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The genus Citrus is native to South Asia, East Asia, Southeast Asia, Melanesia, and Australia. Various citrus species have been utilized and domesticated by indigenous cultures in these areas since ancient times. From there its cultivation spread into Micronesia and Polynesia by the Austronesian expansion (c. 30001500 BCE); and to the Middle East and the Mediterranean (c. 1200 BCE) via the incense trade route, and onwards to Europe.
This was later followed by the spread of citrus species into Taiwan and Japan in the Early Pliocene (5.33 to 3.6 mya), resulting in the tachibana orange (C. tachibana); and beyond the Wallace Line into Papua New Guinea and Australia during the Early Pleistocene (2.5 million to 800,000 years ago), where further speciation events occurred resulting in the Australian limes.
Lemons, pomelos, and sour oranges are believed to have been introduced to the Mediterranean later by Arab traders at around the 10th century CE; and sweet oranges by the Genoese and Portuguese from Asia during the 15th to 16th century. Mandarins were not introduced until the 19th century. This group of species has reached great importance in some of the Mediterranean countries, and in the case of orange, mandarin, and lemon trees, they found here soil and climatic conditions which allow them to achieve a high level of fruit quality, even better than in the regions from where they came.
Citrus fruits are notable for their fragrance, partly due to flavonoids and limonoids (which in turn are terpenes) contained in the rind, and most are juice-laden. The juice contains a high quantity of citric acid giving them their characteristic sharp flavour. The genus is commercially important as many species are cultivated for their fruit, which is eaten fresh, pressed for juice, or preserved in marmalades and pickles.
Citrus fruits are nonclimacteric and respiration slowly declines and the production and release of ethylene is gradual. The fruits do not go through a ripening process in the sense that they become "tree ripe". Some fruits, for example cherries, physically mature and then continue to ripen on the tree. Other fruits, such as pears, are picked when mature, but before they ripen, then continue to ripen off the tree. Citrus fruits pass from immaturity to maturity to overmaturity while still on the tree. Once they are separated from the tree, they do not increase in sweetness or continue to ripen. The only way change may happen after being picked is that they eventually start to decay.
Citrus plants can also develop a deficiency condition called chlorosis, characterized by yellowing leaves highlighted by contrasting leaf veins. The shriveling leaves eventually fall, and if the plant loses too many, it will slowly die. This condition is often caused by an excessively high pH (alkaline soil), which prevents the plant from absorbing iron, magnesium, zinc, or other nutrients it needs to produce chlorophyll. This condition can be cured by adding an appropriate acidic fertilizer formulated for citrus, which can sometimes revive a plant to produce new leaves and even flower buds within a few weeks under optimum conditions. A soil which is too acidic can also cause problems; citrus prefers neutral soil (pH between 6 and 8). Citrus plants are also sensitive to excessive salt in the soil. Soil testing may be necessary to properly diagnose nutrient-deficiency diseases.