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University of KwaZulu-Natal (2016)

South African plants as a source of herbicides : identification of a compound with phytotoxic activity from artemisia afra jacq. ex willd.

Alhassan, Mahama

Titre : South African plants as a source of herbicides : identification of a compound with phytotoxic activity from artemisia afra jacq. ex willd.

Auteur : Alhassan, Mahama.

Université de soutenance : University of KwaZulu-Natal

Grade : Master of Science in Chemistry 2017

Résumé
In this study, six medicinal plant species namely, Artemisia afra Jacq. ex Willd, Eriocephalus africanus L, Vernonia natalensis Sch.Bip. ex Walp belonging to the Asteraceae family and Leonotis leonurus (L) R.Br, Plectranthus fruticosus L’Hér. ’James’ and Tetradenia riparia (Hochst.) Codd of the Lamiaceae were harvested and extracted using dichloromethane and methanol. These extracts were assayed for the germination inhibition of seeds of four different plants, namely, lettuce (Lactuca sativa), radish (Raphanus sativus), ryegrass (Lolium rididum) and teff (Eragrostic tef). The inhibition of germination was considered a preliminary indication of phytotoxicity. Based on the results obtained, the most active plant, A. afra was investigated further. The bioassays were conducted in Petri dishes with 9 cm filter papers impregnated with extract at concentrations of 25 mg/paper, 50 mg/paper and 75 mg/paper. The seeds, 10 each Lactuca sativa and Raphanus sativus and 20 and 100 each of Lolium rididum and Eragrostic teff, respectively, were used in each Petri dishes and monitored for 7 days. The DCM-MeOH (1:1) extract of A. afra showed the highest inhibition of germination, with 50% germination at the lowest concentration and no germination for the two higher concentrations for L. sativa and 40% germination at the lowest concentration and no germination at the two higher concentrations for R. sativus. In the bioassay with Eragrostic teff, the extract showed 80%, 52% and 22% germination for the three concentrations, whereas with ryegrass 95%, 80% and 0% germination at the three concentrations were observed. A bioassay-guided fractionation of the extract lead to the isolation of 7-hydroxy- 6-methoxycoumarin was identified as the compound responsible for the herbicidal property of the extract. In the bioassay, the root and shoot lengths of the germinated seeds were also measured. The pure compound was assayed and it gave the highest inhibition of root and shoot lengths of Lactuca sativa at 75 mg/dish with a Pcal value of 0.000 for each compared with the control while a commercially available coumarin (5,7-dimethoxycoumarin) at the same concentration gave for shoot length Pcal 0.003 and root length Pcal 0.167. Comparing the activity of the two compounds, the isolated coumarin from A. afra showed inhibition of both the root and shoot lengths whereas 5,7-dimethoxycoumarin only inhibited the shoot lengths and did not affect the root lengths in all the concentrations used. Two other compounds, the flavone acacetin and the sesquiterpene lactone lα,4α-dihydroxyguaia- 2,10(14),11(13)-trien-12,6α-olide were also isolated from the extract of A. afra and characterised by spectroscopic techniques. The 13C NMR data of the latter compound are reported for the first time

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