Towards the Optimal Mineral N Fertilization for Improving Peeled Tomato Quality Grown in Southern Italy

Abstract

Nitrogen (N) fertilization has often been used in excess by farmers to improve commercial yield and the profitability of processing tomato crops. However, N fertilizers greatly affect the overall tomato quality, including technological traits, nutritional characteristics, and mineral fruit composition. The aim of this work was to study the effects of increasing mineral N fertilization rates on processing tomato yield and quality when grown in Southern Italy conditions. The study was carried out at Battipaglia (Southern Italy) cropping cultivar “Messapico”, suitable for peeled tomato, and fruit quality was evaluated at the ripening stage. Results showed that N fertilization reduced sunburned fruits and resulted in increasing total yield, average fruit weight, as well as other fruit parameters (size, firmness, color indexes, pH, N, and calcium content). On the other hand, N fertilization negatively affected other fruit traits, such as dry matter and soluble solids content, total sugar index, ascorbic acid, and sodium content, while it had no effect on potassium and magnesium concentration in tomato fruits. Our results show that, in the investigated area, 200 kg N ha⁻¹ is the best compromise to satisfy farmers and processors expectations, improve yield, and at the same time maintain good fruit quality attributes.

1. Introduction

Tomato (Solanum lycopersicum L.) is the most productive vegetable crop worldwide, reaching a yield of almost 187 million tons and an area of 5 million hectares [1]. In this context, processing tomatoes represents almost 21% of total tomato production with over 38.7 million tons [2] mostly used for paste, peeled, and tomato juice. Among European countries, Italy is the first producer of processed tomatoes and ensures a sixth of the global yield (6 million tons in 2021) [2,3]. On average, Italy produces between 70 t ha⁻¹, more than 110 t ha⁻¹ of processed tomato [4], and is the greatest exporting country of processed tomatoes worldwide: numbers have continuously grown since 1961, reaching a peak of 61 million tons of canned products in 2019 (peeled tomatoes mainly, plus paste and juice) [1,5]. In the Italian context, Southern Italy embodies the leading area of production [6,7,8] and the Campania region contributed a cultivated surface of almost 4000 hectares, a total production of 246,000 tons in 2019 [9].

Tomato is mainly composed of water (92.5–95.0%), while the remaining part is represented by dry matter. The latter is made up of sugars (48%: fructose 25%, glucose 22% and sucrose 1%), structural materials (17%), organic acids (13%: 9% citric acid, 4% malic acid), protein, lipids and dicarboxylic amino acids (12%), minerals (8%), and other substances (2%), as reported by Davies and Hobson [10]. Among vegetables, tomato is one of the richest in potassium (K), phosphorus (P), selenium (Se), magnesium (Mg), and calcium (Ca): on average, one serving of fresh tomato (200 g) contains 517 mg of K (15% of Adequate Intake—(AI)—for adults), 48.5 mg of P (8.8% of AI for adults), 24.4 mg of Ca (2.5% of Population Reference Intake—(PRI)—for adults), and 21.5 mg of Mg (4% of AI for adults) [10]. Tomato, and its industrial products, have been recognized as important sources of antioxidant compounds, such as ascorbic acid (AsA, 40.7 mg per serving [10], 37% of PRI for adults), carotenoids (almost totally represented by lycopene: 1.4–40 mg per serving [10,11,12]), and polyphenols (among which naringenin chalcone stands out: 1.8–36.4 mg per serving [13,14]). Lycopene and vitamin C have been extensively studied as two key antioxidant compounds in tomato, confirming their protection against numerous health-related disorders, such as cancers and heart diseases, as deeply reviewed by Collins [15]. Moreover, vitamin C was also found to be important to make iron (Fe) more available for absorption in the human diet [16]. Noteworthy is the effect of different agronomic practices (e.g., fertilization, irrigation, etc.) on the accumulation of these health-related compounds in tomato fruits [17,18].

Since tomato is a high nitrogen (N) demanding vegetable, N fertilizers have often been used in excess to ensure high profitability of crops, with few concerns about environmental issues caused by over-fertilization [19,20]. Furthermore, N supplies greatly affect the overall tomato quality, as reported by an extensive literature [21,22,23,24,25]. Rate, time, application procedures and forms of N fertilizers represent relevant factors impacting on the quality attributes of tomato fruit suitable for processing (i.e., dry matter, soluble solids, titratable acidity, pH, and color) [4,18,26,27,28,29,30]. These parameters may enhance the quality and the shelf-life of the final product (e.g., lower pH penalizes the development of microorganisms causing spoilage of tomato products, or higher sugar content ensures better taste) and/or optimize different industrial processes (e.g., higher fruit dry matter accelerates paste concentration).

N fertilization also enhances N-derived organic compounds in tomato fruits (nitrites and nitrates), which represent a serious threat to human health. Indeed, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) and Scientific Committee on Food (SCF) classified nitrate and nitrite as possible toxic compounds and established an Acceptable Daily Intake (ADI) of 0–3.7 mg kg⁻¹ bodyweight for nitrates (NO₃), and an ADI of 0–0.07 mg kg⁻¹ bodyweight for nitrites (NO₂) [31,32].

Taking into consideration the importance of N as a crucial element for production and quality improvement in a context of variable factors (climate, soil, cultivar, time, and method of N application, form of N fertilizer, etc.), it appears necessary to optimize the rate of N provided to the crop, improving yield and quality-related parameters at the same time.

Therefore, the aims of this work were to study the effect N fertilization rates on: (i) marketable yield, (ii) technological traits, (iii) nutritional quality and quality indexes, and (iv) mineral composition. The final goal was to identify the optimal N rate representing the best compromise between high marketable yield, and high fruit quality, on a conventionally-managed processing tomato in an important area of production of Southern Italy (Campania region) [33]. Peeled tomato cultivar “Messapico” was chosen in this work because of its widespread use in Southern Italy, and as it is resistant to Tomato spotted wilt orthotospovirus (TSWV), which may have devastating effects in Mediterranean conditions [34]. This cv is also characterized by a remarkable fruit weight (80–85 g) and an elongated shape. Those characteristics are extremely suitable for large packages of tinned peeled tomatoes (2650 mL capacity), which are broadly used in large retail distribution (HoReCa) [34].

2. Materials and Methods

2.1. Location of the Trial

The study was carried out in an open-field trial at Sele Valley (40°35′03.8″ N, 14°58′48.6′′ E) (Battipaglia, Southern Italy) in typical Haploxerepts soil (Soil Taxonomy; USDA, 2014) [35] . The tomato cultivation was performed during the spring and summer seasons in 2015.

Soil was analyzed in its physical and chemical properties and the results were as follows: sand 26.8%, silt 40.8%, clay 32.4%, limestone 2.4%, pH 7.8, organic matter 1.6%, total nitrogen 1.3‰, P₂O₅ 126 mg kg⁻¹, and K₂O 324 mg kg⁻¹. The minimum and maximum air temperatures, occurring throughout the crop cycle, were below 20 °C and 30 °C, respectively, until the third decade of July. Between this period and the first decade of August, the maximum temperature frequently exceeded 30 °C. Total rainfall (from transplant to harvest) was 104 mm, most of which (71 mm) occurred within 30 days after transplanting (Figure 1).

2.2. Crop Production

Seedlings of peeled tomato cv “Messapico” [Nunhems, Sant’Agata Bolognese (BO), Italy] were transplanted on 9th May in paired rows with a density of 3.36 plants m⁻², as reported by Ronga et al., 2019 [4]. Based on soil analysis and nutrient requirements, the total K and P supplies were established (150 kg ha⁻¹ and 200 kg ha⁻¹, respectively) and applied at ploughing. Six rates of N fertilizer (0, 50, 100, 150, 200, and 250 kg N ha⁻¹) were applied in a randomized block design with four replicates, each one containing 68 plants (4.0 m × 5.1 m). Three N applications were progressively performed to provide the total N amount during the crop cycle: 40% at transplanting (9 May), 30% at 30 days after transplanting (6 June) and 30% at full blossom (28 June). N was applied as ammonium sulphate at the first supply, and afterwards as ammonium nitrate.

Once the ETo had been determined with the Hargreaves and Samani method [36], and the Kc crop coefficient had been adjusted for tomato crop, the environmental conditions and crop growth stage [37], and the evapotranspiration of the crop (ETc) was calculated as ETc = ETo × Kc. According to Doorenbos and Pruitt [38], 100% ETc was restored in each plot when 40% of the total available water was depleted. Based on this irrigation scheduling, a total of 294 mm of water was applied during the crop cycle through a drip irrigation system. Weed control and plant protection were performed according to the cultivation protocols of the Campania Region (Italy).

A single manual harvest was conducted on 12 August, when ripe fruits were accounting for approximately 90% of the total.

2.3. Fruit Yield and Merceological Assessment

Yield and its components of marketable (ripe), unmarketable (un-ripe), and rotten fruits were assessed for each plot, according to what was described by Ronga et al., 2019 [4]. However, here we reported only marketable yield per plant (FY).

Average fruit weight (FRW) was calculated on 100 mature fruits plot⁻¹. Furthermore, on the same sample, the following merceological evaluations were carried out: sunscald fruits (SSF, expressed in %), size homogeneity (SZH), and fruit firmness (FRF). The last two of these were estimated on a subjective scale from 1 (worse value) to 5 (best value). The average number of marketable fruits per plant (FN) was then derived by dividing FY per FRW [39].

2.4. Fruit Analyses

2.4.1. Technological Characteristics

Well-ripened fruits (2 kg per plot) were washed and dried, and then homogenized in a Waring blender (2 L capacity; Model HGB140, PartsTown, Addison, IL, USA) for 5 min.

For the determination of the fruit dry matter content (DM), 20 g of homogenized tomato sample was dried in a stove at 72 °C until constant weight; the soluble solids content (SSC) was assessed using a digital refractometer (Refracto 30PX, Mettler-Toledo, Novate Milanese, IT), and the results were expressed as °Brix (on 100 g of fresh weight, fw). The pH-Matic 23^® titroprocessor, equipped with a pH electrode (model 5011T) (Crison Instruments, Barcelona, Spain), was used to measure pH and titratable acidity (TTA), expressed as g of citric acid 100 g⁻¹ juice. Color measurements were performed on the equatorial region of the fruits (10 fruits/plot) using a Minolta CR 300 Chroma portable colorimeter (Minolta Co., Osaka, Japan) with C illuminant, expressing them through CIELAB L*, a*, b* values. The colorimeter was calibrated with a white standard calibration plate (Y = 93.9, x = 0.3134, y = 0.3208) before use [39].

CIE L*a*b* values were then used to calculate four color indexes, including CIE L*C*h* color space variables: a*/b* ratio [27]; Hue angle (HUE, Equation (1)) expressed as degrees [40]; Croma (CHR, Equation (2)) [41]; Color Index or Modified Yeatman Index (CI, Equation (3)) [42].

$${\mathrm{HUE}=\tan }^{-1}\left(\frac{{\mathrm{b}}^{*}}{{\mathrm{a}}^{*}}\right)\times \frac{180}{\mathsf{\pi }}$$

(1)

$$CHR=\sqrt{a^{*}{}^2+b^{*}{}^2}$$

(2)

$$CI=\frac{2000\times a^{*}}{L^{*}\times \sqrt{a^{*}{}^2+b^{*}{}^2}}$$

(3)

The electrical conductivity (EC) of tomato juice was measured using a conductivity meter (MultiMeter MM41 by Crison Instruments, Barcelona, Spain) equipped with a platinum cell (mod. 5073 by Crison Instruments, Barcelona, Spain), and expressed as mS cm⁻¹.

2.4.2. Reducing Sugars and Fruit Sweetness

The determination of glucose (GLU) and fructose (FRU) contents (g 100 g of fresh weight⁻¹, fw) was performed according to the analytical methods proposed by Li et al. 2002 [43]. A HPLC (liquid chromatograph, mod. 600E, by Waters, Sesto San Giovanni, Italy) equipped with a Lichrosorb-NH₂ (10 µm) (Merck, Darmstadt, Germany) and a conductivity detector (model 2414 by Waters, Dublin, Ireland) was used. The eluent was an acetonitrile/water mixture (80:20; v/v) [39].

Sugars, and their influence on taste, were gauged by different indexes: SSC to TTA ratio (SSC/TTA); total sweetness index (TSI, Equation (4)), expressed as g 100 g fw⁻¹; TSI to TTA ratio (TSI/TTA); and TTA to DM ratio (TTA/DM; %) [27,44,45,46]. In addition, the FRU to GLU ratio (FRU/GLU) was reported.

$$\mathrm{TSI}=\left(0.76\times \mathrm{GLU}\right)+(1.5\times \mathrm{FRU})$$

(4)

2.4.3. Mineral Composition

Approximately 10.0 ± 1.0 g of freeze-dried sample was weighed and placed in a muffle furnace at 550 ± 10 °C for 24 h until complete conversion to white ash [46]. The incinerated samples were cooled in a desiccator until room temperature, then recovered and boiled in 10 mL of a hydrochloric acid solution in water (1 + 3). Mg, Ca, K, and Na were determined using a Varian Spectra AA-10 Plus atomic absorption spectrometer (Varian Inc., Segrate, Italy), equipped with a D2 lamp background correction system and an air–acetylene flame [47], and expressed as parts per million (ppm).

Finally, total N fruit concentration (N_Kh) was determined by the Kjeldahl method [48], after mineralization with sulphuric acid (96%).

2.4.4. Ascorbic Acid

Ascorbic acid (AsA) was determined from an aqueous extract of tomato fruit (1 g + 3 mL of 6% metaphosphoric acid in distilled water), homogenized for 30 s using an Ultra-Turrax (IKA, Wilmington, NC, USA), then centrifuged at 1975 rpm for 15 min. The extraction and analyses were carried out according to the protocol reported by Parisi et al., 2017 [49]. The AsA contents were expressed as mg% of fw.

2.5. Data Analysis

Analysis of variance (one-way ANOVA) was applied on all recorded data. A Duncan test was used to separate means, when the F test of ANOVA for treatment was significant at p < 0.05. Additionally, a Principal Component Analysis (PCA) model was used to analyze all collected data [50,51], and thus to study the relationships between objects and variables through a biplot graph. For statistical analysis, GENSTAT 17th software package (VSN International, Hemel Hempstead, UK) was used.

3. Results

The effect of N rates on marketable yield, fruit external appearance, and technological quality is reported in

Table 1. Effects of nitrogen fertilization on plant productivity, fruit external appearance, and technological traits of tomato, cv “Messapico”.

Treatm.	FY [kg]		FRW [g]		NF [-]		SSF [%]			SZH (1–5)		FRF (1–5)		DM [%]		SSC [°Brix]		TTA [g 100g−1]		pH		L*		a*/b*		HUE [°]		CHR		CI
N-0	1.41	d	57.67	c	24.95	a		16.67	a	1.33	c	2.33	b	5.49	a	4.91	a	0.23	a	4.47	bd	24.96	a	2.39	d	22.74	a	31.91	a	73.90	c
N-50	2.02	c	73.80	b	27.38	a		14.67	a	2.17	bc	3.33	a	5.40	ab	4.91	a	0.23	a	4.48	bcd	23.70	b	2.45	cd	22.34	a	30.52	bc	78.07	b
N-100	2.32	bc	86.67	a	26.91	a		5.33	b	2.50	abc	3.33	a	5.39	ab	4.86	a	0.23	a	4.53	abcd	23.20	bc	2.52	bc	21.63	b	30.12	c	80.17	ab
N-150	2.48	b	86.60	a	28.66	a		4.00	b	3.67	a	3.83	a	5.37	ab	4.85	a	0.25	a	4.55	abc	22.80	c	2.58	ab	20.96	bc	30.66	bc	81.92	a
N-200	2.60	ab	86.17	a	30.17	a		6.00	b	3.17	ab	3.67	a	5.24	c	4.75	ab	0.25	a	4.55	abc	23.14	bc	2.58	ab	21.17	bc	30.74	bc	80.60	a
N-250	2.89	a	85.73	a	33.72	a		7.00	b	3.17	ab	3.17	a	5.16	c	4.67	b	0.22	a	4.58	a	23.31	bc	2.61	a	20.78	c	31.20	ab	80.25	ab
Mean	2.29		79.44		28.63		8.94			2.67		3.28		5.34		4.83		0.24		4.53		23.52		2.52		21.60		30.86		79.15
Signif.	***		***		NS		**			**		*		***		*		NS		*		***		***		**		**		***

FY = marketable yield per plant, FRW = mean fruit weight, NF = fruit number per plant, SSF = sunscald fruits, SZH = size homogeneity, FRF = fruit firmness, DM = fruit dry matter, SSC = soluble solids content, TTA = titratable acidity, pH, L* = lightness, a*/b* ratio, HUE = hue angle, CHR = chroma, CI = color index. NS, *, **, *** = non-significant or significant at p ≤ 0.05, 0.01, and 0.001, respectively. Different letters within each column indicate significant differences between nitrogen fertilization rates according to Duncan’s multiple range test (p ≤ 0.05).

. Fruit yield increased with the application of N rates from 0 to 250 kg N ha⁻¹, highlighting the significantly lowest value in control plots (1.41 kg of fruits plant⁻¹). Instead, the highest yield was observed for N-250, which did not differ significantly from the N-200 rate. FY significantly improved with application of N-200 and N-250, in comparison with N-50 treatment (Figure 2).

Dry matter and soluble solid contents decreased from N-50 to N-250 treatments (5.40 g% and 4.91°Bx at N-50, and 5.16 g% and 4.67°Bx at N-250 for DM and SSC, respectively); however, significant differences for DM were found for N-200–250 rates, compared to previous ones. Concerning SSC, only N-250 differed from other treatments, except for N-200 supply. TTA was not significantly affected by N fertilization, while increments in pH values were observed with application of N rates from 0–50 kg ha⁻¹ (4.47 and 4.48) to 250 kg ha⁻¹ (4.58).

Fruit color lightness (L*) decreased from N-0 (24.96) to N-150 treatment (22.80), while the a*/b* ratio increased with application of N rates up to 250 kg ha⁻¹. However, for the latter parameter, significant differences were found between the N-0-50 (2.39 and 2.45, respectively) and N-150-200-250 rates (from 2.58 to 2.61). Concerning the other color indexes, HUE (°) significantly decreased from N-0 (22.74°) to N-250 rate (20.78°) showing significant differences between N-0-50, N-100, and N-250. CHR was negatively affected by N fertilization with significant differences between N-0 (31.91) and N-50-100-150-200 (from 30.12 to 30.74). CI significantly increased in all fertilized treatments, with respect to the control (73.90). To sum up, all color indexes for the control treatment significantly differed from N rates comprised between 100 and 200 kg ha⁻¹.

As shown in

Table 2. Effects of nitrogen fertilization (N-0, N-50, N-100, N-150, N-200, and N-250) on sugars, quality indexes, electrical conductivity, ascorbic acid, and mineral contents of tomato fruits, cv “Messapico”.

Treatm.	GLU [g 100 g fw−1]		FRU [g 100 g fw−1]		FRU/GLU		TSI [g 100 g fw−1]		TSI/TTA		SSC/TTA		TTA/DM [%]		EC [mS cm−1]		AsA [ppm]		Mg [ppm]		Ca [ppm]		K [ppm]		Na [ppm]		NKh [ppm]
N-0	1.55	a	1.76	a	1.14	a	3.82	a	16.40	a	21.06	ab	4.25	c	4.45	c	20.86	a	75.45	a	36.71	b	2794.54	a	77.29	a	0.83	bc
N-50	1.49	ab	1.75	a	1.18	a	3.76	a	16.62	a	21.71	a	4.13	c	4.54	bc	22.01	a	74.27	a	36.12	b	2941.34	a	43.76	bc	0.93	b
N-100	1.43	bc	1.68	ab	1.18	a	3.60	b	15.47	ab	20.87	ab	4.33	bc	4.62	ab	20.25	a	72.83	a	34.88	b	2992.22	a	44.73	bc	0.91	b
N-150	1.43	bc	1.66	bc	1.16	a	3.58	b	14.16	b	19.20	b	4.72	ab	4.67	ab	19.48	a	78.07	a	42.86	ab	3013.00	a	60.26	ab	1.07	a
N-200	1.42	bc	1.57	cd	1.11	a	3.44	bc	13.78	b	19,03	b	4.77	a	4.70	a	19.59	a	74.03	a	35.60	b	2935.98	a	39.77	bc	1.07	a
N-250	1.38	c	1.56	d	1.13	a	3.39	c	15.22	ab	20,96	ab	4.33	bc	4.63	ab	14.66	b	81.78	a	50.15	a	2918.83	a	35.03	c	0.74	c
Mean	1.45		1.67		1.15		3.60		15.28		20.47		4.42		4.60		19.48		76.07		39.39		2932.65		50.14		0.93
Signif.	**		***		NS		***		**		*		*		**		**		NS		*		NS		**		***

GLU = glucose, FRU = fructose, FRU/GLU ratio, TSI = total sweetness index. TSI/TTA = total sweetness index to titratable acidity ratio, SSC/TTA = soluble solids content to titratable acidity ratio, TTAA/DM = titratable acidity to dry matter ratio, EC = electrical conductivity of juice AsA = ascorbic acid, Mg = magnesium, Ca = calcium, K = potassium, Na = sodium, N_Kh = nitrogen (Kjeldahl method). NS, *, **, *** = non-significant or significant at p ≤ 0.05, 0.01, and 0.001, respectively. Different letters within each column indicate significant differences between nitrogen fertilization rates according to Duncan’s multiple range test (p ≤ 0.05).

, GLU and FRU contents decreased with increasing N fertilization rates, with significant differences between N-0-50 (1.55 and 1.49 g 100 g fw⁻¹, respectively) and N-250 (1.38 g 100 g fw⁻¹) for GLU, and between N-0-50 (1.76 and 1.75 g 100 g fw⁻¹, respectively) and the rates higher than 150 kg ha⁻¹ of N for FRU (from 1.66 and 1.56 g 100 g fw⁻¹). For TSI, the lowest value was found at N-250 (3.39); furthermore, significant decreases were also observed for N-100-150-200 treatments, with respect to N-50 and control (3.76 and 3.82 g 100 gr fw⁻¹, respectively). For TSI/TTA, SSC/TTA and TTA/DM quality indexes, significant differences were found between N-150-200 and N-50 fruit samples. Considering GLU and FRU contents and quality attributes obtained combining FRU, GLU, SSC, DM, and TTA, the values found for N-50 treatment did not significantly differ from control.

EC measured on tomato juice was enhanced by N fertilization, with significant differences between N-0-50 (4.45 and 4.54 mS cm⁻¹) and N-250 (4.70 mS cm⁻¹) supplies.

Lowest AsA content was detected at N-250 rates (14.66 mg 100 g fw⁻¹), compared to the previous ones (from 20.86 mg 100 g fw⁻¹ of N-0 to 19.59 mg 100 g fw⁻¹ of N-200 supply).

Significant differences in Ca, Na, and N_Kh contents were detected in relation to N fertilization. For the first element, the highest value was found in N-250 (50.15 ppm), which resulted in significant differences from all other treatments, except for N-150 rate (42.86 ppm). Decrement in Na fruit content was observed with raising N levels (from 77.29 ppm of N-0 to 35.03 ppm of N-250 rate); however, an unusually high content was detected at N-150 rate (60.26 ppm), which did not statistically differ from control. An increment in N_Kh content was found, moving from N-0 (0.83 ppm) to N-200 (1.07 ppm), while a sudden decline was observed at N-250 (0.74 ppm). No significant variations according to N fertilization were detected for Mg and K contents (76.07 and 2932.65 ppm on average, respectively).

Relationships between Treatments and Evaluated Parameters

The correlations between N rates and productive and quality traits were reported in Figure 3. The principal component analysis (PCA) explained 81.35% of the total variability (Figure 3). Most of the variation was explained by the PC1 (62.43%), which discriminated N-0 and N-50 levels (on the positive side) from N-100, N-150, N-200, and N-250 (on the negative side). PC2 revealed 18.92% of the total variance and discriminated N-0 and N-250 (on the negative side) from N-50, N-100, N-150, and N-200. Indeed, N-100, N-150, and N-200 were located in the upper-left quarter and more correlated with yield, fruit physical traits (FRW, SZH, and FRF), titratable acidity (TTA), TTA/DM ratio, EC, color indexes (CI), and some minerals (N_Kh and K). N-250 was identified in the lower-left quarter, together with Mg and Ca fruit concentration. N-50 was located in the upper-right quarter and associated with Na content, HUE, canning traits (DM, SSC, FRU, GLU, (FRU/GLU, TSI), and nutritional parameters (AsA). Finally, N-0 was identified in the lower-right quarter and correlated with fruit defects (SSF), TSI/TTA ratio, and two-color parameters (L*, CHR).

4. Discussion

Processing tomato is a high input demanding herbaceous crop, which is frequently over-fertilized to boost yield and therefore crop profitability [52,53]. However, an improper N management can cause environmental issues, mainly regarding the accumulation of soil nitrates, which enhances risk of salinization and N losses through various hydrological and gaseous pathways [54,55,56]. Consequently, good practices should be applied to achieve an environmentally sustainable crop management, especially under high fertility soil conditions (Ronga et al., 2020 [39]).

An extensive literature analyzed the effect of time, form, and way of administration of N fertilizers on yield, and several fruit quality attributes of processing tomato [20,21,29,57]. Dry matter, soluble solids, titratable acidity, pH, and color represent important technological traits as they impact on industrial processes, and therefore the prize, recognized by canning industries to farmers. Furthermore, considering tomato products as sources of alimentary compounds (sugars and acids), antioxidants (AsA, phenols, and carotenoids), and minerals (Ca, Mg, K, P), and the effects of agronomic practices on these traits, a suitable N management for increasing yield and preserving the overall quality of raw material is highly advisable. Another negligible aspect concerns the accumulation of organic nitrogen compounds in tomato fruits, such as nitrates. These are involved in health-related (enhanced risks of stomach and intestine cancers in human adults and methemoglobinemia in children [58]) and technological aspects (detinning in canned tomatoes in the absence of internal epoxy-based coatings [59,60]), and suggest a reduction in the concentration of N-derived compounds (nitrates and nitrites) in raw materials.

The results of our investigation confirmed the positive effects of N fertilization on FY and FRW, rather than NF trait. In a two-year field trial (2002–2003), performed on ‘Galeon’ tomato hybrid in the same area of cultivation and subjected to N fertilization from 0 to 250 kg ha⁻¹, a different response was found highlighting the significant effect of N rate on NF in enhancing FY trait [27]. The different behavior can be related to the genetic background of tomato hybrid, as well as the higher N soil content and the lower total irrigation volume (data not published) than the present investigation. N fertilization resulted in better FRF and lower SSF; furthermore, for the latter quality-related attribute, a strong reduction of sunscald fruits (SSF) was appreciated. Colla et al., 1999, and Parisi et al., 2006 [27,61] highlighted the effect of N fertilization on the enhancement of vegetative biomass, which ensured a good fruits covering and a better protection from sunscald damages (Figure 2) with respect to the control. Results concerning fruit firmness partially agreed with data reported in other works [27,62] and confirmed the positive effect of moderate N fertilization, ensuring reduced fruit loss after harvest (shipping, storage, and retailing) that is greatly appreciated by tomato growers and retailers. However, the effect of N fertilization on FRF is often contradictory [39,63,64] due to unclear effects of N on cell turgor mechanisms and wall characteristics [65].

Taking into account the effects on technological parameters, N-250 rate significantly decreased both fruit dry matter (DM) and soluble solids content (SSC), with respect to the control. The present results agreed with some works [22,66,67] and disagreed with findings by Parisi et al., 2006, and even more by Ronga et al., 2020 [27,39]. The explanation of these differences can be related to different factors such as N soil endowment, irrigation systems and supplies, and genetic background of tomato varieties, etc.

TTA was not significantly affected by N management, while a worsening in pH values was detected under the highest N level (N-250), in full accordance with previous research by Parisi et al., 2006, Kaniszewski et al., 2019, and Kobryń and Hallmann, 2005 [27,62,67]. Juice electrical conductivity (EC) was enhanced by N fertilization without significant differences between N-100 and N-200 rate, similarly to what found by Borin et al., 1990 [68]. The authors studied the response of processing tomato crop to two mineral fertilization levels, compared to the control, and highlighted the positive effect of enhanced EC values, since lower amounts of sodium chloride are needed to improve taste perception of tomato sauce. This is also encouraged by several international guidelines to reduce sugar, fat, and salt in processed foods [69]. Increments in EC values with respect to the control were also reported in tomato crop fertilized with different type of N supplies [23].

All color parameters were significantly affected by N fertilization. In particular, the a*/b* ratio progressively increased up to the highest N rate, indicating a slight change from yellow (b*) to red (a*), as confirmed by HUE, moving towards 0° value [70]. CI, which considers redness, lightness and color intensity, significantly increased in all fertilized treatments, with respect to the control N-0, as also observed by Massantini et al., 2021 [64]. These findings highlighted the positive effect of N fertilization on the red color of tomato fruit, reasonably related to the increment of lycopene concentration [21,71], although it was not directly measured. Therefore, N fertilization may be also considered as agronomic practice to boost tomato antioxidant activity, since lycopene is an important health-related compound of this fruiting-vegetable species [15,72,73].

Concerning nutritional composition and taste-quality indexes of the fruits, N fertilization resulted in lowering fructose (FRU) and glucose (GLU) contents, and thus in decreasing TSI trait and TSI/TTA ratio. Both quality indexes are highly correlated with the sensing of fruit taste [22]. These findings agreed with research performed in different growing environments [22,62,74]. Furthermore, the SSC/TTA ratio was reduced by increasing N fertilization level, according to Parisi et al., 2006 [27] and Ronga et al., 2020 [39], which investigated the effect of N administration under normal and high fertility soil conditions in Southern Italy. Simonne et al., 2008, and Baldwin et al., 2015 [75,76] highlighted that tomatoes with lower SSC/TTA received lower sweetness and flavor ratings in sensorial tests, and overall preference compared to the ones with higher ratio.

No variations of GLU/FRU ratio were noticed in relation to the increasing N inputs. Reductions in the GLU/FRU ratio in tomato fruits by enhancing FRU accumulation are advisable, as fructose is approximately twice as sweet as its isomer [77]. However, based on our results, N fertilization does not seem to be an effective means to alter GLU and FRU proportion, unlike genetical approaches [78]. To sum up, considering both sugar contents and taste-perception traits, a better tomato quality was observed between the N-50 and N-100 rate.

Few reports are available in literature on the effect of N fertilization on mineral contents of tomato fruits. In our experiment, Ca, Na, and N_Kh amounts were affected by raising N rates, which were different from Mg and K concentrations. Schmidt and Zinkernagel, 2021 [79] studied the effect of reduced N supply (by 50% to recommendations) on two cocktail tomato varieties and reported no effect of N rate on fruit mineral composition (P, K, Mg, Fe, Zn, Mn, and Cu). Wang et al., 2008 [29] showed that the over-application of N decreases nutritional quality of tomato due to lower vitamin C, soluble sugar, soluble solids, Mg, and Ca concentrations. Our results, as well as those of the above-mentioned studies, highlighted the extreme uncertainty characterizing the relationship between N fertilizers management and fruit mineral contents. N_Kh increased according to N, supplying up to N-200, in accordance with data already observed by Ronga et al. 2019 [4]. In fact, the authors [4] showed a lower translocation of the N into fruit when plants are overfertilized with N. Pasković et al., 2021 [80] reported that under scarce fertility soil conditions, even a low N supply input (60 kg ha⁻¹) results in a significant increase in N content of tomato fruits. Furthermore, also under high N soil endowment, N fertilization induces the accumulation of N-organic compounds in the fruits [39]. Since these molecules represent a serious threat to human health [58,81], it is advisable to adopt agronomic strategies to reduce N accumulation in tomato fruit, and to limit it below JECFA and SCF ADI thresholds (0–3.7 mg kg⁻¹ body weight (bw) for NO₃ and 0–0.07 mg kg⁻¹ bw for NO₂) [31,32]. It is also noteworthy to consider that canning processes may double (at least theoretically) nitrate concentration in the sauce, causing unwanted detinning phenomena [59,82,83].

Regarding ascorbic acid (AsA), comparable concentrations were maintained increasing N supplies up to N-200, while the highest level of fertilization (N-250) had a detrimental effect. The AsA accumulation in tomato fruit is still uncertain as it is affected by different environmental variables and agronomical practices. Cheng et al., 2021 [21] reviewed that ascorbic acid is significantly influenced by soil pH and average annual temperature [33], and that nitrogen effects on AsA concentration in tomato fruits are more pronounced in greenhouse/pot-grown tomato, rather than field-grown tomato. Consequently, several hypotheses have been proposed. Stefanelli et al., 2010 [84] suggested a dilution effect related to a growth in fruit size. However, the most recurring theory is the shading effect on fruits caused by an increased vegetative growth under high N fertilization levels (Figure 2), which reduces exposure of fruits to sunlight [17,18,85,86,87]. Finally, considering the beneficial effects of AsA on human health [15,88,89], the necessity to maintain a good concentration of this bioactive compound, without any negative impact on fruit yield, represents a desired goal in N fertilization management of different vegetables

Seeking the optimal N rate to enhance yield and ensure low environmental impacts and good quality traits of raw materials, is a key objective in the management of processing tomato crop. In the Southern Italy context, previous works identified 200 kg N ha⁻¹ [4,90] as the best compromise between improved marketable yield and reduced environmental effects. However, in the same growing area, the optimal N supply can be considered between 125 and 200 kg ha⁻¹ depending on N soil endowment, irrigation regime, annual climatic fluctuations, and N-efficiency of adopted genotype [39].

5. Conclusions

Nitrogen management represents a key agricultural practice to enhance yield and profitability in a context of variable factors (climate, soil, cultivar, time and method of N application, and form of N fertilizer, etc.), and in a perspective of crop production sustainability.

Our findings showed that N fertilization positively affected fruit yield and mean fruit weight with respect to the control treatment on treated cv “Messapico” plants. Additionally, fruit size homogeneity and fruit firmness were enhanced by N administration (N > 150 kg ha⁻¹). Sunscald tomatoes were reduced by nitrogen rates higher than 100 kg N ha⁻¹, probably as the increased canopy shaded fruits. However, as highlighted by other authors, a high N fertilizer level (250 kg N ha⁻¹) negatively affected fruit dry matter, pH, glucose, fructose, soluble solids, and ascorbic acid contents. Therefore, in the growing area of our research, we identify 200 kg N ha⁻¹ as the best compromise for improving yield, maintaining good fruit quality attributes.

Further studies should be realized, performing panel tests to understand the effect of several quality-related parameters, which are not currently considered in canning processes (e.g., TSI, TSI/TTA), on the sensorial characteristics of tomato industrial products.