Plant Protection Department, Faculty of Agriculture, University of Jordan, Amman, Jordan.
Allelopathic activity of weeds (Amaranthus gracilis Desf., Convolvulus arvensis L., Lactuca serriola L. and Portulaca oleracea L.) was studied on selected vegetable crops (cabbage, carrot, onion, pepper, tomato and squash) under glasshouse conditions. The root exudates of weeds released into the soil had variable effects on different test crops. However, L. serriola root exudates were found most toxic to growth of different crops. Soilincorporated dried shoot residues, reduced the seedlings growth of the test crops and the degree of inhibition was residue rate dependent. Soil-surface placed weed residues delayed seedlings emergence, but their effects on seedlings growth was lower than soil-incorporated residues. Decayed residues of different weeds inhibited the seedlings growth but their effect was less as compared to un-decayed residues. In all experiments, roots were more sensitive than shoots to allelopathic effects and both positive and negative effects were observed. The weeds differed in their allelopathic effects, depending on the method used and allelochemicals concentration in the medium. The effect however, could be changed and depended on the test weed and crop species used. C. arvensis proved most harmful to all test crops, while cabbage, onion and tomato were most affected than other crops.
