Plant measurements
Main stem heights were measured at 7–12-d intervals from late July to mid-September and finally in early October. To determine plant biomass, artemisinin concentration, artemisinin yield and leaf mineral concentration, plants from both experiments were harvested in October. Leaves and stems from a portion of the main stem of each plant, e.g. one side, were removed from the plant base to the tip. The leaf and stem material was collected from each of the three plants per plot and bulked. The fresh weight of this material was recorded. The material was then dried, in a ventilated oven, at 40–42 8C for at least 48 h, and then passed through a sieve (5-mm mesh) to separate the leaf lamina from pedicles. The dry weights of the two components were recorded. Leaf and stem material was stored, at room temperature, in sealed polythene grip bags to await mineral and artemisinin analysis. The stems and the leaves that remained on the three plants were removed and the fresh weight of the material was recorded. The dry weight of this portion of the plant was estimated using the fresh weight to dry weight ratio obtained from the portion of the plant that was dried. The fresh weight of the main stems, for the three plants, was also bulked and recorded, and these were dried as above. Total fresh weight and estimated dry weight per plant were determined.