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Natsudaidai (Natsumikan) is a natural hybrid of pomelo and sour orange.
The original seedling tree of this fruit is said to have been found in a garden in Yamaguchi Prefecture, Japan, toward the end of the 17th century and is reported still alive. The value of its late-ripening characteristic was not appreciated until approximately a century later and is reflected in the names most commonly used for it: (natsu means summer) Natsudaidai, summer sour orange and Natsumikan summer orange.
Fruit medium to medium-large (grapefruit size), sometimes with very short collared neck and apex slightly depressed; moderately seedy. Color yellowish-orange. Rind medium-thick; surface coarsely pebbled slightly rough. Segments fairly numerous (12); axis large and semi-hollow at maturity. Flavor acid and refreshing. Late in maturity (summer-maturing in most climates). Holds well on the tree and improves in storage.
The Natsudaidai tree is reported to be less cold resistant than the satsuma mandarin in Japan. Heat requirement for fruit maturity somewhat less than that of the grapefruit and comparable with the so-called Poorman orange and Wheeny grapefruit, both of which attain acceptable quality in climates too cool for satisfactory maturity of the grapefruit. Nevertheless, even at full maturity the Natsudaidai remains too acid for some palates.
Natsudaidai is grown commercially in the Japanese coastal regions of mildest winters in Kumamoto and Ehime prefecture. The rough textured fruit is easy to peel and is commonly eaten fresh. It is currently second in importance only to the satsuma mandarin. It is also used for wide variety of products ranging from marmalades to alcoholic beverages.
Numerous unnamed clones and selections are grown, some of which exhibit minor differences, but only two derivative varieties—Kawano and Tajima— are propagated commercially. Kawano differs from the common Natsudaidai in the fact that the fruit is less acid (and hence sweeter), matures much earlier, and loses quality if held on the tree after maturity. Tajima is a new and very juicy, late-ripening, high acid variety of much less importance, but considered to be promising.
The best-known varieties are:
Amanatsu, Beni Amanatsu, Kawano, Tajima and Natsudaidai.
Natsudaidai is also known as Natsukan, Daidai mikan, Ri ben ku ju, Tajima.
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From
http://citruspages.free.fr/souroranges.html