herein reveal that lowering the partial oxygen pressure could benefit retarding fruit softening, whereas there was no impact on soluble solid content (Figure 1A,B), thus confirming the results reported by [PERSON] et al. [12]. After fruit harvest, sugars are the secondary energy source during early storage as there are sufficient organic acids participating in the tricarboxylic acid cycle during respiration [24]. Many apple cultivars are suitable for long-term cold storage partly because the high content of organic acids in fruit at harvest can provide a sufficient primary energy source at low temperatures for a long period [25]. Generally, most apple cultivars have more abundant aroma volatiles in skin tissue compared with flesh tissue [26; 27]. In GD apples, the amount of aroma volatiles, especially esters, in the skin was over two-fold higher than in the flesh tissues (Figure 3 and Tables S2-S4). Volatile esters are critical compounds for aroma development in apple fruit and are responsible for the characteristic fruity or floral flavors [28]. Under 2 kPa O\({}_{2}\) hypoxic conditions, the emissions of esters were dramatically suppressed in both the skin and flesh of GD apples (Figure 3). The flesh tissue of apples under low-oxygen conditions is likely to suffer an oxygen shortage due to a relatively low diffusion rate of oxygen in the apple tissue [29; 30], which further affects aroma biosynthesis in the apples. Therefore, considering the oxygen gradient in fruit tissue, the availability of oxygen may be a critical factor that affects aroma biosynthesis in apples [14; 15; 29]. During the shelf-life period, the residual impact of storage under 2 kPaO\({}_{2}\) low oxygen was more pronounced on the straight-chain esters in GD apples, such as butyl acetate, hexyl acetate, and butyl butyrate, rather than the branched-chain ester 2-methylbutyl acetate (Figure 2). In apples, straight-chain esters are synthesized via the lipoxygenase pathway, while branched-chain esters are synthesized via amino acid metabolic pathways [4]. Previous research revealed that low-oxygen storage led to decreasing the production of fatty acids, which are the precursors of corresponding alcohols and straight-chain esters in apples [31]. In the present study, the reduced transcripts of genes encoding lipoxygenase and alcohol acetyltransferase in GD apples suggest that storage under 2 kPa O\({}_{2}\) repressed the lipoxygenase pathway (Figures 2 and 4). Even so, the increase in _MdLOX1a_ and _MdAATs_ expression during the subsequent shelf-life stage did not promote the production of esters in the apples (Figures 2, 4 and 6). Similarly, [PERSON] et al. found that the differential production of volatile esters in 'Pink Lady' apples could be attributed to biochemical modifications rather than the recovery of ester-synthesizing capability upon being transferred to air conditions [32]. Additionally, apple flesh tissue had a better capability of ester biosynthesis recovery than the skin tissue after low-oxygen storage (Figure 2).