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Artemisinin and sesquiterpene precursors in dead and green leaves of Artemisia annua L. crops.

This paper analyses the accumulation and concentrations of the antimalarial artemisinin in green and dead leaves of Artemisia annua crops in two field experiments. Concentration differences were analysed as being determined by (a) the total production of artemisinin plus its upstream precursors dihydroartemisinic acid, dihydroartemisinic aldehyde, artemisinic aldehyde and artemisinic alcohol and (b) the conversion of precursors towards artemisinin. Concentrations of the total of artemisinin plus its precursors were higher in green leaves than in dead leaves in the younger crop stages, but were comparable at the final harvests. In every crop stage, the conversion of precursors to artemisinin was more advanced in dead leaves than in green leaves. This resulted in the molar concentrations of artemisinin being higher in dead leaves than in green leaves at the final harvests. The molar quantity of dihydroartemisinic acid, the last enzymatically produced precursor, was higher than that of artemisinin in green leaves, but only 19 - 27% of that of artemisinin in dead leaves. Dead leaves were very important for the final artemisinin yield. They constituted on average 34% of the total leaf dry matter and 47% of the total artemisinin yield at the final harvests. The possibility to convert a larger part of dihydroartemisinic acid into artemisinin during post-harvest handling is discussed.