Decrease in growth of redroot pigweed (Amaranthus retroflexus) using extracts of houseplants with allelopathic properties

Allelopathy is how one plant, by producing chemical compounds, affects another plant, either in a harmful or beneficial way, directly or indirectly (Prati and Bossdorf, 2004). Allelopathic phenomena have existed since the distant past and there are estimates in connection with weed control using this material. Pigweed (Amaranthus retroflexus) is one of the most important weeds (McNaughton et al., 2005). This study aimed to control pigweed using allelopathic substances (alcohol and aqueous extracts) of five species of poisonous houseplants.
This experiment was conducted as a factorial in a completely randomized design at the Agricultural Faculty, Tarbiat Modarres University in 2014. Experimental factors included five poisonous houseplants (oleander (Nerium oleanderDieffenbachia amoena, Caladium candidum, Lilium ledebouri and Chrysanthemum indicum) at three concentrations of 5%, 15% and 25% of aqueous and alcohol extract, as well as their control. To evaluate the allelopathic potential of different concentrations in the alcoholic and aqueous phases of five poisonous species, root and shoot length, maximum germination, germination rate, uniformity of germination, germination percentage, seedling length and vigor index were compared. A glasshouse study was conducted to evaluate the effect of the potential of allelopathic extractions on the growth and physiological parameters such as the fresh and dry weight of leaves, stems and inflorescence, and the leaf area of redroot pigweed.
Results showed that the longest roots and shoots, maximum germination, germination rate, germination uniformity, germination percentage, seedling length and vigor index were observed in the treatment without extract, while the different extract treatments caused a sharp decline in trait values. The highest leaf dry weight was observed in a foliar application of 5% aqueous extract of diphenhydramine. The highest dry weight of inflorescences was seen in an alcoholic extract treatment that was applied to the soil with 5% of the Lilium extract. The highest dry and fresh weight of stem was observed in caladium aqueous extract with 5% of the extract applied to the soil. The highest leaf area occurred with a foliar application of 5% caladium extract. The highest fresh weight of leaf and inflorescences was obtained in aqueous soil treatment of 5% oleander extract. Previous studies showed that the effect of plants from the cabbage family on canola seed germination and seedling length are significant. The results showed that canola had maximum and cress minimum potential of inhibition. However there was no significant difference in their effects on root growth (Rezaei-Nodehi et al., 2003). There were significant differences between ornamental plants in terms of germination percentage and seedling length. Generally, the higher concentration of the extract decreased the traits values which can be increased by increasing the amount and toxicity of allelochemicals. Also, it can be deduced that allelochemicals may be selective. This point is emphasized in the topic of weed control. On the other hand, root growth in redroot pigweed was inhibited more than shoot growth. Compared to the aqueous extracts, alcoholic extracts were more effective in decreasing the values of traits in this experiment.
The results of this research confirm the previous reports that the growth of the root was more sensitive than the shoot and all plant traits can be affected negatively by allelopathic substances and their growth reduced.