Studies on Desmodium Species for the Allelochemicals involved in Striga suppression

Abstract

The fodder legumes Desmodium uncinatum and D. intortum suppress the growth of Striga hermonthica (witch-weed) via an allelopathic mechanism that involves Striga germination stimulants and post-germination growth inhibitors. In this study, the root extracts (dichloromethane, acetone and methanol) from the two Desmodium species were investigated for Striga germination stimulation and post-germination radicle growth inhibition activities. The less polar extract (dichloromethane) showed Striga germination stimulation activity of over 40% at 10 ppm, but did not show any post-germination radicle growth inhibition at the same concentration. On the other hand, the polar extracts (acetone and methanol) were found to exhibit post-germination radicle growth inhibition activities of over 45% at 10 ppm. Chromatographic separation of the dichloromethane and acetone extracts of D. uncinatum roots yielded eleven compounds. These include, two pterocarpans [1,9-dihydroxy-3-methoxy-2-methylpterocarpan (uncinacarpan, 1) and 3,9-dihydroxy-1-methoxy-2-(3-methylbut-2-enyl)pterocarpan (edudiol, 2)], two isoflavanones [5,7-dihydroxy-2’,3’,4’-trimethoxy-6-(3-methylbut-2-enyl)isoflavanone (uncinanone D, 3) and 5,4’-dihydroxy-7,2’-dimethoxy-6-methylisoflavanone (uncinanone E, 4)], three abietane diterpenes [7-oxo-15-hydroxydehydroabietic acid (5), 7α-hydroxycallitrisic acid (6) and 7,15-dihydroxy-8,11,13-abietatrien-18-oic acid (7)], a phytosterol [sitosterol (8)], a pentacyclic triterpene [lupeol (9)], a long chain fatty acid [hexadecanoic acid (10)] and a flavone-C-glycoside [vitexin (11)]. Among these compounds, uncinacarpan (1), uncinanone D (3) and uncinanone E (4) are novel. The dichloromethane and acetone extracts of D. intortum roots afforded five compounds, four isoflavanones [7,2’,4’-trihydroxy-2”,2”-dimethylpyrano[5,6:6,7]isoflavanone (intortunone, 12), 5,7,2’,4’-tetrahydroxyisoflavanone (dalbergioidin, 13), 5,7,2’,4’-tetrahydroxy-6-(3-methylbut-2-enyl)isoflavanone (uncinanone A, 14) and 4”,5”-dihydro-5’,2’,4’-trihydroxy-5”-isopropenylfurano-(2”,3”:7,6)isoflavanone (uncinanone B, 15)] and an abietane diterpene [7-oxodehydroabietic acid (16)]. Intortunone (12) is a novel compound. The isolated compounds were characterized by use of a combination of spectroscopic techniques (UV, IR, MS, 1D- and 2D-NMR) and by chemical derivatization. The isoflavanones, intortunone, dalbergioidin and uncinanone B from D. intortum were found to exhibit weak Striga germination stimulation activities (below 25% at 10 ppm) whereas the flavone-C-glycoside, vitexin was found to mildly inhibit the post-germination radicle growth of Striga by 37 % at 100 ppm.