Effect of iron fertilization on nut traits and nutrient composition of ‘Tombul’ hazelnut (Corylus avellana L.) and its potential value for human nutrition

Abstract

Iron (Fe) is an essential element for plants and its deficiency is a well-documented problem, causing decreased yields and nutritional quality. An orchard experiment was carried out to investigate the effect of iron fertilization on some nut traits and mineral compositions of ‘Tombul’ hazelnut (Corylus avellana L.) variety cultivated in the Black Sea Region of Turkey. In addition, the contributions of these nuts on human nutrition were determined. Hazelnuts were fertilized with 0, 4, 8, 12, and 16 kg ha⁻¹ iron in each year during three consecutive years. Iron fertilization significantly affected nut traits and mineral compositions of the nuts. The amounts of total oil, kernel percentage, and kernel weight increased and the amounts of empty and wrinkle nuts decreased with 12 kg ha⁻¹ Fe fertilization. Different iron doses caused slightly different protein content and kernel shape in the kernels. The phosphorus, iron, and boron contents of nut increased significantly with 8 kg ha⁻¹ Fe fertilization. In order to improve some nut traits and mineral compositions of hazelnut, 8 kg ha⁻¹ and 12 kg ha⁻¹ Fe fertilizations could be recommended for practice. According to daily nutrition element requirements, the quantity of 100 g hazelnut provided about 43.12% for P, 12.93% for K, 19.09% for Ca, 36.97% for Mg, 0.18% for Na, 50.88% for Fe, 25.00% for Zn, 14.15% for B, and 77.33% for Mo of the recommended dietary allowances. Cu and Mn contents of 100 g hazelnut were higher than the respective daily requirements. This research indicated that hazelnut is a rich source of mineral elements, and ‘Tombul’ hazelnut can be an important source of mineral elements for human nutrition with health beneficial impact.

Keywords:

• Corylus avellana L.
• iron
• fertilizer
• nut traits
• nutrient
• human nutrition

Introduction

Hazelnut (Corylus avellana L.) is a well-known tree nut worldwide. Hazelnuts are mainly produced in Turkey, Italy, Spain, USA, Portugal, and France. Likewise, hazelnut can also be cultivated in some other countries such as Azerbaijan, Georgia, Australia, New Zealand, China, Chile, and Iran, among others. Turkey dominates world hazelnut production and is the genetic origin of the hazelnut for wild species and cultivated varieties. Hazelnut production in Turkey utilizes over 690,000 hectares of land and mainly distributed along the coasts of the West, Middle, and East Black Sea region. Turkey provides 65–70% of the world hazelnut requirement and annual production is about 580,000 tons. Hazelnut has a very important place among our country's horticultural products in terms of the amount of production and export values, having a very significant impact on the world nut trade and industry.

Iron (Fe) plays essential roles in numerous biochemical processes, due to its affinity to organic ligands and its capacity to change the oxidation state form (II) to (III). Iron is an essential element for microorganisms and plants. Although it is relatively abundant in soils, Fe availability is constrained by the low solubility of Fe (III) species and the slow dissolution of Fe minerals, particularly under nonacidic conditions (Kraemer et al. 2006; Lemanceau et al. 2009). Iron (Fe) is absorbed from soil in both Fe²⁺ and Fe³⁺ forms (Gupta & Gupta 2005). The availability of Fe³⁺ in calcareous soil decreases markedly when the pH of the soil is high (Lucena 2003; Rashid & Ryan 2004). Iron deficiency chlorosis is an important nutritional disorder that results not only from a low level of available Fe in calcareous soils but also from impaired acquisition and use of this metal by plants (Pestana et al. 2003). Iron deficiency causes various morphological and physiological changes in plants. This is due to the fundamental role of Fe in a series of processes. Iron is a constituent of electron-transport chains in both mitochondria and chloroplasts. Therefore, one might conclude that the shortage of physiologically active Fe leads to decreased electron-transport activity in general (Bertamini & Nedunchezhian 2005). In iron deficiency, fruit set and nut yield are reduced, and nut are small, pointed misshapen, and poorly colored. Iron deficiency is a widespread phenomenon throughout the world and may affect soil conditions. The need to correct Fe chlorosis is related to its effects on yield, nut traits, nutrient compositions, and consequently on the economic impact of iron deficiency chlorosis. Correction of the iron deficiency carried out by applications of synthetic Fe chelates to soil and plant (Tagliavini et al. 2000; Alvarez-Fernandez et al. 2006; Pestana et al. 2011). Iron deficiency is one of the most common nutritional disorders in Turkish nuts (C. avellana L.) cultivation. Due to the effects of iron on yield and quality in hazelnut agriculture, it is crucial that iron fertilization be done correctly and sufficiently. Soil and foliar application of Fe are both effective methods for increasing the iron concentrations in hazelnut.

Several studies considered that the major, trace, and heavy metal concentrations in hazelnut were K, P, Ca, Mg, Fe, Zn, Cu, Mn, Se, Al, Ni, Cd, Co, etc. (Açkurt et al. 1999; Özdemir et al. 2001; Köksal et al. 2006; Güneş et al. 2010; Özkutlu et al. 2011; Cosmulescu et al. 2013). Moreover, many studies indicated that the nut mineral composition, protein, oil, and ash contents of hazelnut are affected by climate, variety, composition of soil, uses of fertilizer and irrigation, methods of cultivation, harvest year, and geographical origin (Parcerisa et al. 1998; Açkurt et al. 1999; Özdemir et al. 2001; Dündar et al. 2002; Amaral et al. 2006; Köksal et al. 2006; Seyhan et al. 2007; Cristofori et al. 2008; Özkutlu et al. 2011). In spite of the known major mineral composition of hazelnuts, there is scarce research on its growing techniques, especially fertilization. Good fertilizing program is necessary in order to obtain high quantity and high quality of the yield in hazelnut. There are very few studies on the influence of iron fertilization on mineral composition of hazelnut and its effects on human nutrition.

Hazelnuts are widely used in the food industry such as chocolate, confectionary, baking, ice cream, and dairy products. Hazelnuts can be added to a wide array of dishes from cereals and breads to yogurts, soups, salads, and main dishes to confections, ice creams, and pastries. They are famed for their use in pies and their affinity with chocolate (Fallico et al. 2003; Özkutlu et al. 2011). The nutritional and sensory properties of hazelnuts make them a unique and ideal raw material for food products. Hazelnuts have significant place among the types of dried nuts in terms of nutrition and health owing to their special composition of minerals, protein, fats, carbohydrates, vitamins, dietary fibers, and phenolic antioxidants (Demirbas 2007; Şimşek & Aykut 2007; Li & Parry 2011; Solar & Stampar 2011). Hazelnuts may play an important role in human nutrition and health due to their very special nutritional value. Minerals are essential in maintaining the healthy nerve function and for keeping the body systems, bones, and cells healthy and well balanced. Hazelnut is good source of protein and fat (Garcia et al. 1994; Özdemir et al. 2001; Alasalvar et al. 2006; Balta et al. 2006; Kornsteiner et al. 2006; Cristofori et al. 2008; Güneş et al. 2010; Jakopic et al. 2011). Hazelnut is the best source of essential elements, amino acids, vitamin B and E among tree nuts (Açkurt et al. 1999; Oliveira et al. 2008) and serves as a good source of natural antioxidants (Contini et al. 2008; Li & Parry 2011; Schmitzer et al. 2011). One hundred grams of hazelnut supplies 600–650 kcal, minerals (2–3.5%), and vitamins for human consumption (Souci et al. 2000; Köksal et al. 2006).

The objective of this research was to analyze nut traits and some nutritional composition of ‘Tombul’ hazelnuts variety widely grown throughout the hazelnut plantations of Turkey. Another aim was to determine the effects of raising iron fertilizer applications on mineral composition and nut properties of hazelnut (C. avellana L.) and its potential value for human nutrition.

Materials and methods

Background information for the experiment orchard

The orchard experiment was carried out at Hazelnut Research Station located in Giresun province in East Black Sea region of Turkey (40°54′33″ N, 38°21′01″ E) between the years 2009 and 2011. The elevation was 20–30 m. The monthly climatic parameters of Giresun province were given in Table 1. The amount of total precipitation and the number of rainy days were 1263.5 mm, and 163.9, respectively. The mean temperature and the relative humidity were 14.5°C and 75.5%. The experimental region has a temperate climate and is known to be favorable for hazelnut cultivation.

Table 1. Some monthly climatic parameters of long-term period in Giresun province (1954–2013).

Month	Precipitation (kg m^−2)	Rainy days (number of days)	Temperature (°C)	Humidity (%)
January	122.7	15.0	7.3	69.0
February	96.8	14.2	7.1	71.8
March	90.0	16.2	8.2	74.7
April	78.9	15.6	11.4	78.1
May	68.8	14.5	15.6	80.8
June	78.1	11.8	20.2	78.2
July	77.4	10.5	22.9	77.5
August	88.9	10.7	23.1	78.1
September	124.0	13.1	20.0	78.3
October	166.9	14.2	16.3	77.9
November	145.8	13.5	12.6	73.2
December	125.2	14.6	9.5	68.7
Sum	1263.5	163.9
Mean			14.5	75.5

Source: http://www.mgm.gov.tr/veridegerlendirme/il-ve-ilceler-istatistik.aspx?m=GIRESUN#sfB.

Plant materials

In the trial, the prime quality ‘Tombul’ hazelnut (C. avellana L.) variety was investigated. The orchard experiment was established the average of 20–25 years old ‘Tombul’ hazelnut ocak with four branches based on planting system.

Experimental design

The experiment was carried out according to the design of randomized complete blocks of iron nutrient element in five different doses and three replications per treatment. Each treatment had 15 trees and the total numbers of treatment were 45 trees. Iron fertilizers (Fe-EDHHA) were applied in 0, 4, 8, 12, and 16 kg ha⁻¹ Fe. Applied iron fertilizer doses were abbreviated as Fe 0, Fe 4, Fe 8, Fe 12, and Fe 16. The other fertilizers were applied at required rates in hazelnut orchards according to the results of soil analysis. Applied fertilizers were as follows: calcium ammonium nitrate (26% N), triple super phosphate (42–44% P₂O₅), potassium sulfate (%48–52 K₂O), calcium carbonate (90% CaO), magnesium sulfate (9.8% Mg), borax (11% B), zinc sulfate (35% Zn), copper sulfate (35% Cu), manganese sulfate (25% Mn), and ammonium molybdate (54% Mo).

Sampling

Initial soil samples were taken over two depths of 0–20 cm and 20–40 cm to determine baseline soil properties before iron (Fe) fertilizer applications. Soil samples were air-dried, crushed, and passed through a 2-mm sieve prior to analysis. Each a hazelnut ocak was harvested separately in the first week of August depending on climate during 2009 and 2011 harvest period. Harvested hazelnuts were separated from husk by using mechanical sorters and then, dried by being spread over the ground in natural conditions. Dried hazelnut samples were packed into porous bags, stored as in-shell at room temperature conditions for about one month, and were cracked by using mechanical crackers. Kernel samples were kept in a refrigerator until analyses were performed. All reagents were of analytical grade. All analyses were conducted in triplicate.

Soil analysis

Texture analyses of the experiment soils were performed according to Soil Survey Staff (1951). Soil pH and electrical conductivity (EC) were measured in saturation extracts according to US Salinity Lab Staff (1954) and Rhoades (1996). CaCO₃ analysis was made according to Çağlar (1958). Soil organic matter was determined by using the Walkley–Black method according to Nelson and Sommers (1982). Available phosphorus (P) was determined by using the Bray and Kurtz method according to Bray and Kurtz (1945). Available potassium (K) was determined by extracting the soil sample with 1N ammonium acetate (adjusted pH 7.0), and K concentration in the extracts was measured by Atomic Absorption Spectrophotometry (AAS; Knudsen et al. 1982). Total iron in the soils was determined by diethylene triamine pentaacetic acid extraction method (Lindsay & Norvell 1978; Table 2).

Table 2. Some soil properties before iron fertilizer applications.

Texture	Clay loam
Soil pH	5.72
Salinity (dS m^−1)	0.11
CaCO3 (%)	0.09
Organic matter (%)	4.93
Available phosphorus (mg kg^−1)	84.49
Available potassium (mg kg^−1)	67.15
Total iron (mg kg^−1)	54.42

Nut traits, biochemical, and mineral analysis

In-shell and kernel weight were recorded on one hundred randomly selected subsamples of nuts for each year. In-shell and kernel width, thickness, and height were recorded on 20 randomly selected subsamples of nuts for each year. In-shell and kernel shape were calculated as ((width + thickness/2)/height). Shell thickness was measured by digital caliper. Kernel percentage was determined by the ratio of the in-shell weight to the kernel weight (Kernel weight/In-shell weight × 100). Three replicates of 100 nuts in per treatment were used to calculate the percentage of empty and wrinkle nuts (Köksal 2002).

Total oil in the kernel was extracted from the samples as described in Association of Official Analytical Chemists (AOAC 1990). Oils were extracted with n-hexane (boiling 60°C) for 6 h by using a Soxhlet extractor. A 10 g portion of finely crushed kernels was placed in a cellulose thimble and extracted with 200 ml of hexane for 6 h in a Soxhlet apparatus. After extraction the solvent was evaporated and the residual oil was weighed. Protein contents were determined by micro-Kjeldahl method and were calculated as total N × 6.25 (James 1995). Total ash was determined by drying of the samples for 12 h at 75°C in an oven and then transferring the crucible to a muffle furnace. The temperature was gradually raised to 550°C, the samples were being ash for 24 h to a white color, and then nut ashes were weighed (AOAC 1990).

Mineral element analyses were carried out by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) (Perkin Elmer, Optical Emission Spectrophotometer, Optima-2100 DV). Microwave system was used for solid nuts sample mineralization. The amounts of approximately 0.5 g per nut sample, 6 ml of 65% nitric acid and 2 ml of 33% oxygenated water, were introduced in Teflon recipients and were put under thermic treatment programmer under pressure; heating them up to 180°C by a rate of 4.5°C min⁻¹ and keeping them for 20 minutes at 180°C. After cooling down, the liquid samples were transferred into marked glass balloons. They were brought to 50 ml volume, by using ultrapure water, and were analyzed according to specific procedures. Control sample was made of 6 ml nitric acid 65% and 2 ml oxygenated water 33%, and it was processed under the same conditions as the analyzed sample. Mineral elements K, Ca, Mg, Na, Fe, Cu, Zn, B, Mn, and Mo were determined according to standard AOAC method (AOAC 2000). Phosphorus was spectrophotometrically analyzed in the form of vanadium phosphomolybdate (James 1995).

Statistical analysis

Statistical analyses were performed by using analysis of variance in Minitab Statistical software. Results were expressed as means ± standard error mean (SE). The statistical significance (t test) was determined using Minitab Statistical software. Differences at P < 0.05 were considered to be significant. All data obtained were the mean from the three years.

Results and discussion

Before iron fertilizer applications, the experiment hazelnut orchard soil samples were analyzed. The some soil properties were given in Table 2. The soils of the experiment orchards have clay loam texture, none salinity, and very few CaCO₃. The soil reaction was found to be an average of 5.72, organic matter contents average of 4.93%, available phosphorus contents average of 84.49 mg kg⁻¹, available potassium contents average of 67.15 mg kg⁻¹, and total iron contents average of 54.42 mg kg⁻¹.

Iron is the most important nutrient, and an adequate supply of fertilizer is required for maximum yield, high quality nuts, and especially for rich mineral contents. The protein, oil, ash, some nut traits, and nutrient compositions of ‘Tombul’ hazelnuts were determined and the data were given in Tables 3 and 4. There were significant (P < 0.05 and P < 0.01) differences among the raising iron fertilizers in terms of protein content, total oil content, kernel ratio, percentage of empty and wrinkle nuts, kernel weight, kernel shape, phosphorus (P), iron (Fe), copper (Cu), boron (B), manganese (Mn), and molybdenum (Mo) contents of nuts according to variance analyses.

Table 3. Protein, total oil and ash content, and some nut traits of hazelnut cultivar ‘Tombul’ after fertilization with five Fe doses.

	Iron fertilization (kg ha^−1, Fe)
	0	4	8	12	16	P
Protein (%)	18.55 ± 0.144a	17.33 ± 0.342b	17.48 ± 0.092b	17.63 ± 0.329b	18.38 ± 0.329a	0.001**
Total oil (%)	60.08 ± 0.918b	60.05 ± 0.629b	62.43 ± 0.104a	61.97 ± 0.444a	61.64 ± 0.939ab	0.004**
Total ash (%)	1.95 ± 0.146	2.20 ± 0.233	2.09 ± 0.087	2.13 ± 0.065	2.21 ± 0.077	ns
Kernel percentage (%)	49.13 ± 0.347c	50.21 ± 0.697bc	50.76 ± 0.350ab	51.73 ± 0.261a	51.06 ± 0.176ab	0.000**
Empty nut (%)	8.27 ± 1.148a	6.24 ± 0.977ab	7.57 ± 1.214ab	5.30 ± 0.767b	6.16 ± 1.015ab	0.035*
Wrinkle nut (%)	3.11 ± 0.255a	2.64 ± 0.567ab	1.83 ± 0.289ab	1.70 ± 0.653b	2.41 ± 0.540ab	0.028*
100 In-shell weight (g)	188.62 ± 1.71	194.68 ± 11.54	194.70 ± 11.05	199.53 ± 11.67	195.23 ± 10.50	ns
100 Kernel weight (g)	92.55 ± 0.96c	97.19 ± 1.61bc	99.98 ± 2.07ab	103.57 ± 2.27a	98.76 ± 1.74b	0.000**
Shell thickness (mm)	1.33 ± 0.075	1.27 ± 0.102	1.37 ± 0.035	1.30 ± 0.070	1.29 ± 0.035	ns
In-shell shape	1.08 ± 0.020	1.09 ± 0.025	1.10 ± 0.032	1.09 ± 0.032	1.10 ± 0.020	ns
Kernel shape	1.01 ± 0.0115c	1.03 ± 0.0208bc	1.06 ± 0.0153ab	1.06 ± 0.0173ab	1.08 ± 0.0058a	0.001**

Note: All data represent the mean of three years and three replications. Each data are expressed as means ± SE of three determinations. Data in the same column with different lowercase letters are significantly different at p < 0.05; ns: not significant.

* Significant levels at 5%.

** Significant levels at 1%.

Table 4. Nutrient compositions of ‘Tombul’ kernels after fertilization with five Fe doses.

	Iron fertilization (kg ha^−1, Fe)
Nutrient elements (mg 100 g^−1)	0	4	8	12	16	P
Phosphorous (P)	298.87 ± 4.48ab	310.30 ± 13.77ab	314.23 ± 8.85a	296.42 ± 3.12ab	289.37 ± 7.13b	0.024*
Potassium (K)	609.90 ± 26.70	606.12 ± 42.30	623.32 ± 41.30	603.84 ± 39.20	595.20 ± 28.40	ns
Calcium (Ca)	192.33 ± 10.15	196.83 ± 16.85	202.10 ± 28.40	182.12 ± 13.24	181.00 ± 17.60	ns
Magnesium (Mg)	148.00 ± 13.26	146.90 ± 13.00	147.03 ± 15.44	150.70 ± 11.22	146.83 ± 13.01	ns
Sodium (Na)	2.55 ± 0.218	2.51 ± 0.156	2.63 ± 0.260	2.78 ± 0.155	2.82 ± 0.214	ns
Iron (Fe)	3.75 ± 0.136b	4.08 ± 0.098ab	4.21 ± 0.074a	4.15 ± 0.155ab	4.16 ± 0.247ab	0.026*
Copper (Cu)	1.87 ± 0.101a	1.74 ± 0.075ab	1.64 ± 0.156ab	1.53 ± 0.121b	1.49 ± 0.074b	0.009**
Zinc (Zn)	2.56 ± 0.085	2.60 ± 0.131	2.82 ± 0.240	2.92 ± 0.150	2.83 ± 0.306	ns
Boron (B)	2.66 ± 0.042b	2.80 ± 0.065ab	3.01 ± 0.102a	2.78 ± 0.036ab	2.88 ± 0.160ab	0.011*
Manganese (Mn)	6.69 ± 0.736a	5.88 ± 0.668ab	5.66 ± 0.456ab	5.45 ± 0.433ab	5.04 ± 0.279b	0.039*
Molybdenum (Mo)	0.036 ± 0.004ab	0.039 ± 0.005a	0.037 ± 0.002ab	0.029 ± 0.002bc	0.022 ± 0.003c	0.000**

Note: All data represent the mean of three years and replications. Each data are expressed as means ± SE of three determinations. Data in the same column with different lowercase letters are significantly different at p < 0.05; ns: not significant.

* Significant levels at 5%.

** Significant levels at 1%.

Iron fertilizer doses significantly affected protein content of the kernels. It ranged between 17.33% and 18.55% in iron fertilization applications (Table 3). Significantly, the highest protein contents were found in Fe 0 (18.55%) and Fe 16 (18.38%) iron applications. The doses from Fe 4 to Fe 12 decreased protein contents of the kernels. Using excess iron fertilizer (Fe 16) increased again protein content of the kernels. Ozdemir and Akinci (2004) reported that hazelnuts were good source of energy and were rich in protein. When these results were compared with previous studies on ‘Tombul’ hazelnut variety, the protein contents found in this study were a little high and similar to those reported by Köksal (2002), Özenç and Çaycı (2005), Köksal et al. (2006), and Güneş et al. (2010) for ‘Tombul’ hazelnut variety.

Significant differences were observed among the raising iron fertilizations regarding the total oil content (P < 0.01). It was the highest in the Fe 8 fertilization dose, although statistically it was in the same group with Fe 8 and Fe 12 fertilizations with 62.43% and 61.97%. The lowest oil content was found in Fe 0 dose (60.08%) and in Fe 4 dose (60.05%) (Table 3). The total oil content of the samples was above 50% and was the high values of 62.43% in iron fertilization. These results consistent with the findings of most of the other researchers (Özdemir et al. 2001; Alasalvar et al. 2003; Amaral et al. 2006; Köksal et al. 2006; Oliveira et al. 2008; Bacchetta et al. 2013).

Kernel percentages were significantly different between the iron fertilizer applications (Table 3). The lowest kernel percentage was found in Fe 0 dose (49.13%) and the highest was found in Fe 12 dose (51.73%). Köksal (2002), Ozdemir and Akinci (2004), Özenç and Çaycı (2005), and Cristofori et al. (2008) reported that the kernel percentages were found to be 49.90%, 47.31%, 51.66–54.93% for ‘Tombul’ cultivars, 37.42–48.92% (24 Italian cultivars) and 51.74% (‘Tombul’ cultivar), respectively.

Each iron fertilizer doses had significantly different empty and wrinkle nuts (P < 0.05) (Table 3). Significantly, the lowest empty nut percentage was found in Fe 12 dose (5.30%) and the highest was found in Fe 0 dose (8.27%). The lowest wrinkle nut percentage was found in Fe 12 application (1.70%) and the highest percentage was found in Fe 0 control application (3.11%). Through this research, it was determined that empty and wrinkle nut amounts were decreased by raising iron fertilizations, especially with Fe 12 dose.

Considerably differences were observed among the raising iron fertilizations regarding kernel weight and kernel shape (P < 0.01; Table 3). After iron fertilizations, the highest amount of kernel weights was found as 103.57 g with Fe 12 dose and the lowest amount was found as 92.55 g with Fe 0 dose. Kernel shapes were significantly changed with iron fertilizations. While the kernel shape was the highest in the Fe 16 fertilization with 1.08, it was the lowest in the control with 1.01.

Iron fertilizers did not have a significant effect on the total ash, in-shell weight, shell thickness, and in-shell shape (Table 3). Total ash amounts and 100 in-shell weights were ranged from 1.95% (with Fe 0) to 2.21% (with Fe 16) and from 188.62 g (with Fe 0) to 199.53 g (with Fe 12), respectively. Total ash content was reported as 2.34% and 2.03% for cultivars from West and East Black Sea region (Köksal et al. 2006; Güneş et al. 2010). In this study, total ash contents of the kernels were increased to 2.21% with iron fertilization in the East Black Sea region. Özenç and Çaycı (2005) and Cristofori et al. (2008) reported the in-shell weights were found to be 157.62 g for ‘Tombul’ cultivars and from 1.62 to 3.02 g (24 Italian cultivars) and 1.65 g (‘Tombul’ cultivar). Shell thickness and in-shell shape were changed from 1.27 (with Fe 4) to 1.37 mm (with Fe 8) and from 1.08 (with Fe 0) to 1.10 (with Fe 8 and Fe 16 doses), respectively. Özenç and Çaycı (2005) reported that shell thickness of ‘Tombul’ hazelnuts was found to be 1.02 mm. Through this research, it was determined that total ash amounts, in-shell weights, shell thickness, and in-shell shapes were not affected by iron fertilizations.

The nutrient compositions of ‘Tombul’ kernels, affected by iron fertilization, were given in Table 4. Significant differences were observed in the iron fertilization doses regarding phosphorus (P), iron (Fe), copper (Cu), boron (B), manganese (Mn), and molybdenum (Mo) contents in nuts (P < 0.05 and P < 0.01) (Table 4). Minerals are of interest due to their pro-oxidant activity and health benefits (Parcerisa et al. 1995; Pershern et al. 1995; Alphan et al. 1997; Li & Parry 2011; Cosmulescu et al. 2013). Iron fertilization significantly affected the phosphorus content of the kernels. The highest phosphorus content of the kernels were found 314.23 mg 100 g⁻¹ (with Fe 8) and the lowest were found 289.37 mg 100 g⁻¹ (with Fe 16) (Table 4). While up to Fe 8 fertilization was increased phosphorus amounts of the kernels, using excess iron fertilizer was declined these element concentrations in the kernels. The average of phosphorus content in the kernels was found 301.84 mg 100 g⁻¹ in iron fertilizer applications (Table 5). Phosphorus content was higher than that one found 288 mg 100 g⁻¹ by Köksal et al. (2006) and was lower than that two found 329.8 mg 100 g⁻¹ by Ozdemir and Akinci (2004) and 300.67–455.06 mg 100 g⁻¹ (Romanian cultivars) by Cosmulescu et al. (2013). Phosphorus is a component of bones and cells in energy processing and many other functions. The recommended intake of phosphorus was 700 mg daily for both adult males and females (Table 5). According to the present study results, the consumption of 100 g hazelnut supplies about 43.12% of phosphorus daily intake.

Table 5. Recommended dietary allowances (RDA) level of nutrients and average nutrient contents of ‘Tombul’ kernels after fertilization with five Fe doses.

	Recommended dietary allowances
	Unit	RDA	Means of nutrient compositions (mg 100 g^−1)	Daily intake (%)
Phosphorus (P)	mg	700	301.84	43.12
Potassium (K)	mg	4700	607.66	12.93
Calcium (Ca)	mg	1000	190.88	19.09
Magnesium (Mg)	mg	400	147.89	36.97
Sodium (Na)	mg	1500	2.66	0.18
Iron (Fe)	mg	8	4.07	50.88
Copper (Cu)	mg	0.9	1.65	183.33
Zinc (Zn)	mg	11	2.75	25.00
Boron (B)	mg	20	2.83	14.15
Manganese (Mn)	mg	2.3	6.69	290.87
Molybdenum (Mo)	mg	0.045	0.033	73.33

Source: http://en.wikipedia.org/wiki/Dietary_Reference_Intake#Vitaminsandminerals.

Significant differences were observed in the raising iron fertilizer doses with respect to iron composition of the kernels (P < 0.05). The highest iron concentrations of ‘Tombul’ hazelnut were found at Fe 8 fertilizer (4.21 mg 100 g⁻¹), the lowest were found at Fe 0 fertilizer (3.75 mg 100 g⁻¹). Also, the other iron contents in the present study were changed to 4.08 mg 100 g⁻¹ (with Fe 4 dose), 4.15 mg 100 g⁻¹ (with Fe 12 dose), and 4.16 mg 100 g⁻¹ (with Fe 16 dose) (Table 4). The average of iron content in the kernels was 4.07 mg 100 g⁻¹ in iron fertilizations (Table 5). Iron is a constituent of hemoglobin, myoglobin, and a number of enzymes and, therefore, is an essential nutrient for humans. Iron contents in this study were lower than those reported by Cosmulescu et al. (2013) for Romanian varieties (7.47 mg 100 g⁻¹) and were similar to Köksal et al. (2006) and Özkutlu et al. (2011) for ‘Tombul’ variety (4.2 mg 100 g⁻¹ and 4.91 mg 100 g⁻¹, respectively). The recommended intake of iron for all adults is 8 mg per each day. The results showed that ‘Tombul’ hazelnut can be important for completing the deficiency of Fe in daily intake about 50.88% levels.

Boron appears to affect calcium and magnesium and may be for membrane function in plants. Boron deficiency signs may be related to the level of vitamin D and possibly other nutrients in the diet (Özkutlu et al. 2011). The B intake from hazelnut may promote bone and joint health, particularly in women (Hunt 1996). Raisins, leafy vegetables, fruits, and nuts such as almond, pistachio, and walnut are recommended as good source of B by various researchers (Souci et al. 2000; Şimşek et al. 2003). The kernels showed significantly different boron contents caused by different Fe doses. The highest boron content of the kernels was found 3.01 mg 100 g⁻¹ with Fe 8 application and the lowest was found 2.66 mg 100 g⁻¹ with Fe 0 application (Table 4). The average boron content was 2.83 mg 100 g⁻¹ (Table 5), which was much higher than 1.73 mg 100 g⁻¹, found by Özkutlu et al. (2011). The recommended amount of boron for adults is 20 mg daily, and consuming the recommended daily amount (RDA) of 100 g hazelnut supplies 14.15% of boron daily intake (Table 5).

The copper (Cu), manganese (Mn), and molybdenum (Mo) content of the kernels were significantly declined with iron fertilizer applications (P < 0.05 and P < 0.01) (Table 4). The highest Cu, Mn, and Mo compositions of the kernels were found 1.87 mg 100 g⁻¹ Cu (Fe 0 dose), 6.69 mg 100 g⁻¹ Mn (Fe 0 dose), and 0.039 mg 100 g⁻¹ Mo (Fe 4 dose), respectively and the lowest were found 1.53 mg 100 g⁻¹ Cu and 1.49 mg 100 g⁻¹ Cu (Fe 12 and 16 doses), 5.04 mg 100 g⁻¹ Mn (Fe 16 dose), and 0.022 mg 100 g⁻¹ Mo (Fe 16 dose), respectively. While up to control applications increased the copper and manganese amounts of kernels, using excess iron fertilization declined these element concentrations in kernels. Increasing iron fertilizations increased slightly and then decreased again in the composition of molybdenum of the kernels. The average of copper, manganese, and molybdenum contents in the kernels were found 1.65, 6.69, and 0.033 mg 100 g⁻¹, respectively in iron fertilizations (Table 5). The average Cu compositions in ‘Tombul’ hazelnuts were found to be lowest to those reported by Özdemir et al. (2001), Ozdemir and Akinci (2004), Köksal et al. (2006), Güneş et al. (2010), and Cosmulescu et al. (2013). The manganese content in the experiment kernels was lower than that two found 7.7 mg 100 g⁻¹ by Köksal et al. (2006), 8.53 mg 100 g⁻¹ by Güneş et al. (2010) and was similar to 6.65 mg 100 g⁻¹ by Özkutlu et al. (2011). The molybdenum composition in this study's hazelnuts was higher than the one found 0.011 mg 100 g⁻¹ Mo by Özkutlu et al. (2011).

Copper is a trace minerals, essential for important biochemical functions and necessary for maintaining health throughout life (Ma & Betts 2000). Copper were involved in lipid oxidation and linoleic acid biosynthesis pathway and, especially, for electrolyte balance (Fennema 1985; Parcerisa et al. 1995). Manganese is needed for bone development, healthy nerves, metabolism, a healthy immune system, and regulating the blood sugar. Signs of deficiency include poor reproductive performance, growth retardation, abnormal formation of bone and cartilage, and impaired glucose tolerance. Molybdenum plays a biochemical role as a constituent of several mammalian enzymes. The results of this study showed that ‘Tombul’ hazelnut can be important for completing the deficiency of Cu, Mn, and Mo in daily intake above 100% and about 73.33%. The recommended amount of copper, manganese, and molybdenum for adults are 0.9, 2.3, and 0.045 mg daily, respectively, and consuming the RDA of 100 g hazelnut supplies 100% of copper and manganese and 73.33% of molybdenum (Table 5). ‘Tombul’ hazelnut has got high copper, manganese, and molybdenum potential and may supply all of human daily needs.

While the iron fertilization was not a significant change in potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), and zinc (Zn) content of the kernels, the rising doses of iron fertilizers led to an effect in these nutrient contents of nut. Potassium contents in the kernel were changed from 595.20 g 100 g⁻¹ in Fe 16 to 623.32 g 100 g⁻¹ in Fe 8 fertilization (Table 4). The average of potassium contents in nut was found 607.66 mg 100 g⁻¹ in Fe fertilizations (Table 5). Among the studied elements, the most abundant mineral in nuts was potassium. Potassium contents in this study were higher than those reported by Açkurt et al. (1999), Ozdemir and Akinci (2004), and Güneş et al. (2010) for Tombul variety (526.72, 551.60, and 560.20 mg 100 g⁻¹, respectively) and were similar to Cosmulescu et al. (2013) for Romanian varieties (617.34 mg 100 g⁻¹) and were lowest by Köksal et al. (2006) for ‘Tombul’ variety (814 mg 100 g⁻¹). Potassium was important for a healthy nervous system and a normal heart rate (Demigne et al. 2004). The recommended intake of potassium for all adult is 4.7 g per each day (Table 5). The consumption of 100 g ‘Tombul’ hazelnut supplies about 12.93% of potassium intake.

The average of calcium content in the kernels was found 190.88 mg 100 g⁻¹ (Table 5). Ozdemir and Akinci (2004) and Köksal et al. (2006) reported that calcium contents of ‘Tombul’ hazelnut variety changed 234.0 mg 100 g⁻¹ and 217 mg 100 g⁻¹; Açkurt et al. (1999) and Güneş et al. (2010) changed 78.83 mg 100 g⁻¹ and 124.23 mg 100 g⁻¹; and Cosmulescu et al. (2013) found average 104.81 mg 100 g⁻¹ for Romanian varieties. Calcium is important for a healthy bone structure. The recommended amount of calcium for adults is 1000 mg daily, and consuming the RDA of 100 g hazelnut supplies 19.09% of calcium.

Magnesium contents of the kernels were changed from 146.83 to 150.70 mg 100 g⁻¹ (Table 4). The average of magnesium content in the kernels was found 147.89 mg 100 g⁻¹ in Fe fertilizations (Table 5). Magnesium contents were higher than that two found 137.51 mg 100 g⁻¹ by Açkurt et al. (1999), 54.12 mg 100 g⁻¹ by Güneş et al. (2010) and were lower than that two found 190 mg 100 g⁻¹ by Özdemir et al. (2001) and 278.82 mg 100 g⁻¹ by Cosmulescu et al. (2013). Magnesium plays an essential role in reducing the risk of cardio-vascular disease. The recommended intake of magnesium for all adults is 400 mg per each day. The consumption of 100 g hazelnut supplies about 36.97% of magnesium intake.

Sodium contents of the kernels were changed from 2.51 mg 100 g⁻¹ with Fe 4 application to 2.82 mg 100 g⁻¹ with Fe 16 application (Table 4). The average of sodium content in the kernels was found 2.66 mg 100 g⁻¹ (Table 5). The health benefits of sodium play a vital role in enzyme operation and muscle contraction. It is important for osmoregulation and fluid maintenance of the human body. Other health benefits of sodium include heart performance, nervous system, and glucose absorption (Murphy & Eisner 2009). Sodium contents in this study's hazelnuts were lower than those reported by Ozdemir and Akinci (2004) and Güneş et al. (2010) for ‘Tombul’ variety (50.85 mg 100 g⁻¹ and 13.58 mg 100 g⁻¹, respectively) and were similar to Özdemir et al. (2001) and Köksal et al. (2006) for ‘Tombul’ variety and were highest by Açkurt et al. (1999) and Cosmulescu et al. (2013) for Romanian varieties (0.62 mg 100 g⁻¹ and 0.64 mg 100 g⁻¹, respectively). The recommended amount of sodium for adults is 1500 mg daily, and consuming the RDA of 100 g hazelnut supplies 0.18% of sodium (Table 5).

The highest amount of zinc in the kernels was found to be 2.92 mg 100 g⁻¹ with Fe 12 dose and the lowest amount was found to be 2.56 mg 100 g⁻¹ with control (Table 4). The experiment nuts did not show important changes based on zinc compositions with iron fertilizer applications. The zinc compositions of the kernels were increased slightly in iron fertilizer applications. The average of zinc content in the kernels was found 2.75 mg 100 g⁻¹ (Table 5). Zn contents in the kernels were found to be similar to those reported by Özdemir et al. (2001), Köksal et al. (2006) and Cosmulescu et al. (2013) and highest to those reported by Açkurt et al. (1999) and Güneş et al. (2010). Zinc is a trace mineral, essential for important biochemical functions and necessary for maintaining health throughout life (Ma & Betts 2000). Zinc is involved in numerous aspects of cellular metabolism. Also, zinc a constituent of enzymes involved in most major metabolic pathways is an essential for plants, animals, and humans (Hambidge et al. 1986). The recommended amount of zinc for adults is 11 mg daily, and consuming the RDA of 100 g hazelnut supplies 25.00% of zinc (Table 5).

This present study showed the influence of iron fertilizers on some nut traits and mineral composition of hazelnut kernels and their potential role for human nutrition. The results do confirm that hazelnuts are a rich source of a number of important biochemical compounds; macro- and microelements and their compositions are significantly affected by iron fertilizations. The raising doses of iron considerably changed the protein and total oil contents, as well as some nut traits such as kernel ratio, empty and wrinkle nut, kernel nut weight, and kernel shape. It also improved the phosphorus, iron, copper, boron, manganese, and molybdenum contents of the ‘Tombul’ kernels. In order to obtain rich content, nuts were essential to the using of sufficient amounts of iron fertilizer in hazelnut cultivation. The results showed that ‘Tombul’ nuts variety had a very high nutritional potential, and their some mineral content of 100 g nut was greater than that of human daily consumption for this required level. According to daily macro and micro element requirements, the amounts of 100 g hazelnut supplied about 43.12% for P, 12.93% for K, 19.09% for Ca, 36.97% for Mg, 0.18% for Na, 50.88% for Fe, 100% for Cu, 25.00% for Zn, 14.15% for B, 100% for Mn, and 77.33% for Mo of the RDA. Cu and Mn contents of 100 g hazelnuts were higher than the respective daily requirements, but slight overdoses of these elements were nontoxic for human health. As a result, particularly an adequate amount of iron fertilizers, especially 8–12 kg ha⁻¹ Fe application doses should be applied for obtaining rich biochemical and nutritional compositions of nuts and for obtaining well nut traits.

Acknowledgement

We thank the journal's reviewers for their constructive comments on our manuscript.

Funding

This research is the part of a research project supported by Ministry of Food, Agriculture and Livestock, General Directorate of Agricultural Researches and Policies, Hazelnut Research Station.

Additional information

Funding

Funding: This research is the part of a research project supported by Ministry of Food, Agriculture and Livestock, General Directorate of Agricultural Researches and Policies, Hazelnut Research Station.