Experiments were conducted under laboratory and greenhouse conditions to investigate the allelopathic activity of aqueous extracts of dry and fresh leaves of Eucalyptus citriodora on purslane weed growth and nematode Meloidogyne incognita infecting sunflower plants cv. Giza 102. A Petri dish biotest showed that the aqueous extracts significantly reduced purslane (Portulaca oleracea L.) seedling length, with the degree of inhibition being dependent on the extract concentration. The fresh and dry leaf extracts of E. citriodora standard solution "S" caused the highest net mortality percentage of 100% after 72 hrs of exposure. Greenhouse studies in 2008 and 2009, indicated the greatest significant inhibition in purslane growth as well as the number of galls and egg masses of infecting nematode affected the increase in sunflower growth and yield. The studies indicated increase in the endogenous contents of total phenols in purslane tissues. This increase, correlated with growth inhibition due to treatment with leaf extract of E. citriodora. Chemical analysis indicated an increase in the contents of carbohydrates, protein and oil in sunflower seeds. The analysis of fatty acid composition by Gas Liquid Chromatography (GLC) indicated increases in the percentage of oleic and linoleic acid in sunflower seeds when fresh leaf extract of E. citriodora was used. A high-performance liquid chromatography analysis showed that the following acids; caffeic, ferulic, coumaric, benzoic, vanelic, chlorogenic, and hydroxybenzoic were present in Eucalyptus extracts.
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