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.
Economic botany
Citrus has been a major commercial product in Florida since the 19th century. Florida produces over 70% of the U.S. citrus supply. The color of oranges is not related to ripening, but is a serious component for sales. The orange color only develops in areas with cool nighttime temperatures. In tropical climates, growers often expose the fruit to ethylene, to promote the loss of chlorophyll and expose the beta-carotenes (the orange color).