Nutritional quality of tomatoes as a function of nitrogen sources and doses

Vegetable consumers are increasingly demanding high quality products. Among factors that can influence the nutritional quality of tomato fruits, mineral nutrition stands out. The objective of this study was to evaluate the effect of nitrogen sources and doses on the nutritional quality of tomato fruits. The experiment was carried out in pots in an experimental area at the Universidade Federal de Viçosa Paranaíba Campus, in Rio Paranaíba (Minas Gerais State, Brazil). The commercial hybrid Dominador was cultivated with four plants per pot. The treatments consisted of nitrogen fertilizer doses with 50 and 200 mg dm 3 of N, combined with four sources (urea, ammonium sulfate, ammonium nitrate and calcium nitrate), in a randomized block design with four repetitions. A (4 x 2) + 1 (four sources combined with two doses of N, plus one treatment without the application of N) factorial scheme was used. We evaluated the increase in oBrix and titratable acidity values with the increase of the N dose applied. Urea and the ammonium nitrate resulted in higher pH values in the tomato fruits. The potassium, lycopene and total carotenoid contents in the tomatoes did not present significant differences in relation to the sources and doses used. The sources and doses of nitrogen fertilizers affected the nutritional quality of the tomato fruits, influencing parameters such as oBrix, pH, titratable acidity and sodium content.


INTRODUCTION
In vegetable production, demand for high quality products has steadily increased (Iglesias et al., 2015).Both organoleptic and functional properties are required, with the latter considered a source for preventing specific diseases (Lahoz et al., 2016).In this way, tomato consumption is considered a way to improve health, because of the ingestion of diverse compounds (Dorais et al., 2008;Adalid et al., 2010), such as antioxidants, which help to eliminate free radicals, thus reducing cellular damage (Ding et al., 2016).
Other properties of tomato fruits, such as soluble solids concentration, acidity content, sugars and organic acids *Corresponding author.E-mail: mariasenafernandes@gmail.com.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License are used to evaluate the nutritional state of the fruits (Scibisz et al., 2011;Ding et al., 2016).Thus, their physico-chemical constituents may influence nutritional and sensorial properties, conferring different attributes to the tomatoes and promoting greater or lesser acceptance of the fruits by both consumers and industry (Rosa et al., 2011).
The soluble solids content confers sweetness to the tomato fruit (Baldwin et al., 2008).The pH determines the organic acids content in the fruits (Ayvaz et al., 2016), which also contributes to the peculiar acidic flavor of the tomato and is a product security parameter (Anthon et al., 2011).Both parameters influence fruit acceptability by consumers (Baldwin et al., 2008).Titratable acidity also is an important characteristic for determining the nutritional quality of tomatoes (Anthon and Barrett, 2012).
The presence of minerals in the tomato fruits is highly relevant for human consumption, since mineral consumption aids in the intake of antioxidant compounds and fibers and contributes to adequate intake of certain minerals (Hernández-Suárez et al., 2007;Erba et al., 2013), such as potassium (K) and sodium (Na).
Tomato fruits are rich in carotenoids, which are required for human consumption.Lycopene is the principal carotenoid present in tomatoes, characterized by having beneficial health properties (Eh and Teoh, 2012).In addition to presenting high nutritional value (Adalid et al., 2010), lycopene can help prevent some types of cancer, such as prostate and lung cancer, in addition to cardiovascular diseases (Dillingham and Rao, 2009;Ford and Erdman, 2012).
Various factors may influence the nutritional quality of tomato fruits.Among them are mineral nutrition, with nitrogen (N) being one of the most required nutrients by the tomato plant, contributing to growth, plant development and crop reproduction (Ferreira et al., 2010;Mehmood et al., 2012;Kumar et al., 2013;Kuscu et al., 2014), in addition to influencing characteristics that confer quality to the fruits (Amans et al., 2011).
Adjusting nitrogen fertilization is very complex, because of both the doses applied and the sources used.The availability of N in the soil depends on various factors, among them the processes of nitrification, leaching, volatilization and denitrification, which is responsible for the loss of this nutrient.In addition to this, the various forms of N, nitrate (NO 3 -), ammonium (NH 4 + ) and amide (NH 2 ), differ in terms of costs, leaching potential, soil acidification, volatilization and plant absorption (Marouelli et al., 2014), making it difficult to choose the best source for a given crop condition.As such, the objective was to evaluate the effects of nitrogen sources and doses on the nutritional quality of the tomato fruit.

MATERIALS AND METHODS
The experiment was conducted in an experimental area at the Assunção et al. 997 Universidade Federal de Viçosa -Rio Paranaíba Campus, in Rio Paranaíba (Minas Gerais State, Brazil) (19°12'53"S and 46°13'56"W, altitude 1140 m) in the period from September to December 2015, corresponding to the spring planting.The soil used is classified as Red-Yellow Latosol of very clayey texture, with the following chemical attributes: pH (water) = 5.5; P (Mehlich-1) = 18.4 mg dm -3 ; S = 11.6 mg dm -3 ; Ca 2+ , Mg 2+ , K + , H+Al and CTC potential = 30; 9; 1.6; 53 and 93.6 mmolc dm -3 ; organic material = 2.9 dag kg -1 ; B, Cu, Fe, Mn and Zn = 0.3; 1.1; 37; 9.3 and 2.9 mg dm -3 .The commercial hybrid Dominador was cultivated in pots of 150 dm³ (87 cm diameter and 43 cm height), with seedlings transplanted in the central area, with a total of four seedlings per pot arranged in a zigzag.The pots were used to impede N leaching, since they were not bored through and remained covered with canvas.Each plant was tutored with bamboo and led to the 4th raceme, without thinning the fruits.The first bunch was removed, with the intention of redirecting photoassimilates for other plant organs (Guimarães et al., 2009).Other cultural practices such as weeding, thinning, mooring, irrigation, pest and disease management and weeding were done according to the recommendations for this crop (Silva and Vale, 2007).
Fertilizer was distributed manually in the experimental pots.The nitrogen fertilizer doses were 50 and 200 mg dm -³ of N, equivalent to 100 and 400 kg ha -1 of N when considering the layer from 0 to 20 cm.The doses of N were combined with four sources (urea, ammonium sulphate, ammonium nitrate and calcium nitrate).The doses were calculated based on the total contents of N in the sources and distributed in four coverage according to the emission of the bunchs.
One mg dm -³ of boron and copper and 3 mg dm -³ of zinc were applied to the planting throughout the volume of the soil in the pot.In a central groove 8 cm in depth, 300 mg dm -³ of P was deposited.The total dose of K2O was 240 mg dm -³, where 90 mg dm -³ was applied to the transplanting of the seedlings and the remainder was distributed in four coverings along with the N.
The treatments were distributed in a (4 x 2) + 1 factorial scheme (two doses of N combined with four sources, plus one treatment without the application of N) in a randomized block design with four repetitions.For the analysis, two fruits per plot were collected, totaling eight fruits per treatment at 82 days after transplanting.
For the analyses, the fruits were crushed and passed through a 230 mm sieve to determine soluble solids contents, with the values expressed in °Brix, measured in portable digital refractometer (PAL-1) and the pH of the pulp was measured with the help of a countertop pHmeter (MS Tecnopon Instrumentos mPA-210P) (AOAC, 1997), totaling three repetitions for both variables.
Titratable acidity (TA) was determined in accordance with the method described in AOAC (1997).A sample of 20 g of pulp was taken and diluted in 50 mL of distilled water.This mixture was titrated with standardized solution of NaOH at 0.05 M, with phenolphthalein as an indicator (pH 8.1).TA was expressed as a % of citric acid, by the following formula: Where, V = volume of the NaOH solution used to reach pH 8.1 (mL); N = normality of the NaOH solution; E = gram-equivalent of the predominant acid (64.02 for citric acid); 10 = constant; M = mass of sample used (g).
The K and Na contents of the fruits were determined.The tomatoes were washed in deionized water and dried in an incubator with forced air ventilation at 70ºC for 72 h.Afterward, the samples were crushed in Wiley type mill equipped with a sieve of 1.27 mm and the nutrients analyzed after mineralization by nitric-perchloric digestion.Thus, K and Na were measured by flame emission photometry according to the methodology of Malavolta et al. (1997).
The lycopene and total carotenoids content were determined based on the methodology proposed by Rodriguez-Amaya ( 2001), obtained by spectrophotometric analyses.After the fruits were crushed in a blender, 5 g samples of the pulp were taken and 40 mL of acetone was added (P.A.).The mixture was agitated for 1 h using the MMS Multi Shaker at 200 rpm.Afterward, the solution was vacuum filtered with the help of a Kitasato flask wrapped in aluminum foil, in order to avoid photo-oxidation of the pigment.Each sample was washed three times with acetone, aiming for total extraction of the pigments.Forty five millilitre of petroleum ether was added to the funnel of separation.After filtering, the lower phase was discarded and the samples were washed to remove all of the acetone.The solution of the pigments was transferred to a 100 mL volumetric flask, with the volume completed with petroleum ether.The spectrophotometer reading was done at a wavelength of 470 nm.
The lycopene content was obtained by the following formula (Carvalho et al., 2005): Where, A = absorbance of the solution at the wavelength of 470 nm; V = final volume of the solution; 1,000,000 = constant; = the extinction coefficient or the absorptivity coefficient (3450) and M = sample mass taken for analyses; 100 = constant.The total carotenoid concentration (Ct) was calculated from the following formula (Rosa et al., 2011): Where, Abs = absorbance of the solution at the wavelength of 470 nm; Dil.= dilution of the extract; Vol.= volume of the volumetric flask used (mL); 10,000 = constant; 2,592 = extinction coefficient; ma = sample mass (g).The data obtained were subjected to analysis of variance (ANOVA), with the source means compared the Tukey test and the doses compared by the F test, both at 5%.Additional comparisons of the control and factorial mean were done by means of contrasts tested by the t test.The program R was used for the statistical analyses.

RESULTS AND DISCUSSION
The 200 mg dm -³ dose provided higher mean ºBrix values (6.14) for all sources of N evaluated, conferring higher sugar content to the fruits (Table 1).In a similar way, Kuscu et al. (2014) observed the increase of the soluble solids content with the dose of N applied to evaluate the response of three levels of irrigation and four doses of N (0, 60, 120 and 180 kg ha -1 ) in the yield and quality of tomato fruits in two years of crops.This result may be explained by the higher photosynthetic rate with increasing doses of N, which causes increased production of photosynthates, which may be stored as reducing sugars (Wang et al., 2007).Contrary results were reported however, in a manner that the soluble solids content increased with a reduced supply of N (Bénard et al., 2009).On the other hand, when applying increasing doses of N (0, 80, 160, 240, 320, 400 kg ha -1 ), no change was observed in the ºBrix value, which was maintained at an average of 4.6 (Marouelli et al., 2014).
For the 50 mg dm -³ dose, the highest fruit pH values were found with the urea and ammonium nitrate applications.For the 200 mg dm -³ dose, only the ammonium nitrate presented the highest value (Table 1).This result may be associated to the large accumulation of mineral solutes in the tomato fruit pulp, because of the presence of NH 4 + , resulting in the consumption of organic acids in the assimilation of N (Porto, 2013).Regarding the dose, only with the use of the urea did the 50 mg dm -³ dose provide higher pH.
The 200 mg dm -³ dose provided higher values of titratable acidity relative to the 50 and 0 mg dm -³ doses.With regard to the sources, for the 50 mg dm -³ dose, the calcium nitrate together with the urea presented the highest values, while the same occurred for the 200 mg dm -³ dose when the ammonium nitrate was used (Table 1).It is worth mentioning that the values presented in this work were lower than those in the Brazilian literature.This however demonstrates that the crop conditions used, as well as the hybrid chosen, led to fruits with low titratable acidity.
In a similar way, Kuscu et al. (2014) observed a significant increase of titratable acidity with the applied N dose.This increase in the N dose provided an increase in both the titratable acidity as well as that of the soluble solids content (Wang et al., 2007).Different results however were evidenced when evaluating the impact of the reduction of the N doses on the yield and quality of the tomato fruits, reducing titratable acidity by 10% (Bénard et al., 2009).
In this experiment there was no significant difference for the K content in the fruits, in relation to the sources and doses of N (Table 1).The same occurs when evaluating the influence of the proportion of NO 3 -:NH 4 + in the contents of macro and micronutrients in the fruits, where no significant differences were observed (Borgognone et al., 2013).It is worth mentioning that various factors may influence the mineral composition of tomato fruits such as the hybrid used, water availability, climatic conditions and cultivation method (Hernández-Suárez et al., 2007).Different results however were found by Hernández-Suárez et al. (2007) who determined the influence of the mineral composition and analyzed the influence of crops, growth medium and fruit sampling period on the mineral contents, with low mineral contents found in the fruits, except for K and Mg.
The Na content was higher for the 50 mg dm -³ dose with the ammonium sulfate application and higher for the 200 mg dm -³ dose with the use of urea and calcium nitrate (Table 1).The presence of NH 4 + tends to reduce the absorption of cations because of competition for absorption sites.This probably did not significantly occur Table 1.Mean values of ºBrix, pH, titratable acidity (%), K (g kg -1 ) Na (mg kg -1 ), lycopene and total carotenoids content (μg g -1 ) in tomatoes as a function of the sources and doses of nitrogen.Rio Paranaíba -Minas Gerais State, Brazil.There was no difference in the lycopene and total carotenoids content in relation to the sources and doses of N (Table 1).Kuscu et al. (2014) observed that the N application caused an increase in the lycopene and total carotenoids content until the 120 kg ha -1 of N dose and values were reduced with the 180 kg ha -1 N application.

Conclusions
The 400 kg ha -1 N dose provided higher ºBrix and titratable acidity values in tomato fruits.The sources containing NH 4 + resulted in higher pH values.The sources and doses of N did not influence the K, licopene and total carotenoids content in the fruits.