With the evolution of agriculture, technologies such as greenhouses and hydroponic systems have been created, which, when well managed, allow for increasing the productivity of almost any crop, and there are cases in which production exceeds ten times that of the same crop in the open field (Resh, 2013). However, due to the high installation and operating costs, the economic profitability of these systems must consider efficient space and time management to maximize the yield and quality of the products obtained (Sánchez-del Castillo & Moreno-Pérez, 2017).
In Mexico, tomato (Solanum lycopersicum L.) production grew at an average annual rate of 3.6 % between 2007 and 2017. In that same period, the area cultivated in open fields decreased at an average annual rate of 5.9 %, from 64,663 to 35,175 ha, while with protected agriculture (shade nets and greenhouses) it went from 1,973 to 15,198 ha, corresponding to an average annual growth rate of 22.7 %. Thus, the production obtained with the use of these technologies went from 0.9 % in 2003, to 32.2 % in 2010, and to 63.3 % of the total volume in 2017 (Servicio de Información Agroalimentaria y Pesquera (SIAP), 2018).
Conventional greenhouse tomato management consists of establishing only 3 plants·m-2 and using indeterminate varieties that are allowed to grow more than 7 m in length to harvest 20 to 25 clusters per plant in a single growing cycle per year (Castellanos & Borbón, 2009). In general, the system is used in high-cost, medium to high-tech greenhouses, and requires strict technical management due to cultural practices such as lowering plants, maintaining a constant leaf area index and having the required phytosanitary control, all this derived from the long cycle that is managed (Sánchez-del Castillo, Moreno-Pérez, & Contreras-Magaña, 2012; Sánchez-del Castillo & Moreno-Pérez, 2017).
An alternative management system to the one mentioned above was developed at the Universidad Autónoma Chapingo. This system consists of the early blunting of the apex of the plants to leave only three clusters in each plant, and the transplanting of seedlings at 40 days after sowing (das), which shortens the cycle from transplanting to end of harvest to approximately four months, making it possible to obtain three growing cycles per year. The lower yield per plant is partially compensated by increasing the population density to 8 or 9 plants·m-2. With this relatively simple technology, higher annual yields can be obtained than with the conventional system (Sánchez-del Castillo et al., 2014), by obtaining around 16 kg·m-2 per growing cycle, with the potential for almost 500 t·ha-1·yr-1 by establishing three cycles (Sánchez-del Castillo et al., 2012). Although this yield is high, it is possible to significantly increase it through two strategies: 1) make the transplant with older seedlings (50 to 60 days) and thereby shorten the time from transplant to end of harvest to less than 90 days, which would make it possible to achieve four cycles per year and a 25 % higher annual yield, and 2) promote a greater number of flowers and fruits in each inflorescence without reducing the average weight per fruit. Due to the high population density managed (8 plants·m-2 in greenhouse), with one more fruit per cluster and four growing cycles per year, a potential increase in annual yield of 10 kg·m-2 (100 t·ha-1) could be obtained.
The technical feasibility of transplanting at 60 das has already been experimentally verified with 500 to 750 mL containers (pots) (López-Valencia, Sánchez-del Castillo, & Contreras-Magaña, 2002; Sánchez-del Castillo et al., 2012). However, at the commercial level, this size of container takes up a lot of space in the seedbed and the cost of labor for transplanting is high, due to handling high population densities and seedlings with very large root balls that make this work difficult and costly.
Regarding the formation of more flowers and fruits per inflorescence, Heuvelink, Li, and Dorais (2018) note that, although this trait has a genetic component, this increase can be promoted through temporary modifications of the environment or with the management of source-demand relationships, this by achieving a greater distribution of sugars towards the meristem at the time that floral differentiation is taking place. With greater spacing between seedlings (due to a lower population density), each seedling is expected to receive more photosynthetically active radiation and increase its rate of photosynthesis, which would leave more sugar available for the developing flower primordia and lead to more flowers per inflorescence (Heuvelink & Okello, 2018).
It should be noted that available space and use time in the seedbed are important factors in production efficiency, so the number of seedlings per unit area is normally increased by using trays with a greater number of cavities and therefore lower individual volume. However, the consequence is that seedlings established at a higher density, as they remain longer in the seedbed, compete more for the incident solar radiation and lose quality, since etiolated seedlings are formed (Wien, 1999) that are weak and susceptible to disease during or after transplantation (Kozai, 2016). On the contrary, using higher-volume trays increases the number of roots formed, which favors greater absorption of water and nutrients (Urrestarazu, 2015; Wien, 1999). A larger container volume, combined with a larger space between seedlings, favors both the root and aerial environments, factors that can increase the final yield (Heuvelink & Okello, 2018; Sánchez-del Castillo et al., 2012).
Therefore, this research aimed to evaluate the effect of container volume and population density on the quality of tomato seedlings to be transplanted at 60 das, as well as the number of flowers, number of fruits and yield per plant. The purpose is to contribute to the development of a practical and low-cost method to shorten the cycle from transplanting to end of harvest to less than 90 days, leaving three clusters in each plant.
Materials and methods
The study was carried out under greenhouse and hydroponic conditions at the Universidad Autónoma Chapingo experimental field (19° 29’ 35” NL and 98° 52’ 21” WL, at 2250 m a. s. l.).
The greenhouses used, both in the seedbed and production phases, were covered with high light dispersion thermal polyethylene. The temperature during the day was kept between 15 and 25 °C, and at night between 10 and 16 °C, with relative humidity of 60 and 80 % during the day and night, respectively, this by the opening and closing of windows with polyethylene curtains and anti-aphid mesh, as well as with a heating system. In addition, the seedbed had evaporative cooling through a wet pad with extractors.
The saladette-type 'Bullseye' tomato hybrid (from the Seminis company) with a semi-determinate growth habit was used. Sowing was done in July 2018 in polystyrene trays filled with a mixture of peat-moss, perlite and fine tezontle sand as substrate at a 1:1:1 ratio. From sowing to the first 10 days, the seedlings were irrigated only with water. For the next 15 days, irrigation was done with a nutrient solution containing the following nutrients (mg·L-1): nitrogen (100), phosphorus (25), potassium (100), calcium (125), magnesium (25), sulfur (75), iron (2), manganese (1), boron (0.5), copper (0.1) and zinc (0.1). Subsequently, and until the end of harvest, double the concentration of each macronutrient and the same concentration of micronutrients (complete nutrient solution) were applied.
The seedlings were transplanted into culture beds filled with a 25 cm layer of red tezontle sand, with grain size from 1 to 3 mm in diameter. The width of the beds was 1 m, with 33 cm spacing between rows and 25 cm between plants, resulting in a population density of 12 plants·m-2 of useful space (8 plants·m-2 of greenhouse if 0.5 m wide paths are considered). Before transplanting, a drip irrigation system was installed in each bed, which consisted of strips with integrated drippers every 20 cm to provide irrigation with complete nutrient solution. Depending on environmental conditions, five to seven daily irrigations were given at a dose of 1 L·m-2 per irrigation.
After transplanting, the plants were tutored by holding them from the stem with a plastic ring that supported a raffia cord tied to wires placed along the beds at 1.5 m high. From 60 das onwards, the side shoots of the plants were pruned to leave them at a single stem. Between 70 and 80 das, each plant’s three lower leaves were removed, and at 80 das, with the third inflorescence already formed, the plants were blunted (removal of the apical bud) to leave only two leaves above the third cluster. Pollination was done manually; to do this, every day during anthesis the plants were shaken to release the pollen grain and cause its deposit on the stigma.
Eight treatments were evaluated, with the first four corresponding to seedlings handled in 200-cavity polystyrene trays with 25 mL capacity per cavity and transplanted at 35 das; the other four were in 60-cavity polystyrene trays with 250 mL capacity per cavity and transplanted at 60 das. Within each container volume, four population densities were handled, so that the treatments were as follows: 1) 1,000 seedlings·m-2 in 200-cavity trays, 2) 750 seedlings·m-2 in 200-cavity trays, 3) 500 seedlings·m-2 in 200-cavity trays, 4) 250 seedlings·m-2 in 200-cavity trays, 5) 300 seedlings·m-2 in 60-cavity trays, 6) 200 seedlings·m-2 in 60-cavity trays, 7) 150 seedlings·m-2 in 60-cavity trays and 8) 75 seedlings·m-2 in 60-cavity trays.
A split-plot randomized block experimental design with four replicates was used. In the large plot the number of cavities per tray (200 or 60, corresponding to 25 or 250 mL per cavity, respectively), and in the small plots the population density treatments (250, 500, 750 and 1,000 plants·m-2 for 200-cavity trays, and 75, 150, 200 and 300 seedlings·m-2 for 60-cavity trays). The size of the small plot experimental unit was 15 seedlings.
The following variables were evaluated at 30, 45 and 60 das:
Seedling height (cm): it was measured from the base of the plant to the apical meristem with a tape measure.
Stem diameter (mm): it was measured at the height of the internode of the third and fourth leaves with an electronic Vernier caliper.
Leaf area per seedling (cm2): it was determined with a leaf area meter (LI-3100, LI-COR®, USA).
Total dry weight per seedling (g): it was obtained from the same seedlings used to obtain leaf area; to do this, the seedlings were placed in paper bags and dried in an oven at 70 °C until constant weight.
Leaf area ratio (cm2 leaf area·g-1 dry weight): it was estimated as the leaf area formed per gram of total dry weight of the seedling.
Yield variables and components were also determined: number of flowers per plant, number of fruits harvested per plant, fruit weight (g) and yield per plant (g·plant-1).
The data obtained were subjected to analysis of variance and Tukey’s comparison of means test (P ≤ 0.05), for which SAS version 9.1 statistical software (SAS Institute, 2002) was used.
Results and discussion
Morphological variables and dry weight in seedlings
The analysis of variance (data not shown) indicated that at 30 and 45 das there were highly significant differences between cavity volumes in all morphological variables and dry weight, while at 60 das there were highly significant differences for leaf area and leaf area ratio, and significant differences for seedling height and stem diameter, but not for seedling dry weight.
The comparison of means tests (Table 1) show that at 30 das the seedlings grown in trays with 250 mL cavities had 1.41 cm more height, 0.88 mm thicker stems, a 140 % increase in leaf area, double the dry weight and 15 cm2 more leaf area for each g of total dry weight formed compared to the seedlings produced in trays with 25 mL cavities; these differences were significant. At 45 das, the differences between the two treatments were further accentuated: seedling height in 250 mL containers was 13.8 cm greater, stem diameter was 1.42 mm thicker, leaf area was 96 cm2 greater and they had double the dry weight; only the leaf area ratio was 13.1 cm2 lower.
|Container volume (mL)||Height (cm)||Stem diameter (mm)||Leaf area (cm2)||Dry weight (g)||Leaf area ratio (cm2·g-1)|
|25||7.15 bz||2.55 b||24.48 b||0.42 b||55.9 b|
|250||8.56 a||3.43 a||58.83 a||0.84 a||70.7 a|
|25||18.60 b||4.38 b||123.02 b||0.88 b||140.0 a|
|250||32.45 a||5.80 a||219.40 a||1.79 a||126.9 b|
|25||42.56 b||5.81 b||628.67 a||5.62 a||111.7 a|
|250||47.90 a||6.26 a||461.71 b||5.41 a||86.8 b|
The results obtained, both at 30 and 45 das, can be explained because in the treatments with a higher container volume (250 mL per cavity), during the first 35 das (when the seedlings were transplanted into 25 mL cavities), the root grew with fewer water, oxygen and nutrient limitations than the seedlings that grew in 25 mL of substrate, as also pointed out by Ruff, Krizek, Mirecki, and Inouye (1987) and Wien (1999) in tomato cultivation. According to Sakurai, Ogawa, Kawashima, and Chino (2007), the relatively low densities with which the seedlings were handled in 250 mL cavities (between 75 and 300 seedlings·m2) also had an impact by allowing for greater interception of solar radiation per plant. This resulted in a higher rate of photoassimilate production and, consequently, of accumulated dry matter (dry weight) for greater growth and development of the seedlings compared to those placed in 25 mL trays at higher population densities (between 250 and 1,000 seedlings·m-2).
At 60 das, seedlings grown in 60-cavity trays, with a container volume of 250 mL, were also taller (5.3 cm) and had a larger stem diameter (0.45 cm) compared to plants grown in 200-cavity trays, despite the fact that the latter had been transplanted 25 days earlier and were therefore in a less limited environment both in the aerial part and in the root. On the contrary, the leaf area per plant in the 60-cavity trays was 167 cm2 smaller, but no statistical differences were observed in weight. With the highest container volume, the leaf area ratio remained significantly lower at 25 cm2 (Table 1).
Regarding the population densities in each cavity volume, the analyses of variance (data not shown) indicate that, at 30 das, highly significant differences were found for seedling height, leaf area and leaf area ratio, but not for stem diameter and seedling dry weight. At 45 das, there were highly significant differences in seedling height, and with the 250 mL container there were significant differences in leaf area ratio. At 60 das, only highly significant differences in seedling height were detected.
Table 2 shows the comparisons of means of the population density treatments (1,000, 750, 500 and 250 seedlings·m-2) that were evaluated in 200-cavity trays with 25 mL capacity per cavity and that were transplanted at 35 das.
|Density (seedlings·m-2)||Height (cm)||Stem diameter (mm)||Leaf area (cm2)||Dry weight (g)||Leaf area ratio (cm2·g-1)|
|1,000||7.95 az||2.77 a||34.25 a||0.52 a||63.7 a|
|750||7.70 a||2.52 a||22.30 ab||0.35 a||62.1 ab|
|500||7.50 a||2.55 a||24.25 ab||0.45 a||52.4 bc|
|250||5.47 b||2.35 a||17.15 b||0.37 a||45.2 c|
|1,000||20.82 a||4.32 a||134.05 a||0.90 a||146.75 a|
|750||18.82 b||4.45 a||116.40 a||0.80 a||144.75 a|
|500||19.50 ab||4.60 a||125.23 a||0.92 a||135.50 a|
|250||15.27 c||4.17 a||116.40 a||0.90 a||133.00 a|
|1,000||45.2 a||5.65 a||696.4 a||6.12 a||114 a|
|750||42.9 ab||5.62 a||673.3 a||6.12 a||110 a|
|500||44.5 ab||5.97 a||573.3 a||5.27 a||109 a|
|250||37.7 b||6.02 a||571.8 a||4.97 a||114 a|
Table 2 shows that, at 30 das, seedling height with the lowest density (250 seedlings·m-2) was significantly lower than the control (1,000 seedlings·m-2); the same occurred with leaf area and leaf area ratio. No statistical differences were detected in stem diameter and dry weight. At 45 and 60 das, the differences in height between these treatments were more notable and statistically significant (5.5 cm difference between the two treatments at 45 das, and 7.5 cm difference at 60 das). These results suggest that with high densities in the seedbed there is competition for light, which favors seedling elongation (Higuchi, Sumitomo, Oda, Shimizu, & Hisamatsu, 2012). Therefore, the treatment with a lower population density (250 seedlings·m-2) reduced the effect of competition for light, which was manifested as a decrease in seedling height.
From this, it can be inferred that the lower population density generates a pattern of light quality that allows the lower leaves of the canopy, which are usually the most disadvantaged by natural radiation, to also receive sufficient light (Jishi, 2018). In addition, it must be considered that the light inside the canopy is richer in red and blue, colors that when predominant reduce elongation (Tewolde et al., 2018). According to Taiz, Zeiger, Møller, and Murphy (2015), from a certain population density threshold, competition for light causes symptoms of etiolation, with stem elongation being the most visible.
The other variables evaluated did not show statistical differences between densities at 45 and 60 das.
The comparison of means of the population densities of seedlings grown in 60-cavity trays with 250 mL capacity per cavity (Table 3) shows that at 30 das the seedlings with the highest density (300 seedlings·m-2) were significantly taller compared to the rest of the densities, where the height decreased as the seedling density was reduced. Significant decreases were also found in leaf area and leaf area ratio as population density was reduced. Stem diameter and plant dry weight did not show significant differences.
|Density (seedlings·m-2)||Height (cm)||Stem diameter (mm)||Leaf area (cm2)||Dry weight (g)||Leaf area ratio (cm2·g-1)|
|300||10.57 az||3.77 a||59.47 ab||0.82 a||70.4 ab|
|200||8.77 b||3.47 a||72.40 a||0.87 a||85.4 a|
|150||7.52 c||3.30 a||52.57 b||0.80 a||66.9 b|
|75||7.37 c||3.17 a||50.90 b||0.87 a||60.1 b|
|300||44.47 a||6.05 a||230.60 a||1.52 a||158 a|
|200||35.90 b||5.95 a||239.35 a||1.82 a||130 ab|
|150||28.67 c||5.60 a||215.48 a||1.80 a||123 bc|
|75||20.75 d||5.62 a||192.18 a||2.02 a||96.5 c|
|300||58.1 a||5.77 a||428.00 a||4.85 a||91.1 a|
|200||54.8 a||6.37 a||541.28 a||5.90 a||93.2 a|
|150||42.1 b||6.42 a||457.05 a||5.12 a||90.0 a|
|75||36.6 b||6.50 a||420.53 a||5.77 a||73.1 a|
At 45 das, seedling height at the highest density (300 seedlings·m-2) increased 24 cm with respect to the lowest density (75 seedlings·m-2). Likewise, leaf area ratio decreased significantly with decreasing density, but in the variables stem diameter, leaf area and dry weight, no statistical differences were found among densities. At 60 das, there were also significant differences in height between the highest and lowest density treatments, while no statistical differences were observed in the rest of the variables. The differences in height are explained by increased seedling competition for light as population density increases, a competition that is accentuated in the late seedbed stages and that causes elongation (Kozai, 2016; Wien, 1999).
All treatments in 60-cavity trays with 250 mL capacity per cavity allowed transplanting up to 60 das, an essential requirement to achieve four growing cycles per year instead of the three that can be achieved with younger transplants (Sánchez-del Castillo et al., 2012). However, the transplant work was more complicated with the seedlings that were handled at high density (300 and 200 seedlings·m-2), because they had greater height (58.1 and 54.8 cm, respectively) and long internodes, so they required immediate tutoring to avoid physical damage to the seedlings. This did not occur in the seedlings grown at the lowest density (75 seedlings seedlings·m-2), which had a transplant height of 36.6 cm. With the density of 150 seedlings·m-2, seedling height at 60 das was 5.5 cm more than with 75 seedlings·m-2, with no statistical difference recorded. In addition, with 150 seedlings·m-2, 50 % less space is taken up in the seedbed, so this density can also be recommended to achieve four growing cycles per year without affecting seedling quality at transplanting.
Yield variables and components
According to the analyses of variance (data not shown) and comparisons of means, yield variables and components (number of flowers, number of fruits and weight per fruit) were not statistically different between treatments in terms of the cavity volumes studied (Table 4). This indicates that extending transplanting to 60 das in 60-cavity trays with 250 mL capacity per cavity allows shortening the cycle from transplanting to end of harvest to less than 90 days, so that four growing cycles per year can be obtained with tomato plants managed at three clusters without affecting fruit weight and yield per plant.
|Container volume (mL)||Number of flowers per plant||Number of fruits per plant||Fruit weight (g)||Yield (g·plant-1)|
|25||19 az||18 a||124 a||2249 a|
|250||19 a||17 a||127 a||2200 a|
Normally, if seedlings are grown in trays with 25 mL cavities and are transplanted at 35 das, there is a cycle from transplanting to end of harvest of between 110 and 115 days, so that only three complete cycles per year can be achieved with the blunting system to three clusters per plant.
The average yield obtained in seedlings from 60-cavity trays and established at a density of 8 plants·m-2 under greenhouse conditions was 2,200 g (17.6 kg·m-2) in a cycle that lasted 85 days from transplanting to end of harvest. With four cycles per year, on a commercial scale, yields of approximately 700 t·ha-1·yr-1 could be obtained in a greenhouse (with relatively simple technology). This yield is twice what is normally obtained in an annual cycle with a well-managed conventional production system in Mexico (Castellanos & Borbón, 2009), and even slightly higher than that reported in other countries such as the Netherlands with high-tech greenhouses (Heuvelink et al., 2018). Similar yields per cycle, for saladette tomato managed at three clusters per plant at high population density, have been reported in other studies (Sánchez-del Castillo et al., 2012; Sánchez-del Castillo et al., 2014; Sánchez-del Castillo, Moreno-Pérez, Vázquez-Rodríguez, & González-Núñez, 2017).
Transplanting at 60 das opens up the possibility of establishing four growing cycles per year, without using containers in the seedbed with a large volume of substrate per seedling, which reduces space and facilitates seedbed management and transplantation.
The comparison of means tests of the population density treatments carried out in trays with 25 mL cavities (Table 5) show that there were no significant differences in the yield variables and components. Nor were statistical differences found in these variables when the seedlings were grown in 60-cavity trays with 250 mL capacity per cavity (Table 6).
|Density (seedlings·m-2)||Number of flowers per plant||Number of fruits per plant||Fruit weight (g)||Yield (g·plant-1)|
|1,000||19 az||19 a||117 a||2175 a|
|750||19 a||18 a||128 a||2305 a|
|500||20 a||18 a||129 a||2325 a|
|250||19 a||18 a||120 a||2202 a|
|Density (seedlings·m-2)||Number of flowers per plant||Number of fruits per plant||Fruit weight (g)||Yield (g·plant-1)|
|300||19 az||18 a||124 a||2232 a|
|200||19 a||16 a||133 a||2142 a|
|150||19 a||17 a||127 a||2160 a|
|75||19 a||18 a||126 a||2300 a|
These last results indicate that, in the range of population densities evaluated in the seedbed, it is possible to extend the transplant age to 60 das without adverse effects on final yield. This makes it possible to program four growing cycles per year and increase yield by 25 % compared to the three-cycle system.
Regarding population densities of 75 and 150 seedlings·m-2, the former is recommended because plants of lower height are formed, which facilitates the handling of the plants at the time of transplantation.
Under the conditions in which the experiment was conducted, the volume of the tray cavities and the population densities did not influence the number of flowers or fruits per plant, nor the fruit weight or the final yield per plant.
In the population densities evaluated in 60-cavity trays with 250 mL capacity per cavity, the 60 das seedlings were successfully transplanted and without adverse effects on growth and yield per plant.
For ease of handling, the 60-cavity trays with 250 mL capacity per cavity and the density of 75 seedlings·m-2 were the most suitable because shorter plants were produced.