the effect of mycorrhizal fungi and humic acid on yield and yield components of sunflower

introduction cultivated sunflower is one of the largest oilseed crops in the world. sunflower seed is the third largest source of vegetable oil worldwide, following soybean and canola. nitrogen is one of the most important elements for crops to achieve optimum yields and quality. phosphorus (p), next to nitrogen, is often the most limiting nutrient for crop and forage production. phosphorus availability is controlled by three primary factors: soil ph, amount of organic matter and plant species (reddy et al., 2003). arbuscular mycorrhizal fungi are one of the most important microorganisms in majority of the undamaged soils so that about 70% of the soil microbial biomass is formed by the mycelium of these fungi. mycorrhizal association promotes plant absorption of scarce or immobile minerals, especially phosphorus, resulting in enhanced plant growth. humic acids are dark brown to black, and are soluble in waterunder neutral and alkaline conditions. they are complex aromatic macromolecules with amino acid, amino sugar, peptide and aliphatic compounds linked to the aromatic groups. humic acid contains nitrogen, phosphorous, calcium, magnesium, sulphur, copper and zinc (subramanian et al., 2009). materials and methods experiment was conducted as split plot factorial based on randomized block design with three replications in 2011-2012. the main plots consisted of nitrogen and phosphorus application levels (zero percent or no chemical fertilizer application, 50% equivalent to 37.5 kg.ha-1 urea + 25 kg.ha-1 super phosphate triple and 100% equivalent to 75 kg.ha-1 urea + 25 kg.ha-1 super phosphate triple). two species of mycorrhizal include (g. mosseae) and (g. interaradices) with three levels of humic acid (0, 8 and 16 kg.ha-1) were placed in subplots. the measured traits were: plant height, seed number per head, head diameter, seed oil content, thousand seed weight and seed yield. the data were analyzed using the mstat-c statistical software. mean comparison was performed using lsd method (at 5% level). the figures were prepared by microsoft excel. results and discussion mean comparisons showed that highest seed number per head belonged to plants under g. mosseae, without chemical fertilizer applying and without the use of humic acid (fig. 5). the highest thousand seed weight obtained from using 8 and 16 kg.ha-1 humic acid without chemical fertilizer and applying 0 and 8 kg.ha-1 humic acid and using 50 and 100% chemical fertilizer. it seems that humic acid has antagonistic effects with chemical fertilizer in high levels. interaction of humic acid and mycorrhiza species also showed that the highest thousand seed weight belonged to 8 kg.ha-1 humic acid and g. intraradices. samarbakhash et al. (2009) in maize showed that inoculation with mycorrhizal fungi significantly increased the average seed weight. the highest seed yield obtained from applying 50% chemical fertilizer and 8 kg.ha-1 humic acid. mean comparison of interaction effect of humic acid levels and mycorrhiza strains also showed that the highest seed yield belonged to 8 kg.ha-1 humic acid and g. intraradices. this increase may be attributed to the extensive root development and hyphae that reduce the distance fordiffusion of essential elements thus enhancing the nutrient absorption. mean comparison of interaction effect of chemical fertilizer levels and mycorrhiza strains indicated that chemical fertilizer levels had no considerable effect on g. mosseae efficiency from viewpoint of seed oil content but applying 50 and 100% chemical fertilizer along with using g. intraradices significantly increased seed oil content. mirzakhani et al. (2008) in sprig safflower showed that mycorrhiza can solubilize phosphorus in soil and enhance absorb elements by plant. conclusion in general, among applied humic acid levels, using 8 kg.ha-1 had positive effects on the studied traits of sunflower than its other levels. furthermore, inoculation of seeds with mycorrhiza and using proportionate chemical fertilizers can cause increased grain yield and seed oil content.