Silverleaf nightshade (Solanum elaeagnifolium), a perennial broadleaf weed, has become a serious pest in many semi-arid areas of the world. Control of silverleaf nightshade is confounded by its ability to produce thousands of viable seeds from a single mother plant, a deep and well-developed root system and the ability to propagate vegetatively from creeping lateral roots, root fragments and from rhizomes. Exacerbating factors are longevity of its propagules in soil, particularly under adverse environmental conditions. Currently, in South Africa, it is one of the more serious weeds in cotton (Gossypium hirsutum L.) production, where its interference results in remarkable loss of product quantity and quality. Research has been done on the competition aspect of silverleaf nightshade with cotton but there is a serious lack of information on the allelopathic aspect. In this study, a series of experiments that included laboratory and glasshouse experiments were conducted to evaluate the allelopathic interference of silverleaf nightshade on early growth of cotton. Cotton seeds were exposed to silverleaf nightshade extract solutions or planted in soil into which silverleaf nightshade leaf material or ripe berries were incorporated. In all the experiments attempts were made to avoid or to reduce, at least, the influence of factors that could be potentially confounding. In this regard, PEG-6000 was used to evaluate the sensitivity of crop parameters to the osmolality of test solutions in order to ensure that osmotic inhibition was not a confounding factor in the bioassays where the biological activity (phytotoxicity) of plant extracts were assessed. Competition for growth factors was the other major potentially confounding factor that was considered throughout, and steps were taken to negate its influence. Germination and early seedling growth of cotton cultivars Sicala, CA 223, Siokra V15, Tetra and Delta Opal were inhibited by test solutions (silverleaf nightshade extracts) and by soil-incorporated residues of silverleaf nightshade. Inhibitory effects of silverleaf nightshade solutions were observed when either a layer of filter paper or a thin layer of soil or quartz sand was used as substrate. In both laboratory and pot experiments, it appeared that cotton cultivar Sicala was the most sensitive to allelochemicals contained in extracts and residues of silverleaf nightshade. This finding of differential tolerance of crop cultivars towards allelochemicals contained in a weed is a rare occurrence in allelopathy research. Berries of silverleaf nightshade were generally more inhibitory to cotton than leaf material. For both types of plant material used, residues lost their inhibitory effect over time, probably as decomposition of allelopathic compounds in soil progressed. Information obtained from this study can be viewed as knowledge that contributes to the bridging of the gap between identification and isolation of allelochemicals from silverleaf nightshade, and confirmation of silverleaf nightshade allelopathy under natural conditions. Experiments involving soil as growth medium, in particular fieldwork, are needed to verify the validity of these findings under natural conditions.
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