Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)

The content of the cysteine sulphoxides methiin, alliin, isoalliin, and propiin was studied in 72 Allium species, subspecies, cultivars, and land-races, three Tulbaghia species, Triteleia laxa, and the Brassicaceae Alliaria petiolata. Methiin was present in all investigated samples. In the genus Allium, methiin-dominated species (rarely used by man) were common, but the occurrence of the other cysteine sulphoxides was variable and was largely correlated with use as spices or vegetables. Two major chemical types (named according to the species where they occur) could be distinguished, and at least two more may be recognized. Isoalliin dominates in the widely used “onion-type”, which includes chive ( A. schoenoprasum) and top onion ( A. × proliferum). Pearl onion and leek ( A. ampeloprasum) have higher relative amounts of methiin and propiin, respectively. Alliin dominates in the widely used “garlic-type”, which includes wild leek ( A. obliquum) and sand leek ( A. scorodoprasum). Alliin and isoalliin rarely co-dominate, being only found in the cultivated Chinese leek ( A. tuberosum). A triple mix of almost equal amounts of methiin, alliin and isoalliin is present in ramson ( A. ursinum). General trends in the three evolutionary lines of the genus Allium were apparent. In the first lineage, high amounts of methiin were more frequent, and propiin, although being a minor component in all three evolutionary lines, was present in the highest amounts. Most of the species in the second line showed only traces of cysteine sulphoxides. In the third line, the “onion-type” dominates, the “garlic type” is characteristic for subgenus Allium, and co-dominating alliin and isoalliin also occur. Generally, the total cysteine sulphoxide amount increased, and the complexity of cysteine sulphoxide patterns decreased in the transition from the first to the third evolutionary line.