Phytochemistry of Acacia—sensu lato

Abstract

Little is known about the chemistry of most species of the genus Acacia, although the genus is quite large and widespread in the warm subarid and arid portions of the world. As presently defined, Acacia is a cosmopolitan genus containing in excess of 1350 species. Taxonomic relationships and identification of Acacia species are difficult; new studies of the genus confirm that Acacia is an agglomeration of at least five discrete groups. The major elements of this ‘genus’ are the groups now recognized as the subgenus Acacia, the genus Faidherbia, the subgenus ‘Aculeiferum’, relatives of Acacia coulteri, Bentham’s series Filicinae, the subgenus Phyllodineae, and possibly others, each with somewhat distinct chemistry. A number of secondary metabolites have been reported from various Acacia species including amines and alkaloids, cyanogenic glycosides, cyclitols, fatty acids and seed oils, fluoroacetate, gums, non-protein amino acids, terpenes (including essential oils, diterpenes, phytosterol and triterpene genins and saponins), hydrolyzable tannins, flavonoids and condensed tannins. The most evident and best known are polysaccharides (gums) and complex phenolic substances (condensed tannins).

Introduction

Although the genus Acacia is quite large and is widespread in the warm subarid and arid parts of the world, relatively little is known about the chemistry of most species. The most evident substances in many Acacia species are complex phenolic compounds (condensed tannins) and polysaccharides (gums). The difficulty of structure elucidation and the overall lack of toxicity of these substances undoubtedly has contributed to a dearth of chemical study of these plants. The situation is further complicated because identification of Acacia species is difficult and their taxonomic relationships are not clear. Until relatively recently, the monotypic genus Faidherbia had been treated as a species of Acacia, but the independent generic status of Faidherbia is no longer in doubt (Maslin and Stirton, 1997).

As presently defined, Acacia is a cosmopolitan genus containing in excess of 1350 species that together with the African and the Middle Eastern monotypic genus Faidherbia Chev. comprises tribe Acacieae of subfamily Mimosoideae (Fabaceae).

Cladistic analyses of chloroplast genes and morphological data have been especially informative in developing a better understanding of phylogenetic relationships of this ‘genus’ (Bukhari et al., 1999, Chappill and Maslin, 1995, Clarke et al., 2000, Grimes, 1999, Luckow et al., 2000, Miller and Bayer, 2000, Miller and Bayer, 2001, Miller and Bayer, in press, Robinson and Harris, 2000). Based on these new data and analyses, five monophyletic groups are recognized within Acacia sensu lato. These five genera correspond to those recognized by Pedley (1986) except that Senegalia sensu lato is now regarded as comprising three genera, namely Senegalia sensu stricto, Acaciella Britton and Rose (based on Acacia subgenus Aculeiferum section Filicinae) and an as yet undescribed new genus based on a group of species related to Acacia coulteri (Jawad et al., 2000). Acacia subgenus Acacia appears to be rooted in taxa generally placed in tribe Mimoseae (Luckow et al., 2000). The relationships of subgenus Phyllodineae, subgenus Aculeiferum sensu stricto, sect. Filicinae, the A. coulteri group and Faidherbia are not fully resolved, although in all studies cited above these groups are shown to be monophyletic. On the basis of the recent molecular and cladistic studies above, and from consideration of previous morphological, biochemical and other evidence, it is clear that the genus Acacia, as currently defined, is polyphyletic and cannot be sustained as a single genus (Maslin et al., in press).

Pending revision of the genus, the current classification of Acacia, which views the genus as comprising three large subgenera, namely subgenus Acacia (ca. 161 species, pantropical), subgenus Aculeiferum Vassal (235 species, pantropical) and subgenus Phyllodineae (DC.) Seringe (syn. subgenus Heterophyllum Vassal) (960 species, largely confined to Australia) (Maslin et al., in press), will be accepted for this review, except that Acacia subgenus Aculeiferum sensu lato will be viewed as three groups: subgenus Aculeiferum sensu stricto, series Filicinae Bentham, and a group of species related to A. coulteri.