• flavonoids
• fruit
• peach
• phenolics
• Prunus persica
• secondary metabolites
• shelf-life
• UV-B radiation
• UVR8

Increasing people’s demand of fresh and healthy food has made fruit and vegetable producers interested about eco-friendly ways to increase the content of health-promoting compounds. From this perspective, UV-B radiation has become popular during the last few years thanks to its effects on plant secondary metabolites, especially phenolic compounds, even in post-harvest fruit and vegetable. This work aimed to investigate the perception of UV-B radiation by fruit, particularly peaches which were scarcely studied before, and the impact of different UV-B treatments on several classes of secondary metabolites. Furthermore, the UV-B-induced changes on textural and firmness changes of peaches were assessed. This aspect was deepened by evaluating the activity of several cell wall dismantling enzymes, as well as the expression of their biosynthetic genes, following UV-B exposure. This research was conducted in the light of our previous studies on tomato (Solanum lycopersicum L., cv. Roma) in which it was observed that a post-harvest UV-B treatment induced an accumulation of phenolics and flavonoids, as well as an upregulation of some flavonoid biosynthetic genes, such as CHS and CHI, in the skin. Peach fruit (Prunus persica L. Batsch, cv. Fairtime, melting flesh phenotype) were exposed to 10 min (1.39 kJ m-2) or 60 min (8.33 kJ m-2) of UV-B narrowband lamps, providing additional photosynthetically active radiation (PAR) for all the duration of the UV-B treatment. Fruit were then stored under PAR at room temperature for up to 36 h. Control fruits underwent the same condition, but during the exposure time they were irradiated only with PAR. Phenolics were highly affected by UV-B treatment, differently according to the phenolic subclass considered. Overall, phenolics concentration was lower after 24 h from the UV-B treatment, probably due to a UV-B-related oxidation of such compounds, but then they generally increased after 36 h. Such changes were mainly visible for anthocyanins, flavones and dihydroflavonols, which are well-known to be strong antioxidants. Molecular studies revealed an activation of several genes involved in the phenylpropanoid pathway (e.g. CHS, F3H, F3’H, DFR), in the UVR8 pathway (HY5, COP1) and in regulatory genes (some MYBs). Moreover, UV-B was shown also to impact several other classes of secondary metabolites, such as terpenoids and alkaloids. Among terpenoids, the most UV-B affected were carotenoids, which decreased in parallel with an increase of apocarotenoids o possible degradation products of carotenoids due to a UV-B-induced oxidative stress. Among alkaloids, some pteridines accumulated, likely derived from folates degradation, while indole alkaloids decreased. Also, several lipids involved in the architecture of the cell membrane decrease, possibly due to a UV-B-driven lipid peroxidation, while their degradation products or intermediate in their biosynthesis increased. Although UV-B exposure did not affect soluble solid concentration or titratable acidity, it reduced the loss of firmness after 24 h from the exposure, especially by affecting the activity of the cell-wall related enzymes pectin methylesterase, β-galactosidase and polygalacturonase. Several UV-B irradiations, from 1 h to 12 h, were applied to test whether a pre-UV-B exposure on peach fruit might modulate the phenolic response towards Monilinia fructicola, a very aggressive fungal pathogen for peaches. Our study revealed that the modulation was class-dependent, with different optimal UV-B doses according to each phenolic subclass considered. With a pre-treatment of 1 h of UV-B radiation, the phenolic accumulation was enhanced by the presence of the fungus, while it decreased with 3 h of UV-B exposure, especially for flavonols. Canonical discriminant analysis (CDA) and Pearson correlation revealed that the phenolic compounds significantly regulated by the combination of pre-UV-B exposure and pathogen were the procyanidin trimer, both chlorogenic and neochlorogenic acids, the quercetin-3-galactoside, the quercetin-3-glucoside, the isorhamnetin-3-galactoside and the cyanidin-3-glucoside. However, when the two factors were applied individually, the responsive phenolic compounds were different. Modulation of pathogen-induced phenolics also far from inoculation site might suggest a migration of signalling molecules from the infected area to healthy tissues. Furthermore, although the infection itself was able to increase phenolics content, UV-B effects were overwhelming. Based on our results, UV-B radiation might represent an effective eco-friendly tool to enhance nutraceutical quality of peach fruit, as well as to extend their shelf-life.