Introduction: Millet is one of the drought tolerant plants that nitrogen fertilizer plays an important role in its yield. Water deficit stress as the most important abiotic stress has very adverse effects on nutrient uptake, plant growth and yield. Iran is climatically regarded as an arid and semi-arid region in the world, where the lack of precipitation and its inappropriate distribution, high temperature and extensive evaporation makes the irrigation the main way for meeting plants water demand.  Nitrogen is one of the effective factors on the development of the leaf surface and consequently the development of the shade of the plant, which improves the economic yield of the plant by increasing the leaf surface index, leaf area duration and photosynthesis. Therefore, this research was conducted in order to study the effect of irrigation regimes and nitrogen fertilizer rates on the yield and water use efficiency of millet.
Materials and Methods: This research as a split plot experiment was conducted based on a randomized complete block design with three replications in research farm of Birjand branch, Islamic Azad University, Birjand, Iran in 2016. In this experiment, irrigation regim (50 and 100 percent of water requirement) as main factor and nitrogen rate (0, 50, 100, 150 kg N ha^-1) as sub factor were considered. Each sub-plot consisted of 4 planting lines with a length of 5 meters and a row spacing of 30 cm and a plant spacing of 3 cm (density of 111 plants m^-2). Land preparation was done with moldboard in the early spring; the tillage operation included shallow plowing, double disc plow, and full leveling. Seed millet planting was done on June 1st. Millet seeds of Peshahang variety (Pennisetum miliaceum) were used for planting. The first irrigation was done immediately after planting. After the complete establishment of the plant (four-leaf stage), irrigation treatments were applied. The water requirement was determined with the help of the FAO method and using evaporation statistics from the class A pan. The amounts of water consumed in two irrigation treatments of 50 and 100 percent of the plant's water requirement in the entire growth period were 122.9 and 210.3 liters per m², respectively. Traits of plant height, chlorophyll index, relative water content, panicle number per m², number of seeds per panicle, 1000-seed weight, seed yield, biological yield and water use efficiency for seed and biomass were measured. Finally, the data were analyzed by software MSTAT-C for each trait and the means were compared by Duncan Multiple Range Test at 5% level.
Results and Discussion: The results showed that water deficit stress reduced the relative leaf water content by 29.4%. Under optimal irrigation conditions with increasing nitrogen application from zero to 100 kg N ha^-1 chlorophyll index, plant height, number of seeds per panicle, seed yield and water use efficiency for seed significantly increased by 32.7, 20.4, 23, 35.4 and 27% respectively. Also, in optimal irrigation conditions, the application of 50 kg N ha^-1, the number of panicles per m², 1000-seed weight, biological yield and water use efficiency of biomass increased by 9.7, 26.8, 26.5 and 12.7%, respectively. In conditions of supply 50 percent of water requirement with increasing nitrogen application from zero to 50 kg N ha^-1 seed yield, water use efficiency of seed and biomass significantly increased by 24.9, 17.3 and 7.7%, respectively, compared to the control. In general, the results showed that in order to achieve maximum seed yield by considering water use efficiency, optimal irrigation treatment and 100 kg N ha^-1 in Birjand region can be used.
Conclusion: In general, the results of this research showed that drought stress can lead to a significant decrease in seed yield and the efficiency of water consumption for seed production due to the inhibitory effect of stress on plant reproductive growth. Also, the lack of nitrogen (as one of the most essential nutrients for plants), in the presence of sufficient moisture, plays an important role in reducing yield and yield components and water use efficiency for seed production. However, in the conditions of water deficit stress (supply percent of water requirement), due to the disturbance in the effective absorption of applied nitrogen, the use of this fertilizer did not result in significant changes in most of the traits. Based on the results of this research, in order to achieve the maximum seed yield, taking into account the efficiency of water consumption, the optimal irrigation treatment and application of 100 kg N ha^-1 can be used in Birjand region.