The Influence of Biostimulants on Tomato Plants Cultivated under Hydroponic Systems

A greenhouse experiment was conducted with the tomato hybrid cultivar ‘Merlice’, which is recommended for greenhouse cultivation on artificial substrates (De Ruiter, the Netherlands). The tomato seeds were sown on plastic trays filled with artificial wool substrates and fertilized with a nutrient solution (EC = 2.5 dS·m⁻¹, pH 5.5). Through six weeks, tomato seedlings were kept in the greenhouse with temperatures from 18 °C to 22 °C during the day and from 16 °C to 18 °C at night at 60–65% relative air humidity. The greenhouse was continuously ventilated to avoid plant diseases. Seedlings with 4–5 true leaves were transplanted to the hydroponic system, using mineral wool as a substrate. After setting the seedlings on plastic boxes filled with wool substrate, they were fertilized with 100 ml per plant per day of a Hoagland nutrient solution (0.54 g·L⁻¹ of KNO₃, 0.84 g·L⁻¹ of Ca(NO₃)₂, 0.14 g·L⁻¹ of KH₂PO₄, 0.25 g·L⁻¹ of MgSO₄, and 0.2 g·L⁻¹ of Fe). In the vegetative phase, the EC of the nutrient solution was 3.5 dS·m⁻¹, and in the flowering phase, it was 5.0 dS·m⁻¹; pH was adjusted weekly to pH 5.0–5.5. The light was set for 16 hours with an intensity of 500 μmol·m⁻²·s⁻¹ and temperature at 25 °C day/night, with 60–65% relative humidity.

The experiment was conducted as a completely randomized process, with three replications per treatment and 50 plants per replication. Ten plants of the replicate were taken for analysis of growth parameters. The following concentrations of each biostimulant were studied: Maxifol (MF 2.5, SW 5.0, and 7.5 mg·L⁻¹) and Radifarm (RF 2.5, 5.0, and 7.5 mg·L⁻¹). They were applied three times: for soaking seeds before seeding and spraying in a volume of 65 ml per plant at the flowering and early fruit development stages. The control plants were sprayed with water. Tomato seeds were soaked in biostimulant solutions for 30 minutes before seeding, while control seeds were soaked in distilled water for the same time.

Biometric parameters of 30 plants per treatment were each analyzed three times: before transplanting seedlings, at fruit set, and at fruit formation phase (35, 70, and 105 days after seeding, respectively). The following parameters were scored: root length (only at transplanting), the plant height, number of the leaves per plant, total leaf area per plant, and fresh and dry weights calculated by multiple average leaf area and number of leaves per plant. The dry weight of whole plants was obtained after washing with deionized water and drying in an oven at 60 °C for more than 48 hours (10 samples per replicate). The average leaf area was measured using the Image J program as described by Abdelkader et al. (2019a). The relative growth rate (RGR) was calculated according to the formula: RGR = (ln W₂ − ln W₁)/(t₂ − t₁), and expressed as g·g⁻¹·week⁻¹. The crop growth rate (CGR) is the total dry matter produced per plant over a period of time. It was calculated by the following formula: CGR = (W₂ − W₁)/(t₂ − t₁), where W₁ = dry weight of the plant at time t₁, and W₂ = dry weight of the plant at time t₂ and expressed as g·week⁻¹·plant⁻¹. The net assimilation rate (NAR) was measured using the following equation: NAR = (W₂ − W₁)(lnL₂ − lnL₁)/(t₂ − t₁)(L₂ − L₁), and expressed as (g·m⁻²·week⁻¹), where ln = natural log, W₁ = dry weight of plant (g) at time t₁, and W₂ = dry weight of plant (g) at time t₂ (Usuda 2004). Yield components were calculated by measuring the following parameters: number of fruits per plant, weight of individual fruit, and fruit yield per plant.

Ten tomato fruits at the red-ripening stage were collected from each replicate to conduct biochemical analyses. Three replicates for each treatment and three separate analyses for each replication were conducted. Biochemical properties were evaluated to determine the following parameters: fresh and dry mass of fruit (after drying at 105 °C), the titratable acidity (TTA), expressed as a percentage of citric acid by the methods described by the Association of Analytical Communities (Considine 2005), the concentration of ascorbic acid, expressed as mg·100 g⁻¹, determined by the 2,6-dichlorophenolindophenol method, carotenoids (expressed as mg·100 g⁻¹), and nitrate (mg·kg⁻¹). Sugar to acid ratio (maturity index) was calculated by dividing total soluble solid, measured as °Brix using a refractometer, by the titratable acidity of the given samples under analysis as described by Mohammed et al. (1999). The taste index was calculated using the equation of Navez et al. (1999) using the formula: Taste index=Brix level20 (acidity)+acidity. {\rm{Taste}}\;{\rm{index}} = {{{\rm{Brix}}\;{\rm{level}}} \over {20\;\left( {{\rm{acidity}}} \right)}} + {\rm{acidity}}.

Data were analyzed using one-way analysis of variance (ANOVA) in a completely randomized design followed by Tukey’s HSD test ± SD (p = 0.05) with the help of the MINITAB (v. 19.0) program.

Soaking tomato seeds in a biostimulator solution before sowing influenced the root length of the seedlings (Fig. 1). Radifarm increased the root length by 4.3–7.8% and RutfarmMaxifol by 1.2–2.4% in comparison with the control. The seedlings soaked in Radifarm with a concentration of 7.5 mg·L⁻¹ had the maximum root length (7.6 cm). RutfarmMaxifol increased the seedling root length only when it was used at a concentration of 7.5 mg·L⁻¹, but to a lesser extent than Radifarm. Generally, the application of RutfarmMaxifol and Radifarm significantly enhanced growth and other biometric parameters if applied in concentrations of 5 and 7.5 g·L⁻¹ compared with the control, and the effect increased with concentration (Table 1). RutfarmMaxifol was more effective than Radifarm in the increase of values of the studied parameters.

Figure 1

Figure 2 shows RGR, CGR, and NAR of tomato plants grown under the influence of biostimulants during two periods of growth expressed as weeks after planting (WAP) – the first (5–10 WAP) and second (10–15 WAP). The RGR ranged between 400 and 500 mg·g⁻¹·week⁻¹ at the first growth stage, while in the second growth stage, RGR values decreased to 120–180 mg·g⁻¹·week⁻¹. At the first growth stage, RF 5.0 and RF 7.5 treatments produced the highest RGR, but this effect differed at the second stage. There was a contradictory relationship between RGR values and the age of plants. Generally, RGR declines with increasing plant size (Fig. 2A). Concerning CGR, both biostimulants increased this parameter at the first stage, with a tendency to increase with concentration. At the second growth stage, only Radifarm had the same pattern; the effect of RutfarmMaxifol was different. The concentrations 5 and 7.5 mg·L⁻¹ did not increase this parameter (Fig. 2B). Biostimulants in this experiment stimulated NAR only at the first stage of growth; this parameter in the older plants was not influenced by the biostimulants (Fig. 2C).

Figure 2

For the determination of tomato yield components, the number of fruits per plant (pcs), fruit weight (g), and tomato fruit yield (kg per plant) were measured (Fig. 3). The values of all the above parameters increased with increasing concentration of biostimulators. RutfarmMaxifol was more effective than Radifarm at each concentration. The highest fruit number, fruit weight, and fruit yield gained the highest values at 7.5 g·L⁻¹ of RutfarmMaxifol. The highest concentrations of biostimulants increased fruit number by 15 (Radifarm) and 20 (RutfarmMaxifol) over the control. The single fruit weight increased at the highest concentrations by 14% (Radifarm) and 18% (RutfarmMaxifol). The total yield of the plant increased by 20–70% when biostimulants were applied.

Figure 3

Radifarm enhanced dry matter of tomato fruits from 6.35% to 6.85% and RutfarmMaxifol by 6.25–6.45% (Fig. 4A). The value higher by 14% over control was obtained with Radifarm 7.5 g·L⁻¹. The concentrations of total soluble solids (TSSs) and ascorbic acid increased with biostimulator concentration with the same pattern, more with the application of Radifarm (Fig. 4B, C).

Figure 4

In the present study, the concentration of carotenoids increased significantly only as an effect of the application of 5.0 and 7.5 g·L⁻¹ of Radifarm (Fig. 4D). The acidity rate tended to decrease as an effect of RutfarmMaxifol treatment, less so in Radifarm treatment. Nitrate concentration in the tomato fruits increased with the concentration of Radifarm by 12% at 7.5 g·L⁻¹ (Fig. 4F). Both biostimulants affected maturity degree and taste index in the same way (Fig. 5A, B). Significant increases were obtained at 5.0 and 7.5 g·L⁻¹, more with the application of Radifarm.

Figure 5