Scoparone and scopoletin accumulation and ultraviolet-C induced resistanceto postharvest decay in oranges as influenced by harvest date

'Washington Navel', 'Biondo Comune', 'Tarocco', and 'Valencia Late' orange [Citrus sinensis (L.) Obsek] fruit, harvested at various periods of time, w ere subjected to ultraviolet-C (UV-C) irradiation at 0.5, 1.5, or 3.0 kJ.m( -2) doses and then stored at 7 degrees C and 90% to 95% relative humidity ( RH) far 4 weeks plus one additional week at 20 degrees C and approximate to 80% RH. Following UV-C treatment, there was varying amounts of rind browni ng and necrotic peel damage, depending on cultivar, treatment dose, and har vest date. 'Tarocco' fruit were damaged more easily by UV-C treatment than the other cultivars. 'Valencia L.' were the mast resistant to UV-C irradiat ion, showing no adverse effects at the lowest dosage and having the lowest percentages of damaged fruit at higher dosages. 'Washington Navel' and 'Bio ndo Comune' oranges showed an intermediate susceptibility to UV-C treatment , with negligible differences between these cultivars. The percentage of da maged fruit after irradiation at the higher UV-C dosages decreased in most fruit samples as the season progressed. UV-C irradiation at 0.5 kJ.m(-2) ef fectively reduced decay development compared with nontreated fruit. Irradia tion with 1.5 kJ.m(-2) was more effective compared with 0.5 kJ.m(-2) only i n early harvested fruit. In 'Washington Navel' and 'Biondo Comune' oranges in the later harvests, treatment with 3.0 kJ.m(-2) improved decay control f urther, compared with 0.5 kJ.m(-2). Following UV-C treatments the phytoalex ins, scoparone and scopoletin, accumulated in flavedo tissue depending on t he cultivar, fruit age, and UV-C treatment. Both phytoalexins displayed a s imilar accumulation pattern, however, the levels of scopoletin were very lo w compared with scoparane. Concentrations of phytoalexins rose as the irrad iation dose increased. No scoparone and scopoletin could be detected in non treated fruit. The highest concentration of phytoalexins among cultivars wa s recorded in 'Valencia Late' oranges, the lowest in 'Taracco' with similar intermediate accumulations in 'Washington Navel' and 'Biondo Comune'. In ' Washington Navel','Biondo Comune', and 'Tarocco' oranges, the rate of scopa rone accumulation was significantly higher in fruit harvested earlier in th e season while 'Valencia late' oranges exhibited an opposite trend.