Artemisia annua L. (Asteraceae)


Annual wormwood

Annual wormwood growing in Oxford Botanic Garden. Annual wormwood flowering in Oxford Botanic Garden.

Without showy flowers, only small, pale green, wind-pollinated capitula, it would be easy to overlook Artemisia annua. It can be hard to believe annual wormwood is in the same family as showy sunflowers, dahlias and chrysanthemums. In fact, the head-like capitulum, the inflorescence, is the common character of the family Asteraceae. The capitulum is made up of many, small flowers, known as florets, surrounded by a ring of bracts (involucre). The Asteraceae is a large and diverse family and includes familiar, ornamental plants, as well as some important economic crops.

In Artemisia annua all of the florets are of the same type; only under a hand-lens does each floret take on the appearance of a more familiar flower, with corolla, stamens and ovary being distinguished. The genus Artemisia includes economic plants such as Artemisia absinthium, the flavour of absinthe and vermouth, and Artemisia dracunculus, Russian tarragon. Like other Artemisia species, annual wormwood has highly aromatic leaves. The essential oils making up the particular aroma are stored in special glandular hairs (trichomes) covering the surfaces of the fern-like leaves, and other plant parts. The trichomes are also the storage site of an even more significant substance, artemisinin.

In the 1960s the Chinese government initiated a systematic investigation of the Chinese Materia Medica with the goal of identifying an anti-malarial medicine; such a medicine was found. Malaria is a threat to almost half of the World's population. Every year approximately 100 million people suffer from malaria and one million people die from the disease (predominantly African children). The parasite Plasmodium falciparum is responsible for the majority of these cases and is transmitted when female Anopheles mosquitoes, infected with the parasite, bite people.

Of all the hundreds of plants the Chinese tested, only annual wormwood was found to have any activity against the malarial parasite. Further investigation revealed the activity was due to artemisinin, the structure of which was determined in 1972. Artemisinin, and its more water-soluble derivatives artemether and artensunate, have become the treatments of choice for malaria. To delay the appearance of resistant malarial parasites, artemisinin-based treatments are always used in combination with other therapies. Artemisinin for medical purposes is grown worldwide but yields are low. A genetic map of annual wormwood has revealed loci affecting the yield of artemisinin. Such information is likely to be important for the genetic improvement of annual woodworm, especially the breeding of high artemisinin-producing plants.

Further reading

Graham IA et al. 2010. The genetic map of Artemisia annua L. identifies loci affecting yield of the antimalarial drug artemisinin. Science 327: 328-331.

Hsu E 2010. Qinh hao (Herba Artemisiae annuae) in the Chinese Materia Medica. In: Hsu E & Harris S (eds) Plants, health and healing: on the interface of ethnobotany and medical anthropology. Berghahn Books, pp. 83-130.

Alison Foster