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Article

Using Saline Water for Sustainable Floriculture: Identifying Physiological Thresholds and Floral Performance in Eight Asteraceae Species

by
María Rita Guzman
1,*,
Xavier Rojas-Ruilova
2,
Catarina Gomes-Domingues
3 and
Isabel Marques
4,*
1
Estación de Biodiversidad La Ceiba, Chisec 16013, Guatemala
2
Facultad de la Salud Humana, Universidad Nacional de Loja, C. Manuel Monteros, Loja 110103, Ecuador
3
Linking Landscape, Environment, Agriculture and Food (LEAF) Research Center & Associate Laboratory TERRA, Instituto Superior de Agronomia (ISA), Universidade de Lisboa (UL), Tapada da Ajuda, 1349-017 Lisboa, Portugal
4
Forest Research Center (CEF) & Associate Laboratory TERRA, Instituto Superior de Agronomia (ISA), Universidade de Lisboa (UL), Tapada da Ajuda, 1349-017 Lisbon, Portugal
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(8), 1802; https://doi.org/10.3390/agronomy15081802
Submission received: 19 June 2025 / Revised: 10 July 2025 / Accepted: 23 July 2025 / Published: 25 July 2025
(This article belongs to the Special Issue Effect of Brackish and Marginal Water on Irrigated Agriculture)

Abstract

Water scarcity challenges floriculture, which depends on quality irrigation for ornamental value. This study assessed short-term salinity tolerance in eight Asteraceae species by measuring physiological (proline levels, antioxidant enzyme activity) and morphological (plant height, flower number, and size) responses. Plants were irrigated with 0, 50, 100, or 300 mM NaCl for 10 days. Salinity significantly enhanced proline content and the activity of key antioxidant enzymes (catalase, peroxidase, and ascorbate peroxidase), reflecting the activation of stress defense mechanisms. However, these defenses failed to fully protect reproductive organs. Flower number and size were consistently more sensitive to salinity than vegetative traits, with significant reductions observed even at 50 mM NaCl. Responses varied between species, with Zinnia elegans and Calendula officinalis exhibiting pronounced sensitivity to salinity, whereas Tagetes patula showed relative tolerance, particularly under moderate stress conditions. The results show that flower structures are more vulnerable to ionic and osmotic disturbances than vegetative tissues, likely due to their higher metabolic demands and developmental sensitivity. Their heightened vulnerability underscores the need to prioritize reproductive performance when evaluating stress tolerance. Incorporating these traits into breeding programs is essential for developing salt-tolerant floriculture species that maintain aesthetic quality under limited water availability.

1. Introduction

The global shortage of freshwater resources represents a critical challenge with severe social, economic, and environmental implications, lying at the core of multiple Sustainable Developmental Goals [1]. The increasing frequency and severity of droughts is expected to directly affect between 1.7 to 2.4 billion people by 2050 [2]. In addition, agricultural water scarcity is projected to intensify across more than 80% of global croplands, compromising production systems [3]. This is particularly pressing in the ornamental horticulture industry, which traditionally relies on high-quality freshwater sources for plant production [4]. This sector has a high economic value for several countries. For example, in the United States, floriculture (USD 6.19 billion), nursery production (USD 5.91 billion) and plant propagation (USD 1.19 billion) are significant contributors to the national economy [5], with the overall market projected to reach USD 101.9 billion by 2032 [6]. Similarly, in Europe, the Netherlands has led global floriculture production and exports for decades [7]. Recently, due to high demand, cut flower production has expanded to emerging markets such as China, Colombia, India, and Ecuador [7]. However, the shortage of freshwater and the increasing occurrence of droughts are driving global interest in finding alternative sustainable solutions for crop irrigation, including the use of seawater and brackish water sources [8].
Nonetheless, the use of saline water comes with its own set of challenges. The elevated concentrations of water-soluble salts can interfere with the uptake of water and essential nutrients, increasing plant osmotic stress and negatively affecting plant growth, yield, and quality in most salt-sensitive crops [9,10]. Salinity stress is also associated with elevated levels of reactive oxygen species (ROS) in plant cells, inducing oxidative stress and affecting both the photochemical and biochemical phases of photosynthesis [11,12]. It also increases lipid peroxidation, reducing the integrity of cell membranes [11]. To counteract these effects, plants have developed a complex antioxidant defense system to scavenge the excess of ROS. These include an array of enzymatic antioxidants, such as catalase (CAT), glutathione (GSH), superoxide dismutase (SOD), ascorbate peroxidase (APX), and guaiacol peroxidase (GPX), as well as non-enzymatic antioxidants such as ascorbate, carotenoids, and flavonoids, that altogether mitigate the deleterious effects of oxidative damage [10,11,13]. To cope with stressful conditions, plants also accumulate compatible solutes, or osmoprotectants such as non-structural carbohydrates and proline, which help reduce osmotic stress [14]. These protective mechanisms vary widely across species, and are strongly influenced by a plant’s degree of salt tolerance [14,15]. Notably, the enhanced production of antioxidant compounds under stress conditions may benefit human health since many of these bioactive compounds are now widely used as nutraceutical ingredients, to enhance well-being, prevent chronic diseases, and slow the aging process [16].
Despite the ornamental and economic value of flowers, research on the effects of salinity has predominantly focused on vegetative traits such as plant height, biomass, or leaf morphology. While some studies have addressed reproductive parameters, including flowering time, flower number, and size, these remain limited in scope, are often species-specific, and are rarely integrated with physiological analyses [9,17,18,19]. Moreover, few studies provide a comprehensive evaluation that simultaneously examines both vegetative and floral traits under controlled salinity gradients, or that link floral responses to underlying physiological stress markers. This fragmented understanding hampers the development of guidelines for the sustainable use of saline water in ornamental horticulture. Yet, the limited studies indicate that reproductive traits are often more sensitive to salinity than vegetative traits. For example, salinity significantly reduced flower number, and delayed bud formation in Catharanthus roseus and Zinnia elegans, two widely cultivated ornamental species [20]. In the ornamentals, Tagetes patula and Ageratum mexicanum, an earlier and shorter flowering period was reported under salinity conditions [21]. Similar results were found in Lilium × elegans [22]. Salinity also decreased flower weight and total flower production in Calendula officinalis [23,24]. Even some halophyte species, which are highly adapted to saline conditions, suffer constraints in their flowering stage [25]. For instance, salinity significantly reduced the number of inflorescences in Crithmum maritimum while flowering started earlier in the salt-sensitive genotypes [26]. In the halophyte Limonium algarvense, a reduce number and a decrease in the height of flower steams were also reported in plants subjected to high salinity [27,28]. Thus, these findings highlight the importance of understanding floral responses to salinity, especially if saline irrigation is to be integrated into sustainably ornamental plant production [9]. That deserves further investigation, especially given the aesthetic and commercial importance of flowering traits in ornamentals.
In this context, our study aims to address the existing knowledge gap by providing an integrated analysis of both vegetative and reproductive responses to short-term exposure to salinity in eight widely cultivated ornamental species from the Asteraceae family. The Asteraceae family is one of the largest plant families, with over 23,000 species globally, many of which are naturally adapted to diverse and often harsh environments, including arid and saline regions. Some members of this family, such as Helianthus annuus, have shown physiological adaptations to salinity stress, suggesting the presence of intrinsic salt-tolerance mechanisms within the family. Moreover, Asteraceae species are of considerable importance in the floriculture industry due to their striking inflorescences (hereafter, flowers) which are composite floral structures composed of numerous florets with a wide array of colors, forms, and flowering durations. Despite their ornamental and commercial relevance [29,30], the response of many Asteraceae species to salinity stress remains poorly understood, especially in terms of floral traits, which are crucial for their market value.
To contribute to this underexplored area, we investigated the effects of varying salinity levels on eight representative ornamental Asteraceae species: Aster oblongifolius Nutt. (=Symphyotrichum oblongifolium, aromatic aster), Calendula officinalis L. (pot marigold), Callistephus chinensis (L.) Nees (China aster), Coreopsis tinctoria Nutt. (plains coreopsis), Tagetes patula L. (French marigold), Tetraneuris scaposa (DC.) Greene (four-nerve daisy), Viguiera stenoloba S.F.Blake (skeleton-leaf goldeneye), and Zinnia elegans Jacq. (elegant zinnia). Although these species are widely cultivated, their tolerance to salinity stress has not been systematically evaluated, particularly regarding physiological analysis and even less considering floral traits (but see [19,31,32,33]). This knowledge gap was a primary reason for their inclusion in this study.
Thus, this study aims to: (1) compare the salinity tolerance of these eight ornamental Asteraceae species under controlled conditions; (2) evaluate both vegetative and floral reproductive traits to provide a global assessment of plant performance; and (3) identify salinity thresholds that can guide the use of saline irrigation in ornamental horticulture. For this, we assessed the impact of salinity on key vegetative and reproductive traits, including plant height, flower number, and flower size. We also monitored changes in the levels of proline and in the activity of key antioxidant enzymes, catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX), to evaluate the plants’ oxidative stress responses under salinity stress. This study is novel in several key aspects. First, it evaluates eight ornamental Asteraceae species in a unified comparative framework. Second, it combines morphological and physiological traits to provide a comprehensive understanding of plant performance under salinity conditions. Third, it adopts an application-oriented perspective by identifying irrigation salinity thresholds relevant for sustainable ornamental horticulture.

2. Materials and Methods

2.1. Plant Material and Growing Conditions

Eight ornamental species from the Asteraceae family were selected for this study and grown under controlled environmental conditions: Aster oblongifolius ‘cv.’ Sky Blue (AST), Calendula officinalis L. ‘cv.’ Indian Prince (CAL), Callistephus chinensis ‘cv.’ Blue ribbon (CLI), Coreopsis tinctoria ‘cv.’ Moonlight (COR), Tagetes patula ‘cv.’ Red Aurora (TAG), Tetraneuris scaposa ‘cv.’ Golden Yellow (TET), Viguiera stenoloba ‘cv.’ Golden Eye (VIG), and Zinnia elegans ‘cv.’ Big red (ZIN).
Seeds of each cultivar were germinated in 2 L pots filled with a standardized substrate mixture of 50% peat, 25% perlite, and 25% vermiculite. Prior to sowing, seeds were surface-sterilized with 1% sodium hypochlorite for 5 min and rinsed thoroughly with sterile water [20]. Germination and early growth were conducted in a nursery environment under the following controlled conditions: temperature ranged from 20.5 to 25.3 °C, relative humidity was maintained between 72 and 76%, and a 16 h light/8 h dark photoperiod was applied. After three weeks, when the seedlings reached the two-true-leaf stage, the plants were transplanted into individual 25 L pots and transferred to a controlled-environment chamber. The growth chamber was set to a long-day photoperiod (16 h light), with day/night temperatures of 23 °C and 19 °C, respectively, and relative humidity maintained at 72–76%. Photosynthetically active radiation (PAR) was maintained between 700 and 800 μmol m−2 s−1 using a Tenney WITH-983 system (New Columbia, PA, USA). Each drained pot contained one single plant supported by a polyethylene tray. The same substrate (50% peat, 25% perlite, 25% vermiculite) was used as the growing medium in all pots. Prior to transplanting, the medium was adjusted to a pH range of 5.7–5.9 using a dilute KOH solution, and the electrical conductivity (EC) was set to 2.1 mS cm−1 by incorporating a standardized nutrient solution (see below). The plants were irrigated with 230 mL of sterile water per pot twice a week and allowed to acclimate for one month before the salinity treatments were initiated.

2.2. Salinity-Stress Experiments

To assess the effects of salinity on plant development and flower-related traits, salinity-stress experiments were conducted using juvenile plants that were one month old and at a uniform vegetative stage, prior to floral bud initiation. The plants were exposed to four salinity levels: 0 (control), 50, 100, and 300 mM NaCl, applied via irrigation water. These levels were selected to represent a range from mild to severe salinity stress, with the 300 mM treatment included to test potential tolerance thresholds. Salinity treatments were performed by irrigating each pot with 230 mL of sterile water containing the respective NaCl concentration, twice per week, as previously described. This irrigation regime was maintained throughout the experimental period, ensuring consistent salinity exposure across treatments. Each treatment included 10 biological replicates per species, resulting in a total of 320 plants: 8 species × 4 salinity levels × 10 replicates.
For the short-term salinity assay, plants were irrigated with the respective NaCl solutions for a period of 10 days. The electrical conductivity (EC) was maintained constant for each treatment throughout the stress period: 2.0 mS cm−1 for the control (0 mM NaCl), 6.0 mS cm−1 for 50 mM NaCl, 10.0 mS cm−1 for 100 mM NaCl, and 28.0 mS cm−1 for 300 mM NaCl. While the salinity treatments (50, 100, 300 mM NaCl) were not arithmetically equidistant, they were deliberately selected to reflect ecologically and physiologically distinct stress thresholds, mild, moderate, and severe, as is common in plant salinity research. To maintain the EC of the substrate, we irrigated the plants using a calibrated solution of macronutrients (N–P–K: 15–5–30) and micronutrients (Fe, Mn, Zn, Cu, B, Mo), monitoring the leachate EC every two days using a portable EC Hanna soil test meter (Model HI98331, Woonsocket, RI, USA). At the end of the six-week salinity treatment, 300 mg of fresh weight (FW) of leaves were collected from each treatment and from each plant studied to assess the levels of proline and enzyme activities. Samples were collected from the middle third of the most recently fully expanded leaf on each plant to ensure consistency and physiological relevance.

2.3. Determination of Proline Levels

Proline content was quantified following the acid-ninhydrin method using toluene as the extraction solvent [34]. Briefly, leaf tissue was homogenized and reacted with the acid-ninhydrin reagent under heating to develop the characteristic chromophore. After cooling, the reaction mixture was extracted with toluene, and the absorbance of the organic phase was measured spectrophotometrically at 520 nm. Results were calculated and expressed as micrograms of proline per gram of dry weight (DW).

2.4. Determination of Enzyme Activities

To evaluate the antioxidant responses under salinity stress, the samples were first homogenized in an extraction buffer containing 200 mM Tris-HCl (pH 8.0), 10 mM MgCl2·6H2O, 30 mM β-mercaptoethanol, 4 mM dithiothreitol (DTT), 2% Triton X-100, 10% glycerol, and a protease inhibitor cocktail (EDTA-free, 2 tablets per 50 mL; Complete™, Roche, Basel, Switzerland). Additionally, 1% (v/v) polyvinylpolypyrrolidone (PVPP) was added to each sample during homogenization to remove phenolic compounds. The homogenate was centrifuged at 13,000× g for 20 min at 4 °C, and the resulting supernatant was used for enzymatic assays.
Catalase (CAT) activity was assayed in a 1.5 mL reaction mixture containing 10 mM H2O2 and 50 mM phosphate buffer (pH 7.0), with the enzyme extract added immediately before initiating the reaction that followed [35]. The decomposition of H2O2 was monitored spectrophotometrically at 240 nm, and enzyme activity was calculated based on the rate of absorbance decrease. The results are expressed as CAT units per mg of protein dry weight (DW), where one unit corresponds to the decomposition of 1 mM of H2O2 per minute.
Peroxidase (POX) activity was determined by measuring the rate of H2O2-dependent substrate oxidation at 430 nm, following the method of [36]. The extinction coefficient of 2.47 mM−1 cm−1 was used to calculate enzyme activity. One POX unit was defined as the amount of enzyme required to decompose 1 μmol of H2O2 per minute. The results are expressed in units per mg of protein DW.
Ascorbate peroxidase (APX) activity was assayed spectrophotometrically at 290 nm using a 1 mL reaction mixture containing 20 mM ascorbate, 0.1 mM H2O2, 50 mM phosphate buffer (pH 7.8), and 10 μL of enzyme extract. The oxidation of ascorbate in the presence of H2O2 was monitored, and enzyme activity was calculated using an extinction coefficient of 2.8 mM−1 cm−1, according to [37]. The results are expressed as APX units per mg of protein DW.

2.5. Effects on Plant Height, Flower Production, and Size

At the end of the six-week experimental period, final measurements were taken to assess the effects of salinity on key morphological traits. Specifically, plant height (cm) measured from the base of the stem (at soil level) to the apex of the tallest inflorescence, the total number of flowers per plant, and flower size (measured as flower diameter in cm) were recorded for each individual plant. These measurements were taken immediately prior to harvest to capture the cumulative impact of salinity over time. Following measurement, all aboveground plant material was harvested and oven-dried at 65 °C in a thermo-ventilated chamber until a constant weight was achieved.

2.6. Statistical Analysis

All data are presented as mean values ± standard error (SE), calculated from ten biological replicates per species and treatment. Statistical analyses were performed using IBM SPSS Statistics software, version 28 [30]. Data were first tested for normality using the Shapiro–Wilk test and for homogeneity of variances using Levene’s test. To assess the effects of species, salinity level, as well as their interactions, a two-way analysis of variance (ANOVA) was conducted. The model included all the main effects and interaction terms. When significant effects were detected (p < 0.05), differences among group means were further evaluated using Tukey’s HSD test for post hoc comparisons. Additionally, to understand the relation between proline content and antioxidant enzyme activities, Pearson correlation coefficients (r) were considered for each species across salinity treatments.

3. Results

3.1. Proline Contents Under Different Salinity Levels

Leaf proline content generally increased under saline conditions (Figure 1), although the magnitude and pattern of this response varied significantly among species (F7,128 = 319.385, p < 0.001) and across salinity levels (F3,128 = 460.619, p < 0.001; Table S1). Compared to the control plants, most species exhibited elevated proline levels under salt stress. In Aster oblongifolius, Calendula officinalis, and Callistephus chinensis, proline peaked at the lowest tested concentration (50 mM NaCl), then declined at higher levels (100, 300 mM), even reaching control values in the case of the first two species, suggesting a physiological threshold beyond which proline biosynthesis or accumulation may be impaired. In contrast, the remaining species displayed a steady increase in proline with rising salinity, indicating a more robust osmoprotective response through continued proline accumulation (Figure 1). However, no significant differences in proline were detected between controlled and 50 mM conditions in Tagetes patula and Tetraneuris scaposa. The interaction between species and salinity was also significant (F21,128 = 127.477, p < 0.001), also suggesting species-specific strategies for osmotic adjustment under salt stress.

3.2. Enzyme Activities Under Different Salinity Levels

Overall, salinity induced a significant increase in the activity of catalase (CAT), peroxidase (POX), and ascorbate peroxidase (APX), in the leaves of the eight ornamental species studied (Figure 2).
CAT activity showed a marked increase with rising NaCl concentrations in several species, with the most pronounced response observed at 300 mM NaCl, especially in Calendula officinalis, Tagetes patula, and Tetraneuris scaposa, which exhibited 2- to 3-fold increases compared to the controlled conditions (0 mM). In Coreopsis tinctoria and Viguiera stenoloba, enzyme activity was only enhanced at 100 mM NaCl and then stabilized. In contrast, all the remaining species displayed significant increases in the activity of CAT already at 100 mM NaCl.
POX activity was also significantly enhanced under salt stress, particularly at 300 mM NaCl, where Tetraneuris scaposa, and Zinnia elegans exhibited the highest activity. Several species such as Calendula officinalis, Callistephus chinensis, and Tagetes patula showed a sharp increase of POX at 100 mM NaCl followed by stabilization, indicating that a threshold of oxidative response may be reached at moderate salinity levels. Notably, as observed for CAT, POX activity in Aster oblongifolius increased significantly at the highest salinity level (300 mM), suggesting a delayed or more conservative enzymatic response to oxidative stress in this species.
APX activity followed a similar increasing trend across treatments, with Tagetes patula, and Zinnia elegans displaying the highest enzymatic responses at 300 mM NaCl. In contrast, several species, including Calendula officinalis, Callistephus chinensis, Coreopsis tinctoria, Tetraneuris scaposa, and Viguiera stenoloba, exhibited an early rise in APX activity at 100 mM NaCl, with values stabilizing at higher salinity levels, a pattern consistent with a rapid but saturable antioxidative adjustment. Aster oblongifolius also showed a rise in APX activity at 100 mM NaCl but the levels continued to rise at 300 mM NaCl.
Overall, the three enzymes displayed species- and enzyme-specific patterns of induction in response to salinity. We found significant positive relationships between proline content and antioxidant enzyme activities (CAT, POX, and APX) across salinity levels but species-level correlation analysis indicates that the magnitude of this response is highly genotype-dependent (Table S2).

3.3. Effects of Salinity on Plant Height, Flower Production, and Flower Size

Short-term exposure to salinity significantly affected plant height, flower production, and flower size across all species, with responses varying depending on both the species and NaCl concentration (Figure 3A–C).
Plant height was significantly influenced by salinity (Figure 3A). At 50 mM NaCl, most species showed no significant reduction in height compared to the control conditions. The only exceptions were Viguiera stenoloba, Calendula officinalis, and Zinnia elegans, which showed reductions of 9.1%, 5.2%, and 7.1%, respectively (Figure 3A). At 100 mM NaCl, all species displayed significant height reductions, ranging from 9.1% in Tetraneuris scaposa to 34.2% in Calendula officinalis. The most severe effects occurred at 300 mM NaCl, where reductions ranged from 16.1% in Callistephus chinensis to 44.7% in Calendula officinalis (Figure 3A). Wilting was typically observed in older leaves, especially at the highest concentrations.
Flower production also declined with increasing salinity levels (Figure 3B). Even at 50 mM NaCl, flower number was significantly reduced in most species, except for Calendula officinalis, Coreopsis tinctoria, and Tagetes patula, where no significant differences were observed compared to the controlled conditions (Figure 3B). Reductions ranged from 0.6% (Tagetes patula) to 23.5% (Zinnia elegans). At 100 mM NaCl, flower number declined further, ranging from 15.9% in Tagetes patula to 52.9% in Zinnia elegans. At the highest salinity level (300 mM NaCl), flower production was dramatically affected, with reductions ranging from 28.6% in Tetraneuris scaposa to 76.5% in Zinnia elegans. Notably, flower production in Zinnia elegans was almost entirely inhibited under 300 mM NaCl (Figure 3B).
The size of flowers was also negatively affected by salinity, particularly at higher concentrations (Figure 3C). At 50 mM NaCl, flower size was not significantly altered in most species, except for Tetraneuris scaposa, Zinnia elegans, and Callistephus chinensis which showed reductions of 29.7%, 15.3%, and 5.6%, respectively. At 100 mM NaCl, flower diameter decreased significantly across species, with reductions ranging from 8.5% (Tagetes patula) to 33.3% (Tetraneuris scaposa). The effects were more pronounced at 300 mM NaCl, where flower size declined by 17.4% in Tagetes patula and up to 53.2% in Viguiera stenoloba.

4. Discussion

4.1. Antioxidant and Osmoprotective Responses Under Short-Term Salinity Stress

Salinity is a major abiotic stress that compromises plant growth and productivity through osmotic imbalance, ionic toxicity, and oxidative damage [38,39]. Our findings, alongside previously reported results, reinforce that both enzymatic and non-enzymatic antioxidant systems are indispensable in modulating ROS homeostasis under saline conditions [40,41,42]. In our study, we show that catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POX), along with osmolyte proline (Figure 1 and Figure 2), play critical roles in safeguarding cellular function in response to NaCl exposure. CAT plays a central role in detoxifying hydrogen peroxide (H2O2), especially during oxidative bursts triggered by salt stress [43]. Its high turnover rate and substrate independence make CAT particularly effective during acute oxidative bursts [44], being in fact one of the most reactive enzymes in this study. Additionally, POX was also found to be activated by salinity. POX represents a broad class of enzymes involved in H2O2 scavenging and cell wall fortification through lignin biosynthesis [45], playing a dual role in oxidative stress response and structural adaptation [46]. APX, part of the ascorbate–glutathione cycle, uses ascorbate as an electron donor to reduce H2O2, thereby maintaining redox homeostasis [47]. Therefore, its higher activity found in this study as salinity increased, also indicates its involvement in cell detoxification activities.
Enzymatic defenses in the studied species were complemented by increased proline accumulation, which functions as an osmoprotectant, stabilizes proteins and membranes, buffers redox potential, and scavenges hydroxyl radicals [39]. Proline also helps buffer cellular redox potential and serves as a reservoir of energy and carbon during stress recovery [48]. The strong positive correlation observed between antioxidant activity and proline content (Table S2) supports the hypothesis that these mechanisms acted synergistically to maintain cellular integrity under salt stress. This integrated antioxidant and osmoprotective response has also been documented in other stress-tolerant species. For example, Moringa oleifera exposed to 50–100 mM NaCl exhibited increased CAT and superoxide dismutase (SOD) activities alongside elevated glutathione and flavonoid levels, indicating the integrated participation of enzymatic and non-enzymatic antioxidant defenses [49]. In Chenopodium quinoa, a species with high abiotic stress tolerance, antioxidant activity mitigated the effects of progressive salinity stress (100–400 mM NaCl), despite a decline in CO2 net assimilation [50]. Contrastingly, in several salt-sensitive plants, their antioxidant systems failed to respond adequately under salinity stress. For instance, in the roots of the cultivated tomato Lycopersicon esculentum Mill. ‘cv.’ M82, antioxidant activities did not increase under salt stress, leading to severe oxidative damage, whereas in the wild salt-tolerant species, Lycopersicon pennellii (Corr.) D’Arcy accession Atico, the antioxidant response was significantly activated under salinity stress [51]. Overall, the results reinforce the notion that an effective antioxidant and osmoprotective network is a hallmark of salinity tolerance, enabling plants to mitigate oxidative damage, maintain metabolic functions, and recover from stress. However, it is important to note that the current findings are based on short-term NaCl exposure, which primarily reflects immediate physiological responses. While our study provides valuable insights into the early defense mechanisms activated during acute salt stress, long-term exposure to salinity involves complex metabolic and physiological adjustments. For instance, a 15-year field experiment on saline water irrigation in cotton revealed substantial salt accumulation in the soil’s profile, especially under high salinity irrigation regimes (14.1 and 17.7 dS m−1), with salinity levels increasing dramatically to depths of 3.0 m within the root zone [52]. Such prolonged exposure not only challenges plant ion homeostasis but also alters soil structure and nutrient dynamics. Similarly, studies on Ziziphus spina-christi, a drought- and salt-tolerant tree species showed that acclimation to prolonged stress mainly involves shifts in stomatal and mesophyll conductance while biochemical factors contributed less [53]. Notably, under long-term salinity, there was an increase in leaf Na+ content, indicating that ionic accumulation impairs deeper physiological processes beyond ROS scavenging [53]. These findings emphasize the need to examine traits like turgor maintenance and hydraulic conductance in long-term contexts, to better understand plant responses under prolonged salinity exposure.

4.2. Floral Development Is Highly Sensitive to Salinity Despite High Antioxidant Activity

One of the most critical findings in this study was the marked sensitivity of floral traits to salinity, even when antioxidant defenses were strongly activated. While enzymatic (CAT, APX, POX) and non-enzymatic (proline) responses played an important role in maintaining the integrity of vegetative structures, they were insufficient to prevent significant declines in flower number, size, and overall reproductive output—especially under 100 mM NaCl. Floral traits proved to be more sensitive than vegetative growth across all the species tested. For example, in Zinnia elegans and Calendula officinalis, flower production was almost completely suppressed at higher salinity levels, even when plant height remained relatively stable. These results are in line with findings in three hybrid lily (Lilium × elegans) cultivars exposed to increasing salinity (0.65, 3, and 6 dS m−1), where flower size, number of blooms per stem, and vase life diminished significantly [54]. However, some aesthetic traits such as petal coloration (e.g., increased redness and yellowness) were enhanced under high salinity [54]. In other Tagetes patula cultivars, salinity stress (up to 300 mM NaCl) led to declines in plant growth and flowering across three cultivars (Aurora Orange, Fireball, Safari Scarlet), even though antioxidant enzyme activities (CAT, POX, APX) and the levels of carotenoids, polyphenols, and flavonoids increased under salinity stress [19]. Similarly, in Calendula officinalis, different genotypes (Indian Prince, Golden Emperor, Orange Prince, Sun Glow) subjected to several drought conditions exhibited severe reductions in flower production, despite elevated antioxidant activity, proline accumulation, and increased lipid peroxidation [24].
This differential sensitivity highlights that floral organs are more vulnerable than vegetative parts and are likely governed by stress-sensitive pathways [55]. Previous reports showed that reproductive organs rely on highly regulated hormonal and developmental pathways that are easily disrupted under stress [10]. Among these are hormonal imbalances, such as altered levels of auxins, gibberellins, abscisic acid (ABA), and ethylene, which play crucial roles in flower initiation, development, and senescence [56]. In Hibiscus rosa-sinensis, for instance, salt stress at 200 mM NaCl markedly impaired flower production in the sensitive cultivar ‘Sunny Wind’, while the more tolerant ‘Porto’ cultivar maintained flowering capacity [57]. The tolerant cultivar exhibited lower electrolyte leakage and ABA accumulation and higher proline levels in its leaves. However, ABA concentration was higher in the petals and ovary of flowers in both cultivars, especially in the sensitive one, highlighting that hormonal regulation and tissue-specific ion exclusion are likely critical for preserving flower development [57].
Beyond hormonal imbalances, salinity may also compromise floral meristem activity [58], reduce carbohydrate allocation to reproductive organs [59], and disrupt gene expression involved in floral identity and development [60]. These molecular disruptions can result in early floral abortion, impaired gametogenesis, reduced fertility, and ultimately, reproductive failure [61], which could also explain the drop in flower production seen in this study. This complexity underscores that antioxidant activation, while necessary, is not sufficient to ensure reproductive success under salinity stress.
Collectively, these findings highlight a critical gap in our current understanding of salinity tolerance: while much research has focused on vegetative stress responses, reproductive development remains a major bottleneck for stress resilience. From a breeding and agronomic perspective, it is essential to expand our focus to include reproductive-specific tolerance mechanisms. Conventional breeding methods, such as selection and crossing, could be used to develop salt-tolerant varieties. Marker-assisted selection and more specifically, Marker-Assisted Backcrossing where molecular markers are used to track and select specific genomic regions (QTLs) from a salt-tolerant donor to a recurrent (salt-sensitive) parent, allows for the targeted introduction of traits linked to floral tolerance while minimizing the introduction of undesirable traits from the donor parent [62]. Thus, future strategies should aim not only to enhance antioxidant defenses but also target key genetic and hormonal regulators of flowering. This includes ABA metabolism, sugar transporters, floral meristem identity genes, and transcription factors that govern flower initiation and maintenance under stress. Additionally, wild relatives with inherent floral-stage salinity tolerance could be valuable genetic resources in breeding efforts [63].

4.3. Species-Specific Strategies Reflect Genotypic Variation in Tolerance

The differential responses among the eight Asteraceae species revealed different adaptive strategies to cope with saline conditions. Overall, we found a species-specific pattern of induction in response to salinity. For instance, while all species showed some degree of enzymatic upregulation, Tagetes patula consistently displayed strong responses across CAT, POX, and APX activities, suggesting a particularly robust and coordinated antioxidant defense system. In contrast, species such as Callistephus chinensis, Coreopsis tinctoria, and Viguiera stenoloba demonstrated early enzyme activation at moderate salinity levels (100 mM NaCl), followed by stabilization, indicative of a rapid but possibly saturable oxidative stress response. Meanwhile, Aster oblongifolius was characterized by a delayed enzymatic response, with significant increases only at the highest salinity level (300 mM NaCl), pointing to a more conservative or high-threshold stress perception mechanism. Collectively, these findings reinforce that increased antioxidant enzyme activity forms a central component of salinity tolerance in these ornamental species, with variations in timing and intensity reflecting species-specific strategies.
In relation to floral traits, while Zinnia elegans and Aster oblongifolius were highly sensitive to salinity conditions, having substantial declines in growth and flower production, other species such as Tagetes patula and Tetraneuris scaposa exhibited some tolerance, maintaining vegetative and floral development under at least moderate salinity conditions (Figure 3). This suggests the potential of these species as candidates for cultivation in regions where brackish or low-salinity water must be used for irrigation.
These contrasting responses reflect species-specific and genotype-dependent strategies in stress perception, ion exclusion, osmotic adjustment, and redox regulation. For instance, although chickpea (Cicer arietinum L.) is usually very sensitive to salinity with most genotypes dying in just 25 mM NaCl, some can survive under 100 mM NaCl in hydroponics [64]. Previous studies also highlighted that in many salt-sensitive crop genotypes, antioxidant responses are delayed or insufficient, resulting in excessive ROS accumulation and membrane damage [46]. For instance, in salt-tolerant rice cultivars, higher proline levels and antioxidant enzyme activities, including those of CAT and APX, were associated with improved stress tolerance compared to the sensitive cultivar, in which CAT and APX activities were significantly reduced under salinity stress [65]. Comparable findings were reported in a study involving ten sorghum (Sorghum bicolor L.) genotypes, where exposure to different salinity levels revealed significant variation in stress responses [66]. These observations reinforce that while antioxidant activation is a common protective mechanism, its efficiency, magnitude, and timing are highly genotype-dependent. Breeding for salt tolerance in ornamental and crop species will benefit from a multifactorial approach that captures this complexity.

4.4. Implications of Short-Term Saline Irrigation in Ornamental Floriculture and Future Directions

Irrigating with saline water presents a multifaceted challenge for the ornamental horticulture industry, offering both potential applications and significant limitations [18]. For instance, in semi-arid and arid regions, moderate saline irrigation has been explored as a cost-effective technique to modulate plant architecture, such as limiting excessive stem elongation, to promote more compact and visually uniform plants that are often preferred in commercial markets [67]. However, the implications for ornamental plant quality are also significant. Our findings underscore a critical limitation: salinity induced significant reductions in flower number, size, and diameter, directly compromising the visual appeal and marketability of these species. Furthermore, floral color, symmetry, longevity, and scent, although not evaluated in the present study, are also key quality traits in many commercial ornamentals, which may also be affected by salt stress. Unlike vegetative traits, which may remain visually acceptable under stress, changes in these traits, as well as in petal pigmentation due to ion toxicity or osmotic imbalance, can potentially diminish color vibrancy or alter hue, reducing consumer acceptance. These effects not only reduce the sustainability of ornamental production systems but may also limit the feasibility of reusing substrates or growing successive crops.
To ensure the sustainable application of saline irrigation in ornamental horticulture, it is essential to systematically evaluate floral responses under different salinity regimes. This includes not only quantifying flower number and size but also assessing floral color intensity, pigment profiles, opening duration, and reproductive fertility. Such a comprehensive evaluation should be standardized across species to allow for better comparison and selection of salt-tolerant genotypes with retained ornamental quality. Moreover, we should also test the effects of prolonged or repeated exposure to salinity in flowers. Thus, a multifaceted strategy is needed: integrating the screening of salt-tolerant cultivars, refining growth media to improve buffering capacity, and employing precision irrigation systems that limit salt accumulation while meeting water requirements.

5. Conclusions

This study demonstrates that short-term exposure to salinity triggers species-specific responses in eight ornamental Asteraceae species, revealing a complex interplay between osmotic regulation, antioxidant defense, and morphological performance.
Proline increased significantly under salt stress in all species, though the pattern varied. For example, in Aster oblongifolius, Calendula officinalis, and Callistephus chinensis, proline peaked at 50 mM NaCl but declined at higher concentrations. In contrast, Tagetes patula and Zinnia elegans showed a continuous increase, suggesting stronger osmoprotective mechanisms. Antioxidant enzyme activities (CAT, POX, APX) also rose in response to salinity, with increases up to 2–3-fold observed at 300 mM NaCl in species such as Calendula officinalis and Tetraneuris scaposa. These biochemical responses suggest the activation of oxidative stress defenses but showed variation in onset and magnitude among species.
Morphologically, reproductive traits were particularly sensitive. At 300 mM NaCl, flower production was reduced by up to 76.5% in Zinnia elegans, and flower size declined by over 50% in Viguiera stenoloba. In contrast, plant height was less affected at moderate salinity levels, with significant reductions only appearing at or above 100 mM NaCl.
Given the central importance of flowers in ornamental value, these findings highlight a key limitation of saline irrigation: reproductive traits are more vulnerable than vegetative traits, directly affecting plant marketability. To ensure ornamental production under saline conditions, it is essential to combine salt-tolerant genotype selection, substrate management, and precision irrigation.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/agronomy15081802/s1: Table S1. One-way ANOVA results of the effect of salinity level on leaf proline content within each species. For each species, salinity treatments (0, 50, 100, and 300 mM NaCl) were compared to assess their impact on proline accumulation. Table S2. Pearson correlation coefficients (r) between proline content and antioxidant enzyme activities (CAT, POX, APX) for each species across salinity treatments.

Author Contributions

Conceptualization, M.R.G.; methodology; M.R.G.; formal analysis, C.G.-D., X.R.-R. and M.R.G.; investigation, C.G.-D. and M.R.G.; data curation, X.R.-R. and M.R.G.; writing—original draft preparation, M.R.G. and I.M.; writing—review and editing, all.; supervision, I.M.; project administration, I.M.; funding acquisition, I.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received national funds through the FCT—Fundação para a Ciência e a Tecnologia, I.P., Portugal, through the research unit UID/00239 (CEF), LA/P/0092/2020 (TERRA) and under the Scientific Employment Stimulus—Individual Call (CEEC Individual)—2021.01107.CEECIND/CP1689/CT0001 (IM).

Data Availability Statement

Data are contained within this article or the Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of this study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Increase in proline content (μg gDW−1) after a 10-day exposure to 50, 100, and 300 mM NaCl in comparison to control conditions (0 mM NaCl) on the leaves of Aster oblongifolius (AST), Calendula officinalis (CAL), Callistephus chinensis (CLI), Coreopsis tinctoria (COR), Tagetes patula (TAG), Tetraneuris scaposa (TET), Viguiera stenoloba (VIG), and Zinnia elegans (ZIN). Results are expressed as mean ± SE (n = 10). Different superscript letters indicate statistically significant differences among salinity treatments within the same species (p < 0.05).
Figure 1. Increase in proline content (μg gDW−1) after a 10-day exposure to 50, 100, and 300 mM NaCl in comparison to control conditions (0 mM NaCl) on the leaves of Aster oblongifolius (AST), Calendula officinalis (CAL), Callistephus chinensis (CLI), Coreopsis tinctoria (COR), Tagetes patula (TAG), Tetraneuris scaposa (TET), Viguiera stenoloba (VIG), and Zinnia elegans (ZIN). Results are expressed as mean ± SE (n = 10). Different superscript letters indicate statistically significant differences among salinity treatments within the same species (p < 0.05).
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Figure 2. Increase in the activity of the enzymes catalase (CAT), peroxidase (POX), and ascorbate peroxidase (APX) after a 10-day exposure to 50, 100, and 300 mM NaCl in comparison to control conditions (0 mM NaCl) on the leaves of Aster oblongifolius (AST), Calendula officinalis (CAL), Callistephus chinensis (CLI), Coreopsis tinctoria (COR), Tagetes patula (TAG), Tetraneuris scaposa (TET), Viguiera stenoloba (VIG), and Zinnia elegans (ZIN). Results are expressed as mean ± SE (n = 10). Different superscript letters indicate statistically significant differences among salinity treatments within the same species (p < 0.05).
Figure 2. Increase in the activity of the enzymes catalase (CAT), peroxidase (POX), and ascorbate peroxidase (APX) after a 10-day exposure to 50, 100, and 300 mM NaCl in comparison to control conditions (0 mM NaCl) on the leaves of Aster oblongifolius (AST), Calendula officinalis (CAL), Callistephus chinensis (CLI), Coreopsis tinctoria (COR), Tagetes patula (TAG), Tetraneuris scaposa (TET), Viguiera stenoloba (VIG), and Zinnia elegans (ZIN). Results are expressed as mean ± SE (n = 10). Different superscript letters indicate statistically significant differences among salinity treatments within the same species (p < 0.05).
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Figure 3. Impact of salinity (0, 50, 100, and 300 mM NaCl) on plant height (A), number of flowers (B), and flower diameter (C) on plants of Aster oblongifolius (AST), Calendula officinalis (CAL), Callistephus chinensis (CLI), Coreopsis tinctoria (COR), Tagetes patula (TAG), Tetraneuris scaposa (TET), Viguiera stenoloba (VIG), and Zinnia elegans (ZIN), grown under controlled conditions for 10 days. Results are expressed as mean ± SE (n = 10). Different superscripts indicate significant differences between salinity levels for the same species (p < 0.05). Different superscript letters indicate statistically significant differences among salinity treatments within the same species (p < 0.05). A visual comparison was selected to highlight the contrasting differences between the inflorescences grown under non-saline, controlled conditions (left: 0 mM NaCl) and the highest salinity level tested (right: 300 mM NaCl).
Figure 3. Impact of salinity (0, 50, 100, and 300 mM NaCl) on plant height (A), number of flowers (B), and flower diameter (C) on plants of Aster oblongifolius (AST), Calendula officinalis (CAL), Callistephus chinensis (CLI), Coreopsis tinctoria (COR), Tagetes patula (TAG), Tetraneuris scaposa (TET), Viguiera stenoloba (VIG), and Zinnia elegans (ZIN), grown under controlled conditions for 10 days. Results are expressed as mean ± SE (n = 10). Different superscripts indicate significant differences between salinity levels for the same species (p < 0.05). Different superscript letters indicate statistically significant differences among salinity treatments within the same species (p < 0.05). A visual comparison was selected to highlight the contrasting differences between the inflorescences grown under non-saline, controlled conditions (left: 0 mM NaCl) and the highest salinity level tested (right: 300 mM NaCl).
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Guzman, M.R.; Rojas-Ruilova, X.; Gomes-Domingues, C.; Marques, I. Using Saline Water for Sustainable Floriculture: Identifying Physiological Thresholds and Floral Performance in Eight Asteraceae Species. Agronomy 2025, 15, 1802. https://doi.org/10.3390/agronomy15081802

AMA Style

Guzman MR, Rojas-Ruilova X, Gomes-Domingues C, Marques I. Using Saline Water for Sustainable Floriculture: Identifying Physiological Thresholds and Floral Performance in Eight Asteraceae Species. Agronomy. 2025; 15(8):1802. https://doi.org/10.3390/agronomy15081802

Chicago/Turabian Style

Guzman, María Rita, Xavier Rojas-Ruilova, Catarina Gomes-Domingues, and Isabel Marques. 2025. "Using Saline Water for Sustainable Floriculture: Identifying Physiological Thresholds and Floral Performance in Eight Asteraceae Species" Agronomy 15, no. 8: 1802. https://doi.org/10.3390/agronomy15081802

APA Style

Guzman, M. R., Rojas-Ruilova, X., Gomes-Domingues, C., & Marques, I. (2025). Using Saline Water for Sustainable Floriculture: Identifying Physiological Thresholds and Floral Performance in Eight Asteraceae Species. Agronomy, 15(8), 1802. https://doi.org/10.3390/agronomy15081802

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