## 3 Results
### Expressions of Genes Involved in Fatty Acid Metabolism  
The expression level of _MdLOX1a_ in both the 2 kPa O\({}_{2}\) and air groups increased along with storage time (Figure 4A). Specifically, in Skin, _MdLOX1a_ expression was inhibited in apples under low oxygen during early storage. Meanwhile, for flesh tissues, especially Flesh1, _MdLOX1a_ expression was inhibited throughout the storage period (Figure 4A). During the shelf-life period, the expression of _MdLOX1a_ in Skin and Flesh2 of 2 kPa O\({}_{2}\)-stored fruit was upregulated. The results indicate that _MdLOX1a_ expression was sensitive to the change in oxygen partial pressure in the surrounding environment, which is similar to the discussion by [PERSON] et al. [2].  
The expression of _MdHPL_ in all apples continuously decreased during storage but increased during the shelf-life period (Figure 4B). In the 2 kPa O\({}_{2}\)-stored fruit skin, the _MdHPL_ had a higher expression than that in the air group, being consistent with the content of hexanal and (E)-2-hexenal (Figure 3E). In contrast, Flesh2 in the 2 kPa O\({}_{2}\) group had lower levels of _MdHPL_ expression than did the air-stored apples throughout their storage and shelf life.  
Alcohol dehydrogenase is an enzyme involved in fermentation and is highly sensitive to changes in oxygen level [22]. On day 14 of storage, the expression of _MdADH1_ and _MdADH2_ in 2 kPa O\({}_{2}\)-stored apples was dramatically upregulated, especially in flesh tissues (Figure 4C,D), which could explain the decrease in alcohol levels and increase in aldehyde levels (Figure 3). However, the levels of _MdADH1_ and _MdADH2_ expression in all tissues decreased after being transferred to the shelf-life stage (Figure 4D).  
Figure 4: Expressions of genes involved in fatty acid metabolism pathway coding for enzymes found in the skin and flesh of ‘Golden Delicious’ apple fruit throughout 56-day storage under 2 kPa O\({}_{2}+98\) kPa N\({}_{2}\) at 4 \({}^{\circ}\)C and subsequent 44-day shelf life at 20 \({}^{\circ}\)C. The expressions of _MdLOX1a_, _MdHPL_, _MdADH1_, _MdADH2_, _MdAAAT1_ and _MAAAT2_ are shown in (**A\({}_{1}\)-A\({}_{3}\), B\({}_{1}\)-B\({}_{3}\), C\({}_{1}\)-C\({}_{3}\), D\({}_{1}\)-D\({}_{3}\), E\({}_{1}\)-E\({}_{3}\)** and F\({}_{1}\)-F\({}_{3}\)), respectively. Air represents apple fruit stored in air; 2 kPa O\({}_{2}\) represents fruit stored under 2 kPa O\({}_{2}+98\) kPa N\({}_{2}\). Skin indicates tissue obtained 0\(-\)0.1 mm of top pericarp; Flesh1 and Flesh2 indicate flesh tissues obtained, respectively, 0.1\(-\)1.0 cm and 2.0\(-\)3.0 cm below the exocarp. Bars indicate standard deviation for each mean value from at least three replicates. Asterisks indicate significant difference between air-stored and 2 kPa O\({}_{2}\)-stored fruit at \(p<0.05\) ([PERSON]’s multiple range test).  
For _MdAATs_ (_MdAAT1_ and _MdAAT2_), the expression levels in all apples increased during storage, and their peaks occurred on day 56 + 9 of the shelf-life period, decreasing, thereafter, during the late stage of shelf life (Figure 4E,F). During storage, the expression of _MdAATs_, especially _MdAAT2_, in 2 kPa O\({}_{2}\)-stored fruit were at lower levels compared with that in air-stored fruit, suggesting that low-oxygen conditions could downregulate ester biosynthesis by reducing the expression of _MdAATs_. On day 56 + 9 of the shelf-life period, the expression of _MdAAT2_ in the 2 kPa O\({}_{2}\)-stored fruit increased (Figure 4F), although its levels were still lower than those of the air group (Figure 3A). At the end of the shelf-life period, the level of _MdAAT1_ expression decreased but was higher in the 2 kPa O\({}_{2}\) group than in the air group, especially for Flesh2 tissue, confirming the better recovery of ester components in apple flesh tissue.