Introduction

In Mexico there are more than 500 species of wild edible plants, whose use dates back to pre-Hispanic times given its great cultural, culinary and nutritional relevance, including quelites, nahuatl ‘quiltil’, of which only tender leaves, stems and flowers are consumed (^{Bye and Linares, 2002}; ^{Balcázar-Quiñones et al., 2020}), as they are an important source of nutrition (Ca, K, Mg, Fe, Mn, Zn, P), protein, dietary fiber, vitamins A, C and bioactive compounds, such as phenols, betalains, flavonoids, alkaloids, omega 3, omega 6, carotenes, ascorbic acid, α-tocopherol, glutathione, etc., which give these plants a high antioxidant capacity, in addition to providing dishes with different fragrance, colors and flavors (^{Bye and Linares, 2002}; ^{Santiago-Saenz et al., 2019}; ^{Balcázar-Quiñones et al., 2020}).

Given their high productive potential and the existence of an ancestral culture of consumption of some of these crops (verdolagas, quintoniles, papalos, epazotes, etc.) in the Mexican population of the center of the country, where there are 25 million inhabitants in The City and The State of Mexico alone (^{INEGI, 2015}), the Educational and Research Institutions must develop technology, varieties , intensive production systems and agronomic practices that provide a comprehensive knowledge of the different species of quelites and that help to promote the optimal use of their economic potential and as a food plant.

In Mexico, commercial verdolaga production is concentrated in the states of Morelos (171 ha), Mexico City (158 ha), State of Mexico (45 ha) and Baja California (46 ha) with yields ranging from 12 to 14.86 udm ha^-1 and a production of 5 474.15 t, with a production value of just over 26 361.30 thousand pesos (^{SIAP, 2019}).

In our country, there are two well-defined producing regions (contrasting in climate) with local creole varieties domesticated under cultivation: the region of Xochimilco-San Gregorio-Mixquic (temperate climate) and that of Cuautla, Morelos (warm climate) (^{Villanueva and Ramírez, 2003}). The active and continuous domestication of verdolaga in the different producing regions has allowed the obtaining of genotypes with thicker leaves, habits of erect growth, greater precocity and resistance to diseases, such as the crop ‘Chapingo’ of verdolaga, whose bromatological analyses have shown that it also contains high levels of iron, magnesium, omega 3 and omega 6 (^{Solís et al., 2016}).

Studies conducted under greenhouse (without heating) during the winter cycle indicate that it is possible to produce up to 20 kg m^-2 of vegetables in 50 days (200 t ha^-1), with a population density of 500 m^-2 (^{Villanueva and Ramirez, 2003}). The value of the winter verdolaga harvest can reach up to $8.00 per kilogram to the producer, although it drops dramatically at times of the year of its natural production (^{Villanueva and Ramirez, 2003}; ^{Mera et al., 2010}).

The study of the growth of verdolaga and the factors that affect it is of great importance, since its yield will be determined mostly by the development of the vegetative part of the plant, which in turn will depend on multiple factors, some of them typical of the genotype, others of the environment and various conditions of crop management.

Recently ^{Montoya-García et al. (2017)} assessed the effect of N, P and K fertilization on verdolaga production, found that the height and yield of verdolaga increase in response to the application of applied nitrogen and the harvest date. ^{Montoya-García et al. (2018)} report that it is possible to maximize the nutraceutical content, as well as the concentration of fatty acids and antioxidants with fertilizations with nitrogen, phosphorus and potassium, so factors such as fertilization not only influence the growth of the plant, but also on the potentiation of its properties for the benefit of health.

Knowledge about crop growth forms the basis for the optimal planning and execution of cultural work to be carried out during the production process, in order to obtain the best yields. The genotype, planting date, planting density, locality and climate in which plants develop are factors that can influence the amount of foliar tissue, growth habit, part of the plant and development of other structures.

The most important factors affecting the duration of the stages of verdolaga development include the genotype (inside and between populations), the environment (climate and soil) and its interactions, which can vary the growth habit and precocity of the plant. Some other characteristics of the plant such as the habit of growth, flavor, phyllotaxis and stem, may vary depending on whether the plant develops as weeds or cultivated (^{Villanueva and Ramírez, 2003}). Plant growth involves at the physiological level a series of changes and irreversible reactions in the plant, on which the final agronomic characteristics (phenotype) and the potential yield of the different genotypes will depend.

The evaluation of the growth of a plant can be carried out through direct measures (plant height, stem diameter, number of leaves, number of branches, foliar area, fresh or dry mass, etc.) and indirect (net assimilation rate, crop growth rate, relative growth rate, etc.) (^{Rodríguez, 2000}).

Planting density is a factor that can influence crop yield such as verdolaga, as it interferes with plant physiological events and directly with accumulation of dry matter from the different organs of the plant, and also determines competition between plants by light, water and nutrition (^{Rodríguez, 2000}; ^{Ayala et al., 2004}). Based on the above, the objective of this research was to evaluate the effect of three planting densities on the growth of the crop 'Chapingo' of verdolaga, as well as characterize its dynamics and habit of growth.

Materials and methods

The experiment was established in a batch of the Experimental Agricultural Field of the Chapingo Autonomous University, Mexico at 19° 29’ 27” north latitude and 98° 52’ 23” west longitude, at an altitude of 2 240 meters above sea level (msnm). The modifications made by ^{Garcia (1988)} to the Köppen climate classification system describe the site of the establishment of the experiment as a C(W)(w)b(i)g, which means that it is the driest of temperate subhumid with rains in summer, an average annual temperature of 15.6 °C and -3° to 18 °C in the coldest month (January), a cool summer, the temperature of the warmest month is less than 22 °C (may), with little annual thermal oscillation ranging from 5° to 7 °C. The soil is classified as Mollic Ustifluvent, with a depth of 150 cm and a slimy clay loam texture on the Horizon Ap (^{Survey Soil Staff, 2010}).

40 furrows ±1 of 30 m long and a distance between furrows 0.6 m with verdolaga of the crop ‘Chapingo’ were planting. The sampling sites were established at random, so that each experimental unit had a different planting density, it is delimited 1 m for each experimental unit and it is counted the number of plants in each experimental unit, leaving three different planting densities (treatments): high (295 plants. linear meter), medium (250 plants m^-1 linear) and low (134 plants m^-1 linear). In each experimental unit, data were taken to 20 plants (repeats).

The following variables were evaluated every 4 days from planting and up to 48 days after planting (dds). Variables studied were total plant height (AT), was measured in centimeters (cm), from ground level to the end of the main stem; number of branches (NR), the branches that were inserted directly into the main stem were counted; stem diameter (DT), was measured in millimeters (mm) with the help of a Vernier, in the second internode of the plant, just below the third knot; number of large leaves and small leaves (NHG and NHCH, respectively), the large and small leaves of the plant were counted, considering as large leaves those between 1.5 to 2 cm wide and 2.5 to 3 cm long. Average length of the branches of the n knot (LPRn), was taken the average, in centimeters, of the length of the two branches that were inserted into the n knot of the main stem, numbering the knots from the base to the tip of the plant. The average number of leaves of the n knot branches (NHPn), refers to the average number of leaves of each of the branches that were inserted into the n knot of the main stem, numbering the knots of the base to the tip of the plant.

For statistical analysis, data were taken for three phenological stages of verdolaga development: vegetative development (19 dds; 52.1 Units Heat, UC), vegetative maturity (31 dds, 100.65 UC) and flowering (39 dds, 39 142.55 UC) of verdolaga of the crop ‘Chapingo’. The data was analyzed with the SAS System for Windows 9.1 (^{SAS Institute, 2002}) package, to determine whether there were statistically significant differences between the averages of the three planting densities evaluated an Anova was performed and to determine whether there was equality of its variances the Levene method was used, both with a significance level of p≤ 0.05.

Results and discussion

Figure 1 shows the growth of the verdolaga in plant height (AT), number of branches (NR) and stem diameter (DT); in Figure 2, the number of large leaves (NHG) and number of small leaves (NHCH), while in Figure 3, the growth of the branches of the main stem at the height of the 1^st and 4^th knot, counting from the base to the tip of the plant. The growth dynamics represented in these figures show that it is between 20 and 40 dds where the growth of the verdolaga is accelerated so, although the variables were evaluated every 4 days, the Anova and the Levene test of variance comparison was performed with the data obtained in the phenological stages covering that period (vegetative development, vegetative maturity and flowering).

Figure 1 Growth dynamics of a) total plant height (cm), b) number of branches and c) stem diameter (in cm) of the verdolaga in three planting densities (high: 295; medium: 250 and low: 134 plants m linear, respectively). 

Figure 2 Growth dynamics of a) number of large leaves and c) number of small leaves in three planting densities (high: 295; medium: 250 and low: 134 plants m^-1 linear, respectively). 

Figure 3 Growth dynamics of the average length of the two branches of knot a) 1 (LPR1) and c) 4 (LPR4) and number of average leaves of the two branches of knot a) 1 (NHP1) and d) 4 (NHP4) of the verdolaga in three planting densities (high: 295; medium: 250 and low: 134 plants m linear, respectively). 

Conclusions

There are significant differences between planting densities for total height (AT), number of small leaves (NHCH), average length of the two branches of knot 1 and 4 (LPR₁ and LPR₁, respectively) and number of average leaves of the two branches of knot 1 (NHP₁). High planting densities generate taller plants, smaller stem diameter, fewer leaves (large and small) and less branch development (branch length); however, the number of leaves developed in the branches depends on the level of the canopy where they are located.