# Effects of Melatonin Treatment of Postharvest Pear Fruit on Aromatic Volatile Biosynthesis
## 3 Discussion  
Numerous studies of effects of MT in a range of postharvest fruit during storage, including peach [18], strawberry [19], litchi [20], and pear [7; 21], have shown it inhibits ethylene synthesis, scavenges excessive ROS, retards fruit senescence, and maintains fruit quality. Although it is known that aroma compounds are closely related to ethylene production in postharvest fruit, their response to MT was unclear. In this study and regardless of treatment, we found 10 volatile compounds dominated in 'Korla' pear. Among them, esters are ethyl acetate; ethyl butyrate; butyl acetate; hexyl acetate; alcohols are hex-2-yn-1-ol; hexan-1-ol; aldehydes are hexanal; (_E_)-hex-2-enal; ketones are 6-methylhept-5-en-2-one. Fifteen dominated in 'Abbe Fetel' pear IAT; among them, esters are ethyl acetate; propyl acetate; butyl acetate; 2-methylbutyl acetate; pentyl acetate; hexyl acetate; heptyl formate; alcohols are hex-2-yn-1-ol; 2-methylbutan-1-ol; hexan-1-ol; aldehydes are acetaldehyde; hexanal; 2-methylPent-4-enal; (_E_)-hex-2-enal; ketones is 6-methylhept-5-en-2-one. At PRC, content of four volatiles (2-methylpent-4-enal; 6-methylhept-5-en-2-one) were not detected (Table 1). Content of esters were greater, and aldehydes were lower, respectively, in 'Abbe Fetel' than in 'Korla' IAT. At PRC, content of esters generally increased in 'Abbe Fetel', but there was no change in volatile content of 'Korla' IAT or at PRC. These results indicate that aroma contents of 'Abbe Fetel' pear increase during ripening, whereas those of 'Korla' pear remain stable during storage. When we analyzed the effect of MT on the aroma components in the two varieties of pear, we found they were dominated by hexanal and (_E_)-hex-2-enal, supporting previous research that similarly showed hexanal is the principal volatile in pear fruit [22]. Among the volatile compounds in pear fruit, C\({}_{6}\) aldehydes and alcohols are characterized as green-note volatiles, and following treatment with MT, content of hexanal and (_E_)-hex-2-enal decreased in the two varieties. We showed that the fatty acid pathway produced C\({}_{6}\) aldehydes and alcohols, indicated by linoleic and linoleic acids as precursors catalyzed by LOX. In 'Korla', MT increased content of linolenic and linoleic acids, but decreased content of hexanal. Although there was no difference shown in LOX activity, melatonin significantly downregulated HPL activity, indicating MT had reduced oxidation of the fatty acids that led to the lowered hexanal content. Although MT downregulated the _PbLOX2_ gene, there was no change in LOX activity in 'Korla' pear. HPL is known to catalyze the fatty acid hydroperoxide to generate hexanal and hexanal [23], and we found that HPL activity was inhibited by MT, while expression of _PbHPL_ was downregulated IAT. These results indicate that MT regulates biosynthesis of aldehyde in 'Korla' pear by downregulating expression of HPL. In contrast, MT reduced content of (_E_)-hex-2-enal, reduced LOX activity, and downregulated _PbLOX1_ and _PbLOX2_ in 'Abbe Fetel' pear IAT, but there was no effect on content of linoleic and linolenic acids, HPL enzyme activity, or _PbHPL_ gene expression. These results indicate that MT regulates content of (_E_)-hex-2-enal and hexanal by inhibiting LOX activity in 'Abbe Fetel' pear. Although MT effects on (_E_)-hex-2-enal and hexanal were consistent between 'Korla' and 'Abbe Fetel', the mechanisms differed, possibly due to variation in ethylene release driven by LOX activity that increases with ripening in ethylene-induced fruits; an increase in enzyme activity is a precursor to an increase in endogenous ethylene in climacter fruits [24].