Poncirus as a rootstock in Asia is very old, going back 1000s years BC.
It has resistance to Phytophthora root rot, and also to Citrus Tristeza Virus (CTV): good cold tolerance: adaptation to heavy soils: and gives small size to trees budded on it. Perhaps Poncirus major importance is as a parent in intergeneric hybridization with Citrus, producing the very important citranges. In the USA, the genus is commonly divided into the two catagories as small flowered and large flowered varieties. One variety of P. trifoliata of particular interest is "Flying Dragon" (P.trifoliata var. monstrosa), known in Japan as 'Hiryu'. This is a dwarf variety with very slender leaves and crooked, tortuously curved branches and thorns. Although All trifoliates produce a small tree when used as a rootstock, 'Flying Dragon' is the only variety that is a true dwarfing root stock. Trees budded on 'Flying Dragon' seldom exceed 8 feet (2.5m) in height. In 1943 Swingle stated that he had introduced it to the USA in 1915. Data by Cheng and Roose (1995) demonstrated that the dwarfing characteristics are from a single dominant gene. The curved thorns and curved trunk appear to be pleiotropid effects of this same dwarfing gene. Poncirus was for many decades a monotypic genus. Recently, a putative new species, P. polyandra has been published. This new variety differed from P. trifoliata in having larger leaves, some floral differences and, most strikingly, in being evergreen year around. Perhaps this last characteristic is related to its providence in Yunnan, the southernmost Province of china. Poncirrus polyandra is little known in the west (and perhaps in China). - Millet (AF N.R.)
(Citrus Genetics, Breeding and Biotechnology)
