Programmed citrus breeding in Japan began at the National Institute of Fruit Science in 1937. The intent was both to improve fruit quality and productivity and to broaden fruit ripening time. Citrus is a woody perennial crop that requires a lengthy period of time during cross-breeding for the cross-seedlings to flower and bear fruit. Polyembryony is a fatal obstacle specific to citrus cross-breeding in many cultivars in which a high percentage of seedlings are not hybrids, but rather identical to the female parent, which severely hinders cross-breeding. Therefore, cultivar improvement was achieved through identification of bud sports (natural mutation) among the tree branches.
On the other hand, in 1979, cross-breeding efforts slowly continued and succeeded in producing ‘Kiyomi’ tangor, which is a hybrid of ‘Miyagawa-wase’ mandarin and ‘Trovita’ orange. ‘Kiyomi’ is the first tangor cultivar to be released in Japan. It has excellent eating quality, high juiciness, and fragrant flavor. Fortunately, ‘Kiyomi’ seeds are monoembryonic, meaning that all the seedlings raised are hybrids.
In addition, since the latter half of the 1990’s, new cultivars bearing fruit containing functional constituents, including carotenoids such as β-cryptoxanthin and/or polymethoxyflavonoids such as nobiletin, have been released. ‘Kankitsu Chukanbohon Nou 6 gou’ fruit has excellent eating quality when fresh, contains high levels of functional constituents, including β-cryptoxanthin, nobiletin, tangeretin, and phenylpropanoids, and is suitable for value-added fruit processing due to its functional constituents.