1. Introduction
In the last 20 years, hazelnut cultivation expanded to various areas of the world [
1]. This increase was mainly driven by the confectionery industry, which also opened up to new markets. Nevertheless, the expansion to new areas and the increased pressure of climate change highlighted the need to improve cultivation techniques to increase the efficiency of orchards. Among the various agronomic practices, fertilization certainly plays a key role. A correct supply of nutrients improves plants’ vegetative and productive efficiency. Numerous studies on various plant species highlighted the importance of “micronutrients”, such as iron, boron and zinc, and “macronutrients” in optimizing yield and nut quality. The ordinary fertilization techniques involve the incorporation of fertilizers into the soil. However, roots’ nutrient absorption abilities depend on several environmental factors, such as water availability in the soil [
2], particular soil reaction [
3] and soil aeration conditions [
4]. In addition, in mature orchards, adult trees increase the depth of newly produced roots, with those with better absorption capacity, thus, resulting in less efficient nutrition. In these cases, foliar fertilization becomes an important tool to support granular soil fertilization [
5].
Due to the spread of hazelnut cultivation to various areas of the world, it is necessary to reassess orchard management practices, including the nutritional requirements and nutrient application methods [
6]. In addition, the hazelnut mineral nutritional requirements were defined for the most well-known and used varieties in the traditional productive areas [
7]; however, data are missing for new cultivation contexts, thus requiring deeper insight.
The mineral nutrition of the hazelnut tree has important effects on nut yield and quality [
8]. Like for other tree crops, nitrogen is the macronutrient for which greater accuracy of management is required in the quantities administered, as it is particularly mobile and influences the plant’s vegetative activity [
9]. Nitrogen also plays an important role in kernels’ development, reaching contents higher than 5% of the dry mass during its filling phase, thus suggesting the importance of nutrient availability during this physiological phase [
9].
Among the microelements, boron is also crucial for growth and reproductive development in nut plants, such as pecan [
10], macadamia [
11] and almond tree [
12]. In hazelnut, boron is considered one of the essential nutrients for an excellent fruit set and for improving the nut quality [
13,
14]. Studies conducted in Oregon show that it increases fruit set and improves the quality of hazelnuts [
15,
16,
17], while in Mediterranean cultivation conditions, boron is observed to have effects ranging from null to significant [
13,
18,
19] on the reduction in blank fruits [
20]. Further studies showed that foliar sprayings of boron with macro-nutrients, such as nitrogen, and other micronutrients, such as zinc and iron, improve the fruit set, yield and quality of hazelnuts [
21,
22,
23,
24,
25,
26], with the best results obtained using boron concentrations between 300 and 600 ppm [
17,
20,
23].
On the other hand, in the climatic changes scenario, the application of mineral fertilization at the foliar level is particularly useful in conditions where the absorption of nutrients from the soil is limited, representing an additional way to supply nutrients during the critical growth phases [
6].
The growing interest in foliar nutrition applied to hazelnut orchards revived some experimental activities to optimize intervention protocols, while also taking into account the varietal characteristics of the plants, as shown by recent acquisitions in “Tonda Gentile” [
27], “Tonda Romana” and “Nocchione” [
28], “Barcelona” [
29], and various cultivars of American origin [
21]. This intervention strategy, which is also being tested for applications of TFN (total foliar nutrition) to hazelnut orchards [
28], is supported by scientific evidence that certifies how the leaves can rapidly absorb nutrients, contributing to a significant attenuation of the drifts’ phenomena and environmental pollution. Targeted foliar fertilization can also be carried out in a mixture with other substances, such as pesticides, osmolytes and biostimulants, to promote the resilience of the hazelnut orchard [
30,
31].
Based on these premises, this work aims to evaluate, for the first time, a complex foliar fertilizer, based on urea, organic nitrogen, boric acid, zinc EDTA, iron EDTA and a commercial product, known as Coryl Dry Veg and from now on referred to as CD, before comparing them to a foliar fertilizer based only on boric acid (B), which is usually adopted in soils where boron is scarcely available.
Therefore, a three-year trial was carried out to evaluate yield and some chemical quality parameters of the hazelnut (
Corylus avellana L.) cultivar Mortarella. This cultivar is the highest quality not-rounded kernel cultivar grown in the Campania region, Italy, which is the second most important hazelnut-producing country in the world [
1].
4. Materials and Methods
4.1. Plant Material and Experimental Site
The trial was conducted in a 10-year-old private hazelnut orchard (Corylus avellana L.), cv. ‘Mortarella’, which is grown at open vase and located in Caianello, Caserta, Southern Italy (41°18′00″ N 14°05′00″ E), during the years 2011–2013. Trees were spaced 3 m × 4 m, and tree rows were North–South oriented.
Soil samples were collected, from 0 to 30 cm depths, at the beginning of the experimental trial (late winter 2011) and analyzed for physical and chemical properties (
Table 6). Based on USDA classification [
57], orchard soil texture was clay-loam.
Soil fertility management (417 kg ha−1 20:20:20 NPK at bud swell; 417 kg ha−1 urea 46% in mid-June) and other agricultural practices followed local ordinary practices. The orchard was rainfed.
4.2. Foliar Nutrition Treatments and Foliar Mineral Analysis
Three experimental plots of 300 m2 each were subjected to foliar nutrition treatments as follows: control (distilled water), Coryl-Dry Veg (8% CH4N2O, 1% N organic, 0.5% B soluble, 0.5% Zn EDTA, 0.05% Fe EDTA, Biochemie International, Giffoni Valle Piana, Italy) and Boromin 135 gel (boron ethanolamine 10%, Biolchim, Bologna, Italy). The experimental design included 3 randomized blocks of 5 trees per treatment, with a total of 15 plants treated per treatment, that were sprayed with a distilled water solution of Coryl-Dry (CD) at 2.5%, with a solution 300 ppm of boron ethanolamine (B) or with only water (Control). To avoid any derivatives, measurements were taken from the three central trees.
The spraying was performed three times per year in 2011, 2012 and 2013 with a backpack pump pressure sprayer, starting from the fifth leaf stage (14 April 2011, 17 April 2012 and 16 April 2013) and occurring every 3–4 weeks (12 May and 17 June 2011; 19 May and 13 June 2012; 12 May and 18 June 2013).
4.3. Foliar Analyses
Leaf micronutrient content analyses were performed once per year (June 2011, June 2012 and June 2013) by collecting three samples of five adult leaves randomly selected from the trees included in the blocks of the three treatments.
Macro and micro elements (N-NO
3, N-NH
4, P, K, Ca, Mg, Fe, Zn, B, and Mn) in leaf samples were measured using an inductively coupled plasma mass spectrometer (ICP-OES Spectroblue, Spectro Ametek, Berwyn, PA, USA), as reported by Pannico et al. (2019) [
58]. Briefly, 1000 mg of lyophilized leaves were fully digested in a microwave digestion system (MLS-1200 Microwave Laboratory Systems, Milestone, Shelton, CT, USA) with the addition of a mixture of HNO3 (65%) and HCl (37%) (9:3,
v/
v; 12 mL), and the resulting solutions were transferred to 100-mL volumetric flasks and diluted to the fixed volume (50 mL) with ultrapure water (Milli-Q, Merck Millipore, Darmstadt, Germany). The calibration curve was prepared using a working standard solution, with concentrations ranging from 1.0 to 100 µg L
−1 for all elements. The results were expressed as mg kg
−1 dw and g 100 g
−1 dw for micro and macro elements, respectively.
4.4. Yield, Nut and Kernel Biometrical Traits
At harvest (22 September 2011, 25 September 2012 and 18 September 2013), yield per tree was weighed when commercial moisture was reached (6%), and samples of 1 kg of nuts per tree were collected and stored at −20 °C until analysis, in the three experimental years.
The number of nuts per kilogram per plant was counted to estimate the number of fruits per tree. Nut and kernel dry weight were measured after being dried in a ventilated oven at 60 °C until reaching constant weights.
4.5. Kernel Constituents and Fatty Acid Composition
Ash, total fat, total protein, carbohydrate and total phenols of kernels subjected to different treatments were determined. Total ash was determined by incinerating the dry samples (500 mg of finely chopped kernels) for 3 h at 550 °C in muffle furnace according to the AOAC method (1995) [
59]. Total fat was extracted using a Soxhlet extractor; 5 g of finely crushed kernels were placed in a cellulose thimble and extracted with 30 mL of petroleum ether (boiling point 40–60 °C) for 6 h [
59]. Oil extracted was stored at −20 °C until analysis. Total crude protein was determined via the macro Kjeldahl method; protein content was calculated as Total N × 4.38. Carbohydrate content was obtained using the following formula: carbohydrate content = 100% − (% moisture + % protein + % fat + % ash), according to Olivera et al. (2008) [
60]. Total phenols were determined according to the method of Jakopic et al. (2011) [
61] with some modifications. Hazelnut flour (5 g) was extracted for 45 min with 30 mL of methanol/water (80:20
v/
v) in a water bath using sonification. The hazelnut extracts were centrifuged at 7000 rpm for 10 min, and the supernatant was filtered through a 0.45 μm membrane filter. In total, 5 ml of extract was mixed with 5 mL n-hexane for 3 min in a vortex apparatus, and the mixture was centrifuged at 7000 rpm for 5 min to remove lipid fraction. The procedure was repeated twice with 5 mL of n-hexane. The total phenolic content of the extracts was assessed using the Folin–Ciocalteau phenol reagent method (Singleton and Rossi, 1965 [
62]. Next, 10 ml of bi-distilled water and 500 μL of Folin–Ciocalteau reagent were added to 2 mL of diluted extract (1:5 in water), and 1 mL of sodium carbonate (20%,
w/
v) was added after 3 min. The absorbance at 765 nm was measured after 30 min in the dark. The total phenolic content was expressed as gallic acid equivalents (GAE) in mg 100 g
−1 of hazelnut.
Fatty acid composition was analyzed via gas chromatography after derivatization to fatty acid methyl ester (FAME), according to the IUPAC standard method [
63] slightly modified following Pannico et al. [
64]. A GC Perkin Elmer AutoSystem XL (PerkinElmer, MA, USA.) equipped with a programmed temperature vaporizer, a flame ionization detector (FID), and a capillary column with 100 m × 0.25 mm ID and a film thickness of 0.20 μm using a stationary phase of 50% cianopropyl methyl silicone (Supelco, Bellofonte, PA, USA) was used. The carrier gas, i.e., helium, was introduced at a flow rate of 20 cm/s. The oven temperature program was as follows: 120 °C for 5 min, 5 °C/min ramp-up to 165 °C for 5 min, and then 10 °C/min ramp-up to 240 °C for 20 min. The split ratio was 1/60, and the FID temperature was 260 °C. Fatty acids were identified via comparison with retention times of external standards (SupelcoTM 37 component FAME MIX). Fatty acid concentrations were calculated through a comparison with the pure standard retention time and were based on response factors used to convert peak areas into weight percentages.
4.6. Analyses of Lipid Alteration and Oxidation
The detection of primary and secondary oxidation was performed spectrophotometrically [
65]. This analysis considered the measurements of two variables: K
232 and K
270. K
232 was a measure of the level of conjugated dienes and was indicative of the primary oxidation. K
270 was a measure of the level of conjugated trienes, which was indicative of the secondary oxidation. UV specific extinction determination permitted a good approximation of the oxidation process in unsaturated oils [
65,
66]. The specific extinction coefficients, which were set at 232 nm and 270 nm, were measured according to the following procedure. An oil sample of 100 mg was placed in a 10 mL flask and diluted to 10 mL with spectrophotometric grade hexane (Sigma-Aldrich). The sample was then homogenized, and the absorbance was measured with a UV-Vis 4000 spectrophotometer (Varian, Palo Alto, CA, USA) using pure solvent as blank [
67]. Free fatty acids (FFA) were measured via direct titration of the nuts oil extract with 0.1 N NaOH, using phenolphthalein as an indicator. Free fatty acid contents of oil samples were determined in accordance with method no. 2.201 of IUPAC (1987) [
63]. The peroxide value (PV) was determined using the extracted oil and estimated via iodometric titration assay, which is based on the oxidation of the iodide ion using hydroperoxides (ROOH). A saturated solution of potassium iodide was added to oil samples to react with hydroperoxides. The liberated iodine was then titrated with a standardized solution of sodium thiosulfate and starch as an endpoint indicator. The PV was obtained via calculation and reported as milliequivalents of oxygen per kilogram of samples (meq/kg); the official determination was described in method no. 2.501 of IUPAC (1987) [
63]. All analyzes were performed in triplicate.
4.7. Statistical Analysis
All data were subjected to bifactorial analysis of variance (two-way ANOVA) (year (Y) ×fertilization (F)), using a general linear model generated using the SPSS software package (SPSS version 22, Chicago, IL, USA). Mean effects and interactions were separated according to Tukey’s HSD test (
p = 0.05). For
Table 3,
Table 4 and
Table 5, the mean effects of the two years were compared according to Student’s
t-test.
5. Conclusions
As demonstrated in some previous experiences, in hazelnut, foliar nutrition improves some characteristics of plant production and kernel quality, which, in turn, can positively affect the commercial value of yield. Integrated foliar application of nitrogen and micronutrients, such as boron and zinc, may enhance some qualitative and nutritional characteristics of nuts at harvest, namely kernel dry weight, fat content, and fatty acid composition, with the latter enhanced by increasing the relative content of monounsaturated fatty acids. However, despite the higher concentration of MUFA, foliar nutrient applications, mainly CD, positively influenced kernel fat stability during oxidation, with a relevant effect on hazelnut storability also recorded. Kernel produced under foliar nutrition sprayings were probably resulted more protected from fatty acid oxidation due to a higher concentration of polyphenols. Since the applied foliar fertilization treatments differentially affected the yield and nutritional value of hazelnut cultivar Mortarella, which is grown in the cultivation area of Southern Italy, it appears valuable to consider the possibility of integrating the ordinary practices of orchard nutrition management using complex foliar fertilizers, instead of boron foliar sprayings, to obtain a more economically valuable and healthy production of nuts, both at harvest and during the post-harvest period.