Introduction

The cultivation of tomatoes is a vegetable cultivated in large tracts, of which they obtain important volumes of production and that also participates of important way in the international economy, aspect that could not be obtained without quality of the fruit; achieved in great form through proper nutrition and in particular by the application of potassium sources to the crop. In the year 2013 a value of $8 803 million was generated in the international trade of tomato; of which Mexico participated with 46.7% being the maximum exporter in that year. On the other hand, the United States imported 1 537 403 t, being the main importer (^{FAOSTAT, 2013a}).

In per capita consumption, the United States of America reaches up to 45.8 kg, while Mexico reaches 14.2 kg; however, if one takes into account profitability, Mexico has an average yield of 38 t ha^-1; ie 92% less than that obtained Netherlands, where they reach 484 t ha^-1 according FAOSTAT (^2013b). The high demand of the product worldwide represents an opportunity for tomato producers, however it is imperative to improve cultivation techniques to obtain better yields.

The quality of tomato production is a determinant of price and acceptance in the market, so that fresh tomato is valued for flavor, aroma and texture. It is important to note that variations between quality in tomato fruits are due to many factors such as: the production system regarding hydroponics and soil, ^{Arana et al. (2006)}; ^{Casierra and Aguilar (2008)}; genotype, ^{Hernandez et al.(2013)}; ^{López et al. (2015)}, the dose in the potassium nutrition, ^{Bugarin et al.(2002}a); ^{Ramírez et al. (2011)}; the organically or mineral fertilizer, ^{Cano et al.(2004)}, ^{Rodríguez et al. (2009)}; ^{Márquez et al. (2013)}: foliar applications of organic compounds, ^{Arteaga et al. (2006)}; ^{Terry and Ruiz (2010)}; climate regarding planting season, ^{Gaspar et al. (2012)}; irrigation, ^{Fortes et al. (2013)}, postharvest handling, ^{Gómez and Camelo (2002)} and even fruit coatings ^{Amaya et al. (2009)}. Likewise, the content of lycopene in tomato fruit is important because it is a carotenoid that, because of its antioxidant capacity, protects cells from oxidation by the free radicals present in the body, which helps prevent diseases such as cancer, cardiovascular problems and accelerated aging (^{Ordoñez et al., 2009}). The objective of the present investigation was to quantify the application of foliar fertilizers in two Saladette tomato genotypes to increase yield and quality parameters such as lycopene, soluble solids (TSS), titratable acidity, fruit and potassium pH in fruit, under hydroponics greenhouse.

Materials and methods

The present work was carried out in a postgraduate greenhouse of the College of Postgraduates Campus Montecillo, Texcoco, State of Mexico and is located at a latitude north of 19° 21’, longitude 98° 54’ west and altitude 2 240 meters during the spring-summer period 2015. In a greenhouse overhead type with polyethylene cover an area of 180 m² (9 * 20 m).

A thermohygrometer was used to determine the average temperature, which was 23 °C with variations between 8 and 44 °C. The same instrument allowed to determine the relative humidity, registering values between 17 and 97%. These maximum and minimum values, in the case of the temperature frequently occurred between 2:00 and 4:00 pm and 1:00 and 7:00 am, respectively; however, for the relative humidity were presented inversely; the maximum in the early hours of the day and the minimum in the afternoon. The culture was established in polyethylene bags of black color 35 centimeters in diameter by 35 centimeters in length, which were filled with tezontle as a substrate with a particle size smaller than 8 mm in diameter.

The irrigation system was automated drip between plant and plant a self compensating dropper whose spending is 3 L h^-1 attached to a distributor placed four outputs; of which two were placed in each plant so that the expenditure per minute of irrigation was 25 ml per plant.

The indeterminate tomato hybrids Saladette type Azhura and Cid were used, the first with tolerant to black spot tomato and fusarium caused by Pseudomonas syringae and Fusarium oxysporum f. sp. Lycopersici race 1 and 2. The second high resistance verticilocis, fusarium, root galls and tomato mosaic caused by Verticillium albo-atrum V. Dahlie, Fusarium oxysporum f. sp. Lycopersici race 1 and 2, Meloidogyne arenaria, M. javanica y M. incognita and tomato mosaic virus.

Three foliar fertilizers were selected: foligral^®, nutri K-80^® and nutri humus^®, which are commercial products on the market, recommended for horticultural crops such as tomatoes and increase quality and performance of the same. The nutrient solution for fertigation used is based on Steiner (1981), which is used commercially and experimentally due to the effectiveness of the same according to the applied EC. The formulation was Ca (NO₃)₂ 4H₂O (9 meq L^-1), KNO₃ (3 meq L^-1) K₂SO₄ (3 meq L^-1), MgSO₄ 5H₂O (4 meq L^-1) and KH₂PO₄ (4 meq L^-1) which provides a CE of 2 dS m^-1.

The experiment evaluated two factors: genotypes (“Azhura” and “El Cid”) and three foliar fertilizers (foligral, nutriK-80 and nutri humus) and one control of each genotype without foliar application. Eight treatments were generated by a 2*4 bifactorial arrangement. The experimental design was randomized complete blocks with four replicates; the treatments were randomly distributed in each replicate generating 32 experimental units. Each repetition was defined by eight experimental units at a distance of 1.2 m from each other with a length of 4 m. The experimental unit consisted of seven plants spaced 65 cm apart. Three plants were selected from the center of each row (useful plot), from which all measurements of the variables were taken. The transplant was performed on April 30 in the afternoon to avoid dehydration of the seedling and irrigation of 500 ml per bag.

The plants were taken to a stem, placing the tutoring placement after seven days after transplantation. The fruits were pruned to homogenize the size of the fruits; so that by bunch there would be only six fruits. The pollination was carried out by the slight tapping of the raffia in the mornings when the temperature ranged between 24 and 27 °C.

During the whole cycle, the amount of water applied by the product of the irrigation time was quantified by the expenditure of the dropper, thus calculating the consumption applied by irrigation, per day and total during the experiment. The amount of irrigation applied to the crop ranged from 0.22 to 1.4 L per plant per day according to the phenological stage. At the end of the growing season the amount of water applied reached 171 L planta^-1 (Figura 1).

Figure 1 Water consumption (nutrient solution) per plant per day during the growing season.  

The fruit was harvested when it reached a deep red color and was fully ripe (stage 6 maturation in tomato fruit), it should be noted that the plants that make up the useful plot were harvested about 36 fruits per plant from the first to the sixth cluster. The yield was quantified with all fruits from the first to the sixth cluster, as the quality variables were determined by the fruits three and four of the third and sixth cluster. To estimate the yield, the harvested fruits were weighed with a digital scale (Remo). Also once he harvested until the sixth cluster yield per hectare was calculated considering the density of 13 890 plants ha^-1 obtained with the distribution of plants in the experiment.

Determination of quality variables. Lycopene was determined by colorimetry, a colorimeter (Hunterlab) was used which was calibrated at baseline to determine L, a^* and b^* color measurements reported in the International Color System (CIE). Each harvested fruit for quality determinations were also determined by these parameters and the formula described by ^{Arias et al. (2000)} lycopene was determined.

The soluble solids (°Brix) were determined on each fruit, split in half and extracted one drop of juice for the respective measurement in a digital refractometer (Atago). The pH of the fruit was determined by extracting and centrifuging the juice of tomatoes from each experimental unit in groups of three; i.e., two samples per bunch. Finally without dilution, the reading was taken with the aid of a potentiometer (HANNA).

The titratable acidity was determined using the methodology of ^{Gómez and Camelo (2002)} using the following formula.

Where: A= acidity in (%) citric acid; V= volume of NAOH 0.1 N spent (cm³), and G= amount of sample (g).

Finally, the potassium in fruit was determined using half of each fruit harvested for the previous determinations. It was cut into thin sheets without seed, placed in aluminum plates of 22 cm in diameter and introduced to the drying oven at a temperature of 65 °C for five days. The samples were then ground in a mill (General Electric) until pulverized. A wet digestion was then performed with a biacid solution based on H₂SO₄ and HClO₄, in ratio 2:1 of which 2 ml per sample was added to which 1 ml of H₂O₂ was also added. At the end of the digestion, 10 ml of deionized water was filtered and analyzed to finally analyze the potassium content in a mass spectrometer (ICP) (VALIAN 725 ES).

For the statistical analysis, the data of the variables were subjected to the analysis of variance by the Statistical Analysis System (SAS); according to the experimental design of random blocks (^{Martínez, 1988}).

Results and discussion

In the analysis of variance (ANOVA), in Table 1, there were no significant differences in the genotypes, but in the application of foliar fertilizers and 95% at Tuckey test indicates that the application of foliar fertilizers with potassium causes an increase in the described variables.

Table 1 Analysis of variance in tomato fruit in response to genotype and foliar fertilizer.  

ns= no significativo (p> 0.05); ^*= significativo (p> 0.01 y p≤ 0.05); ^**= altamente significativo (p≤ 0.01). Valores con la misma letra son estadísticamente iguales

The yield is low (Figure 2) compared to what was reported by other authors in Saladette tomato; ^{Flores et al. (2007)} (200 t ha^-1), ^{Bugarín et al. (2002)} (190 t ha^-1) and ^{Márquez et al. (2013)} (137 t ha^-1); however, the main reason for this magnitude in difference is mainly due to the density of sowing, which for this experiment was about 30% of the commercial density and secondly to the number of clusters harvested, which in this case were six. It should be noted that considering this last point, the production per plant was higher with respect to the mentioned authors having 3.6 kg up to the sixth cluster.

Figure 2 Total yield (t ha^-1) in tomato fruit in response to genotype and foliar fertilizer.  

In particular tomato Saladette type there is variation in the content of lycopene in each cluster harvested, this follows a polynomial trend decreasing reported to the cluster ninth (^{Ramírez et al., 2011}) and that is why the results of this experiment, the third cluster was higher than the sixth cluster in this variable. The tuckey test by a Foligral^® and Nutri K-80^® is identified as causing a further increase in the concentration of lycopene, then Nutri Humus^® located above the control (Figure 3); this variation may be due to the concentration of K⁺ in fertilizers and also that in its formulation there are other constituents such as amino acids, phytohormones and humic and fulvic acids that could participate in such differences.

Figure 3 Lycopene content (mg 100 g^-1) in tomato fruit in response to genotype and foliar fertilizer.  

It should be noted that the two foliar fertilizers that caused higher concentration of lycopene differed in 15% concentration of K⁺ but one of them (Nutri K-80^®) contains aminoacids, vitamins and plant hormones that could participate in the increase. Although the concentration of lycopene in the fruit is related to maturation; due to the increase of said carotenoid and decrease of chlorophyll as the fruit passes from green to red (^{Bramley, 2002}); the potassium is involved as a moisturizing agent that causes an osmotic gradient, facilitating the entry of water, resulting in cellular turgor (Alcántar and Trejo, 2013), a mechanism that allows the developing cells to reach the appropriate size, related in this case to the size of the fruit, increasing the yield of tomato and amount of lycopene.

The results are among those cited by ^{Gaspar et al. (2012)}, ^{George et al. (2004)} and ^{Ramírez et al. (2011)}, and indicate a variation of 4.2 to 16.8 mg 100 g^-1, in this respect the latter author indicates that their results were similar to others realized based on the increase of carotenoids with respect to the addition of K in the fertilization. So that is why the lycopene content was increased due to the addition of foliar fertilizers formulated with potassium, achieving a more significant increase in those whose concentration of such cation was greater.

Of the soluble solids, approximately 60% are sugars, mainly glucose and fructose (^{Gómez and Camelo, 2002}), since the plant is highly necessary an adequate supply of these, nutrition and management are very important to achieve this goal (^{Reuscher et al., 2014}). Among other important factors, nutrition is essential for an adequate concentration of °Brix in marketable fruits. The Tuckey means comparison test for soluble solids (°Brix) indicates that Foligral and Nutri K-80 were the fertilizers that caused the higher values. The differences between treatments lie in the concentration of potassium in fertilizers, an element that favors the translocation of photosynthates (among them sugars) from the leaves to the demanding organs, in this case the fruits; so that sufficient nutrition K⁺ generates an adequate rate of translocation promotes higher concentration of the fruit sugars (Alcántar and Trejo, 2013).

In addition, the foliar application of potassium is an advantage since it is applied directly to the leaves and accelerates the process (^{Trinidad and Aguilar, 1999}). Furthermore, ^{Ramírez et al. (2011)} indicate that there is variation between soluble solids content (°Brix) of each cluster harvested in indeterminate tomato Saladette type; being that there is a detriment in the content of degrees from the first to the fifth cluster that later has a rebound in the seventh and ninth clusters, this justifies the fact that in the experiment the third cluster has lower values than the sixth cluster in the content of soluble solid. The results obtained (Figure 4) agree in range with ^{Arana et al. (2006)} (4.21 y 5.3), ^{Arteaga et al. (2006)} (4.45 a 5.27), ^{Bugarín et al. (2002)} (3.9 a 4.2), ^{Casierra and Aguilar (2008)} (3.8 a 5), ^{Gaspar et al. (2012)} (3.9 a 5.2), ^{George et al. (2004)} (5), ^{Gómez and Camelo (2002)} (3.8 a 4.53), ^{Hernández et al. (2013)} (3.9 a 5.2), ^{Márquez et al. (2013)} (4.2 a 4.7) and ^{Ramírez et al. (2011)} (4.8 a 5.5).

Figure 4 Soluble solids (°Brix) in tomato fruit in response to genotype and foliar fertilizer.  

The genotypes used alone have a significant soluble solids content; however, this increases to higher values cited by the authors; with the addition of potassium foliar fertilizers as basic formulation, so that those whose K⁺ concentration was higher able to produce a significant increase of these sugars in the plant.

The potassium content in g kg^-1 increased depending on the application of foliar highest concentration of that nutrient. Being outstanding Foligral^® and Nutri K-80^®, this is evident as potassium applications contributed to meet the demand for this nutrient by the plant. ^{Bugarín et al. (2002a)} indicates that the amount of K⁺ significantly influences the quality of production in a range of 3 to 6 meq L^-1 in the nutrient solution. Also, between 70 and 80% of K⁺ is demanded by the developing fruit this because potassium is necessary for uniform ripening, accumulate organic acids in the fruit to enhance flavor and to encourage the entry of water fruit (Alcántar and Trejo, 2013).

Also, in each cluster harvested differs potassium concentration, it follows a polynomial trend upward to the fifth cluster which decreases in the later clusters (^{Ramírez et al., 2011}), which in the results obtained in this experiment justifies variation between the third and sixth cluster. The results (Figure 5) match ^{Betancourt and Pierre (2013)} (3 to 14 g kg^-1) but differ from ^{Bugarín et al.(2002a)} (42 to 60 g kg^-1), both determinate tomato, they are also below those reported by ^{Ramírez et al. (2011)} (28.3 a 56.1 g kg^-1) which can be explained taking to account the variation between the concentration of K⁺ in tomato has among other causes (climate and management) growth habit of the plant and cycle culture (^{Bugarín et al., 2002b}).

Figure 5 Potassium content (g kg^-1) in tomato fruit in response to genotype and foliar fertilizer.  

The same author points out that the accumulation of potassium by the fruit follows a growing trend corresponding to a polynomial function where the fastest increase is found in the first crops and decreases at the end of the crop cycle. It also points out that the accumulation of K in the fruit is approximately 60% of the total accumulation in the plant, so that the contribution of this element is of great importance due to its effect on the quality of the fruit as well as the turgidity of the cells (Alcántar and Trejo, 2013).

Conclusions

The amount of water applied is greater than that quoted by the literature for tomato cultivation in such period, however; it would also be necessary to quantify the water requirement for the plantation density evaluated in this experiment. The foliar applications of products enriched with potassium favor the accumulation of sugars (°Brix), lycopene and potassium in the fruit, in addition they manage to increase the yield of the crop and the quality of the fruit. Furthermore, it is observed that there is a proportional relationship between the concentration of K⁺ in the foliar fertilizer and increasing said variable quality and performance.