International Journal of Biology January, 2009 71 Study of Bioassay the Allelopathical Effect of Neem (Azadirachta indica) n-hexane, Acetone and Water-soluble Extracts on Six Weeds Zoheir.Y.Ashrafi (Corresponding author) M. Sc. of Weed Science Faculty of Agronomy and Animal Science University college of Agricultural and Natural Resource, Karaj Paradaise University of Tehran and Lecture at Payame Noor University (PNU), Tehran, Iran Tel: 98-261-280-1011 E-mail: zoheir1980@gmail.com Sedigheh Sadeghi M. Sc. of Weed Science Faculty of Agronomy and Animal Science University college of Agricultural and Natural Resource, Karaj Paradaise University of Tehran and Lecture at Payame Noor University(PNU), Tehran, Iran Tel: 98-229-236-1357 E-mail: sadeghi58@gmail.com Hassan.M.Alizade Professor of Department Weed Science, Faculty of Agronomy and Animal Science University college of Agricultural and Natural Resource, Karaj Paradaise, University of Tehran Hamid.R.Mashhadi Professor of Department Weed Science, Faculty of Agronomy and Animal Science University college of Agricultural and Natural Resource, Karaj Paradaise, University of Tehran. Ebrahim.R.Mohamadi M. Sc. of Weed Science Faculty of Agronomy and Animal Science University college of Agricultural and Natural Resource, Karaj Paradaise, University of Tehran The research is financed by University of Tehran (Sponsoring information) Abstract Azadirachta indica, or Neem Tree, is an evergreen tree native to Southeast Asia. All parts of the tree have been used medicinally for centuries. The allelopathic potential of extracts of Azadirachta indica L., which is one of the most dominant weeds in tropical regions of South-west Asia, was investigated under laboratory conditions. The n-hexane- soluble, acetone-soluble and water-soluble fractions obtained from the acetone extract of A. indica shoots inhibited the germination and the growth of roots and shoots of six test plant species. The inhibitory activity of the water-soluble fraction was greatest, followed by that of the n-hexane-soluble and acetone-soluble fractions in all bioassays. Significant reductions in the germination and growth of the roots and hypocotyls were observed as the extract concentration increased. The concentration-dependent responses of the test plants to the fractions suggested that all three fractions might contain allelochemicals, but that the greatest potential was in the water-soluble fraction. These