Induction of flavonoids and phenolic acids in apple by UV-B and temperature

The effect of UV-B irradiation at 10 degrees C and 20 degrees C on the quer cetin glycosides procyanidins, chlorogenic acid and anthocyanin levels in t he skin of detached fruit of five apple cultivars was investigated. UV-B re sponsiveness was compared between sides of the fruit that were outwardly fa cing (exposed) and inwardly facing (shaded) when on the tree. There were no common effects of UV-B irradiation and temperature across all cultivars. U V-B irradiation increased the quercetin glycoside levels in the shaded side of 'Gala', 'Royal Gala' and 'Braeburn' fruit and only at 20 degrees C. UV- B irradiation of the exposed sides of the five cultivars, of either side at 10 degrees C, and of 'Pacific Rose' and 'Aurora' at either 10 degrees C or 20 degrees C had no significant effect on quercetin glycoside levels. Ther e was no effect of UV-B irradiation on procyanidin levels at any combinatio n of treatments. However, levels of chlorogenic acid, although one tenth of those of quercetin glycosides and procyanidins, increased markedly with UV -B irradiation. Both exposed and shaded sides of fruit of all cultivars inc reased in chlorogenic acid levels at 10 degrees C and 20 degrees C after UV -B irradiation. UV-B irradiation increased anthocyanin levels in exposed an d shaded sides of fruit of all cultivars at 20 degrees C, and on 'Braeburn' , and 'Aurora' at both 20 degrees C and 10 degrees C. UV-B irradiation at e ither temperature had no significant effect on either the relative proporti ons of the glycosides of quercetin (galactoside, rhamnoglucoside, glucoside , xyloside, arabinoside and rhamnoside) or the relative proportions of the procyanidins (Procyanidins B2, B5, epicatechin and catechin). The effect of UV-B irradiation was variable, depending on cultivar, previous light expos ure, temperature and class of flavonoids examined.