WATER AVAILABILITY AND POTASSIUM DOSES IN CHERRY TOMATO QUALITY

The quality of cherry tomato fruits is directly related to the management strategies used in the production system, such as irrigation and fertilization. This study aimed to assess the quality of cherry tomato fruits cultivated under water availability and potassium fertilization. The experiment was carried out in a greenhouse in pots of 12 dm of an Oxisol. The experimental design was a randomized block design in a 5 fractional factorial arrangement with five water availabilities in the soil (4, 14, 24, 34, and 44 -kPa), five potassium doses (0, 125, 250, 375, and 500 mg dm) and with four blocks. Irrigation was performed with a semi-automated drip irrigation system with soil moisture monitoring by the Diviner 2000 capacitance probe. The assessed variables in fruits were longitudinal and transversal diameter and pulp thickness, fruit shape index, total soluble solids (SS), titratable acidity (TA), the SS/TA ratio, and vitamin C. The data were submitted to statistical analyses at 5% probability error, with analysis of variance by the F-test and polynomial regression. Fruit size presents a reduction as water availability decreased. The total soluble solids have a higher concentration at a potassium dose of 326 mg dm. The quality of cherry tomato fruits cultivated in an Oxisol is influenced by water availability and potassium doses.


INTRODUCTION
Cherry tomato (Lycopersicon esculentum Mill.) has a characteristic flavor, which is outstanding in the specialized cooking, and a high added value due to the high quality of fruits required by consumers.Thus, the cherry tomato is as a good alternative of profitability for producers since its plants have a good adaptation to different productive systems, with less use of fertilizers and pesticides, especially in protected cultivation (Rocha et al., 2010;Lúcio et al., 2016;Sari et al., 2017).
Among the main indicators used to measure the quality of tomato fruit, the most relevant are those related to appearance, size, and flavor.At the time of purchase, the appearance and pattern of structural characteristics of fruits are the main criteria adopted by consumers.However, according to Yuri et al. (2016), the total soluble solids (SS), titratable acidity (TA), and the SS/AT ratio are the main variables analyzed to assess fruit flavor quality.
The irrigation of tomatoes is managed mainly for a high productivity, being fruit quality, in some cases, neglected in the management strategies.According to Chen et al. (2013), fruit quality is directly affected when tomato plants suffer some water deficit, mainly in fruit formation.Thus, the water restriction applied at the maturation stage of fruits may lead to gains in quality parameters.
The characteristics of cherry tomato fruits are also influenced by soil fertility, especially the availability of potassium.This nutrient plays important roles in plants, such as osmotic regulation, enzyme activation, photoassimilate transport, carbon dioxide assimilation, and transpiration (Pervez et al, 2004;Coskun et al. 2016;Ziaul-Hassan et al., 2016), being directly related to fruit quality.
Considering that, the aim of this study was to assess the quality of cherry tomato fruits cultivated under water availability and potassium doses in a pot containing Oxisol under a protected environment.

MATERIAL AND METHODS
The experiment was carried out at the Institute Institute of Agricultural and Technology of the Federal University of Mato Grosso, Rondonópolis, MT, Brazil, Engenharia Agrícola, Jaboticabal, v.38, n.5, p.657-664, sep./oct. 2018 located at the geographical coordinates 16°27′51″ S and 54°34′50″ W, with an altitude of 284 m.
The soil was collected at a depth of 0.0-0.2m in a Cerrado area and was classified as an Oxisol (Brazilian Soil Classification, Latossolo Vermelho distrófico).Subsequently, this soil was sieved in a 2 mm sieve for chemical and particle size characterization (EMBRAPA, 1997) and in a 4 mm sieve for liming and pot accommodation.
Fertilizations with macro and micronutrients were adapted from Alvarenga (2013) (Table 1), considering that the volume of soil to be exploited by the root system would be limited.In this sense, all recommended fertilizations were split (Figure 1) to avoid possible risks of salinization, in addition to balancing nutrients with the use of different sources.A previous calibration was performed to determine the relationship between soil water tension and volumetric moisture, as well as the relationship between the volumetric moisture obtained with the capacitance probe and the standard volumetric moisture.
The relationship between the standard volumetric moisture and soil water tension (Equation 1) (R 2 = 0.97) was used to determine the water availability considering the soil water tension specified by the treatment (Table 2).θ is the soil volumetric moisture obtained by the standard method (cm 3 cm −3 ) and T is the soil water tension (−kPa).TABLE 2. Soil volumetric moisture as a function of each treatment of water availability according to the soil water tension (θ = 0.5994***T −0.418 ).

Soil tension
(−kPa) 4 14 24 34 44 θ (cm 3 cm −3 ) 0.3357 0.1989 0.1589 0.1373 0.1232 In the experiment, a daily irrigation management was carried out by monitoring soil moisture with the capacitance probe.Thus, the volumetric moisture for determining the irrigation water depth per experimental unit was obtained by the relationship between the standard volumetric moisture and the volumetric moisture of the capacitance probe (Equation 2) (R 2 = 97.3).
Where, θ is the soil volumetric moisture obtained by the standard method (cm 3 cm −3 ), and θDiviner2000 is the volumetric moisture of the Diviner 2000 ® capacitance probe (cm 3 cm −3 ).Based on the soil volumetric moisture (standard) and the soil volume in the pot, the required water depth was calculated (Equation 3) to reach the desired volumetric moisture (Table 2) according to the treatment of water availability.
Where, V is the water volume (cm 3 ); θtreat is the desired volumetric moisture according to the treatments (cm 3 cm −3 ) (Table 2), and θcurrent is the current volumetric moisture (cm 3 cm −3 ).
Irrigation was performed with a semi-automated drip irrigation system.This system consisted of a 1000 L tank with a hydraulic float, a motor pump of 0.5 HP, a 125 micron (120 mesh) disc filter, a ball valve, a manometer at the exit of the control stand, PVC pipes and fittings (polyvinyl chloride), solenoid valves, relief valves on the control stand and at the end of the lines, pressure regulator, microtube, and self-compensating dripper of 4 L h −1 .At seven days after transplanting, the treatments were differentiated according to the water availabilities.Plants were conducted using a narrow ribbon and single stem to about two meters in height in relation to the neck of the plant, from the elimination of side sprouts (Marim et al., 2005;Viol et al., 2017).
Fruits were harvested periodically according to ripening, with fruits presenting a red external coloration.The structural characteristics assessed in the fruits were the average longitudinal and transversal diameter of all fruits produced in the cycle and the pulp thickness sampled in about 20% of the produced fruits.These measurements were obtained with an analog caliper and the data were expressed in (± 0.05) mm.The shape index was calculated by the ratio of longitudinal/transversal diameters.
Engenharia Agrícola, Jaboticabal, v.38, n.5, p.657-664, sep./oct. 2018 The content of soluble solids (SS) was determined with a portable refractometer (LI, Model 2159), being expressed in °Brix (± 0.5).The titratable acidity (TA) was assessed by titration with sodium hydroxide at a concentration of 0.1 M and expressed in g citric acid 100 mL −1 of pulp.In order to establish the balance between sugar content and acidity of fruits, the SS/TA ratio was determined.Vitamin C was determined by titration with potassium iodate at a concentration of 0.002 M and expressed in mg 100 g −1 of pulp (Pregnolatto & Pregnolatto, 1985).For the parameters determined by titration, a graduated burette of 25 (± 0.5) mL was used.Twelve fruits were sampled per experimental unit for determining the soluble solids and six fruits per experimental unit for titratable acidity and vitamin C, throughout the crop cycle.
The data were submitted to the analysis of variance by the F-test at 5% probability level.Subsequently, a response surface was performed when the interaction was significant, or a fitting to a linear and quadratic regression model when an isolated significance was observed by the factor.The analyses were performed with the software Statistical Analysis System version 8.2.

RESULTS AND DISCUSSION
The average fruit mass and pulp thickness presented an isolated significance for the water availability, with a fitting to a linear regression model.The highest average fruit mass (9.84 g fruit −1 ) was observed in the water availability of −4 kPa, with a 41% increase when compared to the water availability of −44 kPa (Figure 4A).The same occurs for the fruit pulp thickness, with the highest value of 3.9 mm, which represents an increase of 18% (Figure 4B).Fruit fresh mass presented a similar behavior to those observed by Candido et al. (2015), who observed an increase in fruit fresh mass as the water depth increased.Bogale et al. (2016), assessed two tomato cultivars under water deficit conditions and observed a reduction of mass per fruit as water availability decreased.
For cherry tomatoes, the consumer does not always opt for larger fruits because of their specific uses but tend to observe more uniformity of the lot regarding color and size.However, the producer looks for larger fruits to obtain a higher yield.
Similar results were found by Soares et al. (2013), who observed in the tomato cultivar Super Marmande a reduction in pulp thickness with irrigation in deficit in relation to the crop evapotranspiration.
Fruit pulp thickness is relevant because it contributes with a higher mass per fruit.In this context, Resende et al. (2004) also pointed out that tomatoes with a larger pulp thickness lead to fruits with a higher firmness, which reflects in a longer shelf life, reducing possible waste due to the post-harvest perishability.
The transversal and longitudinal diameter of fruits presented an isolated significant difference for the water  The reduction in the transversal diameter of tomato with a reduction in the available water was also observed by Soares et al. (2011), Soares et al. (2013), andCandido et al. (2015), who worked with tomato under different water depths.Soares et al. (2011) also observed a reduction of the longitudinal diameter as the water availability reduced.
In the case of cherry tomato, the sale is carried out in differentiated packages, preventing the consumer to choose each fruit, as observed in other tomato groups.Therefore, criteria of uniformity for the lot contained in the package are possibly more adopted (Alvarenga, 2013).
Tomato fruit size depends on the rate and duration of cell growth since water availability directly influences the physiological processes of plants.A reduction in fruit size is justified by an increase in soil water availability, which also negatively affects the average fruit mass (Chevalier et al., 2011).
Possibly, the reduction in fruit size as soil water availability decreased was due to modifications in the source-drain relation since an increase in soil water tension (soil water potential more negative) restricts the absorption of water and nutrients.In addition, it can reduce the flow of elaborated sap to meet the demand for carbohydrates and mineral nutrients in the fruits and, consequently, reduce fruit size (Rodrigues et al., 2014;Coskun et al., 2016).
Fruit shape index did not present a significant difference for water availabilities and potassium doses, with a general average among fruits of 0.95.This is possibly a characteristic more influenced by varieties and genotypes (Kuşçu et al., 2014;Nnungu & Uguru, 2016) than water availability or potassium fertilization.
The total soluble solids presented an isolated significant difference for potassium doses, with a fitting to the quadratic regression model.In this case, the highest concentration of soluble solids (5.7 °Brix) was observed at a potassium dose of 326 mg dm −3 , reaching a 25% increase in relation to the absence of potassium fertilization (Figure 6A).The increased contents of soluble solids in fruits with the increased potassium fertilization occurs by the transport of sucrose in the phloem, which is favored by potassium.This, in turn, potentiates the transport from leaves to fruits, meeting the source-drain relation (Coskun et al., 2016;Martínez-Andújar et al., 2016).Rebouças Neto et al. (2016) worked with the hybrid Dominador F1 of the persimmon group and also observed an increase in the concentration of total soluble solids as potassium doses increased.According to Yuri et al. (2016), the content of total soluble solids is one of the main indicators to be analyzed to assess the commercial quality of tomato.
The total titratable acidity did not present a significant difference for water availabilities and potassium doses, with an average of 0.38 g citric acid 100 mL −1 of pulp.However, the SS/TA ratio presented an isolated significant difference for water availability, providing a better fit to the linear regression model.The highest ratio (16.5) was observed in the water availability of −4 kPa, with a 25% increase when compared to that of −44 kPa (Figure 6B).Rebouças Neto et al. (2016) observed ratios between 7.5 and 12 for the hybrid Dominador F1 of the persimmon group.
According to Monteiro et al. (2008), a high SS/TA ratio indicates that tomato fruits have a combination of sugar and acid that correlate with a mild flavor, with sugars standing out in taste.According to Anthon & Barrett (2003), the flavor in tomato fruits is the result of the interaction between soluble solids, acidity, and volatile components.
However, consumer preference may be related to the purposes given to cherry tomatoes, as some culinary dishes require fruits with more acidic flavors, which please the taste of consumers when considering the tomato along with other ingredients.
The content of vitamin C in cherry tomato fruits did not present a significant difference for water availabilities and potassium doses.The average value of vitamin C was 27.5 mg 100 g −1 of pulp.According to Agbna et al. (2017), vitamin C is a vital antioxidant in the human diet, being one of the main factors that characterize the nutritional quality of fruits.In our research, the cherry tomato cultivar BRS Iracema showed a higher concentration of vitamin C when compared to other cultivars and varieties assessed in other researches.Soares et al. (2013), for instance, quantified for the cultivar Super Marmande a concentration of vitamin C between 9 to 14 mg 100 g −1 of pulp.Zhang et al. (2016) observed for the tomato cultivar Tunhe vitamin C values of 10 to 15 mg 100 g −1 of pulp.In turn, Bogale et al. (2016) found to the cultivar Matina about 2 mg 100 g −1 of pulp and for the cultivar Cochoro values of 3 to 8 mg 100 g −1 of pulp.
According to Ruggieri et al. (2016), the concentrations of vitamin C in tomato fruits range from 10 to 88 mg 100 g −1 of pulp, including non-domesticated species.On the other hand, the range is generally lower in commercial cultivars, which present values of 10 to 40 mg 100 g −1 of pulp, demonstrating that the variability of vitamin C in relation to the genotype is normal.

CONCLUSIONS
The structural quality, soluble solids, and SS/TA ratio of cherry tomato cultivated in pots with an Oxisol under a protected environment is influenced by water availabilities and potassium doses.
Fruit size was reduced as soil water availability decreased.Thus, water availability should be maintained close to the soil water potential of −4 kPa in order to obtain larger fruits.On the other hand, lower water availabilities with soil water potential of −34 and −44kPa if the consumer market opts for smaller fruits.
The total soluble solids of cherry tomato had a higher concentration at a potassium dose of 326 mg dm −3 .However, the total titratable acidity and the concentration and vitamin C were not influenced by water availability and potassium fertilization.
with a diameter of 20 cm and a height of 40 cm.At a height of 25 cm, holes of 5 cm in diameter were made to install the access tube, which allowed monitoring the soil moisture with the Diviner 2000 ® capacitance probe.Subsequently, the experimental units were filled with 12 dm 3 of soil per pot.The access tubes were installed horizontally with four pots per tube for using the Diviner 2000 ® probe, as inPereira et al. (2016).Considering readings performed every 0.1 m, pots with a diameter of 0.2 m, and a space between pots of 0.1 m, each profile provided four usual readings at the center of each pot (Figure2).

FIGURE 3 .
FIGURE 3. Components of the irrigation system.
availabilities.A fitting to the linear regression model was observed for both diameters.The largest transversal (24.1 mm) and longitudinal (25.3 mm) were observed in the water availability of −4 kPa, with increases of 18 and 16% in relation to water availability of −44 kPa, respectively (Figure 5).

FIGURE 6 .
FIGURE 6.Total soluble solids (SS) as a function of potassium doses (A) and SS/TA ratio (STR) as a function of water

TABLE 1 .
Recommendation and sources for fertilization with macronutrients and micronutrients.