Physiology and growth of maize under salinity of water and application of hydrogen peroxide¹

Fisiologia e crescimento do milho sob salinidade da água e aplicação de peróxido de hidrogênio

ABSTRACT

The salinity of irrigation water or soil is the abiotic factor that most negatively impacts the yield of crops, including green maize, so it is necessary to find alternatives to ensure production. In this context, this study aimed to evaluate gas exchange, photosynthetic pigments, and growth of maize under salinity of irrigation water and application of hydrogen peroxide. The experiment was conducted at the Center of Sciences and Agrifood Technology, Pombal, PB, belonging to the Federal University of Campina Grande, in the period from January to February 2015, using the maize hybrid ‘AG 1051’. The treatments consisted of two salinity levels of irrigation water (0.3 and 2.0 dS m^-1) and five concentrations of hydrogen peroxide (0, 40, 80, 160, and 320 µmol L^-1) applied via irrigation water. The experimental design was completely randomized, in a 2 × 5 factorial scheme, with four replicates. ECw of 2.0 dS m^-1 reduces transpiration, stomatal conductance, total chlorophyll, carotenoids, and initial growth, but does not affect the dry mass accumulation of maize plants. Application of H₂O₂ via soil varying from 0 to 320 µmol L^-1 causes reductions in the CO₂ assimilation rate and transpiration, as well as at concentrations from 0 to 160 µmol L^-1 for stomatal conductance of plants irrigated with ECw of 2.0 dS m^-1. Application of H₂O₂ via soil up to a concentration of 320 µmol L^-1 increases the plant height, but reduces culm diameter of maize.

Key words:
Zea mays L.; saline water; mitigator

RESUMO

A salinidade da água de irrigação ou do solo são fatores abióticos que mais impacta negativamente na produtividade das culturas, entre elas, o milho verde, sendo necessário encontrar alternativas para garantir a produção. Nesse contexto, objetivou-se avaliar as trocas gasosas, os pigmentos fotossintéticos e o crescimento do milho sob salinidade da água de irrigação e aplicação de peróxido de hidrogênio. O experimento foi conduzido no Centro de Ciências e Tecnologia Agroalimentar, Pombal, PB, pertencente à Universidade Federal de Campina Grande, no período de janeiro a fevereiro de 2015, utilizando-se do híbrido de milho ‘AG 1051’. Os tratamentos foram constituídos por dois níveis de salinidade da água de irrigação (0,3 e 2,0 dS m^-1) e cinco concentrações de peróxido de hidrogênio (0, 40, 80, 160 e 320 µmol L^-1) aplicadas via água de irrigação. O delineamento experimental foi o inteiramente casualizado, no esquema fatorial 2 × 5, com quatro repetições. A CEa de 2,0 dS m^-1 reduz a transpiração, condutância estomática, clorofila total, carotenóides e crescimento inicial, mas não afeta o acúmulo de massa seca das plantas de milho. A aplicação de H₂O₂ via solo variando de 0 a 320 µmol L^-1 promoveu redução na taxa de assimilação de CO₂ e transpiração, bem como em concentrações de 0 a 160 µmol L^-1 para condutância estomática de plantas irrigadas com CEa de 2,0 dS m^-1. Aplicação de H₂O₂ via solo até a concentração de 320 µmol L^-1 aumentou a altura de plantas, porém reduziu o diâmetro do colmo de milho.

Palavras-chave:
Zea mays L.; águas salinas; atenuante

HIGHLIGHTS:

Electrical conductivity of water (ECw) of 2.0 dS m^-1 reduces growth and chlorophyll content in maize.

Hydrogen peroxide up to a concentration of 171.66 µmol L^-1 increases leaf area and total biomass in maize.

Salt stress combined with high H₂O₂ concentrations inhibits stomatal conductance, transpiration, and CO₂ assimilation rate.

Introduction

In the current scenario of climate change, the low and irregular distribution of rainfall, together with high temperatures and high evapotranspiration, has limited the availability of good quality water in semi-arid regions. As an alternative to meet the water needs of crops, it is necessary to use irrigation; however, when managed improperly, it can lead to soil salinization, reducing the availability of water for plants, in addition to causing problems such as nutritional imbalance and toxicity of specific ions (Kaya et al., 2015b).

In physiology, the first response of the plant to salt stress is stomatal closure, as a way to prevent water loss, resulting in changes in stomatal conductance, internal carbon concentration, transpiration, and CO₂ assimilation rate (Arif et al., 2020). In addition to limiting gas exchange, stomatal closure induces the production of reactive oxygen species (ROS) due to the imbalance between the photochemical phase and the Calvin cycle, with consequences for plant metabolism (Taiz et al., 2017; Foyer, 2018).

However, it is possible to consider that the production of maize in regions with high levels of salts in the soil and/or water can be optimized using strategies to mitigate the deleterious effects of salinity, since this crop is moderately sensitive, with water salinity threshold of 1.1 dS m^-1 and soil salinity (saturation extract) threshold of 1.7 dS m^-1. In this regard, the application of hydrogen peroxide (H₂O₂) can increase the plant’s ability to survive adverse conditions (Silva et al., 2019b), following the example of studies conducted by Gondim et al. (2013) and Silva et al. (2016), who evaluated maize plants from seeds pre-treated with H₂O₂ and subjected to salinity and observed a reduction in the sensitivity of plants to salinity.

Thus, it is necessary to assess the effectiveness of H₂O₂ in activating the plant’s defense system and its effect on initial growth and gas exchange. This study aimed to evaluate gas exchange, photosynthetic pigments, and growth of maize under salinity of irrigation water and application of hydrogen peroxide.

Material and Methods

The study was conducted during January and February 2015, under open sky conditions, at the Center for Sciences and Agrifood Technology of the Federal University of Campina Grande (CCTA/UFCG), located in Pombal, PB, Brazil, whose altitude is 144 m, and coordinates 6º 48’ 16” S and 37º 49’ 15” W. The climatic characteristics recorded during the experiment are shown in Figure 1.

Figure 1
Maximum (T max.) and minimum (T min.) temperature (ºC), maximum and minimum relative humidity of air (RH - %), and rainfall (mm) observed during the experimental period

The treatments consisted of a combination of two levels of irrigation water salinity (0.3 and 2.0 dS m^-1) and five concentrations of hydrogen peroxide (0, 40, 80, 160, and 320 µmol L^-1), forming a 2 × 5 factorial, totaling 10 treatments and 40 experimental units, distributed in a completely randomized design, with four repetitions. The experimental unit consisted of two plants per pot. The salinity levels were based on the threshold water salinity of the maize crop (1.1 dS m^-1), selecting a low salinity level and another level above the threshold. Hydrogen peroxide concentrations were based on the study conducted by Godim et al. (2013) in maize plants using seeds pre-treated with hydrogen peroxide, but we used lower concentrations.

Considering that in the literature there are already studies with peroxide application via leaves and they obtained promising results, we chose to test the application via irrigation water, to check if it has the same effectiveness.

Three applications of hydrogen peroxide were made, one per day, during the first three days after sowing (DAS) via irrigation water, using an A.R. source. The H₂O₂ solutions were prepared from a 1 mmol L^-1 solution of H₂O₂, based on molecular weight, until reaching the desired concentrations. In each event, 0.3 L of the hydrogen peroxide solution was applied. As the application of the product was carried out via irrigation water, it was not necessary to isolate the treatments, since there was no possibility of drift.

The cultivation was performed in pots with capacity of eight cubic decimeters (8 dm³), filled with soil classified as Vertisol, with clayey texture and cation exchange capacity (CEC): 49.15 cmol_c dm^-3; exchangeable sodium percentage (ESP): 2%; pH (in soil:water 1:2.5): 7.14; electrical conductivity of the saturation extract (EC): 0.64 dS m^-1; P: 98.7 mg dm^-3; K: 399.5 mg dm^-3, Na: 214.3 mg dm^-3; Ca: 36.60 cmol_c dm^-3; Mg: 10.60 cmol_c dm^-3; H + Al: 0 cmol_c dm^-3.

The maize seeds used were the double-crossed hybrid ‘AG 1051’, as it is a genetic material indicated for the production of green corn and silage. Five seeds were sown per pot at a depth of 2.0 cm. Emergence in more than 50% of the plants was observed four days after sowing, and thinning was performed seven days after sowing, leaving two plants per pot.

The irrigations were performed manually twice a day (at 8:00 a.m. and 5:00 p.m.), and the volume to be applied was determined by drainage lysimetry. The amount of water applied per pot varied from 0.5 to 2.0 L per day during the experiment. The irrigation water with 2.0 dS m^-1 was prepared by adding NaCl to the local municipal water, which had an electrical conductivity of 0.3 dS m^-1, based on the relationship between ECw and the concentration of salts (mmol_c L^-1 = 10 × ECw) (Richards, 1954). Irrigation with water of different salinities began at 10 DAS, when the emergence of plants had stabilized.

For nutritional management, macro and micronutrients were applied in the amounts of 730, 495, and 748 mg dm^-3 of the soil of N, P, and K, respectively, in the form of urea, ammonium sulfate, monopotassium phosphate, and potassium chloride, divided into 4 applications, the first as basal and the others at 8, 18, and 28 days after emergence (DAE), via fertigation. For fertilization with micronutrients, boric acid and zinc sulfate were used as sources. Cultural practices and phytosanitary control were carried out whenever necessary in accordance with the needs of the crop (Cruz & Pereira Filho, 2002).

The evaluations were performed at 30 DAE. At this time, gas exchange was determined based on CO₂ assimilation rate (A - μmol CO₂ m^-2 s^-1), stomatal conductance (gs - mol H₂O m^-2 s^-1), transpiration (E - mmol H₂O m^-2 s^-1), and intercellular CO₂ concentration (Ci - μmol CO₂ mol^-1), measured with an infrared gas analyzer (IRGA) LCpro (Analytical Development, Kings Lynn, UK), using a constant light source of 1200 µmol photons m^-2 s^-1 and CO₂ concentration of the environment. For the IRGA readings, the intermediate leaves were chosen, the 4^th or 5^th leaf counted from the apex of the plant, so as not to underestimate the values using old or very young leaves, evaluating one plant per plot. Readings were taken between 7 and 9 a.m.

To determine the contents of chlorophyll and carotenoids, leaf samples were collected at 30 DAE and sent to the Plant Physiology Laboratory of CCTA/UFCG to extract the pigments in 80% acetone. After being macerated and filtered in 0.45-micron filter paper, they were quantified by spectrophotometry, as described in Lichtenthaler (1987).

For the evaluation of growth and accumulation of dry mass, at 30 DAE the plants were collected by cutting them close to the ground. In these plants, plant height, culm diameter, number of leaves per plant, leaf area, leaf and culm dry mass, and total dry mass of the shoot were evaluated. Plant height was measured with a tape measure, from the collar, that is, approximately close to the ground, to the apex of the plant. Culm diameter was measured with a digital Vernier caliper. The number of leaves was obtained by counting only the fully formed leaves (visible ligule).

The leaf area (cm² per plant) was determined from the collection of eight leaf discs of known area (1.4 cm²), summing the area of all discs, multiplying the value by the leaf dry mass, and dividing the result by the dry mass of the discs. The discs were dried in an oven with forced-air circulation at 70 ºC for 72 hours.

The total dry biomass was determined by adding the dry mass of the leaves and the culm, also obtained after drying in an oven with forced air circulation at 65 ºC for 72 hours.

For statistical analyses, the data were preliminarily subjected to the normality test (Kolmogorov-Smirnoff) using Sisvar software (Ferreira, 2019), followed by an analysis of variance at 0.05 probability level and, in cases of significance, linear and quadratic regression analysis was performed, using the statistical program SAEG, Version 9.1.

Results and Discussion

There were significant effects of isolated factors as well as that of interaction between irrigation water salinity and H₂O₂ levels for CO₂ assimilation rate (A), transpiration (E), stomatal conductance (gs), and intercellular CO₂ concentration (Ci) (p ≤ 0.01).

It was observed that the plants irrigated with water of 0.3 dS m^-1 obtained a higher CO₂ assimilation rate (29.5 μmol CO₂ m^-2 s^-1) at the estimated H₂O₂ concentration of 71 µmol L^-1. For plants irrigated with 2.0 dS m^-1 water, the data did not obtain adequate fits (y = 23.928 - 0.0011^nsx, R² = 0.0194), with an average CO₂ assimilation rate of 23.8 μmol CO₂ m^-2 s^-1, a value considered normal for plants with C4 metabolism (Figure 2A). According to Silva et al. (2021a), when H₂O₂ is applied at an appropriate concentration, it stimulates some physiological processes, including CO₂ assimilation rate and stomatal conductance.

When analyzing the interaction between the electrical conductivity of irrigation water and hydrogen peroxide concentrations (Figure 2A), it was observed that the CO₂ assimilation rate of plants subjected to ECw of 0.3 dS m^-1 differs significantly from that of plants irrigated with water of 2.0 dS m^-1. Plants grown under ECw of 0.3 dS m^-1 stood out with the highest CO₂ assimilation rate, regardless of the concentration of H₂O₂ applied.

Table 1
Summary of F test for CO₂ assimilation rate (A), transpiration (E), stomatal conductance (gs), and intercellular CO₂ concentration (Ci) in maize plants subjected to different levels of irrigation water salinity and concentrations of hydrogen peroxide (H₂O₂) 30 days after emergence

Source of variation	F Test
	A	E	gs	Ci
Salinity (ECw)	**	**	**	**
Hydrogen peroxide (H2O2)	**	**	**	*
Interaction (ECw × H2O2)	**	**	**	**
CV (%)	3.60	9.07	7.98	15.99

• CV (%) - Coefficient of variation; ** - significant at 0.01 probability level, by the F test

Figure 2
CO₂ assimilation rate (A) and transpiration (B) of maize plants subjected to different salinity levels (ECw) of irrigation water and concentrations of hydrogen peroxide (H₂O₂), 30 days after emergence

The low CO₂ assimilation rate, recorded in plants subjected to ECw of 2.0 dS m^-1, occurred due to the osmotic adjustment that the plant performs to continue the metabolic processes (Sousa et al., 2021); no benefit was observed from the application of H₂O₂ in attenuating the deleterious effects of salinity on this physiological parameter. Such results are in agreement with those found by Sousa et al. (2021) who evaluated the influence of nitrogen doses on growth and gas exchange of the ‘AG 1051’ maize crop irrigated with saline water and observed that the ECw of 3.0 dS m^-1 reduced the parameters of gas exchange, in particular the CO₂ assimilation rate, at 30 DAS.

Regarding transpiration, when the plants were irrigated with water of 0.3 dS m^-1, there was a quadratic behavior with the increase in the concentration of H₂O₂, with maximum transpiration of 5.07 mmol H₂O m^-2 s^-1 at the estimated H₂O₂ concentration of 100 µmol L^-1 (Figure 2B). When the plants were irrigated with water of 2.0 dS m^-1, there was a reduction in the transpiration rate of 2.44% for each 40 µmol L^-1 increase of H₂O₂ (Figure 2B). Thus, under stress conditions, the increase in the concentration of this compound may have helped the plants to identify the stress and reduce the transpiration rate, especially because there was no significant effect on CO₂ assimilation rate, inducing acclimation of plants to salt stress.

When analyzing the effect of ECw at each concentration of H₂O₂ (Figure 2B), it was observed that transpiration was negatively influenced by ECw, with lower values obtained at ECw of 2.0 dS m^-1, with greater intensity at concentrations of 160 and 320 µmol L^-1. This can be explained by the fact that, when plants are under stress, there is an increase in the production of ROS, including H₂O₂, which in excess can induce toxicity and plant death. Thus, the application of high concentrations of H₂O₂ probably may have contributed to the reduction of transpiration of the plants. Significant differences were observed in transpiration rate at all doses of peroxide, except for the control treatment.

For stomatal conductance (Figure 3A), it was found that the increase in the concentration of H₂O₂ caused a reduction in the values of ‘gs’ at both levels of salinity (ECw = 0.3 and 2.0 dS m^-1), an interesting fact because when the plants were irrigated with low-salinity water, the decrease in CO₂ assimilation rate and transpiration only occurred when they were under the concentration of 320 µmol L^-1, which denotes an adaptation. When plants were irrigated with water of higher salinity, the behavior of gs was similar to that of transpiration, emphasizing that the CO₂ assimilation rate was not affected, which may be related to greater efficiency in the use of CO₂.

Figure 3
Stomatal conductance (A) and intercellular CO₂ concentration (B) of maize plants subjected to different salinity levels (ECw) of irrigation water and concentrations of hydrogen peroxide (H₂O₂), 30 days after emergence

When analyzing the irrigation water salinity levels at each H₂O₂ concentration (Figure 3A), there was a reduction in gs with the ECw 2.0 dS m^-1 at the concentrations of 0, 40, 80, and 160 µmol L^-1, compared to the ECw of 3.0 dS m^-1.

For intercellular CO₂ concentration, there was a significant difference between salinity levels for H₂O₂ concentrations 80, 160, and 320 µmol L^-1. When studying the intercellular concentration of CO₂, one notices an opposite behavior to that of CO₂ assimilation rate. In this context, when detailing the result observed in plants irrigated with water of 0.3 dS m^-1 (Figure 3B), one notices that the lowest values were observed with the application of 178.70 µmol L^-1 of H₂O₂; in this case, the reduction in Ci and the increase in CO₂ assimilation rate mean an increase in the efficiency of carboxylation, a situation raised when studying the ‘gs’, which was reduced with the increase in the concentration of H₂O₂, that is, the increase of this compound may have enabled better adaptation of the plants to the environment that, by itself, can cause other abiotic stresses by temperature and relative humidity of the air (Taiz et al., 2017).

For plants irrigated with water of 2.0 dS m^-1, the highest intercellular CO₂ concentration was 119.28 μmol CO₂ mol^-1 at a concentration of 198.91 µmol L^-1 H₂O₂, promoting a 26.08% increase compared to the concentration of 0 µmol L^-1 of H₂O₂ (Figure 3B); in this case, as there was no reduction in CO₂ assimilation rate, but there was a decrease in the values of ‘gs’ and ‘E’, it can be said that the accumulated CO₂ was strategic for the acclimatization of plants, ensuring its absorption and assimilation.

The physiological parameters serve to check for alterations in plants subjected to stress conditions. Therefore, the reduction observed in these variables when the ECw was increased from 0.3 to 2.0 dS m^-1 is common; however, it was expected that the increase in the concentration of H₂O₂ could be more significant in favoring these variables, alleviating the salt stress in pre-treated maize plants, as verified by Gondim et al. (2013), who reported that the pre-treatment of seeds with H₂O₂ was able to increase the values of photosynthetic rate and stomatal conductance, compared to plants that did not have pre-treated seeds.

The application of H₂O₂ at low concentrations, as a pretreatment before subjecting plants to abiotic stresses, activates physiological responses that allow them to tolerate salinity (Gondim et al., 2012; Silva et al., 2016). Silva et al. (2021a) observed that the use of hydrogen peroxide, via soaking of seeds of passion fruit plants at concentrations between 10 and 30 μM, mitigated the deleterious effects of salinity on stomatal conductance, transpiration, CO₂ assimilation rate, and instantaneous carboxylation efficiency.

Silva et al. (2019a), when studying the foliar application of hydrogen peroxide in yellow passion fruit under salinity, observed that the 25 µM concentration favored transpiration, CO₂ assimilation rate, and internal carbon concentration of the plants.

In this study, when plants were irrigated with water of electrical conductivity 2.0 dS m^-1, a reduction in some physiological parameters was observed in plants from the H₂O₂ treatment, but the CO₂ assimilation rate was maintained. This effect is related to the association of salt stress together with the high concentration of H₂O₂ above 70 µmol L^-1, because H₂O₂ is a ROS which in turn is already produced naturally in the plant; however, in excess ROS can induce toxicity and plant death (Petrov & van Breusegem, 2012; Smirnoff & Arnaud, 2019).

There was no significant interaction between irrigation water salinity and H₂O₂ concentration for chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid contents in maize plants. However, there were significant effects of irrigation water salinity levels on total chlorophyll and carotenoids. For hydrogen peroxide concentrations, no significant effect was observed (Table 2).

Table 2
Summary of F test for chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (Chl t) and carotenoids (Car) in maize plants subjected to different levels of irrigation water salinity and concentrations of hydrogen peroxide (H₂O₂) 30 days after emergence

Source of variation	F Test
	Chl a	Chl b	Chl t	Car
Salinity (ECw)	ns	ns	**	*
Hydrogen peroxide (H2O2)	ns	ns	ns	ns
Interaction (ECw × H2O2)	ns	ns	ns	ns
CV (%)	17.98	18.11	20.10	20.27

• CV (%) - Coefficient of variation; * and **- significant at 0.05 and 0.01 probability level, respectively, by the F test

When studying the total chlorophyll and carotenoid contents as a function of salinity, it was noted that the increase in the concentration of salts in the irrigation water caused a reduction in the contents of these variables, that is, there was a degradation of total chlorophyll and carotenoids with increasing salinity, a fact that explains the lower gas exchange observed in plants under salt stress, regardless of the concentration of H₂O₂ applied (Figure 4). The decrease in pigment synthesis in plants under high salinity conditions possibly stimulated the activity of the enzyme chlorophyllase, which acts on the degradation of photosynthetic pigments and destroys the structure of chloroplasts, also causing imbalance and loss of activity of pigment proteins (Silva et al., 2021b; Lima et al., 2021).

Figure 4
Total chlorophyll (A) and carotenoids (B) contents of maize plants subjected to different levels of irrigation water salinity and concentrations of hydrogen peroxide (H₂O₂), 30 days after emergence

High levels of carotenoids can contribute to the maintenance of chlorophyll levels in plants under salt stress (Kim et al., 2012; Kachout et al., 2013). Thus, it can be said that the reduced contents of carotenoids may have contributed to the non-significance in the contents of chlorophyll a and b, especially in plants irrigated with water of 2.0 dS m^-1, since the concentration of total chlorophyll in maize plants decreases with increase in water salinity (Kaya et al., 2015a).

There was no significant interaction between the salinity of irrigation water and levels of H₂O₂ for plant height, culm diameter, number of leaves, leaf area, leaf dry biomass, leaf dry biomass, and total dry biomass. However, the salinity and H₂O₂ factors were significant for plant height and culm diameter, and for the number of leaves, leaf area, and total dry biomass of plant there was a significant effect of the H₂O₂ factor (Table 3).

Table 3
Summary of F test for plant height (PH), culm diameter (CD), number of leaves (NL), leaf area (LA), culm dry biomass (CDB), leaf dry biomass (LDB), and total dry biomass (TDB) of maize plants subjected to different levels of irrigation water salinity and concentrations of hydrogen peroxide (H₂O₂) 30 days after emergence

Source of variation	F Test
	PH	CD	NL	LA	CDB	LDB	TDB
Salinity (ECw)	**	**	ns	ns	ns	ns	ns
Hydrogen peroxide (H2O2)	**	**	*	*	ns	ns	*
Interaction (ECw × H2O2)	ns	ns	ns	ns	ns	ns	ns
CV (%)	6.42	2.87	5.14	16.83	17.79	16.59	16.55

• CV (%) - Coefficient of variation; * and ** significant at p ≤ 0.05 and 0.01, respectively by F test

Considering the values of plant height and culm diameter at the salinity levels of irrigation water, it can be observed that plants irrigated with 0.3 dS m^-1 water have higher mean values than plants irrigated with 2.0 dS m^-1 water (Figures 5A and B). The reduction in growth under salt stress can be attributed to the reduction of the osmotic potential caused by the increased concentration of soluble salts in the soil solution, which directly affects water absorption by plants. In addition, the absorption of nutrients by plants is affected, as specific ions such as Na⁺ may compete with other essential nutrients (Byrt et al., 2018).

Figure 5
Plant height (A) and culm diameter (B) of maize plants subjected to different levels of irrigation water salinity and concentrations of hydrogen peroxide (H₂O₂) 30 days after emergence

Plant height increased linearly with increasing H₂O₂ concentration, obtaining 22.29% when compared to the concentration of 320 µmol L^-1 with 0 µmol L^-1 (Figure 6A), which indicates that regardless of whether the water is saline or not, the increase in H₂O₂ concentration favored the primary growth of the plants. According to Farouk & Amira, (2018), this is because H₂O₂, when used in adequate concentrations, has a beneficial effect on plants, favoring greater absorption of water and nutrients, including essential ions for plant growth and development, such as N, P, and K.

Figure 6
Plant height (A), culm diameter (B), number of leaves (C), leaf area (D), and total dry biomass (E) of maize plants under different levels of irrigation water salinity and concentrations of hydrogen peroxide (H₂O₂), 30 days after emergence

For culm diameter, the opposite situation occurred, with a linear reduction of 5.36% when comparing the concentration of 0 µmol L^-1 with that of 320 µmol L^-1 of H₂O₂ (Figure 6B). It can be observed that high concentrations of H₂O₂, combined with salt stress, caused greater elongation of the plants, but smaller diameter.

The number of leaves per plant increased with increasing H₂O₂ concentration up to 151.66 µmol L^-1, promoting an increment of 5.8% compared to the concentration of 0 µmol L^-1 of H₂O₂ (Figure 6C). The leaf area also increased with the increment in the concentration of H₂O₂, with a value of 2325.55 cm² at the estimated concentration of 172.69 µmol L^-1, an increment of the order of 15.67% in leaf area compared to the 0 µmol L^-1 concentration of H₂O₂, regardless of the salinity of the irrigation water (Figure 6D).

In this study, even though there was no significant interaction, the highest total dry biomass was 16.96 g at the estimated concentration of 171.66 µmol L^-1, promoting an increase of 12.16% in the total dry mass of the shoot compared to the concentration of 0 µmol L^-1 (Figure 6E).

The addition of H₂O₂ to leaf tissues exogenously or its endogenous induction acts as an induction signal for the expression of genes referring to catalase, ascorbate peroxidase, guaiacol peroxidase, and glutathione reductase (Petrov & van Breusegem, 2012), whereby the accumulation of H₂O₂ in appropriate amounts benefits plants by mediating acclimation and tolerance to biotic and abiotic stresses. In this context, the application of H₂O₂ at the time of sowing can induce the emergence of more resistant seedlings by accelerating enzymatic activity and metabolic processes, before the plants are exposed to salt stress.

In some studies, H₂O₂ has been used to combat salinity. For example, Souza et al. (2019), when studying the exogenous application of hydrogen peroxide at 20 µM, found beneficial effects on the accumulation of phytomass and quality of cashew rootstocks. Silva et al. (2019c) found that foliar application of 15 µM promoted better leaf area in the melon plant under saline conditions. Gondim et al. (2011), when spraying maize seedlings with H₂O₂ at a concentration of 10 mM, under salinity of 0 and 80 mM, found that H₂O₂ induced the acclimation of maize plants to salt stress, partially reversing the deleterious effects of salinity on growth.

Working with maize plants from seeds pretreated with H₂O₂ and subjected to salinity, Godim et al. (2013) observed that pretreatment conferred salinity tolerance to plants. Silva et al. (2019d), studying the application of hydrogen peroxide in sunflower plants under salt stress, found that pretreatment with hydrogen peroxide via seed and the combination via seed + leaves were able to reduce the deleterious effects of salinity, promoting higher production of biomass. In yellow passion fruit, Silva et al. (2021a) found that the use of hydrogen peroxide, via seed soaking at concentrations between 10 and 30 µM induces the acclimatization of plants to salt stress, mitigating the deleterious effects of salinity on the relative growth rate in culm diameter and leaf area.

Conclusions

1. Irrigation water salinity of 2.0 dS m^-1 reduces transpiration, stomatal conductance, total chlorophyll, carotenoids, and initial growth, but does not affect the dry mass accumulation of maize plants.

2. Application of H₂O₂ via soil varying from 0 to 320 µmol L^-1 causes reductions in the CO₂ assimilation rate and transpiration, as well as at concentrations from 0 to 160 µmol L^-1 for stomatal conductance of plants irrigated with ECw of 2.0 dS m^-1

3. Application of H₂O₂ via soil up to a concentration of 320 µmol L^-1 increases the plant height, but reduces culm diameter of maize.

Acknowledgements

The authors thank the Graduate Program in Tropical Horticulture of the Universidade Federal de Campina Grande (UFCG) and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for providing necessary infrastructure and funding to conduct this research.

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• ¹ Research developed at Universidade Federal de Campina Grande, Unidade Acadêmica de Ciências Agrárias, Campus Pombal, Pombal, PB, Brazil

Edited by

Editors: Geovani Soares de Lima & Hans Raj Gheyi

Publication Dates

• Publication in this collection
  08 Aug 2022
• Date of issue
  Nov 2022

History

• Received
  20 Feb 2022
• Accepted
  03 June 2022
• Published
  06 July 2022