Effect of the timing of nitrogen fertilizer application on grain yield and quality of bread wheat (Triticum aestivum L.) cultivars under terminal drought stress conditions

To investigate the effect of  the timing of nitrogen fertilizer application on grain yield and quality of bread wheat cultivars under terminal drought stress conditions, two field experiments were conducted at Agricultural Research Station of Miandoab in West Azerbaijan, Iran, in 2013-2014 and 2014-2015 cropping seasons. Experimental treatments were arranged as split plot factorial in randomized complete block design with three replications. Irrigation levels I1 = full irrigation, I2 = irrigation withholding from heading, I3 = irrigation witholding from anthesis stage) were assigned to main plots, 120 kg.ha-1 nitrogen at four application times (N1=20 kg.ha-1 at planting and 100 kg.ha-1 at the tillerign stage, N2 = 20 kg.ha-1 at the planting and 100 kg.ha-1 at stem elongation stage, N3 = 20 kg.ha-1 at the planting, 50 kg.ha-1 at the tillering stage with 50 kg.ha-1 at stem elongation stage, N4 = 20 kg.ha-1 at planting, 50 kg.ha-1 at tillering stage with 50 kg.ha-1 at heading stage) and five bread wheat cultivars (G1 = Zarrin, G2 = Pishgam, G3 = Orum, G4 = Zareh and G5 = Mihan) as factorial were randomized in subplots. Results showd that under terminal drought stress conditions (I2), concentrations of N, P and Zn of grain increased by 23%, 37% and 18%, respectively, however severe water shortage also reduced the concentrations of potassium (11%) and magnesium (45%). The timing of N3 and N4 fertilizer application times increased N, P, Fe and Mg concentrations of grain, while in the N1 fertilizer application time, only K concentration of grain increased. The highest amounts of N, P, Fe, Zn and Mg in severe water stress conditions (I2) were observed in the N3 fertilizer application time with drought tolerant cultivars cv. Mihan and cv. Pishgam, however under full irrigation conditions the distribution of nitrogen at spike emergence stage (N4) increased  N, P and Mg contents of grain. The highest grain yield (7026 kg.ha-1) obtained under N3 nitrogen fertilizer application time. The results of this experiment showed that with integrating of appropriate timing of nitrogen fertilizer application under terminal drought stress conditions and planting of new improved cultivars such as cv. Pishgam and cv. Mihan, bread wheat grain yield and its quality can be improved, in areas with similar environmental conditions.