The research was conducted in an experimental field of the Department of Agricultural, Food, and Forest Sciences of Palermo (SAAF), at Marsala, Trapani Province (longitude 12°26'E, latitude 37°47'N, altitude 37 m) in the North-western coast of Sicily (Italy). A 27 × 50 m, north-south oriented, multi-span greenhouse covered with polyvinyl chloride was used for the experiment. The high-tech greenhouse was equipped with a fan-and-pad evaporative cooling, high-pressure fogging and over-head air heating systems.

On 18th February 2016, seedlings of four tomato hybrids [“Bybal” F₁ (Syngenta Seed, Basel, Switzerland) belonging to the round tomato group, “TyTy” F₁ (Syngenta Seed, Basel, Switzerland) belonging to the cherry tomato group, “Paride” F₁ (MedHermes, Ragusa, Italy), and “Ornela” F₁ (Vilmorin Italia, Bologna, Italy) belonging to the ellipsoid tomato group], raised in peat, were transferred to a perlite grow-bags [Agripan perlite (Perlite Italiana, Milan, Italy)] at the stage of four to five true leaves. Each bag was 1.0 m in length, 0.25 m in width, and 33 L in volume and accommodated four tomato plants. The plant density was 3.3 plants m⁻².

As a second treatment factor, Mo in the nutrient solution (NS) was adjusted to a null, a low (standard) and two high concentrations corresponding to 0.0, 0.5, 2.0, and 4.0 μM Mo, respectively, from the beginning to the end of the experiment. The different Mo levels were attained by adding appropriate amounts of ammonium molybdate tetrahydrate to the nutrient solution. The concentrations of all other nutrients in the solution initially introduced into the system were identical for all NS treatments and the composition was as follows: 1.2 mM NH4+, 9.5 mM K⁺, 5.4 mM Ca²⁺, 2.4 mM Mg ²⁺, 16.0 mM NO3-, 4.4 mM SO42-, 1.5 mM H₂PO₄, 15.0 μM Fe, 10.0 μM Mn, 5.0 μM Zn, 30 μM B, and 0.75 μM Cu (Sonneveld and Voogt, 2009). In addition, the irrigation water contained 9.5 mM of Na and 9.0 mM of Cl. The electrical conductivity (EC) and pH in the above NS were 3.60 mS cm⁻¹ and 5.6, respectively. The pH in the NS was adjusted to 5.6 to 5.7 daily by adding appropriate amount of HNO₃. Plants were fed by complete NS given daily via drip irrigation system. In-line emitters with discharge rate of 2.0 L h⁻¹ at 0.25 m spacing on lateral were used. The amount of water was estimated according to the solar radiation of the previous day (Boztok et al., 1984; Gül and Sevgican, 1992). A leaching fraction of 40% was adopted. The drainage was collected in a reservoir tank, however, it was not reutilized (open cycle management).

The four Mo treatments were combined with the four tomato genotype treatments in a two-factorial experimental design rendering 16 treatments. Each treatment was replicated four times and contained four plants. Thus, the total number of tomato plants was 256. Fruit setting was facilitated by vibration of the trusses at approximately midday two times a week. Climate conditions inside the greenhouse were adjusted via computer controller. In order to avoid limitations in fruit setting resulting from insufficient pollen production or pollen tube growth, the inside air temperature was set to 16 ± 1°C during the night and 24 ± 1°C during the day. Relative humidity was kept between 60 and 70% during the growing season of tomato. The cumulated greenhouse global radiation was 1615.1 MJ m⁻².

Plant vigor was assessed by plant height at 50 days after transplanting (DAT) and aboveground biomass produced at the end of fruit harvest [including total yield and vegetative part produced (weight of the plant at the end of harvests plus vegetative part removed by pruning)]. First truss emission (expressed as DAT) wasalso collected.

Immediately after harvesting fruits were weighed. Total yield (kg plant⁻¹) and marketable yield (kg plant⁻¹) were estimated. Average fruit weight (g) was also calculated.

Immediately after harvesting, fruit color (L^*, a^*, and b^* parameters -CIELab) was measured on four replications of five fruits per treatment. The records were taken on two opposite point of tomato fruit skin (equatorial zone) by a colorimeter (Chroma-meter CR-400, Minolta Corporation, Ltd., Osaka, Japan). The colorimeter was calibrated with a white standard calibration plate (Y = 93.9, x = 0.3134, y = 0.3208) before use. L^* corresponds to a dark/light scale (0 = black, 100 = white) and represents the relative lightness of colors, being low for dark colors, and high for light colors (McGuire, 1992; Lancaster et al., 1997).

Sampling for the fruit quality analysis was conducted as described by Sabatino et al. (2016, 2018) for eggplant. Thus, 3–5 commercially mature fruits for each replication from the second and third harvest were used; only healthy fruits were chosen. Care was taken to ensure that each sample contained the same percentage weight of apical, middle, and distal parts of the fruits. Qualitative fruit characteristic analyses were conducted on fruits harvested from labeled fruits (the flowers were labeled at the fruit set stage) and all fruits were harvested after 35 days from labeling (fruit commercial maturity stage).

Samples of the fruit pulp were squeezed by hand with a garlic squeezer. The juice was filtered and soluble solids content (SSC) was measured using a digital refractometer (MTD-045 nD, Three-In-One Enterprises Co. Ltd. Taiwan).

Titratable acidity (TA) was determined using 10 g aliquots of tomato fruits poured in 50 mL of distilled water and titrated with 0.1 N NaOH to an end-point of pH 8.1. TA was expressed as percentage of citric acid and was calculated using the method reported by Han et al. (2004). The SSC/TA ratio was also calculated.

Fruit water content was determined in samples dried at 80°C for the first two days and subsequently dried at 105°C until constant weight using a thermo-ventilated oven (Memmert, Serie standard, Venice, Italy). Water content (%) was calculated from the difference in the masses before and after drying.

Ascorbic acid content was measured from tomato samples by reflectometer Merck RQflex^* 10 meter using Reflectoquant Ascorbic Acid Test Strips. One gram of fruit juice was dissolved in distilled water, maked up to 10 mL, and mixed; then dipped appropriate test strip into the sample and inserted it into the meter. Results were expressed as mg of ascorbic acid per 100 g fresh weigh.

Total phenolic content was measured by using 2 g of each sample which was weighed out and extracted with 50 ml of methanol. The extraction was conducted under stirring for 60 min at 60°C. The mixture was filtered through filter paper (Whatman No. 3), filled in a 50 ml volumetric flask and allowed to set in the dark until analysis. Total phenolic content was determined according to the Folin-Ciocalteu method (Slinkard and Singleton, 1997) with slight modifications. The standard or sample extract (100 μL; triplicate) was mixed with 0.4 mL Folin-Ciocalteu reagent. After 3 min reaction 0.8 mL of 10 % Na₂CO₃ was added. The tubes were allowed to stand for 30 min at room temperature, and the absorption was measured at 765 nm using a spectrophotometer (CELL, model CE 1020, Cambridge, UK). Gallic acid was used as calibration standard, and the results were calculated as gallic acid equivalent (GAE; mg 100 g⁻¹ dry weight basis).

Lycopene content was determined as described by Sadler et al. (1990). Briefly, 5 g of homogenized sample was extracted adding 50 ml of a mixture of hexane/acetone/ethanol (2:1:1, v/v/v) for 30 min. The total lycopene content expressed in mg 100 g⁻¹ fresh weight was obtained by measuring the absorbance of the lycopene hexane fraction at 472 nm. Pure lycopene (Sigma, St. Louis, MO) was used for the preparation of calibration curves.

Nitrogen (N) content was obtained from the Kjeldahl method. In particular, a sample rate was subjected to acid-catalyzed mineralization to turn the organic nitrogen into ammoniacal nitrogen. The ammoniacal nitrogen was then distilled in an alkaline pH. The ammonia formed during this distillation was collected in a boric acid solution and determined through titrimetric dosage.

The Mo was determined by ICP-MS instrument (Plasma Quant MS Elite, Jena, Germany), roughing pump, re-circulator, data acquisition and analysis software, equipped with a low liquid uptake nebulizer, a free-running radio frequency (RF) plasma generator, automated X, Y, Z torch positioning, and a four-stage vacuum system. A MARS6 (CEM, USA) high-throughput closed microwave digestion workstation was used for dissolving metal and preparing reference solution.

Iron (Fe) and copper (Cu) were determined using atomic absorption spectroscopy (SavantAA, ERRECI, Milan, Italy) following wet mineralization as reported by Morand and Gullo (1970).

The data were subjected to two-factorial analysis of variance (genotype × Mo concentration) using the SPSS software package version 14.0 (StatSoft, Inc., Chicago, USA). For data expressed in percentage, the arcsin transformation before ANOVA analysis [Ø = arcsin(p/100)^1/2] was applied. When the genotype and/or the Mo supply level were significant, the means were separated using Tukey HSD test (P < 0.05). The same test was used to separate the 16 means from all experimental treatments when the interaction for a particular measured characteristic was significant.

As regard total yield, a significant interaction was found between genotype and Mo concentration (Table 1). The highest total yields were identified in “Bybal” F₁ plants grown with 2.0 μmol Mo L⁻¹ and in “Bybal” F₁ plants grown with 2.0 μmol Mo L⁻¹. The lowest total yield was observed in ‘Tyty' F₁ tomato genotype grown using a NS with 0.0 μmol Mo L⁻¹.

The data collected on marketable yield supported the trend established for total yield (Table 1).

Regardless of the genotype, Mo concentration did not significantly affect average fruit weight (Table 2). Conversely, the genotype significantly influenced the average fruit weight which was highest in fruits from “Bybal” F₁ plants and lowest in “Ornela” F₁ plants. No significant interaction was found between genotype and Mo concentration in terms of average fruit weight.

ANOVA for aboveground biomass showed a significant effect of the interaction genotype × Mo concentration (Table 1). “Bybal” F₁ plants grown using a NS with a Mo concentration of 2.0 μmol L⁻¹ had the highest aboveground biomass value, followed by plants of the same cultivar grown at a Mo concentration of 0.5 μmol L⁻¹. “Tyty” F₁ plants grown using a NS with a Mo concentration of 0.0 μmol L⁻¹ had the lowest aboveground biomass.

Regardless of the Mo concentration, “Bybal” F₁, “Paride” F₁, and “Ornela” F₁ showed the highest values in terms of plant height at 50 DAT, whereas, “Tyty” F₁ showed the lowest one (Table 2). Irrespective of the genotype, tomato plants grown using a NS with a Mo concentration of 2.0 μmol L⁻¹ displayed the highest plant height. Plants grown with 0.0 μmol Mo L⁻¹ showed the lowest value in terms of plant height. ANOVA for plant height at 50 DAT did not show a significant interaction genotype × Mo concentration.

Ignoring of the Mo concentration, “Bybal” F₁ plants gave the shortest time of first truss emission, whereas, “Tyty” F₁ and “Paride” F₁ revealed the longest first truss emission time (Table 2). Irrespective of the genotype, tomato plants grown using a NS with a Mo concentration of 0.0 and 0.5 μmol L⁻¹ displayed a shorter time in terms of first truss emission compared to tomato plants grown with 2.0 and 4.0 μmol Mo L⁻¹. No significant interaction genotype × Mo concentration was found.

Regardless of the Mo concentration (Table 2), “Bybal” F₁, “Tyty” F₁, and “Ornela” F₁ displayed the highest L^* color coordinate, whereas, “Paride” F₁ showed the lowest one. Irrespective of the genotype, tomato plants grown using a NS with a Mo concentration of 0.0, 0.5, and 2.0 μmol L⁻¹ revealed the highest L^* color coordinate. Whereas, tomato plants grown using NS with a Mo concentration of 4.0 μmol L⁻¹ showed the lowest L^* color coordinate. ANOVA for L^* color coordinate did not show a significant interaction genotype × Mo concentration.

ANOVA for a^* color coordinate showed a significant effect of the interaction genotype × Mo concentration (Table 1). “Ornela” F₁ plants grown using a NS with a Mo concentration of 0.0, 0.5, 2.0, and 4.0 μmol L⁻¹ had the highest a^* fruit color coordinate value (Table 1), followed by “Bybal” F₁ grown with 4.0 μmol Mo L⁻¹ which in turn showed values 7.1% higher than plants of the same cultivar grown at a Mo concentration of 2.0 μmol Mo L⁻¹. “Paride” F₁ plants grown using a NS with a Mo concentration of 4.0 μmol L⁻¹ had the lowest a^* fruit color coordinate.

Regardless of the genotype, Mo concentration did not significantly affect b^* fruit color coordinate (Table 2). On the contrary, the genotype significantly influenced the aforementioned parameter. The highest b^* fruit color coordinate value was recorded from “Bybal” F₁, followed by “Ornela” F₁. The lowest b^* fruit color coordinate was recorded from “Paride” F₁. No significant interaction was found between genotype and Mo concentration in terms of b^* fruit color coordinate.

Disregarding of the Mo concentration (Table 2), fruits from “Tyty” F₁ and “Paride” F₁ showed the highest lycopene content, while, fruits from “Ornela” F₁ revealed the lowest values. However, fruits from “Bybal” F₁ did not show significant difference neither from fruits from “Tyty” F₁ and “Paride” F₁ nor from those from “Ornela” F₁ in terms of lycopene content. Without regard of the genotype, fruits from tomato plants grown using a NS with 0.0, 0.5, and 2.0 μmol Mo L⁻¹ gave the highest lycopene content, whereas, those from tomato plants grown with 4.0 μmol Mo L⁻¹ gave the lowest values (Table 2). No significant interaction was found between genotype and Mo concentration in terms of fruit lycopene content.

Ignoring of the Mo concentration, “Paride” F₁, displayed the highest polyphenol content, while, “Bybal” F₁, “Tyty” F₁, and “Ornela” F₁ showed the lowest ones (Table 2). Irrespective of the genotype, at 4.0 μmol Mo L⁻¹ polyphenol content was 4.0% significantly higher than that recorded in fruits from plants grown with a Mo concentration of 2.0 μmol L⁻¹. The lowest polyphenol content was found in fruits from tomato plants grown using a NS with 0.0 and 0.5 μmol Mo L⁻¹. ANOVA for polyphenol content did not show a significant interaction genotype × Mo concentration (Table 2).

With respect to ascorbic acid, ANOVA showed a significant effect of the interaction genotype × Mo concentration (Table 1). The combination “Ornela” F₁ × 2.0 μmol Mo L⁻¹ had the highest ascorbic acid value, followed by “Ornela” F₁, “Paride” F₁, and “Bybal” F₁ grown with 0.0 and 0.5 μmol Mo L⁻¹ which in turn showed a higher ascorbic acid value than “Tyty” F₁ grown with 0.0 and 0.5 μmol Mo L⁻¹ and “Ornela” F₁ grown using a NS with 4.0 μmol Mo L⁻¹. The combination “Tyty” F₁ × 2.0 μmol L⁻¹ produced the lowest ascorbic acid level.

As regarding the fruit water content, ignoring of the Mo concentration the highest value was recorded from “Paride” F₁, followed by “Bybal” F₁ (Table 2). The lowest fruit water content was recorded from “Ornela” F₁. Disregarding of the genotype, tomato plants grown at 0.0 and at 0.5 μmol Mo L⁻¹ displayed the highest fruit water content value which in turn was significantly higher value than thet recorded in plants grown at a Mo concentration of 2.0 μmol L⁻¹. Plants grown with 4.0 μmol Mo L⁻¹ showed the lowest fruit water content. No significant interaction was found between genotype and Mo concentration in terms of fruit water content.

ANOVA for SSC revealed a significant effect of the interaction genotype × Mo concentration (Table 1). “Ornela” F₁ grown at a Mo concentration of 0.5 μmol L⁻¹ had the highest SSC value, which was 4.3, 4.3, and 1.0%, respectively higher than that recorded in “Ornela” F₁ grown with 2.0 μmol Mo L⁻¹, in “Paride” F₁ grown with 2.0 μmol Mo L⁻¹ and in “Tyty” F₁ grown with 0.5 μmol Mo L⁻¹. “Bybal” F₁ grown using a NS with a Mo concentration of 4.0 μmol L⁻¹ showed the lowest SSC value.

ANOVA for TA showed a significant effect of the interaction genotype × Mo concentration (Table 1). The highest TA was found in fruits from “Ornela” F₁ and “Tyty” F₁ which in turn was significantly higher than in fruits from “Paride” F₁ grown using a NS with a Mo concentration of 2.0 and 4.0 μmol L⁻¹. The lowest values of TA were recorded in fruits from “Bybal” F₁ grown with 0.0, 0.5 (standard NS) and 2.0 μmol Mo L⁻¹ and in fruits from “Paride” F₁ grown with 0.0, 0.5 μmol Mo L⁻¹.

In respect to the SSC/TA ratio, a significant interaction was found between genotype and Mo concentration (Table 1). The highest SSC/TA ratio was detected in fruits from “Paride” F₁ grown using a NS with a Mo concentration of 2.0 μmol L⁻¹, whereas, the lowest one was identified in fruits from “Bybal” F₁ grown with 4.0 μmol Mo L⁻¹.

ANOVA for N fruit concentration showed a significant interaction genotype × Mo concentration (Table 1). Fruits from “Paride” F₁ plants grown using a NS with a Mo concentration of 2.0 μmol L⁻¹ showed the highest N content followed by those from “Ornela” F₁ grown at 2.0 μmol Mo L⁻¹ which in turn had a N content 8.4% higher than fruits from “Bybal” F₁ grown with 2.0 μmol Mo L⁻¹. The lowest fruit N content was observed in fruits from “Tyty” F₁ grown using a NS with a Mo concentration of 0.0 μmol L⁻¹.

ANOVA for fruit Mo content showed a significant effect of the interaction genotype × Mo concentration (Table 1). Tomato fruits from “Paride” F₁ grown using a NS with a Mo concentration of 4.0 μmol L⁻¹ had the highest fruit Mo content, followed by those from “Ornela” F₁ grown with 4.0 μmol Mo L⁻¹ which in turn were 83.3, 57.1, 57.1, 83.3, 83.3, and 57.1%, respectively higher than those from “Bybal” F₁ grown using a NS with 0.5 (standard NS), 2.0 and 4.0 μmol Mo L⁻¹, “Tyty” F₁ grown with 4.0 μmol Mo L⁻¹ and “Paride” F₁ grown with 0.5 (standard NS) and 2.0 μmol Mo L⁻¹. Fruits from “Bybal” F₁ and “Tyty” F₁ plants grown using a NS with a Mo concentration of 0.0 μmol L⁻¹ had the lowest fruit Mo content.

Ignoring of the Mo concentration, genotype did not significantly affect fruit Fe content (Table 2). On the contrary, the Mo concentration significantly influenced the abovementioned parameter. The highest fruit Fe content was recorded in fruits from plants grown with 2.0 μmol Mo L⁻¹, followed by those from plants grown using a NS with a Mo concentration of 4.0 μmol Mo L⁻¹ which in turn showed a significantly higher value than those from plants grown with 0.0 μmol Mo L⁻¹. Nevertheless, tomato fruits from plants grown with 0.5 (standard NS) μmol Mo L⁻¹ did not show significant difference neither from fruits from plants grown with 4.0 μmol Mo L⁻¹ nor from those from plants grown with 0.0 μmol Mo L⁻¹. No significant interaction was found between genotype and Mo concentration in terms of fruit Fe content.

Regardless of the Mo concentration (Table 2), the highest values in terms of fruit Cu content were collected in fruits from “Bybal” F₁, “Paride” F₁, and “Ornela” F₁, whereas, “Tyty” F₁ revealed the lowest ones. Irrespective of the genotype, fruits from tomato plants grown using a NS with 0.0 and 0.5 (standard NS) μmol Mo L⁻¹ revealed the highest values in terms of fruit Cu content followed by those from plants grown with 2.0 μmol Mo L⁻¹. The lowest fruit Cu content was observed in tomato fruits from plants grown with 4.0 μmol Mo L⁻¹. No significant interaction was found between genotype and Mo concentration in terms of fruit Cu content.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.