Introduction

Cucumber (Cucumis sativus) is the sixth horticultural product with the highest production worldwide, after potatoes (Solanum tuberosum), cassava (Manihot esculenta), tomatoes (Solanum lycopersicum), watermelon (Citrullus lanatus) and sweet potatoes (Ipomea batatas). In 2014, 74.98 million tons (t) of cucumbers were produced in 2.18 million hectares (ha) harvested. In Mexico, 16 808 ha were harvested and produced 707 632 t (^{FAO, 2016}), of which Sinaloa, the main national producer, accounts for 24.5% of the harvested area and 43.1% of the production obtained (^{SIAP, 2016}).

The protected crop represents the best option to increase the production of cucumber, by promoting a less restrictive environment for the growth and development of the plants than the one that occurs in the open sky (^{Smitha and Sunil, 2016}). Due to the high costs of the facilities and management it is necessary to develop and apply specific agricultural practices, such as optimizing the density of plants per unit area, for a maximum expression of the productive potential of the crop (^{Ortiz et al., 2009}).

Cucumber in a protected environment with plants tutored translates into a better arrangement of leaves to take advantage of light energy and greater ventilation, which promotes a lower incidence of pests and diseases, facilitates the harvest and allows the use of higher densities of population to obtain high yields of fruits with higher quality (^{Olalde et al., 2014}). Although different types of trellises have been used in this crop, in a protected environment the support of plants is usually done with polypropylene thread (raffia) fixed in the basal part of the plant (bundle, knotted or with rings) and wire or horizontal tutor located at a certain height above the vegetal canopy (^{Grijalva et al., 2010}).

In commercial greenhouse cucumber production, like indeterminate tomato cultivars, they are predominantly grown with a single main stem and axillary buds are eliminated on a regular basis (^{Maboko et al., 2011}; ^{Max et al., 2016}; ^{Mendoza-Pérez et al., 2018}). The increase in the density of plants can be obtained, both by a greater number of plants m ^-2, and by letting lateral stems develop to increase the number of stems m ^-2 (^{Kleinhenz et al., 2006}; ^{Rahmatian et al., 2014}). The culture with low initial density of plants and then increased the effective density by developing lateral stems, seems to be a promising way to increase the number of fruits m ^-2 and has the additional advantage of using a smaller number of plants on the crop surface. Previous research indicates that, in general, the addition of lateral stems decreases the average weight of the fruits, reduces the variability of the average weight of the fruits, which turn out to be of more uniform size during the crop cycle, although it reduces the production of fruits larger (^{Peil and Gálvez, 2004}).

For slicer-type cucumber, grown under greenhouse conditions and commonly handled to a stem, spacings of 1.5 to 2 m between rows and 0.2 to 0.3 m between plants are used; that is, from 1.7 to 3.3 plants m ^-2. However, few studies have been carried out to evaluate the effects of planting density of new varieties, and it is necessary to optimize plant density in cucumber production, especially in those varieties with high seed costs (^{López-Elias et al., 2011}). In addition, there is limited information on cucumber production under protected conditions and agronomic management variables associated with its productivity, such as the number of stems.

The research was carried out with the objective of knowing the influence of plant density and stem pruning on plant growth and yield of parthenocarpic cucumber slicer grown under greenhouse conditions.

Materials and methods

The work was carried out in a greenhouse located in the experimental field of the Faculty of Agronomy of the Autonomous University of Sinaloa, geographically located at 24° 48’ 30” North latitude, 107° 24’ 30” West longitude and 38.54 m altitude. The greenhouse (Baticenital 740-350®, ACEA, Mexico) is oriented from North to South, with a metal structure, with 3.5 m of rainwater and 6.3 m of total height. It consists of three ships of 7.4 m wide in a total area of 1 480 m². The cover, in the form of a double arch, has translucent white plastic, LDPE 180 μ, UV stabilization type Hals, 91% of global transmission of visible light and 35% of visible light dispersion. It has ventilation on sides and headboards (3.3 m), as well as zenith (1.15 m), with anti-aphids mesh (16 x 16 cm ^-2).

The soil of the site is of the chromic vertisol type with regular surface drainage. The field capacity of the soil is 64% and the point of permanent wilting of 39%, so that the usable humidity 25%, based on the weight of dry soil. Its content of organic matter is less than 1%; with less than 0.002% nitrogen, around 17.5 and 300 mg kg ^-1 of phosphorus and potassium, respectively, pH between 7.5 and 8 and electrical conductivity less than 1 dS m ^-1. The climate of Culiacan is BS1(h’)w(w) (e): very hot semi-dry extreme with summer rains, percent of winter precipitation with respect to the annual total less than five (^{García, 2004}).

After the preparation of the soil, beds of culture were formed separated to 1.8 m, to which was incorporated vermicompost in doses of 12.5 t ha ^-1, on top of it were placed two irrigation tapes with drippers every 30 cm (1.5 L h ^-1) and finally they were padded with white/black plastic. ‘Alanis RZ F1’ cucumber seed of the parthenocarpic slicer type was used. Planting was carried out in polystyrene trays of 128 cavities, filled with peat moistened with a solution of Bacillus subtilis [Agrobacilo^® (4 x 10¹⁰ ufc mL ^-1), Agrobiologica, Mexico] in doses of 500 mL m^-3. The seeds were covered with vermiculite.

From the appearance of the first true leaf, the cucumber seedlings were fertigated with 50 mg L ^-1 of NPK, increasing by 50 mg every third day until reaching 250, using the fertilizer 18N ^-18P ^-18K ^-1Mg ^-1S-0.04Fe-0.02Zn-0.02Mn-0.01Mo-0.01Cu-0.01B (Ultrasol Multi-purpose®, SQM, Mexico). Between each fertirrigation the foliar fertilizer 9N-45P ^-15K-0.5Mg ^-1S-0.01Cu-0.01Mn-0.05Fe-0.01Mo-0.01Zn-0.01B (Speedfol Starter®, SQM, Mexico), was applied, in doses of 0.3 to 0.5 g L ^-1. The transplant was performed 22 days after sowing. After this, the water and nutrient inputs were made by fertigation.

The volume of water, amount of fertilizers and the periodicity of the irrigation depended on the climatic conditions, the phenology of the plants and the humidity of the soil measured with tensiometers (2725ARL, Soilmoisture Equipment Corp., USA). The irrigation application was made when the humidity tension reached values of 20 to 25 kPa in the tensiometers placed at 30 cm depth. At the end of the study 268, 109, 378, 182, 70 and 56 kg ha ^-1 of N, P, K, Ca, Mg and S, respectively, had been applied.

A phytosanitary control was carried out with emphasis on prevention, through a program of applications of Allium sp. extract, Azadirachta indica seed extract, Cinnamomum zeylanicum extract and potassium salts of fatty acids, against insect pests (Bemisia sp., Frankliniella occidentalis, Polyphagotarsonemus latus and others), as well as Bacillus subtilis, Trichoderma sp., potassium bicarbonate, potassium phosphite, Larrea tridentata extract, among others, against pathogens causing diseases (Oidium sp. and Pseudoperonospora cubensis) common cucumber grown in the region.

The cucumber plants were pruned to one and two stems per plant. In the plants with pruning to a stem all axillary shoots of the main stem were eliminated. While in the plants with pruning to two stems one of the first axillary buds was allowed to grow, which became the second stem, then in both stems the axillary shoots were eliminated and they were tutored with raffia and plastic rings. At 78 days after the transplant (ddt) the stems were blunted, when they reached the height of the horizontal tutors of the greenhouse, delimiting with it the crop cycle.

A randomized complete block experimental design with a 2 x 2 factorial arrangement was used. Factor A corresponded to the density of plants m ^-2, with two levels: 1.68 and 2.22 plants m ^-2 (0.33 and 0.25 m between plants, respectively). The factor B was the pruning of stems, with two levels: pruning to one and two stems plant ^-1. The treatments: T1 (A¹ x B¹)= 1.68 m ^-2 plants with pruning to one stem, T2 (A¹ x B²)= 1.68 plants m ^-2 with pruning to two stems (3.37 stems m ^-2), T3 (A² x B¹)= 2.22 plants m ^-2 with pruning to one stem, and T4 (A² x B²)= 2.22 plants m ^-2 (4.44 stems m ^-2), were established with four repetitions in 216 m² of experimental area (23 and 30 plants per repetition in the respective densities of 1.68 and 2.22 plants m ^-2).

The growth of the plants was evaluated, every 15 ddt, through the following response variables: stem length in centimeters, considered from the base of the stem to the distal end, basal diameter and apical stem in millimeters (Mitutoyo, Kanawaga, Japan, precision ± 0.01 mm), measured at an intermediate position of the first basal internode and internode forming the third and fourth leaf counted from the apex of the stem, respectively; number of leaves plant ^-1, foliar area plant ^-1 measured with an integrator (LI-3000A, LI-COR Inc., USA), leaf area index (IAF) in m² of leaf area per m² of soil surface, obtained by dividing the plant leaf area between the surface occupied by each plant, according to its density (0.59 m² for 1.68 plants m ^-2 and 0.45 m² for 2.22 plants m ^-2) and foliar verdure (SPAD-502, Konica Minolta, Japan), obtained from leaves of the low, medium and high strata of each side (east and west) of the plants. The accumulation of fresh and dry biomass of leaves and stems (Sartorius, Goettingen, Germany; precision ±0.001 g) after drying in oven (FE ^-292, Felisa, Mexico) was also obtained up to constant dry weight of 16 plants per treatment, at the end of the crop cycle.

Flower plant ^-1 production was also evaluated every third day and weekly growth of selected fruits, measuring their length and diameter from anthesis to harvest, of 16 plants per treatment. To determine the yield of cucumber, the harvested fruits were weighed (Sartorius, Goettingen, Germany, precision ± 0.01 g) and classified into sizes: small (19.1 ^-20.3 cm long and 3.81-5.08 cm in diameter), select (20.3 ^-21.6 cm long and 5.08-6.03 cm in diameter), super select (21.6 ^-22.8 cm long and 5.08-6.03 cm in diameter), large (22.8 ^-24.1 cm long and 4.8-5.7 cm in diameter) and extra-large (24.1 ^-25.4 cm long and 5.71-6.98 cm in diameter).

The results of the variables evaluated were processed with the Statistica 7.0 package (^{StatSoft, 2004}); through the analysis of variance and the Tukey test (p≤ 0.05) for the separation of means.

Results and discussion

Conclusions

The basal and apical stem diameter, fruit length, and large and extra-large cucumber yield were not significantly influenced by plant density and stem pruning. The fresh biomass of stem plant ^-1 and dry leaves plant ^-1 decreased with increasing plant density, but increased in plants with two stems. The length and dry biomass of stem, leaves plant, fresh biomass of leaves plant and production of flowers plant ^-1 increased, while leaf greenery decreased, with two stems plant ^-1. Both factors caused decreases in the diameter of the fruit. However, the yield of cucumber was positively influenced both by the density of plants and by the pruning of stems. The total yield, as well as select and super select cucumber, were obtained when the density of 2.22 plants m ^-2 was used with pruning to two stems plant ^-1.