Location of the experiments

The experiment was carried out in a greenhouse of the experimental field of the Faculty of Agricultural Sciences (18° 58’ 51” north latitude, 99° 13’ 57” west longitude, 1 868 meters above sea level) at the Autonomous University of the state of Morelos, Cuernavaca, Morelos, Mexico, during the months of April-September 2015.

Vegetal material

For the production of seedlings seeds of habanero ‘Jaguar’ chili were planted on polyethylene trays. This variety presents plants with height of 80-90 cm in open field and up to 1.8 m in systems of protected agriculture with tutoreo. From 70 to 85 days to flowering and from 115 to 120 days to the first harvest. It has one to three flowers per knot, which can give the same amount of fruit. It presents uniform fruits of emerald green color in physiological maturity that happen to intense orange in the total maturity, reaching a hue value (Hue) of 54. The fruits have a length of 3.8 to 5.5 cm, diameter of 2.5 to 3.0 cm and of 6.5 to 10 g of weight, with yields ranging from 18.3 to 36 t ha^-1 (^{Ramírez et al., 2012}).

Management of the experiment

Ten nutritional regimes were developed for habanero chili (Table 1). These consisted in modifying the nitrate (NO₃ ^-) concentrations in the vegetative stage, which comprised from the transplant until 50% of the plants presented the first flower. Subsequently, the nitrate:phosphate:sulfate (NO₃ ^-:H₂PO₄ ^-:SO₄ ^2-) ratios were modified in the flowering stage, since 50% of the plants presented the first flower until 50% of the plants had the first fruit with a length of 10 ±1mm.

Table 1 Nutritional regimes evaluated in habanero chili plants. 

Diseño experimental completamente al azar con 10 repeticiones. * = tratamiento testigo.

Finally, in the fruiting stage the nitrate: potassium (NO₃ ^-:K⁺) ratio was modified, since the first fruits were 10 ±1 mm in length until the end of the harvest. These modifications were made by maintaining the mutual relations between anions and between cations as indicated by ^{Steiner (1984)} with the ratio 12:1:7:7:9:4 of NO₃ ^-:H₂PO₄ ^-:SO₄ ^2-:K⁺:Ca²⁺:Mg²⁺ respectively. In addition, each regimen was formulated with the Fe (Fe-EDTA), B (H₃BO₃), Mn (MnCl₂), Zn (ZnSO₄), Cu (CuSO₄) and Mo (H₂MoO₄) micronutrients in concentrations of 5, 2.88, 1.81, 0.22, 0.18, 0.02 mg L^-1 respectively.

A completely randomized experimental design with 10 replicates per treatment was used and the 100% Steiner solution was applied throughout the growing cycle as a control treatment. The experimental unit was a black polyethylene bag 25.5 cm in diameter by 30 cm in height with a capacity of 15.14 L, filled with red tezontle in granulometry ≤1 cm in diameter, considered a chemical inert substrate (^{Ojodeagua et al., 2008}).

In each bag was placed a seedling of habanero chili, taking as a parameter for the transplant when they had the well developed root and with ten true leaves, the tenth with 5 ±1 mm in length. The irrigations were performed using a drip irrigation system (with a Netafim self-compensating dropper and flow rate of 8 L per hour), with a timer of eight irrigations per day, with a duration of 1 min each irrigation the first days after transplantation and increasing the irrigation time as the plant required during its growth (reaching up to 4 min for each irrigation in the fruiting stage).

Chemicals were used to prevent the attack of pests and diseases. Confidor^® (ia. Imidacloprid) was applied at a dose of 2 mL L^-1 to prevent the occurrence of possible pests such as whitefly or thrips (Bemisia sp., Trialeurodes sp., Frankliniella spp., Liryomisa sp.) and Promyl^® (ia. Benomilo) 2 g L^-1, to prevent diseases such as cenicilla (Oidium spp.), anthracnose (Colletotrichum phomoides), leaf spot (Septoria lycopersici) or decay (Botrytis cinerea).

Response variables

The height of the plant was measured with a tape measure, from the base of the stem to the apex of the longest stem; the diameter of the stem was measured with a watch-type vernier and obtained from the base of the main stem, 2 cm above the substrate, the chlorophyll content was measured with an SPAD-502 (Konica Minolta) considering the fifth leaf after the apex of each stem; the leaf area was determined with a leaf area integrator (LI-COR, LI3100C). The mechanical strength of the stem was obtained with a Shimadzu Ez Test texturometer, taking as a point of measurement the first internode of each secondary stem and a penetration of 3 mm with a conical strut, the weight of fresh and dry matter of leaf, stem and root was obtained with an ADAM Core digital scale CQT5000.

To obtain the dry matter weight, the different plant organs (stem, leaf and root) were placed in brown paper bags and dried in a forced circulation oven at 65 °C until the dry matter weight remained stable. In order to count the number of flowers per plant, the first four branches of each secondary stem were taken as the parameter for all plants, and from that point the percentage of floral abortion was obtained.

The volume of fresh root was measured with a graduated 2 L specimen and amounting to 1 L, for which the root was introduced into the water and the volume difference was that corresponding to the involved organ. The first three variables were analyzed during the development of the crop, the variables on fruit quality were analyzed after each fruit cut and the destructive variables were analyzed at 170 days after transplant (DDT).

The fruits were harvested at 133 DDT when they showed intense orange color, making a cut per week for a period of six weeks. Once harvested, the fruits of each plant were counted and weighed on an ADAM Core CQT5000 digital scale. The total weight was divided between the number of fruits of each plant and the average weight per fruit was obtained. The length of the fruit was measured from the peduncle to the apex of the fruit with a watch type vernier as well as the diameter in the middle part of the fruit. The color of the fruit was obtained with a portable spectrophotometer X-Rite SP-60. While yield per plant was obtained with the sum of harvested during the six-week period.

Statistic analysis

A variance analysis was performed with the SAS program (Version 9.0) and the Tukey test (p≤ 0.05) was used for statistical data.

It was tried to give greater understanding to the symbology of the treatments

As a result of the nutritional regime 14 (NO₃ ^-), 12:1:7 (NO₃ ^-:H₂PO₄ ^-:SO₄ ^2-) and 14:5 (NO₃ ^-:K⁺) me L^- in the vegetative, reproductive and fruiting stages respectively, plant height was 122.8 cm at 169 DDT (Table 2). Also due to the effect of the same regime, there were significant differences in the diameter of the main stem, as well as in the leaf area, where also the regime of 14 (NO₃ ^-), 14:1.25:4.75 (NO₃ ^-:H2PO₄ ^-:SO₄ ^2-) and 14:5 (NO₃ ^-:K⁺) me L^-1, caused an increase in leaf area compared to the rest of the treatments, achieving the highest number of flowers per plant (57 flowers/plant) to the fourth bifurcation of each stem secondary.

Table 2 Morphological characteristics, stem resistance to mechanical damage, chlorophyll concentration and flower abortion in response to the different nutritional regimes of habanero chili. 

AP= altura de planta; DTP= diámetro de tallo principal; AF= área foliar; RMT= resistencia mecánica del tallo; CRC= contenido relativo de clorofila; NFP= número de flores por planta; ADF= aborto de flor; * = solución Steiner como tratamiento testigo; CV= coeficiente de variación. Medias con la misma literal son iguales estadísticamente de acuerdo con la prueba de Tukey (p≤ 0.05).

However, with these regimens, the highest percentages were obtained in flower abortion, reaching 54.5% (Table 2). ^{Ramírez-Luna et al. (2005)} indicate that in chili habanero there is 40% of flower abortion, which can decrease with the application of growth regulators such as gibberellins, which under specific environmental conditions of light and temperature induce the formation of flowers.

For its part ^{Medina-Lara et al. (2008)}, report up to 85% of flower abortion in habanero chili, attributing this loss to nitrogen deficiencies. However, ^{Cruz et al. (2012)} indicate that high temperatures cause physiological disturbances in plants of habanero chili (Capsicum chinense Jacq.) Causing a high flower abortion. Similarly, ^{Jaimez et al. (2010)} report the same effect on paprika (Capsicum annuum) but argue that the phenomenon is due to an effect of the pollen and pollen germination process, besides that this process may also be different among cultivars.

In the root volume, statistical differences between the different treatments were also found (Table 3), with 14 (NO₃ ^-), 14:1.25:4.75 (NO₃ ^-:H₂PO₄ ^-:SO₄ ^2-) and 14:5 (NO₃ ^-:K⁺) me L^-1, with a volume 20% greater than the root volume produced by the Steiner solution. In addition, the total weight of dry matter was higher when the plants were fed at the same rate. Of the total weight of dry matter per plant, 29% corresponded to the leaves, 53% to the stem and 18% to the root (Table 3).

Table 3 Volume of root and weight of dry matter of the organs of the plant by effect of different nutritional regimes. 

AP= altura de planta; DTP= diámetro de tallo principal; AF= área foliar; RMT= resistencia mecánica del tallo; CRC= contenido relativo de clorofila; NFP= número de flores por planta; ADF= aborto de flor; * = solución Steiner como tratamiento testigo; CV= coeficiente de variación. Medias con la misma literal son iguales estadísticamente de acuerdo con la prueba de Tukey (p≤ 0.05).

According to ^{Peil and Gálvez (2005)}, the conditions of cultivation with artificial substrates in the greenhouse, with optimal water and nutrients contribution, achieve a maximum growth of the plants with a reduced radical system.

In the case of cucumber, this fraction varies between 8% and 15%, in the stage of vegetative growth and between 3% and 7% during the reproductive stage. In the case of tomato, the dry matter fraction for the roots varies between 17% and 20% in the vegetative stage and between 1% and 10% in the reproductive stage. For their part ^{Peña and Zenner (2015)}, indicate that high dry matter percentages of the aerial part of the plants of Capsicum annuum indicate a greater number of leaves, source and production of photo-assimilates for the filling of the demanding organs.

There was an increase in the number of fruits per plant (Figure 1) as well as in yield (Figure 2) as a result of the nutritional regime where nitrogen in the three phenological stages and phosphorus in the flowering stage were higher than the Steiner solution. With the regimen 14 (NO₃ ^-), 14:1.25:4.75 (NO₃ ^-:H₂PO₄ ^-:SO₄ ^2-) and 14:5 (NO₃ ^-:K⁺) me L^-1 in the vegetative, flowering and fruiting stages respectively, 15.5% more fruits than with Steiner solution throughout the growing cycle. While the highest yield was 1 054 g plant^-1, 66.4% higher than the yield obtained with Steiner solution.

Figure 1 Number of fruits per plant accumulated in six cuts of habanero chili. NO₃-, 10 me L^-1 on R1, R2 y R3, 12 me L^-1 on R4, R5, R6 y R10, 14 me L^-1 on R7, R8 and R9; plus NO₃-:H₂PO₄-:SO₄ ² -, 14:1.25:4.75 me L^-1 en R1, R3, R4, R6, R7 and R9, 12:1:7 me L^-1 on R2, R5, R8 y R10; plus NO₃-:K⁺, 14:5 me L^-1 on R1, R2, R4, R5, R7 and R8, 12:7 me L^-1 on R3, R6, R9 y R10. Means with the same literal are statistically the same according to Tukey’s test (p≤ 0.05). 

Figure 2 Yield accumulated in six weeks of habanero chili cut. NO₃ - , 10 me L^-1 en R1, R2 y R3, 12 me L^-1 en R4, R5, R6 y R10, 14 me L^-1 en R7, R8 y R9; más NO₃ - :H₂PO₄ - :SO4 2-, 14:1.25:4.75 me L^-1 en R1, R3, R4, R6, R7 y R9, 12:1:7 me L^-1 en R2, R5, R8 y R10; más NO₃ - :K⁺ , 14:5 me L^-1 en R1, R2, R4, R5, R7 y R8, 12:7 me L^-1 en R3, R6, R9 y R10. Means with the same literal are statistically the same according to Tukey’s test (p≤ 0.05)  

On the other hand, ^{Khan et al. (2010)}, tested a nutrition with three concentrations of phosphorus, four nitrogen and the interaction of these two elements on the amount of fruits per plant in Capsicum annuum and found that because of the higher level of phosphorus, as with the greater nitrogen level, the number of fruits per plant was increased. Similarly, ^{Nieves-González et al. (2015)} indicate that when the concentration of phosphate (1 me L^-1) in the Steiner solution is modified to 1.5m L^-1, greater production of chili habanero var. “Big Brother”.

Something similar in terms of the greater contribution of nitrogen and phosphorus is reported by ^{Borges-Gómez et al. (2010)}, who tested three nutrient formulas N-P₂O₅-K₂O (240-240-240, 120-120-120 and 000-000-000) along with three levels of soil moisture (100, 75 and 50% of usable humidity) and yielded 10 fruit cuts (weekly), an average yield of 1 391 g plant^-1 with the highest fertilizer dose and the highest moisture level. On the other hand, ^{Tucuch-Haas et al. (2012)}, report that supplying the Steiner solution with an NH₄ ⁺:NO₃ ^- ratio of 1: 9 respectively, in three cuts of fruit, a yield of habanero cv. “Criollo Naranja” of 302 g plant^-1. While ^{Nieves-González et al. (2015)} indicate that with Steiner solution, by modifying the concentration of H₂PO₄ ^- at 1.5 me L^-1 a yield of chili habanero var. “Big Brother” 851.71 g plant^-1.

In the present study, the best response in fruit size and weight was obtained with the 14 (NO₃ ^-), 12:1:7 (NO₃ ^-:H₂PO₄ ^-:SO₄ ^2-) y 14:5 (NO₃-:K⁺) me L^-1 (Table 4), achieving on average fruits of 3.48 cm in length, 2.29 cm in diameter and a weight of 3.45 g fruit^-1. On the other hand, ^{Tucuch-Haas et al. (2012)}, indicate that the size of the habanero chili cv. “Criollo Naranja” grown under greenhouse has an average of 3.58 cm in length, 2.47 cm in diameter and a weight of 5.51 g fruit^-1, produced in conditions similar to the present study. While ^{Tapia-Vargas et al. (2016)} mention that the black habanero chili produced under a greenhouse in a hydroponic system and with application of a hormonal complex (Formax-F^®) the size of the fruit is 2.92 cm long and 2.44 in diameter. However, according to ^{Ramirez et al. (2012)}, the length of the habanero chili fruit, ‘Jaguar’ variety is 3.8 to 5.5 cm, the diameter of 2.5 to 3 cm and 6.5 to 10 g fruit^-1, however, it is not indicated if that fruit size is obtained under greenhouse conditions.

Table 4 Quality of the habanero chili fruit in size, weight and color, due to the effect of the nutritional regime. 

L= luminosidad; C= cromaticidad; h= matiz; CV= coeficiente de variación. Medias con la misma literal son iguales estadísticamente de acuerdo con la prueba de Tukey (p≤ 0.05).

On the other hand, ^{Ramirez-Luna et al. (2005)}, mention that Habanero chili plants grown in greenhouses have a higher number of flowers and fruits, but with small fruit size, in contrast to field production, where larger fruits are obtained; attributing the smallest size of fruit, to the low intensity of light that is in the greenhouse, a condition that favors larger plants, stems thinner, but small fruits. However, in tomato cultivation (Lycopersicon esculentum Mill.), ^{Aldana et al. (2007)} state that the size of the fruit depends directly on the amount of pollen grains deposited in the stigma, that is, with less pollen, produce smaller, deformed or low seed fruits. This is related since the cultivation of habanero chili under plastic cover, unlike open pit conditions, does not receive the same pollination by wind or by insects.