Integrated Physiological and Ornamental Responses of Cestrum nocturnum to Salinity Stress: Identifying Tolerance Thresholds Under Saline Irrigation

ABSTRACT

The use of saline water for irrigation is an increasingly important strategy to reduce freshwater consumption in ornamental horticulture; however, the tolerance and physiological responses of many species remain poorly understood. This study provides the first comprehensive assessment of the response of Cestrum nocturnum to saline irrigation under nursery conditions, integrating physiological, nutritional, and ornamental quality parameters. Plants were subjected to three salinity levels (2.1, 4.7, and 7.2 dS m⁻¹) over a 10‐week period. The results reveal that C. nocturnum activates early adaptive mechanisms, including stomatal regulation and changes in leaf hydraulic function, in response to salinity. A key finding is the identification of a threshold response: moderate salinity (4.7 dS m⁻¹) caused only minor reductions in biomass with limited impact on plant quality, whereas high salinity (7.2 dS m⁻¹) led to severe photochemical stress, ion toxicity (Na and Cl accumulation), chlorosis, necrosis, and a strong decline in growth and ornamental value. By linking physiological processes with commercial quality traits, this study fills a critical knowledge gap and establishes practical salinity limits for this species. These results support the use of moderately saline water in C. nocturnum production while defining the thresholds beyond which plant performance is compromised.

1. Introduction

Nurseries play a crucial role in the global effort to mitigate and adapt to climate change, as they serve as foundational systems for propagating plant species that can thrive under increasingly unpredictable environmental conditions. Climate change has already altered temperature regimes, rainfall patterns, and soil dynamics, which directly affect plant survival and productivity (Allen et al. 2010). Species respond to these adverse conditions through a combination of physiological, morphological, and genetic adaptations; for example, shifts in phenology, increased abiotic stress resistance, and enhanced root development are commonly observed (Nicotra et al. 2010; Acosta‐Motos et al. 2017). However, not all species exhibit sufficient plasticity, and many face heightened vulnerability to extreme events such as prolonged droughts, heatwaves, or salinity stress (Parmesan 2006). Within this context, plant nurseries function not only as sites for mass propagation but also as testing grounds for evaluating resilience and adaptive traits (Leakey et al. 2019). Through controlled experimentation, nurseries can identify genotypes and populations with superior tolerance to stress, which can then be prioritized in reforestation, restoration, or agricultural projects (Aitken and Whitlock 2013). Furthermore, advances in plant biotechnology and selective breeding are accelerating the development of more tolerant species, thereby increasing the adaptive capacity of ecosystems and food systems (Zhu 2016). The selection and cultivation of such resilient plant material in nurseries enhance carbon sequestration, support biodiversity, and safeguard ecosystem services critical for human well‐being (Bauhus et al. 2010). Thus, nurseries represent a vital interface between conservation science, ecological restoration, and climate adaptation strategies, bridging the gap between experimental research and large‐scale environmental solutions.

The use of saline or low‐quality water in the production of ornamental plants presents significant challenges that can negatively affect plant growth, flowering, biomass accumulation, and overall esthetic value. HS levels can lead to ion toxicity, osmotic stress, and nutrient imbalances, resulting in visible symptoms such as chlorosis, necrosis, early senescence, and reduced flowering (Niu and Cabrera 2010; Rouphael et al. 2012), reducing plant biomass and quality (Grieve et al. 2012). These physiological disturbances ultimately compromise the marketability of ornamental crops, which heavily depend on their visual appeal.

To mitigate these adverse effects, growers can adopt several management strategies, including the selection of salt‐tolerant cultivars (Shannon et al. 1994; Rouphael and Colla 2020). Indeed, plants may exhibit diverse physiological and biochemical strategies to cope with salinity stress, which vary even between varieties from the same species. In this sense, salt tolerance in species of pharmaceutical and ornamental interest remains underexplored, despite its enormous potential for crop diversification in areas affected by salt (soil salinization or the use of water with high electrical conductivity) (Emami Bistgani et al. 2023). For example, in ornamental species, the flowering and esthetic value can be compromised very quickly at salinity levels as low as 50 mM NaCl, as documented in a study with eight species of the Asteraceae family, some of them with medicinal uses, where a reduction in the number and size of flowers was evident (Guzmán and Marques 2023; Guzmán et al. 2025). Similarly, a medicinal plant, such as Grangea maderaspatana, has demonstrated surprising tolerance up to 400 mM NaCl, with mechanisms of osmotic adjustment, ionic homeostasis, and activation of antioxidant defenses, opening the door to its cultivation in saline environments (Tran, Van Hoang, and Phan 2024). However, these cases remain isolated, and generalizing to other species of pharmaceutical or ornamental interest is very difficult due to the scarcity of studies. This lack of information presents a clear opportunity to highlight the value of these species: identifying tolerant genotypes, characterizing their physiological and biochemical mechanisms in response to saline stress, and leveraging these qualities to develop sustainable crops in degraded soils or with saline water (Ghassemi‐Golezani and Abdoli 2022). This would not only contribute to the conservation of undervalued species but also to the diversification of the pharmaceutical, phytocosmetic, and ornamental markets under marginal growing conditions.

C. nocturnum L., commonly known as night‐blooming jessamine, is an evergreen shrub native to Central America and the Caribbean, whose physiological response to irrigation with saline water is still unknown. It thrives in tropical and subtropical climates, growing in moist forests, disturbed areas, and along forest edges. C. nocturnum prefers well‐drained, neutral pH soils and grows best in full sun to partial shade. Morphologically, it reaches up to 5 m in height, with lanceolate, glabrous leaves and tubular, greenish‐white flowers that emit a strong fragrance at night. It is used in gardens, as hedges and for esthetic landscaping. On the top of its ornamental value, various parts of the plant are used in traditional medicine for their purported pharmacological properties (Fakhrah et al. 2024) (antioxidant, hepatoprotective, analgesic, antibacterial, antifungal, anticonvulsant, anti‐hyperlipidemic, larvicidal, etc.). However, these uses are often empirical, and caution is advised because parts of the plant are known to be toxic. It is also used in the perfume industry as flower extracts (e.g., attars), although its toxicity can limit certain uses.

This study aimed to address the lack of information on salinity tolerance in C. nocturnum by evaluating the viability of saline irrigation under nursery conditions, combining physiological, nutritional, and ornamental quality assessments.

2. Materials and Methods

2.1. Plant Material and Growing Conditions

The experiment was carried out in a greenhouse located at the experimental farm of the Polytechnic University of Cartagena (Cartagena, Murcia, Spain). C. nocturnum L. plants measuring approximately 10 cm in height were selected from a nearby nursery (Viveros Bermejo S.L., Spain) and transplanted into 2.5 L black plastic pots with a substrate composed of peat, coconut fiber, and perlite in a 60:30:10 (v/v) ratio. Transplanting took place during the third week of January 2024 and plants were growing until the last week of April. The pots were placed in holes of the metal grids, supported by concrete blocks that kept them 80 cm above the ground. The temperature and relative humidity (RH) inside the greenhouse were recorded using a LOG 32 TH data logger (Dostmann Electronic). Microclimatic data recorded in the greenhouse during the period from February 21 to April 24 indicated that RH fluctuated between a minimum of 15% and a maximum of 90%, with a mean of 45%–50%. Regarding temperatures, values ranged from a minimum of 14°C to a maximum of 27°C, with a mean of 22°C.

2.2. Experimental Design and Application of Treatments

A total of 90 plants were randomly attributed to three treatments (30 plants per treatment). Each replication consisted of a row of 10 plants (3 replications per treatment). The sensors were inserted vertically into randomly selected pots, always at the same cardinal position and distance from the emitter. There was one sensor per pot and three sensors per treatment (each per replication).

On February 14, 2024, three irrigation treatments were applied: Control (C, 2.1 dS m⁻¹), moderate salinity (MS, 4.7 dS m⁻¹), and HS (7.2 dS m⁻¹). Nutrient solutions were prepared from irrigation water with an initial electrical conductivity (EC) of 1.2 dS m⁻¹, to which a liquid fertilizer (4–1.7‐4.5‐4‐4‐1.4 N‐P₂O₅‐K₂O‐CaO‐MgO) was added until an increase of 0.8 dS m⁻¹ in EC was achieved. To achieve the electrical conductivities corresponding to each treatment, sodium chloride (NaCl) was incorporated into the irrigation water (Table 1). The pH was adjusted to approximately 6 using nitric acid. Following salt addition, sodium concentration increased from 118.3 mg L⁻¹ in the Control treatment to 591.2 mg L⁻¹ in MS treatment and 1073.1 mg L⁻¹ in HS treatment. Similarly, chloride content increased from 209.5 to 1030.3 and 1886.2 mg L⁻¹, respectively. The experiment ended on April 24, 2024, after 10 weeks from the imposition of treatments, and 13 weeks from the transplanting.

TABLE 1.

Physico‐chemical analyses of irrigation water.

Parameter	C	MS	HS
pH	5.98	5.88	5.84
EC (dS m−1)	2095	4701	7185
Ca (mg L−1)	516.16	484,22	477.31
Mg (mg L−1)	35.95	33.37	34.11
Na (mg L−1)	118.30	591.20	1073.10
Fe (mg L−1)	0.59	0.48	0.45
K (mg L−1)	81.63	78.72	79.25
P (mg L−1)	18.73	16.86	16.22
Mn (mg L−1)	0.55	0.53	0.51
B (mg L−1)	4.38	4.67	3.50
S (mg L−1)	126.60	126.20	130.10
Zn (mg L−1)	0.45	0.38	0.42
Ni (mg L−1)	0.01	< 0.01	< 0.01
Cl− (mg L−1)	209.52	1030.30	1886.20
NO2 − (mg L−1)	< 0.1	< 0.1	< 0.1
NO3 − (mg L−1)	420.00	349.20	354.70
PO4 3− (mg L−1)	58.04	48.68	52.98
SO4 2− (mg L−1)	407.80	418.50	424.10
Br− (mg L−1)	0.64	0.63	< 0.05

2.3. Irrigation Management

A drip irrigation system with a self‐compensating, anti‐drain emitter (Netafim Ltd.) of 2.3 L h⁻¹ per pot was used. Irrigation was managed independently for each treatment based on a volumetric water content (VWC) threshold of 36%, calculated as the average of the three Teros‐12 sensors per treatment. Teros‐12 sensors were installed in the potting soil and connected to a CR1000 datalogger to estimate VWC. The VWC was estimated using a substrate‐specific calibration equation developed from permittivity and moisture content measurements, following the methodology of Valdés et al. (2014). To facilitate leachate collection and ensure proper system verification across all treatments, VWC levels were checked daily at a fixed time (11:00 AM). The volume of water applied per irrigation event was the same for all treatments. But the irrigation was triggered only in treatments in which average VWC reached the threshold at that time. There was a total of 37, 25, and 17 irrigation events for the control, MS, and HS treatments, respectively. Therefore, irrigation frequency depended on the specific rate of water depletion in the substrate for each treatment, allowing for independent irrigation scheduling. The daily evolution of VWC, monitored using Teros 12 sensors, is shown in Figure 1. Overall, substrate moisture remained within adequate levels, suggesting no water limitation for the plants. However, clear differences were observed among treatments in the temporal pattern of VWC.

FIGURE 1.

Evolution of the daily volumetric water content (VWC) for the 3 salinity levels studied. Values are means of three pots per treatment. Vertical bars indicate standard errors and arrows indicate the timing of the physiological measurements performed.

The total amount of water applied was estimated considering the dripper flow rate and irrigation time, with quantities that reached 27.83 L pot⁻¹ in the control treatment, 18.48 in MS treatment and 12.34 L pot⁻¹ in HS treatment. Leachate was collected weekly in plastic containers under the pots, and its volume and EC were determined. Leachate volumes in the experiment ranged from 6.05 L pot⁻¹ in the control to 3.46 L pot⁻¹ in the HS treatment. The leaching fraction was quantified as the ratio of leached water divided by the total water applied to the pot. Consequently, leaching fractions were 21.74%, 29.64%, and 28.04% for the Control, MS, and HS treatments, respectively, indicating a differentiated adjustment of the water balance under saline stress. Water consumption was calculated by subtracting the total leachate from the total water applied, with values of 21.78, 13.00, and 8.88 L pot⁻¹ for the control, MS, and HS treatments, respectively.

2.4. Physiological Determinations

2.4.1. Water Relations

Leaf water relations were measured throughout the experiment in six plants per treatment (two plants per replication). Leaf water potential (Ψ_leaf) was measured at midday, collecting a mature leaf according to Scholander et al. (1965) using a pressure chamber (Model 3000; Soil Moisture Equipment Co.). Leaves were placed in the chamber within 20 s of collection and pressurized at a rate of 0.02 MPa s⁻¹ (Turner 1988). Stem water potential (Ψ_stem) was measured in the same way as leaf water potential was measured, with the difference that leaves were covered with foil approximately 2 h before the measurement. Adjacent leaves were also collected, placed in distilled water for 24–48 h to reach full saturation, and frozen in liquid nitrogen (−196°C). After thawing, leaf osmotic potential at full turgor (Ψ_100s) was measured in the extracted sap using a WESCOR 5520 vapor pressure osmometer (Wescor Inc.), according to Gucci et al. (1991).

Leaf stomatal conductance and fluorescence parameters: Leaf stomatal conductance (g _s), transpiration rate (E), and the effective quantum efficiency of photosystem II (Phi PSII) were measured periodically at midday with a porometer LI‐600, in nine plants per treatment (three per replication).

Leaf hydraulic conductance (K _leaf; mmol m⁻² s⁻¹ MPa⁻¹) was measured periodically on four to five plants per treatment using the evaporative flux method according to Brodribb and Holbrook (2003):

$$K_{\text{leaf}}=E/{\text{∆Ψ}}_{\text{stem}-\text{leaf}}$$

where E is the transpiration rate (mmol m⁻² s⁻¹), and ΔΨ_stem–leaf is the difference between stem water potential (Ψ_stem; MPa) and leaf water potential (Ψ_leaf; MPa).

2.5. Nutritional Uptake

At the end of the experiment, the inorganic solute content in leaves was determined from the dry mass in five plants per treatment. The concentrations of Na⁺, K⁺, P, N, Ca²⁺, Mg²⁺ and B were determined in a digestion extract with HNO₃:HClO₄ (2:1, v/v) by Inductively Coupled Plasma optical emission spectrometer (ICP‐OES IRIS INTREPID II XDL Thermo, England). The concentration of Cl⁻ was analyzed by chloride analyzer (Chloride Analyser Model 926, Sherwood Scientific Ltd.) in the aqueous extracts obtained after mixing 100 mg of dry vegetable powder with 40 mL of water and then shaking for 30 min and filtering.

2.6. Growth and Plant Quality Indicators

2.6.1. Growth Parameters

At the end of the experiment, five plants per treatment were selected to determine growth parameters. To calculate dry biomass, shoots (leaves and stems) and roots were separated and washed with water before being oven‐dried at 80°C until constant weight (Scale Sartorius Entris). Leaf area was calculated using Easy Leaf Area software (Easlon and Bloom 2014), based on digital image analysis. The number of leaves and shoots was counted and shoot length was measured.

2.6.2. Plant Quality Indicators

At the end of the experiment, the number of plants with inflorescences and the number of plants with foliar necrosis were determined. To assess the presence of leaf chlorosis, the relative chlorophyll content (RCC, SPAD) in leaves was determined in six plants per treatment using a Minolta SPAD‐502 chlorophyll meter (Konica Minolta Sensing Inc.) and the leaf color was determined in six plants per treatment with a Konica Minolta CR‐10 colorimeter (Konica Minolta Sensing Europe B.V.), which provided the color coordinates lightness (L*), chroma (C*) and hue angle (h°) (McGuire 1992).

2.7. Statistical Analysis

Plants were distributed using a randomized complete block design, with 30 plants per treatment and three replications per treatment (10 plants per replication). Data were analyzed using a one‐way ANOVA with Statgraphics Centurion software (v. XVI, StatPoint Technologies). When significant differences were detected (p < 0.05), the least significant difference (LSD) test was applied to compare treatment means. All graphs were created using SigmaPlot software (v. 14.5, Systat Software Inc.).

3. Results

3.1. Physiological Determinations

Seven weeks after the application of the saline treatments, the plants subjected to MS and HS treatment showed a decrease in the leaf water potential (Ψ_leaf) and stem water potential (Ψ_stem) compared to those irrigated (control), although the values from these parameters did not reach 1 MPa (Figure 2A,B). The values of the osmotic potential at full turgor in plants of the HS treatment gradually decreased from week 3 compared to plants from the control treatment (Figure 2C). The lowest values corresponded to the HS treatment at the end of the experiment, reaching around −1.58 MPa. A decreasing trend in this parameter was observed in the MS treatment compared to the control treatment (Figure 2C).

FIGURE 2.

Leaf water potential (Ψ_leaf) (A), stem water potential (Ψ_stem) (B), and leaf osmotic potential at full turgor (Ψ_100s) (C) of Cestrum nocturnum plants irrigated with 3 salinity levels throughout the experiment. Values are means of six plants per treatment. Vertical bars indicate standard errors, and different lowercase letters indicate significant differences between treatments according to Duncan0.05 test.

While plants from control treatment reached maximum values of leaf stomatal conductance up to 91.5 mmol m⁻² s⁻¹ (week 1), a significant reduction of leaf stomatal conductance (g _s) was observed in plants of MS and HS treatments compared to those irrigated with control treatment from week 3 onwards (Figure 3A). This reduction was more evident in HS treatment, whose g _s values were close to 4 mmol m⁻² s⁻¹ at the end of the experiment. From week 7, the effective quantum efficiency of photosystem II (Phi PSII) showed lower values in plants from HS treatment than in plants from Control and MS treatments (Figure 3B).

FIGURE 3.

Leaf stomatal conductance (g _s) (A) and the effective quantum efficiency of photosystem II (PhiPS II) (B) of Cestrum nocturnum plants irrigated with 3 salinity levels throughout the experiment. Values are means of nine plants per treatment. Vertical bars indicate standard errors and different lowercase letters indicate significant differences between treatments according to Duncan0.05 test.

The transpiration rate (E) followed the same dynamics as the stomatal conductance throughout the experiment, with the lowest values corresponding to the HS treatment (Figure 4A). In general, leaf hydraulic conductance was higher at the beginning of the experiment, and it declined in all treatments throughout the experiment (Figure 4B). Curiously, plants from the HS treatment showed the highest values at weeks 3 and 5, and the lowest values were observed at weeks 7 and 10. In addition, a strong positive correlation between the difference between stem and leaf water potential (∆Ψ_stem‐leaf) and transpiration (E) was observed for each treatment, especially for the HS treatment (Figure 5).

FIGURE 4.

Leaf transpiration rate (E) (A) and leaf hydraulic conductance (K _leaf) (B) of Cestrum nocturnum plants irrigated with 3 salinity levels throughout the experiment. In (A), values are means of nine plants per treatment. Vertical bars indicate standard errors, and different lowercase letters indicate significant differences between treatments according to the Duncan0.05 test.

FIGURE 5.

Relationship between the difference between stem water potential and leaf water potential (ΔΨ_stem–leaf) and the transpiration rate of Cestrum nocturnum plants irrigated with 3 salinity levels.

3.2. Nutritional Uptake

At the end of the experiment, Na accumulated in the roots of plants in the MS and HS treatments to a greater extent than in plants in the control treatment (Figure 6A). In general, this accumulation was higher in roots than in leaves, although plants in the HS treatment accumulated more Na in leaves than the plants of the control treatment (multiplying by 71) and those of the MS treatment (multiplying by 10) (Figure 6B). The accumulation of Cl followed a similar trend to the accumulation of Na, although Cl concentrations in both roots and leaves were much higher than those of Na (Figure 6C,D). The lowest values of K/Na and Ca/Na ratios in roots belonged to the MS treatment (Figure 6E,G), while the lowest values in leaves were found in the HS treatment (Figure 6F,H). In leaves, these ratios decreased as irrigation water salinity increased.

FIGURE 6.

Na and Cl content and K/Na and Ca/Na ratios in roots (A, C, E, G) and leaves (B, D, F, H) of Cestrum nocturnum plants irrigated with 3 salinity levels at the end of the experiment. Values are means of five plants per treatment. Vertical bars indicate standard errors and different lowercase letters indicate significant differences between treatments according to Duncan0.05 test.

At the end of the experiment, leaves accumulated more K, Mg, Ca, and P than roots (Figure 7). In the roots, HS‐treated plants had similar or lower concentrations of K, Mg, and Ca than plants of the control and MS treatments. However, in leaves, they had the highest concentrations of K and Ca, while the concentration of Mg was similar between treatments. Instead, the content of P in plants of the HS treatment decreased in both roots and leaves compared to those from control and MS treatment (Figure 7G,H).

FIGURE 7.

K, Mg, Ca and P content in roots (A, C, E, G) and leaves (B, D, F, H) of Cestrum nocturnum plants irrigated with 3 salinity levels at the end of the experiment. Values are means of five plants per treatment. Vertical bars indicate standard errors and different lowercase letters indicate significant differences between treatments according to Duncan0.05 test.

3.3. Growth and Plant Quality Indicators

Plant, shoot and root dry weight decreased as the salinity level in irrigation water increased (Table 2). Plants of the MS and HS treatment reduced their dry weight by 42% and 67%, respectively, compared to the control treatment. The number of shoots per plant, shoot length and leaf area were also reduced by salinity, especially leaf area (Table 2). Specifically, the leaf area of plants of the HS treatment was reduced by 63% compared to the leaf area of the control plants. The plants of MS treatment doubled the percentage of leaves with necrosis compared to the control treatment plants, while this percentage was 10 times higher in the plants of HS treatment (Table 2). The plants of MS and HS treatment increased the percentage of plants with inflorescences by 42% and 28%, respectively, compared to plants of the control treatment (Table 2).

TABLE 2.

Growth parameters and plant quality indicators of Cestrum nocturnum plants irrigated with 3 salinity levels at the end of the experiment.

Parameter	C	MS	HS	P
Plant DW (g)	80.00a	46.24b	26.80c	***
Shoot DW (g)	63.40a	35.80b	20.60c	**
Root DW (g)	16.60a	10.44b	6.20c	**
Number of shoots plant−1	9.20a	9.00a	7.40b	*
Shoot length (cm)	82.70a	67.70b	49.21c	**
Leaf área (dm2)	42.63a	26.40b	15.93c	**
Leaves with necrosis (%)	2.32c	4.87b	23.87a	***
Plants with inflorescences (%)	50.11c	71.40a	64.30b	*

Note: Different lowercase letters indicate significant differences between treatments according to the least significant difference (LSD) test at p ≤ 0.05. p, probability level; n.s., not significant.

^* p ≤ 0.05.

^** p ≤ 0.01.

^*** p ≤ 0.001.

From week 5, the RCC (SPAD) significantly decreased in leaves of MS and HS treatment compared to those irrigated with the control treatment (Figure 8). At the end of the experiment, this reduction was more evident. Plants of Control, MS, and HS treatment showed RCC values around 43%, 37%, and 25% (Figure 8). These results were associated with those found for leaf color parameters. From week 7, luminosity and chroma parameters increased as salt content in the irrigation water increased, HS treatment showing the highest values, while hue angle decreased only in leaves of the HS treatment with respect to those irrigated with control and MS treatment (Figure 9).

FIGURE 8.

Relative chlorophyll content (RCC) of Cestrum nocturnum plants irrigated with 3 salinity levels throughout the experiment. Values are means of six plants per treatment. Vertical bars indicate standard errors and different lowercase letters indicate significant differences between treatments according to Duncan0.05 test.

FIGURE 9.

Color parameters such as luminosity (A), chroma (B) and hue angle (C) of Cestrum nocturnum plants irrigated with 3 salinity levels throughout the experiment. Values are means of six plants per treatment. Vertical bars indicate standard errors and different lowercase letters indicate significant differences between treatments according to the Duncan0.05 test.

4. Discussion

The existing literature on C. nocturnum plants reveals that most studies conducted to date have focused primarily on its pharmacological properties, phytochemical composition, and toxic potential (Fakhrah et al. 2024; Satapathy et al. 2024; Zambrano Vera et al. 2025). However, despite the growing interest in the medicinal and chemical applications of this species, as well as its potential and demand as an ornamental plant, there has been a notable paucity of research aimed at understanding its physiological responses and behavior under salinity stress conditions. Considering that soil and water salinity are one of the main limiting factors for plant productivity in many regions of the world (Acosta‐Motos et al. 2017; Majeed and Muhammad 2019), the lack of information on this species' tolerance to this type of stress represents a significant gap in the literature.

Plants irrigated with both salinity levels did not show symptoms of water stress until week 7, when the leaf and stem water potential decreased as a consequence of the reduction of the water potential gradient between soil and plant and, therefore, a lower water absorption. Nevertheless, the reduction of the Ψ_100s was observed from week 3. This could be interpreted as the plant adjusted its internal osmotic potential before actual leaf water loss occurred. During the first few weeks, it maintained a relatively stable leaf water potential thanks to a preventive or adaptive osmotic adjustment, possibly involving the accumulation of compatible solutes. Several ornamental plants have shown that they are able to osmotically adjust to maintain cell turgor (Acosta‐Motos et al. 2015a, 2015b; Gómez‐Bellot et al. 2021). Nevertheless, these plants ( Myrtus communis , Eugenia myrtifolia , Crithmum maritimum and Atriplex halimus and Hebe andersonii), upon detecting stress (salinity or water stress), did not accumulate solutes in advance to delay the decline in turgor. In parallel, Cestrum plants exhibited an immediate and strong stomatal regulation, with stomatal closure occurring from week 3 in the MS treatment and being more pronounced under HS. This rapid response highlights a coordinated water‐saving strategy operating from the onset of stress, combining osmotic adjustment and stomatal control to delay hydraulic failure. Overall, these results reveal an early, integrated hydraulic–stomatal response that had not previously been described for C. nocturnum under saline conditions.

A novel aspect of this study is the integration of temporal stomatal regulation with photochemical performance, revealing that, in C. nocturnum , the onset of strong stomatal restriction is closely linked to the subsequent collapse of photochemical efficiency under HS. While plants under MS maintained chlorophyll fluorescence values similar to the control, indicating preserved electron transport efficiency and photosynthetic functionality (Hanelt 2018), those exposed to HS showed a marked decline associated with strong stomatal limitation from week 7 onward. This response led to severe photochemical stress (Lucena et al. 2012), reflecting a reduced capacity to convert absorbed light energy into chemical energy. This result pattern aligns with other findings in other ornamental plants such as Scabiosa columbaria , Lobelia cardinalis , Aquilegia canadensis , Caryopteris x clandonensis “Dark knight,” Cestrum “orange peel” and Cymbopogon citratus , which reduced their stomatal conductance by around 30% and 50% when they were subjected to irrigation water at 5 and 10 dS m⁻¹, respectively (Wu et al. 2016; Xing et al. 2025). Instead, other ornamental species barely experienced changes in gas exchange. These differences between species depend on the degree of tolerance of each species, the salinity degree, water source and duration of the experiment (Wu et al. 2016; Gómez‐Bellot et al. 2021, 2022).

Interestingly, while transpiration followed the same dynamics as stomatal conductance, leaf hydraulic conductance increased between weeks 3 and 5 in the highest salinity treatment. This indicates that, despite reducing g _s, these plants could maintain a high K _leaf during the first weeks by implementing a water efficiency strategy as an adaptive compensation; that is, achieving a greater capacity of the leaf hydraulic system to supply the cells that maintain their turgor and metabolism. The mechanisms used could be physiological (through the activation of aquaporins, or osmotic adjustment, which maintains the Ψ gradient between xylem and cells) or anatomical to reinforce xylem integrity (Heinen et al. 2009; Simonin et al. 2015; Li et al. 2024). A strong correlation between the difference between stem and leaf water potential and transpiration was observed for each treatment, especially for plants subjected to the HS treatment, which indicates that, despite the osmotic stress, there was a close link between water transport dynamics and transpiration water loss. A small difference between stem water potential and leaf water potential corresponded to low transpiration, and vice versa. Simonin et al. (2015) reported that as Ψ_leaf falls when plants are under an abiotic stress, the subsequent drop in the xylem hydraulic conductivity will dominate and the leaf will exhibit the classical pattern of falling E with declining K _leaf, which is in line with our results of Ψ_leaf, E and K _leaf starting from week 7. Overall, our results challenge the effectiveness of early hydraulic adjustments as a protective mechanism in C. nocturnum , revealing that such responses delay but do not prevent functional collapse under prolonged HS.

The analysis of ionic accumulation at the end of the experiment revealed that C. nocturnum possesses a partial exclusion mechanism, restricting Na and, to a lesser extent, Cl transport to the aerial parts through root retention. This ability to restrict salt mobility to the leaves has been found in other ornamental species and could be an important adaptive advantage in saline environments (Chavarria et al. 2021; Liu et al. 2023; Costa et al. 2024). However, this study provides novel evidence that this exclusion capacity is not sufficient to prevent ionic dysfunction under HS, particularly due to the marked translocation of Cl at 7.2 dS m⁻¹. This differential behavior between ions highlights a previously underexplored aspect of salinity response in this species: a greater physiological sensitivity to chloride compared with sodium. Despite some degree of ion compartmentalization, the maintenance of leaf K/Na and Ca/Na ratios close to control levels was not achieved, indicating a breakdown of ionic homeostasis under stress. The reduction in these ratios has been reported in several ornamental plants under salinity conditions (Li et al. 2016; Henschke 2017; Gómez‐Bellot et al. 2020; Alexopoulos et al. 2021). Importantly, the present results show that growth and physiological impairment under HS are not solely explained by nutrient depletion or general ionic imbalance, but are more closely associated with toxic ion accumulation, particularly Cl (Bhuiyan et al. 2017). A key contribution of this work is the clarification of the physiological basis underlying the loss of ornamental quality: although most essential nutrients remained stable or even increased (except phosphorus), ionic toxicity rather than nutritional deficiency appears to be the primary driver of functional decline. This finding refines the interpretation of salinity damage in ornamental species, where nutrient‐based explanations have often been overemphasized. Overall, these results position Cestrum nocturnum as a species with moderate ion exclusion capacity but limited ability to maintain ionic homeostasis under HS, reinforcing that its tolerance threshold is constrained by toxic ion accumulation rather than nutrient imbalance.

HS resulted in a substantial reduction in plant performance, with biomass and leaf area decreasing by 67% and 63%, respectively. This magnitude of reduction highlights the strong sensitivity of Cestrum nocturnum under severe saline conditions and reflects the cumulative physiological impairments described previously (Saddiq et al. 2021; Costa et al. 2024; Hassan 2024; Wang, Nepal, et al. 2025). A key contribution of this study is the integration of growth responses with ornamental quality traits, revealing that salinity effects are not limited to biomass reduction but also involve complex shifts in plant developmental strategy (Finger et al. 2016; Kumar et al. 2023). In particular, flowering increased under both moderate and HS, suggesting an adaptive reallocation of resources towards reproduction (Álvarez and Sánchez‐Blanco 2015; Kazan and Lyons 2016; Chen et al. 2023; Tran, Pham, et al. 2024). This response supports the hypothesis that Cestrum nocturnum may accelerate flowering under stress as a survival strategy, but only within its tolerance limits. In fact, our results are consistent with the findings of Soliman et al. (2024), who stated that this phenomenon can be explained by the plant accelerating flower formation to ensure the species' survival under stress conditions, up to a certain tolerance level. In addition, this work clarifies that ornamental deterioration under HS is primarily driven by visible foliar damage rather than growth reduction alone. Severe marginal and apical necrosis observed under HS conditions reflects the combined effects of ionic toxicity and osmotic stress, directly compromising esthetic value (Wang, Poudyal, et al. 2025). In contrast, plants under MS maintained acceptable visual quality, with limited necrosis despite some growth reduction. Overall, these findings highlight that salinity impacts ornamental value through multiple coordinated pathways—growth inhibition, reproductive adjustment, and visual deterioration—and identify clear differences between moderate and HS responses in Cestrum nocturnum that had not been previously characterized in an integrated way.

The decline in SPAD values with increasing salinity (up to 41% under HS) provides new evidence of the sensitivity of Cestrum nocturnum chlorophyll stability to salt stress. This reduction was associated with chlorophyll degradation (Choi et al. 2018; Wang, Poudyal, et al. 2025) and was visually expressed as chlorosis and marked alterations in leaves (in our experiment, in lighter, more yellowish leaves and more saturated colors), which directly compromise ornamental quality coloration (Zhang et al. 2019; Hossain et al. 2022; Xiong et al. 2023). A novel contribution of this study is the demonstration that MS can actually enhance key ornamental traits despite slight growth reductions. Under MS conditions, plant architecture was improved through increased compactness, as shoot number remained stable while biomass and shoot length were moderately reduced (Álvarez et al. 2013). In addition, flowering was stimulated without inducing significant foliar damage, indicating that sub‐lethal salinity can positively modulate ornamental performance in this species. Importantly, this work provides the first integrated evaluation of physiological responses and commercial quality in Cestrum nocturnum under saline irrigation, a species previously neglected in this context.

5. Conclusions

This study provides novel insights into the salinity tolerance of Cestrum nocturnum , a species for which physiological responses to saline irrigation had not been previously characterized under nursery conditions. The results demonstrate that this species activates adaptive mechanisms, including osmotic adjustment, stomatal regulation, and partial ion exclusion, allowing it to cope with MS levels. Importantly, this work identifies a clear threshold of tolerance: while irrigation at 4.7 dS m⁻¹ is compatible with maintaining plant quality—and may even enhance ornamental traits such as compactness and flowering—higher salinity (7.2 dS m⁻¹) leads to ion toxicity, severe physiological impairment, and loss of commercial value. By linking physiological processes with growth and esthetic responses, this study not only fills a key knowledge gap but also provides practical guidelines for the sustainable use of saline water in ornamental horticulture. These findings support the use of moderately saline irrigation as a viable strategy, while defining the limits beyond which plant performance is compromised.

Author Contributions

M.J.S.‐B. and M.F.O. conceived the research. M.J.G.‐B. and S.B. performed the research and compiled the data. M.J.G.‐B. and S.B. analyzed part of the data. M.J.G.‐B. wrote the draft, with relevant contributions from M.J.S.‐B., M.F.O., and S.B. All authors edited and revised the final version of the manuscript.

Funding

This work was supported by the Spanish Ministry of Science, Innovation and Universities under the project PID2022‐141821OB‐I00 funded by MCIN/AEI/10.13039/501100011033.

Conflicts of Interest

The authors declare no conflicts of interest.

Gómez‐Bellot, M. J. , Sánchez‐Blanco M. J., Ortuño M. F., and Bañón S.. 2026. “Integrated Physiological and Ornamental Responses of Cestrum nocturnum to Salinity Stress: Identifying Tolerance Thresholds Under Saline Irrigation.” Physiologia Plantarum 178, no. 4: e70996. 10.1111/ppl.70996.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.