عنوان مقاله [English]
In a sustainable agricultural system, the utilization of organic matters such as humic acid had the minimum damage to the environment and with its hormonal like activity; it can absorb water, mineral nutrients, and ultimately increase crop yields. To investigate the effect of humic acid on quantitative yield and some qualitative characteristics of sunflower (Var. Farrokh Hybrid) under drought stress, a split-plot experiment was conducted in a randomized complete block design with three replications at Shahroud Agricultural Research Center, Shahroud, Iran. Treatments were three levels of drought stress: control (irrigation by 100% water requirement), mild stress (irrigation by80% water requirement) and high tension (irrigation by 60% water requirement) as the main plots and five levels of humic acid: control (no humic acid), two concentrations of foliar application (1.5 and 3 gr/ liter of water), two levels of field application (15 and 30 kg/ha) as a subplots. Results showed that drought stress reduced grain yield (45.5%), 1000-seed weight (26.7%), head (23.3%) and stem diameters (25.9%), biological yield (44.6%), seed phosphorus (16.2%), and leaf chlorophyll “b” (50.7%) and carotenoids (29.8%). Foliar and soil applied humic acid had significant effects on plant height, head diameter, grain yield, biological yield, chlorophyll “a,” and P and K percentages in seeds and increased them. According to the results, it can be concluded that the application of humic acid can reduce the effects of drought stress on sunflower. At first, the application of 30 kg/h of humic acid and next, 1.5 gr humic acid /liter of water had the highest efficiency in improving the sunflower yield under water stress conditions.