Silverleaf nightshade is a serious problem weed in many semi-arid regions of the world. Although many research efforts have been devoted to this weed’s interference with crop species, possible chemical interference (allelopathy) has not been thoroughly investigated yet. In the present study, allelopathic potential of silverleaf nightshade foliage was assessed by means of germination bioassays. Preliminary experimentation was necessary to evaluate procedures of the bioassay method to be used, as many different approaches are described in literature. Water infusions and crude water-soluble extracts of silverleaf nightshade foliage inhibited germination and root growth of cotton and lettuce respectively. Osmolalities of the infusions or extracts used were not inhibitory to germination or root growth of either cotton or lettuce. Preliminary exploration of the nature of the chemical substances implicated in this phytotoxic activity suggests that more than one compound is involved. This includes an alkaloid, a saponin and several flavonoidic constituents, implying the presence of a synergistic effect for crude extracts. An anatomical study was conducted in an attempt to link the allelopathic potential of silverleaf nightshade foliage to specific cells or structures in the leaves. It was considered that glandular trichomes, abundant on both leaf surfaces, might harbour phytotoxic secondary metabolites. It was furthermore speculated that stellate trichomes with intrusive basal cells observed to reach the vascular bundles, might be involved in excreting alkaloids contained in the vascular bundle sheath onto the leaf surface. The results of this study represent the first step in showing the allelopathic potential of silverleaf nightshade and laid a foundation for continuing with field studies and more in-depth chemical analyses.
Dissertation (MSc (Agric) Horticulture)--University of Pretoria, 2006.