Artemisinin is a sesquiterpene lactone endoperoxide and an important antimalarial drug produced in Artemisia annua. To unravel the diverse processes determining artemisinin yield in A. annua crops, artemisinin accumulation during the development of individual leaves was studied in two field experiments. During the life cycle of a leaf, artemisinin was always present. Quantities were low at leaf appearance and increased steadily. In leaves studied until after senescence, maximum quantities and concentrations were achieved after the leaf had turned brown. The total quantity of possible artemisinin precursors per leaf (dihydroartemisinic acid and other upstream precursors) was highest early in the leaf cycle when the leaf was still expanding. Dihydroartemisinic acid was more abundant than the other compounds and its quantity declined during leaf development whereas that of artemisinin increased. Dihydroartemisinic acid was not converted directly into artemisinin, because on a per leaf basis the decline in molar quantity of precursors in the earliest formed leaves was not compensated for by a simultaneous increase in artemisinin. Our results suggest that a (putative) intermediate such as dihydroartemisinic acid hydroperoxide temporarily may have accumulated in considerable quantities. The number of mature, capitate trichomes on the adaxial leaf side increased after leaf appearance until the end of leaf expansion, and then decreased, probably due to collapse of trichomes. Artemisinin production thus (also) occurred when trichomes were collapsing. Later formed leaves achieved higher concentrations of artemisinin than earlier formed leaves, because of a higher trichome density and a higher capacity per trichome.

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The quantitative insight in processes underlying yield and concentrations of interesting secondary metabolites in crops is still limited. Yet, this insight is essential to further improve crops and commercial production of target metabolites. Artemisia annua L. (annual or sweet wormwood, Asteraceae) was used to conceptualize a model to describe the processes determining and limiting the production of target metabolites during crop growth. A. annua is an annual herb producing the antimalarial artemisinin, a sesquiterpene lactone with an endoperoxide bridge. Artemisinin is predominantly produced in glandular trichomes present on the leaves and inflorescences. Leaves are the most important organs harvested in commercial production. The accumulation of artemisinin in the crop was analysed as the resultant of the key processes determining leaf dry weight production, accumulation of artemisinin in the leaves and losses after synthesis. A yield formation modelling approach was used to quantify artemisinin yield as a function of the individual processes and to study which processes limited production. Leaf dry weight production was limited by low total dry matter production of the crop because of poor radiation interception during early canopy expansion, and by a high proportion of dry matter allocated to stems that do not contribute to artemisinin production. Production of the target metabolite in the leaves was limited because the total production of artemisinin precursors per unit leaf dry weight was low and because the conversion of precursors to artemisinin was only partial. Possibilities to increase the values of the different yield components are discussed. A new, simple, model for explaining variation in artemisinin yield based on leaf and trichome production in combination with biosynthesis of artemisinin is proposed. INTRODUCTIONInterest in plant secondary metabolites that have a protective or curative effect on human diseases has been renewed. These compounds can be taken in as drugs or as part of food and beverages. Yields and concentrations of target metabolites in crops or crop products are usually low and highly variable over studies. Also within crops, concentrations vary. The causes of these fluctuations are often unknown because commercial production methods are commonly based on applied dose-effect studies with limited attention to the crop physiology behind the effects. A great need exists for more insight into the different processes underlying the accumulation of the secondary metabolites of interest. An improved understanding can lead to more stable yield levels and concentrations, better breeding strategies, more founded selection of production environments, and agronomical and technological innovations in the production chain.To achieve a full and quantitative understanding of accumulation of secondary metabolites is difficult because mechanisms underlying their yields and concentrations in crops are more complex than those underlying yield of plant organs, yield of primary