Citron
There are also a number of citron hybrids; for example, ponderosa lemon, the lumia and rhobs el Arsa are known citron hybrids, some are claiming that even the Florentine citron is not pure citron, but a citron hybrid.
Citrus taxonomy
The trifoliate orange is a cold hardy plant distinguishable by its compound leaves with three leaflets and its deciduous nature, but is close enough to the genus "Citrus" to be used as a rootstock. Swingle moved the trifoliate orange from "Citrus" to its own genus, "Poncirus". It was long thought to be the only member of this genus, until a second species, "Poncirus polyandra", was discovered in Yunnan (China) in the 1980s. Early phylogenetic analysis nested "Poncirus" within the citrus, suggesting they all belong to a single genus, but the genomic sequencing of Wu, "et al.", placed it outside the cluster representing "Citrus", and the authors retained a separate "Poncirus" genus.