Laboratory and pot experiments were done to evaluate the allelopathic potential of shoot aqueous extracts of eucalyptus (Eucalyptus camaldulensis L.), sunflower (Helianthus annuus L.) and sugar beet (Beta vulgaris L.) on germination, growth and photosynthesis of redroot pigweed (Amaranthus retroflexus L.). Germination, growth and photosynthesis of A. retroflexus were inhibited by aqueous extract of the tested plant species. Sugar beet extract was more inhibitory to seed germination and seedling growth than sunflower and eucalyptus. Seed germination of A. retroflexus was significantly decreased by 57.4% and 92% at 10 and 20 gL-1 extract concentration, respectively than control. While sunflower and eucalyptus extracts reduced the weed seed germination by 39% and 38%, respectively at 20gL-1 concentration. High extract concentration (20gL-1) of sugar beet reduced the plant height, leaf area, shoot dry weight and leaf photosynthesis of A. retroflexus by 34.6%, 56.3%, 39.9% and 47.3%, respectively over the control, while, sunflower and eucalyptus extracts reduced the weed plant height by 23.1% and 20.6%, leaf area by 24.5% and 21%, shoot dry weight by 31.5% and 29.8% and leaf photosynthesis by 27% and 22.3%, respectively compared with control.

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