OCCURRENCE OF THE UNUSUAL AMINO COMPOUND S-METHYL CYSTEINE IN AUSTRALIAN MEMBERS OF THE RESTIONACEAE - BIOLOGICAL AND TAXONOMIC SIGNIFICANCE

The non-protein amino acid S-methyl cysteine (CH3-S-CH2 CH (NH2) COOH) was identified and assayed by HPLC and GC/MS analyses of ethanolic ex tracts of freshly-collected culms and rhizomes of Australian species o f Restionaceae. Of 140 species examined, only 20 proved positive and t wo probably positive for the compound. Amounts in culms and rhizomes o f these positive species varied from a trace to 74% of the ninhydrin-p ositive soluble amino N or from a trace to 33 mu mol per g fwt of tiss ue. Certain species showed substantial Variations in amounts of S-meth yl cysteine between sites and seasons. Xylem bleeding sap was collecte d and analysed from 32 species. The nine species whose xylem sap was p ositive for S-methyl cysteine (0.5-23.5% of xylem total amino N) also rated positive in culms and rhizomes. Investment of N in S-methyl cyst eine ranged from 0.1 to 6.3% of culm total N or 0.2-6.7% of rhizome to tal N. A possible role of the compound in protection against herbivory was examined. Positive species came from only three of the 19 current ly recognized genera, Lepyrodia (16 positive, two possibly positive), Restio (one positive species) Loxocarya, (one positive species) and a further three as yet unnamed species. Under a proposed revision (B. Br iggs and L. Johnson, pers. comm.), the positive species of Restio and the three unnamed species would be ascribed to a redefined genus (Loxo carya sens. orig.), thus restricting all species containing S-methyl c ysteine to this new genus and Lepyrodia. The latter genus is changed u nder the proposed classification by transfer of certain species to the genus Sporadanthus. All six species proposed for such transfer lack t he compound. Taxonomic implications of the findings are discussed.