Effect of Nitrogen Fertilizer and Plant Density on Seed Yield and Oil Yield of Sesame Using a Central Composite Design

Sesame (Sesamum indicum L.) is known as the king of oil seeds due to the high oil content (50-60%) of its seed. Nitrogen (N) is one of the most important nutrients in crop production systems. Excessive use of N in crop production causes a declining trend in nitrogen use efficiency (NUE). It has been reported that not more than 33% of N applied is used by the plant, while the remainder is lost and causes environmental pollution as well as emission of greenhouse. Optimization of nitrogen and plant density is a management approach to conserve resources and decline environmental pollutions. Response surface methodology (RSM) is defined as a set of mathematical and statistical techniques that are used to develop, to improve or to optimize a product. RSM is a statistical method for optimization of multiple factors which determine optimum process conditions by combining experimental designs. In this work, optimization of nitrogen fertilizer and plant density of sesame using central composite design for Response surface methodology was done.
  This research was conducted using central composite design with 13 treatments and two replications at the Research Field of Ferdowsi University of Mashhad during the growing season of 2015-2016. The treatments were allocated based on low and high levels of plant density (10 and 40 plants.m-2, respectively) and nitrogen (0 and 100 kg Urea ha-1, respectively). Plant height, yield components, seed yield, biological yield, harvest index, oil percentage, oil yield, protein percentage and, protein yield were calculated as dependent variables and changes of these variables were evaluated by a regression model. Lack-of-fit test was used to evaluate the quality of the fitted model. The adequacy of the model was tested by analysis of variance. In general, the full quadratic polynomial equation was tested to determine the significance of the model and the component of the model (linear, squared, first-order interaction terms). The quality of the fitted model was judged using the determination coefficient (R2).
  The results showed that effect of linear component was significant on all studied characteristics. Effect of square component was significant on all studied criteria except harvest index, seed No. per capsule and 1000-seed weight. Interaction effect of full quadratic was significant on plant height, seed No. per plant and protein percentage. Lack of fit test had no significant effect on the studied traits. The full square model for the response variables gave insignificant lack-of-fit indicating that the data of experimental were satisfactorily explained. The highest estimated and observed values of seed yield were obtained for 25 plants.m-2 and 50 kg Urea ha-1 and 25 plants.m-2 and 100 kg Urea ha-1 with 1320.5 and 1272.4 kg.ha-1, respectively. The highest estimated values of oil percentage and protein percentage were obtained for 10 plants.m-2 and without Urea application (46. 7%) and 40 plants.m-2 and 100 kg Urea ha-1 kg Urea ha-1 (25.3%) and these maximum observed amounts were recorded in 10 plants.m-2 and without Urea application (46.5%) and 40 plants.m-2 and 100 kg Urea ha-1 kg Urea ha-1 (25.0%), respectively.
  The slope of seed yield increased by an increase in density up to 25 plants.m-2 and was higher under high levels of N fertilizer (optimum level= 50 kg Urea ha-1) than under low levels, because the plant growth was improved in high amount of N fertilizer and resulted in high yield components and seed yield. Therefore, the effect of plant density on yield improvement could be increased at high levels of N fertilizer which clearly suggest the importance of N for higher seed production in sesame. In general, it seems that resource use optimization based on the central composite design may be suitable cropping approach for sustainable production of sesame.
This research was funded by Vice Chancellor for Research of Ferdowsi University of Mashhad, which is hereby acknowledged.