Studies on Desmodium Species for the Allelochemicals involved in Striga suppression

Date of Award

2007

Document Type

Dissertation

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.

This document is currently not available here.

Share

COinS