Foliar Application of Salicylic Acid Mitigates Saline Stress on Physiology, Production, and Post-Harvest Quality of Hydroponic Japanese Cucumber

Abstract

Salicylic acid (SA) is a phenolic compound capable of inducing physiological and metabolic changes that enhance the tolerance of plants to saline stress associated with using a hydroponic system and enable the use of saline water in semi-arid regions. In this context, this assay aimed to evaluate the impact of the foliar application of SA on mitigating salt stress effects on Japanese cucumber cultivated in a hydroponic system. The experiment was carried out in a protected ambient (greenhouse), using the Nutrient Film Technique—NFT hydroponic system. A completely randomized design was performed in a 4 × 4 split-plot scheme, with four levels of electrical conductivity of the nutrient solution—ECns (2.1, 3.6, 5.1, and 6.6 dS m⁻¹)—considered as plots and four SA concentrations (0, 1.8, 3.6, and 5.4 mM), regarded as subplots, with four replicates and two plants per plot. An increase in the ECns negatively affected the physiology, production components, and post-harvest quality of cucumber. However, the application of SA to leaves at concentrations between 1.4 and 2.0 mM reduced the deleterious effects of saline stress and promoted an increase in the production of and improvement in the post-harvest quality of cucumber fruits.

1. Introduction

Freshwater scarcity is common in both arid and semi-arid zones worldwide, due to irregular distribution of water resources, degradation of water quality by anthropic activities, and increased consumption of water resulting from population growth [1,2]. Thus, the use of brackish water becomes necessary to ensure agricultural production in these regions and meet the need for food [3].

However, excess salts in water and/or soil are becoming an increasing threat to global agricultural production and affect nearly 20% of irrigated land of the world, causing severe losses in food production and quality [4]. The high concentrations of salts inhibit water absorption by roots, resulting in water deficit, a decrease in leaf area, and stomatal conductance, which reduces photosynthesis and plant growth [5]. Salt stress also modifies electron transport and alters the activity of photosystem II, which is responsible for oxidizing water molecules to produce electrons [6]. In addition, the absorption and excessive accumulation of Na⁺ in cells cause ionic imbalance, lipid peroxidation, and damage to the cell membrane [7]. The water content in the fruit can also be affected by salt stress, causing changes in the concentrations of soluble solids, ascorbic acid, and titratable acidity.

Phytohormones play a fundamental role in signaling and attenuating biotic and abiotic stresses [4]. Among the phytohormones, salicylic acid (SA), a phenolic compound, stands out as a regulator of plant growth and development [8]. SA is considered a key component of the plant’s antioxidant system and plays an important role in regulating the metabolism of reactive oxygen species (ROS) and in the balance of the redox system [9]. However, its effect depends on the concentration, plant species, stage of crop development, and mode of application [10,11].

The beneficial effect of salicylic acid as an attenuator of salt stress has been reported in recent years in several studies with vegetables, such as bell pepper [12], tomato [13], eggplant [14], melon [15], okra [3], and basil [16]. However, information on its use in Japanese cucumber crops, particularly in hydroponic cultivation, is still incipient.

The use of hydroponic systems is becoming more widespread, due to greater control over the rhizosphere conditions compared to cultivation in soil, which results in gains in the quantity, quality, and safety of production [17]. Hydroponic cultivation also promotes greater efficiency in the use of water and nutrientsa and the absence of matric potential minimizes the effects of salinity on plants, which enables the use of brackish water [18]. Among the hydroponics systems, the NFT (Nutrient Film Technique), a closed system with recirculation of the nutrient solution, stands out as the most used in the cultivation of fast-growing vegetables [3,19].

Cucumber (Cucumis sativus L.) is one of the most popular vegetables. It has a moderate sensitivity to salinity [20,21], with a threshold salinity of 2.5 dS m⁻¹ in irrigation water and 3.5 dS m⁻¹ in the saturation extract of soil [22,23]. Its fruits have a high nutritional value and are rich in proteins, carbohydrates, vitamin C, and minerals [24,25]. In addition, cucumber has anti-inflammatory, antioxidant, and anticancer properties that help in the treatment of several diseases [26].

Previous studies demonstrate that salt stress inhibits germination, growth, biomass production, and cucumber yield [21]. In research conducted by Brengi et al. [27], limitations in growth and chlorophyll synthesis were verified due to salt stress. Chen et al. [28] reported reductions in cucumber yield corresponding to a 13% per unit increment of electrical conductivity above 2.5 dS m⁻¹, demonstrating its sensitivity to salt stress.

This study hypothesizes that the application of salicylic acid on leaves induces tolerance to salt stress in Japanese cucumbers cultivated in a hydroponic system through the regulation of physiological and biochemical processes, which result in gains in production and post-harvest fruit quality. In this context, this study was conducted to evaluate the influence of the foliar application of salicylic acid on mitigating salt stress effects on the physiology, production, and post-harvest quality of Japanese cucumbers in the NFT hydroponic system.

2. Materials and Methods

2.1. Experiment Site

The study was carried out in a protected ambient (greenhouse) belonging to the Center of Science and Agri-Food Technology (CCTA) of the Federal University of Campina Grande (UFCG), in Pombal (6°46′13″ S, 37°48′6″ W, 184 m a.s.1), Paraíba, Brazil. The daily temperature (maximum, mean, and minimum) and average relative humidity during the experimental period (from May to June 2022) are shown in Figure 1.

2.2. Cultivar Studied

‘Hiroshi’ Japanese cucumber seeds from Isla^® were used in this assay. This variety has a cycle of approximately 60 days, with vigorous and highly productive plants, as well as adaptability to different growing regions in Brazil. It produces cylindrical and uniform fruits of bright dark green color, with from 18 to 22 cm length and diameters between 30 and 40 mm [29].

2.3. Experimental Design and Treatments

The treatments included four levels of electrical conductivity of the nutrient solution—ECns (2.1, 3.6, 5.1, and 6.6 dS m⁻¹) and four concentrations of salicylic acid—SA (0, 1.8, 3.6, and 5.4 mM). The treatments were distributed in a completely randomized design in a split-plot scheme, with the ECns levels as plots and salicylic acid concentrations as subplots with four replicates and two plants per plot. Salicylic acid was applied using foliar spraying.

The concentrations of SA used in this study were based on a study conducted with melon [15], while the salinity levels of the nutrient solution were adapted from the assay carried out by [30] with cucumber cv. ‘Hokushin’.

2.4. Setting Up and Management of Experiment

The hydroponic system used was the Nutrient Film Technique—NFT type, performed using six-meter-long polyvinyl chloride (PVC) tubes of 100 mm diameter spaced 0.40 m apart. In the hydroponic profile, the circular planting cells had a diameter of 54.17 mm at a distance of 0.50 m and the spacing between treatments (subsystems) was 1.0 m. The hydroponic profiles were supported by 0.60 m high sawhorses with 4% slopes to permit the flow of nutrient solution (Figure 2). At the end of each subsystem, a 150-L polyethylene recipient was placed to collect the excess returning nutrient solution and to recirculate it into the system. The nutrient solution was injected into the hydroponic profile at the top of each channel with a pump of 35 W at a flow rate of 3 L min⁻¹. A timer was used to program the circulation of the nutrient solution into the system, with an intermittent flow of 15 minutes at hourly intervals during the day and 30 minutes at night.

In the study, the nutrient solution recommended by Hoagland and Arnon [31] was used containing N, P, K, Ca, Mg, S, B, Mn, Zn, Cu, Mo, and Fe in concentrations of 210, 31, 234, 200, 48, 64, 0.5, 0.5, 0.05, 0.02, 0.01, and 5 mg L⁻¹, respectively. The fertilizers used as sources of macronutrients in the preparation of the solution were monobasic potassium phosphate (KH₂PO₄), potassium nitrate (KNO₃), calcium nitrate (Ca(NO₃)₂.4H₂O), and magnesium sulfate (MgSO₄.7H₂O). As sources of micronutrients, boric acid (H₃BO₃), manganese sulfate (MnSO₄.4H₂O), zinc sulfate (ZnSO₄.7H₂O), copper sulfate (CuSO₄.5H₂O), ammonium molybdate ((NH₄)₆Mo₇O₂₄.4H₂O ), ferrous sulfate (FeSO₄), and EDTA-Na, respectively, were employed.

The sowing was conducted in polyethylene cups of 50 mL capacity containing vegetable sponges of the genus Luffa (Luffa aegytiaca) arranged in trays. The sponges were sanitized using sodium hypochlorite (2.5%) before sowing. From germination to the emergence of the first true leaf (on average ten days after sowing), a half-strength (50%) nutrient solution was used. The vegetable sponge was removed after the emergence of the first true leaf, the seedlings were inserted directly into the hydroponic channels, and a full-strength nutrient solution was employed.

The saline solutions used in the experiment were obtained from the addition of sodium chloride (NaCl), calcium chloride (CaCl₂.2H₂O), and magnesium chloride (MgCl₂.6H₂O) in the equivalent proportion of 7:2:1, respectively, to the nutrient solution prepared in the municipal supply water as described above. The proportion of Na, Ca, and Mg added is commonly found in the waters used for irrigation in the semi-arid region of northeastern Brazil [32]. The amounts of the individual salts added to the nutrient solution are shown in

Table 1. Quantities of salts incorporated per 100 L of nutrient solution.

ECns (dS m−1)	NaCl	CaCl2.2H2O	MgCl2.6H2O
	g
2.1	-	-	-
3.6	61.43	20.55	15.23
5.1	122.85	41.10	30.45
6.6	184.28	61.65	45.68

.

At intervals of eight days, the nutrient solution was completely replaced. The electrical conductivity and pH were monitored daily and, when necessary, the nutrient solution was adjusted by the addition of local-supply water or nutrient solution, maintaining the ECns as per treatments established initially. The pH was maintained between 5.5 and 6.5 by adding either 0.1 M potassium hydroxide (KOH) or hydrochloric acid (HCl). The aplants were cultivated using a vertical support fixed with a plastic string (number 10) (Figure 3).

The salicylic acid solution (100 mM) was prepared by dissolution in 30% ethyl alcohol. The solutions of adequate concentrations, as per treatment, were prepared by diluting this solution in water before each application event. To reduce the surface tension of the drops on the leaf surface, a Wil-fix adjuvant was added (0.5 mL L⁻¹) to the solution. The first application was performed 48 h after transferring the seedlings and 72 h before the application of the saline nutrient solution. The other applications were carried out at 10-day intervals until the beginning of the flowering stage, spraying the abaxial and adaxial leaf surfaces. The sprayings were performed between 17:00 and 18:00 h and the average volume of solution applied per plant was 80 mL. During the spraying of salicylic acid, a plastic tarpaulin structure was used to prevent the solution from drifting onto neighboring plants.

2.5. Traits Analyzed

At 40 days after transplanting (DAT), the relative water content, percentage of electrolyte leakage in the leaf blade, leaf gas exchange, photosynthetic pigments, and chlorophyll a fluorescence were evaluated. Harvest began at 43 DAT; the following production components were obtained: number of fruits per plant, average fruit weight, total production, fruit length, and fruit diameter. In addition, the following post-harvest characteristics were determined in the fruit pulp: pH, titratable acidity, ascorbic acid, and soluble solids contents.

2.5.1. Relative Water Content

For the determination of the relative water content (RWC), two leaves were removed from the middle third of the main branch obtaining ten discs 12 mm in diameter. The disks were immediately weighed for the fresh mass (FM); then, the discs were transferred to a beaker and immersed in 50 mL of distilled water for 24 h. After this period, with a paper towel, the excess water was removed from the disks and the turgid mass (TM) was determined. The discs were dried at a temperature of ≈65 ± 3 °C in an oven until constant weight to obtain the dry mass (DM). The relative water content was determined using Equation (1) as recommended by [33].

$$\mathrm{RWC}=\left(\left(\mathrm{FM}-\mathrm{DM}\right)/\left(\mathrm{TM}-\mathrm{MS}\right)\right)\times 100$$

(1)

where:

• RWC—relative water content (%);
• FM—fresh mass of leaves (g);
• TM—turgid mass (g);
• DM—dry mass (g).

2.5.2. Percentage of Electrolyte Leakage

The electrolyte leakage (EL) was obtained using a copper hole puncher to produce five leaf discs with an area of 1.54 cm² each. The discs were washed and placed in an Erlenmeyer^® flask containing 50 mL of distilled water. After closing with aluminum foil, the Erlenmeyer^® flasks were kept at a temperature of 25 °C for 24 h and then the initial electrical conductivity of the medium (Xi) was measured using a benchtop conductivity meter (MB11, MS Techonopon^®, Piracicaba—SP, Brazil). Later, the Erlenmeyer^® flasks were subjected to a temperature of 80 °C for 120 minutes in an oven (SL100/336, SOLAB^®) and, after cooling, the final electrical conductivity (Xf) was determined. In turn, the percentage of electrolyte leakage (% EL) was obtained through Equation (2), as recommended by [34].

$$\%\mathrm{EL}=(\mathrm{Xi}/\left(\mathrm{Xf}\right)\times 100$$

(2)

where:

• % EL—percentage of electrolyte leakage (%);
• Xi—initial electrical conductivity;
• Xf—final electrical conductivity.

2.5.3. Photosynthetic Pigments

The quantification of the photosynthetic pigments (chlorophyll a, chlorophyll b, chlorophyll total, and carotenoids) was carried out according to [35], with extracts from disc samples of the third mature leaf blade from the apex. In each sample, 6.0 mL of 80% acetone PA was added. Through these extracts, the concentrations of chlorophyll and carotenoids were determined using a spectrophotometer (UV/VIS—UV17030, AKSO^®, São Leopoldo—RS, Brazil) at the absorbance wavelength (470, 647, and 663 nm), using Equations (3)–(6), with results expressed in μg mL⁻¹.

$$\mathrm{Chl}a=\left(12.25\times \mathrm{ABS}663\right)-\left(2.79\times \mathrm{ABS}647\right)$$

(3)

$$\mathrm{Chl}b=\left(21.5\times \mathrm{ABS}647\right)-\left(5.10\times \mathrm{ABS}647\right)$$

(4)

$$\mathrm{Chl}t=\left(7.15\times \mathrm{ABS}663\right)+\left(18.71\times \mathrm{ABS}647\right)$$

(5)

$$\mathrm{Car}=\frac{\left|\left(1000\times \mathrm{ABS}470\right)-\left(1.82\times \mathrm{Cl}a\right)-\left(85.02\times \mathrm{Cl}b\right)\right|}{198}$$

(6)

where:

• Chl a—chlorophyll a;
• Chl b—chlorophyll b;
• Chl t—chlorophyll total;
• Car—carotenoids.

2.5.4. Leaf Gas Exchange Parameters

The internal CO₂ concentration (Ci, μmol CO₂ m⁻² s⁻¹), stomatal conductance (gs, mol H₂O m⁻² s⁻¹), transpiration (E, mmol H₂O m⁻² s⁻¹), and CO₂ assimilation rate (A, μmol CO₂ m⁻² s⁻¹) were determined on the third mature leaf counted from the apex of the main branch of the plant, using an irradiation of 1200 μmol m⁻² s⁻¹, obtained from the photosynthetic light saturation curve, and airflow of 200 mL min⁻¹, using the portable photosynthesis meter “LCPro+” from ADC BioScientific Ltd. The leaf gas exchange determinations were performed between 08:00 and 10:00 a.m., under ambient conditions of temperature and CO₂ concentration.

2.5.5. Chlorophyll Fluorescence

The chlorophyll fluorescence was performed on the third mature leaf, counted from the apex of the main branch of the plant between 08:00 and 10:00 a.m., using an OS5p pulse-modulated fluorimeter from Opti Science, employing the Fv/Fm protocol to obtain the variables—initial fluorescence (F0), maximum fluorescence (Fm), variable fluorescence (Fv = Fm − F0), and quantum efficiency of photosystem II (Fv/Fm). This protocol was performed after adaptation of the leaves to the dark for 30 minutes, using a clip of the device, to ensure that all the acceptors were oxidized, i.e., with the reaction centers open. Subsequently, the evaluations were determined under light conditions, using an actinic light source with a multi-flash saturating pulse coupled to a clip to determine the initial fluorescence before the saturation pulse (Fs), maximum fluorescence after adaptation to saturating light (Fms), electron transport rate (ETR), and quantum efficiency of photosystem II (Y_II).

2.5.6. Production Components

The fruits were harvested from each plant based on their degree of maturation when they were from 18 to 22 cm in length [36]. The following production components were obtained: number of fruits per plant, average fruit weight (g per fruit), total production per plant (g per plant), average fruit length (cm), and average fruit diameter (mm). The average diameter of the cucumber fruit was measured in the center of the fruit.

2.5.7. Post-Harvest

The pH of the pulp was determined immediately after harvest, using a digital pH meter (COMBO5, AKSO^®, São Leopoldo—RS, Brazil) previously calibrated at pH 4.0 and 7.0 with buffer solutions; the soluble solids (°Brix) were determined by direct reading using a digital refractometer (MA871, AKSO^®, São Leopoldo—RS, Brazil); and the ascorbic acid content (mg per 100g of pulp) was obtained using titration. The determinations were created using the methodologies recommended by [37]. The titratable acidity was expressed as a percentage of citric acid.

2.6. Data Analysis

For the data collected in this study regarding relative water content, the percentage of electrolyte leakage, photosynthetic pigments, leaf gas exchange parameters, chlorophyll fluorescence, production components and yield, and post-harvest quality, the levels of sources of variation, electrical conductivity of the nutrient solution (ECns), and salicylic acid (SA) concentrations were explored and submitted to the distribution normality test (Shapiro–Wilk) to verify if the data obeyed normal distribution. Then, the analysis of variance (ANOVA) was realized and, in cases of significance, being the quantitative factors, linear and quadratic regression analyzes were performed using the SISVAR-ESAL statistical program [38]. The definition of the model (linear or quadratic) was based on the values of the coefficient of determination (R²) and the biological significance of the phenomenon. Further, the effects of the interaction (ECns × SA) were analyzed using response surface curves, prepared with the SigmaPlot v.12.5 software.

3. Results

The relative water content (RWC), the percentage of electrolyte leakage in the leaf blade (% EL), and all the variables of leaf gas exchange were affected significantly (p ≤ 0.01) by the interaction between the electrical conductivity of the nutrient solution and the concentrations of salicylic acid (ECns × SA) (

Table 2. Summary of the analysis of variance (ANOVA) for relative water content (RWC), percentage of electrolyte leakage (% EL), internal CO₂ concentration (Ci), stomatal conductance (gs), transpiration (E), and CO₂ assimilation rate (A) of Japanese cucumber grown in a hydroponic system with saline nutrient solution and foliar application of salicylic acid, 40 days after transplanting.

Source of Variation	DF	Mean Squares
		RWC	% EL	Ci	gs	E	A
Saline nutrient solution (ECns)	3	1607.72 **	106.41 **	19,058.13 **	0.012 **	2.93 **	237.04 **
Linear regression	1	4218.93 **	307.07 **	51,606.70 **	0.03 **	7.64 **	685.88 **
Quadratic regression	1	601.59 ns	8.19 *	3835.94 *	0.001 ns	0.79 ns	25.11 ns
Residual 1	9	24.42	0.06	180.69	2.0 × 10−6	0.01	0.05
Salicylic acid (SA)	3	11.26 ns	136.83 **	4900.61 **	0.002 **	0.38 **	130.82 **
Linear regression	1	-	60.31 *	1070.47 ns	0.001 *	0.07 *	141.21 *
Quadratic regression	1	-	223.59 **	11,184.12 **	0.004 **	1.04 **	165.86 **
Interaction (ECns × SA)	9	107.06 *	0.33 **	1300.44 *	6.0 × 10−6 *	3.52 **	10.86 **
Residual 2	36	29.07	3.40	415.59	5.7 × 10−4	0.28	0.64
CV 1 (%)		6.62	3.66	5.93	10.55	6.32	5.81
CV 2 (%)		7.23	5.10	8.99	13.89	12.32	9.83

DF: degree of freedom; CV: Coefficient of variation; ^ns, *, and **, respectively, not significant, significant at a p ≤ 0.05 and p ≤ 0.01.

).

The foliar application of salicylic acid (SA) up to concentrations of 2.0 mM promoted an increase in the RWC, even when plants were cultivated with the highest ECns (6.6 dS m⁻¹) (Figure 4A). The highest RWC (85.05%) was observed in plants subjected to ECns of 2.1 dS m⁻¹ and SA concentration of 2.0 mM, corresponding to an increase of 3.34% compared to plants cultivated with the same ECns (2.1 dS m⁻¹) and without SA application (0 mM). However, the application of SA on leaves at concentrations greater than 2.0 mM intensified the harmful effects of salt stress on RWC, and the lowest value (59.03%) was obtained in plants that received ECns of 6.6 dS m⁻¹ and SA concentration of 5.4 mM.

The increase in the electrical conductivity of the nutrient solution increased electrolyte leakage in the leaf blade (% EL), regardless of the concentration of SA (Figure 4B). The application of SA at concentrations greater than 1.5 mM intensified the harmful effects of saline stress with the highest value of % EL (42.77%) obtained in plants subjected to ECns of 6.6 dS m⁻¹ and SA concentration of 5.4 mM. However, cucumber plants subjected to the highest level of ECns (6.6 dS m⁻¹) and SA concentration of 1.4 mM showed an EL of 37.41%, i.e., a reduction of 12.53% compared to plants cultivated with the same ECns and SA application of 5.4 mM, demonstrating the beneficial effect of SA on the acclimatization of plants to saline stress when applied at appropriate concentrations.

The internal CO₂ concentration (Ci) was reduced by the application of SA up to the concentration of 2.0 mM, regardless of the ECns level (Figure 5A). The lowest value of the internal CO₂ concentration (164.7 μmol CO₂ m⁻² s⁻¹) was observed in plants subjected to ECns of 2.1 dS m⁻¹ and SA concentration of 2.0 mM. Cucumber plants subjected to the highest level of ECns (6.6 dS m⁻¹) and SA concentration of 2.0 mM showed a reduction of 9.1% (24.0 μmol CO₂ m⁻² s⁻¹) in the internal CO₂ concentration compared to plants cultivated with the same ECns and without SA application (0 mM).

The foliar application of salicylic acid with concentrations of up to 2.0 mM promoted an increase in stomatal conductance (gs), even when plants were cultivated under ECns of 6.6 dS m⁻¹ (Figure 5B). The highest value of stomatal conductance (0.305 mol H₂O m⁻² s⁻¹) was verified in plants cultivated with ECns of 2.1 dS m⁻¹ and an SA concentration of 2.0 mM, corresponding to an increase of 4.81% (0.014 mol H₂O m⁻² s⁻¹) compared to plants cultivated with the same salinity level (2.1 dS m⁻¹) and without SA application (0 mM). However, the foliar application of SA at concentrations greater than 2.0 mM intensified the harmful effects of salt stress on stomatal conductance; the lowest value (0.221 mol H₂O m⁻² s⁻¹) was obtained in plants subjected to ECns of 6.6 dS m⁻¹ and SA concentration of 5.4 mM.

The transpiration (E) and CO₂ assimilation rate (A) of cucumber plants were also favored by the application of salicylic acid on leaves up to a concentration of 2.0 mM, regardless of the ECns (Figure 5C and 5D). The plants subjected to SA concentration of 2.0 mM and ECns of 2.1 dS m⁻¹ attained the highest transpiration (4.7 mmol H₂O m⁻² s⁻¹) and CO₂ assimilation rate (34.3 μmol CO₂ m⁻² s⁻¹). In relative terms, the transpiration and CO₂ assimilation rate of plants cultivated with ECns of 2.1 dS m⁻¹ and subjected to SA concentration of 2.0 mM compared to those cultivated under the same level of salinity and without SA application (0 mM) enabled increments of 1.52% (0.07 mmol H₂O m⁻² s⁻¹) and 5.9% (1.90 μmol CO₂ m⁻² s⁻¹) to be observed, respectively.

There was a significant effect (p ≤ 0.01) of the interaction between the ECns and the concentrations of SA only for chlorophyll a and chlorophyll total (

Table 3. Summary of the analysis of variance (ANOVA) for chlorophyll a (Chl a), chlorophyll b (Chl b), chlorophyll total (Chl t), and carotenoids (Car) of Japanese cucumber grown in a hydroponic system with saline nutrient solution and foliar application of salicylic acid, 40 days after transplanting.

Source of Variation	DF	Mean Squares
		Chl a	Chl b	Chl t	Car
Saline nutrient solution (ECns)	3	54.47 **	27.09 **	154.76 **	1.90 **
Linear regression	1	158.72 **	67.54 **	433.61 **	5.41 **
Quadratic regression	1	4.16 ns	12.06 ns	30.36 ns	0.26 ns
Residual 1	9	3.94	0.59	6.59	0.62
Salicylic acid (SA)	3	0.49 ns	1.26 ns	9.73 ns	1.01 ns
Interaction (ECns × SA)	9	5.42 *	0.76 ns	18.33 *	0.29 ns
Residual 2	36	3.24	1.20	7.52	0.38
CV 1 (%)		10.87	12.02	10.43	13.65
CV 2 (%)		9.87	17.19	11.14	10.48

DF: degree of freedom; CV: Coefficient of variation; ^ns, *, and **, respectively, not significant, significant at a p ≤ 0.05 and p ≤ 0.01.

). On the other hand, the electrical conductivity of the nutrient solution significantly influenced (p ≤ 0.01) all the variables of photosynthetic pigments, while the concentrations of salicylic acid alone did not affect the chlorophyll and carotenoid contents of Japanese cucumber, 40 days after transplanting.

The increase in the ECns reduced the chlorophyll a (Figure 6A) and chlorophyll total (Figure 6B) contents of cucumber plants. However, the foliar application of SA up to concentration of 2.0 mM reduced the effects of saline stress, promoting increments in Chl a and Chl t, even when plants were subjected to the highest ECns (6.6 dS m⁻¹). The highest values of chlorophyll a (21.17 μg mL⁻¹) and chlorophyll total (28.76 μg mL⁻¹) were obtained in plants cultivated with ECns of 2.1 dS m⁻¹ and SA concentration of 2.0 mM, corresponding to increments of 3.61% (0.74 μg mL⁻¹) in chlorophyll a and 3.50% (0.97 μg mL⁻¹) in chlorophyll total for plants cultivated with the same salinity level (2.1 dS m⁻¹) and without the application of SA (0 mM). On the other hand, the application of SA on leaves at concentrations greater than 2.0 mM intensified the harmful effects of saline stress, with the lowest values of chlorophyll a (15.66 μg mL⁻¹) and chlorophyll total (20.23 μg mL⁻¹) in plants subjected to ECns of 6.6 dS m⁻¹ and SA concentration of 5.4 mM.

The synthesis of chlorophyll b (Figure 6C) and carotenoids (Figure 6D) of cucumber plants were negatively affected by the increase in the electrical conductivity of the nutrient solution, with reductions, per unit increment in ECns, of 7.46% in chlorophyll b content and 3.82% in carotenoid content. Comparing the chlorophyll b and carotenoid contents of plants grown under ECns of 6.6 dS m⁻¹ to those of plants subjected to ECns of 2.1 dS m⁻¹, reductions of 39.9% (3.21 μg mL⁻¹) and 18.17% (1.17 μg mL⁻¹) were observed, respectively.

According to the summary of the analysis of variance, there was a significant effect (p ≤ 0.01) of the interaction between the ECns and the concentrations of SA only on the quantum efficiency of photosystem II (Fv/Fm) (

Table 4. Summary of the analysis of variance (ANOVA) for initial fluorescence (F0), maximum fluorescence (Fm), variable fluorescence (Fv), quantum efficiency of photosystem II (Fv/Fm), initial fluorescence before saturation pulse (Fs), quantum efficiency of photosystem II in the light phase (Y_II), and electron transport rate (ETR) of Japanese cucumber grown in a hydroponic system with saline nutrient solution and foliar application of salicylic acid, 40 days after transplanting.

Source of Variation	DF	Mean Squares
		F0	Fm	Fv	Fv/Fm	Fs	YII	ETR
Saline nutrient solution (ECns)	3	16,234.27 **	878,366.35 **	889,782.38 **	0.021 **	174.32 **	0.017 **	120.39 ns
Linear regression	1	47,458.15 **	225,506.26 **	20,197.31 **	0.028 *	505.99 **	0.006 ns	-
Quadratic regression	1	669.52 ns	73,902.41 ns	178,325 ns	0.034 **	10.51 ns	0.037 **	-
Residual 1	9	1368.75	46,782.09	178.25	72.0 × 10−6	4.73	0.001	53.01
Salicylic acid (SA)	3	1906.64 ns	44,209.09 ns	695.02 ns	25.6 × 10−5 ns	93.97 ns	0.0001 ns	19.61 ns
Interaction (ECns × SA)	9	2315.81 ns	113,099.59 ns	26,025.02 ns	0.007 **	37.07 ns	0.002 ns	62.11 ns
Residual 2	36	2395.35	64,148.96	15,211.43	11.8 × 10−5	5.87	0.001	31.74
CV 1 (%)		6.44	9.73	9.94	10.21	5.41	5.94	12.94
CV 2 (%)		8.51	11.40	8.66	11.42	6.79	5.08	10.01

DF: degree of freedom; CV: Coefficient of variation; ^ns, *, and **, respectively, not significant, significant at a p ≤ 0.05 and p ≤ 0.01.

). As a single factor, the levels of ECns significantly influenced (p ≤ 0.01) all variables, except the electron transport rate (ETR). On the other hand, SA concentrations had no significant effect on any of the chlorophyll fluorescence variables.

The increase in the ECns had an increasing linear effect on the initial fluorescence (F0) of cucumber plants (Figure 7A), with an increment of 3.22% per unit increase in ECns. The plants cultivated with ECns of 6.6 dS m⁻¹ had an increase of 13.57% (73.06) compared to those grown with ECns of 2.1 dS m⁻¹. Unlike the effect observed on the initial fluorescence (Figure 7A), the maximum fluorescence was reduced as the ECns increased (Figure 7B). The plants grown under ECns of 6.6 dS m⁻¹ showed a reduction of 20.68% (512.6) compared to those cultivated with ECns of 2.1 dS m⁻¹.

The variable fluorescence (Fv) of cucumber plants decreased with the increase in the electrical conductivity of the nutrient solution (Figure 7C). The plants cultivated with ECns of 2.1 dS m⁻¹ had a variable fluorescence of 1697.34, while the minimum value (1152.08) was verified under ECns of 6.6 dS m⁻¹, i.e., there was a reduction of 545.26 (32.12%) under the highest salinity level. The application of salicylic acid on leaves at the concentration of 2.0 mM promoted an increase in the quantum efficiency of photosystem II (Figure 7D), even in plants cultivated with the highest ECns level (6.6 dS m⁻¹); however, the highest value of Fv/Fm (0.802) was obtained in plants cultivated with ECns of 3.8 dS m⁻¹, corresponding to an increase of 8.52% (0.063) compared to plants subjected to ECns of 2.1 dS m⁻¹ and without SA application (0 mM). The lowest quantum efficiency of photosystem II (0.684) was recorded in plants cultivated with ECns of 6.6 dS m⁻¹ and SA concentration of 5.4 mM.

For the initial fluorescence before the saturation pulse (Fs) (Figure 8A), it was observed that the increase in ECns promoted an increment of 12.55% in plants cultivated with ECns of 6.6 dS m⁻¹ compared to those under ECns of 2.1 dS m⁻¹. On the other hand, the quantum efficiency of photosystem II in the light phase (Figure 8B) decreased when nutrient solutions with electrical conductivity above 2.1 dS m⁻¹ were used, with the lowest value of Y_II (0.565) observed in plants cultivated with ECns of 6.6 dS m⁻¹.

The interaction between the ECns and SA concentrations significantly influenced (p ≤ 0.05) the number of fruits, total production per plant, and average fruit weight (p ≤ 0.01) (

Table 5. Summary of the analysis of variance (ANOVA) for the number of fruits (NF), average fruit weight (AFW), total production per plant (TPP), average fruit length (AFL), and average fruit diameter (AFD) of Japanese cucumber grown in a hydroponic system with saline nutrient solution and foliar application of salicylic acid, 45 days after transplanting.

Source of Variation	DF	Mean Squares
		NF	AFW	TPP	AFL	AFD
Saline nutrient solution (ECns)	3	40.18 **	13,910.47 **	5,869,190.94 **	49.47 **	126.64 **
Linear regression	1	117.83 **	40,951.25 **	16,913,963.71 **	143.91 **	364.10 **
Quadratic regression	1	2.71 ns	481.58 ns	409,158.52 ns	4.36 *	11.78 ns
Residual 1	9	0.47	753.89	184,044.58	0.64	2.42
Salicylic acid (SA)	3	0.17 ns	527.18 ns	66,638.93 ns	3.88 ns	2.62 ns
Interaction (ECns × SA)	9	1.67 *	5877.88 **	543,499.86 *	1.01 ns	1.60 ns
Residual 2	36	0.42	3054.67	89,665.21	0.63	2.79
CV 1 (%)		21.29	8.44	19.76	4.63	4.93
CV 2 (%)		19.42	16.98	17.75	3.89	5.29

DF: degree of freedom; CV: Coefficient of variation; ^ns, *, and **, respectively, not significant, significant at a p ≤ 0.05 and p ≤ 0.01.

). The levels of ECns significantly influenced (p ≤ 0.01) all the variables of the production components, while the concentrations of salicylic acid alone did not affect any variable of the production components of cucumber.

The increase in the ECns negatively affected the number of cucumber fruits per plant (NFP), regardless of the SA concentration (Figure 9A). It is worth pointing out that the foliar application of SA above the estimated concentration of 2.3 mM intensified the effects of saline stress, with the lowest value of NFP (0.90 fruits per plant) in plants cultivated with ECns of 6.6 dS m⁻¹ and SA concentration of 5.4 mM, while the highest number of fruits per plant was 5.05, observed in plants subjected to ECns of 2.1 dS m⁻¹ and an SA concentration of 2.0 mM.

The foliar application of SA at the estimated concentration of 2.0 mM also promoted increments in average fruit weight (Figure 9B) and total production per plant—TPP (Figure 9C). The plants cultivated with ECns of 2.1 dS m⁻¹ and SA concentrations of 2.0 mM stood out with the highest values of AFW (383.40 g per fruit) and TPP (1932.59 g plant), corresponding to increments of 7.95% in AFW and 7.66% in TPP, compared to plants cultivated with the same level of ECns (2.1 dS m⁻¹) but without SA application (0 mM).

However, it is worth noting that the application of SA on leaves at concentrations greater than 2.1 mM, associated with increased ECns, reduced the AFW and TPP of cucumber (Figure 9B,C), with the lowest values of AFW (276.54 g per fruit) and TPP (263.35 g per plant) in plants cultivated with ECns of 6.6 dS m⁻¹ and SA concentrations of 5.4 mM.

The ECns negatively affected the length and diameter of cucumber fruits (Figure 10A and 10B), with reductions of 3.78% in the average fruit length—AFL—and 3.76% in the average fruit diameter—AFD—per unit increment in ECns. Comparing the AFL and AFD of plants cultivated with ECns of 6.6 dS m⁻¹ to those of plants subjected to ECns of 2.1 dS m⁻¹, reductions of 18.5% (4.03 cm) and 18.4% (6.40 mm) were observed in the length and diameter of fruit, respectively.

There was a significant effect (

Table 6. Summary of the analysis of variance (ANOVA) for hydrogen potential (pH), soluble solids (SS), ascorbic acid (AA), and titratable acidity (TA) of fruits of Japanese cucumber grown in a hydroponic system with saline nutrient solution and foliar application of salicylic acid, 45 days after transplanting.

Source of Variation	DF	Mean Squares
		pH	SS	AA	TA
Saline nutrient solution (ECns)	3	0.53 **	17.97 **	0.73 **	0.41 **
Linear regression	1	1.56 **	52.91 **	1.99 **	1.16 **
Quadratic regression	1	0.01 ns	0.99 ns	0.16 ns	0.005 ns
Residual 1	9	0.03	0.004	0.006	0.001
Salicylic acid (SA)	3	0.35 **	0.009 ns	0.35 **	0.65 **
Linear regression	1	0.17 ns	-	0.49 *	0.69 *
Quadratic regression	1	0.55 **	-	0.45 **	1.21 **
Interaction (ECns × SA)	9	0.13 *	0.45 **	0.006 **	0.04 *
Residual 2	36	0.02	0.05	0.002	0.008
CV 1 (%)		3.24	3.42	2.27	2.74
CV 2 (%)		3.00	4.62	4.86	6.75

DF: degree of freedom; CV: Coefficient of variation; ^ns, *, and **, respectively, not significant, significant at a p ≤ 0.05 and p ≤ 0.01.

) of the interaction between the ECns and the concentrations of SA on the hydrogen potential (pH), soluble solids (SS), ascorbic acid (AA), and titratable acidity (TA) of fruits of Japanese cucumber cultivated in a hydroponic system, at 45 days after transplanting.

The increase in the ECns reduced the pH of the cucumber fruit pulp (Figure 11A), and the reductions were intensified with salicylic acid concentrations greater than 2.1 mM, with the lowest pH value (5.12) verified in plants cultivated with ECns of 6.6 dS m⁻¹ and an SA concentration of 5.4 mM. However, it is observed that the foliar application of SA up to the concentration of 2.0 mM promoted an increase in pH, with the highest pH value (5.92) obtained in plants subjected to ECns of 2.1 dS m⁻¹ and SA concentrations of 2.0 mM. The soluble solids (Figure 11B) of cucumber fruits were also reduced by the increase in the ECns, with the highest value of soluble solids (5.92 °Brix) observed in plants cultivated with ECns of 2.1 dS m⁻¹ and SA concentration of 2.0 mM, while the fruits of plants subjected to the same concentration of SA (2.0 mM) and ECns of 6.6 dS m⁻¹ showed a 43.9% (2.60 °Brix) reduction in soluble solids compared to the fruits of cucumber plants under ECns of 2.1 dS m⁻¹.

The foliar application of SA at the estimated concentration of 2.0 mM promoted an increase in the ascorbic acid content (Figure 11C) and titratable acidity (Figure 11D) of cucumber fruits, even when plants were subjected to the highest salinity of nutrient solution (6.6 dS m⁻¹). However, the highest values of ascorbic acid (1.44 mg 100g⁻¹ pulp) and titratable acidity (1.67%, Figure 11D) were obtained in plants grown under ECns of 2.1 dS m⁻¹, corresponding to an increase of 6.67% (0.09 mg 100g⁻¹ pulp) in AA and 7.05% (0.11%) in TA compared to the fruits of plants cultivated with the same level of ECns but without an SA application (0 mM). The lowest value of AA (0.67 mg 100g⁻¹ pulp) and TA (0.88%) were obtained in plants cultivated with ECns of 6.6 dS m⁻¹ and an SA concentration of 5.4 mM.

4. Discussion

The employment of saline water in crop production, either for supplemental irrigation or in the preparation of nutrient solutions in hydroponic systems, has long been debated [2,3]. Several studies have indicated that the use of brackish water can lead to saline stress and reduced crop yields [39,40]. This study demonstrated that saline stress caused by increased ECns has negative impacts on the physiology, production, and post-harvest quality of fruits of Japanese cucumber grown in an NFT hydroponic system. However, the harmful effects of salt stress were to some extent attenuated by the application of salicylic acid on leaves.

The root is the main organ that remains in direct contact with the nutrient solution and therefore accumulates most of the potentially toxic elements [41]. Under conditions of saline stress, an excess of toxic ions is common, especially Na⁺ and Cl⁻, which restrict the capacity of roots to absorb water [3,42]. The reduction in water absorption leads to reduced leaf status, as observed in this study through the relative water content (Figure 4A). Similar results were reported for different vegetable crops cultivated in the hydroponic system, such as melon [15], okra [3], and tomatoes [43].

An increase in the ECns caused an increase in the percentage of electrolyte leakage (EL) in the leaf blade (Figure 4B). Salt stress increases the production of reactive oxygen species (ROS) and damages proteins and nucleic acids, as well as cell membrane lipids, causing lipid peroxidation [44,45], and, consequently, greater EL in the leaf blade. However, the increase observed in the present study did not cause injuries to the cell membranes of leaf tissues, as damage is considered to only occur when the EL exceeds 50% [46]. Corroborating the present study, no injuries to the cell membrane of leaves were observed in hydroponic okra and melon plants under salt stress [3,47], with electrolyte leakage in the leaf blade less than 50%.

The harmful effects of salt stress on the RWC and percentage of EL in the leaf blade were attenuated by the foliar application of SA at the concentration of 2.0 mM, i.e., salicylic acid increased the RWC (Figure 4A) and reduced the percentage of EL in the leaf blade (Figure 4B). The beneficial effects of foliar application of salicylic acid have also been reported by [3], in the hydroponic cultivation of okra plants under salt stress (ECns ranging from 2.1 to 9.0 dS m⁻¹). These authors found that the foliar application of SA at a concentration of 1.5 mM was able to increase the RWC and reduce the percentage of EL in the leaf blade. Salicylic acid is one of the principal phenolic compounds and acts as a growth regulator, playing a unique role in various physiological and biochemical processes [9,48]. In addition, it acts in the reduction in lipid peroxidation and can interact with other plant hormones to increase the tolerance of plants to salt stress [49,50].

The leaf gas exchange in cucumber plants was negatively impacted by the increase in ECns (Figure 5). Under conditions of abiotic stress such as saline stress, there is an imbalance between the production of ROS and antioxidant defense, leading to oxidative stress [51]. Thus, the availability of water to plants decreases and the stomata are partially closed (Figure 5B) as a mechanism to reduce the transpiration rate and salt absorption (Figure 5C) [52]. This directly reduces the activity of the ribulose-1,5-bisphosphate carboxylase/oxygenase enzyme in the Calvin cycle and increases the production of ROS by incomplete oxygen recovery, reducing the CO₂ assimilation rate (Figure 5D) [52]. Reductions in the leaf gas exchange in cucumber plants as a function of salt stress have also been observed in other studies, such as [53,54].

On the other hand, the application of SA on leaves at the concentration of 2.0 mM alleviated the effect of salt stress on leaf gas exchange (Figure 5). Salicylic acid can increase the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase as well as potassium absorption and ATP content, maintaining adequate Na⁺/K⁺ ratio in plants, thus favoring tolerance to saline stress [55]. The SA also increases the accumulation of osmoprotectants, improving turgor in plant cells under stress, and the activation of antioxidant enzymes, resulting in better photosynthetic activity [56].

Photosynthetic pigments play an essential role in energy assimilation in plants and their levels are significantly altered under salt stress [57]. The results of the present study reveal that the increase in ECns negatively affected the synthesis of chlorophyll and carotenoids in cucumber plants (Figure 6). However, the foliar application of SA at a concentration of 2.0 mM mitigated the effects of salt stress, promoting increments in chlorophyll a (Figure 6A) and chlorophyll total (Figure 6B).

The beneficial effect of SA is associated with its role as a signaling molecule and in the activation of the plant’s defense system, which includes osmoregulation, elimination of ROS, and ionic homeostasis [58]. Salicylic acid can stimulate chlorophyll biosynthesis and/or reduce its degradation [59]. An increase in chlorophyll synthesis as a function of the foliar application of SA has also been observed in studies with bell peppers [15], cherry tomatoes [18], and strawberries [60].

Salt stress damages the chloroplast structure, promotes non-radiative heat dissipation, and inhibits electron transfer in photosystem II [2]. The results obtained in the present study demonstrate that increasing the ECns reduced the maximum fluorescence (Figure 7B), indicating damage to the light-harvesting complex of photosystem II. An increase in F0 (Figure 7A) is also an indication of damage to the reaction center of photosystem II or a reduction in the capacity to transfer excitation energy from the light-harvesting system to the reaction center of photosystem II (PSII) [61].

Maximum fluorescence (Figure 7B) and variable fluorescence (Figure 7C) were reduced by the increase in the ECns, with no influence of SA concentrations. Fm is the point at which the fluorescence of the plant reaches its maximum capacity and practically all quinone is reduced [62]. The decrease in Fm reflects a reduction in maximum light energy absorbed by photosystem II and the degradation of photosynthetic pigments [63,64], as observed in the present study (Figure 6). Variable fluorescence refers to the plant’s capacity to transfer the energy of electrons ejected from pigment molecules to the formation of NADPH, ATP, and reduced ferredoxin, so its reduction may indicate that the photosynthetic apparatus was damaged by saline stress, compromising photosystem II, with negative effects on the photosynthetic process [65].

The foliar application of SA at the concentration of 2.0 mM increased the quantum efficiency of photosystem II in the dark phase (Figure 7D), regardless of the level of ECns. Thus, the results reveal that the Fv/Fm of plants sprayed with SA at a concentration of 2.0 mM was not compromised up to 5.0 dS m⁻¹, as the values of Fv/Fm ranged from 0.75 to 0.80, i.e., they were greater than or equal to 0.75. Several authors consider Fv/Fm values between 0.75 and 0.85 as normal in non-stressed plants [66,67].

Salicylic acid, in addition to signaling antioxidant genes and proteins under salt stress conditions, can lead to a greater accumulation of ions responsible for osmoregulation and membrane structure, such as K⁺ and Ca²⁺, and reduce the concentration of toxic Na⁺ and Cl⁻ ions [16]. This may be related to increased photochemical efficiency in cucumber plants sprayed with SA at a concentration of 2.0 mM [68,69]. Corroborating the present study, Mendonça et al. [3] evaluated the effect of foliar application of SA on hydroponic okra plants under salt stress (ECns ranging from 2.1 to 9.0 dS m⁻¹) and reported that the application of SA at a concentration of 1.2 mM promoted an increase in Fv/Fm.

The reductions in the initial fluorescence before the saturation pulse and in the quantum efficiency of photosystem II in the light phase (Y_II) (Figure 8) due to the increase in the ECns indicate a decrease in photosynthetic activity, which corroborates the reductions observed in the CO₂ assimilation rate (Figure 5D) of plants subjected to the highest levels of ECns.

The results of this study indicated that an increase in the electrical conductivity of the nutrient solution negatively affected the production components of Japanese cucumber, with reductions in the NFP, AFW, TPP, AFL, and AFD. These results are a consequence of the osmotic and ionic effects induced by the high salinity of the nutrient solution [3]. The reduction in osmotic potential causes water stress and accumulation of Na⁺ and Cl⁻ ions, leading to nutritional imbalance and a consequent reduction in production components [70]. Reductions in production components as a function of salt stress in hydroponic cultivation have also been observed in other studies with okra [71], cucumber [72], ‘Biquinho’ pepper [73], and Italian zucchini [40].

The foliar application of SA at a concentration of 2.0 mM promoted an increase in the number of fruits, average fruit weight, and total production per plant, especially in plants cultivated under ECns of 2.1 dS m⁻¹. The increase in production components observed in plants treated with SA (2.0 mM) may be associated with chlorophyll a fluorescence and gas exchange responses. In summary, the foliar application of SA in cucumber plants acted as an elicitor, increased leaf turgor (Figure 4A), and reduced lipid peroxidation (Figure 4B), consequently protecting the photosynthetic apparatus of the plants. In addition, this protection was observed through the higher quantum efficiency of photosystem II (Figure 7D). Finally, the plants treated with SA at a concentration of 2.0 mM showed the highest CO₂ assimilation rate (Figure 5D) and this photo-assimilated carbon was translocated to the fruits, resulting in a greater number of fruits, average fruit weight, and total production per plant (Figure 9).

It is worth mentioning that, in this study, SA concentrations above 2.1 mM intensified the deleterious effects of salt stress, causing reductions in the production components, the quantum efficiency of photosystem II, and CO₂ assimilation rate with lower values obtained in plants cultivated under ECns of 6.6 dS m⁻¹ and sprayed with SA at a concentration of 5.4 mM. Therefore, the beneficial effect of SA depends on several factors, including concentration and mode of application [10,11]. According to Aires et al. [74], high concentrations of SA can cause high levels of oxidative stress, leading to a reduction in stress tolerance.

From the results obtained in this study, it can be suggested that the application of SA on leaves at adequate concentrations can increase the production of Japanese cucumber cultivated in a hydroponic system, probably by stimulating physiological processes involved in the active transfer of photosynthetic products from the source to the sink, which is consistent with the results obtained by [75]. This beneficial effect of SA on the production components of cucumber may be related to its role in reducing the absorption of Na⁺ and increasing the absorption of N, P, K, Ca, and Mg by plants [76]. Increases in production components as a function of the foliar application of SA have also been observed in hydroponic okra [3] and melon [15].

Average fruit length and diameter are variables of great interest to the consumer because they define the size of the edible part and the classification of the fruit. In this study, there was no positive or negative influence of the application of SA on the diameter and length of the cucumber fruit, although the SA increased the number of fruits, their average weight, and, consequently, the total production per plant. However, the length and diameter of cucumber fruits were reduced by the increase in the ECns. These results are a consequence of the high salinity of the nutrient solution, which can cause a water deficit by lowering the osmotic potential and toxicity of specific ions such as Cl⁻ and Na⁺ [13].

Post-harvest variables play an important role because they are responsible for the organoleptic characteristics that provide the feeling of freshness and palatability [77]. The pH is an important characteristic in fruit quality evaluation because low values can guarantee pulp conservation without the need for heat treatment, hence avoiding the loss of nutritional quality. Thus, the reduction observed in pH with an increase in the ECns shows that the saline stress imposed on the cucumber increased the acidic character of the pulp, besides reducing the concentration of the total soluble solids. It is worth pointing out that changes in the post-harvest quality of the fruits produced under salt stress conditions occur due to the action of the osmotic effect of the nutrient solution, inhibiting the absorption of water and nutrients by plants and their photosynthetic capacity [78].

The foliar application of SA at a concentration of 2.0 mM promoted an increase in soluble solids, especially in plants grown under ECns of 2.1 dS m⁻¹. The increase in soluble solids may be related to the role of SA in reducing the rate of degradation of polysaccharides and, consequently, the greater availability of simple sugars, besides delaying fruit maturity by inhibiting the production and effects of ethylene [79]. An increase in soluble solids content due to SA application was also reported by [46] in the melon fruits cultivated in a hydroponic system with brackish water. The ascorbic acid content and titratable acidity were also increased as a function of the application of SA at a concentration of 2.0 mM. The increase in these variables is a highly valued characteristic, particularly when one intends to use cucumber for industrial processing, as it reduces the employment of acidifiers, thus improving nutritional and organoleptic quality, being considered a principal attribute for post-harvest evaluation [80].

5. Conclusions

Cucumber is a vegetable crop that is sensitive to saline stress even when grown in a hydroponic system, being negatively affected by the electrical conductivity of nutrient solution above 2.1 dS m⁻¹. However, the application of salicylic acid on leaves between concentrations of 1.4 and 2.0 mM not only promotes an increase in the relative water content in the leaf blade but also positively influences the synthesis of photosynthetic pigments, leaf gas exchange, and the quantum efficiency of photosystem II, reducing the percentage of electrolyte leakage in the leaf blade. In addition, salicylic acid increases the production and postharvest quality of cucumber fruits. These observations reinforce the hypothesis that the foliar application of salicylic acid in adequate concentrations can act as a crucial signaling molecule in the attenuation of saline stress in cucumber plants, which can enhance the use of brackish water in hydroponic cultivation, especially in regions with the scarcity of freshwater of low salinity. Further studies are needed to understand how salicylic acid acts in salt stress signaling through biochemical analysis. These studies will help in understanding the metabolic and detoxifying mechanisms that occur in plants to develop efficient strategies to mitigate the harmful effects of saline stress.