Hydrogen peroxide as a saline stress attenuator in okra (Abelmoschus esculentus L.)

Peróxido de hidrogênio como atenuante do estresse salino em quiabeiro (Abelmoschus esculentus L.)

ABSTRACT:

Okra is a vegetable that stands out for its low production cost, high yield, high nutritional and value and socioeconomic importance in income generation, especially in family farming. In this context, the present study evaluated the physiology, production and water use efficiency of okra under irrigation with saline water and exogenous application of hydrogen peroxide. The treatments were distributed in a randomized block design, in a 5 × 3 factorial arrangement, with five levels of electrical conductivity of irrigation water - ECw (0.3; 1.3; 2.3; 3.3 and 4.3 dS m^-1) and three concentrations of hydrogen peroxide - H₂O₂ (0, 25 and 50 μM), with five replicates, totaling 75 plants. Irrigation with ECw above 0.3 dS m^-1 negatively affected stomatal conductance, transpiration, number of fruits, total production and water use efficiency of okra cv. Clemson Americano 80. Hydrogen peroxide at concentration of 22 μM mitigated the effects of salt stress on CO₂ assimilation rate, number of fruits, average fruit weight and total production of okra cv. Clemson Americano 80. Water use efficiency is favored by H₂O₂ application at concentration of 12 μM, especially in plants irrigated with ECw of 0.3 dS m^-1.

Key words: Salinity; H₂O₂; acclimatization.

RESUMO:

O quiabeiro é uma hortaliça que se destaca pelo baixo custo de produção, elevada produtividade, alto valor nutricional e pela importância socioeconômica na geração de renda, principalmente na agricultura familiar. Nesse contexto, o presente trabalho teve como objetivo avaliar a fisiologia, a produção e a eficiência no uso da água de quiabeiro sob irrigação com águas salinas e aplicação exógena de peróxido de hidrogênio. Os tratamentos foram distribuídos no delineamento de blocos casualizados, em arranjo fatorial 5 × 3, sendo cinco níveis de condutividade elétrica da água de irrigação (0,3; 1,3; 2,3; 3,3 e 4,3 dS m^-1) e três concentrações de peróxido de hidrogênio (0, 25 e 50 μM), com cinco repetições, totalizando 75 plantas. A irrigação com CEa acima de 0,3 dS m^-1 afetou negativamente a condutância estomática, a transpiração, o número de frutos, a produção total e a eficiência no uso da água do quiabeiro cv. Clemson Americano 80. O peróxido de hidrogênio na concentração de 22 μM amenizou os efeitos do estresse salino sobre a taxa de assimilação de CO₂, número de frutos, peso médio de frutos e produção total de quiabeiro cv. Clemson Americano 80. A eficiência no uso da água é beneficiada pela aplicação de peróxido de hidrogênio na concentração de 12 μM, sobretudo nas plantas irrigadas com CEa de 0,3 dS m^-1.

Palavras-chave: Salinidade; H₂O₂; aclimatação

INTRODUCTION:

Okra (Abelmoschus esculentus L.) is a fast growing annual crop, cultivated mainly for its tender fruits, being widely grown in tropical and subtropical regions in different countries from Africa to Asia, southern Europe and America (DURAZZO et al., 2019; ISLAM et al., 2019). Okra is an important vegetable in the human diet, being a source of carbohydrates, proteins, fats, minerals, and vitamins (MENDONÇA et al., 2022). It is a vegetable widely produced in Brazil, with about 128,460 tons of okra produced in 2017, and the Northeast is the second largest producer, with 32,337 tons, which corresponds to 25.1% of the national production (IBGE, 2017).

The semi-arid region of northeastern Brazil provides favorable edaphoclimatic conditions for okra production, because of its rusticity and tolerance to high temperatures; however, due to water restrictions in terms of quality and quantity (SOARES et al., 2018; SOARES et al. 2020), it is necessary to perform irrigation using waters with high levels of salts, commonly found in surface and underground water sources, leading to losses in fruit production and quality (LIMA et al., 2020).

The use of saline water can cause osmotic imbalances and ionic, nutritional and oxidative damage, resulting from the excessive accumulation of reactive oxygen species (ROS), such as superoxide radical (O_2• ^-), hydrogen peroxide (H₂O₂) and hydroxyl radical (OH), compromising plant growth and physiological processes (MORAIS et al., 2020, SILVA et al., 2021). However, the effects of salt stress may vary on each crop, depending on irrigation and fertilization management, edaphoclimatic conditions, species or even development stages (LIMA et al., 2016; SOARES et al., 2021).

In this context, alternatives have been sought to mitigate the effects caused by salt stress on plants, among which the exogenous application of hydrogen peroxide (H₂O₂) at low concentrations has proven to be promising in the acclimatization of plants to salt stress (SEMIDA, 2016, SILVA et al., 2019; ANDRADE et al., 2022) In addition, H₂O₂ acts as a signaling molecule in plants under biotic and abiotic stresses, favoring greater accumulation of soluble proteins and carbohydrates that can act as organic solutes, performing the osmotic adjustment of plants under salt stress, thus allowing greater water absorption (ANDRADE et al., 2019; DANTAS et al., 2021; SILVA et al., 2021).

However, there is no information in the literature on the role of this reactive oxygen species in the mitigation of salt stress effects on okra under the semi-arid conditions of northeastern Brazil. In this context, the present research evaluates the effect of hydrogen peroxide on the physiology, production and efficiency of water use in Abelmoschus esculentus L. plants under irrigation with saline water.

MATERIALS AND METHODS:

The experiment was carried out under 70% shading conditions from December 2020 to March 2021, at the Center for Science and Agri-Food Technology - CCTA of the Federal University of Campina Grande - UFCG, located in the municipality of Pombal, Paraíba, Brazil, at the geographic coordinates 6º46’13” S, 37°48’06” W and altitude of 193 m. Data on maximum and minimum air temperature, precipitation and relative humidity during the experimental period are shown in figure 1.

Figure 1
Climatic data of maximum air temperature, minimum air temperature and precipitation and relative humidity during the experimental period.

The experimental design used was randomized blocks, in a 5 × 3 factorial arrangement, referring to five levels of electrical conductivity of water - ECw (0.3; 1.3; 2.3; 3.3 and 4.3 dS m^-1) and three concentrations of hydrogen peroxide - H₂O₂ (0, 25 and 50 μM) with five replicates and one plant per plot. Due to few studies with H₂O₂ in vegetables, the concentrations used were based on studies carried out with zucchini (DANTAS et al., 2021), passion fruit (RAMOS et al. 2021) and passion fruit (Silva et al., 2019), while salinity levels were based on SOARES et al. (2020).

Two seeds of Abelmoschus esculentus L. cv. Clemson Americano 80 were sown at 0.5 cm depth in plastic trays with 162 cells, with capacity of 50 ml. The substrate used was obtained by mixing soil, sand and cattle manure in the proportion of 2:1:1, respectively; in this phase, the plants were irrigated daily with low-salinity water (0.3 dS m^-1) and, subsequently, thinning was performed, leaving only one plant per cell.

At 20 days after sowing (DAS), when the plants reached 10 cm in height and had two pairs of true leaves, they were transplanted to pots adapted as drainage lysimeters with 20 L capacity, which received a 3-cm-thick layer of crushed stone above a geotextile which covered the base of the container, to prevent clogging by soil material. At the base of each container, a 15-mm-diameter hose was installed, as a drain, connected to a plastic container (2 L) to collect drained water. Then, the pots received 22 kg of a Neossolo Flúvico (Fluvent) with sandy loam texture, whose physical and chemical characteristics were determined according to TEIXEIRA et al. (2017): Ca²⁺, Mg²⁺, Na⁺, K⁺, Al³⁺ + H⁺ = 9.07, 2.78, 1.64, 0.23 and 8.61 cmol_c kg^-1, respectively; pH (1:2.5 soil water suspension) = 5.58; ECse = 2.15 dS m^-1; organic matter = 2.93 dag kg^-1; sand, silt and clay = 572.7, 100.7 and 326.6 g kg^-1, respectively; moisture content at 33.42 and 1519.5 kPa = 25.91 and 12.96 dag kg^-1, respectively.

Fertilization with NPK (100 mg N kg^-1 soil; 300 mg P₂O₅ kg^-1 soil and 150 mg K₂O kg^-1 soil) was performed according to the recommendation of NOVAIS et al. (1991), using urea, monoammonium phosphate and potassium chloride as sources, respectively, applied as top-dressing, split into three portions, with the first fertilization at 10 days after transplanting (DAT). Micronutrients were supplied every two weeks, starting at 20 DAT with the commercial product Micro Rexene^® containing: Mg - 1.2%; B - 0.85%; Zn - 4.2%; Fe - 3.4%; Mn - 3.2%; Cu - 0.5% and Mo - 0.06%. The pots were arranged in single rows at spacing of 1.5 m between rows and 1.0 m between plants in the row.

The treatments began to be applied at 20 DAT. Hydrogen peroxide applications were performed every 15 days, in the late afternoon, totaling four applications, with an average volume of 250 mL per plant. Foliar applications were performed on the abaxial and adaxial sides of the leaves, using a manual sprayer with capacity of 1 L.

The water with the lowest electrical conductivity (0.3 dS m^-1) was obtained from the public supply system of Pombal-PB and the other ECw levels were prepared from the dissolution of sodium chloride (NaCl) considering the relationship between ECw and the concentration of salts (mmol_c L^-1 ≈ 10 × ECw) (RHOADES et al., 2000).

Prior to transplantation, the soil moisture content was increased to the level corresponding to the maximum water holding capacity, and irrigation were carried out daily with water of low electrical conductivity (0.3 dS m^-1) until 20 DAT. After this period, irrigation began to be performed with the different salinity levels, at one-day interval, and the applied depth was determined based on the water balance to replace the average daily consumption by plants, with a leaching fraction of 10% every 15 days, obtained by dividing the value of the volume to be applied (mL) by 0.9, to promote the leaching of excess salts supplied by irrigation water from the root zone.

During the experiment, all preparations and phytosanitary practices recommended for the crop were performed, monitoring the emergence of pests and diseases and adopting control measures when necessary.

Gas exchange was measured at 75 DAT based on stomatal conductance - gs (mol CO₂ m^-2 s^-1), internal CO₂ concentration - Ci (μmol CO₂ m^-2 s^-1), transpiration - E (mmol H₂O m^-2 s^-1) and CO₂ assimilation rate - A (μmol CO₂ m^-2 s^-1). These data were then used to determine 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 [ (μmol CO₂ m^-2 s^-1) (μmol CO₂ m^-2 s^-1) ], with an infrared gas analyzer - IRGA (LCpro - SD model from ADC BioScientific, UK).

At 59 DAT, fruits began to be harvested manually as they showed the typical green color of ripe fruits, over a period of 23 days. The number of fruits per plant (NFP), average fruit length (AFL), average fruit weight (AFW) and total production per plant (TPP) were recorded. Fruit length was measured from the tip of the fruit to the insertion point of its peduncle. The fruits obtained in each harvest were weighed on a precision scale (0.01 g) to obtain the fresh matter. The average production per plant was obtained considering the number of plants during the harvest period. Water use efficiency - WUE (g L^-1) was determined through the relationship between production (total production per plant) and accumulated water consumption until the end of the production cycle.

The data obtained were subjected to analysis of variance (F test) at 0.05 and 0.01 probability levels and, in cases of significance, linear and quadratic polynomial regression analysis was performed for the levels of electrical conductivity of irrigation water and means comparison test (Tukey) was performed for H₂O₂ concentrations using the statistical software SISVAR (FERREIRA 2019).

RESULTS AND DISCUSSION:

There was a significant effect of water salinity levels (SL) on stomatal conductance (gs), transpiration (E) and CO₂ assimilation rate (A) of Abelmoschus esculentus L. plants (Table 1). Hydrogen peroxide (H₂O₂) concentrations significantly affected all variables studied. Only the instantaneous water use efficiency (WUEi) of Abelmoschus esculentus L., at 75 days after transplanting, was not significantly influenced by the interaction between factors (SL × H₂O₂).

Table 1
Summary of the 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 Abelmoschus esculentus L. cv. Clemson Americano 80 cultivated with saline waters and hydrogen peroxide concentrations, at 75 days after transplanting.

Sources of variation	------DF------	-----------------------------------------------Mean squares------------------------------------------
		gs 1	E	Ci	A 1	WUEi	CEi 1
Salinity levels (SL)	4	0.003*	0.28*	160.61ns	16.23*	0.76ns	0.001ns
Linear Regression	1	0.01**	0.66**	77.76ns	33.15*	0.008ns	0.01**
Quadratic Regression	1	0.001ns	0.40*	17.14ns	14.36ns	2.59*	0.0006ns
Hydrogen peroxide (H2O2)	2	0.01**	1.53**	1070.56*	74.87**	2.27*	0.008**
Interaction (SL × H2O2)	8	0.002*	0.35**	1172.79**	19.59**	0.62ns	0.003**
Blocks	4	0.001ns	0.42	520.74ns	4.11ns	0.51ns	0.001ns
CV (%)		10.35	6.92	11.16	7.83	9.48	13.88
Mean		0.34	4.33	154.08	30.76	7.13	0.20

• ^{ns, *, **,} respectively not significant and significant at P ≤ 0.05 and P ≤ 0.01; CV= coefficient of variation; DF = degrees of freedom; ¹data transformed into √x.

Stomatal conductance (Figure 2A) decreased with increasing water salinity in plants, regardless of H₂O₂ concentration, and it is also verified that the increase in H₂O₂ concentration intensified the deleterious effects of salinity on gs. While plants irrigated with ECw of 0.3 dS m^-1 and subjected to a concentration of 0 μM H₂O₂ obtained the highest value of gs (0.387 mol H₂O m^-2 s^-1), plants irrigated with ECw of 4.3 dS m^-1 and cultivated with H₂O₂ at concentration of 50 μM had the lowest gs (0.312 mol H₂O m^-2 s^-1), corresponding to a reduction of 19.3% (0.075 mol H₂O m^-2 s^-1) compared to the highest value of gs.

Figure 2 -
Response surface for stomatal conductance - gs (A) and internal CO₂ concentration - Ci (B) of Abelmoschus esculentus L. cv. Clemson Americano 80, as a function of the interaction between electrical conductivity of irrigation water - ECw and hydrogen peroxide concentrations - H₂O₂ at 75 after transplantation. X and Y - ECw and hydrogen peroxide, respectively; ^{*, **} Significant at P ≤ 0.05 and 0.01 by F test.

Stomatal closure occurred because gs is dependent on stomatal cells, that is, when plants cannot absorb water from the soil, their mechanism of functioning is affected, due to the osmotic effect. Stomatal closure is a strategy to prevent water loss to the environment and maintain high water status in the cell (DIAS et al., 2019). Decrease in gs in plants grown under water salinity has also been observed in other crops, such as cowpea (OLIVEIRA et al., 2017) and zucchini (DANTAS et al., 2021).

H₂O₂ concentrations up to 25 μM promoted an increase in the internal CO₂ concentration, regardless of the electrical conductivity of irrigation water (Figure 2B). Plants subjected to a concentration of 25 μM and irrigated with water of 2.1 dS m^-1 obtained the highest value of Ci (166.3 μmol CO₂ m^-2 s^-1). Plants irrigated with ECw of 2.1 dS m^-1 and subjected to H₂O₂ concentration of 25 μM increased their Ci by 13.7% (20.1 μmol CO₂ m^-2 s^-1) compared to those cultivated with ECw of 2.1 dS m^-1 and without H₂O₂ application (0 μM). It is also verified that the increase in ECw combined with H₂O₂ at concentrations above 25 μM reduced Ci, and a value of 137.6 μmol CO₂ m^-2 s^-1 was recorded in plants irrigated with ECw of 4.3 dS m^-1 and subjected to H₂O₂ concentration of 50 μM.

DANTAS et al. (2021), when evaluating gas exchange in Italian zucchini cultivated under salinity of nutrient solutions in hydroponic system and foliar application of H₂O₂, also verified that the internal CO₂ concentration decreased with the increase in salinity levels. Decrease in Ci is indicative that the CO₂ fixed in the mesophyll cell is being consumed in the synthesis of sugars during the photosynthetic process (DIAS et al., 2019).

Effect similar to that observed on gs (Figure 2A) was verified for the transpiration rate (Figure 3A), i.e., the increase in the electrical conductivity of irrigation water reduced E, regardless of H₂O₂ concentration. It is also observed that the increase in H₂O₂ concentration intensifies such reduction, with the lowest value of E (3.73 mmol H₂O m^-2 s^-1) obtained in plants irrigated with ECw of 4.3 dS m^-1 and subjected to H₂O₂ concentration of 50 μM. The reduction of transpiration may be related to the low water potential in the roots caused by osmotic stress, which leads to physiological changes in plants, such as stomatal closure, to avoid dehydration of leaves due to the loss of water to the atmosphere (LIMA et al., 2016).

Figure 3 -
Response surface for transpiration - E (A) and CO₂ assimilation rate - A (B) of Abelmoschus esculentus L. cv. Clemson Americano 80, as a function of the interaction between electrical conductivity of irrigation water - ECw and hydrogen peroxide concentrations - H₂O₂, at 75 after transplantation. X and Y - ECw and hydrogen peroxide, respectively; ^{*, **} Significant at P ≤ 0.05 and 0.01 by F test.

Foliar spraying of H₂O₂ at concentration of 22 μM resulted in the mitigation of the deleterious effects of salinity on the CO₂ assimilation rate (Figure 3B). Plants sprayed with H₂O₂ at concentration of 22 μM and irrigated with ECw of 1.3 dS m^-1 reached the highest value of A (33.08 μmol CO₂ m^-2 s^-1), which corresponded to an increase of 5.7% (1.80 mmol CO₂ m^-2 s^-1) compared to plants irrigated with the same ECw (1.3 dS m^-1) and without H₂O₂ application (0 μM). The beneficial effects of hydrogen peroxide may be related to the fact that it functions as a signaling molecule and acts in the regulation of several pathways, including responses to salt stress (SEMIDA, 2016; SILVA et al., 2021). It is important to highlight that H₂O₂ is a ROS and that its generation is part of the natural metabolism of plants.

For instantaneous water use efficiency (Figure 4A), plants that received foliar application of H₂O₂ at 25 and 50 μM had statically higher WUEi compared to those grown without H₂O₂ (0 μM). However, when comparing the WUEi values of plants that received 25 and 50 μM, it was observed that there was no significant difference between them. According to CARVALHO et al. (2011), pre-exposure of plants to signaling metabolites such as H₂O₂ may promote an increase in metabolites or antioxidative enzymes and, therefore, may result in better physiological performance.

Figure 4 -
Instantaneous water use efficiency - WUEi of Abelmoschus esculentus L. cv. Clemson Americano 80, as a function of hydrogen peroxide concentrations - H₂O₂ (A) and response surface for instantaneous carboxylation efficiency - CEi (B) as a function of the interaction between the levels of electrical conductivity of irrigation water - ECw and H₂O₂ concentrations, at 75 days after transplanting. Means followed by different letters indicate significant difference between treatments by Tukey test (P ≤ 0.05). X and Y - ECw and hydrogen peroxide, respectively; ^{*, **} Significant at P ≤ 0.05 and 0.01 by F test.

Regarding the instantaneous carboxylation efficiency of plants (Figure 4B), the increase in H₂O₂ concentration to 50 μM promoted an increase in CEi, regardless of the electrical conductivity of irrigation water, with the highest value of CEi (0.241 (μmol CO₂ m^-2 s^-1)(μmol CO₂ m^-2 s^-1)^-1) obtained in plants irrigated with ECw of 1.8 dS m^-1 and subjected to H₂O₂ concentration of 50 μM. However, irrigation with ECw above 1.8 dS m^-1 leads to reduction of CEi, and the lowest CEi value (0.173 (μmol CO₂ m^-2 s^-1)(μmol CO₂ m^-2 s^-1)^-1) was recorded in plants irrigated with ECw of 4.3 dS m^-1 and without H₂O₂ application (0 μM).

Reduction in CEi is an indication that factors of non-stomatal origin also influenced the photosynthetic activity of plants, such as low activity of the enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), probably due to limited availability of substrate (ATP and NADPH) for enzyme activation and regeneration (HUSSAIN et al., 2012).

There was a significant effect of the interaction between the factors (SL × H₂O₂) for number of fruits per plant (NFP), total production per plant (TPP), average fruit weight (AFW) and water use efficiency (WUE) (Table 2). As single factor, there was significant effect of water salinity levels on all variables studied, except average fruit length. Hydrogen peroxide concentrations, on the other hand, promoted significant differences in plant production, average fruit weight and water use efficiency.

Table 2
Summary of the analysis of variance for number of fruits per plant (NFP), total production per plant (TPP), average fruit weight (AFW), average fruit length (AFL) and water use efficiency (WUE) of Abelmoschus esculentus L. cv. Clemson Americano 80, cultivated with saline waters and hydrogen peroxide concentrations (H₂O₂), at 82 days after transplanting.

Sources of variation	----DF----	--------------------------------------------Mean squares---------------------------------------------
		NFP	TPP	AFW	AFL	WUE
Salinity levels (SL)	4	87.053**	42299.604**	28.945*	0.789ns	31.927**
Linear Regression	1	340.506**	165810.934**	17.967ns	1.626ns	124.619**
Quadratic Regression	1	0.076ns	549.238ns	8.178ns	0.016ns	1.675ns
Hydrogen peroxide (H2O2)	2	0.280ns	3669.260**	62.634**	1.428ns	4.231**
Interaction (SL × H2O2)	8	1.113**	3300.000**	32.922**	1.024ns	4.011**
Blocks	4	0.886ns	842.520*	7.422ns	1.387ns	1.149ns
CV (%)		9.69	10.51	14.56	9.41	12.64
Mean		6.24	140.92	22.84	12.34	5.45

• ^{ns, *, **,} respectively not significant and significant at P ≤ 0.05 and ≤ 0.01; CV = coefficient of variation.

The increase in the electrical conductivity of irrigation water reduced the number of fruits of Abelmoschus esculentus L., regardless of the H₂O₂ concentration (Figure 5A). However, plants subjected to H₂O₂ concentration of 22 μM and irrigated with ECw of 0.3 dS m^-1 stood out with the highest NFP (9.23 fruits), which corresponded to an increase of 2.92% compared to those irrigated with the same level (0.3 dS m^-1) and without application of H₂O₂ (0 μM). The lowest NFP value (3.53 fruits) was obtained in plants irrigated with ECw of 4.3 dS m^-1 and without application of H₂O₂ (0 μM). The reduction in the number of fruits in Abelmoschus esculentus L. plants may be related to their difficulty in absorbing water and nutrients, due to the decrease in the osmotic potential of the soil solution, caused by excess salts (MENDONÇA et al., 2023). Water salinity induced stomatal closure, a situation verified by the reduction in stomatal conductance and changes in CO₂ assimilation rate.

Figure 5
- Response surface for number of fruits per plant - NFP (A), total production per plant - TPP (B) and average fruit weight - AFW (C) of Abelmoschus esculentus L. cv. Clemson Americano 80, as a function of the interaction between the levels of electrical conductivity of irrigation water - ECw and hydrogen peroxide concentrations - H₂O₂ at 82 days after transplanting. X and Y - ECw and hydrogen peroxide, respectively; ^{*, **} Significant at P ≤ 0.05 and 0.01 by F test.

The total production per plant (Figure 5B) was also reduced by the increase in the electrical conductivity of irrigation water at all H₂O₂ concentrations. However, foliar spraying of H₂O₂ at the concentration of 22 μM promoted the highest TPP (224.3 g per plant) in plants cultivated with ECw of 0.3 dS m^-1, decreasing from this salinity level and reaching the lowest value (88.4 g per plant) under ECw of 4.3 dS m^-1 and H₂O₂ of 50 μM. The reduction in plant production under salt stress conditions is also related to energy diversion to maintain metabolic activities, due to the restriction in the absorption of water and nutrients caused by osmotic and ionic stress, resulting from the high concentration of salts in the soil solution, especially Na⁺ and Cl^- ions, disorganization of the membrane system and production of reactive oxygen species (LUCENA et al., 2012).

Foliar spraying of H₂O₂ at a concentration of 22 μM resulted in the mitigation of the deleterious effects of salinity on the average fruit weight (Figure 5C). Plants sprayed with H₂O₂ at the concentration of 22 μM and irrigated with ECw of 2.5 dS m^-1 reached the highest value of AFW (25.4 g per fruit), which corresponded to an increase of 4.5% (1.1 g per fruit) compared to plants irrigated with the same ECw (2.5 dS m^-1) and without application of H₂O₂ (0 μM). The increase in AFW may be associated with the role of hydrogen peroxide as a signaling molecule under biotic and abiotic stresses. In addition, H₂O₂ acts by inducing the antioxidant enzyme defense system when applied at low concentrations, attenuating the deleterious effects of salinity (PETROV & BREUSEGEM, 2012).

The water use efficiency of plants was negatively affected by the increase in electrical conductivity of irrigation water (Figure 6). Nevertheless, the regression equation showed that the H₂O₂ concentration of 12 μM was able to promote an increase in WUE, with the maximum value of 6.51 g L^-1 obtained in plants irrigated with ECw of 0.3 dS m^-1 and cultivated with H₂O₂ concentration of 12 μM. The lowest WUE (3.49 g L^-1) was recorded in plants irrigated with ECw of 4.3 dS m^-1 and without H₂O₂ application (0 μM).

Figure 6
- Response surface for water use efficiency - WUE of Abelmoschus esculentus L. cv. Clemson Americano 80, as a function of salinity of irrigation water - ECw and hydrogen peroxide concentrations - H₂O₂ at 82 days after transplanting. X and Y - ECw and hydrogen peroxide, respectively; ^{*, **} Significant at P ≤ 0.05 and 0.01 by F test.

Thus, it can be affirmed that the plants had a lower capacity to convert the volume of water actually consumed into photoassimilates, which was observed by the accumulation of biomass, due to higher energy expenditure to maintain the biosynthesis of osmotic solutes and the generation of energy necessary for this biosynthesis, in addition to other important processes for the osmotic adjustment of plants, which contribute to their absorption of water and nutrients (LIMA et al., 2020).

CONCLUSION:

Irrigation with ECw above 0.3 dS m^-1 negatively affects stomatal conductance, transpiration, number of fruits, total production and water use efficiency of Abelmoschus esculentus L. cv. Clemson Americano 80.

Hydrogen peroxide at concentration of 22 μM mitigates the effects of salt stress on the CO₂ assimilation rate, number of fruits, average fruit weight and total production of Abelmoschus esculentus L. cv. Clemson Americano 80.

Water use efficiency is favored by the application of hydrogen peroxide at concentration of 12 μM, especially in plants irrigated with ECw of 0.3 dS m^-1.

ACKNOWLEDGEMENTS

The I.A.P.L. was partial funded by the Brazilian Federal Agencies: Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brazil - Finance Code 001.

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• 0
  CR-2022-0252.R1

Edited by

• Editors: Leandro Souza da Silva (0000-0002-1636-6643) Mauricio Hunsche (0000-0002-9694-0042)

Publication Dates

• Publication in this collection
  18 Dec 2023
• Date of issue
  May 2024

History

• Received
  30 Apr 2022
• Accepted
  18 July 2023
• Reviewed
  17 Sept 2023