Sunflower [Helianthus annuus (L.) Koch.] contains watersoluble allelochemicals that inhibit the ermination and growth of other species. This characteristic could be used in weed management programmes. Greenhouse and laboratory experiments were conducted to determine the effects on wild barley (Hordeum spontaneum Koch.) germination and seedling growth of(i) preceding crops, (ii) fresh sunflower residue incorporation, and (iii) sunflower leaf, stem, flower and root water extract concentrations. Growth of wild barley, as indicated by plant height and weight, was significantly reduced when grown in soil previously cropped to sunflower compared with that cropped to wild barley. Soil incorporation off fresh sunflower roots and both roots and shoots reduced wild barley germination, plant height and weight when compared with a no-residue control. In bioassays, sunflower extracts reduced wild barley hypocotyl length, hypocotyl weight, radicle weight, seed germination, and radicle length by as much as 44, 578, 61, 686 and 79 %, respectively, when compared with a water control. Increasing the water extract concentrations from 4 to 20 g per 100 ml of water of all sunflower parts significantly increased the inhibition of wild barley germination, seedling length and weight. Based on 8-day-old wild barley radicle length, averaged across all extract concentrations, the degree oftoxicity ofdifferent sunflower plant parts can be ranked in the following order of inhibition: leaves > flowers > mixture of all plant parts > stems > roots.

Key words: allelopathy , sunflower — Helianthus annus (L.) Koch. , wild barley ,Hordeum spontaneum Koch., , water extracts, inhibition germination and growth