Effects of post-harvest UV-B treatments on peach fruit polyphenols and expansins: biochemical ad molecular aspects

Peaches are characterized by a moderate content of health-promoting compounds, such as polyphenols. Besides composition and nutritional/nutraceutical properties, the final fruit quality (and the commercial- and shelf-life) is determined by other chemico-physical parameters including flesh firmness. It is recognized that the softening and the melting processes occurring throughout peach fruit ripening are the result of the sequential activation of specific enzymes and changes in the cell-wall associated proteins, including expansins (Exp). The objective of this work was to investigate whether post-harvest UV-B treatments are effective in slowing down the loss of flesh firmness, and, at the same time, affecting the phenol metabolism of the skin. Nectarines (cv ‘Big Top’, classified as slow melting-flesh) were collected at commercial maturity (about 60N) and subjected to irradiation with UV-B light for of 12, 24 and 36 hours (1.68 W/m2, 20 °C). The emission of ethylene was stimulated in UV-B treated fruits after 24h of irradiation. No significant difference was detected between control and treated fruit in terms of flesh firmness loss. However, UV-B treated fruit showed a reduced expression of PpExp1, PpExp2, PpExp3 genes. The content of polyphenolic compounds was assessed by spectrophotometric and chromatographic assays. UV-B treated fruits showed a significant increase in the content of total polyphenols in the skin after 24h of irradiation. In particular, flavonols, flavanols, proanthocyanidins and anthocyanins were in general increased in treated fruit. Expression analyses of genes (PAL, 4CL, CHI, CHS) involved in phenylpropanoid metabolism indicated an up-regulation after 36h of UV-B treatment. In conclusion, the UV-B treatment of harvested peach fruit appears to affect the composition of the skin by increasing the accumulation of secondary metabolites with health-promoting properties, whereas limited or null effects are present in terms of flesh firmness loss.