Distribution of miliacin (olean-18-en-3β-ol methyl ether) and related compounds in broomcorn millet (Panicum miliaceum) and other reputed sources: Implications for the use of sedimentary miliacin as a tracer of millet
Résumé
Using sedimentary miliacin (olean-18-en-3β-ol methyl ether) as a molecular tracer of the history of Panicum miliaceum (broomcorn millet) cultivation depends upon broomcorn millet being sedimentary miliacin's dominant source. It also requires knowledge of the variability in miliacin concentration in broomcorn millet. Finally, it is affected by the presence of other pentacyclic triterpene methyl ethers (PTMEs) that may exist in conjunction with miliacin in other sources but not in broomcorn millet. Miliacin biosynthesis has been proposed for other Panicum species, Setaria italica (Italian or foxtail millet), Pennisetum sp., and Chaetomium olivaceum (an olive green mold). We found miliacin concentration in seeds of different varieties of P. miliaceum to be similarly high (with trace amounts of β- and α-amyrin methyl ethers). It was absent from hulls and roots, and nominally present in leaves and stems. The transfer of miliacin from plant to sediments is therefore mostly from seeds. it was abundant (often with larger amounts of β- and α-amyrin methyl ethers) in all other Panicum species studied, but only in some species of the genus Pennisetum and was absent from Setaria italica. Neither C. olivaceum nor its growth medium (rice) showed any trace of miliacin. Our results, with miliacin absent from S. italica and C. olivaceum, its high miliacin in seed of P. miliaceum relative to other PTMEs and to other grasses and, considering the high biomass that cultivated broomcorn millet has relative to other potential plant sources, support the use of sedimentary records of miliacin in some contexts to track past millet agricultural dynamics.
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