The genus Artemisia includes a large number of species and some have been cultivated as commercial crops with a wide diversity of uses. Some better known examples include antimalarial (Artemisia annua - annual or sweet wormwood), culinary spices (Artemisia dracunculus - French tarragon), liquor flavouring (Artemisia absinthium - absinthe), garden ornamental (A. abrotanum - southernwood) and insect repellent (Artemisia vulgaris - mugwort). However this review will concentrate on the cultivation of Artemisia annua because of its contemporary importance as a source of new and effective antimalarial drugs.
During World War II and in the years immediately following, the world wide incidence of malaria was dramatically reduced. On the one hand the Anopheles mosquito vector was successfully controlled by the advent of the insecticide DDT and on the other the organisms causing human malaria – the single celled Plasmodium species: falciparum, vivax, malariae and ovale - were effectively controlled by the use of synthetic derivatives of quinine. The specific statistics for India illustrate this dramatic reduction. In 1961 the incidence of malaria had fallen to about 100,000 reported cases, however by 1977 the number of reported cases in India had risen to at least 30 million (). The reasons for this dramatic increase were the dual factors of the development of resistance to DDT by the Anopheles mosquito and the development of resistance to quinine and quinine analogues by the Plasmodium. Although the problem of malarial drug resistance in the Plasmodium varies throughout the world – with the worst in South East Asia – the overall situation is that malaria is currently the most serious and devastating tropical disease in the world. At least 300 million clinical cases occur worldwide with up to 2.7 million deaths annually ().
It was because of this background of a massive increase in the incidence of malaria and the serious reduction in effectiveness of standard antimalarial drugs that the possibilities extended by extracts from Artemisia annua were eagerly taken up. Artemisia annua L. is a member of the family Asteraceae (formerly Compositae) and is considered to be a native of Asia with its origin in China () where it forms part of the native plant population of the steppes of Chahar and Suiyuan Provinces (40° N, 109° E) (). Artemisia annua, or Qing Hao (= green herb) as it is referred to in the Chinese literature and quite often in the general literature, has been used as a traditional herbal treatment for malaria in China for centuries with the first recorded use dating back almost 2000 years (). This traditional use of Artemisia annua utilised extraction of the plant with hot or boiling water either alone or more commonly as part of a mixture with other herbs. Chinese scientists studied the effectiveness of the herbal infusion method utilising Artemisia annua in the late 1960’s and found that it was not effective (). However extraction of Artemisia annua leaf with diethyl ether gave a compound with marked anti-malarial activity in animal experiments. This compound was isolated and identified as artemisinin by Chinese scientists in 1972 who also refer to it by its Chinese name of qinghaosu (= extract of green herb) (). Artemisinin is more specifically an uncommon sesquiterpene lactone endoperoxide of the cadinane series (). With the possible exception of A. apiacea () Artemisia annua appears to be the only species in the plant kingdom which contains artemisinin.
The apparent lack of effectiveness of infusions of Artemisia annua in hot or boiling water raises the question of why the plant should have established its reputation as a febrifuge. One explanation offered is that Artemisia annua was commonly used as an infusion in conjunction with other herbs. Extracts from these herbs such as saponins may have acted as detergents and released artemisinin into the water. Flavonoids, which may also have been present in the other herbs, could also have potentiated the antimalarial activity of artemisinin (). One particular mixture recorded in an old Chinese herbal () utilised Artemisia annua, Amy da sinensis, Rebmania gluti-nosa, Anemarrbea aspholeloides and Paeonia suffruticosa. A. much earlier Chinese herbal (Ge Hong, 340 AD) prescribed the top growth of Artemisia annua for the treatment of fevers.
Since the isolation and identification of artemisinin from Artemisia annua as the compound which exerts the antimalarial effect, detailed studies have been carried out on other related compounds in the plant and on ways in which artemisinin can be modified to have more effective medicinal qualities. Artemisinic acid – also known as qinghao acid – a precursor of artemisinin which is present in some parts of the plant at concentrations up to tenfold that of artemisinin () is one of the more important compounds of potential commercial importance. Arteether () and artemether () the ethyl and methyl derivatives respectively of artemisinin are two of the more important conversion products of enhanced antimalarial efficacy which are being developed commercially.
Despite the fact that chemical synthesis of artemisinin can be accomplished () the cost and complexity of the synthesis is such that extraction of artemisinin from Artemisia annua plant material is a much more economic method of production. Although micro propagation of Artemisia annua can be easily accomplished () it is unlikely that the in vitro production of artemisinin will be a commercially viable proposition (). Because of this, the production of artemisinin from field cultivated plants will be the preferred option for the foreseeable future. A. number of useful abbreviated general outlines of Artemisia annua cultivation have been recorded (). This review will give a detailed assessment of the various contributions which have been made to the knowledge of cultivation of Artemisia annua. The review will follow the growth cycle of the plant from seed drilling or transplanting through to vegetative growth, final harvest and post harvest operations.
Artemisia annua is an annual shrub of the family Asteraceae (formerly Compositae). The height ranges from 30-250 cm depending on strain, region of production and a range of agronomic factors particularly plant population density. The leaves are bipinnatifid, glabrous with segments linear and dentate. The inflorescence is a terminal compound panicle. The flower heads are small and typically yellow in color, heterogamous, 2-3 mm wide, globular, bracts linear, oval-acuminate or oval. The marginal flowers are female with a corolla 4 lobed and the disk (central) flowers are hermaphrodite, with the corolla 5 lobed and stamens 5. Glandular trichomes containing a strongly volatile oil are present in both flower heads and leaves of Artemisia annua. The fruit is an obovoid achene often light grey in color, smooth and about 0.5 mm long (). A. very detailed botanical description of Artemisia annua has been recorded by Ferreira et al. ().
Climatic Range of Production
Although Artemisia annua is generally considered to have its origins in the temperate regions of China (40° N) where it forms a part of the natural steppe vegetation at an elevation of 1000-1500 metres above sea level () the range of the plant is much wider. In China it is fairly generally distributed and extends as a native into southern Siberia, Vietnam and northern India (). Outside of Asia Artemisia annua has been introduced and grows wild in a wide range of countries in Europe (Hungary, Bulgaria, Rumania, France), North America (USA) and South America (Argentina) (). In addition, it has been introduced into experimental cultivation in Vietnam, Thailand, Burma, Madagascar, Malaysia, USA, Brazil, Australia (Tasmania) and in Europe into Holland, Switzerland, France and as far north as Finland.
The climatic range of Artemisia annua is of considerable importance in determining areas for potential production. Although Artemisia annua originated in relatively temperate latitudes () it appears that the plant can grow effectively at much lower tropical latitudes with seed selections which are either native to these areas () or which have been adapted by breeding (). However other workers have concluded that cultivation of Artemisia annua would be unadapted to the tropics because the plant is a short day annual which would flower without achieving sufficient biomass (). These conclusions may well need to be reassessed in the light of other evidence from late flowering strains of Artemisia annua. The high artemisinin concentrations (0.5-1.5%) in the leaves of some of these strains could allow high artemisinin yields in tropical latitudes even though the leaf biomass may not be as high as some strains of Artemisia annua grown in temperate latitudes. In Vietnam (near Hanoi), at a latitude of 21° 02′ N, a field experiment achieved adequate leaf dry matter (5.3 t/ha) before flowering with high artemisinin yields equivalent to about 45 kg/ha (). Also in Madagascar at 18° 52′ S adequate leaf dry matter (4.7 t/ha) was also achieved before flowering with a high artemisinin yield of about 40 kg/ha (). In this study the field experiment was carried out at an elevation of ca. 1500 metres above sea level. The beneficial influence that higher altitudes may have on the production of Artemisia annua at tropical latitudes may be a principle which could be applied to other parts of tropical Africa and elsewhere where similar areas could be investigated. The possibility that drugs derived from Artemisia annua may soon be needed in the battle against malaria in Africa should be an incentive to seek out these niche areas of production (see Time of establishment).
Site and Soil Selection
The ideal site selection for Artemisia annua cultivation would depend on the scale of operation and the location of commercial extraction plants. The concentration of artemisinin in Artemisia annua (see Germplasm) is relatively low and the total fresh plant yield from which the chemical is extracted can be as high as 100t/ha and greater. Generally it would seem that the extraction and processing plant should be as close as possible to the area of production, or vice versa depending on circumstances, to minimise transport costs. If ultimately mechanical harvesting is a feasible proposition () then the selection of relatively flat locations would be appropriate. Artemisia annua has been grown in a wide diversity of soils and its effectiveness in colonising waste areas suggests that it is very adaptable. Apart from an intolerance of some Artemisia annua selections to acid soils below pH 5.0-5.5 () many soil types could be utilised.
Since the identification of artemisinin as the compound in Artemisia annua which conveys the antimalarial effect there have been a number of investigations to select strains with high artemisinin. These studies have shown wide variability in artemisinin content which can range from 0.01% () to about 1.0% () and approaching 1.5% (). The economics of commercial development of artemisinin derived drugs and their use in areas of most need hinge on plant raw material with high artemisinin content. Because of this, investigations have been carried out to select seed from high artemisinin producers coupled with other desirable agronomic characteristics. These include good seed and plant vigour, high leaf to stem ratio with high dry matter leaf yield, disease resistance and desirable time of flowering appropriate to the region of production. The seeds of Artemisia annua will maintain their vigour for at least three years if stored under dry, cool conditions ().
Germplasm comparison and selections have been carried out on the basis of (i) plants which have been introduced and established in the investigating countries and (ii) promising high artemisinin lines introduced from countries where Artemisia annua is native (e.g. China and Vietnam). However commercial competitiveness in the possession of high artemisinin lines has limited the widespread availability of these lines at the present time. Likewise the general access to hybrids which have incorporated the high artemisinin (1.1%) but low vigour of Chinese clones with the low artemisinin (0.04-0.22%) but high vigour of a range of European clones () are also generally unavailable. Similarly the more recent hybrids between Chinese and Vietnamese selections with even higher artemisinin (1.0-1.5%) are only available to a limited extent (). The cost and time involved in carrying out accurate assays for artemisinin have also been other limitations on the screening and selection of Artemisia annua lines. Rapid and economical methods of artemisinin assay developed by Charles et al. () and Ferreira and Janick () and the technique of appraising plantlets growing in vitro in the laboratory for artemisinin content () are all strategies by which the process of screening Artemisia annua germplasm could be speeded up. Germplasm assessment and studies of a range of other agronomic factors which have a bearing on the successful cultivation of Artemisia annua have been carried out in Australia (Tasmania) (), Brazil (), India (), Japan (), Madagascar (), Netherlands (), Switzerland (), and in the United States ().
Alternative selection strategies
Artemisinic acid (qinghao acid) a precursor of artemisinin is present in Artemisia annua at concentrations up to tenfold that of artemisinin () and can be converted to artemisinin with an efficiency up to 40% (). Artemisia annua also contains an oil which is used in perfumery and as an anti-microbial (). At the present time the volume of oil traded is relatively small but if the cultivation of Artemisia annua expands, the greater availability of the oil may make it a more commercially attractive commodity. Various strains of Artemisia annua which are high in specific oil components of commercial interest have been identified in studies in the United States (). The screening of Artemisia annua germplasm for both artemisinic acid and oil content as well as artemisinin may be a very useful technique in that it may be possible to extract both oil and artemisinic acid in the one operation ().
Selections from the book: “Artemisia”. Edited by Colin W. Wright. Series: “Medicinal and Aromatic Plants — Industrial Profiles”. 2002.