Japanese citrus

The Kojiki and Nihon Shoki, compiled in the 700s, were the first books in Japan to describe citrus fruits. The Nihon Shoki states that a man named Tajimamori brought back citrus fruits from the Land of immortality (常世の国) on the orders of Emperor Suinin, which is thought to refer to the tachibana orange that grows wild in Japan. The Man'yōshū, a collection of poems from the same period, contains many poems about tachibana orange, and because of its strong acidity at the time, it was dried and used for medicinal and ornamental purposes rather than for food. The Kokin Wakashū, compiled in the 900s, mentions that tachibana orange was burned and used as incense to give a nice fragrance to kimonos. In Japan, tachibana orange is a symbol of eternity and is the motif for the Order of Culture.

History
Citrus fruits are believed to have originated in Assam, India and the foothills of the Himalayas, and later spread throughout the world. Yuzu, kunembo, daidai, and kishu mikan are believed to have been introduced to Japan from China, where Kishu mikan were the mainstay until the unshu mikan became dominant.

Japan's warm summer climate makes it particularly suitable for citrus plants; while they are grown all over the country, the largest citrus growing areas are located in the prefectures of Wakayama, Shizuoka, Tokushima, Kochi, Oita, Miyazaki and Ehime.

The most widely grown variety in Japan today is the unshu mikan (Citrus unshiu), also known as the satsuma orange. There are two theories about the origin of the unshu mikan. One is that unshu mikan originated in Japan, while the other is that it originated in China. According to the Japanese origin theory, several species that would serve as the parents of unshu mikan were introduced from China, and in the 1600s, they were crossed in Nishi-Nakajima, Higo Province (later Nagashima, Kagoshima) in Japan. The Japanese Ministry of Agriculture, Forestry and Fisheries, Ehime Prefecture and several Japanese scientists support this theory of Japanese origin. Genetic research has shown that the unshu mikan is a cross between the kishu and kunembo. During the Edo period (1603–1867), unshu mikan were not very popular due to a superstition that eating seedless unshu mikan would make one infertile. It was not until the Meiji era (1868–1912), when modernization began, that the popularity of unshu mikan increased.

From the Edo period (1603–1867) to the present, the Japanese have created numerous varieties of citrus fruits, collectively known as tyūbankan (中晩柑). It is a general term for citrus fruits shipped from January to May, after the shipping period of unshu mikan, which is from October to February. For example, natsumikan (Citrus natsudaidai) was developed around 1700, hassaku in 1860, and iyokan in 1886. Kiyomi, Setoka and Kanpei are also types of tyūbankan. dekopon, created in 1972, is one of the most popular varieties internationally. Dekopon was introduced to South Korea in the 1990s and became popular as Hallabong, was exported to California in 1998, and has been marketed as "Sumo Citrus" since 2011.

In Japan, various cultivars have been developed based on the unshu mikan, and three cultivars, namely miyagawa wase, okitsu wase, and aoshima unshu, account for nearly half of the production volume of unshu mikan.

Japanese taxonomy
Japan usually follows the botanical names of the taxonomy from Tyôzaburô Tanaka, often referred to as the "Tanaka system", giving for each cultivar a separate name no matter if it is pure or a hybrid of two or more species or varieties. While elsewhere it is more popular to classify the genus citrus into species, and further into varieties, and then into cultivars or hybrid. Such a system was created by Walter Tennyson Swingle from Florida and is called the "Swingle system". The different approaches of the two systems lead to partially-overlapping or nested 'species', for example, Citrus unshiu and Citrus tangerina (Tanaka) versus Citrus reticulata (Swingle). Likewise, common terms, like "mikan", do not always align with these taxonomic groups.

Japanese citrus (partial list)
Japanese citrus fruits include the following: