Introduction

The need to increase agricultural production in Mexico in light of population growth, scarce arable area, and climate and soil problems leads to the use of intensive production systems, such as hydroponics and greenhouses (^{Alpizar-Antillón, 2004}; ^{Resh, 2004}). The cost of production with these technologies is high, so it is only suitable for crops with high economic value and a stable market to achieve high economic profitability.

In Mexico, the vegetable that is most managed under greenhouse and hydroponic conditions is the tomato (Solanum lycopersicum L.), which covers about 70 % of the total area cultivated with this technology (^{Ponce, Molina, Cepeda, Lugo, & Maccleery, 2015}).

The greenhouse tomato production system, which is practiced in European countries and in North America, consists of using intermediate-growth, beef-type cultivars with population densities of 2 to 3 plants∙m^-2 that are left to grow to more than 7 m in height. The aim is to harvest from 20 to 25 clusters in a period of 10 to 11 months, from transplant to harvest, with yields that can reach 500 t·ha^-1·year^-1 in high-technology greenhouses (^{Peet & Welles, 2005}; ^{Resh, 2004}). This system is also the most used by large companies that produce under greenhouse conditions in Mexico. Although the yield is high, its drawback is that the crop cycle is very long, so the plants are exposed for a longer time to possible damage caused by pests and diseases. In addition, agricultural tasks such as pruning and training are technically difficult to perform, and fruit weight decreases as the last clusters are harvested (^{Ponce et al., 2015}).

High technology, which includes very tall greenhouses and sophisticated equipment for environmental control, raises the cost of production per kilogram of fruit, so it is only profitable if there are select markets or if the product is for export. This situation is difficult for small and medium greenhouse producers, which in Mexico account for at least 95 % of the total (^{Ponce et al., 2015}).

At the Universidad Autónoma Chapingo, Mexico, ^{Méndez-Galicia, Sánchez-del Castillo, Sahagún-Castellanos, and Contreras-Magaña (2005)}, ^{Sánchez-del Castillo, Moreno-Pérez, and Cruz-Arellanes (2009)}, ^{Sánchez-del Castillo, Moreno-Pérez, Coatzín-Ramírez, Colinas-León, and Peña-Lomelí (2010)}, ^{Sánchez-del Castillo, Moreno-Pérez, and Contreras-Magaña (2012)} and ^{Sánchez-del Castillo and Ponce-Ocampo (1998)} have developed an alternative, hydroponic greenhouse tomato production system, which is based on shortening the crop cycle, from transplant to final harvest, to less than four months. To do this, early blunting (pruning of the main apex) of the plant is performed to allow only the fruits of the first two or three clusters to grow, in population densities ranging from 8 to 12 plants∙m^-2 and combined with the transplant of seedlings older than those normally used (late transplant).

Comparing the system of short cycles and high population density with the conventional one characterized by low density and many clusters per plant, yield per plant is lower in the former, but per unit area is partially compensated by using a higher population density, which is possible due to the smaller leaf area formed by the plant (^{Sánchez-del Castillo et al., 2010}). The crop cycle, from transplant to final harvest, is shortened so much that it is possible to achieve at least three cycles per year, which allows for high annual productivity, even more than what is obtained with the conventional system (^{Sánchez-del Castillo & Ponce-Ocampo, 1998}; ^{Sánchez-del Castillo et al., 2012}). With the shortened crop cycle, phytosanitary problems are also reduced and it is possible to concentrate the crop in domestic market sales when the price is high, giving the producer a greater economic benefit (^{Sánchez-del Castillo & Corona-Sáez, 1994}). In addition, lower-height greenhouses with fewer technological requirements can be used, which significantly reduces the cost of production.

The use of tomato varieties with a determinate growth habit is not common in greenhouses, but for the proposed blunting systems, where only three clusters are left per plant, these varieties may be appropriate. This is due to the fact that, in general, they have less leaf area per plant, lower height and, consequently, less mutual shading for the high population densities that are managed (^{Méndez-Galicia et al., 2005}).

Previous studies with indeterminate-habit cultivars have allowed the establishment of a suitable population density to achieve the highest yield possible per unit area without reducing average fruit size (^{Sánchez-del Castillo & Corona-Sáez, 1994}; ^{Sánchez-del Castillo & Ponce-Ocampo, 1998}). However, for recent determinate and indeterminate varieties on the market, optimum population densities have not been established with this management.

Therefore, the objective of this study was to evaluate the behavior of agronomic management in tomato varieties (with different fruit type and growth habit), in particular the yield and its components in plants blunted to two and three clusters, established at different population densities.

Materials and methods

This study was carried out in a greenhouse located at the Universidad Autónoma Chapingo Experimental Station in Chapingo, Mexico, at 19° 29’ North latitude, 98° 53’ West longitude and 2,250 masl.

The yield and its components (fruit weight and number), as well as plant height and the leaf area index of four contrasting tomato varieties were compared: ‘Imperial’ (beef-type fruit and indeterminate growth), ‘Pick ripe’ (beef-type fruit and determinate growth), ‘Moctezuma’ (saladette-type fruit and indeterminate growth) and ‘Serengueti’ (saladette-type growth and determinate growth). Each variety was evaluated at two blunting levels (removal of the terminal bud two leaves above the second or third inflorescence formed). Within each blunting level, three population densities were established (20, 16 and 12 plants∙m^-2 when two inflorescences were left, and 13, 10 and 7 plants∙m^-2 when three were left).

To achieve the indicated densities, the plants were placed in 1 m wide growing beds with 50 cm aisles. The distances between plants and rows within the beds were: 22.5 x 22 cm (20 plants∙m^-2), 20 x 30 cm (16 plants∙m^-2), 25.5 x 30 cm (13 plants∙m^-2), 27.5 x 30 cm (12 plants∙m^-2), 33 x 30 cm (10 plants∙m^-2) and 30 x 50 cm (7 plants∙m^-2).

A randomized complete block design with four replicates was used in a split-plot treatment arrangement with nested factors (densities within blunting levels). From the combination of four varieties, with two tipping levels and three population densities, there were 24 treatments. The small-plot experimental unit was 1.5 m long by the width of the bed, which was 1 m (1.5 m²).

Seeding was done in 60-cavity trays (with a rootball volume of approximately 200 cm³), using as substrate a mixture of peat moss with perlite at a ratio of 1:1 (v/v). Once the seed was placed it was covered with a 0.5 cm layer of vermiculite and heavily irrigated with water. From the onset, the seedlings were irrigated with a nutrient solution at 50 % of its normal concentration and 15 days later the 100 % solution was used. The solution contained the following elements and concentrations (mg∙L^-1): N = 250, P = 50, K = 250, Ca = 280, Mg = 50, S = 150, Fe = 2, Mn = 1, B = 0.5, Cu = 0.1 and Zn = 0.1. This was used throughout the crop cycle.

The transplant was performed 40 days after sowing (das). The growing beds were 0.3 m deep and filled with red tezontle sand (with most of its particles between 1 and 3 mm in diameter). The blunting was carried out two leaves above the second or third inflorescence formed, according to the treatment, which occurred between 80 and 90 das.

In due course, the required cultural practices such as leaf and side shoot pruning, training, blunting, pollination, and pest and disease control, among others, were carried out according to a protocol established for the crop.

The variables evaluated were:

The obtained data were submitted to an analysis of variance and Tukey's range test (P ≤ 0.05), using Statistical Analysis System software (SAS, 2002).

Results and discussion

The analysis of variance shows that there were significant effects among varieties, between blunting levels and among population densities for all yield variables and its components, as well as plant height and leaf area index. In addition, there was interaction between variety and blunting level for fruit weight (Figure 1).

Figure 1 Interaction between varieties and blunting level for average fruit weight. 

In the average of blunting levels and population densities, it was observed that the varieties with ball-type fruit yielded significantly more (about 37 %) than the saladette ones (Table 1). The difference was that the ball-type fruits weighed on average 70 g more than the saladette ones, while in the number of fruits per unit area they were similar.

Although saladette tomato is easier to market in Mexico than the beef type, the average annual price it reaches on the domestic market is usually 30 % higher (^{Sistema Nacional de Información e Integración de Mercados [SNIIM], 2016}). Therefore, growing ball-type varieties may be a better option for the country’s small-scale producers. Nevertheless, the saladette type, by customs and traditions, is more used in the preparation of typical Mexican dishes.

As expected, due to the genetic characteristics of the materials, the indeterminate-growth varieties ('Imperial' and 'Moctezuma') reached a higher plant height than the determinate ones. In leaf area index, 'Imperial' statistically outperformed ‘Serengueti’ (Table 1).

Among beef varieties, 'Imperial' was statistically superior in yield to 'Pick ripe' (determinate growth), as it formed more fruits per unit area. Therefore, from the commercial point of view, it is more advisable to use the 'Imperial' variety, which on average yielded 29.6 kg∙m^-2 of cultivated area (equivalent to 19.7 kg∙m^-2 of greenhouse area or 197 t∙ha^-1) in a cycle, from transplant to final harvest, of 100 days, under the established management system.

As for saladette varieties, 'Moctezuma' yielded significantly more (21.6 kg∙m^-2) than 'Serengueti' (17.6 kg∙m^-2), also as a result of a higher number of fruits harvested, since in fruit weight there was no difference. Plant height and leaf area index were also significantly lower for 'Serengueti', which may explain, in part, its lower number of fruits per unit area and consequently its lower yield. ^{Gardner, Pearce, and Mitchel (1990)} and ^{Taiz and Zeiger (2006)} indicate that leaf area per plant is correlated with the photosynthetically active radiation (PAR) interception percentage and therefore with the greater formation of photoasimilates.

The differences found in yield, weight and number of fruits among varieties are probably defined by genetic traits such as fruit type, growth habit, leaf area per plant, and photosynthetic efficiency, among others (^{Grandillo, Zamir, & Tanksley, 1999}; ^{Monamodi, Lungo, & Fite, 2013}; ^{Sánchez-del Castillo, Ortiz-Cereceres, Mendoza-Castillo, González-Hernández, & Colinas-León, 1999}).

In the general average of varieties and population densities (Table 2), yield per unit area between blunting to two and three clusters per plant was statistically the same.

With blunting to leave two clusters per plant, more fruits per unit area were obtained than with the plants blunted to three clusters, but average fruit weight decreased. A possible explanation of the difference observed based on the pruning level is that, according to the dates on which the experiment was carried out (July to December 2012), several cloudy days were recorded during the growth period of the first two clusters, while the solar radiation conditions were more favorable for the growth of the third-cluster fruits.

According to information from the Universidad Autónoma Chapingo weather station, during the logarithmic growth phase of the fruits of the first cluster (between October 16 and November 14) there were 253 hours of sunshine, for the second cluster (between October 26 and November 24) 237 hours and for the third cluster (between 5 November and 4 December) 299 hours. This resulted in greater fruit weight, especially for those beef-type ones of the third cluster.

^{Heuvelink and Dorais (2005)} and ^{Ho and Hewitt (1986)} mention that the growth and size of tomato fruits depends, in addition to their genetic determination (cherry, beef, saladette), on environmental factors, with the daily incident PAR integral and temperature being the main ones in their rapid growth stage. On the other hand, in leaf area index there were no differences between blunting levels (Table 2).

Between the beef tomato varieties, 'Imperial' was superior in yield and number of fruits per unit area to 'Pick ripe', and between the saladette ones, 'Moctezuma' outperformed 'Serengueti' in the same variables. Therefore, only the population density results within each blunting level for the variety of each type with higher yield are presented.

Conclusions

The beef-type varieties yielded on average 37 % more than the saladette ones, having fruits of greater weight. Between varieties with the same type of fruit, 'Imperial' yielded more than 'Pick ripe' and 'Moctezuma' surpassed 'Serengueti'; that is, the indeterminate varieties, within each fruit type, yielded more than the determinate ones. In both cases the component that most influenced yield was the number of fruits harvested per unit area. The highest yield of beef-type 'Imperial' and saladette-type 'Moctezuma' was achieved by establishing a population density of 13 plants∙m^-2 and with plants tipped to the third inflorescence.