Allelopathic activity of some common weeds species in onion fields

Allelopathic activity of some common weed species in onion fields in Yemen Republic was investigated under laboratory and growth chamber. Test weeds spp. were: Amaranthus graecizans L., Alhagi maurorum Medik., Brachiaria reptans L. Gard. & Hubb., Cassia italica (Mill) Lam. Ex. Steus., Corchorus trilocularis L., Cyperus rotundus L., Dactyloctenium aegyptium L. P. Beauv., Digera muricata L. Mart., Echinochloa colonum L. Link., Euphorbia granulata Forsake., Matura innoxia Mill., Phyllanthus maderaspatensis L. and Zaleya pentandra L. Jeffr. In pot culture, the weed residues incorporated at 100 g kg⁻¹ of soil inhibited the germination and growth of onion. The weed residues reduced the plant height, root length and shoot and root dry weight in onion. Shoot residue of A. maurorum was most harmful and drastically reduced the shoot and root dry weights of onion by 64% and 96%, respectively. Root exudates of some weed species were also phytotoxic. The root exudates of A. graecizans were most inhibitory to onion seed germination, C. italica root exudates drastically inhibited the plant height and shoot dry weight and E. colonum exudates decreased the onion root length and dry weight. Although none of the weeds foliage leachates was harmful to onion seed germination but leachates reduced the onion height and shoot and root dry weights. Foliage leachates of A. graecizans reduced the onion height and root length by 59 and 64% of control, respectively. The reduction in shoot and root dry weights of onion planted in the soil after the growth of A. maurorum was 76% and 79% of the control, respectively. It was concluded that shoot residues, root exudates and foliage leachates of most test weed species were phytotoxic to onion.