Citrus peels are important sources of various pleasant aroma compounds and valuable bioactive substances. To investigate differences in the composition and content of Shiikuwasha (Citrus depressa Hayata) peels from different cultivation lines, the composition of volatile aroma components, flavanones, and polymethoxylated flavones (PMFs) in four Shiikuwasha cultivation lines was examined. The composition of volatile aroma components in cold-pressed extracts of Shiikuwasha peels was analyzed using gas chromatography-flame ionization detection and gas chromatography-mass spectrophotometry. The extracts contained mainly monoterpene hydrocarbons (93.40-97.25%), including limonene (46.52-68.26%) and γ-terpinene (21.48-30.52%). Differences in the composition of volatile aroma compounds in the Shiikuwasha cultivation lines were revealed using principal component analysis. Additionally, the composition of flavanones and PMFs was determined using high-performance liquid chromatography methods. Neohesperidin (96.58%) was the predominant flavanone in 'Izumi kugani' peel, while the other peels had high hesperidin contents (89.26-98.66%). Moreover, the PMFs of Shiikuwasha peels were composed of nobiletin (56.74-64.77%) and tangeretin (23.17-34.70%).

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The Okinawa Islands have many wild and locally cultivated citrus genetic resources belonging to the mandarin group. However, these have decreased in recent years. It is essential, therefore, to explore the northern part of Okinawa Island and collect and study these citrus germplasm to ensure the availability of these fruit for future utilization. As a result of our exploration, we collected 30 local citrus genetic resources including wild monoembryonic Citrus tachibana (Makino) Tanaka var. attenuata, wild Citrus depressa Hayata (Shiikuwasha), and introduced and cultivated varieties; each of these exhibited different phenotypes. We thus were able to study the phenotype details and fruit quality of these citrus germplasms. Since polymethoxyflavonoids (PMFs) have been previously studied as a functional component in Citrus, we analyzed of our all samples using HPLC. As a result, we categorized the samples into 3 groups according to their PMFs constituents: 'Shiikuwasha', 'C. keraji hort. ex Tanaka (Kabuchii)', and others, which included C. oto hort. ex. Yu Tanaka and C. nobilis Lour. The 'Shiikuwasha' and 'Kabuchii' groups contained high amounts of PMFs. C. tarogayo hort. ex Yu Tanaka was included in the 'Kabuchii' group. Citrus tachibana (Makino) Tanaka var. attenuata and C. depressa (var. Ishikunibu) were included in the 'Shiikuwasha' group because of their specific constituents, despite having lower amounts of total PMFs than other cultivars in the 'Shiikuwasha' group.

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