Morphophysiology of cucumber under saline nutrient solutions and salicylic acid application in hydroponic system¹

Morfofisiologia de pepino sob soluções nutritivas salinas e aplicação de ácido salicílico em sistema hidropônico

ABSTRACT

In conventional systems, the use of saline water makes vegetable production unfeasible, so hydroponic cultivation is a promising alternative in semi-arid areas, as it minimizes socioeconomic impacts. An alternative for mitigating salt stress effects on plants is the application of salicylic acid. The objective of this study was to evaluate the effect of foliar application of salicylic acid on the physiological aspects and growth of ‘Hiroshi’ japanese cucumber under saline nutrient solutions in a hydroponic system. The study was carried out in a greenhouse. The hydroponic system used was the Nutrient Film Technique - NFT. Treatments consisted of four values of electrical conductivity of the nutrient solution - ECns (2.1, 3.0, 3.9, and 4.8 dS m^-1) and four concentrations of salicylic acid - SA (0, 1.8, 3.6, and 5.4 mM), distributed in a completely randomized design in split plots, with ECns levels considered the plots and SA concentrations considered the subplots, with four replicates. Growth, gas exchange, photosynthetic pigments, relative water content, and electrolyte leakage in the leaf blade were evaluated. Salicylic acid at concentrations ranging from 3.7 to 4.7 mM increased stomatal conductance and carotenoid synthesis, under ECns of 2.1 dS m^-1. ECns above 2.1 dS m^-1 reduced gas exchange, photosynthetic pigment content, relative water content, and growth of Japanese cucumber plants, at 23 days after transplanting.

Key words:
Cucumis sativus L.; phytohormone; saline nutrient solution

HIGHLIGHTS:

Electrical conductivity (EC) of the nutrient solution from 2.1 dS m^-1 limits chlorophyll content, gas exchange, and growth.

Salicylic acid at concentration of 3.7 mM increases stomatal conductance under nutrient solution EC of 2.1 dS m^-1.

Foliar application of 5.4 mM salicylic acid increases chlorophyll a content.

RESUMO

Em sistemas convencionais, o uso de água salina inviabiliza a produção de hortaliças, sendo o cultivo hidropônico uma alternativa promissora em áreas semiáridas, pois minimiza os impactos socioeconômicos. Uma alternativa para mitigação dos efeitos do estresse salino sobre as plantas é a aplicação de ácido salicílico. O objetivo deste estudo foi avaliar o efeito da aplicação foliar do ácido salicílico sobre os aspectos fisiológicos e crescimento de pepino japonês ‘Hiroshi’ sob soluções nutritivas salinas em sistema hidropônico. O trabalho foi conduzido em casa de vegetação. O sistema de cultivo utilizado foi o hidropônico tipo Técnica de Fluxo Laminar de Nutriente - NFT. Os tratamentos consistiram de quatro níveis de condutividade elétrica da solução nutritiva - CEsn (2,1; 3,0; 3,9 e 4,8 dS m^-1) e quatro concentrações de ácido salicílico - AS (0; 1,8; 3,6 e 5,4 mM), distribuídos em delineamento inteiramente casualizados em parcelas subdivididas, sendo os valores de CEsn considerados as parcelas e as concentrações de AS as subparcelas, com quatro repetições. Foram avaliadas as variáveis de crescimento, trocas gasosas, pigmentos fotossintéticos, teor relativo de água e extravasamento de eletrólitos no limbo foliar. O AS nas concentrações variando de 3,7 e 4,7 mM aumentou a condutância estomática e a síntese de carotenoides, sob efeito de CEsn de 2,1 dS m^-1. A CEsn acima de 2,1 dS m^-1 reduziu as trocas gasosas, os teores de pigmentos fotossintéticos, o conteúdo relativo de água e o crescimento das plantas de pepino japonês, 23 dias após o transplantio.

Palavras-chave:
Cucumis sativus L.; fitormônio; solução nutritiva salina

Introduction

Japanese cucumber (Cucumis sativus L.) is a vegetable belonging to the Cucurbitaceae family whose fruits are rich in minerals such as magnesium, phosphorus, and potassium, and vitamins A, B, C, and K, appreciated and consumed worldwide fresh or preserved, in salads (Chacón-Padilla & Monge-Pérez, 2020). It stands out for its precocious character, developing in areas of the Brazilian semi-arid region; however, it is classified as moderately sensitive to salinity of irrigation water (< 2.5 dS m^-1) (Stepien & Klobus, 2006).

In the semi-arid region of Northeast Brazil, the restriction in water availability due to climatic conditions (irregular rainfall and high evapotranspiration rates) limits agricultural production in most months of the year, making cultivation under irrigated conditions a necessity (Soares et al., 2015; Lacerda et al., 2022). Water restriction induces producers to use water generally with high electrical conductivity from wells and dams, where inadequate management alters soil characteristics and crop development, with a negative impact on employment opportunity and revenue generation (Dias et al., 2019).

Using water with high salt content causes an osmotic effect, restricting the absorption of water and nutrients, induces partial closure of the stomata, and causes an ionic effect, triggered by the absorption and accumulation of toxic ions such as Na⁺ and Cl^- in plant tissues, leading to nutritional imbalance, hence interfering in morphophysiological processes and plant production (Soares et al., 2018; Lima et al., 2019).

In conventional soil cultivation, the use of brackish water affects the photosynthetic processes and growth of vegetables, such as mini watermelon (Alves et al., 2023), cucumber, and broccoli (Fonseca et al., 2022). However, an alternative for farmers to use brackish water is hydroponic cultivation, as it allows greater tolerance to the effect of salt stress, with constant oxygenation and a negligible matric potential, allowing plants to have greater absorption and efficiency in the use of water compared to conventional cultivation, facilitating production throughout the year, improving product quality and adding commercial value with less environmental impact (Leal et al., 2020).

Among the alternatives that have been used to mitigate salt stress effects on plants, foliar application of salicylic acid - SA stands out (Mendonça et al., 2022). SA is a phytohormone that acts in the signaling and activation of the defense system of plants through the production of antioxidant enzymes, increasing tolerance to abiotic stresses, with a regulatory function of physiological processes and restoration of membrane potential (Lotfi et al., 2020). Several studies have highlighted the beneficial effects of foliar application of SA, as observed by Soares et al. (2022) and Silva et al. (2022) with melon and cherry tomato crops, respectively.

This study is based on the hypothesis that foliar application of SA promotes activation and production of antioxidant compounds that minimize the effect of salt stress on ‘Hiroshi’ cucumber plants, favoring osmotic and ionic homeostasis, improving photosynthetic activity, and plant growth. The objective of this study was to evaluate the effects of foliar application of salicylic acid on the physiological aspects and growth of ‘Hiroshi’ Japanese cucumber under saline nutrient solutions in a hydroponic system.

Material and Methods

The experiment was carried out from July 1 to August 18, 2022, in greenhouse, with a lattice arch structure, measuring 8 m wide, 20 m long and 3 m high, covered with 200 micron transparent plastic film and 75% black shade net, with the sides also covered with 75% shade net, belonging to the Center of Sciences and Agrifood Technology (CCTA) of the Federal University of Campina Grande (UFCG), in Pombal, PB, Brazil, located at geographic coordinates 6° 46’ 13” South latitude, 37° 48’ 6” West longitude and mean altitude of 184 m. Data of air temperature (maximum and minimum) and mean relative air humidity observed during the experimental period are presented in Figure 1.

Figure 1
Maximum and minimum daily air temperature and mean relative air humidity observed inside the greenhouse during the experimental period (July 1 to August 18, 2022)

Treatments consisted of four levels of electrical conductivity of the nutrient solution - ECns (2.1, 3.0, 3.9, and 4.8 dS m^-1) and four concentrations of salicylic acid - SA (0, 1.8, 3.6, and 5.4 mM), distributed in a completely randomized experimental design in split plot, with ECns considered as plots and SA as subplots, with four replicates. Salicylic acid concentrations were established based on the study conducted by Soares et al. (2022) with melon crop, whereas ECns values were adapted from the study conducted by Dantas et al. (2022) with the Italian zucchini crop.

‘Hiroshi’ Japanese cucumber seeds from ISLA^® were used in the present study. It is a climbing, creeping, annual cultivar, with large, rough, and cordiform green leaves, with good lateral branching and monoecious yellow flowers, producing cylindrical and uniform fruits of bright dark green color, with average length of 18 to 22 cm and diameter of 3 to 4 cm.

The hydroponic system used was of the Nutrient Film Technique (NFT) type, constructed as described by Soares et al. (2022). The nutrient solution was prepared according to the recommendation of Hoagland & Arnon (1950), with concentrations (mg L^-1) of 210, 31, 234, 200, 48, 64, 0.5, 0.5, 0.05, 0.02, 0.01, and 5 of N, P, K, Ca, Mg, S, B, Mn, Zn, Cu, Mo, and Fe, respectively, obtained by dissolving 136.09, 101.10, 236.15, and 246.49 g L^-1 of KH₂PO₄, KNO₃, Ca(NO₃)₂.4H₂O, and MgSO₄.7H₂O respectively, in addition to 3.10, 1.70, 0.22, 0.75, 1.25, 13.9, and 13.9 g L^-1 of H₃BO₃, MnSO₄.4H₂O, ZnSO₄.7H₂O, CuSO₄.5H₂O, (NH₄)₆Mo₇O₂.4H₂O, FeSO₄, and EDTA, respectively. Municipal supply water with electrical conductivity of 0.3 dS m^-1 was used in preparation of nutrient solution, resulting in the lowest ECns value (2.1 dS m^-1).

Japanese cucumber was sown in 50 mL polyethylene containers containing autoclaved sand, arranged in trays. In the stage from germination to emergence of the first true leaf (on average ten days after sowing), a half-strength nutrient solution (50% of the recommendation) was used. After the appearance of the first true leaf, the plants were removed from the sand and inserted into the hydroponic profiles, using the full-strength nutrient solution according to treatments.

The saline solutions used in cucumber cultivation were obtained by adding sodium chloride (NaCl), calcium chloride (CaCl₂.2H₂O), and magnesium chloride (MgCl₂.6H₂O) salts to the nutrient solution prepared in water from the supply system of the municipality of Pombal, Paraíba, Brazil, being incorporated in the equivalent proportion of 7:2:1, respectively. This is the proportion of Na, Ca, and Mg commonly found in the waters used for irrigation in the semi-arid region of Northeast Brazil (Silva Junior et al., 1999). The saline nutrient solutions were prepared considering the relationship between ECw and the concentration of salts (Richards, 1954),

The nutrient solution was completely replaced every eight days; however, the electrical conductivity and pH were monitored daily in the system, and whenever necessary, the solution was adjusted by adding municipal supply water (ECw = 0.3 dS m^-1), always maintaining the ECns according to the established treatments. The pH was maintained between 5.5 and 6.5 by adding 0.1 M KOH or HCl. The plants were cultivated in hydroponic profiles using vertical staking with nylon twine.

Salicylic acid concentrations were obtained by dissolution in 30% ethyl alcohol, prepared on the day of application. The first application of SA was performed five days after the insertion of the plants in the hydroponic profiles between 17:00 and 18:00 hours. The other applications were made at 7-day intervals, spraying on the abaxial and adaxial surfaces of the leaves with a spray bottle, in order to fully wet them, applying on average 50 mL per plant. During the spraying with SA, a plastic tarpaulin structure was used to prevent the SA solution from drifting onto neighboring plants.

At 23 days after transplanting (DAT), in the vegetative phase of the crop, the effect of the treatments was measured by gas exchange, photosynthetic pigments, relative water content, electrolyte leakage in the leaf blade and growth.

Gas exchange was measured through stomatal conductance - gs (mol H₂O m^-2 s^-1), transpiration - E (mmol H₂O m^-2 s^-1), CO₂ assimilation rate - A (μmol CO₂ m^-2 s^-1), and internal CO₂ concentration - Ci (μmol CO₂ m^-2 s^-1), using a portable infrared carbon dioxide analyzer (IRGA), model LCPro+ Portable Photosynthesis System^® (ADC BioScientific Limited, UK), with irradiation of 1200 μmol photons m^-2 s^-1 and air flow of 200 mL min^-1, under atmospheric CO₂ concentration. After data collection, the instantaneous water use efficiency - WUEi - A/E [(μmol CO₂ m^-2 s^-1) (mmol H₂O m^-2 s^-1)^-1] and instantaneous carboxylation efficiency - CEi - A/Ci [(μmol CO₂ m^-2 s^-1) (μmol CO₂ m^-2 s^-1)^-1] were quantified.

The photosynthetic pigments: chlorophyll a (Chl a), chlorophyll b (Chl b), and carotenoids (Car), were determined according to the methodology of Lichtenthaler (1987). Electrolyte leakage in the leaf blade was obtained according to Scotti-Campos et al. (2013). Relative water content (RWC) was determined according to the methodology of Weatherley (1950).

Growth was analyzed by means of: plant height - PH (cm), measured as the distance from the orifice of the hydroponic system to the insertion of the apical meristem; stem diameter - SD (mm), measured three centimeters above the hydroponic profile using a digital caliper; number of leaves, determined by counting the leaves of each plant that had a minimum length of 3 cm; and leaf area - LA (cm²), determined by Eq. 1, as suggested by Yang et al. (1990), where L is leaf length and W is leaf width (> 5 cm).

$$LA=\Sigma 0.739\times L\times W^{0.00104}$$ (1)

The results were subjected to the data normality test (Shapiro-Wilk) and subsequently subjected to analysis of variance by the F test at p ≤ 0.05. In case of significance polynomial regression analysis was performed for ECns and SA concentrations using the statistical software SISVAR - ESAL (Ferreira, 2019). For the significant interaction between ECns values and SA concentrations, SigmaPlot v. 14.5 software was used to construct the graphs of the response surfaces.

Results and Discussion

There was a significant effect of the interaction between the salinity levels of the nutrient solution and salicylic acid concentrations on the electrolyte leakage in the leaf blade and carotenoid contents of Japanese cucumber plants (Table 1). The salinity levels of the nutrient solution significantly influenced the relative water content and the contents of chlorophyll a and b. On the other hand, salicylic acid concentrations significantly affected the chlorophyll a content of cucumber plants.

Table 1
Summary of the analysis of variance for relative water content (RWC), electrolyte leakage in the leaf blade (EL), chlorophyll a (Chl a), chlorophyll b (Chl b), and carotenoids (CAR) of cucumber plants grown with saline nutrient solution (ECns) under foliar application of salicylic acid (SA) in a hydroponic system

Sources of variation	DF	Mean squares
		RWC	EL	Chl a	Chl b	CAR
Saline nutrient solution (ECns)	3	1271.90**	1379.64**	523.95**	10.80**	7.30**
Linear regression	1	3178.85**	4081.58**	1463.20**	30.57**	18.99**
Quadratic regression	1	534.07**	52.58ns	70.37*	1.21ns	2.72*
Salicylic acid (SA)	3	15.98ns	77.61*	60.61**	0.49ns	0.10ns
Linear regression	1	10.38ns	232.71**	181.72**	1.46ns	0.18ns
Quadratic regression	1	32.49ns	0.03ns	0.026ns	0.01ns	0.04ns
Interaction (ECns × SA)	9	42.92ns	89.40**	19.45ns	0.93ns	0.46*
Residual 1		34.36	13.12	7.52	0.49	0.36
Residual 2		24.36	18.43	8.23	0.76	0.16
CV 1 (%)		8.78	11.46	18.01	16.56	8.31
CV 2 (%)		7.40	13.58	18.84	13.23	5.63

• ns, *, **, respectively not significant and significant at p ≤ 0.05 and ≤ 0.01, DF - Degrees of freedom, CV - Coefficient of variation

The increase in the ECns reduced the relative water content in the leaf blade (Figure 2A), and the highest value of 79.08% was obtained in plants cultivated under ECns of 2.1 dS m^-1. In turn, the lowest value of 59.69% was observed in plants cultivated under nutrient solution salinity of 4.4 dS m^-1. The reduction in the relative water content is due to salt stress in plants, caused by the osmotic effect, which destabilizes water balance and interferes with water accumulation in the cell (Lima et al., 2020). Guedes et al. (2024), in a study with cherry tomato ‘Laranja’ using saline nutrient solutions (ECns ranging from 2.1 to 4.2 dS m^-1), also observed reduction in RWC with the increase in ECns above 2.1 dS m^-1.

Figure 2
Relative water content - RWC (A) of cucumber plants as a function of electrical conductivity values of nutrient solution - ECns and electrolyte leakage - EL (B) in the leaf blade, as a function of the interaction between ECns values and concentrations of salicylic acid - SA in hydroponic cultivation

The increments in the electrical conductivity of the saline nutrient solution and concentration of salicylic acid resulted in an increase in electrolyte leakage in the leaf blade of 21.32% (Figure 2B), when comparing the maximum and minimum values (43.26 and 21.94%), obtained in plants cultivated under ECns of 4.8 and 2.1 dS m^-1 with application of SA concentrations of 5.4 and 0 mM, respectively. Salt stress causes nutritional imbalance in plants reducing the absorption of nutrients, accumulation of toxic ions in plant tissues, increasing the production of reactive oxygen species (ROS), which destabilizes and reduces the production of antioxidant enzymes, increasing the oxidation of proteins, nucleic acids, and lipids (Lima et al., 2017). The intensity of these effects may have increased with the high concentration of SA, since it has a differentiated action in several physiological processes, and may promote or inhibit them, depending on the concentration used and the method of application, according to Syeed et al. (2021).

The increase in the electrical conductivity of the saline nutrient solution reduced chlorophyll a content (Figure 3A), with the highest value of 21.87 mg g^-1 FM obtained in plants cultivated under ECns of 2.1 dS m^-1. On the other hand, the lowest value of 9.85 mg g^-1 FM was observed in plants subjected to ECns of 4.8 dS m^-1. The increase in ECns also negatively affected the chlorophyll b content of cucumber plants (Figure 3B), whose reduction was 7.67% per unit increase in ECns, i.e., there was a decrease of 24.80% when comparing plants subjected to the highest salinity value of the nutrient solution (4.8 dS m^-1) to those that received 2.1 dS m^-1. The decrease in the contents of photosynthetic pigments results from the activity of the enzyme chlorophyllase, which degrades chloroplast membranes, caused by toxic ions present in the saline nutrient solution, which in large amounts result in oxidative stress, causing instability of photosynthetic pigments (Lima et al., 2020).

Figure 3
Contents of chlorophylls a - Chl a (A) and b - Chl b (B) of cucumber plants as a function of the electrical conductivity of the nutrient solution - ECns and chlorophyll a - Chl a (C) as a function of the concentrations of salicylic acid - SA, in hydroponic cultivation

Foliar application of salicylic acid increased linearly the chlorophyll a content of cucumber plants (Figure 3C), equal to 6.46% per unit increment. When comparing plants subjected to foliar application of salicylic acid at concentration of 5.4 mM to those that received 0 mM, an increase of 34.88% was observed. The increase in chlorophyll a content is possibly related to the beneficial action of salicylic acid in gene signaling, which induces the accumulation of K⁺ and soluble sugars in cells to maintain membrane potential, improving photosynthetic activity and consequently enhancing pigment content (Rasheed et al., 2022).

Foliar application of salicylic acid at concentration of 5.4 mM promoted an increase in carotenoid content in plants cultivated under an estimated nutrient solution salinity of 4.8 dS m^-1, which obtained the maximum value (8.11 mg g^-1 FM) (Figure 4). On the other hand, the minimum value of 6.17 mg g^-1 FM was observed in plants subjected to nutrient solution salinity of 4.8 dS m^-1 without application of salicylic acid (0 mM). Salicylic acid acts on the synthesis of carotenoids, altering the dimensions of the antenna complex in relation to the reaction center, which improves RuBisCO activity (Farheen et al., 2018). The increase in carotenoid content minimizes photosynthetic damage in plants under salt stress, inhibiting the production of ¹O₂ and accumulation of ROS, as a defensive response to prevent the photooxidation of chlorophyll molecules, increasing the antioxidant capacity with the application of salicylic acid, which signals the production of enzymes such as superoxide dismutase, ascorbate peroxidase, and glutathione reductase (Syeed et al., 2021).

Figure 4
Contents of carotenoids - CAR of cucumber plants, as a function of the interaction between electrical conductivity of nutrient solution (ECns) and concentrations of salicylic acid (SA), in hydroponic cultivation

There was a significant effect of the interaction between the salinity values of the nutrient solution and foliar application of salicylic acid on the stomatal conductance of cucumber plants (Table 2). The salinity values of the nutrient solution significantly influenced transpiration, CO₂ assimilation rate, and instantaneous water use efficiency. Salicylic acid had a significant effect on the internal CO₂ concentration and instantaneous carboxylation efficiency of cucumber plants.

Table 2
Summary of analysis of variance for stomatal conductance (gs), transpiration (E), internal CO₂ concentration (Ci), CO₂ assimilation rate (A), instantaneous water use efficiency (WUEi), and instantaneous carboxylation efficiency (CEi) of cucumber plants cultivated with saline nutrient solution (ECns) and foliar application of salicylic acid (SA) in a hydroponic system

Sources of variation	DF	Mean squares
		gs	E	Ci	A	WUEi	CEi
Saline nutrient solution (ECns)	3	0.039**	2.950**	1782.97ns	92.61**	5.46ns	0.010**
Linear regression	1	0.794**	8.227**	4911.44ns	249.04**	5.95ns	0.027**
Quadratic regression	1	0.369**	0.392*	345.86ns	28.72ns	0.16ns	0.003ns
Salicylic acid (SA)	3	0.011**	0.092ns	1930.88*	116.57ns	10.94**	0.003ns
Linear regression	1	0.028**	0.016ns	2768.42*	21.89ns	10.39*	0.000ns
Quadratic regression	1	0.006ns	0.243ns	2610.45*	120.94ns	13.68**	0.000ns
Interaction (ECns × SA)	9	0.006**	0.104ns	1520.67ns	34.22ns	5.09ns	0.001ns
Residual 1		0.001	0.06	522.70	11.07	0.52	0.0004
Residual 2		0.001	0.12	179.51	10.04	1.43	0.0009
CV 1 (%)		19.00	8.62	14.04	16.06	10.00	23.48
CV 2 (%)		19.17	11.82	8.23	15.30	16.59	15.55

• ^ns, ^*, ^**, respectively not significant and significant at p ≤ 0.05 and ≤ 0.01, DF - Degrees of freedom, CV - Coefficient of variation

The plants obtained higher stomatal conductance (0.50 mol H₂O m^-2 s^-1) with the application of salicylic acid at concentration of 5.4 mM and nutrient solution salinity of 2.1 dS m^-1 (Figure 5), reflecting the beneficial effect of salicylic acid on the production of phenolic compounds, which favors greater stomatal opening (Nóbrega et al., 2020). On the other hand, the lowest stomatal conductance (0.13 mol H₂O m^-2 s^-1) occurred in plants subjected to ECns of 3.8 dS m^-1 without application of salicylic acid (0 mM).

Figure 5
Stomatal conductance - gs of cucumber plants, as a function of the interaction between electrical conductivity of nutrient solution - ECns and concentrations of salicylic acid - SA, in hydroponic cultivation

Plants under salt stress possibly activate defense mechanisms, partially closing their stomata to prevent excessive water loss and absorption of toxic ions such as Na⁺ and Cl^- (Lima et al., 2019). Reduction in stomatal conductance in plants cultivated under salt stress was also reported by Mendonça et al. (2022), who evaluated hydroponic cultivation of okra using saline nutrient solution and found reduction of 45.67% with the increase in ECns from 3.0 to 9.0 dS m^-1.

Plant transpiration decreased with the increase in ECns (Figure 6A), and the maximum value of 79.08 mmol H₂O m^-2 s^-1 was obtained under the electrical conductivity of the nutrient solution of 2.1 dS m^-1. On the other hand, the minimum value of 59.69 mmol H₂O m^-2 s^-1 was observed in plants cultivated under ECns of 4.8 dS m^-1. The reduction in transpiration is due to the partial closure of the stomata, which limits the absorption of water by the roots with the decrease in the osmotic potential of the saline nutrient solution, reducing the loss of water in the form of vapor to the atmosphere, avoiding the dehydration of the cells (Freire et al., 2021).

Figure 6
Transpiration - E (A), CO₂ assimilation rate - A (B) and instantaneous carboxylation efficiency - CEi (C) of cucumber plants, as a function of electrical conductivity of nutrient solution - ECns, in hydroponic cultivation

Regarding the CO₂ assimilation rate (Figure 6B) and instantaneous carboxylation efficiency (Figure 6C), reductions were observed with the increase in ECns, equal to 7.12 and 11% per unit increment in ECns, respectively. When comparing the A and CEi of plants cultivated under ECns of 4.8 dS m^-1 to those of plants that received saline nutrient solution of 2.1 dS m^-1, reductions of 17.60 and 31.25% were observed, respectively. The reduction in CO₂ assimilation rate is due to the partial closure of the stomata in plants under the negative effect of salt stress, restricting the entry of CO₂ into the substomatal chamber; consequently, the efficiency of carbon fixation is reduced by modifications in the Calvin-Benson cycle, which affects RuBisCO activity (Figueiredo et al., 2023).

For internal CO₂ concentration (Figure 7A) and instantaneous water use efficiency (Figure 7B), the maximum estimated values of 176.47 μmol CO₂ m^-2 s^-1 and 8.21 [(μmol CO₂ m^-2 s^-1) (mmol H₂O m^-2 s^-1)^-1] were obtained in plants cultivated without foliar application of salicylic acid. Reductions of 10.23 and 20.09% were observed when comparing the maximum values with the minimum values (158.41 μmol CO₂ m^-2 s^-1 and 6.56 [(μmol CO₂ m^-2 s^-1) (mmol H₂O m^-2 s^-1)^-1]), obtained in plants subjected to SA concentrations of 4.2 and 3.4 mM, respectively (Figure 7A and B). The reduction in CO₂ concentration with the increase in SA application is attributed to the synergistic effect between the production of antioxidant compounds and high production of ROS such as H₂O₂ (Fatima et al., 2023).

Figure 7
Internal CO₂ concentration - Ci (A) and instantaneous water use efficiency - WUEi (B) of cucumber plants, as a function of concentrations of salicylic acid - SA, in hydroponic cultivation

The electrical conductivity of the nutrient solution significantly influenced the number of leaves, leaf area, plant height, and stem diameter of cucumber plants, 23 days after transplanting (Table 3). SA concentrations and the interaction between the factors (ECns × SA) did not significantly influence any of the growth variables measured (Table 3).

Table 3
Summary of the analysis of variance for number of leaves (NL), leaf area (LA), plant height (PH), and stem diameter (SD) of cucumber plants cultivated with saline nutrient solution (ECns) and concentrations of salicylic acid (SA) in hydroponic system

Sources of variation	DF	Mean squares
		NL	LA	PH	SD
Saline nutrient solution (ECns)	3	182.47**	10378.84**	9105.64**	6.97**
Linear regression	1	497.50**	28154.07**	25080.90**	17.72**
Quadratic regression	1	26.26ns	2980.47*	2173.89**	1.50*
Salicylic acid (SA)	3	4.01ns	663.93ns	158.44ns	1.57ns
Linear regression	1	6.90ns	537.29ns	34.45ns	0.28ns
Quadratic regression	1	0.39ns	63.62ns	370.56ns	0.63ns
Interaction (ECns × SA)	9	8.87ns	649.57ns	109.27ns	0.58ns
Residual 1		6.36	461.86	144.77	0.29
Residual 2		5.23	354.04	118.09	0.45
CV 1 (%)		18.58	12.88	10.40	7.43
CV 2 (%)		16.84	11.28	9.39	9.18

• ns, ^*, ^**, respectively not significant and significant at p ≤ 0.05 and ≤ 0.01, CV - Coefficient of variation, DF - Degrees of freedom

The number of leaves of cucumber plants decreased linearly with the increase in the electrical conductivity of the saline nutrient solution (Figure 8A), by 11.97% per unit increase in ECns. Leaf area data were described by a quadratic equation (Figure 8B), and the maximum value of 201.77 cm² was obtained in plants cultivated under ECns of 2.1 dS m^-1. On the other hand, the estimated minimum value of 145.39 cm² was observed in plants subjected to ECns of 4.7 dS m^-1. The inhibition in leaf production reflects the changes observed in stomatal conductance and transpiration with the increase in ECns, affecting growth with the reduction of transpiring surface and number of leaves, avoiding dehydration of cells to maintain metabolic activity under stress conditions (Nóbrega et al., 2022).

Figure 8
Number of leaves (A), leaf area (B), plant height (C), and stem diameter (D) of cucumber plants as a function of electrical conductivity values of nutrient solution - ECns in hydroponic cultivation

Plant height and stem diameter were also negatively influenced by the increase in ECns values (Figures 8C and D), with maximum values of 148.09 cm and 8.17 mm, respectively, in plants under ECns of 2.1 dS m^-1. On the other hand, the minimum values of 94.97 cm and 6.76 mm were found at the ECns of 4.8 dS m^-1, with decreases of 35.87 and 17.26% in comparison with the maximum value, respectively. Growth inhibition relates the reduction of water absorption due to the decrease in the osmotic potential of the solution, in addition to affecting the metabolism of essential nutrients (N, K, Ca, P, and S), as toxic ions such as Na⁺ and Cl^- cause competition with these nutrients in the carriers of plasma membranes of roots, affecting the photosynthetic activity, compromising cell division and elongation of stem diameter and plant height (Rasheed et al., 2022). Negative effect of salt stress on growth was also reported by Fátima et al. (2023), who observed that the increase in ECns from 2.1 dS m^-1 reduced stem diameter and main branch length in ‘Gaúcho’ melon.

Conclusions

1. Electrical conductivity of the nutrient solution (ECns) above 2.1 dS m^-1 reduces the relative water content in the leaf blade, synthesis of chlorophyll a and b, and gas exchange, and inhibits the growth of ‘Hiroshi’ Japanese cucumber plants.

2. Salicylic acid at concentrations ranging from 3.7 to 4.7 mM increased stomatal conductance and carotenoid synthesis, at ECns of 2.1 dS m^-1, in ‘Hiroshi’ Japanese cucumber, at 23 days after transplanting.

3. ECns of 4.8 dS m^-1 associated with salicylic acid concentration of 5.4 mM negatively affects ‘Hiroshi’ Japanese cucumber plants, in hydroponic cultivation, increasing electrolyte leakage in the leaf blade.

Acknowledgments

To the National Council for Scientific and Technological Development - CNPq, Brazil for providing the financial support (Proc. CNPq 408511/2023-0).

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• ¹ Research developed at Universidade Federal de Campina Grande, Centro de Ciências e Tecnologia Agroalimentar, Pombal, PB, Brazil

Supplementary documents

• There are no supplementary sources.

Edited by

• Editors: Ítalo Herbet Lucena Cavalcante & Walter Esfrain Pereira

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Publication Dates

• Publication in this collection
  20 Jan 2025
• Date of issue
  May 2025

History

• Received
  18 June 2024
• Accepted
  25 Oct 2024
• Published
  28 Nov 2024