Extended Abstract
Introduction: Water limitation under is one of the most important factors limiting crop production in arid and semi-arid regions, particularly in rainfed production systems. Several criteria have been proposed to increase the resistance of crops against water deficit. Application of iron nanooxide and humic acid can improve the performance of crop plants (including bread wheat) under water deficit conditions. Iron (Fe) is an essential micronutrient that plays a key role in the regulation of plant growth and development. The availability of Fe for plants is greatly influenced by the soil's pH level, leading to a frequent occurrence of Fe deficiency in plants grown in various types of soil. Fe uptake in plants is also severely affected under water deficit condition. Thus, the application of nano-Fe in various forms (Fe, FeO, Fe2O3, or Fe3O4) under water limitation conditions may provide an efficient way of mitigating symptoms of Fe deficiency and enhancing plant growth and agronomic performance. Also, the alleviation of the devastating effects of water limitation in arid and semi arid region ‎on plants using humic acid is an appropriate solution ‎for sustainable, less costly, and organic agriculture and plays a crucial role in reducing the impact of abiotic stresses such as salt and drought. Thus, this research aimed to evaluate the effect of Fe nanooxide and humic acid on relative water content, electrical conductivity and dry matter remobilization of the stem and shoots in relation to grain yield of rainfed wheat. 
Materials and Methods: In order to study of the effects of nano iron oxide and humic acid on grain yield and quality and some physiological traits of rainfed wheat (Baran cultivar), an factorial experiment was conducted based on a randomized complete block design with three replications under field conditions of Khatonabad, Sarab, East Azarbayjan, northwest of Iran during 2018-2019. The area is located at 37˚ 57′ N latitude and 47˚ 34′ E longitude with an elevation of 1680 m above mean sea level. The area is characterized by a semi-arid temperate climate with cold winters and moderate summers. Experimental factors included Fe nanooxide foliar application at four levels (foliar application of water as control, 0.4, 0.8 and 1.2 g L-1 of Fe nanooxide) and application of humic acid in four levels (foliar application of water as control, 100, 200 and 400 mg L-1 of humic acid) in tillering and booting stages (coincident to 24 and 40 Zadoks codes, respectively). The bread wheat cultivar ‘Baran’ was employed in the experiment with a target planting density of 380 seeds m-2. The Fe nanooxide was purchased from the Pishgaman Nanomaterials Company, Iran. The applied Fe nanooxide had an average particle size < 30 nm g-1 and a specific surface area>30 m2. To have a high-quality solution, deionized water was combined with Fe nanooxide and placed on a shaker with ultrasonic equipment (40 kHz and 100 W). At the stages of spike emergence and grain filling (coincident with 59 and 69 Zadoks codes, respectively), the flag leaves of the plants were separated to measure relative water content and electrical conductivity. 
The dry matter and remobilization of stem reserves to grain yield were determined as: Dry matter remobilization to grain (g/plant) = maximum shoot dry matter after anthesis (g/plant)–shoot dry matter (without grains) in maturity (g/plant)                                                                     
Dry matter contribution of assimilates to grain (%) = [remobilization/grain yield] × 100                 
Stem reserve remobilization to grain yield (g/plant) = maximum stem dry matter after anthesis (g/plant) – stem dry matter in maturity (g/plant)                                                                         
Stem reserve contribution to grain yield (%)=[stem dry matter remobilization/grain yield] ×100  
The SAS software was utilized to conduct the analysis of variances and mean comparisons. The interactions and main effects were tested by applying the least significant difference (LSD) test at the probability level of P < 0.05.
Results and Discussion: Application of Fe nanooxide and humic acid at the highest level, increased plant height (23%), 1000-grain weight (20.9%), and relative water content at ear emergence and grain filling (42.5 and 37%, respectively) in comparison with no application of Fe nanooxide and humic acid. The highest level of Fe nanooxide increased ear length (7.4%), grain protein and iron content and grain yield (by approximately 27, 10.7 and 27%, respectively) in comparison with no application of Fe nanooxide. More or less, similar results were obtained for humic acid application. Though, application of Fe nanooxide decreased dry matter remobilization from shoot and stem (74 and 30%, respectively) and contribution of remobilization to grain yield (22.4%), compared to no application of Fe nanooxide. 
Conclusions: Based on the aforegone findings, foliar application of 1.2 g L-1 of Fe nanooxide and 400 mg L-1 humic acid could increase grain yield and quality of the examined bread wheat cultivar under rainfed conditions, due mainly to improving some physiological functions and thus grain yield attributes.