*Article* **Ionic Response and Sorghum Production under Water and Saline Stress in a Semi-Arid Environment**

**Rodrigo Rafael da Silva 1, José Francismar de Medeiros 1,\*, Gabriela Carvalho Maia de Queiroz 1, Leonardo Vieira de Sousa 1, Maria Vanessa Pires de Souza 2, Milena de Almeida Bastos do Nascimento 3, Francimar Maik da Silva Morais 1, Renan Ferreira da Nóbrega 4, Lucas Melo e Silva 5, Fagner Nogueira Ferreira 1, Maria Isabela Batista Clemente 1, Carla Jamile Xavier Cordeiro 1, Jéssica Christie de Castro Granjeiro 1, Dárcio Cesar Constante <sup>1</sup> and Francisco Vanies da Silva Sá <sup>1</sup>**


**Abstract:** The increase in water demand in regions with limited good-quality water resources makes it necessary to study the effect of low-quality water on plant metabolism. Therefore, the objective of this study was to evaluate the effect of water and salt stress on the levels of mineral elements and accumulation of toxic elements Na<sup>+</sup> and Cl<sup>−</sup> in the leaves and their consequences on the production variables of the sorghum cultivar IPA SF-15. The design adopted was randomized blocks in a factorial scheme (4 <sup>×</sup> 4), with four salt concentrations (1.5; 3.0; 4.5, and 6.0 dS m<sup>−</sup>1) and four irrigation depths (51.3; 70.6; 90.0, and 118.4% of crop evapotranspiration ETc) in three repetitions. To obtain nutrient, sodium, and chlorine contents in the leaf, we collected the diagnosis leaf from six plants per plot. For production data, we performed two harvests at 76 and 95 days after planting (silage point and for sucrose extraction). We evaluated the dry mass, fresh mass yield, and total dry mass for the two cutting periods and applied the F-test at the 5% significance level. There was an effect of water stress but not saline, making it possible to use saline water for sorghum irrigation. As for the toxicity of ions, the plant showed tolerance behavior to Na<sup>+</sup> and Cl<sup>−</sup> ions. The grain filling phase was more sensitive than the final phase of the crop cycle.

**Keywords:** *Sorghum bicolor* (L.) Moench; nutrient accumulation; water deficit

## **1. Introduction**

Sorghum (Sorghum bicolor) is an important food crop in many countries, mainly in Africa and Asia, and is used worldwide in the biofuel industry and livestock [1]. In the Brazilian semi-arid region, it has stood out in terms of forage production with its cultivation in the rainy season and during the dry season, under irrigation, for the high productivity achieved per volume of water spent on crop irrigation [2–4].

Sorghum can adapt to different edaphoclimatic conditions due to its rusticity and ability to adapt to water scarcity [5]. According to [6], in a study with sorghum subjected to irrigation depths, it was observed that a 20% reduction in the standard irrigation depth did not influence the productivity of the crop, and it was possible to expand the production area using the same volume of water. Similar studies [7,8] obtained similar results, demonstrating the tolerance of sorghum to water stress conditions.

The considerably high resistance to drought and salinity becomes essential compared to other crops in terms of yield. [9]; it is a critical factor affecting the entire production mechanism [10]. However, sorghum has the characteristic of adjusting the osmotic balance

**Citation:** da Silva, R.R.; de Medeiros, J.F.; de Queiroz, G.C.M.; de Sousa, L.V.; de Souza, M.V.P.; de Almeida Bastos do Nascimento, M.; da Silva Morais, F.M.; da Nóbrega, R.F.; Silva, L.M.e.; Ferreira, F.N.; et al. Ionic Response and Sorghum Production under Water and Saline Stress in a Semi-Arid Environment. *Agriculture* **2023**, *13*, 1127. https://doi.org/ 10.3390/agriculture13061127

Academic Editor: Andres Belver Cano

Received: 31 March 2023 Revised: 22 May 2023 Accepted: 23 May 2023 Published: 27 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

in the cell, which induces a salinity tolerance strategy. In this sense, sorghum's strategies to express its productive performance under water and saline restrictions have been studied [6,7,9,10], showing safe results for using these resources in sorghum cultivation.

The high use of water for human and agricultural consumption demands low-quality water for irrigation [11]. In this way, it is necessary to explore the knowledge of good practices for using these waters to maintain the productive potential of the culture and the quality of the soil and production. The difficulty in using these waters in agricultural production is because of high concentrations of soluble salts since they are in contact with soluble materials from soil and rocks [12]. Salinity affects plants because of its osmotic effect, reducing water availability and providing high concentrations of Na+ and Cl<sup>−</sup> in the soil solution, which, when absorbed at high levels by crops, can trigger a toxic effect on plants [10,13], besides that, an ionic imbalance in the generated soil can affect the overall nutrition of the plant.

According to [14], sorghum tolerates a water and soil salinity of 4.5 and 6.8 dS m−1, respectively. For values above these limits, a reduction of around 16% is expected for each unit increase in soil salinity [15]. Sorghum stands out compared with other grasses, such as maize, which is moderately tolerant to water salinity up to 4.5 dS m−<sup>1</sup> [14,16], and barley, which tolerates salinity up to 6.0 dS m−<sup>1</sup> [17]. Studies also show that in a saline environment, sorghum can exclude Na<sup>+</sup> from the xylem to the roots and compartmentalize it in cell vacuoles as an osmolyte to adjust cell osmosis [18–22]. Under saline conditions, the selective uptake of Ca2+ and K<sup>+</sup> over Na<sup>+</sup> is an additional mechanism for salt stress tolerance [21]. Excess Na+ can lead to accumulation in the leaf and affect the translocation of Ca2+, K+, and Mg2+ ions [22], impairing photosynthetic activity and plant development [23].

Different management strategies to ensure the use of saline water in irrigation are necessary. Using saline water associated with an irrigation depth that provides water availability for the plant seeks to minimize the effects of saline stress on plant metabolism [24]. Therefore, using plants tolerant to water and saline deficit is a way to deal with the problem of water salinity in the semi-arid region [25]. Given this, the objective was to evaluate the effect of water and salt stress on the levels of mineral elements and accumulation of toxic elements Na<sup>+</sup> and Cl<sup>−</sup> in the leaves and their consequences on the production variables of the sorghum cultivar IPA SF-15.

#### **2. Materials and Methods**

The study was conducted in the experimental area at the Cumaru site (5◦33 30 S, 37◦11 56 W, altitude of 110 m) in the municipality of Upanema-RN. According to the Köppen classification, the region's climate is BSh—hot semi-arid with autumn rains and average monthly air temperature consistently above 18 ◦C. Because of the low latitudes, the region has two well-defined seasons: wet (January to May) and dry (June to December). The average annual precipitation is 650 mm, characterized by high space–time variability. The soil in the area was classified as Cambisol [26], with chemical and physical characteristics in the 0.00–0.20 cm layer before planting, as shown in Table 1.

The crop investigated as sorghum (*Sorghum bicolor* (L.) Moench), cultivar IPA SF-15, with an aptitude for forage production. The experiment was conducted in the dry season, from September to December 2019. The preparation of the area consisted of plowing followed by harrowing, opening the planting furrows, and carrying out the basal fertilization with 180 kg ha−<sup>1</sup> of MAP (10–50–00). Soil fertilization was done according to soil analysis recommendations and crop nutritional requirements. In fertirrigation, 60 kg ha−<sup>1</sup> of N was applied using urea. To meet the demand for potassium, 30 kg ha−<sup>1</sup> of K2O was applied, using KCl as fertilizer. Fertilizers were applied at 21, 28, and 35 days after planting.


**Table 1.** Physical and chemical soil attributes in the study area before the experiment.

EC—Electrical conductivity.

The experimental design was in randomized blocks with three replications in a 4 × 4 factorial scheme, with four concentrations of salts expressed in electrical conductivity of irrigation water (1.5, 3.0, 4.5, and 6.0 dS m<sup>−</sup>1) and four irrigation depths (51.3, 70.6, 90.0, and 118.4% of ETc). The experimental units comprised two double rows of seven meters, totaling 168 plants. The outer rows of each plot were considered borders.

The water with the lowest salinity level (1.5 dS m−1) came from a tube well, and the water with the highest salinity (6.0 dS m<sup>−</sup>1) was defined based on the salinity tolerance of the sorghum crop for yield than 50% of its productive potential [27]. The two other levels were 3.0 and 4.5 dS m<sup>−</sup>1, corresponding to intermediate and equidistant points of the two extreme values. To obtain the three highest salinity levels, stock solutions were prepared at a concentration of 200 g L−<sup>1</sup> of NaCl (3.42 mol L−1), CaCl.2H2O (1.36 mol L−1), and MgSO4.7H2O (0.81 mol L−1) and added quantities of the stock solution so that the final proportion was 6.3:2.7:1 of Na, Ca, and Mg, which represents the average composition of the waters in the region which exploits the Jandaíra Limestone Aquifer [28] (Table 2). Salinity levels were monitored daily using a portable conductivity meter.

**Table 2.** Chemical composition of natural water and after adding salt solutions used in the experiment.


EC—Electrical conductivity, Na+—Sodium, Ca2+—Calcium, Mg2+—Magnesium, K+—Potassium, Cl<sup>−</sup>—Chlorine, SO4 <sup>2</sup>−—Sulfates, HCO3−—Bicarbonates.

The irrigation depths were estimated by the percentage of crop evapotranspiration (ETc) (Figure 1A), adjusting for field conditions and operation of the irrigation system. ETc was calculated daily from the calculation of daily reference evapotranspiration, using the Penman-Monteith method [29], and the daily crop coefficient (Kc) (Figure 1B) by the dual Kc method. Because it is a localized irrigation system (drip), we adopted an irrigation efficiency of 95% to calculate the standard irrigation depth. ETo was estimated from the data collected at a meteorological station near the experiment. To obtain the different depths, drip hoses spaced between lines of 1.65 m were used, with different spacing between emitters (20, 30, 40 cm) and flow rates (1.69, 1.65, 3.46, and 3.90 L h<sup>−</sup>1), to provide flows per linear meter proportional to the required depths.

**Figure 1.** Crop coefficient (**A**) and crop evapotranspiration (**B**) were calculated and adopted. The total irrigation depth for 100% ETc was 6% more.

Sowing was carried out directly, placing five seeds per hole. Thinning was performed ten days after sowing, leaving three plants per hole spaced in double rows 1.40 × 0.25 × 0.30 m. In addition, two manual weedings and an application of Chlorantraniliprole and Imidacloprid were carried out through fertirrigation to control fall armyworms (*Spodoptera frugiperda*) and aphid (*Aphis gossypii*).

To obtain nutrient, sodium, and chlorine contents in the leaf, we collected the diagnosis leaf, the fourth leaf, from six different plants (Figure 2). First, the leaves were dehydrated in a forced circulation oven at a temperature of 65 ◦C until they reached constant mass; then, they were processed in a Willey SL-31 type mill to determine the P, K+, Ca2+, Mg2+ contents, Cl−, and Na+. The nutrient/ion contents were extracted using the dry digestion method [30]. The concentration of sodium and potassium was determined by the technique of flame photometry and phosphorus by the colorimetric method of molybdate-vanadate in a spectrophotometer. Ca2+ and Mg2+ were determined by atomic absorption spectrophotometry [30]. Chlorine concentration was determined by the MOHR method, extracted by calcium nitrate solution (Ca(NO3)2.4H2O), in the form of chloride ion, titrated with a standardized solution of silver nitrate (AgNO3), in the presence of potassium dichromate (K2CrO4) as an indicator.

**Figure 2.** Timeline of evaluations during the experiment.

The first harvest occurred 76 days after planting (flowering), and the second was 95 days after planting (silage point and sucrose extraction). Production was quantified by counting the number of plants and weighing the plant material (leaves, stems, and inflorescences) collected ina3m central row. Weightings were carried out in the field using a portable digital hook scale with a capacity of 50 kg and a resolution of 10 g. Based on these data, the fresh mass yield was considered based on the average stand of plants in the area at the time of each harvest, and the values were expressed in Mg ha<sup>−</sup>1.

The percentage of dry mass was obtained from six plants in the valuable area of the experimental plot. First, the plants were separated into leaves, stems, and inflorescences and weighed to obtain the total fresh mass. Then, samples of the respective fresh materials were placed in a forced circulation oven at a temperature of 65 ◦C until they reached the constant mass to obtain the dry mass. Each organ's dry mass percentage was estimated with these data, and the weighted average was calculated for the entire plant. The total dry mass, expressed in Mg ha−1, resulted from the product between fresh mass yield and the dry mass content in the plant.

Data were analyzed for normal distribution using the Shapiro–Wilk test at a 5% significance level and performed the Pearson correlation matrix. Then, the results were submitted to analysis of variance by the F-test at the 5% significance level; in case of significant effects, regression analysis was carried out using the statistical software RStudio version 4.2.2.1 through the ExpDes.pt package.

## **3. Results**

#### *3.1. Content of Mineral Elements in Sorghum Diagnosis Leaf*

Water deficit linearly interfered with the K<sup>+</sup> and P contents, in which the increase in the irrigation depth provided a reduction in the K+ content (*<sup>p</sup>* ≤ 0.01) (Figure 3A) concomitant with an increase in the P content in the leaves (*p* ≤ 0.01) (Figure 3B). Salinity linearly reduced the Cl<sup>−</sup> content (*p* ≤ 0.01) in the diagnosis leaf (Table 3), as can be seen in Figure 4, but did not affect the other ions studied. It is worth emphasizing that no interactive effect of salt × irrigation was observed in this study, which may indicate the adoption of brackish water as long as it is in good quantity or reduce the irrigation depth as long as the water used is of good quality.

**Figure 3.** Potassium (**A**) and phosphorus (**B**) content in the diagnosis leaf for cultivar IPA SF-15 as a function of crop evapotranspiration levels.


**Table 3.** Summary of analysis of variance for nutrient content traits in sorghum (*Sorghum bicolor* (L.) Moench) IPA SF-15 diagnosis leaf as a function of water and salt stress.

SV—Source of variation, DF—Degree of freedom, CV—Doefficient of variation, Salt—Effect of salinity; Irrig.—Effect of water stress; Comp. L and Q—Linear and quadratic component, significance (0.01 ≥ *p* ≤ 0.05) by F-test.

**Figure 4.** The chlorine content in the diagnosis leaf for the IPA SF-15 cultivar as a function of the electrical conductivity of the irrigation water.

#### 3.1.1. Potassium and Phosphorus

Under an irrigation depth of 51.3% ETc, the observed potassium content was 14.86 g kg<sup>−</sup>1, dropping sharply to 14.03 and 13.20 g kg−<sup>1</sup> for the irrigation depths of 70.6 and 90.0% of ETc, respectively (Figure 3A). According to the linear model generated (K+ = 17.0519 − 0.0428\*\*x; R2 = 0.75), for each unit increase in %ETc, the K+ content decreased by approximately 0.04 g g kg−<sup>1</sup> in dry mass (1.24%). The lowest K+ content (12 g kg−1) was obtained underwater control conditions of 118% ETc, approximately 19% lower than the maximum value observed.

Increasing the irrigation depth provided higher levels of P in the leaves (Figure 3B), with estimated values ranging from 1.06 to 1.99 g kg−<sup>1</sup> of P in dry mass for a depth of 51.3 to 118% of ETc respectively, that is, an increase of 87% about lower water availability. From the regression analysis, a significant effect of the irrigation depth was observed with a linear rise in 0.014 g kg−<sup>1</sup> of P in the dry mass (0.53%) in the leaf of the sorghum plant for each unit variation in the water depth irrigation.

## 3.1.2. Chlorine

Salt stress reduced the chlorine content in the leaf (Figure 4), which ranged from 24.73 to 20.37 g kg−<sup>1</sup> for electrical conductivity from 1.5 to 6.0 dS m−1, respectively, expressing a decrease of 18% about higher electrical conductivity. The regression model that best fit was linear decreasing (Cl<sup>−</sup> = 26.1646 − 0.9664\*\*x; R2 = 0.97), showing an estimated decrease of approximately 0.96 g kg−<sup>1</sup> of Cl<sup>−</sup> in dry mass (−0.21%).

## *3.2. Production Variables*

Regarding production analyses, analysis of variance (Table 4) identifies that salinity interfered only with the total fresh mass yield at 76 DAP (*p* ≤ 0.05). Despite being significant, the tested linear and quadratic regression models did not fit the data in Figure 5A with very shallow R2 values. This means some model was significant but would probably have no plausible physiological explanation for this behavior. As there was no adjustment of the models tested for the variables, the Tukey test was applied (*p* ≤ 0.05) for the salinity factor for the variables' yield of fresh matter and total dry mass, where it was verified that there was no significant difference, as shown in Figure 5. As for the effect of the depth, it was observed that it interfered with the yield and total dry mass at 76 and 95 DAP (*p* ≤ 0.01) since the interaction was not significant for any of the variables in any of the studied periods.

**Table 4.** Summary of analysis of variance: dry mass percentage (DMP), plant yield (Yield), total dry mass (TDM) at 76 and 95 days after planting (DAP) for sorghum (*Sorghum bicolor L.* Moench) IPA SF-15 as a function of water and salt stress.


SV—source of variation, DF—degree of freedom, CV—coefficient of variation, Salt—Effect of salinity; Irrig—Effect of water stress; Comp. L and Q—linear and quadratic component, significance (0.01 ≥ *p* ≤ 0.05) by F-test.

It was observed that the fresh matter yields at 76 and 95 DAP were not affected by the saline levels applied with values ranging from 60.04, 71.02 and 81.72 to 84.55 Mg ha<sup>−</sup>1, respectively Figure 5A. The total dry mass at 76 and 95 DAP also showed no significant difference for the salinity factor; the values ranged from 17.48, 21.49 and 29.97 to 33.57 Mg ha<sup>−</sup>1, respectively, for saline levels (Figure 5B).

The yield of fresh matter and total dry mass for the two sorghum cutting periods increased linearly with the increase in the irrigation depth, regardless of the salinity level of the irrigation water (Figure 6A). It ranged from 52.92 to 83.00 Mg ha−<sup>1</sup> for a water regime of 51.3% of ETc (244 mm) and 118% of Etc (547 mm), representing a 57% increase in yield compared to lower water availability. For each unit variation in %Etc, there is an estimated increase of approximately 0.46 Mg kg−<sup>1</sup> in fresh mass yield (0.64%).

**Figure 5.** The yield of fresh matter (**A**) and total dry mass (**B**) for the sorghum cultivar (*Sorghum bicolor* (L.) Moench) IPA SF-15, as a function of the electrical conductivity of the irrigation water for cutting performed at 76 days (red) and 95 days (blue) after planting.

**Figure 6.** The yield of fresh matter (**A**) and total dry mass (**B**) for the sorghum cultivar (*Sorghum bicolor* (L.) Moench) IPA SF-15 as a function of the total irrigation depths for cutting performed at 76 days (red) and 95 days (blue) after planting.

Water availability significantly influenced dry mass production (Figure 6B), with a variation from 14.96 to 25.68 Mg ha−<sup>1</sup> for the lowest and highest water availability condition. This variation promoted an increase of 72% in the production of total dry mass. Applying 90% of the ETc (419 mm), the dry mass production was 21.18 Mg ha<sup>−</sup>1, only 7% lower than the production generated by the irrigation depth of 464 mm (100% of the ETc), which was 22.78 Mg ha<sup>−</sup>1.

In the cutting period at 95 DAP, yield behavior was linearly increasing (Figure 6A). The variation ranged from 72.2 to 97.64 Mg ha<sup>−</sup>1, representing a 35% increase in production from the lowest to the highest applied depth, representing an applied water volume of 311 to 713 mm, respectively. When comparing the two cutting periods, the increase in yield for the second cut was lower than the first, as was the volume of water applied. In regions with limited water volume, it was cutting 76 days after planting is necessary.

## *3.3. Pearson Correlation Matrix*

The mineral elements K<sup>+</sup> and Mg2+ were strongly correlated with Na<sup>+</sup> and Ca2+ in the plant leaf (r = 0.53 and r = 0.80) (Table 5). As for the production variables, total dry mass and yield at 76 DAP correlated positively with the phosphorus content in the leaf (r = 0.52 and r = 0.50), while for the K<sup>+</sup> content, the correlations were negative for the percentage of mass, yield at 76 DAP (r = −0.36 and r = −0.42) and yield and total dry mass at 95 DAP (r = 0.34 and 0.41). In addition, other correlations were verified; however, the correlative effect was shallow both positively and negatively between the elements and the production variables for the two analyzed periods.


**Table 5.** Pearson correlation matrix for variables measured at 76 e 95 DAP.

Na+—Sodium, K+—Potassium, Ca2+—Calcium, Mg2+—Magnesium, P—Phosphorus, Cl−—Chlorine, dry mass percentage (DMP), Yield plant yield (Yield), total dry mass (TDM) at 76 and 95 days after planting (DAP).

## **4. Discussion**

In conditions of extreme water deficit, 51% of the water needs the K+ content was maximum; this can be justified because the K+ is used to control the opening and closing of the stomata. May notice that until water availability at 90% of the ETc, the K+ content was within the limit of 13 g kg−<sup>1</sup> in the dry mass recommended by [31] as a critical level of K+ for the species. Despite the K+ content being at the limit, no nutrient deficiency symptoms were observed in the plant.

According to water deficit, the increase in K+ content in the leaf may be related to concentration effects, as the plant limits water loss through better stomatal regulation [32]. Furthermore, the response to water deficit of reducing shoot growth promotes root development, increasing the plant's resistance to water deficit [33–35]; therefore, this nutrient is more concentrated in the dry mass. Although researchers such as [36] have observed a reduction in K+ in the aerial part of sorghum subjected to water stress, we understand that the different results found in this work can be explained by the fact that the K+ evaluation was carried out directly from the diagnosis leaf.

Underwater scarcity, a form of plant defense, is the stomata's closing to avoid water loss to the atmosphere. In this sense, the increase in K<sup>+</sup> concentration is due to the greater demand by the plant to maintain photosynthesis and protect chloroplasts [37]. In addition, in leaves, K<sup>+</sup> acts in osmotic regulation, mainly under conditions of water deficiency, where the presence of the ion guarantees the turgor of the guard cells, reducing the osmotic potential [38] and, therefore, promoting the flow of water from the roots for the leaves. This osmotic adjustment function is widely known due to the high solubility of K+ at the

potential in the role of fundamental osmoregulation [39] for the osmotic control of the plant subjected to water stress. All these adjustments that the plant starts to execute are measures to reduce the stress in which it is submitted.

The reduction in P content according to water stress may be associated with its low mobility, given that, as it is absorbed by diffuse flow, it depends on soil moisture to be absorbed by the plant. Therefore, the increase in water availability contributes to the absorption of P by the plant. In underwater scarcity conditions, the root system tends to expand to increase P interception increasing plant uptake [40].

In a situation of salt stress, the plant reduces chlorine displacement to the leaf blade as a defense [41], despite being an essential nutrient for the plant. This is because, even though it is essential, an excess of the element can cause unpredictable damage to the plant [42]. In this way, potentially toxic ions are retained in the stem and root, showing the efficiency of sorghum in preventing displacement to the leaf depth [41–43] and contributing to greater tolerance to saline stress. Therefore, reducing Cl− contents in the leaves with increased EC of water is a mechanism to avoid accumulating toxic levels for the plant.

Chlorine levels in the leaf are in the range of critical toxicity concentration in leaf tissues for tolerant plants, which varies from 15 to 50 g kg−<sup>1</sup> [44]. However, it did not affect the production data. High concentrations of Cl− induce nutritional disturbances [42], such as interference in the absorption of ions (Ca2+, Mg2+, Mn2+, Zn2+, and B), in addition to toxicity in foliar tissues [45]. In addition, the high concentration of Cl− in the leaf is directly related to the increase in leaf succulence [24,46], an efficient protection mechanism to increase the partial dilution of salts by increasing the degree of juiciness. Another point to consider is that the Cl<sup>−</sup> content absorbed by sorghum was higher than that of Na+, which may be due to the high mobility of this anion and its free transport in the plant.

The water stress condition caused by the irrigation deficit during early flowering results in stomatal closure, reducing transpiration, affecting plant development, and reducing yield [47]. Ref. [48] A reduction in production for wheat cultivars was observed when subjected to water deficit during the grain filling period. This may be related to lower nutrient availability in the solution caused by reduced soil moisture. Limited water availability justifies its use. According to [7], applying irrigation depth above 200 mm may be insignificant to increase yield. Reinforcing that even if it reduces production, it is justifiable to use the smallest blade possible to obtain an adequate yield of sorghum production.

The reduction in production when subjected to water deficit can be explained due to the closure of stomata and impairment of plant photosynthesis [49,50]; or by causing direct damage to the photosynthetic metabolism of the plant [51,52]. However, applying a depth of 50 and 71% of the crop's need, despite the reduction in production, there is a gain in water savings, mainly for regions where water availability is limited. Ref. [2] found a dry mass production of 27.02 Mg ha−<sup>1</sup> using 75% of the reference evapotranspiration, values close to those found in this work, making possible better use of water in periods where water availability is limited.

Pearson's correlations showed few relationships between the variables; however, the mineral elements that presented a correlation were potassium with sodium, an effect that may be related to the high levels of potassium found in the soil and leaf. Potassium is responsible for the stomatal regulation of the plant in the process of opening and closing the plant's stomata. In addition, magnesium and calcium showed a strong correlation with each other. Calcium is fundamental in the plant's response to abiotic stress, acting as a messenger signaling the stress signal [22,53]. Despite the mineral elements provided in the irrigation water in more significant quantities, such as calcium, magnesium, and toxic sodium and chlorine ions, they did not affect sorghum production in the two cutting periods. However, phosphorus positively affected plant production at 76 DAP. As for potassium, its effect on all production variables was negative; this can be explained by the high levels of available potassium harming the plant.

## **5. Conclusions**

The study showed saline stress did not affect sorghum production characteristics, making using saline water in irrigation possible. As for the toxicity of ions, the plant manifested a tolerance behavior to Na+ and Cl<sup>−</sup> ions, reducing the accumulation in the leaf and possibly directing it to the stem and roots, reinforcing that the crop is tolerant to salinity.

Water stress reduced the production of IPA SF-15; however, when seeking to reduce water costs and its limitation of water availability, the supply of 71% of the ETc of the crop can be adopted without significant losses for the sorghum crop. The threshold depth acceptable for the crop can be defined by the estimated depth of 90% of the maximum yield obtained in the experiment for the two planting times. The effect is more at grain filling than at the end of the crop cycle.

For semi-arid conditions where there are water limitations, the result of sorghum production for a water supply of up to 51% of the water requirement of the crop is an alternative to maintain the cultivation of sorghum IPA SF-15, mainly for small producers who live in these conditions of water scarcity and problems with the salinity of the available water sources.

**Author Contributions:** Conceptualization, R.R.d.S., G.C.M.d.Q. and J.F.d.M.; methodology, R.R.d.S., G.C.M.d.Q., J.F.d.M., L.V.d.S., M.V.P.d.S., M.d.A.B.d.N., F.M.d.S.M., R.F.d.N., L.M.e.S., F.N.F., M.I.B.C., C.J.X.C., J.C.d.C.G., D.C.C. and F.V.d.S.S.; validation, J.F.d.M., F.V.d.S.S.; formal analysis, R.R.d.S., G.C.M.d.Q. and J.F.d.M.; investigation, R.R.d.S., G.C.M.d.Q., J.F.d.M., L.V.d.S., M.V.P.d.S., M.d.A.B.d.N., F.M.d.S.M., R.F.d.N., L.M.e.S., F.N.F., M.I.B.C., C.J.X.C., J.C.d.C.G., D.C.C. and F.V.d.S.S.; resources, J.F.d.M.; data curation, R.R.d.S. and J.F.d.M.; writing—original draft preparation, R.R.d.S. and M.V.P.d.S.; writing—review and editing, R.R.d.S., J.F.d.M., G.C.M.d.Q.; supervision, J.F.d.M.; project administration, J.F.d.M.; and funding acquisition, J.F.d.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Council for Scientific and Technological Development, CNPq. Notice 01/2016 process (432.570/2016-0) and Fundação de Amparo à Pesquisa do Rio Grande do Norte—FAPERN, Process No.: 10910019.000263/2021-43.

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** The data used to support the findings of this study are included in the article.

**Acknowledgments:** The authors would like to thank the National Council for Scientific and Technological Development (CNPq), Coordination for the Improvement of Higher Education Personnel (CAPES), and the Federal Rural University of the Semi-Arid (UFERSA) for the financial support provided to this research.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


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