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Citrus is the most extensively produced tree fruit crop in the world. There are two clearly differentiated markets: the fresh fruit and the processed juice market. Citrus species are essentially diploids (2n = 2x = 18) and were domesticated in Southeast Asia several thousand years ago and then spread throughout the world. Most of the cultivated citrus species are part of the Citrus genus containing, depending on the taxonomist, between 16 and 156 species. The relative complexity of these classifications results from the conjunction of a broad morphological diversity, total sexual interspecific compatibility within the genus and apomixis.
Most of the citrus scion cultivars result from the selection of spontaneous bud mutations identified in production orchards. Today several projects of ploidy manipulation are developed in different countries to select seedless triploid mandarins. Fruit quality (size, color, easy-peeling), seedlessness, and the extension of the harvest season are the main selection objectives for fresh market cultivars. The majority of the rootstocks used for propagation are original species or ancient natural hybrids. However, intergeneric hybrids (Citrus × Poncirus) such as citranges, citrumelos, and citrandarins have an increasing importance. The first rootstock breeding objective is adaptation to soil conditions and soil pathogens. The most widespread needs would be tolerance to CTV. Biotechnology is strongly integrated into breeding and propagation schemes. Shoot tip grafting is widely used for sanitation of the mother plants in certification propagation schemes. Somatic hybridization is an effective tool used for scion and rootstock breeding. Functional genomics studies ESTs, microarray platforms, and a full genome sequence are available. Efficient and reliable transformation systems for several economically important citrus species exist. KeywordsOrange–Grapefruit–Mandarin–Lemon–Lime–Pummelo–Citron–Poncirus–Rootstock–Scion–Shoot tip grafting–Somatic hybridization–Ploidy manipulation, genomics