Origin, diversity and breeding of cultivated Citrus species (Citrus spp.)

Presently almost 103 million tons of citrus fruits are being produced on an area of 7 million hectares. Among them is the orange (Citrus sinensis) which is consumed as fresh food and juice the most important. With a production of more than 17 million tons, Brazil is the largest producer of oranges. Moreover in many countries of the tropics and subtropics lemons, limes, mandarins, Clementines, satsumas, pummelos, grapefruits and sour oranges are being grown and are in part an important merchandise. Almost all citrus varieties origanated from the southeast Asian region and have been known for more than 4.000 years. While the majority of citrus species arose through hybridization, citron (C. medica), mandarin (C. reticulata) and pummelo (C. grandis) has been assigned as real species. Almost all citrus species are being threatened by diseases and pests. The currently most important threats for citrus cultivation are the tristeza and the sudden death disease. As the pathogens particularly infect the root stocks, presently millions of root stocks must be eradicated and substituted by resistant lines. Another large threat for citrus cultivation are abiotic stress factors such as frost, drought and salt. With the help of classical genetics, cytogentics and cell biology large efforts are being undertaken in breeding research in order to solve these problems. Old cultivars, breeding lines and related species are an important potential of genetic resources to combat the threat of stress factors in citrus species in the future.