Introduction

The strawberry (Fragaria x ananassa Duch.) is in great demand in Mexico and around the world, especially in developed countries; 9 223 815 t are produced in Mexico alone (^{Romero-Romano et al., 2012}; ^{FAOSTAT, 2020}). The main producing states nationwide are Michoacán, Baja California, Baja California Sur, State of Mexico and Morelos (^{SIAP, 2020a}). Due to the importance of fresh strawberry consumption, the physical and above all biochemical quality is of great importance, due to its large amount of sugars and minerals, in addition to having nutraceutical compounds such as phenols and flavonoids, which have antioxidant properties with capacity to capture free radicals (^{Vásquez et al., 2007}; ^{Luna-Zapien et al., 2016}). ^{Llacuna y Mach (2012}) report that plant products with a high nutraceutical content are important for human health by promoting physiological balance, as well as reducing the risk of developing chronic-degenerative diseases, diabetes and cancer.

To obtain the quality parameters in strawberry, it is of great importance to control the nutritional regime during the cultivation cycle (^{Jara and Suni, 1999}; ^{Avitia-García et al., 2014}); that is, the supply of macro and micronutrients to the phenological phase. In this investigation, the criteria of Stenier (1984) on the mutual relationships between anions (NO₃ ^-:H₂PO₄ ^-:SO₄ ^2-) and cations (K⁺:Ca²⁺:Mg²⁺) were considered, in addition to keeping the total concentration of anions constant (20 meq L^-1) and that of cations (20 meq L^-1).

N is one of the most limiting nutrients in strawberry production, in such a way that farmers apply high doses of nitrogen fertilization in order to obtain outstanding yields (^{Cárdenas-Navarro et al., 2004}), which is why they increase production costs and contamination of groundwater tables by nitrate leaching (^{Vázquez-Gálvez et al., 2008}). N has an essential function in the vegetative growth, productivity and quality of the strawberry; its functions are structural and osmotic. This nutrient is absorbed mainly in the form of NO₃ ^-. If deficiencies are present, the vigor of the plants and the productivity decrease, but the organoleptic quality of the fruit improves; on the other hand, if there is an excess of N, Zn deficiency is induced (^{Kirschbaum and Bórquez, 2006}; ^{Eyal, 2008}; ^{Chávez- Sánchez et al., 2014}).

P is an essential nutrient for plants, although it is a poorly mobile element in the soil, it benefits the plant by stimulating radical development and flowering, as it is the primary constituent of the systems responsible for energy collection, storage and transfer. It is part of the essential macromolecule structures, such as nucleic acids and phospholipids, so it participates in all physiological processes.

Plants absorb it as a primary orthophosphate ion (H₂PO₄ ^-) or as a secondary orthophosphate (HPO₄ ^2-) (^{Fernández, 2007}). P intervenes in biochemical processes such as: biogenesis of glycosides, biosynthesis of lipids, chlorophylls and carotenoid compounds, in glycolysis and the metabolism of organic acids; which translates into the acidity, aroma and color of the fruits (^{Díaz et al., 2017}). The deficiency of P diminishes the number and diameter of the flowers causing a reduction of 50% in the yield, the maturation is delayed, the size and firmness of the fruits decreases, and the content of vitamin C decreases; but high levels of P induce Zn deficiency and inactivate Fe (^{Kirschbaum and Bórquez, 2006}; ^{Eyal, 2008}; ^{Díaz et al., 2017}).

K is known as the quality nutrient for its effect on the size, shape, color, flavor and storage resistance that it confers on the fruits (^{Chávez-Sánchez et al., 2014}). It is involved in the absorption of water by the roots, influences photosynthesis and regulates the opening of stomata; it is a structural component of lignin and cellulose; it also affects the starch and sugar contents, it is involved in resistance to diseases and insects. It is absorbed by the plant as K⁺, it is a mobile element in plants, its availability is critical in leaves and growing fruits. Deficiency induces a decrease in fruit vigor, yield and quality due to affecting pigmentation (^{Kirschbaum and Bórquez, 2006}). K directly affects the quality of the fruit because high levels increase apical rot and reduce the firmness of cell walls (^{Hernández et al., 2009}).

Mexico has 14 771 ha of cultivated berries (strawberry, raspberry, blackberry and blueberry) in macrotunel. Of the previous surface, 11 091 ha is strawberry, of which 89.78% is mechanized and 65.63% has plant health technology (^{SIAP, 2020b}). This technified system allows to obtain 50% more yield compared to traditional cultivation (open pit and manual agricultural work), in addition to prolonging the harvest period (^{SAGARPA, 2016}).

Strawberry production in technical systems also allows controlling the nutritional regime; that is, the amount of nutrients for each phenological phase of the strawberry and thereby optimize the development, yield and quality of the fruits (^{Manqueros-Aviles, 2015}). The objective of the investigation was to determine the nutritional regime to favor the growth, yield and physical and biochemical quality of strawberry produced in a system without soil under plastic cover.

Materials and methods

Results and discussion

The analysis of variance (p≤ 0.05) carried out on the variables of growth, production and fruit quality indicated that at least one nutritional regime exerted statistically significant differences from the rest of the regimes in the relative content of chlorophyll, leaf area, dry biomass of leaves, flower diameter, length and diameter of the fruit, average weight of the fruit, yield per plant and concentration of total soluble solids in the fruit.

The variables that did not show significant statistical differences (p≤ 0.05) were the number of leaves, root volume, dry root biomass and number of flowers per plant (unpublished data), which indicates that these characteristics are more influenced by the genetic component than the nutritional one, since all the treatments were in the same physical-chemical environment (temperature, light intensity, relative humidity, availability of nutritive solution in the substrate, among others).

Regarding the relative content of chlorophyll expressed in SPAD units, the nutritional regime (in meq L^-1) 14:1.25:9 of NO₃ ^-:H₂PO₄ ^-:K⁺ in the vegetative, reproductive and fruiting phases, respectively, was 47.71 , 8.03% higher than in plants with the 10:0.75:7, regime, which presented the lowest value of 44.16, in the other treatments, including the control, presented similar value (46.39% on average) (Table 2). The relative content of chlorophyll is an indicator between the relationship of the degree of supply and availability of nutrients, ^{Juárez-Rosete et al. (2007}) report readings of up to 43.23 SPAD, nourishing the plant with the Steiner nutritive solution, also mention that as the cultivation cycle passed, the SPAD readings decreased in the different phenological phases of strawberry cv. Chandler.

Regarding the leaf area, with 12:0.75:7 it was 1 819.92 cm², 165% more with respect to the plants that received 10:1.25:5 and 12:1.25:5 (686.75 cm², on average) (Table 2). This result differs from that obtained by ^{Caso et al. (2010}) who report a leaf area of 920.52 cm² in strawberry cultivated in pumice substrate (100%) and with La Molina nutrient solution. For their part, ^{Casierra-Posada and Poveda (2005}) obtained a leaf area of 600 cm² when cultivating the strawberry var. Camarosa with highly soluble compound fertilizer. From the diversity of values in the reported strawberry leaf area, the effect of both nutrition and variety can be inferred, aspects to consider in the case of establishing a commercial crop.

In relation to biomass accumulation, as a parameter to quantify growth (^{Urrestarazu et al., 1999}; ^{Villegas-Torres et al., 2005}), a significant difference was found in leaves. The nutritional regime that led to this variable (26.75 g) was 14:1:7, which represented an increase of 148% compared to plants (10.75 g) fed with the 12:1.25:5 regime (Table 2). ^{Caso et al. (2010}) reported dry biomass of leaves of 3.8 g, this in strawberries grown with the pumice substrate and La Molina hydroponic solution.

Regarding the flower size, the 14:0.75:7 regime favored the plants to present larger flowers (2.37 cm in diameter), a difference of 37.79% with respect to the flower diameter of strawberries fed with 10:1.25:5. Fruiting started ten days after flowering, while the harvest lasted 185 days. The 14:0.75:9 regimen induced the strawberry to produce the greatest number of fruits per plant (12.67), the difference was 66.05% greater with respect to the plants fed with 10:0.75:7 (Table 3).

^{Caso et al. (2010}) reported 68.17 fruits per plant over a period of 270 days in strawberries grown in pumice using the La Molina hydroponic nutrient solution. The amount of fruit produced by plants can vary since temperatures of 24 to 32 °C cause abortion of fruits in some plants, therefore, the amount of fruit decreases (^{Taylor, 2002}; ^{Romero-Romano et al., 2012}).

There were also significant statistical differences in length and diameter in strawberry fruits var. San Andreas, the regime that favored both variables (3.59 and 2.96 cm respectively) was 10:1.25:7. ^{Chávez-Sánchez et al. (2014}) reported values in the length of strawberry fruits 3.68 cm and for the diameter 2.73 cm applying a concentration of NO₃ ^- of 9 milliMol (mM) in the nutritive solution. ^{Caso et al. (2010}) obtained in the diameter and length values of 2.99 cm and 4.11 cm, respectively, in strawberry fruits with the rice husk substrate and La Molina hydroponic solution.

The nutritional regime 10:1.25:7 had a significant effect (p≤ 0.05) on the average weight per fruit (15.15 g), which was 147% higher than the fruits (6.13 g) of plants fed with 10:1.25:5. In this case, the variation in the response was due to the concentration of SO₄ ^2- in the fruiting phase. ^{Casierra-Posada and Poveda (2005}) obtained fruit weights of up to 10.7 g, however, they also mention that radiation and photoperiod affect the weight of strawberry fruits.

Regarding the yield of fruits per plant, the outstanding value (289.28 g) was registered with the regime (in meq L^-1) 10:1:7, which was 322.24% higher than that registered in plants fed with 14:1:9 (Table 3). ^{Moor et al. (2004}) reported fruit yield of 252 g per plant, fertilizing with the Kemfos^® and Kemira Ferticare^® products in the different phenological phases of strawberry var. Bounty, while ^{Romero-Romano et al. (2012}) obtained 189.42 g using organic-mineral nutrition (chemical fertilizer + fulvic acids + growth regulator + vermicompost), while Furlani and ^{Fernández (2007}) indicated yields of 50 to 300 g.

In relation to the nutritional regime, it would be expected that with the highest concentration of NO₃ ^- (14 meq L^-1) in the vegetative phase and K⁺ (9 meq L^-1) in the fruiting phase, the fruit yield per plant was higher than with the lower values of both nutrients: 10 and 7, respectively; however, the data showed otherwise. From the above, it follows that the relationships between the concentration of NO₃ ^- in the vegetative phase and that of K⁺ in the fruiting phase is more important than the absolute value of each of the nutrients involved.

The physical characteristics of the fruit are important, but so is the degree of sweetness, among other biochemical attributes (^{Juárez-Rosete et al., 2007}). Table 3 shows that the regime that favored the CSST was (in meq L^-1) 14:1.25:5, with a value of 11.75 °Brix, in relation to that obtained in fruits of plants fed with 14:1.00:5, was 710% higher (Table 3). ^{Roudeillac and Trajkovski (2004}) point out that the strawberry must be between 7 and 12 °Brix, to be among the post-harvest quality recommendations. ^{Giraldo (2006}) reported 9.3 °Brix in strawberry fruits, while ^{Martínez-Bolaños et al. (2008}) reached values of up to 8.48 °Brix with the Mexican strawberry cultivar CP-Roxana. ^{Núñez-Castellano et al. (2012}) evaluated strawberry fruits where the result was 9.50 °Brix with the treatment without calcium immersion, with plastic cover.

It is important to note that with the decrease of H₂PO₄ ^- in the flowering phase, with the same concentration of NO₃ ^- and K⁺ in the vegetative and fruiting phases, respectively, the CSST decreased significantly, which may indicate that, in the metabolism of sugars, organic acids, among others, the participation of H₂PO₄ ^- is important as is its relative concentration with the other two ions (NO₃ ^- and K⁺).

Conclusions

Strawberry plants produced in hydroponics with nutritional regimen showed a significantly different response in the relative content of chlorophyll, leaf area, dry biomass of leaves, flower diameter, length and equatorial diameter of the fruit, average fruit weight, yield per plant and concentration. of total soluble solids in fruit, depending on the relative concentrations between NO₃ ^-:H₂PO₄ ^-:K⁺, in the vegetative, reproductive and fruiting phases, respectively.

The nutritional regime of 10 meq L^-1 of NO₃ ^- in the vegetative phase, 1 meq L^-1 of H₂PO₄ ^- in the reproductive phase and 7 meq L^-1 of K⁺ in fruiting, is recommended to produce strawberry in soilless system because the diameter of the fruit and the yield per plant increased significantly.