Introduction

Agriculture in Mexico faces constant challenges that limit its productivity and generate economic uncertainty, including rugged topography, scarcity and random rainfall, low temperatures, and frosts. These factors have driven the adoption of technologies such as the use of greenhouses and hydroponic systems, which offer greater security and profitability to horticultural producers (Sánchez-del Castillo & Moreno-Pérez, 2017). The implementation of these technologies in high-value crops allows for considerable yields and profits, in addition to generating 10 full-time jobs per hectare, which contributes to regional economic development (Pratt & Ortega, 2019).

Among the vegetables grown under protected agriculture, the tomato (Solanum lycopersicum L.) stands out for its high demand in national and international markets, as well as for its high yield and quality under intensive production systems. In Mexico, it is the most important vegetable in terms of surface area, economic value, and volume managed in greenhouses and hydroponic systems (Costa & Heuvelink, 2018; Agri-Food and Fisheries Information Service [SIAP], 2020). In 2018, 1.68 million tons of tomatoes were exported, representing 48.7 % of national production (estimated at just over 3 million tons), of which 99.7 % was destined for the United States market (Trusts Instituted in Relation to Agriculture [FIRA], 2019).

In response to the need to improve profitability for small and medium-sized producers with limited market access, an alternative greenhouse hydroponic tomato production system has been commercially validated. This system, without increasing production costs per ton, allows the annual yield to increase by 50 % compared to the conventional system, which translates into greater economic benefits (Sánchez-del Castillo & Moreno-Pérez, 2017; Sánchez-del Castillo et al., 2010, 2012).

The proposed alternative system (Figure 1) is characterized by two cultivation stages: the initial seedling stage in the nursery and the production stage (from transplant to harvest). This scheme allows for up to three cultivation cycles per year, in contrast to the single cycle of the conventional system. To achieve this, cultural practices have been implemented that delay transplanting until 45-50 days after sowing (das) and reduce the transplant-to-harvest period to 100-110 days by topping (removing the terminal bud) above the third inflorescence and pruning lateral shoots to leave a single stem. This practice limits each plant to producing only three clusters. Although this reduces the yield per plant, the smaller leaf area facilitates the increase in population density up to 8 plants∙m^-2, which partially compensates for this reduction. Furthermore, shortening the production stage to less than 110 days allows for three crop cycles per year (Sánchez-del Castillo et al., 2012, 2017) with a potential annual yield close to 450 t∙ha^-1 (Moreno-Pérez et al., 2022; Sánchez-del Castillo et al., 2021).

Due to the high population density required by this system, the demand for seeds, germination trays, and substrate for germination and seedling development increases with each cycle. However, production costs per cycle have been, on average, lower than those of the traditional system of one cycle per year, such that costs per kilogram produced remain similar, which allows for greater economic profit per year (Sánchez-del Castillo et al., 2012, 2021).

Although the system of three clusters per plant and three cycles per year has proven to be highly productive, there is still room for improvement in crop efficiency. Reducing the length of the production stage would allow for more cycles per year, which would increase annual productivity. In this regard, the early topping of plants to limit them to one or two clusters and proportionally increase population density has been evaluated. However, this strategy has not proven practical, as it increases production costs per cycle (mainly due to the high cost of improved hybrid seed) and reduces yield per plant (Sánchez-del Castillo et al., 2017; Sánchez-del Castillo & Ponce-Ocampo, 1998).

In this sense, as mentioned in the article by Cabañas-Díaz et al. (2025), a tomato production system has been proposed that incorporates three cultivation stages: seedbed, high-density intermediate, and production (Figure 2), which allows increasing the number of production cycles per year. However, by prolonging the duration of the intermediate stage under high-density conditions (18 plants∙m^-2), the leaf area per plant and, consequently, the leaf area index (LAI) can reach such high values that they would cause competition stress, thereby decreasing growth and yield. According to Heuvelink et al. (2018), Mendoza-Pérez et al. (2022), and Won and Jong (2020), the optimal LAI to maximize tomato production is in the range of 3 to 4.

Considering the aforementioned, the objective of this study was to determine the optimal duration of the intermediate stage at high population density without negatively affecting growth and yield, as well as to estimate the potential number of crop cycles per year based on the duration of the production stage, under a hypothetical scenario of optimal climate or high-tech climate-controlled greenhouses.

Materials and methods

The experiment was set up at the facilities of the Horticulture Institute of the Autonomous University of Chapingo, Mexico (19° 29’ 35’’ N and 98° 52’ 19’’ W, at 2 250 m a. s. l.), under greenhouse and hydroponic conditions for 140 days from sowing to harvest (March to July 2023). A chapel-type greenhouse with a metal structure and a high-light-scattering thermal polyethylene cover, oriented north to south, was used. The climate control system included LP gas heating, a wet-wall cooling system, and ventilation with exhaust fans located on the opposite wall, maintaining a temperature between 20 and 30 °C during the day and between 10 and 20 °C at night. Relative humidity ranged from 50-80 %. The soil was covered with white polypropylene ground cover. The three growing stages (seedbed, high-density intermediate, and final production) took place in different spaces within the same greenhouse.

The commercial tomato hybrid 'Condor' was used, with a saladette-type fruit and an indeterminate growth habit, suitable for greenhouse cultivation. The seeds were sown in 60-well polystyrene trays, with a volume of 200 mL per well and spaced 5 cm apart. Before sowing, the trays were washed and disinfected.

The germination substrate consisted of a 1:1 (v/v) mixture of peat moss (Cosmopeat®, pH = 5.0-6.5 and electrical conductivity [EC] = 0.2-0.8 dS∙m^-1) and perlite (Multiperl). The seeds were covered with a layer of vermiculite approximately 0.5 cm thick. The trays were placed on metal frames raised 50 cm above the ground.

The seedlings were irrigated, one to two times a day, with a 50 % concentration nutrient solution for the first 40 das. Subsequently, the 100 % nutrient solution was applied, which contained the following nutrient concentrations (mg∙L^-1): N = 200, P = 60, K = 250, Ca = 250, Mg = 50, S = 150, Fe = 3, Mn = 1, B = 0.5, Cu = 0.2, and Zn = 0.2. The nutrient sources used were calcium nitrate, potassium sulfate, 85 % phosphoric acid, magnesium sulfate, ferrous sulfate, manganese sulfate, boric acid, copper sulfate, and zinc sulfate. The pH of the solution was adjusted between 5.5 and 6.5 with 10 % dilute sulfuric acid, and the water for the solution had an EC of 0.3 dS∙m^-1. The resulting EC of the nutrient solution ranged between 2.2 and 2.5 dS∙m^-1.

At 40 das, seedlings were transplanted into 20 cm diameter and 21 cm high pots (6.8 L) to begin the intermediate stage of high-density cultivation (18 plants∙m^-2). Volcanic sand (red tezontle) with particles between 1 and 3 mm in diameter was used as a substrate. Each plant was supported by a 1.2 m long wire rod stake with plastic rings. The nutrient solution was prepared in a cistern and distributed using drip tapes (Toro® cal. 8000) with a flow rate of 3 800 mL∙h^-1 per dripper. The spacing between drippers was 20 cm during the intermediate stage and 30 cm during the production stage. Irrigation was adjusted daily to ensure 20 % drainage relative to the volume of solution applied at each irrigation. Between four and eight daily irrigations were provided according to the weather conditions and the phenological stage of the crop.

The plants were topped (removed from the terminal bud) at 75 days of age, leaving two leaves above the third inflorescence. The crop was grown on a single stem per plant. Harvesting began at 120 days of age, when the fruits turned pink, and successive cuts were made over a period of 20 days.

Six treatments were evaluated, all with a 40-day seedling stage in the seedbed; the seedlings were subsequently transplanted to begin the intermediate stage at high density (18 plants∙m^-2), except for the control (T1), which was transplanted directly to the production stage at a density of 6 plants∙m^-2. The remaining treatments consisted of different durations (days) of the intermediate and production stages: 25 and 75 (T2), 30 and 70 (T3), 35 and 65 (T4), 40 and 60 (T5), and 45 and 55 (T6), respectively. To initiate the production stage, the intermediate-stage plants (T2-T6) were relocated with their pot and tutored to establish a density of 6 plants∙m^-2. They remained in this area until the end of the crop cycle. The experimental design was randomized complete blocks with four replications and 18 plants per experimental unit.

At 85 das, when plants in all treatments were in production, growth variables were recorded, and at the end of the production cycle, yield and its main components were quantified. The growth variables analyzed were:

• 1.

  Plant height (cm): measured with a tape measure from the base of the stem to the apex.

• 2.

  Stem diameter (mm): measured with an electronic vernier caliper (model 14388, Truper®, China) between the fourth and fifth node of the main stem.

• 3.

  Leaf area per plant (cm²): This was determined at 40 days (beginning of the intermediate stage) and every five days from 65 to 85 days using indirect estimation. Initially, destructive sampling of two plants per treatment was carried out, measuring the length and width of each leaf and examining them with a leaf area integrator (LI-3000A, LI-COR®, USA). A linear regression analysis was performed with the data obtained, resulting in the following equation:

• 4.

  Estimated leaf area   = 0.2889x + 10.813                                               R 2   = 0.9477

• 5.

  where x corresponds to the product of leaf length × width. The leaf area per plant corresponded to the sum of the estimated areas of all leaves.

• 6.

  Leaf area index (LAI, m²∙m^-2): obtained by dividing the total leaf area of the plants (m² of leaves) by the surface area of the experimental unit.

The yield variables analyzed were:

• 1.

  Number of fruits per unit area (fruits∙m^-2): calculated from the number of fruits per plant × the population density at the production stage (6 plants∙m²).

• 2.

  Average fruit weight (g): obtained by dividing the total weight of the fruit per unit of surface (g∙m^-2 of greenhouse) by the number of fruits harvested on that surface (fruits∙m^-2 of greenhouse).

• 3.

  Yield per cycle (kg∙m^-2): calculated by multiplying the yield per plant × the population density of each treatment at the production stage.

The data obtained were subjected to analysis of variance and comparison of means tests (Tukey, α = 0.05). The analyses were performed using the SAS Institute statistical program (2002).

Results and discussion

At 85 das, plants in the control treatment showed lower height and greater stem diameter compared to the treatments that included three stages, except for the treatment with 25 days in the intermediate stage, which showed a stem diameter similar to the control (Table 1). This is explained by the temporal effect of competition for light among plants caused by the high population density in the intermediate stage. According to Jishi (2018) and Martínez-García et al. (2014), competition for light can lead to “shade avoidance syndrome,” characterized by an increase in stem height and a reduction in stem thickness. This occurs because the production of photoassimilates decreases, and a greater proportion of them is used for stem elongation. Furthermore, the synthesis of auxins and gibberellins is stimulated, which induces cell elongation (Bhatla, 2018; Kalra & Bhatla, 2018).

Table 1. Comparison of means of the effect of the duration of the intermediate stage on the growth of tomato plants at 85 days after sowing.

Leaf area per plant, and therefore LAI, were similar in all treatments at the beginning of the production stage. This was due to the topping of the plants at 75 das, which prevented the formation of new leaves. The remaining leaves reached their maximum size before 85 days of age, the date on which the measurements were taken. At the time of transplanting to initiate the intermediate stage at 40 days of growth, plants had a relatively small leaf area, with a population density of 18 plants∙m^-2 and a LAI of 0.5 m²∙m^-2, so competition for light had not yet begun. However, as the intermediate stage prolonged, the LAI increased rapidly, reaching values above 9 m²∙m^-2 in the 40-day intermediate stage treatment (Figure 3). This increase intensified competition for light and, consequently, reduced yield in treatments with a longer duration of the intermediate stage.

When a crop's LAI exceeds a certain threshold, plants do not receive sufficient incident solar radiation (Mendoza-Pérez et al., 2022). Under these conditions, the lower leaves of the canopy receive less light, so they consume more photoassimilates than they can produce. This results in a reduction in the absolute growth rate (daily biomass accumulation per plant) (Heuvelink et al., 2018, 2020). When a crop's LAI exceeds a certain threshold, plants do not receive sufficient incident solar radiation (Mendoza-Pérez et al., 2022). Under these conditions, the lower leaves of the canopy receive less light, so they consume more photoassimilates than they can produce. This results in a reduction in the absolute growth rate (daily biomass accumulation per plant) (Heuvelink et al., 2018, 2020).

The highest yield in a complete production cycle was obtained with the control (without intermediate stage), as well as in the treatments with 25 and 75 days, and with 30 and 70 days in the intermediate and production stages, respectively (Table 2), even with the competition for light produced by the high population density during the intermediate stage. In contrast, treatments with longer durations of the intermediate stage significantly and progressively reduced fruit yield. The 45-day treatment at the intermediate stage reduced fruit yield by 33 % compared to the 25 and 30-day treatments (Table 2).

Table 2. Comparison of means of the effect of the duration of the intermediate stage on yield and its components in tomato plants.

The three-cluster, two-stage system, considered the control, had a yield duration of 105 days from transplant to final harvest. Meanwhile, the 25- and 30-day intermediate-stage treatments (75 and 70 days in production, respectively) showed similar yield and duration. This suggests the possibility of achieving more crop cycles per year with higher yields and, possibly, greater profitability. Therefore, it is necessary to evaluate the economic potential of these production systems.

The short-cycle system proposed in this research could be economically viable in locations with extreme climates like Sinaloa, where tomatoes can only be grown in greenhouses for seven months of the year. During this period, the conventional system produces an average of 140 t∙ha^-1 (FIRA, 2019). With proper planning, the three-stage system would allow for up to three crop cycles in seven months, with similar yields per cycle.

Regarding fruit number, a tendency toward higher values was observed in plants without an intermediate stage (control) or with an intermediate stage limited to 25 or 30 days. Likewise, average fruit weight increased significantly in these three treatments (Table 2). According to these results, the most appropriate tomato crop management would be a 40-day initial seedling stage, a 30-day intermediate stage, and a 70-day production stage. This scheme achieves a yield similar to the control (9.8 versus 9.5 kg∙m^-2 per cycle), although with the possibility of conducting up to five cycles per year.

Several authors have reported higher tomato fruit yields than those obtained in this study. Sánchez-del Castillo et al. (2017) reported yields of 15 to 18 kg∙m^-2 per cycle with the cultivar ‘Moctezuma’ trained to three clusters in two stages (seedbed and production) and a density of 7 plants∙m^-2. Moreno-Pérez et al. (2022) obtained 13 kg∙m^-2 with the cultivar ‘El Cid’ managed with 8 plants∙m^-2. In both cases, beds with tezontle sand with a capacity of 25 to 30 L of substrate per plant were used as containers. In contrast, in the present study, the population density during the production stage was limited to 6 plants∙m^-2 and the substrate volume to 6.8 L per plant, conditions that probably limited yield.

According to Heuvelink et al. (2018, 2020) and Won and Jong (2020), the highest yield can be achieved with an LAI close to 4, so it would be convenient to explore the agronomic behavior with population densities higher than 6 plants∙m^-2, without exceeding 18 plants∙m^-2 in the intermediate stage. Likewise, it is advisable to explore the use of bags or pots with a greater volume of substrate per plant, since this favors radical development and the availability of oxygen, in addition to improving the retention of the nutrient solution and reducing oscillations in pH and electrical conductivity (Suazo-López et al., 2014).

To optimize this production system, it is recommended to evaluate different population densities at the production stage along with larger container volumes.

Conclusions

The prolongation of the intermediate stage at high plant density was accompanied by a higher leaf area index, taller plants, and thinner stems, resulting in a reduction in yield per unit area due to a lower number of fruits per plant. Among the treatments evaluated, the 30-day intermediate stage and 70-day production treatment showed the best agronomic performance, with a yield of 9.5 kg∙m^-2.