Introduction

Pepper is one of the main horticultural crops produced worldwide in open field and greenhouse production. Mexico is the main supplier of this product to the markets of the United States and Canada (^{Díaz et al., 2013}). The states with the highest production are Sinaloa, Sonora and Guanajuato, with a contribution of 152.4, 2.1 and 1.6 thousand tons, respectively, and an approximate value of 3,600 million pesos (^{SIAP, 2016}). In Mexico there are different pepper conduction systems. Most of the greenhouses use the Dutch type or pruning in “V”, which consists in guiding the plant to two stems by pruning one of the branches in each bifurcation, during 10 months approximately. The system is delicate in its management and the cost of production per kilogram is high (^{Sánchez et al., 2017}). An alternative system of lower cost is the early pruning, which allows to stop the growth of the plants by eliminating the growth apices, once the first three or four bifurcations have been formed. By means of this procedure, the cultivation cycle is shortened to four or five months after the transplant and results in the establishment of at least two to three cultivation cycles per year. The yield per plant with this system is usually smaller, but it is compensated per surface unit, given that the population density is higher compared to what is used commercially (^{Ortíz et al., 2009}). At experimental level, the pruning above the third and fourth bifurcation has been used in pepper crops, demonstrating that they can potentially achieve an annual productivity equal to or greater than European pruning systems, but with simpler technologies and lower production costs (^{Cruz et al., 2009}). Another important factor in the production of pepper in soilless crop production is the composition and concentration of ions in the nutrient solution. In particular, the nitrogen nutrition and the ionic form of NO₃^- and NH₄⁺ in a proper ratio, can increase the growth and yield of crops compared with either of the two forms of nitrogen separately (^{Sheng-Xiu et al., 2013}). The beneficial effect of the proportion NO₃^-/NH₄⁺ varies between species, as well as the degree of development of the crops, environmental conditions, total concentration of nitrogen and total ionic concentration of the nutrient solution (^{Chang et al., 2010}; ^{Tucuch-Haas et al., 2012}; ^{Chen et al., 2015}; ^{Liu et al., 2017}). An inadequate supply of the NO₃^-/NH₄⁺ ratio may be harmful to the crop due to the effects of toxicity and alterations in metabolism that may occur by high concentrations of nitrogen (^{Sheng-Xiu et al., 2013}). The correct ammoniacal nutrition can become an effective tool to improve the nitrogen absorption in plants, starting from the fact that the assimilation of nitrogen by the absorption of the ammoniacal form is faster (^{Bonete & Martínez, 2009}) and with a lower energetic cost for the plants (^{Degiovanni et al., 2010}). In tomato, ^{Dong et al. (2004)} reported that a 25/75 ratio of NH₄⁺ and NO₃^- increased the fresh and dry fruit weight. On their part, ^{Claussen (2002)} and Xu et al. (2001) reported that in hydroponic greenhouse pepper and tomato crops, maximum growth and yield was obtained when the NH₄⁺ concentration did not exceed 30 %. The objective of the present study was to evaluate the effect of different NO₃^-/NH₄⁺ ratios in the nutrient solution, in the accumulated total dry biomass and yield of Bell pepper fruit with management of early pruning, in an open-circuit hydroponic system.

Material and Methods

The study was carried out in a modified tunnel-type greenhouse without climatic control and with passive zenith and lateral ventilation, located in the Academic Unit of Agriculture, at 21° 25’ North latitude and 104° 53’ West longitude in Xalisco, state of Nayarit, during the period from August 2017 to January 2018. Two varieties of blocky bell pepper, Avante and Tribeka with ripe red fruits were used, which have high resistance to the Tobacco mosaic virus and intermediate resistance to the Tomato spotted wilt virus and Pepper sadness. The sowing was carried out in polystyrene trays of 200 cavities, placing one seed per cavity; peat (Sunshine®) fine mixture number 3 was used as substrate. Once the seeds germinated, the irrigations and fertilization were carried out when the first true leaves emerged, applying the universal Steiner solution at 25 %. At 45 days after sowing, when the plants had formed two to three true leaves, the transplant was performed in black polyethylene bags of 10 liters capacity, with red basaltic volcanic rock as a substrate with a particle size of 6 mm to fine. The plantation framework consisted of paired lines with a distance of 1.20 m between corridors, 0.60 m between lines and 0.50 m between plants.

The chemical composition, pH and electrical conductivity of the irrigation water used in the experiment is detailed in Table 1; it has a sodium adsorption ratio of 0.86, residual sodium carbonate -0.35 and according to the Riverside Standards it is classified as C₁S₁, good quality water. The ^{Steiner’s universal nutrient solution (Steiner, 1961)} was used modifying the total nitrogen concentration of 12 to 15 meq·L^-1 from its original formulation and the total ionic concentration was reduced to an osmotic potential (Ψo) of -0.037 ± -0.0025 kPa. The percentage proportions of NO₃^-/NH₄⁺ used were 100/0, 90/10, 80/20 and 70/30, which corresponds to a NH₄⁺ concentration of 0, 0.75, 1.5 and 2.25 meq L^-1 respectively, in the cation ratio. Table 2 shows the chemical composition of anions and cations of the nutrient solutions mentioned.

For the preparation of the nutrient solutions (Table 2) in the different treatments, the concentrations of anions and cations present in the irrigation water were considered (Table 1). Fertigation grade commercial fertilizers were used as a source of nutrients: calcium nitrate tetrahydrate, potassium nitrate, potassium sulfate heptahydrate, magnesium sulfate, ammonium sulfate and monopotassium phosphate. As a micronutrient source, Ultrasol micromix® was used. The adjustment of the pH to 6.0 ± 0.1 of the nutritive solutions was made with sulfuric acid. The management of irrigation with nutrient solution or only acidulated water with a pH of 6.0 was carried out by means of a drip irrigation system, placing two emitters of 2 liters per hour in each container, operated with the use of a digital timer SKU model 458148 of 125 V-60Hz. A percentage of total drainage was maintained from 10 to 15% in the vegetative stage and 20 % in the flowering-fruiting stage, to achieve a drainage electrical conductivity of 1.0 to 1.3 dS m^-1 throughout the crop cycle. Irrigations with nutrient solution were daily at 8:00 a.m. and 1:00 p.m., with a volume that ranged from 250 to 500 ml according to the development of the crop, as well as a daily total of 12 micro irrigations, with acidulated water and a volume of 50 to 100 ml per event with a frequency of one hour.

The factors under study were four NO₃^-/NH₄⁺ relations and two blocky bell pepper varieties: Avante and Tribeka. In this way, eight treatments were established in a completely randomized experimental design, with a 4x2 factorial arrangement, with five repetitions. The experimental unit consisted of a plant placed in a container of 10 L capacity with a substrate. The different nutrient solutions were supplied from the first day of the transplant. The plants were cultivated with the growth of all their stems until the fourth bifurcation and the three leaves above it, the removal of the growth apices or pruning was done. The tutoring was carried out with raffia cords held on wire fastened in the structure of the greenhouse at a height of 3.2 m. The variables evaluated were Average fruit weight (AFW, g•fruit^-1), Number of fruits harvested per plant (NFP, Number of fruits•plant^-1), Fruit production per plant (RP, kg•plant^-1), Fruit yield per m² (RM², kg•m^-2), Number of fruits per m² (NFM²) and Accumulated total dry biomass (TDB, g·plant^-1). For the quantification of the total dry biomass, the samples of root, leaf, stem and fruit were dried in an oven with forced air circulation at 70°C until reaching constant weight. For the case of fruit, the dry weight of all the fruits obtained per plant, estimated from fresh weight, was added. The estimators were obtained by linear regressions between dry weight and fresh fruit weight, with five repetitions. The fresh or dry weight was determined using a digital scale. The data obtained from the response variables considered in the experimental treatments were subjected to an analysis of variance and Tukey’s means test (p≤0.05) with the statistical program SAS®.

Results and Discussion

The analysis of variance showed significant differences for the variety factor in all the variables, except for fruit yield per m² (FYM²). The NO₃^-/NH₄⁺ ratio factor and variety interaction with the NO₃^-/NH₄⁺ ratio had significant differences in all the response variables, with the exception of accumulated total dry biomass (Table 3).

Conclusions

The different NO₃^-/NH₄⁺ proportions supplied in the nutrient solution with the same osmotic potential of -0.037 kPa promoted a differential response in most of the evaluated variables related to fruit yield and total accumulated dry biomass in bell pepper with early blunting at the height of the fourth bifurcation of the stem. The best response in the studied variables was obtained with the addition of 10 % of NH₄⁺ that equals 0.75 meq L^-1 in the ratio of cations. In the same way, the concentration of ammonium in the nutrient solution had an interactive effect with the pepper varieties Avante and Tribeka in the quantified variables.