Hydrogel Soil Conditioner as an Input for Ornamental Sunflower Production Under Saline Water Irrigation: An Alternative Use for Low-Quality Water

Abstract

The use of saline water (low-quality water) in irrigation is a reality in many regions, especially in areas where fresh water is scarce, like semi-arid regions. However, it is important to adopt strategies to minimize the damage caused by salt stress to plants. The use of soil conditioners can help improve soil structure and water retention capacity, reducing salinity effects. The objective was to analyze the potential of a soil conditioner (hydrogel) as a mitigator of salty stress by irrigation with saline water in ornamental sunflower. Two sunflower cycles were carried out in a protected environment with a factorial 4 × 4 consisting of four doses of hydrogel polymer (0.0, 0.5, 1.0, and 1.5 g kg⁻¹) and four different levels of irrigation with saline water (0.5, 2.0, 3.5, and 5.0 dS m⁻¹). Plant biomass and physiological parameters, such as chlorophyll fluorescence measurements and gas exchange parameters, stomatal conductance, transpiration, and photosynthesis, were evaluated. Ornamental sunflower showed better performance with a saline water of 0.5 dS m⁻¹ without the use of hydrogel. At higher salinity levels, with a hydrogel dose of 1.5 g kg⁻¹, the sunflower achieved favorable performance, promoting gains in some gas exchange variables in plants irrigated with saline water at 3.5 dS m⁻¹ and in fluorescence-related variables within the range of 2.0 to 3.5 dS m⁻¹. This positive effect of hydrogel indicates its potential as a mitigating strategy against the adverse effects of salinity, contributing to the maintenance of plant vigor and physiological functionality in saline environments.

1. Introduction

Salinity, now one of the primary challenges in agriculture, is present in soil solutions or irrigation water and hampers plant growth and development by inducing water stress, toxicity, and nutrient imbalances [1,2]. It is estimated that approximately thirty crop species provide around 90% of the global food supply, and many of these crops experience significant yield reductions under moderate salinity [3,4]. According to Lessa [5], semi-arid regions of the world suffer from water scarcity and high salinity of groundwater sources, with electrical conductivity (EC) values between 1.0 and 6.0 dS m⁻¹. Bernardo [6] classified irrigation water into four classes based on EC: C1 (0 and 0.25 dS m⁻¹) have low salinity and can be safely used in most crops and soils, requiring only normal leaching; C2 (0.25 to 0.75 dS m⁻¹) have medium salinity and require more careful management, being suitable for moderately salt-tolerant crops; C3 (0.75 to 2.25 dS m⁻¹) represent high salinity and should be used in plants with high tolerance, requiring specific management practices; and C4 (2.25 to 5.00 dS m⁻¹) have very high salinity, being naturally less used for conventional agricultural use.

In this context, seeking alternatives to mitigate the detrimental effects of saline water irrigation becomes essential. The use of soil conditioners is one of the strategies that enhance the physical, chemical, and biological properties of the soil or substrate. The most used conditioners are gypsum, manure, and polyacrylamide-based hydrogels, which were used to improve adaptation to this saline condition by improving the soil physical structure [7,8]. Hydrogel has been gaining prominence due to its high-water retention capacity, particularly as a component of substrates for seedling production [9,10]. However, the use of hydrogel is not universally suitable for all soil types, being mainly evaluated in sandy substrates, where it has shown potential to improve water retention and mitigate water stress [11,12,13].

The effect of the association between saline water and soil conditioners is commonly evaluated based on the consequences of morphological and productive parameters. However, evaluating stress symptoms in plants through physiological analyses related to the photosynthetic apparatus and photochemical efficiency, including gas exchange [14,15] and chlorophyll fluorescence [16,17] can bring a greater and more robust understanding of the effects, providing insights into the plant’s potential adaptations to stress, since physiological, biochemical, and molecular adjustments are essential for survival in saline environments [18,19].

Although primarily grown for oil production, sunflower (Helianthus annuus L.) also comprises ornamental varieties cultivated as cut flowers, garden plants, or in pots. Their commercial value derives mainly from floral attributes, such as color, size, and shape, and they play an important role in the rapidly expanding floriculture sector, which is driven by increasing global demand for aesthetic landscaping and decorative purposes [20,21]. Sunflower is recognized for its tolerance to adverse conditions such as drought, but its productivity is considerably constrained by salinity stress, which affects more than 20% of agricultural land worldwide [22]. Sunflower is considered a moderately salt-tolerant plant. Although it can grow in soils with moderate salt levels, its productivity and quality may be affected under high salinity conditions. Excessive salinity can impair plant growth, photosynthetic capacity, and nutrient uptake, resulting in lower yield and quality of flowers or seeds. The cultivation of ornamental flowers is a market that has achieved economic importance, with some alternative water sources being employed for plant irrigation, mainly involving the use of wastewater [23,24].

Only a few studies address the use of hydrogel and the effects of salinity on sunflower, particularly in oilseed varieties. However, no research has yet explored the integration of hydrogel as a salinity mitigator in ornamental sunflower. This study proposes an innovative approach by evaluating the combined use of hydrogel and saline water (low-quality) in ornamental sunflower cultivation, offering alternatives to reduce the reliance on high-quality water and supporting the development of agricultural policies, especially in floriculture. Accordingly, the objective was to assess the potential of hydrogel in mitigating stress caused by saline irrigation water and to explore its use as an alternative water source in agriculture, by analyzing growth, gas exchange, and chlorophyll fluorescence parameters in ornamental sunflower plants over two growing seasons.

2. Materials and Methods

2.1. Local and Environmental Conditions

The experiment was conducted in the Biosystems Engineering Department, “Luiz de Queiroz” College of Agriculture, University of São Paulo (22°42′ S, 47°37′ W, 546 m altitude), in Piracicaba, Brazil. According to the Köppen classification, the city’s climate falls within the humid subtropical zone (Cw) (average temperature of 21.6 °C, annual relative humidity of 73%, and 1280 mm of annual precipitation) [25]. The experiment was carried out in two crop cycles in a protected environment (18 m × 6 m), covered by 150-micron polyethylene transparent film with a 4 m ceiling height. The first cycle was conducted from September to November 2020, and the second from January to March 2021. During the crop cycles, average daily temperature and relative humidity inside the protected environment were measured using a Hikari’s HTU-21 thermo-hygrometer (Figure 1).

2.2. Treatments and Experimental Design

The treatments consisted of four levels of electrical conductivity of irrigation water—ECw (0.5, 2.0, 3.5, and 5.0 dS m⁻¹) and four doses of a soil conditioner-hydrogel (H1—0.0, H2—0.5, H3—1.0 and H4—1.5 g kg⁻¹), totaling sixteen treatments in a randomized block design and analyzed in a split-plot scheme with four blocks, totaling sixty-four experimental units. Each experimental unit consisted of a plastic pot (14.7 cm in diameter at the top, 9.8 cm in diameter at the bottom, and 11.6 cm in height, with a volume of 1.0 L) and an ornamental sunflower plant. The experimental units were distributed on benches with a height of 1.2 m, and the pots were spaced 0.25 m apart. Figure 2 shows a summary of the main treatments and measurements in ornamental sunflower for the two cycles.

2.3. Substrate and Hydrogel Preparation

The pots were filled with a thin layer of 150 g of type 1 gravel, followed by the placement of a geotextile blanket. Then, the pots were filled with 1.2 kg of substrate. The substrate consisted of three commercially used materials in a 2:1:1 ratio: sand (1.2 to 0.42 mm), humus (100% Terral^® earthworm humus), and soil to reproduce the treatment schemes currently used by farmers, respectively. The soil used is classified as Oxisol by Soil Taxonomy [26], with 76% sand, 4.1% silt, 19.9% clay, and a bulk density of 1.58 g cm⁻³. The chemical characteristics are pH CaCl₂: 5.6; electrical conductivity: 1.0 dS m⁻¹; organic matter: 11.0 g dm⁻³; calcium: 19.6 mmol_c kg⁻¹; magnesium: 9.5 mmol_c kg⁻¹; potassium: 2.3 mmol_c kg⁻¹; phosphorus: 24 mg dm⁻³; and sulfur: 14 mg dm⁻³.

The components were mixed to obtain a homogeneous substrate. Then, the respective doses of hydrogel were added, according to each treatment. The hydrogel used UPDT^®, classified as organic and of vegetable origin, is solid, granular, and yellowish in color. It has a real water capacity of 400%, contains starch and potassium salt in its composition, and exhibits an adsorption capacity of 500 to 700 times its volume in water. For each of the hydrogel treatments, the field capacity point (vessel capacity) for each condition was determined through capillary saturation, in which, after complete saturation and subsequent natural drainage of excess water, the vessels were weighed.

2.4. Saline Water Preparation

To obtain the saline water levels, it was used the water from the treatment and supply center of the Luiz de Queiroz Higher School of Agriculture (Piracicaba, Brazil), which presents the electrical conductivity of 0.5 dS m⁻¹, to which a mixture of three salts, NaCl, CaCl₂·2H₂O and MgCl₂·6H₂O were added, in the proportion of 7:2:1, to achieve the levels of the electrical conductivity, and diluted by solubilizations with constant monitoring of electrical conductivity in 100 L of water.

2.5. Plant Management

To the cultivation of the ornamental sunflower cv Anão de Jardim, initially, seedling production was carried out within the protected environment. Seeds were sown in polyethylene trays using the commercial substrate Biomix^®, recommended for ornamental plants. The seedlings were irrigated daily. 10 days after sowing (DAS), the seedlings were transplanted to the pots. Plant fertilization was carried out according to the recommendations of the culture [27] with potassium sulfate (50% K₂O and 17% S), monoammonium phosphate (47% P₂O₅ and 5% N), magnesium sulfate (11.9% S and 9% Mg) and urea (46% N), applied in the soil; and a mixture with micronutrients (1.82% B; 1.82% Cu; 7.26% Fe; 1.82% Mn; 0.36% Mo; 0.35% Ni) applied by foliar application.

2.6. Irrigation

After transplanting, irrigation was implemented using a self-compensating drip irrigation system, with one emitter per plant spaced at 0.30 m, a flow rate of 1.0 L h⁻¹, and a service pressure of 1 bar. For all the treatments, until 14 DAS, it was irrigated with clear water. From 15 DAS, the treatments with different salinity levels began to be applied. To determine the uniformity of the drip tapes, Christiansen’s Uniformity Coefficient was calculated; the obtained coefficients were 94.4% and 93.5%, respectively, for the first and second cycles. Irrigation management was daily, with replacement of the required irrigation depth obtained by lysimetry [28,29]. To prevent salt accumulation, a leaching fraction equivalent to 20% more than the irrigation depth required for the crop was applied weekly. This procedure aimed to promote the removal of salts from the pot, minimizing their concentration in the root zone.

2.7. Fluorescence Parameters

The fluorescence measurements were performed at 58 DAS from 9:00 a.m. with a JUNIOR-PAM portable fluorometer (Walz, Effeltrich, Germany). The leaves were adapted to the dark for 30 min for the total oxidation of the photosynthetic electron transport system. Initial fluorescence (Fo) and then maximum fluorescence (Fm) were obtained. These measurements were used to calculate the variable fluorescence (Fv), which is the difference between maximum fluorescence and initial fluorescence, and the maximum yield of photosystem II, dividing Fv by Fo.

2.8. Gas Exchange Parameters

The gas exchange parameters of the ornamental sunflower were measured from 10:00 a.m. in fully expanded intermediate leaves exposed to solar radiation at 59 DAS for both cycles using a portable photosynthesis measurement system—infrared gas analyzer (LI6400, Li-Cor, Biosciences Company, Lincoln, NE, USA). The photosynthetic photon flux density (PPFD) used was 1500 μmol m⁻² s⁻¹, with 400 μmol mol⁻¹ of CO₂ concentration inside the chamber and using the light source LI-COR 6400-02, coupled to the measurement chamber. The net photosynthetic rate (A, μmol m⁻² s⁻¹), stomatal conductance (gs, mol m⁻² s⁻¹), intercellular carbon dioxide concentration (Ci, μmol mol⁻¹), and transpiration rate (E, mmol m⁻² s⁻¹).

2.9. Plant Biomass

At the end of the experiment, 60 DAS, the biomass of the aerial part was determined by drying it in a forced air circulation oven at 65 °C and keeping it there until reaching a constant mass, and then weighing it on an analytical balance (0.001 g).

2.10. Statistical Analyses

Data was subjected to analysis of variance by the F test, at a 0.05 probability level. In case of significance, the treatments were compared by regression analysis, while orthogonal contrasts were applied to assess the effects of treatment. All these analyses were performed using SISVAR software, version 5.6 for data processing [30].

3. Results

3.1. Plant Biomass

The imposition of increasing levels of salinity from irrigation water led to a reduction in the dry biomass of the aerial part of ornamental sunflower plants during the cultivation cycles (Figure 3a,b). In contrast, with the application of increasing doses of hydrogel, there was a positive trend in increasing plant biomass (Figure 3c,d).

3.2. Gas Exchange Parameters

The results support strengthening the relevance and applicability of hydrogel as an innovative and sustainable practice, allowing ornamental sunflowers to be successfully cultivated in saline water environments, mitigating adverse effects on the gas exchange parameters.

3.2.1. Photosynthetic Rate

During the first cycle, the analysis of hydrogel doses at each saline water level showed linear decreases in photosynthetic rate, with rates of 1.94, 2.55, and 0.48 mol m⁻² s⁻¹ for H1, H2, and H4, respectively, for each increase in dS m⁻¹. However, under the H3 treatment, photosynthetic rate increased, showing a linear rise despite the increase in the electrical conductivity of the irrigation water (Figure 4a). The analysis of hydrogel doses across treatments revealed a difference only at the highest saline water level, where the highest photosynthetic rate values observed were up to 23% higher with the H3 dose and 14% higher with the H4 dose compared to the control.

In the second cycle, photosynthetic rate was estimated to decrease at rates of 0.73 and 0.64 μmol of CO₂ m⁻² s⁻¹ for each increase in dS m⁻¹ as saline water levels rose, without hydrogel and with the 0.5 g kg⁻¹ treatment, respectively. However, at the highest hydrogel doses, photosynthetic rate increased at rates of 0.36 and 1.26 μmol of CO₂ m⁻² s⁻¹ for each saline water level increment (Figure 4b). Similar to the first cycle, differences between hydrogel doses were only significant at 5.0 dS m⁻¹, with photosynthetic rate being up to 35% higher in H4 compared to the control.

In the first cycle, the photosynthetic rate of sunflower, cv. ‘Anão de Jardim’, under the saline water level of 0.5 dS m⁻¹, was higher without the addition of hydrogel. However, when evaluating the performance of this conditioner under the level of 5 dS m⁻¹, it was observed that the use of hydrogel in the substrate reduced stress, leading to higher photosynthetic rates, especially at the highest doses tested.

3.2.2. Stomatal Conductance

In relation to stomatal conductance during the first cycle, the treatments H1, H2, and H4 resulted in linear reductions as saline water levels increased. In contrast, the treatment H3 exhibited a maximum conductance trend of 0.78 mol m⁻² s⁻¹ at 3.38 dS m⁻¹ (Figure 4c).

As observed in the first cycle, the second cycle also showed that the absence of the conditioner led to decreases in stomatal conductance as the saline water levels increased. However, the application of hydrogel in the substrate promoted maximum stomatal conductance values, estimated at 0.249 mol m⁻² s⁻¹ (at 3.12 dS m⁻¹), 0.446 mol m⁻² s⁻¹ (at 2.83 dS m⁻¹), and 0.491 mol m⁻² s⁻¹ (at 2.97 dS m⁻¹) under treatments H2, H3, and H4, respectively (Figure 4d).

Furthermore, contrast analysis revealed significant differences between hydrogel doses at the two highest electrical conductivities (3.0 and 5.0 dS m⁻¹), with higher values observed for the dose 1.5 g kg⁻¹ at both conductivity levels during the first cycle. Interaction analysis indicated that, particularly in the first cycle (except for the hydrogel dose of 1.0 g kg⁻¹), stomatal conductance decreased. In the second cycle, there was a consistent trend of reduction for each increment of dS m⁻¹ in saline water level without hydrogel application. Conversely, with the application of hydrogel doses, maximum values were found for saline water levels ranging from 2.83 to 3.12 dS m⁻¹. Thus, the hydrogel doses demonstrate maximal mitigating performance within these saline water levels.

3.2.3. Transpiration Rate

During the first cycle, as the hydrogel dose increased, increments in transpiration rate were observed even with rising salinity, particularly without hydrogel and at hydrogel doses of 0.5 and 1.0 g kg⁻¹ (H2 and H3), with estimated rates of 0.74, 0.38, and 0.39 mmol CO₂ m⁻² s⁻¹ for each unit increase in dS m⁻¹, respectively. However, the application of 1.5 g kg⁻¹ resulted in a decrease, at a rate of 0.65 mmol CO₂ m⁻² s⁻¹ per increment of saline water level (Figure 4e).

In the second cycle, increases in transpiration rate were noted without hydrogel and at the lowest dose (0.5 g kg⁻¹) as the saline water levels increased, with gains of 0.28 and 0.44 mmol CO₂ m⁻² s⁻¹ for each dS m⁻¹ increment. In contrast, hydrogel doses of 1.0 and 1.5 g kg⁻¹ led to reductions, at rates of 0.72 and 0.74 mmol CO₂ m⁻² s⁻¹, respectively, for each unit increase in dS m⁻¹ (Figure 4f). A significant difference due to hydrogel use was observed only at an electrical conductivity of 5.0 dS m⁻¹, with a lower transpiration rate recorded, particularly at the highest salinity levels with the 1.5 g kg⁻¹ dose.

3.3. Fluorescence Parameters

3.3.1. Initial Fluorescence

In the first cycle, the initial fluorescence of plants grown with hydrogel reached estimated maximum values of 54.73, 57.60, and 64.23 a.u. at ECw levels of 2.43, 3.07, and 2.89 dS m⁻¹ for treatments H1, H2, and H4, respectively. In treatment H3, there was no significant difference in the interaction (p > 0.05), with an average value of 58.78 a.u. (Figure 5a).

During the second cycle, the initial fluorescence of sunflower plants subjected to increased electrical conductivity without hydrogel decreased by 1.88 a.u. per dS m⁻¹ increment in the irrigation water. In contrast, plants treated with hydrogel showed reductions starting from saline water levels above 2.90, 2.81, and 2.64 dS m⁻¹ for treatments H2, H3, and H4, respectively (Figure 5b). Analyzing the effect of hydrogel at each saline water level, a significant difference was observed only at 2.0 dS m⁻¹ in the first cycle, where lower initial fluorescence values were recorded in the absence of hydrogel (H1). In comparison, higher values were obtained with 1.5 g kg⁻¹ of hydrogel.

3.3.2. Maximum Fluorescence

In sunflower plants, the minimum values estimated of maximum fluorescence values of 116.49 and 139.64 a.u. were observed in the treatment without hydrogel and in the treatment with 1.0 g kg⁻¹ (H3) in the growing medium, respectively, during the first cycle. In contrast, treatments H2 and H4 exhibited the maximum values of maximum fluorescence of 183.51 and 174.10 a.u., respectively (Figure 5c). In the second cycle, the application of hydrogel resulted in higher maximum fluorescence values at salinity levels greater than the control, with maximum values of 148.67, 149.77, and 158.46 a.u. recorded in treatments H2, H3, and H4, respectively (Figure 5d).

It is noteworthy that across both cycles, sunflower plants treated with hydrogel consistently showed higher maximum fluorescence values compared to the control, which used saline water in irrigation. Significant differences, however, were only observed at 2.0 and 3.5 dS m⁻¹, with the highest means achieved using 1.5 g kg⁻¹ of hydrogel. Across both cycles, the range of maximum fluorescence values in the first cycle was broader, ranging from 140 to 180, compared to the second cycle, which ranged from 110 to 150.

3.3.3. Variable Fluorescence

For variable fluorescence, the control treatment and the treatment with 1.0 g kg⁻¹ (H3) exhibited higher values after reaching the minimum points of 61.74 and 81.55 a.u. under the levels of 3.47 and 2.54 dS m⁻¹, respectively. In contrast, treatments containing hydrogel in the growing medium achieved maximum points estimated at 126.25 and 111.05 a.u. under treatments H2 and H4, respectively, for the first cycle (Figure 5e). In the second cycle, treatments with hydrogel reached maximum points estimated at 87.13, 91.86, and 87.74 a.u. for doses H2, H3, and H4 at saline water levels of 2.80, 2.79, and 3.05 dS m⁻¹, respectively (Figure 5f). When comparing the hydrogel effect at different electrical conductivity levels, significant differences were observed at 2.0 dS m⁻¹ in the second cycle and at 2.0 and 3.5 dS m⁻¹ in the first cycle.

The variable fluorescence trends were similar to those observed for maximum fluorescence, except during the second cycle, where the conditioner only caused a difference at 2.0 dS m⁻¹, with higher variable fluorescence values under 0.5 g kg⁻¹. Additionally, in the salinity control treatment, the hydrogel did not influence this variable. Furthermore, in the second cycle, increasing the saline water level did not affect plants without the conditioner in (H1). In summary, the fluorescence variables were influenced by hydrogel doses at saline water levels of 2.0 and 3.5 dS m⁻¹, with the highest hydrogel dose (1.5 g kg⁻¹—H4) leading to increased fluorescence values.

3.4. Maximum Yield of Photosystem II

When analyzing the effect of hydrogel at each saline water level, the maximum yield of photosystem II was influenced by hydrogel doses at the higher saline water level, specifically in the first cycle at 3.5 dS m⁻¹, and in the second cycle at levels of 3.5 and 5.0 dS m⁻¹. In both cycles, the highest maximum yield of photosystem II was achieved with the highest hydrogel dose (1.5 g kg⁻¹—H4). In the first cycle, the maximum yield of photosystem II showed minimum values of 1.27 and 1.66 for H1 and H3, respectively, and a maximum value of 2.27 for H2. The highest dose (H4), however, caused a linear decrease (Figure 6a). Nevertheless, higher absolute values for this variable were observed at the highest hydrogel doses, particularly under higher saline water irrigation levels. Additionally, in the second cycle, the treatments H3 and H4 resulted in maximum points of 1.78 and 1.83 for maximum yield of photosystem II at saline water levels of 2.84 and 3.30 dS m⁻¹, respectively (Figure 6b).

4. Discussion

The results support strengthening the relevance and applicability of hydrogel as an innovative and sustainable practice, allowing ornamental sunflowers to be successfully cultivated in saline water environments, mitigating adverse effects on photosynthesis and plant growth. Irrigation with saline water is known to cause serious effects on the growth and physiology of various plants, especially in regions where access to high-quality water is limited. High levels of salinity in the soil lead to an accumulation of ions in the roots, which interfere with the absorption of essential nutrients and the water balance of plants [31,32,33]. In this context, previous studies indicated that the application of hydrogel contributed to reducing soil salinity, which supported the positive effects observed on the photosynthetic rate under saline irrigation conditions [34].

Despite the presence of water in the substrate, plants cannot use it effectively due to the high osmotic pressure imposed by the salt [35,36]. These effects resulted in decreased vegetative growth of ornamental sunflowers in both crop cycles. Salinity stress effects alter metabolism, producing reactive oxygen species (ROS) in mitochondria and chloroplasts, changes in ion balances, mineral nutrition, stomata behavior, photosynthetic and ultimately causing a decline in plant growth [37]. In ornamental crops irrigated with brackish water, such as gazania and kalanchoe, production decreases have been observed [38]. Similar responses are noted in vegetables like parsley [39] and chives [40]. These findings align with those observed in this study, where hydrogel application demonstrated potential to alleviate the adverse effects of salinity on ornamental sunflower (Helianthus annuus L.) by enhancing physiological parameters and maintaining efficient growth even under saline stress. In addition, studies with tomato plants indicated that hydrogel application significantly increased soil organic carbon, N, P, and K contents, enhancing nutrient uptake and providing favorable conditions for plant development. These effects may explain the improvements observed in the photosynthetic rate at higher hydrogel doses in the present work [41].

Temperatures above 35 °C are known to affect the membrane where chlorophyll resides, disrupting its fluidity and thereby impacting photosynthesis. Hydrogel helped reduce these effects, as observed by increased plant biomass with higher hydrogel doses across both crop cycles. This observation is consistent with the findings of El-Asmar [42], where hydrogel application improved plant biomass in maize plants under saline stress. Moreover, the hydrogel’s ability to enhance net photosynthesis across varying levels of salinity in this study can be attributed to its thermal stability. The acrylamide component of hydrogel, copolymerized with other stable monomers, provides resilience under high temperatures, as noted by Bae [43].

In terms of photosynthetic efficiency, the study recorded a decline in photosynthesis rates (1.94 and 0.73 μmol of CO₂ m⁻² s⁻¹) with increased salinity but observed an attenuation of this reduction when 0.5 g kg⁻¹ hydrogel was applied, suggesting that the hydrogel mitigated salt stress on net photosynthesis. This response is due in part to the interaction between monovalent salts in saline water and the hydrogel, which contains potassium, a monovalent mineral in this study. While higher salinity levels reduce hydrogel effectiveness, the presence of bivalent salts seems to counterbalance this effect, enhancing the conditioner’s efficiency [44]. In addition, hydrogels exhibited relevant ionic transport properties: the potassium in their composition interacted with the monovalent salts in saline water, improving the conditioner’s efficiency. Thus, the structural properties of the hydrogel supported the results obtained, justifying the improvements in physiological parameters and photosynthetic rate of plants under saline stress [45].

Hydrogel retains large amounts of water, gradually releasing it as the plant needs. This property is essential for reducing the impact of salt stress, as it allows the plant access to a continuous water source, lowering the salt concentration in the root zone and, consequently, the osmotic pressure. This water retention is important for maintaining stomatal conductance at more stable levels, enabling adequate gas exchange and a more efficient photosynthetic rate [46,47,48]. Chemical crosslinking increased the material’s mechanical and thermal stability, which may explain the resilience observed in this study under conditions of salinity and high temperatures [45]. It is essential to note that in semi-arid conditions, hydrogel durability can be compromised by salinity, resulting in a reduction in its water absorption potential by up to 85% after 120 days [49].

The study also highlights the hydrogel’s effect on stomatal conductance under saline conditions. Despite the general decrease in stomatal conductance due to salt stress, hydrogel application resulted in smaller reductions, a difference from Gomes [50], who reported losses of 41.26% in ‘Catissol’ sunflower plants under saline water. In this study, the highest stomatal conductance loss observed was 37.14% within the range of 0.5 to 5.0 dS m⁻¹ at a 0.5 g kg⁻¹ hydrogel dose. This further demonstrates the hydrogel’s role in reducing water loss and sustaining gas exchange.

During the second cycle, higher daytime temperature peaks were observed, which may have resulted in lower stomatal conductance compared to the first cycle, likely due to the high temperatures. Gas exchange processes, like stomatal conductance, are sensitive to environmental factors [51], suggesting that elevated temperatures in the second cycle contributed to this reduction in stomatal conductance. In this context, the three-dimensional crosslinked structure of hydrogel, formed by chemical bonds or physical interactions, contributed to its thermal stability and water retention capacity, even under stress conditions. This structural characteristic helped explain the role of hydrogel in reducing water loss and maintaining gas exchange observed in this study [45].

An increase in efficiency was achieved with a hydrogel dose of 1.5 g kg⁻¹. Plants in this treatment exhibited higher yields relative to water consumption due to improved net photosynthesis and stomatal conductance, reducing transpiration rates. This result aligns with Dawes [52], who reported that improved water use efficiency typically corresponds to enhanced growth and carbon fixation.

Hydrogel’s positive impact extended to chlorophyll fluorescence parameters, particularly under the electrical conductivity of 5.0 dS m⁻¹. The maximum yield of photosystem II showed that the hydrogel effectively supported photochemical processes despite elevated salinity levels. The highest hydrogel dose was associated with greater maximum fluorescence, as opposed to the results by Nóbrega [53], where increased salinity led to a decline in fluorescence values in Mesosphaerum suaveolens. This highlights the hydrogel’s role in preventing electron flow disruptions in the maximum yield of photosystem II, commonly observed under salt stress [54].

Similar results were reported in Calendula officinalis, in which hydrogel application, either alone or in combination with salinity levels, promoted significant increases in growth characteristics, biochemical parameters, and enzymatic activity, all essential for salt stress tolerance. In contrast, severe stress drastically reduced these values, reinforcing the hydrogel’s role as a mitigator of the negative effects of salinity. These findings indicated that hydrogel not only improved the physiological performance of ornamental sunflower but also represented a promising alternative for other ornamental and medicinal species, paving the way for future applications in the floriculture sector [37].

Studies show that hydrogel can protect PSII by minimizing the impact of ROS and reducing the risk of photoinhibition. This occurs because the constant availability of water allows the plant to maintain electron flow in PSII, preventing the accumulation of excess energy that could generate ROS. The efficiency of PSII is thus preserved, maintaining a maximum fluorescence rate [55,56]. With the use of hydrogel, an increase in water use efficiency is observed, as the plant can assimilate more CO₂ per unit of water transpired. This effect is especially beneficial under high salinity conditions, where water use efficiency is essential for plant survival and productivity [11,57,58]. Furthermore, by mitigating osmotic stress, the hydrogel facilitates the accumulation of metabolites, such as phenolic compounds, which neutralize free radicals and enhance the plant’s antioxidant defenses, thereby complementing the protection of PSII against the effects of salinity [59]. Figure 7 shows a summary of the main mechanisms of hydrogel along with salinity stress.

Plants treated with 1.5 g kg⁻¹ of hydrogel demonstrated greater physiological performance at saline water values up to 3.5 dS m⁻¹, suggesting that hydrogel effectively mitigated the negative impact of high salinity on photosynthetic apparatus efficiency. Hydrogel application promoted a positive effect on electron flow within photosystem II under the most challenging saline conditions, enhancing sunflower resilience by supporting the plants’ photochemical efficiency. This protective effect was most evident with higher hydrogel doses, underscoring the importance of optimal conditioner concentration in improving plant performance under saline irrigation conditions.

This study provided insights into the effects of the hydrogel under varying salinity levels in the ornamental sunflower. However, variability in soil, climate, and genetic material conditions in the field may affect plant responses, requiring long-term investigations in different soil and climate scenarios. Furthermore, in regions with high salt concentrations in irrigation water (greater than 3.5 dS m⁻¹), the incorporation of the hydrogel was not sufficient to mitigate the negative effects of salinity. Therefore, it’s recommended in locations where the salinity in irrigation water is below 3.5 dS m⁻¹. For future research, it is recommended to conduct an economic feasibility evaluation of integrating hydrogel application with the use of saline water (low-quality).

5. Conclusions

The alternative use of saline water (low-quality) for irrigation in ornamental sunflower cultivation causes a significant reduction in plant biomass and compromises its physiological performance. However, cultivation with the addition of hydrogel in the pot showed benefits even under saline conditions (low-quality), promoting an increase in the expression of physiological variables related to both gas exchange and chlorophyll fluorescence. After two crop cycles, it can be concluded that the ornamental sunflower showed better performance with a saline water of 0.5 dS m⁻¹ without the use of hydrogel. At higher salinity levels, with a hydrogel dose of 1.5 g kg⁻¹, the sunflower achieved favorable performance, promoting gains in some gas exchange variables in plants irrigated with saline water at 3.5 dS m⁻¹ and in fluorescence-related variables within the range of 2.0 to 3.5 dS m⁻¹.

This positive effect of hydrogel indicates its potential as a mitigating strategy against the adverse effects of salinity, contributing to the maintenance of plant vigor and physiological functionality in saline environments. For ornamental sunflower cultivation, it is recommended to use 0.5 or 1.0 g kg⁻¹ of hydrogel under irrigations with electrical conductivities of 2.0 and 3.5 dS m⁻¹, and doses of 1.5 g kg⁻¹ for conductivities above 3.5 dS m⁻¹. This study faced limitations due to the restrictions imposed by the COVID-19 pandemic, which prevented the performance of some laboratory analyses, such as the determination of Na and K concentrations and electrical conductivity at the end of the experiment. In future research, these and other analyses, including enzymatic evaluations, will be considered to deepen the understanding of the physiological responses involved.