Plant material and growing conditions

Saladette tomato seeds cvs. Sahel (Syngenta®) were sowed in polystyrene germination containers with 25 mL cavities filled with wet peat moss (Promix PGXTM, Quebec, Canada). One seed was placed per cavity. Seeds were covered with black plastic for four weeks until germination; the substrate was kept moist through that period. Four-weeks old seedlings were transplanted to a greenhouse located at Torreón, Coahuila, México (25° 36' 36.54" N, 103° 22' 32.28" W and 1123 masl). The circular greenhouse was covered with a plastic polyethylene layer and equipped with semi-automatic cooling.

Experimental plants were grown in 20-L black polyethylene plastic bags containing a blend of river sand and pearl B12 (80:20, v:v) as substrate. Before mixing, the river sand was washed and sterilized with a 5 % sodium hypochlorite solution. A plant density of 4.2 plants m² was used. Pots were drip-irrigated. The total daily irrigation volume was 0.750 L/plant from transplantation to flowering; from flowering to harvest, total daily irrigation volume increased to 2.0 L/plant. Tomato plants were thinned to one stem per plant, and plant support was provided by polypropylene strings tied to the greenhouse ceiling structure. Pollination was performed daily from start of flowering to harvest, from 12:00 to 14:00 h using an electric toothbrush.

Treatments

Four nutritional solutions were applied, (a) Conventional inorganic ^{Steiner nutrient solution (Steiner, 1984}); (b) Compost tea; (c) Vermicompost tea; and (d) Vermicompost leachate. The organic solutions were prepared according to the methodology reported by ^{Edwards et al. (2010)}. pH was adjusted to 5.5 with food-grade citric acid (C₆H₈O₇H₂O) (^{Preciado-Rangel et al., 2011}). Electrical conductivity was adjusted to 2.0 dS m^-1 by dilution with tap water to avoid phytotoxic consequences. Solution composition is shown in Table 1.

Table 1 Chemical composition of the nutritive solutions applied to tomato under greenhouse conditions. 

† Ions contained in water used for solution preparation.

Fruit yield and commercial quality

The following variables were measured and recorded: fruit yield per plant, polar and equatorial diameter and soluble solids content. Fruits from 10 plants per treatment were harvested at consumer maturity (firm texture, red color, and absence of physical or mechanical damages) from the first to the fifth branch. Yield was calculated as total fruit weight per plant. Polar and equatorial diameter was measured on the harvested fruits. Six fruits per treatment were randomly selected to measure soluble solid content (°Bx) using an Atago® refractometer (Atago Inc., Bellevue, WA, USA).

Nutraceutical quality

Extract preparation. One g of fruit tissue was mixed with 5 mL of methanol in a screw cap plastic tube. The tube was placed in a shaker (ATR Inc., USA) for 6 h (20 g) at 5 °C. The tubes were then centrifuged at 3540 X g for 10 min in a centrifuge Abtek® model J12 (ABTEK, Monterrey, N.L., México) and the supernatant removed for analytical tests.

Total phenolic content. The total phenolic content was determined using a modification of the Folin-Ciocalteau method (^{Esparza-Rivera et al., 2006}). Thirty μL of extract were mixed with 270 μL of distilled water and 1.5 mL of diluted (1:15) Folin-Ciocalteau reagent (Sigma-Aldrich®, St. Louis, Missouri, USA). The mixture was mixed by vortexing for 10 s. After letting the mixture incubate for 5 min, 1.2 mL of sodium carbonate (7.5 % w/v) were added, and the tube vortexed for 10 s. The tube was then placed in a hot water bath at 45 °C for 15 min and allowed to cool at room temperature. Absorbance of the solution was read at 765 nm in a spectrophotometer Hach® 4000 (HACH Co., Germany). Phenolic content was calculated using a gallic acid (Sigma-Aldrich®, St. Louis, Missouri, USA) standard curve, as a reference standard, and the results were reported in mg of equivalent gallic acid per g of fresh weight (mg equiv AG per g FW). Analyses were run by triplicate.

Antioxidant capacity equivalent in Trolox (DPPH+ method). Antioxidant capacity was determined using a modification of the DPPH⁺ method published by ^{Brand-Williams et al. (1995)}. A DPPH⁺ methanolic solution was prepared adjusting the absorbance of the solution at 1.100 ± 0.010 at a wavelength of 515 nm. The antioxidant capacity test was run by mixing 50 μL of sample extract and 0.950 mL of DPPH⁺ solution and reading the absorbance of the mixture after 3 min of reaction at a wavelength of 515 nm. A standard curve was prepared with Trolox (Sigma-Aldrich®, St. Louis, Missouri, USA), and the results were reported in equivalent μM in Trolox per g fresh weight (μM equiv Trolox per g FW). Analyses were run by triplicate.

Lycopene content. Lycopene content was determined based on the method proposed by ^{Olives et al. (2006)} and a modified chromatographic method (^{Berra, 2012}). One gram of fresh tomato was mixed with 5 mL of a chloroform:methanol (5:1) solution in a plastic container with screw cap. The mixture was agitated under dark conditions for 24 h at 20 g. Afterwards, the chloroform phase was separated, centrifuged at 4720 Xg for 10 min, and filtered in a separation column packaged with activated sodium sulphate (Sigma-Aldrich®, St. Louis, Missouri, USA). Chloroform was evaporated in a rotary evaporator Buchi® R-210 (Buchi Labortechnik AG, Flawil, Switzerland). The residue was stored at -20 °C until its reconstitution for analysis.

Each residue was reconstituted with HPLC-grade chloroform (Sigma-Aldrich®, St. Louis, Missouri, USA) and filtered through a cellulose-acetate membrane filter (0.20 μm) before its injection into an HPLC chromatograph (Series 1200, Hewlett Packard^TM, Palo Alto, California, USA) using the ChemStation software for LC (Agilent Technologies, Santa Clara, CA, USA). Lycopene was eluted in a C18 Supelco column (150 mm x 5.0 μm x 0.5 cm) with a 0.5 mL min^-1 flow using a 50:50 mixture of acetonitrile and 70 % methanol as a mobile phase. Both solvents were HPLC grade (Sigma-Aldrich®, St. Louis, Missouri, USA). The effluent was monitored at 472 nm in a diode array detector G1315D (Agilent Technologies, Santa Clara, CA, USA). Results were calculated using a standard curve of lycopene (Sigma-Aldrich®, St. Louis, Missouri, USA) and reported as mg of lycopene per g of fresh tomato fruit.

Statistical Analysis

The experimental design was set-up as a random block arrangement, and the obtained data were analyzed through ANOVA. Mean comparison was conducted using the Tukey test (P < 0.05).

Yield

The type of nutrient solution significantly affected fruit weight (P ≤ 0.05, Table 2). The Steiner solution treatment produced the highest yield (Table 2). This result agrees with ^{Preciado-Rangel et al. (2011)}, who indicated that using inorganic nutrient solution produced higher yield of tomato fruits. In addition, these authors also mentioned that regular application of organic fertilizer produces lower yield compared to traditional fertilizers (^{Márquez-Quiroz et al., 2013}). Generally, growers involved in organic agriculture accept these results as a rule (^{Márquez-Quiroz et al., 2014}). However, lower yield from organic agriculture systems is usually compensated by premium prices (^{Leifeld, 2012}).

Table 2 Yield and fruit size of tomato fertilized with different nutrient solutions under greenhouse conditions. 

Values followed by different letter in the columns are significantly different, according to a Tukey test (P ≤ 0.05).

Nitrogen is reported as the main nutritional constrain to high crop yield (^{Fonseca and Piña, 2006}); thus, lower yield in crops fertilized with organic nutrient solutions could be attributed to lower N concentration and bioavailability (^{Preciado-Rangel et al., 2014}). The organic nutrient solutions in this study were diluted before their application to avoid phytotoxic effects caused by salinity (^{Oliva-Llaven et al., 2010}) (Table 1). Therefore, prior evaluation of nonsynthetic fertilizer alternatives that can fulfill nutrimental crop requirements without toxicity to plants or environment is required. Toxicity might be reduced by application of organic nutrient solutions mixtures (^{Márquez-Hernández et al., 2013}) or higher dilutions of such organic liquid extracts (^{González-Solano et al., 2013}).

Commercial fruit quality

Polar and equatorial diameters of fresh tomato fruits were evaluated as indicators of commercial product quality. Both diameters showed differences among treatments (P < 0.05, Table 2). Bigger tomato fruits were obtained from treatments fertilized with Steiner solution. Organic fruits are regularly smaller than those fertilized with inorganic solutions (^{Rodrigues et al., 2010}). However, organic tomato fruits might concentrate more lycopene (^{Zhang et al., 2016}) and have more soluble solids (^{Preciado-Rangel et al., 2011}).

Soluble solid content is an important quality parameter of tomato fruits that mostly suggests sugar content and flavor (^{Rodrigues et al., 2010}). Total soluble solids (TSS) content measured in tomato fruits in this study approximate reported values for fresh or processed tomato (4.5 °Bx) (^{Márquez-Hernández et al., 2008}). Yet, the type of nutrient solution applied affected TSS values in tomato (P < 0.05, Table 2). Fruits obtained from plants fertilized with vermicompost leachate had the highest TSS (6.0 °Bx).

These results agree with those reported by ^{Preciado-Rangel et al. (2011)} who obtained higher TSS in organically-fertilized tomato compared to fruits produced with inorganic fertilization. This effect could be attributed to higher saline concentration of the organic solutions and also to Na⁺ and Cl^-ion concentration in such nutrient solutions (^{Wu and Kubota, 2008}). Higher salinity compromises water and nutrient uptake in tomato plants (^{Hassan et al., 2015}) and causes oxidative stress that affects normal fruit development.

As a response to higher salinity, fruits accumulate organic solutes like simple sugars (glucose, fructose and sacarose) that reduce the cellular osmotic potential and facilitate water absoption (^{Goykovic and Saavedra, 2007}). Alternative organic nutrient solutions with lower salinity and adequate nutrient content, that fulfill the tomato crop nutritional requirements, need to be evaluated.

Nutraceutical fruit quality

Polyphenols and antioxidant compounds present in fruits are nutraceutical indicators mainly determined by genotype (^{George et al., 2004}) and plant nutritional status (^{Zhang et al., 2016}). Nutrient solution applied affected phenolic content and antioxidant capacity of tomato fruits (P ≤ 0.05, Table 3). Other researchers (^{Bunea et al., 2012}; ^{Omar et al., 2012}) have reported that application of organic manure on crops increases fruit phenolic content and antioxidant capacity.

In this research, fruit fertilized with vermicompost leachate had higher phenolic content and antioxidant capacity (higher nutraceutical quality) than inorganically fertilized tomatoes (P ≤ 0.05, Table 3). These results might be the result of high content of humic acids (^{Gutiérrez-Miceli et al., 2007}) and low N content in the organic solutions applied (Table 1).

Table 3 Soluble solids, phenolic content and antioxidant capacity of fresh tomato fruits obtained with different nutritive solutions under greenhouse conditions. 

†Values followed by different letter in the columns are significantly different, according to a Tukey test (P ≤ 0.05). ††Method ABTS.

Plants respond differently to the N supply: when nitrogen requirements for the crop are satisfied, some Ncontaining compounds such as amino acids, proteins and alkaloids will be produced (^{Hallmann and Rembiałkowska, 2012}). Contrary to this behavior, plants grown under N-deficient conditions will produce simple and complex sugars, organic acids, vitamins, secondary metabolites, pigments and antioxidant compounds like terpenoids and phenolics (^{Nguyen and Niemeyer, 2008}).

Additionally, N deficit causes oxidative stress in tomato plants, which results in higher anti-oxidative activity of superoxide dismutase (SOD) (^{García-Hernández et al., 2001}) and higher antioxidant capacity and phenolic content in fruits (^{Oliveira et al., 2013}). Fruits obtained from plants fertilized with Steiner solution, compost tea or vermicompost leachate had the highest lycopene content. Potassium present in the nutrient solutions (Table 1) might influence lycopene content, as it improves carotenoid synthesis, increases lycopene content in fruit (^{Almeselmani et al., 2009}), and is a key enzymatic cofactor in lycopene synthesis from ß-carotene (^{Bramley, 2002}).

Application of organic nutrient solutions represents a feasible option that produces fruits free of chemical agents with better nutraceutical quality. These traits provide marketing advantages and promote environment preservation from reduced usage of chemical fertilizers. Organic nutrient solutions with higher K and N content are viable products for increasing yield and nutraceutical quality of tomato fruits produced under greenhouse conditions.