Allelopathy related to Artemisia plants has already been described 100 years ago (Bode, H. R. 1939. Über die Blattausscheidungen des Wermuts und ihre Wirkung auf andere Pflanzen. Planta 30:567-589). The work of H Bode was completed a few years later (GL Funke. Blumea - Biodiversity, Evolution and Biogeography of Plants, 1943 5, 281-293). Eighteen species of plants, most of which were chosen at random, were sown beside a hedge of Artemisia absinthium; they were severely injured and in one case even killed by the chemical excretions of the latter within a distance of ± 100 cm. The experiments were made during two successive summers and in different surroundings. Seedlings which had survived the proximity of the absinth in their first year developed normally during the next season.
Artemisias are often considered as invasive species. For example Artemisia tridentata, big sagebrush, is predominant in the steppes of Idaho, Washington, Oregon, Nevada, California, Arizona, Utah, covering an area of at least 570 000 km². The authors (TW Weaver et al., The American Midland Naturalist, 1977. 97, 508-512) suggest that the success of A tridentata depends on the production of phytotoxins.
But the nature of these phytotoxins was not known. More recent papers suggest that the allelopathy of Artemisia plants might be related to tannins.
This hypothesis was first advanced in 1964 (HD Zinsmeister,Der Einfluss von chinesischem Tannin auf Wachstum, Planta 1964, 61, 130-141). Indeed in most of the cases the aqueous extracts of various plants have stronger allelopathic properties than extracts obtained with apolar solvents like hexane. Tannins are highly soluble in water and not in hexane. In 1959 already a hydrolysable tannin as germination inihibitor was identified. The hot and cold water extracts of fresh or fallen leaves of Pinus densiflora inhibited the seed germination of several plants. Aqueous extracts of Artemisia tridentata, Artemisia cana, Artemisiia frigida, Artemisia dracunculus all inhibited germination of various plants (GR Hoffman et al., J Range Management, 1977, 30, 134-137). Aqueous extracts of Artemisia monosperma inhibit the germination of Phaseolus vulgaris (A Al-Watban et al., Int Res J Plant Science 2012, 3 158-163) The allelopathic effect of aqueous extract of Artemisia annua was studied on Plantago ovate (Seyed Mohsen et al., Annals of Biol Res 2011, 2, 687-691). A significant effect was noticed on germination percentage and rate, plumule and radicle lengths and fresh and dried leaves of seedlings. The effect was concentration dependant. Another study with the aqueous extract of Artemisia annua showed a remarkable inhibition on the growing of wheat seedlings (Mu Xiaoquian et al., Acta Bot Boreali-Occident. 2005, 25,, 1025-1028). By means of bioassay in laboratory and field, a Chinese team studied the allelopathy of 18 kinds of plants in Gansu Province on the seedlings of wheat, cucumber and radish. The results showed that the aqueous extract of the stems and leaves of Artemisia annua, had the strongest allelopathy on test receptor plants. (Shen H1, Ying Yong Sheng Tai Xue Bao. 2005 Apr;16(4):740-743). Another paper (N Kornilova, 2014 Зб алансоване природокористування) deals with allelopathic impact of of Artemisia absinthium L. on the growth and development on other medicinal plants. It has been determined, that water extracts from Artemisia absinthium L. are characterized by significant allelopathic activity on Calendula officinalis L., Hyssopus officinalis L, Salvia officinalis L., Tagetes patula L., Ocimum basilicum L., Gr., Melissa officinalis L. L., Origanum vulgare L., Nepeta cataria L.,
The plant Sorghum rich in condensed tannins is well known for its allelopathic properties (Ch Summers et al., http://CaliforniaAgriculture.ucop,ed.Jan-Mar 2009, 35-40). As Artemisia plants have shown similar allelopathic properties on might suppose that these plants also are rich in tannins. Tannins might even contribute significantly to the prophylactic and therapeutical properties. Artemisia is one of the plants richest in tannins (proanthocyanidins) , more than other medicinal herbs (T Sunmonu et al., Pakistan J of Nutrition 2012, 11, 520-525). In Algeria Artemisia herba alba also was found to contain a sizeable amount of proanthocynidins (LS Eddine et al., J Pharmacy Res 2016, 10, 58-64) Tannins appear thus to play a key role in allelopathy. They are released by decaying plants and contaminate soils and rivers. They accumulate in soils because they have a high stability, are not destroyed by soil bacteria and easily washed out. Many other flavonoids like casticin or artemetin are unstable (PJ Weathers et al., Planta Med 2012, 78, 1024-26). Tannins inhibit germination of other plants and inhibit nitrate uptake (Elroy L. Rice and Sunil K. Pancholy ,American Journal of Botany 1974Vol. 61, No. 10, 1095-1103). Or more exactly tannic acid in soil drastically reduces Rhizobium and Azotobacter. Available nitrogen declines (G Muthukumar et al., Proc Indian Natn,Sci Acad 1985, B51, 270-281). This was first noticed when effluents of tanneries have gradually made thousands of hectares non productive in some parts of India. A trial was made in Iran to rehabilitate with other plants or herbs areas which had been invaded by Artemisia sieberi. This range improvement failed (R Bagheri et al., Iran J Range and Desert Res, 2011 17, 548). Artemisia sieberi is very similar to Artemisia herba alba growing in Morocco, Algeria, Tunesia.
It is not our intent to interfer with the business of pharmaceutical companies which have launched large Artemisia annua plantatations in Africa. We hope that they have studied the consequences these plantations might have for local farmers if these want to recover these fields for other agricultural needs.
As humanitarian NGO with the objective to see Artemisia plants growing in every garden in Africa as natural drug against malaria, bilharzia tuberculosis we feel responsible for studying the effects Artemisia plants might have on other staple food crops in these family gardens.
Allelopathy can also be used as environmentally friendly method for weed control. The herbibicidal and fungicidal effect of Pomegranate peels which are rich in tannins are well known
See also the following blogs on www.malariaworld.org.
Why are Artemisia plants prophylactic?
The herbicidal hypothesis. Are Artemisia annua plantations killing fields ?