Effect of Irrigation and Nitrogen on Agronomic Parameters, Yield, Grain Quality and Agronomic Nitrogen Use Efficiency of Sunflower (Helianthus annuus L.)

Sunflower (Helianthus annuus L.) is one of the most important oil crops in the world. 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. Hence, studying the effect of irrigation on agriculture is more important in arid and semi-arid regions. In addition, nitrogen is one of the major nutrients which enhance the metabolic processes that lead to increases in vegetative, reproductive growth and yield of crops. Due to their importance and multiple functions in plant living processes, water and nitrogen are considered as important environmental factors affecting plants cultivation and production. Therefore, the current study was carried out to evaluate the effect of irrigation and nitrogen fertilizer on yield, yield components, grain quality and agronomic nitrogen use efficiency of sunflower.
Experiment was carried out as split plot based on randomized complete block design with three replications at Bu-Ali Sina University during 2012 growing season. Irrigation (irrigation till flowering, irrigation till seed formation and irrigation at total growth period) and nitrogen fertilizer (0, 60, 120 and 180 kg. ha-1) were assigned in main and sub plots, respectively.
Results indicated that the effect of irrigation and nitrogen was significant on all traits. The highest plant height, seed weight, oil percentages and agronomic nitrogen use efficiency were obtained at full irrigation treatment. The lowest value of these traits was obtained at irrigation till flowering stage. Moreover, the highest and the lowest protein percentages were revealed at irrigation till flowering stage and irrigation at total growth period, respectively. Mean comparison for nitrogen levels showed that the highest plant height, seed weight and protein percentages were achieved in treatment with180 kg N ha-1 and maximum agronomic nitrogen use efficiency was obtained in the treatment of 120 kg N ha-1. Effect of treatment interactions on anthodium diameter, the number of seeds per anthodium and grain, oil and protein yields were significant. The highest anthodium diameter, the number of seeds per anthodium, grain, oil and protein yields was obtained under irrigation treatment during total growth period and application of 180 kg N ha-1 had no significant difference with irrigation during total growth period and application of 120 kg N ha-1. The lowest values for these traits were revealed under irrigation treatment till flowering stage and non-application of N. Decreasing of plant height and anthodium diameter under water and nitrogen deficiency conditions can be related to the high sensitivity of cell division and growth to drought stress since the decrease in water potential of tissues reduced cell turgor pressure which it was not enough to their enlargement and it led to the decrease in cell enlargement. Since potential flower number is the factor changing seed number per anthodium which is determined particularly by leaf expansion during vegetative growth period. Water deficit stress during growing period decreases plant photosynthesis source and enzymatic activity by the decrease in leaf area and its shedding and then, it decreases anthodium size and flower number per anthodium. It can be said that seed, oil and protein yields decreased under deficit irrigation conditions due to the decrease in plant growing period, seed filling, anthodium diameter, seed number per anthodium and 1000-seeds weight and the increase in empty seeds percentage per anthodium.
It can be concluded that full irrigation and consumption of 120 kg N ha-1 is suitable for production the highest grain and oil yields and it improves agronomic nitrogen use efficiency in sunflower cultivation.