## 2 Results and Discussion  
Contents of Linoleic Acid and Linolenic Acid for Pear Fruits Feeding on Linoleic Acid and Linolenic Acid  
_In vitro_ cultures are useful for examining the biosynthetic pathways of aroma volatiles in fruits [11; 12; 13]. The incubation of pear fruits with linoleic acid and linolenic acid showed that these compounds were efficiently absorbed by pear fruits (Figures 1 and 2) and used for the synthesis of aroma volatiles. However, different absorbabilities were observed for the two pear cultivars. Linoleic acid contents of pear fruits feeding on linoleic acid. **(A)**_P. bretschnrideri_ \"Dangshansuli\"; **(B)**_P. ussuriensis_ \"Nanguoli\"; Values represented means of three replicates, bars were standard deviation from three replicates; * denoted significant difference between treatments and CK, \(p<0.05\); ** denoted significant difference between treatments and CK, \(p<0.01\). Linoleic acid content increased by 7.98% and 3.13% for \"Dangshansuli\" and \"Nanguoli\" fruits fed on linoleic acid, respectively. The content of linoleic acid in \"Dangshansuli\" fruits fed on linoleic acid was 2.19-fold that of the control, while it was 1.23-fold that of the control for \"Nanguoli\" fruits. The content of linolenic acid increased for pear fruits fed on linolenic acid, however the increase was not significant for \"Dangshansuli\" fruits. Linolenic acid content increased by 0.62% and 3.27% for \"Dangshansuli\" and \"Nanguoli\" fruits fed on linolenic acid, respectively. These results show that  
Figure 1: Linoleic acid contents of pear fruits feeding on linoleic acid. **(A)**_P. bretschnrideri_ “Dangshansuli”; **(B)**_P. ussuriensis_ “Nanguoli”; Values represented means of three replicates, bars were standard deviation from three replicates; * denoted significant difference between treatments and CK, \(p<0.01\). Figure 2: Linolenic acid contents of pear fruits feeding on linolenic acid. **(A)**_P. bretschnrideri_ “Dangshansuli”; **(B)**_P. ussuriensis_ “Nanguoli”; Values represented means of three replicates, bar were standard deviation; ** denoted significant difference between treatments and CK, \(p<0.01\). \"Dangshansuli\" fruits absorbed linoleic acid much more effectively than linolenic acid, while \"Nanguoli\" fruits had similar absorption capacities for linolenic acid and linoleic acid. Composition of Aroma Volatiles Derived from Fatty Acid Metabolic Pathway for Pear Fruits Feeding on Linoleic Acid and Linolenic Acid  
The absorbed linoleic acid and linolenic acid were used for the synthesis of straight-chain esters, alcohols and aldehydes. Esters were the class of volatile with the greatest increases in pear fruits. Methyl acetate, methyl hexanoate, methyl 2-hexenoate, ethyl hex-3-enoate, hexyl acetate, and methyl octanoate were all increased in \"Dangshansuli\" fruits fed on linoleic acid. The esters increased in \"Nanguoli\" fruits fed on linoleic acid were ethyl acetate, ethyl butanoate, butyl acetate, methyl hexanoate, and some long-chain ester compounds (Table 1). The total quantity of esters increased to 409.4 ng\(\cdot\)g\({}^{-1}\cdot\)FW and 1104.4 ng\(\cdot\)g\({}^{-1}\cdot\)FW for \"Dangshansuli\" and \"Nanguoli\" fruits fed on linoleic acid, respectively, representing 2.07-fold and 1.66-fold increases, respectively, over control levels. Esters were also increased in pear fruits fed on linolenic acid. Methyl acetate, methyl hexanoate, and the unsaturated esters methyl 2-hexenoate, ethyl hex-3-enoate, (_Z_)-hexen-2-ol acetate, were significantly increased in fruits fed on linolenic acid. More than 10 aroma volatiles, including several long-chain esters were synthesized in pear fruits fed on linolenic acid. The total quantity of esters increased to 285.1 ng\(\cdot\)g\({}^{-1}\cdot\)FW and 1,163.4 ng\(\cdot\)g\({}^{-1}\cdot\)FW for \"Dangshanli\" and \"Nanguoli\" fruits fed on linolenic acid, respectively, representing 1.44-fold and 1.75-fold increases over control fruits. The increases in esters corresponded to the increases of linoleic acid and linolenic acid (Figures 1 and 2).