Determining the mode of action of allelochemicals is one of the challenging aspects in the allelopathic studies. In this research, the methanolic extracts of wild poinsettia (Euphorbia heterophylla Linn.) at different concentrations (0, as untreated control, 8%, 12% and 16%) were tested for genotoxic and inhibitor activity on morphological, biochemical and molecular traits of sunflower (Helianthus annuus L.). Seedling emergence was reduced with wild poinsettia leaf extracts in sunflower. Leaf extracts of wild poinsettia decreased chlorophyll a and b levels and consequently the soluble sugar content was proportional to the increase in the concentrations of the leaf leachates. Some changes occurred in random amplification of polymorphic DNA (RAPD) profiles, profiles of protein bands and total soluble protein of germinated treated seed. These included variation in band intensity, loss of bands and appearance of new bands compared with control. Increased concentration of extracts caused increasing the total soluble protein content, decreasing genome template stability (GTS) value and increasing polymorphism values, the results indicated that they were dose dependent. For instance, the highest and lowest amounts of GTS were observed in 8% and 16%, respectively. In a dendrogram constructed based on genetic similarity coefficients, the treatments were grouped into two main clusters: (a) 8%, 12% and 16% dose clustered together and (b) untreated control grouped alone. Also, we concluded that the basis of interaction between plants, like allelopathy, may be related with genotoxic effects.

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