Evaluation the effect of C3 and C4 weeds and different levels of nitrogen on growth and biomass production of sunflower (Heliantheus annus L.)

In order to consider the effects of nitrogen and weeds interference (pigweed and lambsquarter) on the sunflower growth and biomass production, an experiment was conducted in a greenhouse in Fasa Township in 2010. The experimental design was completely randomized block with factorial treatment arrangements, replicated three times. The first factor, was the 3 levels of nitrogen (0, 46 and 92 kg/ha) and the second factor was the competition of sunflower and weeds in 5 levels including a pure sunflower only without weeds and 2 levels of corn and weeds intercropping (pigweed and lamsquarter) and 2 levels of pure weeds. The results showed that nitrogen application increased shoot biomass of sunflower, but by increasing weeds density, shoot biomass of sunflower decreased. Increase of nitrogen in different levels of sunflower and lambsquarter, benefited the sunflower, because it increased shoot biomass, but in various pigweed densities, 46kg nitrogen application treatment of the accumulation increased the sunflower shoot weight. But applying 92 kg nitrogen decreased sunflower shoot biomass. In fact, it expresses that C4 plants react to Nitrogen more positively. Increasing nitrogen caused increase of sun flower root in all nitrogen levels.C4 weeds were more competitive with the increase of nitrogen level, but in lower levels of nitrogen C3 plants were more successful. Lambsquarter was more competitive for soil sources, especially in the higher nitrogen levels. With the increase of weed accumulations, the ratio of root to shoot increased. Sunflower chlorophyll index in pure and mixed culture with lambsquarter was not significantly affected. But Chlorophyll indices of sun flower and pigweed decreased. Chlorophyll index was independent of weed density but the level of chlorophyll was affected by culture types.The PRY index decreases with the increase of weed density. In the mix of sun flower and lambsquarter the PRY in lambsquarter was lower than that of sun flower and in the mix of sun flower and pigweed the amount of PRY was higher for pigweed.