Oranges, lemons and limes are nearly all mixes of just three wild fruits
Sweet oranges, lemons, grapefruit and limes are not ancient wild species. Nearly every major commercial citrus is a hybrid of three ancestors, the mandarin, the pomelo and the citron, crossed with one another and a few wild relatives over the last few thousand years. The whole family traces back to the Himalayan foothills.
Genetic studies point to a birthplace in the region spanning eastern Assam, northern Myanmar and western Yunnan. Citrus split from the trifoliate orange lineage about 7 million years ago, and a climate shift in the Late Miocene triggered a burst of new species: citrons in South Asia, pomelos in mainland Southeast Asia, mandarins and kumquats in southeastern China, kaffir limes on the islands. Later waves reached Taiwan and Japan, then crossed the Wallace Line into New Guinea and Australia, where native limes evolved.
People carried citrus far beyond that range. Austronesian voyagers took pomelos and kaffir limes east into Micronesia and Polynesia as canoe plants between roughly 3000 and 1500 BCE. The citron reached the Mediterranean early; seeds from a Cypriot site date to about 1200 BCE, and Theophrastus described it around 310 BCE. Arab traders brought lemons, pomelos and sour oranges around the tenth century, Genoese and Portuguese merchants introduced sweet oranges in the fifteenth and sixteenth, and mandarins arrived only in the nineteenth. Northern Europeans raised citrus in orangeries, often more status symbol than farm.
The fruit is a specialised berry called a hesperidium. Its zest is the outer rind layer, the white pith sits beneath, and the segments are packed with juice sacs. Scent comes from oil glands in the peel; sharpness comes from citric and ascorbic acids; grapefruit's bitterness comes from naringin. Oranges only turn orange where winters bring cool nights, so tropical oranges can stay green when fully mature.
Citrus is big business. World production reached 124 million tonnes in 2016, about half of it oranges, and in 2018 citrus made up nearly half of all global fruit trade by value. Most trees are grafted onto hardy rootstocks. Diseases threaten the crop, notably citrus greening, whose bacterium is carried by the Asian citrus psyllid and endangers groves in Florida, California and beyond.
Source: Citrus
