Effects of Melatonin Treatment of Postharvest Pear Fruit on Aromatic Volatile Biosynthesis

Aroma affects the sensory quality of fruit and, consequently, consumer satisfaction. Melatonin (MT) is a plant growth regulator used to delay senescence in postharvest fruit during storage; however, its effect on aroma of pear fruit remains unclear. In this study, we assessed the effects of 0.1 mmol L-1 MT on volatiles and associated gene expression in the fruit of pear cultivars 'Korla' (Pyrus brestschneideri Rehd) and 'Abbe Fetel' (Pyrus communis L.). MT mainly affected the production of C-6 aromatic substances in the two varieties. In 'Korla', MT inhibited expression of PbHPL, and reduced hydroperoxide lyase (HPL) activity and content of hexanal and (E)-hex-2-enal. In contrast, MT inhibited activity of lipoxygenase (LOX), reduced expression of PbLOX1 and PbLOX2, promoted PbAAT gene expression, increased alcohol acyltransferase (AAT) activity, and increased propyl acetate, and hexyl acetate content in 'Abbe Fetel' that similarly led to the reduction in content of hexanal and (E)-hex-2-enal. Content of esters in 'Abbe Fetel' pear increased with increasing postharvest storage period. Although mechanisms differed between the two varieties, effects on aroma volatiles mediated by MT were driven by expression of genes encoding LOX, HPL, and AAT enzymes.