Effect of redroot pigweed (Amaranthus retroflexus L.) competition on some physiological traits and yield of sunflower (Helianthus annus L.)

In order to evaluation of the effect of redroot pigweed density and interference time on some physiological traits of sunflower two factorial experiments were performed with sunflower cultivars (Azarghol, Hysun and Allstar), redroot pigweed densities (8.3, 25 and 41.7 plants/m2) and relative times of weed emergence (emerged with sunflower, 15 and 30 days after sunflower emergence) during 2005-2006. Results revealed that in three sunflower cultivars, at high densities and early interference times of redroot pigweed leaf area index decreased, significantly. In Azarghol, reduction amount of leaf area per plant in emerging of 41.7 weeds/m2 was 12%, compared with control. In Allstar, weed density was more effective than interference time, while, in Azarghol and Hysun interference time was more effective than weed density with a view to effect on leaf area duration. Azarghol could tolerate 41.7 and 8.3 weeds/m2 from 30 and 15 days after sunflower emergence later, respectively, without reduction on yield. While intensity of inter-specific competition between redroot pigweed and sunflower in Allstar was higher than Azarghol and Hysun.