Postharvest temperature influences volatile lactone production via regulation of acyl-CoA oxidases in peach fruit.

The biosynthesis of volatile compounds in plants is affected by environmental conditions. Lactones are considered to be peach-like aroma volatiles; however, no enzymes or genes associated with their biosynthesis have been characterized. White-fleshed (cv. Hujingmilu) and yellow-fleshed (cv. Jinxiu) melting peach (Prunus persica L. Batsch) fruit were used as materials in two successive seasons and responses measured to four different temperature treatments. Five major lactones accumulated during postharvest peach fruit ripening at 20 °C. Peach fruit at 5 °C, which induces chilling injury (CI), had the lowest lactone content during subsequent shelf life after removal, while 0 °C and a low-temperature conditioning (LTC) treatment alleviated development of CI and maintained significantly higher lactone contents. Expression of PpACX1 and activity of acyl-CoA oxidase (ACX) with C16-CoA tended to increase during postharvest ripening both at 20 °C and during shelf life after removal from cold storage when no CI was developed. There was a positive correlation between ACX and lactones in peach fruit postharvest. Changes in lactone production in response to temperatures are suggested to be a consequence of altered expression of PpACX1 and long-chain ACX activity.

Expression of genes associated with aroma formation derived from the fatty acid pathway during peach fruit ripening.

Changes in characteristic aroma volatiles, levels of fatty acids as aroma precursors, and expression patterns of related genes, including lipoxygenase (LOX), hydroperoxide lyase (HPL), alcohol dehydrogenase (ADH), alcohol acyltransferase (AAT), and fatty acid desaturase (FAD), were studied in peach ( Prunus persica L. Batsch., cv. Yulu) fruit during postharvest ripening at 20 degrees C. Concentrations of n-hexanal, (E)-2-hexenal, (E)-2-hexenol, and (Z)-3-hexenol decreased, whereas the production of (Z)-3-hexenyl acetate, gamma-hexalactone, gamma-octalactone, gamma-decalactone, and delta-decalactone increased with fruit ripening. Lactones showed a clear pattern concomitant with the climacteric rise in ethylene production, with gamma-decalactone being the principal volatile compound at the late ripening stage. Of the LOX family genes, PpLOX2 and PpLOX3 had relatively high transcript levels initially followed by a decline with fruit ripening, while levels of PpLOX1 and PpLOX4 transcripts were upregulated by accumulated ethylene production. Expression of PpHPL1, PpADH1, PpADH2, and PpADH3 showed similar decreasing patterns during ripening. Expression levels of PpAAT1 showed a rapid increase during the first 2 days of postharvest ripening followed by a gradual decrease. Contents of polyunsaturated linoleic and linolenic acids increased, and saturated palmitic acid levels tended to decline as the fruit ripened. The increased levels of unsaturated fatty acids closely paralleled increasing expression of PpFAD1 and PpFAD2. The significance of gene expression changes in relation to aroma volatile production is discussed.