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.
Zest (ingredient)
The zest and mesocarp vary with the genetics of the fruit. Fruit with peels that are almost all flavedo are generally mandarines; relatives of pummelos and citrons tend to have thicker mesocarp. The mesocarp of pummelo relatives (grapefruit, orange, etc.) is generally more bitter; the mesocarp of citron relatives (Mexican and Persian limes, alemows etc.) is milder. The lemon is a hybrid of pummelo, citron, and mandarin. The mesocarp is also edible, and is used to make succade.