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

摘要

Washington Navel, Biondo Comune, Tarocco and Valencia Late orange fruits, harvested on a range of dates, were subjected to ultraviolet-C (UV-C) irradiation at 0.5, 1.5 or 3.0 kJ m-2 doses and then stored at 7deg C and 90-95% RH for 4 weeks plus 1 additional week at 20deg C and～80% RH. Following UV-C treatment, there were varying amounts of rind browning and necrotic peel damage, depending on cultivar, treatment dose, and harvest date. Tarocco fruits were damaged more easily by UV-C treatment than the other cultivars. Valencia Late were the most resistant to UV-C irradiation, showing no adverse effects at the lowest dosage and having the lowest percentages of damaged fruits at higher dosages. Washington Navel and Biondo Comune oranges showed an intermediate susceptibility to UV-C treatment, with negligible differences between these cultivars. The percentage of damaged fruits after irradiation at the higher UV-C dosages decreased in most fruit samples as the season progressed. UV-C irradiation at 0.5 kJm-2 effectively reduced decay development compared with nontreated fruits. Irradiation with 1.5 kJ m-2 was more effective than 0.5 kJ m-2 only in early harvested fruits. In Washington Navel and Biondo Comune oranges in the later harvests, treatment with3.0 kJ m-2 improved decay control further, compared with 0.5 kJ m-2. Following UV-C treatments the phytoalexins, scoparone and scopoletin, accumulated in flavedo tissue depending on the cultivar, fruit age, and UV-C treatment. Both phytoalexins displayed a similar accumulation pattern, but the levels of scopoletin were very low compared with scoparone. Concentrations of phytoalexins rose as the irradiation dose increased. No scoparone and scopoletin could be detected in nontreated fruits. The highest concentration of phytoalexins among cultivars was recorded in Valencia Late, the lowest in Tarocco, with similar intermediate accumulation in Washington Navel and Biondo Comune. In Washington Navel, Biondo Comune, and Tarocco oranges, the rate of scoparone accumulation was significantly higher in fruits harvested earlier in the season while Valencia Late oranges exhibited an opposite trend.