The true citrus fruit trees belong to the family of Rutaceae, subfamily Aurantioideae. Rutaceae is one of the four families in Rutales, division Lignosae of the subphylum Dicotyledoneae, with mostly subtropical or tropical genera. Leaves usually possess transparent oil glands and flowers contain an annular disc. Rutaceae contains about 150 genera and 1600 species (Swingle and Reece, 1967).

Aurantioideae, the ‘Orange’ subfamily – one of seven subfamilies in Rutaceae (Engler, 1931) – has been subdivided by W. T. Swingle into two tribes–Clauseneae and Citreae (a single tribe according to Engler, 1896), with 33 genera and 203 species. The main characteristics of the subfamily are: the fruit is a berry (hesperidium) with a leathery rind or hard shell, often with juicy pulp in the subtribe Citrineae. The seeds are without endosperm, sometimes with two or more nucellar (apomictic) embryos. The leaves and bark have schizolysigenous oil glands. They are small or sometimes large trees, rarely shrubs. Incorporating new taxonomic information, the number of species is now estimated at about 220, though a reduction in according species rank to several citrus species will reduce this number (see Table 2.1). Relationships between genera have in the past been based mainly on comparative morphology. Morphological affinities have been supported to some extent by grafting and hybridization performance.

Citrus relatives

An increase in interest in the Aurantioideae has been shown in the quest for wild members of the subfamily as a source of novel genetic variation and as a possible source of rootstocks, as well as for traits for pest and disease resistance.