In this research we studied the allelopathic activity of 56 aromatic and medicinal plants of 22 families that most of them have been used in Persian traditional medicine. The assay method was dish pack with lettuce seed. Results showed that 51 species out of 56 tested plants declined lettuce seedling growth. Ten species completely inhibited lettuce seed germination namely Bunium persicum, Carum copticum, Achillea wilhelms, Pulicaria gnaphalodes, Berberis vulgar var asperma, Lavandula spica, Ziziphora clinopodioides and Ailanthus altissima. Further researches need to be conducted on the detection of main volatile constituents that are responsible for this strong allelopathic activity of the mentioned plants by GC/MS and NMR techniques.

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