Phytotoxic Compounds Isolated and Identified from Weeds

Abstract

Water and methanol extracts of ninety weed and crop species were bioassayed at 3,30 and 300 ppm by weight for their effect on turnip (Brassica rapa L. 'Purple Top') root growth. Eighteen species were significantly inhibitory and six species were significantly stimulatory to turnip root growth. Several species showing activity were chosen for further chemical and biological investigation. Coumarin, o-coumaric acid and melilotic acid were found to be the major active compounds in white sweet clover (Melilotus alba Desr.). Investigation of related compounds showed that activity was inhanced by increasing the side chain length and by inserting ortho-substituted hydroxyl groups, while blockage of the ortho-hydroxyls decreased activity. Rhizome exudates and pure compounds identified in johnsongrass (Sorghum halapense (L.) Pers.) were active against radish (Raphanus soliocus L.) and tomato (Lycopersicon asculentum Miller) root growth as well as growth of nine bacterial species. Leaf extracts and leachates of Illinois bundleflower (Desmanthus illinoensis (Michaux) MacM.) were active against radish and tomato root growth, bacterial growth and tobacco and tomato root growth, bacterial growth and tobacco budworm (Heliothis virescens F.) larval growth. Leaf extract activity varied widely due to the physiological age of the plant.