. Allelopathic Effects of Sunflower (Helianthus annuus) on Germination and Initial Growth of Redroot Pigweed (Amaranthus retroflexus) and Common Lambsquarter (Chenopodium album). Reza Ghorbani1, Kobra Orooji1, Mohammad Rashed1, Hamid Khazaei1, Majid Azizi1; 1Ferdowsi University of Mashhad, Mashhad, Khorasan, Iran Allelopathic effects of sunflower on redroot pigweed and common lambsquarter in two series of laboratory and greenhouse experiments were studied. Under laboratory conditions, leaf, stem and root aqueous extracts of sunflower at 0%, 2.5%, 5%, 7.5 % and 10% (m/v) concentrations were applied to determine their effect on and common lambsquarter seed germination and initial growth under laboratory conditions. The results indicated that germination and mean daily germination were reduced 43% and 50%, respectively. The effects of leaf and stem extracts on germination were more allelopathic than root extracts of sunflower. The percentage and quality of germination were decreased by increasing the concentration of extracts. The root length and shoot length were reduced by 80% following application of sunflower extracts. The results of this study suggest that sunflower have severe allelopathic potential for redroot pigweed and common lambsquarter. In greenhouse experiments, to study the role of decomposition plant debris on release of allelochemicals, stem, root and leaf residues of sunflower were incorporated with pot soil by 5% (w/w) and set under the mist system in greenhouse for 8 weeks. Fresh residues of sunflower werealso mixed with soil. After preparing pots, weed seeds were sown and then were irrigated with tap water as needed. Redroot pigweed and common lambsquarter were grown and samples were taken in five growth stages (2, 4, 6, 8 leaf and mature plant) and dry weight, leaf area and height of weeds were determined. The results of greenhouse experiments indicated that incorporation of sunflower residues in the soil (fresh or decomposed residues) reduced growth of redroot pigweed and common lambsquarter. The difference between fresh and decomposed residues was not significant. Inhibitory effects of sunflower residues on weed in this study, apparently, were depended on weed age, weed species, sunflower organel and degree of decomposition residues.

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