Chemical composition of seed saponins as a guide to the classification of Medicago species

The presence of saponins and their aglycones (sapogenins) was surveyed in seeds of 33 species of Medicago (Fabaceae). The four major subsections of the largest taxonomic section of Medicago, section Spirocarpos, can generally be distinguished by presence and (or) absence of medicagenic acid and zanhic acid. The latter sapogenin was found only in three of the subsections of Spirocarpos and seems to reflect the phylogenetic recency of these groups within Medicago. The distribution of zanhic acid in combination with other considerations proved useful in establishing that Medicago granadensis seems to be the oldest species in subsection Intertextae. The saponin profile of Medicago hybrida, most recently assigned to section Medicago, strongly supports the placement of this species and its relative Medicago sufruticosa in a separate section. Hemolytic saponins proved to be much more common in those species of Medicago that retain their seeds permanently within the fruit compared with species that tend to scatter their seeds, possibly because the former can benefit more from the antiherbivorous and (or) allelopathic properties of the saponins. Key words: hemolytic saponins, chemotaxonomy, taxonomy, ecology, Medicago.