In order to investigate allelopathic effect of foliar aqueous extract of Ephedra major on growth parameters of Cirsium arvense weed, an experiment was carried out in completely randomized design with five replications during spring 2011 in a glasshouse of Shirvan Agricultural Faculty, Ferdowsi University of Mashhad. Four treatments including distilled water as control plus three different foliar aquatic extract concentrations (15%, 30%, 45%) of Ephedra major were used. The results showed that aquatic extracts of E.major extremely decreased leaf area, plant height, leaf chlorophyll, shoot dry weight and root dry weight of Cirsium arvense plants. The comparison means showed that the lowest (15%) and highest (45%) extract concentrations of E. major decreased leaf area of Cirsium arvense plants by 50.8% and 90.9%, plant height by 39.2% and 73.4%, shoot dry weight by 55.9% and 88.8%, root dry weight by 32.5% and 82%, leaf chlorophyll by 14.2% and 46.1% and leaf photosynthesis by 17.9% and 72.4%, respectively compared to control plants.

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