Fats, fatty acids and their metabolic by-products have various significant functions in the human metabolism. They usually provide an efficient source of energy, along with resistance to external factors. They constitute the basic building blocks of cell membranes (Lee, 1994). Since omega-3 and omega-6 fatty acids are not synthesized in the human body, they should be supplied externally. They are called essential fatty acids and play significant roles in the human metabolism (Erdinc et al., 2018).

Vegetable oils are used worldwide in human nutrition, food and industrial purposes. Despite ample sources, the world’s vegetable oil production mostly comes from soybean, sunflower, rapeseed and date palms. Although pumpkin seed oil has sufficient quality attributes for human nutrition and industrial purposes, it has not been used much worldwide (Stevenson et al., 2007). The oil content of pumpkin seeds generally varies from 10.9–54.0% (Stevenson et al., 2007; Murkovic et al., 1996; Seymen et al., 2016). Pumpkin seed oil is generally used as salad dressing in Australia, Slovenia and Hungary. Although pumpkin seeds are largely used either fresh or roasted as appetizers (Murkovic et al., 1996; Ardabili et al., 2011), they have been used for medicinal purposes since ancient times.

Pumpkin seed oil is quite rich in unsaturated fatty acids and that makes it a valuable source of oil for human nutrition. Pumpkin seed oil has high oleic and linoleic unsaturated fatty acid contents and low palmitic and stearic saturated acid contents. Pumpkin seeds are also rich in vitamin E and protein (Ermiş, 2010). With these valuable quality attributes, pumpkin seeds prevent prostate enlargement, arthrolith, reduce cholesterol levels and regulate blood pressure (Stevenson et al., 2007). Pumpkin seeds also prevent hypertension, stimulate hyperglycemic activity and thus prevent diabetes (Stevenson et al., 2007; Fu et al., 2006; Imaeda et al., 1999). Pumpkin seeds are used as an important source of vitamin E (tochopherol) in Japanese diets.

Many physiological, molecular and biochemical reactions of plants are influenced by water stress. Seed fat and fatty acid compositions are largely influenced by the cultivar, soil and climate conditions and cultural practices (Nawirska-Olszanska et al., 2013). Parallel findings were reported in previous studies carried out with different plant species (Kaplan et al., 2017 for maize; Ali and Ullah, 2012 for sunflower; Kırnak et al., 2010, for soybean).

In Turkey, the cultivar Cucurbita pepo is commonly used and Cucurbita moschata is less frequently used for pumpkin seed production. Annually, 42181 tons of pumpkin seeds are produced from 62844.1 ha of land area in Turkey. The Kayseri province, with 15053 tons of production annually, from 31310.1 ha of land area, constitutes about 35.7% of the country’s production (TUIK 2016).

Although a few studies have been conducted regarding water stress on pumpkin growth, seed yield and some seed quality properties such as 1000-seed weight and oil content, no studies have been found about the effects of water stress on the other properties of pumpkin seeds. Therefore, this study was conducted to determine the effects of different irrigation regimes on oil content, fatty acids, protein and vitamin E content of pumpkin seeds produced in the central Anatolia region of Turkey.

Experiments were conducted in the experimental fields of the Agricultural Research and Implementation Center of Erciyes University for two years, in 2015 and 2016. The research site with an altitude of 1094 m is located between 34° 56’ and 36° 59’ east longitude and between 37° 45’ and 38° 18’ north latitude. Long-term and 2015–2016 climate data are respectively provided in Tables 1 and 2. As can be inferred from Table 2, the precipitation throughout the experiments was 142 mm in 2015 and 179.4 mm in 2016.

Soil samples were taken from the 0–120 cm soil profile of 3 different locations within the research site. Water samples were taken from an irrigation water supply well and analyses were performed in accordance with Ayyıldız (1990). Irrigation water has a pH of 7.60 and EC of 242 μS; thus it was classified as C1S1 (Ayyıldız, 1990). The results from the soil analysis are provided in Table 3. The stable rate of infiltration was measured as 23.3 mm/h.

Experiments were conducted in a randomized block design with 3 replications. Sowing was performed with 100-cm-row spacing and 60-cm on-row plant spacing. Since there aren’t any registered varieties, the commonly-used, so-called “framed” seeds were used. Each plot had 8 rows. Two side rows and a plant row from the top and bottom sections of the plots were omitted so as to consider side-effects. About 2 m spacing was provided between the plots and 2.4 m between the blocks to prevent interactions. Based on soil analysis results, DAP fertilizer (12 kg/da N, 12 kg/da P and 10 kg/da K) was supplied through fertigation. Since the soil had sufficient K levels, K fertilization was not performed.

A drip irrigation system was used for irrigation. They system was designed in accordance with Keller and Bliesner (1990). Dripper spacing was 0.33 m, dripper discharge rate was 4 lt/h, lateral diameter was 20 mm, manifold pipe diameter was 40 mm and main pipe line diameter was 63 mm. Water flow meters were placed at the entrance of each block and applied water quantities were measured. The wetted area percentage was calculated as 66% (Keller and Bliesner, 1990).

Soil moisture levels within the 120 cm soil profile were monitored on every other day with a neuron meter (CPN 503DR Hydroprobe) and irrigation was initiated when 35–40% of available water within the root zone was depleted. Plant efficient root depth was taken as 60 cm (Allen et al., 1998). Irrigation intervals varied during the growing season because of irrigation scheduling based on soil moisture depletion. In total, irrigation was carried out 12 times in 2015 and 10 times in 2016.

Different irrigation levels were experimented in this study. Experimental irrigation levels were created through supplying different portions of depleted moisture within the efficient root zone of the plants. The treatments were arranged as supplying 100% (I₁₀₀), 80% (I₈₀), 60% (I₆₀), 40% (I₄₀), 20% (I₂₀) and 0% (I₀) of depleted moisture.

Sowing was performed on 5 May, 2015 and 29 April, 2016 and harvesting was done on 24 August, 2015 and 10 August, 2016. Irrigation programs were initiated on 19 June, 2015 in the first year and on 13 June, 2016 in the second year. Chemicals were not applied since there were not any pests or diseases.

Irrigation water quantities applied to pumpkin seed plants in I₀, I₂₀, I₄₀, I₆₀, I₈₀ and I₁₀₀ treatments were measured as 24, 70.8, 117.9, 164.6, 211.4 and 258.4 mm, respectively, in 2015 and as 13, 104.4, 195.8, 287.3, 378.8 and 470.2 mm, respectively, in 2016.

The vitamin E contents, fatty acids, oil and protein contents of different irrigation treatments are provided in Table 4.

While different irrigation treatments did not have significant effects on the vitamin E contents of the pumpkin seeds in 2015, the effects of irrigation treatments on vitamin E contents were found to be significant in 2016 (Table 5). Vitamin E contents varied from 41.6–55.3 mg/100g in 2015 and from 45.0–55.3 mg/100g in 2016. The lowest vitamin E content was obtained from I₄₀ treatments in 2015 and 2016 and the greatest vitamin E content was obtained from I₂₀ treatment in 2015 and from I₆₀ treatment in 2016.

Linoleic, linolenic, oleic, stearic, palmitic and meristic acids were investigated in pumpkin seeds and linolenic and meristic acids were not encountered in the seeds. Although linolenic and meristic acids are among the main components of pumpkin seed, these acids are included in trace levels and some researchers could not determine linolenic or meristic acid (Potočnik et al., 2018; Meru et al., 2018; Lazos, 1986; Rezig et al., 2012). Irrigation treatments did not result in significant changes in the fatty acid compositions of the pumpkin seeds in either year (Table 5). Linoleic acid contents varied from 33.0–34.35 in 2015 and from 32.4–35.0% in 2016. Oleic acid was the dominant fatty acid in both years. Oleic acid contents varied from 45.3–48.9% in 2015 and from 39.6–44.5% in 2016. Palmitic acid contents varied from 10.7–12.4% in 2015 and from 10.8–12.6% in 216. The stearic acid contents of the pumpkin seeds varied from 6.4–8.5% in 2015 and from 9.1–10.4% in 2016. Fatty acids exhibited similar variations in both years.

Irrigation treatments had significant effects on the oil contents of the pumpkin seeds in both years (Table 5). Oil contents varied from 26–51% in 2015 and from 58–64% in 2016. In 2015, the lowest oil content was obtained from the I₀ treatment and the greatest oil content was obtained from the I₁₀₀ treatment. There were three statistical groups in 2015 and the I₁₀₀, I₈₀, I₆₀ and I₄₀ treatments were placed in the same statistical group. In 2016, the lowest oil content was obtained from I₀ and the greatest oil content was obtained from the I₁₀₀ treatment. There were two statistical groups in 2016 and the I₂₀, I₆₀, I₈₀ and I₁₀₀ treatments were placed in the same statistical group. The oil contents in the seeds increased with increasing irrigation water quantities.

While irrigation treatments did not have significant effects on the protein contents of the pumpkin seeds in 2015, the effects of irrigation treatment on protein contents were found to be significant in 2016 (Table 5). In both years, the greatest protein content was obtained from the I₀ treatment and the lowest oil content was obtained from the I₈₀ treatment. Protein contents decreased with increasing irrigation treatments.

The relationships between the applied irrigation water quantities and the oil and protein contents in the pumpkin seeds are presented in Figure 1. Irrigation water quantities significantly correlated with oil and protein contents. There was a significant logarithmic relationship between irrigation water quantities and oil contents in 2015 (R² = 0.93) and a linear relationship was observed in 2016 (R² = 0.86). On the other hand, there were polynomial relationships between irrigation water quantities and protein contents (R² = 0.91, R² = 0.80, respectively) in both years.

Pumpkin seed production has increasing popularity compared to cereals because of profitability, easy storage and marketing. Rich oil, fatty acids, protein and vitamin E contents increase the significance of pumpkin seed cultivation even more.

Pumpkin seeds are quite rich in vitamin E. Rezig et al., (2012) reported the vitamin E contents of pumpkin seeds (Cucurbita maxima) as 41.9 mg/100 g. Stevenson et al., (2007) worked with pumpkin seeds of 12 different cultivars and reported vitamin E contents between 45.4 and 70.9 mg/100 g. In another study carried out in Iran, the average vitamin E content in pumpkin seeds (Cucurbita pepo Subsp. pepo Var) was reported as 88.8 mg/100g (Ardabili et al., 2007). The present findings comply with those earlier ones. Vitamin E is a natural antioxidant. The vitamin E contents in pumpkin seed oil largely depend on the fatty acid contents and compositions of the seeds. Vitamin E contents are also greatly influenced by post-harvest conditions (Nakic et al., 2006). Environmental conditions affect the fatty acids, amino acids, minerals and vitamins of different pumpkin genotypes (Erdinç et al., 2018). Because the vitamin E content of pumpkin seed depends on water stress and other factors, we may not find regular effects of water stress on vitamin E.

Pumpkin seeds were mostly composed of linoleic, oleic, palmitic and stearic acids. Pumpkin seeds generally had high unsaturated fatty acid (Linoleic+Oleic) contents. Fatty acid contents varied from 79.1–82.65 in 2015 and from 72–78.3% in 2016. Stevenson et al., (2007) reported the unsaturated fatty acid contents of pumpkin seeds as between 73.1–80.5%. Meru et al., (2018) reported the unsaturated fatty acid contents as between 78.6–86.1%. Oleic acid was the dominant fatty acid in the present study and it was followed by linoleic acid. The present findings were in good agreement with the findings of Nakic et al., (2006). The lowest acids were identified as stearic (6.4–10.4%) and palmitic acids (10.7–12.6%). The present findings comply with the results of earlier studies (Lazos 1986; Sekerci et al., 2017; Nawirska-Olszanska et al., 2013; Seymen et al., 2016).

Pumpkin seed oils are mostly unrefined. Therefore, they are not used as cooking oil, but mostly used as salad dressing because of the color, strong aroma and fatty acid composition. Linoleic acid usually reduces the heat balance of the oil, thus it not suitable to be used as cooking oil (Nederal et al., 2012; Potacnik et al., 2018).

The Present oil contents (26–64%) comply with the findings of earlier studies carried out with cucurbita species. Meru et al., (2018) reported the oil contents of pumpkin seeds as between 29.33 and 48.41%. Lazos (1986) reported the average oil content of pumpkin seeds as 45.4% and Ardabili et al., (2011) as 41.6%. In another study, the oil contents of pumpkin seeds were reported as between 10.9 and 30.9% (Stevenson et al., 2007). Differences in oil contents of the seeds largely depend on cultivars, agricultural practices and water management regimes.

The present protein contents also comply with the findings of previous studies. Glew et al., (2006) reported the average protein content of pumpkin seeds as 51.4%, Lazos (1986) as 32% and Ardabili et al., (2011) as 25.4%. Al-Khalifa (1996) reported the protein contents as 26.5% for Cucurbita pepo and 24% for Cucurbita moschata. The Present protein contents varied from 28.5–37.7%. These values are lower than the values reported by Glew et al., (2006), but well comply with the findings of other studies. Differences in protein contents are mostly attributed to differences in cultivars, growing conditions and climate parameters.

The pumpkin seeds of this experiment have high vitamin E content, which makes them desirable for human health. Vitamin E content was not changed with irrigation water amounts. Although water stress decreased the oil’s components, fatty acid content was not significantly changed. Oil content was improved by increasing irrigation amounts and the highest oil content was acquired by applying full irrigation. Oleic, linoleic, palmitic and stearic acids were the main components of the pumpkin seeds. Irrigation also improved pumpkin seed protein content. We concluded that pumpkin seed properties are beneficial for human health and their production could be hastened by proper irrigation management strategies.