## 2 Results and Discussion
### Hexyl Ester and/or Hexanoate Ester Content in Pear Fruits Fed on Hexanal and Hexanol
However, the content of the corresponding hexyl esters in \"Nanguoli\" fruits was two-thirds that in \"Dangshansuli\" fruits (Table 3). The comparatively low hexyl esters content in \"Nanguoli\" fruit fed on hexanol have been due to the greater released from fruits or unknown differences in the capacity for ester formation between cultivars remained unknown. If it was the metabolic difference, the difference may be due to the low levels of acetate or decreased enzyme activity for ester synthesis, as well as feedback inhibition. Feedback inhibition of volatile aroma biosynthesis has been observed in _Antirrhinum majus_[22]. Whether the low hexyl acetate formation for \"Nanguoli\" fruits fed on hexanol resulted from decreased AAT activity _in vitro_ or feedback inhibition requires further analysis. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multirow{2}{*}{**Aroma Volatiles**} & \multicolumn{3}{c}{**Dangshansuli**} & \multicolumn{3}{c}{**Nanguoli**} \\ \cline{2-7}  & **CK** & **Hexanal** & **Hexanol** & **CK** & **Hexanal** & **Hexanol** \\ \hline Methyl hexanoate & \(1.3\pm 1.0\) bAB [FOOTNOTE:]Footnote : The number of hexanol used in the literature is \(1  
Different increases of hexyl esters were observed in different pear fruits feeding on same quantity of hexanal. Higher levels of hexyl ester in \"Dangshansuli\" fruits showed that more hexanol was formed in \"Dangshansuli\" fruits after feeding on hexanal. The activity levels of reductase/oxidase were responsible for the aroma volatiles synthesis. In \"Dangshansuli\" fruits, more hexanal was reduced to hexanol catalyzed by ADH reductase, so the activity of ADH reductase may not the limiting factor in volatile ester biosynthesis for these fruits. The low levels of hexyl esters in \"Nanguoli\" fruits fed on hexanal indicated the limitation of ADH reductase in catalyzing hexanal to hexanol. That ADH reductase was a limiting factor for aroma biosynthesis in \"Nanguoli\" fruits remains a hypothesis and requires further study. The higher ester content in pear fruits fed on hexanal and hexanol than fruits fed on linoleic acid and linolenic acid showed that the former two compounds more efficiently enhanced volatile biosynthesis, which may have been due to comparatively better uptake and metabolism of these compounds. A similar phenomenon has been observed in branched chain flavor volatile synthesis by amino acid metabolism pathway. The application of amino acid to fruit pericarp segments did not stimulate synthesis of the corresponding volatile, however application of the corresponding \(\alpha\)-keto acids resulted in much higher volatile production [23]. Therefore, the content of hexanal and hexanol or the formation rate are likely more important in the production of aroma volatiles than linoleic acid and linolenic acid. Metabolic intermediates have been indicated as more important than precursors in aroma compound synthesis for other fruits [13]. Due to the importance of the precursors hexanal and hexanol, the initial steps (including those catalyzed by the key enzymes LOX and HPL) of hexanal and hexanol formation are critical for ester biosynthesis in pear fruits. In plants, hexanal is synthesized through the conversion of linolenic acid or linoleic acid via the enzymes LOX and HPL. Synthesis of green note aroma compounds through the biotransformation of fatty acids using yeast cells co-expressing lipoxygenase and hydroperoxide lyase indicates that modification of LOX and HPL activity may serve as a promising approach for increasing efficiency of green note aldehyde synthesis [24]. Several studies have demonstrated that LOX plays a key role in volatile ester production [16; 17; 18]. Accordingly, further studies of HPL and ADH, which catalyze aldehydes to their corresponding alcohols in pear fruits, should be taken.