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Introduction
The tomato (Solanum lycopersicum) is one of the most consumed vegetables (Al-Omran et al. 2010). In 2014, 52,374 ha of tomato were planted in México (SIAP 2016), while in the Comarca Lagunera 949 ha were cultivated, of which 802 ha were cultivated under protected agriculture (SAGARPA 2015). In the country, protected agriculture is carried out on 21 530 ha, of which 54.3 % are cultivated under shade netting and 45.7 % in a greenhouse (SAGARPA 2015). Tomato production Systems have been diversified in order to increase yield, incorporating technologies such as plástic covers, drip irrigation and hydroponics, obtaining yields of between 5 and 8 kg plant-1, which is more than three times that obtained under open field conditions, where yields range between 1.5 and 2.0 kg plant-1 (Jaramillo et al. 2006).
Due to the negative effect of fertilizers on the environment and their high prices, there is a strong need to look for alternatives such as organic fertilizers, among which manure, compost and ver micompost stand out (Fortis et al. 2012). The benefits of organic fertilizers as substrates are reported in studies by De la Cruz-Lázaro et al. (2009) , De la Cruz-Lázaro et al. (2010), Márquez et al. (2013) and Moreno et al. (2014), who report having obtained safe tomatoes without the use of synthetic pesticides or fertilizers by using organic substrates such as compost, vermicompost, sand, sawdust and tezontle (Ortega et al. 2010).
Tomato is rich in vitamins A and C, potassium and carotenoids such as lycopene, the latter being responsible for the red color in the fruit, which is used as a quality Índex (Candelas-Cadillo et al. 2005). Lycopene is an antioxidant that, once absorbed by the body, helps prevent and repair damaged cells. An important feature of tomato is lycopene accumulation as a physiological response to stress conditions during its development. These stress factors can be increased salinity, light inten- sity, temperature and/or nutrient limitation, among others (Guevara et al. 2005). Although there is a lot of information related to lycopene produc- tion, very little is known about the synthesis of this compound in response to the use of organic fertilizers. As a result, it is important to conduct research in order to establish the optimal conditions to achieve higher biomass productivity and nutraceutical quality (Bermudez et al. 2002). Therefore, the aim of this study was to determine the proportion of sand, solarized manure, vermicompost and soil suitable for increasing yield and lycopene content in tomato fruits.
Materials and methods
Description of experimental site and crop development
The experiment was set up at the Technological Institute of Torreón in Torreón, Coahuila, México, located between 24° 30' and 27° NL, and 102° 00' and 104° 40' WL, at an elevation of 1120 masl. The experiment was conducted in a metal-framed shade house with a fíat roof, north-south orientation, and Raschel-type 30 % shade mesh. The variety evaluated was Sahel (Syngental®), a saladette tomato of indeterminate growth. The density was 4 plants m2, placing a single plant in a 20-L, 800-gauge black plástic bag. The culture System was a stem, with weekly pruning, phytosanitary control performed preventively and using inputs approved by international standards for organic production (NOP 2002). A drip irrigaron system was used; according to the phenlogical stage, the amount of water applied ranged between 0.5 and 2.0 L bag-1. At the beginning of the anthesis of the flowers, pollination was performed mechanically every day between 12:00 and 14:00 h with an electric vibrator.
Treatments
Mixtures of organic fertilizers were made from livestock manure, compost, vermicompost and river sand. River sand was sterilized with a 5% hypochlo rite solution. The treatments were formed by mixtures of organic fertilizers, which were: TI: 80 % Sand - 20 % Vermicompost (SV), T2: 80 % Sand - 20 % Solarized manure (SSM1), T3: 80 % Sand - 20 % Perlite - Steiner Solution (SPSS), T4: 80 % Sand - 5 % Soil - 15 % Vermicompost (SSoV), T5: 85 % Sand - 15 % Solarized manure (SSM2) and T6: 80 % Sand - 5 % Soil - 15 % Solarized manure (SSoSM). Steiner nutrient solution (Stainer 1984) was prepared with highly soluble commer- cial fertilizers, with pH adjusted to 5.5 with sulfuric acid, and eléctricaI conductivity (EC) of 2.0 dS m-1.
Variables evaluated
Fruit yield, fruit weight, polar and equatorial diameter, total soluble solids content and lycopene content in fruits were evaluated. Fruit yield was determined by the sum of the weight of all the fruits harvested at commercial maturity in each treatment, reporting it in kg plant-1. Average weight, equatorial diameter and polar diameter were obtained in the fruits of five plants obtained from each treatment, which were measured with an AutoTEC® precisión digital Vernier caliper. Fruit weight was determined as the average weight of all the fruits harvested in each treatment and weighed on an Ohaius Valor™ balance. Soluble solids content was determined from a drop of macerated fruit obtained from each treatment, which was placed in an ATAGO refractometer.
Lycopene content was determined by the method of Barba et al. (2006). For lycopene extraction, 10 g of tomatoes at commercial maturity were taken from each treatment, to which a solution of tetrahydrofuran and methanol (1:1 v/v THF:MeOH) was added before the suspensión was vacuum filtered. The fíltrate was transferred to a separating funnel to which petroleum ether and a 10% NaCI solution were added, and then mixed by stirring. Then the top layer of petroleum ether was washed with 100 mi of distilled water. The ether fraction was transferred to a 50-ml flask and evaporated to dryness in a Napco vacuum oven for 14 h at absolute pressure of 60 mm Hg at 50 °C. The residue was dissolved in 6 mi of hexane. Then the fíltrate was filtered and analyzed by high perfor mance liquid chromatography (HPLC) on a Agilent 1100 Series chromatograph, into which a reversedphase C18 Supelco Discovery column (15 cm x 4.6 mm and 5 µm) was installed. An isocratic mobile phase System composed of acetonitrile:methanol: 2-propanol (38:60:2 v/v/v) was used. The flow rate was 1 mi min-1 by injecting 20 µm of the sample. Lycopene was quantified at a wavelength of 470 nm. Lycopene identification was based on the retention time of the Sigma lycopene standard. The concentraron of the standard was 50 µg mI-1. The whole process was conducted under reduced light. Lycopene content was calculated based on the relationship between the known concentration of the standard and the corresponding peak area, to report it as mg of lycopene per 100 g fresh weight (mg 100 g-1FW).
Results and discussion
Statistical differences among the evaluated treatments were found. The control (T3) and organic substrates T5, T4 and TI had the largest yields with an average of 3.13 kg plant-1 (Figure 1), confirming that organic substrates can have yields similar to that obtained with nutrient solution. In this regard, Márquez et al. (2006) mention that the use of organic fertilizers as substrate compo- nents can obtain a good yield in the first tomato cuts, while Rodríguez et al. (2009) indícate that an inert substrate (sand), fertilized with compost tea, obtains similar yields to tomato crops fertilized in a traditional manner. On the other hand, De la Cruz-Lázaro et al. (2009) and (2010) point out differences in yield between inorganic fertilization and organic substrates. These differences can be associated with the differential response of crops to the type and proportion of organic fertilizer used as substrate (Lazcano et al. 2009). The ability of an organic substrate to improve the physical, Chemical and nutritional properties of a crop depends on its source and proportion in the growth médium (Chaoui et al. 2003).
Figure 1: Average tomato yield in pots with mixtures of organic substrates. Different letters indicate statistical diference (Tukey, P ≤ 0.05). The lines on the bars represent the standard error of the mean.
Statistical analysis for total soluble solids content (°Brix) and equatorial diameter showed no statistical difference among treatments, indicating that the control and the treatments containing mixtures of organic substrates behaved similarly (Table 1), while in the variables fruit weight and polar diameter statistical differences were detected; the heaviest fruit weight was obtained by treatments T5, T4, TI and T3 (control), which had an average weight of 0.325 kg. This indicates that the organic substrates used can produce similar fruit weight characteris- tics as the control treatment. In this regard, other research studies indícate better crop development and yield by using less than 30 % compost as a mixture component (Atiyeh et al. 2000), which probably improves the physical properties of the substrate (Hernández et al. 2008) and the gradual release of nutrients (Ao et al. 2008), thereby favoring the presence of natural hormones such as bio-stimulants, growth regulators and humic acids (Azarmi et al. 2008).
Table 1: Average values of fruit weight, total soluble solids content, equatorial and polar diameter of tomato fruit produced in organic substrates.
Different letters within each column indicate signifficant statistical difference (Tukey, P ≤ 0.05).
The fruits of plants in organic substrate mixtures had the largest polar diameter, which is an indicator of pericarp thickness (Coelho et al. 2003). Fruits produced in treatments T5, TI and T2 had the highest polar diameter valúes, averaging 6.19 cm, which is similar to that reported by Márquez et al. (2013), who found valúes between 5.9 and 6.3 cm in tomato fruits produced with organic substrate mixtures.
Figure 2 shows that four of the organic substrate mixtures had the highest lycopene content valúes, averaging 3.45 mg 100 g-1 of lycopene, 26 % higher than the control treatment (T3) and similar to the figures reported by Arias et al. (2000) who determined the lycopene content in slightly red tomato fruits. Overall, in tomato fruits, the lycopene concentration varies between 3.0 and 12.2 mg 100 g-1 depending on the stage of fruit ripening (Martínez-Valverde (2002). In this respect it is known that lycopene content varies with fertilizaron, harvest time, variety and environmental conditions (Waliszewski and Blasco 2010). In this regard the use of organic fertilizers, due to having an excess of soluble salts, can cause stress that increases the metabolism of carotenoids; concerning this, López-Elias et al. (2013) found that the concentration of total carotenoids increased by reducing the concentration of nitrogen. Possibly the level of concentration and availability of this element in the organic mixtures used stimulated the production of carotenoids. The use of organic fertilizers in a shade netting system can reduce dependence on Chemical fertilizers, increasing the yield and improving the quality of tomato fruits.
