Introduction

The black chilhuacle (Capsicum annuum L.) is an endemic fruit of San Juan Bautista Cuicatlán, Oaxaca (^{López et al., 2016}), it is trapezoidal in shape 10 cm long and 8 cm wide, in an immature state it is intense green in color, in the mature state it is bright brown and black when dehydrated (^{López and Pérez, 2015}), it is marketed mainly dry, used as the main ingredient to make the ‘Oaxacan black mole’ (^{García et al., 2017}), one of the seven most popular moles in the country, where it is cited in the oldest books of gastronomy, also maintains national and international recognition as it is served as a gourmet dish.

In recent years, the price of dried fruit has risen due to the scarcity of chilhuacle producers, increased production costs, particularly due to the excessive use of agrochemicals for pest and disease control (^{López et al., 2016}) and the lack of knowledge of crop nutrition, which is of great relevance because a well-nourished plant is more tolerant to pest attack, diseases and viruses (^{Sieiro et al., 2020}), which together are a consequence of poor yield. Fertilization only occurs two or three times during the cycle, where foliar fertilization is complementary and more used, if conditions allow it, an estimated harvest of 1 t ha^-1 is reached (^{López et al., 2016}).

The formulation of nutrient solutions in each phenological stage is carried out in order to provide the necessary nutrients to increase plant growth, yield, quality of the organs of interest and in the case of fruit vegetables reduce flower abortion (^{Moreno-Pérez et al., 2019}). Nutrient solutions not only induce achieving the maximum productive potential in a crop (^{Luna-Fletes et al., 2021}) or reducing the damage caused by the appearance of pathogens that directly affect the commercial quality of the fruits (^{Fajardo et al., 2021}), but also have an impact on the development, yield and quality of the fruits (^{López-Gómez et al., 2017}).

Of the indispensable macronutrients in crops, nitrogen is the nutrient with the highest demand by plants, it is related to growth, development (^{Larios-González et al., 2021}) and crop yields. In the stage of flower formation and fruit set, apart from the need for N, P participates in the formation of roots, flowers, fruits and seed ripening (^{Taiz and Zeiger, 2010}), while S in the form of SO₄ ^2- contributes to the differentiation of flower buds (^{Corrales-Maldonado et al., 2014}), stimulates growth, increases the quality and uniformity of the fruits, favoring resistance to cold, tolerance to drought and salts (^{Nazar et al., 2011}). In the fruiting stage, potassium (K) is related to crop quality and yield (^{Pavón et al., 2021}), increasing vitamin C, the color and taste of fruits, greater size and shelf life.

Due to the specific nutritional requirements during the development of plant species and varieties, the objective of this research was to evaluate the effect of nutrient solutions of different chemical composition in three phenological stages of chilhuacle (vegetative, reproductive and fruiting) in order to favor the growth, yield and quality of the fruits.

Materials and methods

Results and discussion

The vegetative stage lasted 29 days, with an average temperature of 21.4 °C and relative humidity of 49.5%, which can be considered short-lived since ^{Azofeifa and Moreira (2004)} mention 40 days for jalapeño pepper and ^{Noh et al. (2010)} indicate 50 days in habanero pepper. Within C. annuum, the growth and behavior of non-genetically manipulated varieties can be variable, so in chilhuacle the transplant time may not be constant because of the climatic conditions in which it is grown. In nutrition programs, it is important to consider the duration of the vegetative stage according to the species and variety since some important characteristics are cell division and rapid growth of roots, stems and leaves, involving distribution of biomass to these organs and demand for nutrients (^{Azofeifa and Moreira, 2004}), such as N as it is the element responsible for growth and development (^{Larios-González et al., 2021}), especially in the first days after transplantation where growth is accelerated.

Regarding the effect of the concentration of NO₃ ^- in the nutrient solution on the growth of chilhuacle plants, 10, 12 and 14 meq L^-1 had a statistically similar effect (p≤ 0.05) on the relative chlorophyll concentration (50.35 SPAD units), leaf area (292.4 cm²) and root length (18 cm), as well as in the accumulation of dry matter of root (0.5 g) and leaf (1.23 g). The variables plant height, main stem diameter, root volume, dry matter of the stem and of the whole plant were affected by the concentration of NO₃ ^- of the nutrient solution (Table 1). In this sense, with 14 meq L^-1 of NO₃ ^-, the plants were 11.51% taller, an increase of 21% more in the stem diameter was observed, in contrast to the plants nourished with 12 meq L^-1 (control), something similar in root volume, with 14 meq L^-1 of NO₃ ^-, the plants presented 30.55% more root volume than with a nutrition with 10 and 12 meq L^-1 of NO₃ ^- (Table 1).

In relation to the accumulation of dry matter, in stem it increased 40.50% with 14 meq L^-1 compared to plants nourished with 12 meq L^-1 of NO₃ ^-. When considering total dry matter, plants nourished with 14 meq L^-1 of NO₃ ^- produced 22.63% in contrast to a nutrition with 12 meq L^-1; however, with 10 meq L^-1, dry matter production increased by 10.28% than the control treatment (Table 1).

Therefore, supplying 10 and 14 meq L^-1 increases the dry matter available in the organs of chilhuacle plants, where this parameter indicates the amount of cumulative matter in the evaluated organs. The outstanding plants in relation to height, stem diameter, root volume, dry matter of stem and of whole plant (Table 1) allocated 45% of their dry matter to the leaves, 37% to the stem and 18% to the root, a balanced distribution of dry matter between the aerial part in greater proportion and less to stems and roots (^{Peil and Gálvez, 2005}).

According to the analysis of variance (p≤ 0.05) plant height, main stem diameter, relative chlorophyll content and leaf area, the chilhuacle plants nourished with 10:0.75:9.25 and the control (12:1:7) were statistically similar in terms of growth during the period in which they were cultivated.

The NO₃ ^-:H₂PO₄ ^-:SO₄ ^2- ratios supplied to the chilhuacle during the flowering stage had a statistically different effect on the variables of growth and accumulation of dry matter (Table 2). In the flowering stage, the most important variables were the number of flowers and buds. On average, chilhuacle plants had 3.4 flowers/plant regardless of the nutritional ratio with which they were treated. As for the number of buds per plant, the ratio NO₃ ^-:H₂PO₄ ^-:SO₄ ^2- 10:0.75:9.25 meq L^-1 and the control were statistically similar (p≤ 0.05), presenting the same number of buds in 30.55% more with respect to the ratio with 14:0.75:5.25 meq L^-1.

In this same sense, by increasing the number of buds per plant we do not ensure the development of fruits, but the flower abortion decreases by at least 2.05% (Table 2). Therefore, N and P at 10 and 0.75 meq L^-1 and 9.25 meq L^-1 of S reduces the percentage of abortion and increases the number of buds per plant, finding that the availability of N in the plant depends on the formation of buds and the development of flowers, in addition, SO₄ ^2- participates in the formation of flower buds (^{Corrales-Maldonado et al., 2014}). Therefore, as the concentration of NO₃ ^- increases, the flower abortion increases, and the number of fruits produced per plant decreases.

The fruiting stage of chilhuacle lasted 184 days with an average temperature of 20.9 °C. In this period, the components of the yield and quality of the fruit are important. As for the first, the different NO₃ ^-:K⁺ ratios evaluated had a statistically similar effect (p≤ 0.05) on the number of fruits per plant (53), yield of fresh (1.416 kg plant^-1) and dry (0.253 kg plant^-1) matter of fruits (Table 3). The number of fruits on the main stems varied significantly. In this sense, with the ratio NO₃ ^-:K⁺ 14:5 meq L^-1, the plants had the highest number of fruits in the main stems (20.4), which represented 54.54% more than the control plants (12:1:7 meq L^-1).

The same results reported in habanero pepper, where the ratio 14:5 meq L^-1 increased by 15.5% the number of fruits per plant, while the ratio 12:1:7 meq L^-1 increased the yield by 66.4%, harvesting up to 1 054 g plant^-1 (^{López-Gómez et al., 2017}). The chilhuacle plants did not show differences due to the effect of nutrition, but due to the effect of populations on yield and number of fruits, harvesting up to 274.14 g plant^-1 and 13.58 fruits in red chilhuacle and 434.43 g plant^-1 with 29.67 fruits plant^-1 in black chilhuacle (^{Urbina-Sánchez et al., 2020}).

In the case of plants nourished with 14:5 meq L^-1, compared to the ratio 14:9 meq L^-1 of NO₃ ^-: K⁺, it is interesting to consider that with the same concentration of NO₃ ^- (14 me L^-1) but different in K⁺ (5 and 9), it was statistically similar in RCC (63.81 SPAD units), total soluble solids (9.03 °Brix) and weight of dry root matter (31.1 g). Although the plants nourished with the ratio NO₃ ^-:K⁺ 14:5 had 50% more fruits on the main stems compared to the plants that received 14:9, these produced fruits with greater equatorial diameter (5.28%), with a tendency to produce more weight of fresh (7.59%) and dry (8.68%) matter. This ratio favors chilhuacle plants, where the concentration of NO₃ ^- remains constant and demanding (^{Larios-González et al., 2021}) during all their stages, while the contribution of K⁺ increases the quality and yield of the crop (^{Pavón et al., 2021}).

Regarding the percentage of flower abortion, with none of the NO₃ ^-:K⁺ ratios evaluated in this experiment the reduction below 50% was achieved, since the variation was from 50.41% (10:5 me L^-1 of NO₃ ^-:K⁺) to 70.42% (12:9 meq L^-1 of NO₃ ^-:K⁺ ). With the ratio 12:7 (control), the percentage of flower abortion was 65.70, while with 14:9, it was 60.85 (unpublished data). As for the quality of the fruits, the fruit length (60.76 mm), exocarp thickness (2.99 mm) and number of seeds (131) were statistically similar (p< 0.05) regardless of the different NO₃ ^-:K⁺ ratios with which the plants were treated.

The equatorial diameter of the fruit was significantly affected by the NO₃ ^-:K⁺ ratios (Table 3). The ratio 14:9 meq L^-1 caused the plants to have a diameter with 5.64% more than the control (12:7), among other ratios of NO₃ ^-:K⁺ (10:7, 12:5, 12:9, 14:5, 14:7). As for the weight of the fresh matter of the same organ, the plants nourished with 14:9 of NO₃ ^-:K⁺ had fruits 12.88% heavier than those produced with 10:7 and 12:9. The ratio NO₃ ^-:K⁺ 14:9 also caused the fruits to present greater dry matter compared to plants whose nutrient solution was 10 and 12 meq L^-1 (Table 3), which confirms that throughout the production cycle of chilhuacle, it demands N (14 meq L^-1) and K to favor the formation and quality of the fruits (^{Taiz and Zeiger, 2010}).

Chilhuacle fruits are marketed dry, so the size and weight of the dry matter are important. In this regard, the plants nourished with the ratio 14:9 of NO₃ ^-:K⁺ presented a statistical trend to have a greater equatorial diameter, as well as dry matter (Table 3). ^{López-Gómez et al. (2017)} reported that, in habanero pepper var Jaguar, the ratio 14:5 meq L^-1 of NO₃ ^-:K⁺ in the nutrient solution supplied in the fruiting stage increased the yield and weight of fresh matter of the fruit. This difference in the demand for K⁺, with the same concentration of NO₃ ^- (14 meq L^-1), can be attributed to the high demand for N and K required by Capsicum, so in C. chinense, one of the most studied chili peppers, the demand it required was lower than chilhuacle.

In this same sense, ^{Urbina-Sánchez et al. (2020)} fertilized chilhuacle plants with ammonium in concentrations of 0, 1.5 and 3 meq L^-1, the results showed that, statistically, the fresh and dry weight of fruit was not significant but between populations of black chilhuacle (18.66 and 2.13 g) and red chilhuacle (23.22 and 2.2 g).

The content of total soluble solids presented significant differences due to the effects of nutrient solutions, the control treatment (12:7 meq L^-1 of NO₃ ^-:K⁺) increased this content, compared to the plants that were treated with the ratio 12:5 meq L^-1, where said increase was 9.46% more than the fruits of the plants nourished with a different nutrition. As for the content of total soluble solids (TSS), ^{Urbina-Sánchez et al. (2020)} reported higher content in red chilhuacle, where at 0 meq L^-1 of NH₄ ^-, it produced 10.2 ºBrix, while at 3 meq L^-1 of NH₄, the TSS content was 10 ºBrix. In black chilhuacle, they presented on average 9.5 ºBrix with a contribution of 3 meq L^-1 of NH₄, same results obtained in this study.

Regarding plant growth, there were no statistical differences (p≤ 0.05) in the estimated leaf area (1 384 cm² plant^-1) and accumulation of dry matter in leaves (12.6 g), but it was different in terms of the relative chlorophyll content. Plants nourished with 14:9 of NO₃ ^-:K⁺ presented relative chlorophyll content statistically similar to that of the control (12:7) but 30.21% less compared to those nourished with 10:9. The above data indicate that the increase in the concentration of N-NO₃ ^- in the nutrient solution does not directly increase chlorophyll since its effect was modified by the level of K⁺ (Table 3). In relation to the above, the availability of N modifies the relationship with K, as in habanero pepper a concentration of 12 and 7 meq L^-1 increased the chlorophyll content in the leaves, while it decreased when the plants were irrigated with 10 meq L^-1 of NO₃ ^-:K⁺ (^{López-Gómez et al., 2017}).

Although chlorophyll in plants is important because it is the pigment responsible for the capture of light energy during the process of photosynthesis and its concentration is directly related to the intensity of the green color (^{Rincón and Ligarreto, 2010}), photosynthetic efficiency and the partitioning of photoassimilates is also fundamental, favoring the organ of interest. Regarding the production of dry matter, in leaves, stems and in the whole plant, it was statistically similar (p≤ 0.05) between treatments with values of 12.6 g, 124.4 g and 164.9 g, respectively, in root, with the ratio NO₃ ^-:K⁺ 10:7, the plants accumulated 74.78% compared to the root of the plants nourished with the ratio 12:7 (Table 3). It is considered that the effect of the different treatments observed in the plants was due to the ratio NO₃ ^-:K⁺ of the nutrient solution since all the plants were randomized in the same greenhouse.

Conclusions

In the vegetative stage, the concentration with 14 meq L^-1 of NO₃ ^- favored the height, stem diameter, root volume, dry matter of the stem and of the whole plant. In the reproductive stage, the NO₃ ^-:H₂PO₄ ^-:SO₄ ^2- ratios of the nutrient solution significantly modified the number of buds per plant. With 10:0.75:9.25 and 12:1:7 meq L^-1 (control), the largest amount was obtained; however, 10:0.75:9.25 meq L^-1 caused the lowest percentage of flower abortion. In the fruiting stage, the ratio 14:9 meq L^-1 of NO₃ ^-:K⁺ had a different influence on the quality of the chilhuacle fruits in presenting a greater diameter, as well as the weight of the fresh and dry matter.