Methylxanthines have been found in nearly 100 species in 13 orders of the plant
kingdom (Ashihara and Suzuki 2004; Ashihara and Crozier 1999a). Compared with
other plant alkaloids, such as nicotine, morphine and strychnine, purine alkaloids
are distributed widely throughout the plant kingdom although accumulation of high
concentrations is restricted to a limited number of species, including Coffea