We assessed the allelopathic effects of the aqueous extract of Sonchus oleraceus dry shoots on the germination and seedling growth of Trifolium alexandrinum, three weed species (Brassica nigra, Chenopodium murale and Melilotus indicus) and S. oleraceus itself. We assayed four different concentrations of the aqueous extract (w v-1): 1%, 2%, 3% and 4%. To determine whether the effects of the extract were attributable to the presence of allelopathic compounds, its osmotic potential or both, we prepared concentrations of polyethylene glycol (PEG) with osmotic potentials equivalent to those of the aqueous extract. All concentrations of the plant extract completely inhibited the germination and seedling growth of C. murale. The lowest concentration of the plant extract partially inhibited germination and seedling growth of B. nigra, M. indicus and S. oleraceus, whereas the higher concentrations inhibited those parameters completely. The germination of T. alexandrinum was not affected by the aqueous extract at 1% or 2%. In general, the aqueous extracts were more effective in inhibiting seed germination and seedling growth than were the PEG solutions. Phytochemical analyses revealed that phenols and alkaloids were the most abundant compounds in S. oleraceus dry matter. Our results suggest that the aqueous extract of S. oleraceus has an allelopathic effect on some weeds, and its usefulness as a bioherbicide therefore merits further study

Laboratory and greenhouse experiments were conducted to evaluate the phytotoxic effect of black mustard extracts and root exudates on two crops: Trifolium alexandrinum and Triticum aestivum, and two weeds: Phalaris paradoxa and Sisymbrium irio. The seeds were treated with aqueous and ethanolic extracts and chloroform for eight days, or subjected to root exudates of just harvested mustard in a greenhouse for five weeks. High-performance liquid chromatography (HPLC) was used to quantify phytotoxins from plant tissues. Seed germination of P. paradoxa was reduced with the lowest concentration of the different extracts. However, the aqueous extract at 4% completely curtailed the germination of all the target species. In general, plant extracts had a concentration-dependent reduction of seedling growth of the target species. However, the ethanolic extract, at the lowest concentration, has stimulated the shoot length of both T. alexandrinum and T. aestivum, and the root length of the former. Mustard root exudates inhibited emergence and growth of the target species throughout the experiment. Ferulic and syringic acids were the dominant allelochemicals found when HPLC was used.

The floristic composition and species diversity of the germinable soil seed bank were studied in three different habitats (desert salinized land, desert wadi, and reclaimed land) in the Eastern Desert of Egypt. Moreover, the degree of similarity between the seed bank and the above-ground vegetation was determined. The seed bank was studied in 40 stands representing the three habitats. Ten soil samples (each 25 × 20 cm and 5 cm depth) were randomly taken per stand. The seed bank was investigated by the seedling emergence method. Some 61 species belonging to 21 families and 54 genera were identified in the germinable seed bank. The recorded species include 43 annuals and 18 perennials. Ordination of stands by Detrended Correspondence Analysis (DCA) indicates that the stands of the three habitats are markedly distinguishable and show a clear pattern of segregation on the ordination planes. This indicates variations in the species composition among habitats. The results also demonstrate significant associations between the floristic composition of the seed bank and edaphic factors such as CaCO3, electrical conductivity, organic carbon and soil texture. The reclaimed land has the highest values of species richness, Shannon-index of diversity and the density of the germinable seed bank followed by the habitats of desert wadi and desert salinized land. Motyka's similarity index between the seed bank and the above-ground vegetation is significantly higher in reclaimed land (75.1%) compared to desert wadi (38.4%) and desert salinized land (36.5%).