## 2 Results and Discussion
### Hexyl Ester and/or Hexanoate Ester Content in Pear Fruits Fed on Hexanal and Hexanol  
Hexanol and hexanal are important intermediates of the fatty acid metabolic pathway. Some of the hexanol and hexanal in fruits shuttle across cell membranes and alter fruit aroma volatiles, while the others are used as precursors for ester formation. Hexanol can be directly used to synthesis volatile esters catalyzed by AAT. Hexanal, as an intermediate metabolite, is another important substrate of volatile esters biosynthesis. As the reduced form of hexanal, hexanol can be used in the syntheses of hexyl esters. Similarly, as the oxidized form of hexanal, hexanoate can be used to form hexanoate esters. The content of hexyl ester and hexanoate ester were increased in pear fruits feeding on hexanal and hexanol due to the increase in precursor substrates of ester biosynthesis (Table 3). Hexyl esters content, especially those of hexyl acetate, hexyl butanoate, and hexyl hexanoate, were significantly increased in pear fruits fed on hexanol. The total quantity of hexyl ester reached 1524.2 ng\(\cdot\)g\({}^{-1}\) FW and 1127.4 ng\(\cdot\)g\({}^{-1}\) FW for \"Dangshansuli\" and \"Nanguoli\" fruits fed on hexanol, respectively. These values represented increases \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Treatments**} & \multicolumn{2}{c}{**Dangshansuli**} & \multicolumn{2}{c}{**Nanguoli**} \\ \cline{2-5}  & **Hexanal** & **Hexanol** & **Hexanal** & **Hexanol** \\ \hline CK & \(16.7\pm 2.1\) (b) & \(1.9\pm 1.3\) & - (a) & \(37.9\pm 10.0\) \\ Hexanal & \(19.0\pm 0.2\) & \(1909.3\pm 39.8\) **, (c) & \(19.7\pm 10.3\) ** & \(1822.4\pm 748.1\) ** \\ Hexanol & \(20.3\pm 0.3\) & \(2193.9\pm 47.8\) ** & \(11.0\pm 15.6\) ** & \(3024.8\pm 537.3\) ** \\ \hline \end{tabular}
\end{table} 
Table 2: Hexanol and hexanal contents (ng\(\cdot\)g\({}^{-1}\)\(\cdot\)FW) of pear fruits fed on hexanol and hexanal; (a) -, not detected. (b) Values represented means \(\pm\) SD of three replicates. (c) ** denoted significant differences between treatments and CK at the level of 0.01, according to [PERSON]’s test. of 1485.1 ng\(\cdot\)g\({}^{-1}\) FW and 958.6 ng\(\cdot\)g\({}^{-1}\) FW, separately, over the control fruits. The greater increases of hexyl ester content in \"Dangshansuli\" fruits than \"Nanguoli\" fruits fed on the same quantity of hexanol demonstrated that \"Dangshansuli\" fruits were capable of forming more hexyl ester than \"Nanguoli\" fruits when given sufficient hexanol. The dominant esters for pear fruits fed on hexanol were hexal acetate and ethyl hexanoate, followed by hexyl hexanoate. The content of hexyl acetate in \"Dangshansuli' fruits fed on hexanol was 2.6-fold that of ethyl hexanoate, while the contents of ethyl hexanoate and hexyl acetate were similar in \"Nanguoli\" fruits fed on hexanol. The total quantity of hexanoate esters was 804.0 ng\(\cdot\)g\({}^{-1}\cdot\)FW and 840.2 ng\(\cdot\)g\({}^{-1}\cdot\)FW, respectively, for \"Dangshansuli\" and \"Nanguoli\" fruits fed on the same quantity of hexanal, representing increases of 759.5 ng\(\cdot\)g\({}^{-1}\cdot\)FW and 619.4 ng\(\cdot\)g\({}^{-1}\cdot\)FW. The content of hexyl esters were 1716.8 ng\(\cdot\)g\({}^{-1}\cdot\)FW and 767.0 ng\(\cdot\)g\({}^{-1}\cdot\)FW for \"Dangshansuli\" and \"Nanguoli\" fruits fed on the same quantity of hexanal, representing increases of 1677.5 ng\(\cdot\)g\({}^{-1}\cdot\)FW and 598.2 ng\(\cdot\)g\({}^{-1}\cdot\)FW, respectively. Thus, we concluded that the greater amount of hexanal absorbed by \"Dangshansuli\" fruits was reduced to hexanol for the synthesis hexyl esters, but not oxidized to hexanoate for the synthesis hexanoate esters, while similar quantities of hexanol and hexanoate were formed to generate the corresponding esters in \"Nanguoli\" fruits fed on hexanal. The increase of hexanol in \"Nanguoli\" fruits was higher than that in \"Dangshansuli\" fruits when they were fed on the same quantity of hexanol (Table 2).