Light absorption and leaf chlorophyll content in monoculture and interspecific competition of sunflower and redroot pigweed

In order to measure the effects of redroot pigweed emergence time and density to lightabsorption and leaf chlorophyll content at interference with three sunflower hybrids، a two years experiment was conducted during 2005-2006 based on additive design at islamic azad university of Tabriz. Treatments were combination of three different weed densities (8. 3، 25 and 41. 7 plants m-2) at three relative times of weed emergence (coincident with sunflower; and 15 and 30 days after sunflower emergence) using three different sunflower cultivars (Azarghol، Hysun and Allstar). Results revealed that in Azarghol and Hysun mixed canopy closure with increasing of density and early emergence times was happened. Because of short stem height and lower leaf area، Allstar could transmit 8% of total PAR at 60 days after sunflower emergence. In full-season interference of 41. 7 weeds m-2، light absorption by redroot pigweed in three sunflower cultivars increased 23%، 31% and 37%، respectively. With increasing of density and earlier interference times of redroot pigweed، leaf chlorophyll content index decreased in all studied treatments except in treatments of different weed densities at 30 days after sunflower emergence، and the Allstar was tolerated higher negative effect from weed density than other cultivars. With a view to grain yield in sunflower، compatibility of redroot pigweed with Allstar was higher than Azarghol and Hysun cultivars. In Allstar because of lower competitive power with weeds، redroot pigweed control is recommended at early interference times.