H₂O₂ as attenuator of salt stress on the physiology and growth of hydroponic cherry tomato

H₂O₂ como atenuante do estresse salino na fisiologia e crescimento de tomate cereja hidropônico

ABSTRACT

In arid and semi-arid regions, agricultural production is challenging due to the scarcity of water for irrigation, so brackish water is commonly used. However, the use of these waters negatively affects the growth and development of crops. In this context, it is essential to look for strategies to mitigate the effects of salt stress on plants. The objective of this study was to evaluate the effects of foliar application of H₂O₂ on gas exchange, photosynthetic pigments, photochemical efficiency, and growth of cherry tomato plants in hydroponic cultivation with saline nutrient solution. The experiment was carried out in a greenhouse in Pombal-PB, using a Nutrient Film Technique (NFT) hydroponic system. Treatments were distributed in a split-plot scheme, in which the levels of electrical conductivity of the nutrient solution - ECns (2.1, 2.8, 3.5, and 4.2 dS m^-1) were considered the plots and the five concentrations of H₂O₂ (0, 12, 24, 36, and 48 μM) were considered the subplots, with six replicates and two plants per plot. ECns from 2.1 dS m^-1 reduced gas exchange, photochemical efficiency, photosynthetic pigments, relative water content, and growth of cherry tomato. H₂O₂ at concentrations of 36 and 48 μM associated with saline nutrient solution of 2.1 dS m^-1 stimulated plant height, growth, and chlorophyll b synthesis, respectively. Hydrogen peroxide alone did not affect gas exchange, chlorophyll fluorescence, photosynthetic pigments, and growth of cherry tomato.

Keywords:
Solanum lycopersicum ; Acclimatization; Nutrient solution

RESUMO

Em regiões áridas e semiáridas, a produção agrícola é desafiadora devido à escassez de água para irrigação, sendo comum o uso de água salobra. No entanto, o uso dessas águas afeta negativamente o crescimento e o desenvolvimento das culturas. Neste contexto, é essencial a busca por estratégias para amenizar os efeitos do estresse salino nas plantas. O objetivo desta pesquisa foi avaliar efeitos da aplicação foliar de H₂O₂ nas trocas gasosas, nos pigmentos fotossintéticos, na eficiência fotoquímica e no crescimento de plantas de tomate cereja em cultivo hidropônico com solução nutritiva salina. O experimento foi desenvolvido em casa de vegetação em Pombal-PB, utilizando o sistema hidropônico tipo técnica de fluxo laminar de nutriente. Os tratamentos foram distribuidos em esquema de parcelas subdivididas, onde os níveis de condutividade elétrica da solução nutritiva – CEsn (2, 1; 2,8; 3,5 e 4,2 dS m^-1) foram considerados as parcelas e as cinco concentrações de H₂O₂ (0, 12, 24, 36 e 48 μM) como subparcelas, com seis repetições e duas plantas por parcela. A CEsn a partir de 2,1 dS m^-1 reduziu as trocas gasosas, a eficiência fotoquímica, os pigrmentos fotossintéticos, o conteúdo relativo de água e o crescimento do tomate cereja. O H₂O₂ nas concentrações de 36 e 48 μM associadas à solução nutritiva salina, de 2,1 dS m^-1 estimulou o crescimento em altura de plantas e a sintese de clorofila b, respecitvamente. O H₂O₂ de forma isolada não afetou as trocas gasosas, a florescência da clorofila, os pigmentos fotossintéticos e o crescimento do tomate cereja.

Palavras-chave:
Solanum lycopersicum ; Aclimatação; Solução nutritiva

INTRODUCTION

Cherry tomato (Solanum lycopersicum L.) var. Cerasiforme is a table variety that stands out for its sweet flavor and smaller fruit size (GONÇALVES et al., 2018). It is a vegetable of great economic and social importance in Brazil, being cultivated in several regions of the country (FRANCA; LEITÃO; CAMPECHE, 2017). Although the national production of cherry tomato is lower compared to other tomato varieties among the main groups, the added value of cherry fruits is offset by their great acceptance among consumers and high commercial value of the product (CASTAÑEDA et al., 2020).

Despite the economic and social potential of this vegetable crop in the semi-arid region of Northeast Brazil, in this region the quality of water sources used in irrigation is a limiting factor for crop production, since rainfall is poorly distributed in space and time and temperatures lead to a gradual reduction in water availability, both in quality and quantity (PINHEIRO et al., 2022), with common occurrence of water sources with moderate to high salt levels, due to the climate imbalance.

Water salinity can cause changes in plant physiology, significantly reducing yield due to osmotic stress, which causes changes in osmotic homeostasis and ionic toxicity, especially in plants sensitive to salt stress (SUN et al., 2020). The restriction in the absorption of water and nutrients limits the growth and development of plants, with negative effects on yield. Excessive accumulation of toxic ions, especially sodium and chloride, damages the cytoplasm, affecting biochemical and photosynthetic functions (TAVARES FILHO et al., 2020).

Vegetable production through hydroponic cultivation is a sustainable strategy, as it allows the control of production factors, especially water and nutrient management, in addition to enabling the use of brackish water, allowing year-round production in a greenhouse (OLIVEIRA et al., 2022). In hydroponic cultivation, as there is no soil involved, only the osmotic potential of the solution affects growth and the matric potential present in conventional cultivation is not considered. For this reason, this system is suitable to reduce the socioeconomic effects caused by the lack of water in arid regions (COSTA et al., 2020).

In addition, it is necessary to look for alternatives to reduce the deleterious effects of salinity, and one example is foliar application of hydrogen peroxide (H₂O₂), as observed in studies with zucchini (DANTAS et al., 2022a) and mini watermelon (SILVA et al., 2022a). H₂O₂ is a reactive oxygen species that plays a fundamental role in the acclimatization of plants to salt stress (SILVA et al., 2021), contributing to the production of antioxidant enzymes, proteins, and compounds capable of regulating various mechanisms under stress conditions, improving water and nutrient absorption and photosynthetic efficiency, contributing to the maintenance of plant ionic and redox homeostasis (CARVALHO et al., 2011). Dantas et al. (2022b), in a study with zucchini in a hydroponic system, observed that foliar application of H₂O₂ reduced the effects of salt stress on stomatal conductance and internal CO₂ concentration. In another study, these authors also found that foliar spraying of H₂O₂ at concentrations of 60 and 40 μM associated with nutrient solution with electrical conductivity of 2.1 dS m^-1 promoted increments in stem diameter and root length of zucchini plants (DANTAS et al., 2021).

In view of the above, the objective of this study was to evaluate the effects of foliar application of H₂O₂ as an attenuator of salt stress effects on leaf gas exchange, photosynthetic pigments, and growth of cherry tomato in an NFT hydroponic system.

MATERIAL AND METHODS

The experiment was carried out from October 2022 to February 2023, under conditions of arched greenhouse, with a 150-micron low-density polyethylene cover, at the Center of Sciences and Agri-food Technology - CCTA, belonging to the Federal University of Campina Grande - UFCG, in Pombal, PB, Brazil, located at the geographic coordinates 6° 46' 8'' S, 37° 48' 06'' W, average altitude 193 m. According to Köppen’s classification, the municipality of Pombal has a climate classified as hot and humid semi-arid, AW’ (ALVARES et al., 2013). Data on temperature (maximum and minimum) and relative humidity were collected using a digital thermohygrometer inside the greenhouse and are presented in Figure 1.

Figure 1.
Average daily temperature data (maximum, mean, and minimum) and average relative humidity of the air during the experimental period.

The experimental design was completely randomized, in a split-plot scheme, with four levels of electrical conductivity of the nutrient solution – ECns (2.1, 2.8, 3.5, and 4.2 dS m^-1) as plots and the concentrations of hydrogen peroxide – H₂O₂ (0, 12, 24, 36, and 48 μM) as subplots. Each plot contained two plants and the experiment was conducted with six replicates. ECns levels were defined based on a study conducted by Silva et al. (2022a), while H₂O₂ concentrations were established based on the study conducted by Silva et al. (2022b).

The nutrient solution used was that recommended by Hoagland and Arnon (1950), whose composition and nutrient concentrations are shown in Table 1. The solution was prepared in local water supply (0.3 dS m^-1) and had an electrical conductivity of 2.1 dS m^-1 after preparation.

Table 1.
Chemical composition of the nutrient solution recommended by Hoagland and Arnon (1950), used in the hydroponic cultivation of orange cherry tomatoes.

Nutrients	Fertilizers	Quantity (g 1000L-1)
P/K	KH2PO4	136.09
K/N	KNO3	101.10
Ca /N	Ca (NO3)2.4H2O	236.15
Mg	MgSO4.4H2O	246.49
B	H3BO3	3.10
Mn	MnSO4.4H2O	1.70
Zn	ZnSO4.7H2O	0.22
Cu	CuSO4	0.75
Mo	(NH4)6Mo7O24. 4H2O	1.25
	EDTA – Na	13.9
Fe	FeSO4	13.9

The saline nutrient solutions were prepared so as to have an equivalent ratio of 7:2:1 referring to Na:Ca:Mg, respectively, from the salts NaCl, CaCl₂.2H₂O, and MgCl₂.6H₂O, a ratio that prevails in water sources used for irrigation in small farms in the Northeast, considering the relationship between ECw and salt concentration (RICHARDS, 1954), according to Equation 1, to prepare waters in the laboratory:

(1) $\text{Q}\approx 10\times \text{ECw}$

where:

Q = Sum of cations (mmol_c L^-1); and

ECw = Desired electrical conductivity after discounting the ECw of the water from the municipal supply system (dS m^-1).

Seedlings were produced using Blueline® orange cherry tomato seeds (Santo Antônio de Posse/SP, Brazil). This cultivar has a cycle of approximately 90 days, a very productive plant, tasty fruits, and orange color.

Orange cherry tomato seeds were sown in 80 mL plastic cups containing coconut fiber substrate, one seed per cup. Prior to sowing, the coconut fiber was sanitized and washed with hypochlorite (2 to 2.5%). From germination to the appearance of the second pair of true leaves, nutrient solution at a 50% concentration of the recommendation of Hoagland and Arnon (1950) was applied. Then, the plants were removed from the coconut fiber and inserted directly into the hydroponic profiles at 30 days after sowing (DAS), using a vertical staking with nylon string to keep the stem erect.

The hydroponic system used was Nutrient Film Technique (NFT), made from PVC pipes with 100 mm diameter and 6 m length. The system was composed of four subsystems, spaced 0.80 m apart, each with three channels separated by 0.40 m. In the channels, the spacing was 0.50 m between plants and 1.0 m between treatments.

The hydroponic system channels were installed at 0.60 m height, with 4% slope to assist in the nutrient solution flow. At the lowest level of each bench of the hydroponic system, a 150 L polyethylene recipient was installed to collect and conduct the nutrient solution back to the channels. The nutrient solution was pumped to the head of the channels by a 35 W pump, with an average flow rate of 3.0 L min^-1. Nutrient solution circulation was programmed by a timer, with an intermittent flow of 15 min during the day and 45 min at night, according to the needs of the crop.

To ensure that the ECns and pH values were in accordance with the established treatments, the nutrient solution was monitored daily and completely renewed every 12 days. When necessary, ECns was adjusted with the addition of public-supply water with ECw = 0.3 dS m^-1, while pH was regulated by the addition of 0.1 M KOH or HCl solution.

H₂O₂ concentrations were obtained by diluting H₂O₂ in deionized water at each application event. Applications were carried out manually with a spray bottle, so as to fully wet the leaves (abaxial and adaxial sides). A non-ionic adhesive spreader was added to break the surface tension and increase absorption by the leaves. Applications were carried out from 5:00 p.m., applying an average of 19 mL per plant in total, at an interval of 12 days, totaling 3 applications. A cardboard structure was used to avoid the drift of the treatments among the plants. Plants were monitored and phytosanitary practices were carried out when necessary.

H₂O₂ application began 10 days after inserting the plants in the hydroponic profiles. At 72 hours after the H₂O₂ concentrations were applied, the saline nutrient solutions began to be applied, according to the established treatments.

At 64 days after transplanting (DAT), leaf gas exchange was evaluated by determining 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), internal CO₂ concentration - Ci (μmol CO₂ m^-2 s^-1), instantaneous water use efficiency - WUE [(μmol CO₂ m^-2 s^-1) (mmol H₂O m^-2 s^-1)^-1], instantaneous carboxylation efficiency - CEi [(μmol CO₂ m^-2 s^-1) (μmol CO₂ m^-2 s^-1)^-1], intrinsic water use efficiency - WUEi [(μmol CO₂ m^-2 s^-1) (μmol H₂O m^-2 s^-1)^-1]. Gas exchange was determined with a portable carbon dioxide analyzer (IRGA), LCPro+ Portable Photosynthesis System® (ADC BioScientific Limited, UK), on leaves located in the middle third of the plants under ambient temperature conditions, maintaining the atmospheric CO₂ concentration and using an artificial radiation source of 1,200 μmol m^-2 s¹, established through the light-photosynthesis response curve (FERNANDES et al., 2021).

Photosynthetic pigments were quantified according to the methodology of Arnon (1949). From 1 g of fresh matter of the third leaf, homogenized in 4 mL of 80% acetone, after 72 hours hermetically sealed in the dark, the concentrations of chlorophyll and carotenoids in the solutions were determined using a spectrophotometer (Thermo Scientific®, Genesys 20 model) at the absorbance wavelength (ABS) (470, 646, and 663 nm), using Equations 2, 3, 4 and 5. The values obtained for chlorophyll a, chlorophyll b and carotenoids were expressed in mg g^-1 of FM (fresh matter).

(2) $\left(\text{Chl}a\right)=12.21\times {\text{ABS}}_{663}-2.81\times {\text{ABS}}_{646}$

(3) $\left(\text{Chl}b\right)=20.13\times {\text{ABS}}_{646}-5.03\times {\text{ABS}}_{663}$

(4) $\left(\text{Chl}T\right)=17.3\times {\text{ABS}}_{646}+7.18\times {\text{ABS}}_{663}$

(5) $\left(\text{Car}\right)=\frac{\left(1000\times {\text{ABS}}_{470}-1.82\times Chla-85.02\times Chlb\right)}{198}$

Chlorophyll a fluorescence in the dark phase was measured in fully expanded leaves of the middle third and pre -adapted to the dark for 30 min, using a pulse-modulated fluorometer (Sciences Inc.- OS-30p model, Hudson, USA). For this purpose, clips were placed on the leaves for 30 min before the readings to adapt them to the dark, and the initial fluorescence (F₀), maximum fluorescence (F_m), variable fluorescence (F_v), F_v/F₀ ratio, and quantum efficiency of photosystem II (F_v/F_m) were measured.

To determine the relative water content in the leaves (RWC), four leaf discs (113 mm²) were collected and weighed on an analytical balance to obtain fresh mass (FM), turgid mass (TM) after 24 h submerged in distilled water, and dry mass (DM) after 48 h of drying in a forced air ventilation oven at 60 ºC. RWC was determined according to the methodology of Weatherley (1950), using Equation 6.

(6) $\text{RWC}=\frac{\text{FM-DM}}{\text{TM-DM}}\times 100$

Where:

RWC - Relative water content (%);

FM - leaf fresh mass (g);

TM - leaf turgid mass (g); and

DM - leaf dry mass (g).

Plant height was determined at 5 cm from the hydroponic profile to the insertion of the apical meristem, measured with a graduated ruler. Stem diameter was measured at 5 cm from the hydroponic profile with a digital caliper. Number of leaves was obtained by counting the leaves, considering those with a minimum length of 5 cm. At the end of the orange cherry tomato cycle (70 DAT), the plants were removed from the hydroponic profiles, separated into different parts, placed in paper bags and dried in a forced air circulation oven at temperature of 65 °C for 72 h; subsequently, the material was weighed to obtain dry mass of leaves (LDM), stem (SDM), and root (RDM), by weighing on a scale with a precision of 0.01 g.

The data obtained were subjected to the normality test (Shapiro & Wilk), and later analysis of variance was performed by the F test at p ≤ 0.05 probability level; when significant, polynomial regression analysis was performed for the salinity levels of the nutrient solution and concentrations of hydrogen peroxide, using the statistical software SISVAR – ESAL (FERREIRA, 2019). SigmaPlot® software was used to construct the response surface curves when there was a significant effect of the interaction between factors (ECns × H₂O₂).

RESULTS AND DISCUSSION

There were significant effects of the interaction between saline nutrient solution and hydrogen peroxide (ECns × H₂O₂) on the stomatal conductance (gs), internal CO₂ concentration (Ci) and intrinsic water use efficiency (WUEi) of tomato plants (Table 2). ECns significantly affected transpiration (E), CO₂ assimilation rate (A), and instantaneous carboxylation efficiency (CEi). On the other hand, H₂O₂ concentrations did not significantly affect any of the variables analyzed.

Table 2.
Summary of the analysis of variance for stomatal conductance (gs), transpiration (E), CO₂ assimilation rate (A), internal CO₂ concentration (Ci), instantaneous water use efficiency (WUE), instantaneous carboxylation efficiency (CEi), and intrinsic water use efficiency (WUEi) of cherry tomato plants cultivated with saline nutrient solution (ECns) and application of hydrogen peroxide (H₂O₂) in a hydroponic system at 65 days after transplanting.

Sources of Variation	DF	Mean squares
		gs	E	A	Ci	WUE	CEi	WUEi
Saline nutrient solution (ECns)	3	0.0092*	1.623*	22.57**	6719.12**	0.799 ns	0.00012**	1387.0**
Linear regression	1	0.0096**	4.06**	57.80**	14967.2**	2.36 ns	0.00035**	38.72**
Quadratic regression	1	0.00023*	0.60*	9.93ns	4593.7**	0.00ns	0.00073ns	83.66**
Residual 1	6	0.0025	0.060	0.409	31.06	0.016	0.00015	5.44
Hydrogen peroxide (H2O2)	4	0.0071*	0.672ns	1.077ns	645.27*	0.309 ns	0.00018ns	568.3**
Linear regression	1	0.0018*	1.65ns	2.67*	1044.3**	0.91 ns	0.00013ns	38.07*
Quadratic regression	1	0.009*	0.05ns	0.71ns	656.09**	0.01ns	0.00010*	1191.7**
Interaction (ECns × H2O2)	12	0.0029*	0.179ns	0.858ns	175.14*	0.039ns	0.00014ns	118.84**
Residual 2	34	0.0066	0.270	1.091	96.42	0.021	0.00011	37.85
CV 1(%)		5.57	6.56	10.57	2.17	7.89	14.17	5.89
CV 2(%)		5.25	13.92	17.25	3.82	8.90	14.90	15.77

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

The stomatal conductance of orange cherry tomatoes decreased with the increase in ECns, regardless of H₂O₂ concentration (Figure 2A). However, it was observed that foliar application of H₂O₂ at concentration of 48 μM was able to reduce the effects of salt stress, with maximum estimated value of 0.179 mol H₂O m^-2 s^-1 in the gs of plants subjected to ECns of 2.1 dS m-1, corresponding to an increase of 10.49% (0.017 mol H₂O m^-2 s^-1) compared to plants subjected to the same ECns level (2.1 dS m^-1) and without H₂O₂ application. It is worth pointing out that in the present study the application of H₂O₂ at concentration greater than 36 μM intensified the deleterious effects of salt stress on the stomatal conductance of orange cherry tomato plants.

Figure 2.
Stomatal conductance (gs), internal CO₂ concentration (Ci) and intrinsic water use efficiency (WUEi) of orange cherry tomato plants, as a function of the interaction between saline nutrient solution – ECns and concentrations of hydrogen peroxide – H₂O₂ in a hydroponic system, at 65 days after transplanting.

Reduction in stomatal conductance is a strategy to reduce water loss and thus the absorption of water and salts from the soil solution, without affecting photosynthetic activity (DIAS et al., 2019). The positive effects of H₂O₂ on the stomatal conductance of tomato plants up to the concentration of 36 μM may have been due to defense mechanisms that induce signaling and activation of the antioxidant enzyme system and thus attenuate the detrimental effects of salt stress (CARVALHO et al., 2011).

Regarding the internal CO₂ concentration (Figure 2B), plants cultivated under ECns of 2.1 dS m^-1 obtained the highest estimated value (293.24 μmol CO₂ m^-2 s^-1) when subjected to H₂O₂ concentration of 19 μM. On the other hand, the lowest value of Ci (235.34 μmol CO₂ m^-2 s^-1) was observed in plants subjected to H₂O₂ concentration of 48 μM and ECns of 3.7 dS m^-1. Foliar application of H₂O₂ in plants under salt stress can contribute to the activation of the defense system through the activity of antioxidant enzymes, such as superoxide dismutase, catalase, and ascorbate peroxidase, resulting in the reduction of the deleterious effects on leaf gas exchange (CARVALHO et al., 2011). During photosynthesis, CO₂ fixed in the mesophyll cell is consumed in the synthesis of sugars, which results in a reduction (DIAS et al., 2019). Dantas et al. (2022a), in zucchini cultivated with ECns 2.1 to 6.6 dS m^-1 in an NFT hydroponic system, also found that the internal CO₂ concentration decreased with the increase in irrigation water salinity and the lowest value was obtained in plants subjected to ECns of 6.6 dS m^-1.

For the intrinsic water use efficiency of orange cherry tomato (Figure 2C), plants cultivated under ECns of 2.1 dS m^-1 obtained the highest estimated value, 51.11 [(μmol CO₂ m^-2 s^-1) (μmol H₂O m^-2 s^-1)^-1], when subjected to H₂O₂ concentration of 0 μM. The lowest value of 16.59 [(μmol CO₂ m^-2 s^-1) (μmol H₂O m^-2 s^-1)^-1] was found at ECns of 4.2 dS m^-1 at a H₂O₂ concentration of 48 μM, with a reduction of 68.16% when compared to the highest value of intrinsic water use efficiency. The reduction in intrinsic water use efficiency may be related to osmotic stress and absorption of toxic ions, since, under stressful conditions, such as drought or salinity, the decrease in water loss through stomatal closure also restricts the entry of CO₂ (SÁ et al., 2019).

Transpiration was reduced with the increase in the electrical conductivity of the nutrient solution (ECns), with the maximum estimated value of 4.19 mmol H₂O m^-2 s^-1 obtained in plants under saline nutrient solution of 2.1 dS m^-1, while those subjected to ECns of 4.2 dS m^-1 expressed the lowest E (3.49 mmol H₂O m^-2 s^-1) (Figure 3A). Stomatal closure resulted in lower transpiration, a strategy used by plants to reduce water loss as a result of lower absorption of water and nutrients. However, this mechanism can be beneficial under salt stress conditions, as transpiration creates a negative pressure of water in the aerial part that can be b alan ce d through greater absorption of water by the roots, forming a mechanism of tolerance to salt stress (LIMA et al., 2017).

Figure 3.
Transpiration - E (A), CO₂ assimilation rate - A (B), and instantaneous carboxylation efficiency - CEi (C) of orange cherry tomato plants, as a function of salinity levels of nutrient solution - ECns in a hydroponic system, at 65 days after transplanting.

Due to the reductions in stomatal conductance (Figure 2A) and transpiration (Figure 3A), when the plants were subjected to different levels of salinity of the nutrient solution, the CO₂ assimilation rate was compromised, showing a decrease of 12.53% per unit increment in ECns (Figure 3B). Nutrient solution salinity may have induced an imbalance in the photosynthesis process, since tomato plants have C3 photosynthetic metabolism and need to keep their stomata open for longer time to fix CO₂ by the RuBisCO enzyme in the Calvin cycle (GUIMARÃES et al., 2019). Roque et al. (2022) evaluated cherry tomatoes grown in pots under irrigation with saline water and found a decrease in CO₂ assimilation rate with the increase in the salt concentration in the irrigation water, when the plants were irrigated with high-salinity water (4.3 dS m^-1), compared to those grown with low-salinity water (0.3 dS m^-1).

Instantaneous carboxylation efficiency (Figure 3C) decreased linearly with the increase in the salinity levels of the nutrient solution, by 11.69% per unit increment in ECns levels. In relative terms, there was a reduction of 32.53% in the CEi of plants cultivated under ECns of 4.2 dS m^-1 compared to those subjected to the lowest salinity level (2.1 dS m^-1). Instantaneous carboxylation efficiency (CEi) is directly linked to CO₂ assimilation rate and intracellular CO₂ concentration; moreover, as salt stress becomes more severe, the dehydration of mesophyll cells inhibits photosynthesis, mesophyll metabolism is impaired and, consequently, carboxylation efficiency is decreased (JACINTO JÚNIOR et al., 2019).

The salinity levels of the nutrient solution significantly influenced the maximum (F_m) and the variable fluorescence (F_v) of tomato at 65 days after sowing. The concentrations of hydrogen peroxide and the interaction between the factors (ECns × H₂O₂) did not significantly affect any of the variables measured at 65 days after transplanting (Table 3).

Table 3.
Summary of the analysis of variance for initial fluorescence (F₀), maximum fluorescence (F_m), variable fluorescence (F_v), quantum efficiency of photosystem II (F_v/F_m), and maximum efficiency of the photochemical process in photosystem II in the dark phase (F_v/F₀) of tomato cultivated with saline nutrient solution (ECns) and concentrations of hydrogen peroxide (H₂O₂) in a hydroponic system, at 64 days after transplanting.

Sources of Variation	DF	Mean squares
		F0	Fm	Fv	Fv/Fm	Fv/F0
Saline nutrient solution (ECns)	3	2206.7ns	3883.3**	2117.6**	0.004ns	0.109ns
Linear regression	1	5401.7ns	11445.3**	6311.2**	0.014**	0.3181ns
Quadratic regression	1	62.0ns	120.4ns	19.26ns	0.000ns	0.001ns
Residual 1	6	89.76	101.1	14.57	0.000	0.058
Hydrogen peroxide (H2O2)	4	26.94ns	743.8ns	524.41ns	0.002ns	1.995ns
Linear regression	12	16.13ns	800.8ns	73.63ns	0.003ns	5.106ns
Quadratic regression	1	66.88ns	1030.0ns	257.52ns	0.004ns	0.991ns
Interaction (ECns × H2O2)	12	407.85ns	262.5ns	24.07ns	0.000ns	0.055ns
Residual 2	34	47.39	136.30	575.40	0.000	0.042
CV 1(%)		6.87	1.79	0.86	1.93	7.51
CV 2(%)		4.99	2.08	5.43	2.84	6.44

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

The maximum fluorescence (F_m) of orange cherry tomato plants decreased linearly with the increase in the electrical conductivity of the saline nutrient solution, by 2.86% per unit increment in ECns (Figure 4A). In relative terms, there was a 6.4% decrease in the F_m of plants subjected to ECns of 4.2 dS m^-1 compared to those cultivated with a saline nutrient solution of 2.1 dS m^-1.

Figure 4.
Maximum fluorescence (A) and variable fluorescence (B) of cherry tomato plants, as a function of the salinity levels of the nutrient solution - ECns in a hydroponic system, at 65 days after transplanting.

The decrease in F_m is caused by the inactivation of PSII in the thylakoid membranes, which in turn results from the reduction of quinone, which affects the flow of electrons between photosystems. This situation may have had a negative impact on the photochemical activity of the leaves, as a high F_m affects the transfer of energy for the formation of the reducing agent NADPH, ATP, and reduced ferredoxin, promoting a greater capacity for CO₂ assimilation in the biochemical phase of photosynthesis (DIAS et al., 2021). Similar results were found by Mendonça et al. (2022), in a study evaluating the effects of saline nutrient solution under similar conditions on okra plants, in which maximum fluorescence decreased by 4.13% per unit increment in ECns.

The variable fluorescence (F_v) of cherry tomato plants decreased with the increase in the electrical conductivity of the nutrient solution (Figure 4B), with a reduction of 3.05% per unit increment in ECns. Variable fluorescence is related to the ability of the plant to transfer the energy of the electrons released by the pigment molecules for the production of NADPH, AT P, and reduced ferredoxin, so the reduction in F_v due to the increase in salinity indicates that the photosynthetic apparatus was damaged by salt stress, consequently causing negative effects on the photosynthetic process (DIAS et al., 2019).

There was a significant effect of the interaction between saline nutrient solution and hydrogen peroxide (ECns × H₂O₂) concentrations on the contents of chlorophyll a (Chl a) and chlorophyll b (Chl b) of cherry tomato plants (Table 4). The salinity levels of the nutrient solution (ECns) caused a significant effect on the relative water content (RWC) of orange cherry tomato plants at 65 days after transplanting.

Table 4.
Summary of the analysis of variance for the contents of chlorophyll a (Chl a), chlorophyll b (Chl b), chlorophyll a/b (Chl a/b), total chlorophyll (Chl T), carotenoids (Car), and relative water content (RWC) of cherry tomato plants, cultivated with saline nutrient solution (ECns) and concentrations of hydrogen peroxide (H₂O₂) in a hydroponic system, at 65 days after transplanting.

Sources of Variation	DF	Mean squares
		Chl a	Chl b	Chl a/b	Chl T	Car	RWC
Saline nutrient solution (ECns)	3	0.008**	0.004**	0.730 ns	0.028ns	0.00073ns	387.52**
Linear regression	1	0.023**	0.012**	2.14**	0.085**	0.00027*	986.2**
Quadratic regression	1	0.001**	0.000ns	0.028ns	0.00001ns	0.00013ns	154.2**
Residual 1	6	0.000	0.000	0.019	0.001	0.00029	4.69
Hydrogen peroxide (H2O2)	4	0.003**	0.003**	0.391ns	0.0036ns	0.00018ns	20.57ns
Linear regression	1	0.000ns	0.002**	0.448ns	0.007ns	0.00012ns	0.198ns
Quadratic regression	1	0.000*	0.000**	0.781ns	0.00001ns	0.00010ns	80.20**
Interaction (ECns × H2O2)	12	0.001**	0.000*	0.051ns	0.002ns	0.00024ns	14.10ns
Residual 2	34	0.000	0.000	0.029	0.001	0.00052	15.88
CV 1(%)		3.59	6.76	6.22	11.72	6.10	3.80
CV 2(%)		8.81	10.30	7.59	12.08	9.58	6.62

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

Under the highest salinity level of the nutrient solution (4.2 dS m^-1), H₂O₂ application at a concentration of 48 μM significantly reduced the chlorophyll a contents, with a minimum value of 0.193 mg g^-1 FM (Figure 5A). On the other hand, under the lowest salinity of the nutrient solution (2.1 dS m^-1) and without foliar application of H₂O₂, higher contents of Chl a (0.264 mg g^-1 FM) were observed. For chlorophyll b contents, plants subjected to ECns of 2.1 dS m^-1 and H₂O₂ at a concentration of 18 μM obtained the highest value (0.171 mg g^-1 FM) (Figure 5B). The lowest contents of Chl b (0.101 mg g^-1) were observed in plants cultivated under ECns of 4.2 dS m^-1 and 48 μM of H₂O₂.

Figure 5.
Chlorophyll a - Chl a (A) and chlorophyll b - Chl b (B) contents of cherry tomato plants in a hydroponic cultivation, as a function of the interaction between saline nutrient solution - ECns and concentrations of hydrogen peroxide - H₂O₂ and relative water content - RWC in the leaf blade (C), as a function of ECns levels, at 65 days after transplanting.

The results obtained can be attributed to the synthesis of reactive oxygen species, which is favored by the water deficit caused by salt stress, hindering the metabolism of plants, through the oxidation of photosynthetic pigments, including chlorophyll b (SILVA et al., 2016). According to Nóbrega et al. (2020), excessive presence of salts in plant tissues can affect the synthesis of chlorophyll a and b, due to th eir degradation by the activation of the enzyme chlorophyllase, which reduces photosynthesis and the production of pigmentation proteins.

For the relative water content - RWC (Figure 5C), it was observed that cherry tomato plants cultivated under ECns of 2.1 dS m^-1 obtained the maximum estimated value of 67.48%. In contrast, the minimum RWC value was 56.06% at the ECns of 4.2 dS m^-1. The reduction in water content in the leaf blade reflects the action of the osmotic effects due to the high concentration of salts, which restricts the absorption of water and nutrients by the plants.

There was a significant effect of the salinity levels of the nutrient solution on the stem diameter (SD), stem dry mass (SDM), and root dry mass (RDM) of orange cherry tomato plants. The interaction between factors (ECns × H₂O₂) significantly influenced the height of tomato plants. On the other hand, H₂O₂ concentrations did not significantly affect any of the variables evaluated at 70 days after transplanting (Table 5).

Table 5.
Summary of the analysis of variance for plant height (PH), stem diameter (SD), number of leaves (NL), dry mass of leaves (LDM), stem (SDM), and roots (RDM) of cherry tomato plants, cultivated with saline nutrient solution (ECns) and concentrations of hydrogen peroxide (H₂O₂) in a hydroponic system, at 70 days after transplanting.

Sources of Variation	DF	Mean squares
		PH	SD	NL	LDM	SDM	RDM
Saline nutrient solution (ECns)	3	1014.8**	18.82*	60.977ns	10.675ns	16.770*	6.46*
Linear regression	6	2914.0**	51.79**	0.000ns	14.822ns	36.45**	11.46*
Quadratic regression	1	120.41ns	2.59ns	117.60ns	5.787ns	10.79*	0.277ns
Residual 1	1	20.62	0.978	16.09	2.691	1.662	0.61
Hydrogen peroxide (H2O2)	4	36.54ns	0.857ns	22.566ns	3.294ns	3.149ns	1.41ns
Linear regression	1	0.208ns	0.001ns	0.033ns	1.961ns	0.741ns	5.24ns
Quadratic regression	1	118.33ns	1.537ns	14.880ns	7.978ns	7.86ns	0.08ns
Interaction (ECns × H2O2)	12	88.87*	1.632ns	17.144ns	8.194ns	3.358ns	1.42ns
Residual 2	34	27.00	0.525	6.395	3.205	0.926	0.57
CV 1(%)		6.47	10.84	11.85	9.72	10.15	11.07
CV 2(%)		7.40	7.94	7.47	10.60	7.58	10.63

• DF - degrees of freedom; CV (%) - coefficient of variation; *,** significant at p ≤ 0.05 and p ≤ 0.01, respectively ^ns not significant.

The height of orange cherry tomato plants decreased with the increase in the electrical conductivity of the nutrient solution (Figure 6). However, foliar application of H₂O₂ at a concentration of 48 μM was able to reduce the effects of salt stress, with the maximum estimated value of 81.56 cm in PH obtained at ECns of 2.1 dS m^-1, corresponding to an increase of 2.53% (2.06 cm) compared to plants cultivated with the same ECns level (2.1 dS m^-1) and without application of H₂O₂. Plants that grow under salt stress may have a reduction in their water uptake, affecting their development due to osmotic and ionic effects that alter the photosynthetic rate and metabolism of plants (LIMA et al., 2020). However, in this study, the beneficial effect obtained by the foliar application of H₂O₂ at concentration of 36 μM may be related to its signaling role, contributing to the production of proteins and carbohydrates, in addition to boosting the plant defense system, detoxifying reactive oxygen species and aiding plant growth (SILVA et al., 2022b).

Figure 6.
Plant height of cherry tomato as a function of the interaction between the salinity levels of the nutrient solution (ECns) and concentrations of hydrogen peroxide (H₂O₂), at 70 days after transplanting.

The stem diameter of cherry tomato plants decreased linearly with the increase in the salinity levels of the nutrient solution (Figure 7A), by 9.3% per unit increment in ECns. When comparing the SD of plants subjected to ECns of 4.2 dS m^-1 to the value of those that received the lowest salinity level of the nutrient solution (2.1 dS m^-1), a reduction of 24.04% was observed. This decrease in stem diameter growth may be linked to the increase in reactive oxygen species that cause various biochemical disturbances and physiological changes, such as decreased stomatal opening, which imposes limitations on plant growth (DANTAS et al., 2021).

Figure 7.
Stem diameter - SD (A), stem dry mass - SDM (B), and root dry mass - RDM (C) of cherry tomato plants, as a function of the salinity levels of the nutrient solution - ECns, in a hydroponic system, at 70 days after transplanting.

For stem dry mass (Figure 7B), it was observed that plants under ECns of 2.1 dS m^-1 reached the maximum estimated value of 14.17 g per plant, while those subjected to ECns of 4.2 dS m^-1 expressed the lowest SDM accumulation (11.93 g per plant). The reduction in biomass accumulation may be related to the decrease in the photosynthetic rate of plants. Excess of salts can impair the physiological aspects of the plant, causing ionic, osmotic, hormonal, and nutritional changes, consequently leading to suboptimal growth (ROQUE et al., 2022). In a study conducted by Silva et al. (2022a), with cherry tomatoes grown under salt stress in pots, these authors found that plants irrigated with low-salinity water (0.6 dS m^-1) had a 10.72 g higher SDM compared to plants irrigated using water with ECw of 2.6 dS m^-1.

As for the root dry mass of cherry tomato plants (Figure 7C), there was a decreasing linear behavior with the increase in nutrient solution salinity, with a reduction of 6.29% per unit increment in ECns. Plants subjected to 4.2 dS m^-1 had their RDM reduced by 15.21% (2.09 g per plant) compared to those under ECns of 2.1 dS m^-1. The reduction of root dry mass is a result of lower root growth.

The effect of salinity on the root system is due to the direct contact between the roots and the salts in the environment, which slows growth and evapotranspiration rates (LIMA et al., 2014). This reduction may be related to the deleterious effects caused by salt stress, as the high concentrations of sodium salts negatively affect the physiological aspects of the plant, promoting ionic, osmotic, hormonal, and nutritional changes, thus causing reductions in growth and, consequently, in biomass accumulation (SÁ et al., 2019). Mendonça et al. (2022), when evaluating okra plants subjected to salt stress, found a reduction in root dry mass accumulation of 9.11% per unit increment in ECns.

CONCLUSIONS

Nutrient solution salinity from 2.1 dS m^-1 negatively affects leaf gas exchange, photochemical efficiency, photosynthetic pigments, relative water content, and growth of cherry tomato at 65 days after transplanting. Foliar ap p lication of hydrogen peroxide at concentrations of 36 and 48 μM associated with nutrient solution salinity of 2.1 dS m^-1 stimulates growth in plant height and the synthesis of chlorophyll b, respectively, in cherry tomato plants. Hydrogen peroxide alone did not affect gas exchange, chlorophyll fluorescence, photosynthetic pigments, and growth of the orange cherry tomato at 65 days after transplanting.

REFERENCES

• ALVARES, C. A. et al: Köppen climate classification map for Brazil. Meteorologische Zeitschrift, 22: 711-728, 2013.
• ARNON, D. I. Copper enzymes in isolated cloroplasts: polyphenoloxidases in Beta vulgaris Plant Physiology, 24: 1-15, 1949.
• CARVALHO, F. E. L. et al. Aclimatação ao estresse salino em plantas de arroz induzida pelo pré-tratamento com H₂O₂ Revista Brasileira de Engenharia Agrícola e Ambiental, 15: 416-423, 2011.
• CASTAÑEDA, W, D. et al. Production and nutritional quality of tomatoes (Solanum lycopersicum var. Cerasiforme) are improved in the presence of biochar and inoculation with arbuscular mycorrhizae. American Journal of Plant Sciences, 11: 426-436, 2020.
• COSTA, L. F. et al. Cauliflower growth and yield in a hydroponic system with brackish water. Revista Caatinga, 33: 1060-1070, 2020.
• DANTAS, M. V. et al. Gas exchange and hydroponic production of zucchini under salt stress and H₂O₂ application. Revista Caatinga, 35: 436-449, 2022a.
• DANTAS, M. V. et al. Hydrogen peroxide and saline nutrient solution in hydroponic zucchini culture. Semina: Ciências Agrárias, 43: 1167-1186, 2022b.
• DANTAS, M. V. et al. Summer squash morphophysiology under salt stress and exogenous application of H₂O₂ in hydroponic cultivation. Comunicata Scientiae, 12: 1-9, 2021.
• DIAS, A. S. et al. Gas exchanges, quantum yield and photosynthetic pigments of West Indian cherry under salt stress and potassium fertilization. Revista Caatinga, 32: 429-439, 2019.
• DIAS, A. S. et al. Gas exchanges and photochemical efficiency of West Indian cherry cultivated with saline water and potassium fertilization. Revista Brasileira de Engenharia Agrícola e Ambiental, 22: 628-633, 2021.
• FERNANDES, E. A. et al. Cell damage, gas exchange, and growth of Annona squamosa L. under saline water irrigation and potassium fertilization. Semina: Ciências Agrárias, 42: 999-1018, 2021.
• FERREIRA, D. F. SISVAR: A computer analysis system to fixed effects split-plot type designs. Revista Brasileira de Biometria, 37: 529-535, 2019.
• FRANCA, R. J. F.; LEITÃO M. M. V. B. R.; CAMPECHE, L. F. S. M. Produtividade do tomate cereja em ambiente protegido e céu aberto em função das lâminas e intermitências de irrigação. Revista Brasileira de Agricultura Irrigada, 11: 1364-1370, 2017.
• GONÇALVES, D. C. et al. Cultivo tomate cereja sob sistema hidropônico: Influência do turno de rega. Uniciências, 22: 20-23, 2018.
• GUIMARÃES, R. F. B. et al. Trocas gasosas em cultivares de alface crespa em cultivo hidropônico com água salina. Revista Brasileira de Agricultura Irrigada, 13: 3599-3609, 2019.
• HOAGLAND, D. R.; ARNON, D. I. The water-culture method for growing plants without soil 2. ed. Circular. Berkeley: California Agricultural Experiment Station, 1950. n. 347, 32 p.
• JACINTO JÚNIOR, S. G. et al. Respostas fisiológicas de genótipos de fava (Phaseolus Lunatus L.) submetidas ao estresse hídrico cultivadas no estado do Ceará. Revista Brasileira de Meteorologia, 34: 413-422. 2019.
• LIMA, G. S. de et al. Crescimento e componentes de produção da mamoneira sob estresse salino e adubação nitrogenada. Engenharia Agrícola, 34: 854-866, 2014.
• LIMA, G. S. et al. Gas exchange, chloroplast pigments and growth of passion fruit cultivated with saline water and potassium fertilization. Revista Caatinga, 33: 184-194, 2020.
• LIMA, G. S. et al. Trocas gasosas, pigmentos cloroplastídicos e dano celular na mamoneira sob diferentes composições catiônica da água. Irriga, 22: 757- 774, 2017.
• MENDONÇA, A. J. T. et al. Salicylic acid modulates okra tolerance to salt stress in hydroponic system. Agriculture, 12: e1687, 2022.
• NÓBREGA, J. S. et al. Acúmulo de biomassa e pigmentos fotossintéticos em plantas de Mesosphaerum suaveolens (L.) Kuntze sob estresse salino e doses de ácido salicílico. Research, Society and Development, 9: e121953286, 2020.
• OLIVEIRA, V. K. N. et al. Salicylic acid does not mitigate salt stress on the morphophysiology and production of hydroponic melon. Brazilian Journal of Biology, 82: 262-664, 2022.
• PINHEIRO, F. W. A. et al. Gas exchange and yellow passion fruit production under irrigation strategies using brackish water and potassium. Revista Ciência Agronômica, 53: e20217816, 2022.
• RICHARDS, L. A. Diagnosis and improvement of saline and alkali soils Washington: U.S, Department of Agriculture. 1954. 160 p.
• ROQUE, I. A. et al. Biomass, gas exchange and production of cherry tomato cultivated under saline water and nitrogen fertilization. Revista Caatinga, 35: 686–696, 2022.
• SÁ, F. V. et al. Ecophysiology of West Indian cherry irrigated with saline water under phosphorus and nitrogen doses. Bioscience Journal, 35: 211- 221, 2019.
• SILVA, A. A. R et al. Hydrogen peroxide reduces the effect of salt stress on growth and postharvest quality of hydroponic mini watermelon. Water, Air, and Soil Pollution, 233: e198, 2022b.
• SILVA, A. A. R. et al. Induction of salt stress tolerance in cherry tomatoes under different salicylic acid application methods. Semina: Ciências Agrárias, 43: 1145-1166, 2022a.
• SILVA, A. A. R. et al. Hydrogen peroxide in the acclimation of yellow passion fruit seedlings to salt stress. Revista Brasileira de Engenharia Agrícola e Ambiental, 25:116-123, 2021.
• SILVA, A. R. A. et al. Pigmentos fotossintéticos e potencial hídrico foliar em plantas jovens de coqueiro sob estresses hídrico e salino. Agro@mbiente, 10: 317-325, 2016.
• SUN, Y. L. et al. Moderate soil salinity alleviates the impacts of drought on growth and water status of plants. Russian Journal of Plant Physiology, 67: 153-161, 2020.
• TAVARES FILHO, G. et al. Qualidade da água no semiárido e seus efeitos nos atributos do solo e na cultura da Moringa oleifera Lam. Revista de Ciências Agrárias, 43: 293-301, 2020.
• WEATHERLEY, P. E. Studies in water relations of cotton plant. I: The field measurement of water deficits in leaves. New Phytology, 49: 81-97, 1950.

Publication Dates

• Publication in this collection
  23 Feb 2024
• Date of issue
  2024

History

• Received
  06 June 2023
• Accepted
  14 Sept 2023