# Effects of Melatonin Treatment of Postharvest Pear Fruit on Aromatic Volatile Biosynthesis
## 1 Introduction  
Melatonin (MT) (\(N\)-acetyl-5-methoxyryptamine) is a nontoxic biological molecule produced by the pineal gland in animals and in different tissues in plants. Studies have identified a range of functions of MT in plants, including its scavenging of reactive oxygen species (ROS) that improves resistance to biotic and abiotic stresses, such as pathogen attack, extreme temperatures, salinity, drought, waterlogging, and low-sulfur [1; 2; 3; 4; 5; 6]. The shelf-life of postharvest produce declines with time, due to deterioration in quality associated with ripening and senescence processes. As a result, a range of treatments to maintain the quality and shelf life of postharvest fruit has been developed, including the application of MT that reduces ROS activity, increases production of antioxidant enzymes during postharvest storage, and inhibits ethylene synthesis to delay ripening and softening [7].Exogenous application of MT to grapes on the vine changes their polyphenol metabolism, carbohydrate biosynthesis, and, importantly, ethylene signaling [8] that, when reduced, has been shown to improve antioxidant activity in cassava fruit [9] and aroma, color, sugar content, and overall postharvest quality of tomato fruit [10]. Color, level of sugar-acid, and content of aromatic compounds are key quality indexes used in the climacteric pear fruit, and many studies have shown that aroma biosynthesis is dependent on the ethylene biosynthetic pathway. In climacteric fruit, ripening is characterized by an increase in respiration rate, and a greater production of volatiles and ethylene [11]. Ripening of apple fruit is largely driven by ethylene production that regulates the volatile ester biosynthetic pathways. Potential biochemical steps involved in the modulation of ester production under ethylene regulation have been demonstrated in a transgenic line of apple with high suppression of ethylene biosynthesis [12], and it has been shown that treatment of apple fruit with ethylene regulates volatile biosynthesis and gene expression [13]. Similarly, biosynthesis of ethylene in pear fruit is closely related to the production of aromatic volatile compounds. For example, application of the ethylene action inhibitor 1-methylcyclopropene (1-MCP) changed the aroma components in 'Ruanerli' and 'Alexander Lucas' pears [14]. Pear varieties comprise two types: those that are ready to eat immediately upon harvest, where ethylene production has no effect on quality, and those that require ripening and softening triggered by an increase in respiration and ethylene production. 'Korla' pear is an example of the former. It is one of the most popular fruits in China [15] and is exported to many countries around the world. 'Abbe Fetel' is an example of the latter, and is the most important pear variety cultivated in Italy. It is characterized by a strong aroma after softening, indicating it is suitable for consumption. Aromatic components of pear fruit are dominated by C\({}_{6}\) aldehydes and alcohols that are generated by linoleic and linolenic acids as precursors through the lipoxygenase (LOX) pathway [16]. LOX and hydroperoxide lyase (HPL) convert linoleic and linolenic acids to hexanal and hexenal, respectively, via 9- and 13-hydroperoxide isomers, and aldehydes are then reduced to the corresponding C\({}_{6}\) alcohols by alcohol dehydrogenase (ADH). Aroma esters are subsequently produced through alcohol acyltransferase (AAT) that catalyzes the final linkage of an acyl moiety and an alcohol. Although specific enzymes and genes involved in fatty acid-derived lactone (cyclic ester) formation have not been characterized [16], studies have shown that gene expression and enzyme activity are closely related to the synthesis of volatiles in fruit [11; 17]. Here, we hypothesized that MT changes aroma components in pear fruit, because it inhibits the release of ethylene in postharvest fruit.