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A class of nitrogen-containing pigments containing the red betacyanins (e.g. betanidin, the beetroot pigment) and the yellow betaxanthins (e.g. indicaxanthin from the fruits of Opuntia ficusindica). They replace and perform the functions of the floral pigment anthocyanin in certain plants. The possession of betalains has been used as a taxonomic character. Its occurrence is restricted to a group of families (nine or ten depending on the classification system used) which, with the exception of the Cactaceae, have all traditionally been placed in the order Centrospermae (or Caryophyllales). The Cactaceae are sometimes included in the Centrospermae on the basis of their resemblance to members of the Aizoaceae (mesembryanthemum family) but others have placed them in the monotypic order Cactales (or Opuntiales). The occurrence of betalains in cacti is strong evidence for their inclusion in the Centrospermae. Interestingly one family, the Caryophyllaceae, normally included in the Centrospermae, lacks betalains.
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