The center of genetic origin of Citrus is believed to be southeastern Asia. This includes the areas from eastern Arabia to the Philippines, and also from Himalayas south to Indonesia and northern Australia. The vegetation in that region subsumes rain forests and tropical shaded tall-tree habitats, and Citrus species might have thrived for many years in the understory of these forests. Before the fifth century B.C., Citrus fruits were recognized by its medicinal uses. First sailors used fresh citrus fruits to prevent scurvy. But when brought to Europe and the Americas after 1500 A.D., Citrus fruits were used as a general source of food. After the great voyages, plants of this genus became crop plants outside its natural environment. In South America, it was first introduced in northeast Brazil, and in the Andes. By the middle of the past century, São Paulo and Minas Gerais states in southeastern Brazil became one of the hot spots of Citrus production. Differently from lands that had long been used for agriculture, Citrus plantations in Brazil required the removal of native vegetation for producing sweet oranges for juice processing. This strategy, although successful, disregarded many morphological and functional traits of native plants on the southern border of the Brazilian savanna (Cerrado), which was displaced by orange tree plantations. These “invader plants” had to face the sunshiny plains in the center, north, and northwest São Paulo state, where savanna-type vegetation used to grow on soils that are acidic (pH < 4.0), rich in aluminum (Al), poor in macronutrients, and that are also subjected to five-month seasonal droughts. Cerrado woody plants possess long and deep roots, with low specific leaf area, traits which possibly help these species survive droughts and fire events. These species also evolved mechanisms to deal with Al in the metabolism. On the other hand, orange trees, which exhibit traits that are typical of forest species, had to be grafted on rootstocks that attenuate such harsh edaphic conditions. In addition, fertilizers and lime are still being used today as a means of overcoming the low fertility of Cerrado soils. In this chapter we rescue biological history and ecology, and revisit differences between Citrus and Cerrado woody species, which have developed as forest and savanna species, respectively. We focused on functional traits related to nutritional and photosynthetic apparatus of these plants. We believe that these discussions could improve reflections for Citrus breeding programs.
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