Cereal grains are predominantly composed of carbohydrates (generally starch forms), proteins, lipids, vitamins, and minerals and the amount and quality are affected by genetic and environmental conditions (Ali et al., 2010). Among the cereals, maize (Zea mays L.) plays an important role in food, feed and bio fuel processes worldwide. In a growing strategy to obtain high grain and biomass yields from the grains, some applications such as increasing nitrogen (N) fertilizer levels have been used (Zhu et al., 2016). N is an important element which controls the growth of the plant. It is also important for photosynthetic activity and it also has an important function for the growth of oil seeds and oil fruits. Chemical fertilizers have been used to provide an increment in the yield of oil seeds for a long time (Yalcin et al., 2011). At the same time, doses of N play a vital role in plant nutrition and a deficit or excess of N can cause some adverse effects on plant growth. Ashraf et al. (2006) reported that high nitrogen doses cause a decrease in some enzyme activities and thereby the photosynthetic rate decreases. In addition, nitrogen affects the dry matter production of cells by influencing the leaf area development (Shahrokhnia and Sepaskhah 2016).

Maize is well-known as a crop which demands water. High grain yields can be achieved (10–12 t·ha⁻¹) when the water and nutrients are not limited during growing. Maize is a crop which is sensitive to water shortage and grain yield response to deficit irrigation depends on optimum use of N (Pandey et al., 2000). Several researches showed that unsuitable conditions, especially drought, had an effect on the change of the seed composition and related quality parameters such as oil level or structure. It has been reported that the lack of water is the limiting factor for seed growth and affects its composition (Ali et al., 2009; Ali et al., 2010).

The main structural parts of maize kernels are endosperm (82%) and germ (embryo and scutellum) (12%). 80 to 84% of total kernel oil is present in the germ region followed by 12% in the aleurone and 5% in the endosperm. The typical profile of fatty acids in a maize kernel is composed of 57.9% linoleic acid, <1% linolenic acid, 25.2% oleic acid, 11.6% palmitic acid and 1.8% stearic acid (White et al., 2003; Ashgan Abou Gabal and Amera Zaitoun, 2015). In earlier studies it has been reported that water shortage, along with N application at low or high doses can affect the chemical composition of different seeds by different metabolic processes (El-Deen et al., 1997, Yang et al., 2004, Xing et al., 2001 Pavlista et al., 2016a, Pavlista et al., 2016b.). However, the effects of fertilizer and water shortage on the fatty acid composition of maize oil have been poorly investigated. In particular, little or no information has been obtained about the effects of simultaneous application of irrigation and fertilizer on maize oil. Therefore, the objective of this study was to evaluate the change in the crude oil content and fatty acid composition of maize with three levels of fertilizer (N1:100, N2:200 and N3:300 kg·ha^-1), three levels of irrigation (I 50: 50%; I 75: 75%; and I 100: 100% field capacity). The interaction effects of irrigation and fertilizer treatments were also displayed as well as the effect of harvest year.

In this study the effects of different irrigation and nitrogen levels on the crude oil content and fatty acid composition of Zea mays L. were investigated. Table 3 shows the effects and their importance between the interactions and means of fertilizer and irrigation, statistically. Figure 1 shows the irrigation and nitrogen effects on the crude oil content of Zea mays L. The fertilizer doses positively affected positively the crude oil content and the % oil content increased with increasing nitrogen levels. It can be seen that increasing irrigation levels caused an increase in oil content. On the contrary to our results, Sebei et al. (2004) studied four levels of N fertilization (none, low, medium and high) in rapeseed cultivation and they stated that the highest N fertilizer dose decreased the oil content of rapeseed. Ghassemi-Golezani et al. (2015) aimed to evaluate the effects of water deficit and nitrogen fertilizer on the grain yield, oil and protein contents of maize and similarly to our results grain oil percentage significantly decreased as a result of water deficit but unlike in our study, a nitrogen application decreased the oil percentage while oil yield per unit area increased. Ray et al. (2006) aimed to study the effect of high rates of N applied at planting on seed protein and oil content of soybean and as in our study, oil yield increased with the application of fertilizer N at planting. The authors explained these increases due to the increase in seed yield with the application of fertilizer. In agreement with our results, Pavlista et al. (2016a) informed that the highest seed oil content was determined in fully irrigated camelina samples. On the contrary, the same authors concluded that the fatty acid profile was not substantially altered by irrigation in canola seed oil (Pavlista et al. 2016b).

The lowest content among the fatty acids in the maize samples was determined to be myristic acid. The interaction effect between irrigation and fertilizer was not found to be statistically significant on myristic acid content. But the mean value of myristic acid measured in 50% irrigated samples was determined to be lower than the 75 and 100% water (p < 0.05). In addition, the harvest year caused a significant change in its content. White et al. (2007) informed that environmental factors such as temperature, soil type, planting date, year, and location influence the composition of seeds, resulting in quality changes.

Palmitic acid is the third most abundant fatty acid and neither harvesting year nor fertilizing treatment had a significant effect on the amount of it. There was only significant change in palmitic acid content with irrigation treatment. It could be seen that stearic acid values changed with year, irrigation and fertilizer parameters. The interaction effect between irrigation and fertilizer was found to be statistically significant (p < 0.01). Similarly, Kiani et al (2016) reported that there is a strong interaction between irrigation and N for sunflower seed oil. When the fertilizer doses increased the mean values of stearic acid showed a decrement. When the irrigation level was increased from 50% field capacity to 75%, the mean values showed an increment but at the 100% water level the stearic acid value decreased slightly.

As expected, the linoleic and oleic acids were the most abundant fatty acids present in the maize samples. The harvesting year did not affect the oleic acid content while the other parameters were determined to be statistically significant (p < 0.01). The lowest oleic acid value was obtained from a 50% field capacity and 300kg ha-1 fertilizer treatment combination. The oleic acid content tended to increase with 75% field capacity but 100% field capacity treatments caused a decrease in it. The highest linoleic acid content value was determined in a 50% field capacity and 300 kg·ha^-1 fertilizer treatment combination while the lowest value was obtained from a 75% field capacity and 100 kg·ha^-1 fertilizer treatment combination. The mean values of linoleic acid increased with the highest fertilizer dose but no trend was found with the increasing fertilizer doses. Shao-Wen et al. (2004) investigated the effects of the level of N fertilization (125–25 kg·ha^-1) on the fatty acid composition of corn and they pointed out that a moderate N application at 175–225 kg·ha^-1 significantly increased the contents of total fatty acids, unsaturated fatty acids, linoleic acid and oleic acid while higher N treatment levels had a negative effect on these fatty acid contents.

Linoleic acid was not affected by the harvesting year. The mean values of linoleic acid showed a decrement with the increment of irrigation level from 50% field capacity to 75%, but an increment was observed with 100% field capacity treatments once again. In accordance with our results the amount of oleic acid (C18:1) and linoleic acid (C18:2) decreased from 21 to 18% and 20 to 19%, respectively with adding water during the season for camelina (Pavlista et al., 2016a). Ashraf et al. (2006) reported that linoleic acid showed no change in their amounts at varying levels of N for black cumin seed oil.

The combination of 50% field capacity and 300 kg·ha^-1 fertilizer negatively affected the linolenic acid content and the lowest value was obtained. The mean values of linolenic acid decreased with the increasing fertilizer doses, while increasing irrigation levels caused an increase in the mean values of linolenic acid. Pavlista et al. (2016a) determined the growth, seed yield, and oil characteristics of camelina exposed to four levels of applied water and stated that linolenic acid (C18:3) as a main constituent increased with irrigation from 32 to 35% in concordance with our results.

In conclusion, irrigation and nitrogen fertilizer might be considered as visible factors which affected the crude oil and fatty acid composition of maize. Irrigation and nitrogen application altered the oil content and fatty acid composition of maize seed oil but the pattern of increase or decrease in fatty acids at varying N and I levels was not constant. In particular, the interaction effects induced remarkable variations in fatty acids. These results might be used for future breeding studies.