Aluminium accumulation in leaves of Rubiaceae: Systematic and phylogenetic implications

Abstract

Aluminium (Al) accumulators are plants which accumulate more than 1000 ppm Al in their tissues. In addition to earlier analyses on leaves of Rubiaceae, 251 specimens were tested to verify the systematic importance of this character. The distribution of Al accumulators in the family shows that the feature is more or less restricted to a widely circumscribed subfamily Rubioideae including Craterispermeae, Knoxieae, Urophylleae and Pauridiantheae. Other Rubioideae representing strong Al accumulators are the genus Coccocypselum, Coussareeae, Prismatomerideae, and most Psychotrieae. It is hypothesized that the ability to accumulate high levels of Al evolved in an ancestor of the Rubioideae s.l. since the feature is concentrated in basal taxa of this subfamily. Moreover, recent phylogenetic insights in the subfamilial classification of the Rubiaceae based on macromolecular data are confirmed. Al accumulation is reported in only two genera outside Rubioideae s.l., viz. Coptosapelta and Alberta. Al accumulators generally are woody, relatively primitive taxa which occur in tropical forests with relatively high rainfall. In more derived Rubioideae such as Anthospermeae, Hedyotideae, Paederieae, Rubieae and Spermacoceae the tendency to herbaceousness is possibly associated with the absence of Al accumulation, it is likely that this lack is related to their adaptation to more xeric, alkaline soils and their more temperate distribution.