Effects of N fertilization on trichome density, leaf size and artemisinin production in artemisia annua leaves

Artemisia annua is currently the only economically viable source of the antimalarial compound artemisinin. Synthesis of artemisinin takes place in glandular trichomes, primarily on the leaves from where artemisinin is extracted. It is not well understood why yields and concentrations of artemisinin vary across crops in relation to external conditions and agricultural practices. We therefore studied the diverse processes underlying artemisinin synthesis in A. annua crops, focussing on effects of |nitrogen fertilization on processes involved in formation of leaves and trichomes, and production of artemisinin in the individual leaves. In two field experiments, effects of nitrogen application levels (0, 75, 175, 400 kg N ha^-1) on leaves from a selected position at the main stem and a primary branch were studied. Measurements during part of the life cycle of the leaves included: area and dry weight per leaf, trichome density on the abaxial (lower) leaf side, trichome size, and artemisinin concentration. Results showed that effects of N fertilization were generally small, but in line with the hypothesis that at low N levels individual leaves remain smaller but have higher trichome densities. These trends were especially clear in the branch leaves. The total |number of trichomes per leaf usually increased with increase in N application up to at least 175 kg N ha^-1. Within a leaf position, effects of N application on artemisinin concentration in the leaf dry mass were similar to effects on percentage of leaf area covered by trichomes. The total quantity of artemisinin produced per (abaxial) trichome varied, but seemed to decrease linearly with increase in N level. There were no systematic linear or quadratic responses to N application in the total quantity of artemisinin per leaf. The reduction in artemisinin concentration in the leaf mass at higher N levels was therefore caused by increased dry weights per leaf.