A phenolic–taxometric study of Medicago (Leguminosae)

Abstract

Phenolics from dried leaf tissue of 47 species of Medicago were examined using two-dimensional chromatography on polyamide plates. A few samples from the closely related genera Melilotus and Trigonella were also studied. Compounds tentatively identified as cyanidin, quercetin, kaempferol, luteolin, apigenin, chrysoeriol, tricin, and coumestrol were found. Proanthocyanidins were not detected in the leaf tissue of any of the species. Apigenin was found in all species of Medicago except M. cretacea but not in the one species of Melilotus or the two of Trigonella studied. Two additional compounds as yet unidentified were also restricted to Medicago. All of the phenolics were detected as aglycones with the possible exceptions of glucuronides and C-glycosides which are strongly resistant to hydrolysis. Data for the phenolic compounds were subjected to numerical taxonomic analysis. The single species of Melilotus and the two species of Trigonella were dramatically different from Medicago in their phenolic composition. A number of controversial species placed in Medicago by some taxonomists, and in Trigonella by others, proved to have phenolic profiles much like Medicago and quite unlike the species of Trigonella examined. The annual species comprising subgenus Spirocarpos, which are morphologically distinct from the remaining species of Medicago, were not clearly distinguishable from the other species collectively on the basis of phenolic chemistry. However, within subgenus Spirocarpos the most sharply distinguished subgroup on morphological criteria, section Pachyspirae, tended to be different chemically from the remaining species of the subgenus. When the species of Medicago except for subgenus Spirocarpos were analysed separately, it was found that the two major phylads of these tended to be distinguishable chemically. These included a group of perennials (subgenus Medicago section Falcago) and a group of peculiar species (subgenus Orbicularia) tending morphologically towards the genus Trigonella). Appreciable intraspecific diversity was found for phenolic chemistry but significantly less than interspecific variation. In a few cases species which are quite similar morphologically diverged chemically, most notably the two species of subgenus Lupularia.