Plant materials and greenhouse conditions.

The experiment was conducted at the Utah State University (USU) Research Greenhouse in Logan, UT (lat. 41°45′28″N, long. 111°48′48″W, elevation 1409 m) from 10 Jul to 29 Sep 2023. H. syriacus ‘JWNWOOD4’, V. carlesii ‘Spiro’, and V. agnus-castus ‘SMVACBD’ were selected for this study. Thirty plants of each species were purchased from Proven Winners (DeKalb, IL, USA) on 2 Jun 2023. Two days later, the plants were transplanted into 7.6-L injection-molded, polypropylene containers (PC1D-4; Nursery Supplies, Orange, CA, USA) filled with soilless substrate (Metro-Mix^® 820; Canadian sphagnum peatmoss, 35% to 45% composted pine bark, coir, coarse perlite, and dolomitic limestone; SunGro Horticulture, Agawam, MA, USA). The plants were kept in the research greenhouse and irrigated with Logan City water (EC = 0.33 dS·m⁻¹; pH = 8.30) before the experiment commenced. Throughout the experiment, greenhouse temperatures were controlled at 25.1 ± 0.8 °C during the day and 21.2 ± 1.2 °C at night [mean ± standard deviation (SD)]. The daily light integral (DLI) inside the greenhouse was recorded using a full-spectrum quantum sensor (SQ-500-SS; Apogee Instruments, Logan, UT, USA), and the average DLI was 26.1 ± 9.4 mol·m⁻²·d⁻¹ (mean ± SD) from 10 Jul to 29 Sep. When light intensity inside the greenhouse was less than 500 µmol·m⁻²·s⁻¹, supplemental lights with intensities of 373.0 ± 35.2 µmol·m⁻²·s⁻¹ (mean ± SD) at the plant canopy level, measured using a Quantum flux meter (MQ-306X, serial no. 1004; Apogee Instruments), were provided using light-emitting diodes (Luxx Lighting, Jurupa Valley, CA, USA) from 0600 to 2200 HR.

Salinity treatments.

Two saline solution treatments were applied on H. syriacus ‘JWNWOOD4’, V. carlesii ‘Spiro’, and V. agnus-castus ‘SMVACBD’, including saline solutions at EC levels of 5.0 dS·m⁻¹ (EC 5.0) and 10.0 dS·m⁻¹ (EC 10.0). All treatment solutions were prepared in 100-L tanks. The nutrient solution used for the control was prepared by dissolving 0.8 g·L⁻¹ water-soluble fertilizer 15N–2.2P–12.5K (Peters Excel 15-5-15 Ca-Mag Special; ICL Specialty Fertilizers, Dublin, OH, USA) in reverse osmosis (RO) water. The saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹ were prepared by adding NaCl (Fisher Chemical, Waltham, MA, USA) at 0.92 and 2.27 g·L⁻¹ plus CaCl₂·2H₂O (Hi Valley Chemical, Centerville, UT, USA) at 1.17 and 2.80 g·L⁻¹ at a molar ratio of 2:1 to the nutrient solution, respectively. The pH of treatment solutions was adjusted to 5.7 ± 0.3 (mean ± SD) using 1.0 mol·L⁻¹ HNO₃ (Fisher Chemical, Fair Lawn, NJ, USA) or 88% potassium hydroxide pellets (Sigma-Aldrich, St. Louis, MO, USA). The ECs and pHs of treatment solutions were confirmed using an EC and pH meter (LAQUA Twin; Horiba, Kyoto, Japan) before each treatment, respectively. The ECs of irrigation solutions for the control, EC 5.0, and EC 10.0, were 0.90 ± 0.03, 5.13 ± 0.22, and 9.77 ± 0.21 dS·m⁻¹ (mean ± SD), respectively, during the experiment. In this 8-week experiment, 1200 mL of treatment solutions per pot was applied weekly (eight irrigation events in total), resulting in a leachate fraction of 24.7 ± 3.5% (mean ± SD). At least 30 min after each treatment, four plants per species per treatment were chosen for monitoring the leachate EC. RO water (100 mL) was poured onto the substrate surface, and a saucer was placed under the container to collect the leachate, which was used for EC measurements.

Visual score.

The visual quality of all three species was rated weekly according to a score from 0 to 5, where 0 = dead, 1 = severe foliage damage (>90% leaves with burn, necrosis, and discoloration), 2 = moderate foliage damage (50% to 90%), 3 = slight foliage damage (<50%), 4 = acceptable quality with minimal foliage damage (<10%), and 5 = good quality without salt damage. The visual score was determined based solely on the condition of plant foliage.

Growth data collection and harvest.

Destructive harvests were conducted on 16 Aug (37 d after the initial treatment, first harvest) and 4 Sep 2023 (58 d after the initial treatment, second harvest), with five plants harvested per species per treatment. At harvest, the plants were cut at the basal part of the stems at the substrate level. Plant height (centimeter), width (centimeter), and number of shoots (longer than 5 cm) were recorded before the first treatment (initial data) and at both harvests. Height was recorded from the substrate surface to the highest point of the plants. Two perpendicular widths were measured. The growth index (GI) {[height + width 1 (the widest point of canopy) + width 2 (perpendicular width of width 1)]/3} was calculated (Lalk et al. 2023). Plant growth rate was calculated as (final GI − initial GI)/final GI. At harvest, leaf area (square centimeter) was determined using a leaf area meter (LI-3100; LI-COR Bioscience, Lincoln, NE, USA). In addition, shoot and root dry weights (DWs) were measured after plant materials were dried in an oven at 60 °C for 7 d. The root-to-shoot ratio was calculated as the ratio of the DW of the roots to shoots (stems and leaves).

Leaf greenness and photosynthesis parameters.

Leaf greenness was measured using a handheld chlorophyll meter (SPAD-502 Plus; Minolta Camera Co., Osaka, Japan) at harvest. For each measurement, eight healthy and mature leaves from one plant were measured, and the average was recorded. At harvest, stomatal conductance (g_s) was measured using a porometer/fluorometer (LI-600; LI-COR Biosciences). Photosynthesis parameters, including leaf vapor-pressure deficit (VPD) and net photosynthetic rate (P_n), were measured using a portable photosynthesis system (CIRAS-3; PP System, Amesbury, MA, USA) equipped with an automatic universal leaf cuvette (PLC3; PP Systems). For each treatment and each species, four plants were randomly chosen for photosynthesis parameter measurements at both harvests. The maximal photochemical efficiency of photosystem II (F_v/F_m, where F_v is F_m − F_o, F_o is minimal fluorescence, and F_m is maximum fluorescence) was measured using a rapid screening continuous excitation chlorophyll fluorimeter (Hansatech Instruments, Penteny, UK) at both harvests. A fully expanded and healthy leaf from each plant was selected for g_s, photosynthesis parameters (VPD and P_n), and F_v/F_m data collection. At the first harvest, g_s and F_v/F_m measurements were taken for all plants. At the second harvest, the remaining plants were used for g_s and F_v/F_m measurements. However, an exception was made for V. carlesii ‘Spiro’ treated with a saline solution at an EC of 10.0 dS·m⁻¹ at the second harvest, when only one plant was used for data collection due to high mortality.

Canopy temperature.

At both harvests, top-view thermal infrared images of plant canopies were captured at 1200 HR using a thermal image camera (FLIR E5-XT; Teledyne FLIR, Wilsonville, OR, USA). The average canopy temperature of each plant was then calculated using FLIR Thermal Studio Suite (Teledyne FLIR).

Mineral analysis.

In this study, four plants per species per treatment from the second harvest were used for mineral analysis. The dried leaves were finely ground using a stainless Wiley mill (Thomas Scientific, Swedesboro, NJ, USA). The resulting samples were submitted to the USU Analytical Laboratories (Logan, UT, USA) to analyze mineral contents. In brief, chloride (Cl⁻) content was examined by a flow injection analysis and ion chromatography system (QuikChem 8000; Lachat Instrument, Loveland, CO, USA) and reported on a dry plant basis (mg·g⁻¹). Sodium (Na⁺), calcium (Ca²⁺), K⁺, magnesium (Mg²⁺), manganese (Mn²⁺), zinc (Zn²⁺), iron (Fe³⁺), phosphorus (P), and sulfur (S) were quantified following the protocol described by Gavlak et al. (2005). To analyze these elements, 0.5-g ground leaf samples combined with 6 mL of HNO₃ were added in a digestion tube. The mixture was subsequently heated in a digestion block at 80 °C for 10 min and then cooled for 2 min. Following this, the digestion tube was placed again in the digestion block at 130 °C for 1 h after adding 2 mL of 30% hydrogen peroxide (H₂O₂). A vortex stirrer was used to mix the mixture in the digestion tubes. The contents of the digestion tubes were cooled to room temperature before being transferred into a 25-mL volumetric flask. The digest was analyzed using an inductively coupled plasma-optical emission spectrometer (iCAP 6300 ICP-AES; Thermo Scientific, Waltham, MA, USA) and reported on a dry plant basis (mg·g⁻¹).

Experimental design and data analysis.

The experiment was designed as a randomized complete block design involving the three species and three treatments, with ten replications per species per treatment. Each experiment unit comprised one container with one plant. The most appropriate transformation (log) of data were conducted for all parameters, which meet the assumptions of analysis of variance. Back-transformed least-squares mean and standard errors were calculated to aid in interpreting the results. A mixed model analysis was performed to evaluate the effects of salinity levels on measured parameters. Correlation analyses were also conducted among visual score, plants’ growth parameters, g_s, leaf VPD, P_n, and Na⁺ and Cl⁻ contents, as well as K⁺/Na⁺ and Ca²⁺/Na⁺ ratios. All statistical analyses were performed using the PROC MIXED procedure with the Tukey–Kramer method in a SAS Studio (version 3.8; SAS Institute, Cary, NC, USA) with a significant level specified at 0.05.

Visual quality.

Because salinity stress can severely impair the aesthetic appearance and market value of plants, evaluating the salinity tolerance of landscape plants should prioritize their visual quality. At the first and second harvests, foliar salt damage was evident on plants exposed to higher salinity levels, with varying responses among the three species (all P < 0.0001) (Fig. 1; Table 1). Throughout the experiment, all plants maintained excellent visual quality when treated with a nutrient solution, resulting in visual ratings equal to or exceeding 4.0. Regardless of treatments, there was no notable foliar salt damage (leaf burn, discoloration, or necrosis) after the first irrigation event, with an average visual score equal to or above 4.9 for all three species (data not shown). Similar results were observed for all plants after the second and third applications of the saline solution. One week after the second and third irrigation events, saline solution irrigation started to show significant impacts on plant visual score (P < 0.0001), with an interaction observed between salt treatment and species (P < 0.0001).

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Table 1. Summary of variance for the effect of salt treatments and their interactions with species on visual score (VS), height, plant growth rate (PGR), number of shoots and flowers, leaf area (LA), shoot dry weight (DW), root-to-shoot ratio, leaf greenness [Soil Plant Analysis Development (SPAD) reading], maximal photochemical efficiency of photosystem II (F_v/F_m), stomatal conductance (g_s), net photosynthetic rate (P_n), leaf vapor pressure deficit (VPD), and canopy temperature (CT) of H. syriacus ‘JWNWOOD4’, V. carlesii ‘Spiro’, and V. agnus-castus ‘SMVACBD’ irrigated with a nutrient solution at an electrical conductivity (EC) of 0.9 dS·m⁻¹ (control) or saline solution at an EC of 5.0 dS·m⁻¹ (EC 5.0) or 10.0 dS·m⁻¹ (EC 10.0) in a research greenhouse.ⁱ

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At the first harvest, all plants irrigated with saline solutions survived. Most plants treated with a saline solution at an EC of 5.0 dS·m⁻¹ exhibited no or minimal foliar damage, with average visual scores of 4.2, 4.1, and 4.1 for H. syriacus, V. carlesii, and V. agnus-castus, respectively (Table 2). When irrigated with a saline solution at an EC of 10.0 dS·m⁻¹, H. syriacus and V. carlesii exhibited moderate foliage damage, with an average visual score of 3.7 and 3.1, respectively. In contrast, V. agnus-castus showed similar performance to plants at an EC of 5.0 dS·m⁻¹, with an average visual score of 4.2.

Table 2. Visual score of H. syriacus ‘JWNWOOD4’, V. carlesii ‘Spiro’, and V. agnus-castus ‘SMVACBD’ irrigated with a nutrient solution at an electrical conductivity (EC) of 0.9 dS·m⁻¹ (control) or saline solution at an EC of 5.0 dS·m⁻¹ (EC 5.0) or 10.0 dS·m⁻¹ (EC 10.0) after the fourth (first harvest) and the eighth (second harvest) irrigation event in a research greenhouse.ⁱ

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One week after the fifth application of the saline solution, most plants displayed minimal salt damage on leaves, with average visual scores ranging from 3.7 to 4.7, except for V. carlesii irrigated with saline solution at an EC of 10.0 dS·m⁻¹, which exhibited slight to moderate foliage damage, with an average visual score of 2.4. Slight to minimal foliar salt damage was observed in H. syriacus and V. agnus-castus for up to the seventh treatment, with an average visual score remaining at or greater than 3.7. However, 1 week after the sixth irrigation event, moderate foliar damage with an average visual score of 3.2 was observed in V. carlesii when receiving saline solutions at an EC of 5.0 dS·m⁻¹. Meanwhile, V. carlesii irrigated with saline solution at an EC of 10.0 dS·m⁻¹ had severe foliage damage, with an average visual score of 1.0. One week after the seventh application, most plants for all three species had less than 50% of leaves damaged, with an average visual score equal to or higher than 3.0. Unfortunately, when irrigated with a saline solution at an EC of 10.0 dS·m⁻¹, V. carlesii experienced a mortality rate of 57% 1 week after the seventh application.

At the second harvest, H. syriacus irrigated with saline solution at an EC of 5.0 dS·m⁻¹ and V. agnus-castus irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹ maintained good visual quality with average visual scores equal to or greater than 4.0. However, the visual scores of V. carlesii were 3.0 and 0.3, respectively, when irrigated with saline solution at ECs of 5.0 and 10.0 dS·m⁻¹ (Table 2).

The leachate EC increased steadily throughout the 8-week irrigation period with saline solutions at EC levels of 5.0 and 10.0 dS·m⁻¹, while the control remained consistently low (Fig. 2). Prolonged saline irrigation led to progressive salt injury symptoms, as evidenced by declining visual quality and increasing leaf damage.

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Salt stress diminishes a plant’s aesthetic appearance by disrupting osmotic balance, inducing oxidative damage, and causing ion toxicity (Singh et al. 2014). Our observations revealed that both the EC levels of saline solutions and the duration of salinity stress affect the visual score, which serves as an indicator of foliar salt damage. In our study, H. syriacus maintained a good visual quality after 8 weeks of irrigation with saline solution at an EC of 5.0 dS·m⁻¹. Similarly, Liu et al. (2017) observed that H. syriacus ‘ILVOPS’ retained high visual quality after irrigation with a saline solution at an EC of 5.0 dS·m⁻¹ eight times. Comparable results were found in two other H. syriacus cultivars exposed to salinity levels ranging from 50 to 200 mM NaCl (∼4.6 to 14.6 dS·m⁻¹) (Lu et al. 2023), where H. syriacus ‘Duede Brabaul’ (‘Duede Brabaul’ rose of Sharon) did not exhibit any obvious salt damage with up to 100 mM NaCl treatment (∼7.3 dS·m⁻¹), and no significant changes were observed in H. syriacus ‘Blueberry Smoothie’ (Blueberry Smoothie™ rose of Sharon) when treated with 50 mM NaCl, approximately equivalent to 4.6 dS·m⁻¹. Our findings indicated that V. carlesii exhibited around 50% foliage burn and necrosis when irrigated with saline solution at an EC of 5.0 dS·m⁻¹ eight times and more than 90% foliage damage at an EC of 10.0 dS·m⁻¹ for the same duration. In addition, high mortality was observed for V. carlesii at an EC of 10.0 dS·m⁻¹. Sun et al. (2020) investigated the salt tolerance of 12 viburnum taxa and found that 11 of them showed more than 50% foliage damage, with 9 experiencing mortality rates of 60% or greater after 8 weeks of irrigation with saline solution at an EC of 10.0 dS·m⁻¹. These results are consistent with the findings of our study. Regardless of treatment, V. agnus-castus exhibited no or minimal foliage salt damage throughout the entire experiment. A similar conclusion was reached by Doğan et al. (2011), who investigated V. agnus-castus communities naturally distributed in soils with varied salinity levels, ranging from 0.6 to 45.7 dS·m⁻¹. Maintaining an acceptable aesthetic appearance for landscape plants is essential when irrigated with alternative water, which is usually considered low-quality water. Based on this criterion, V. agnus-castus was the most salt tolerant, followed by H. syriacus, whereas V. carlesii was sensitive to salinity stress.

Growth.

At the first harvest, irrigation with saline solutions influenced the height of H. syriacus and V. carlesii, although these differences were not statistically significant. Specifically, H. syriacus irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹ were 16% and 12% shorter than those in the control, respectively (Table 3). Similarly, V. carlesii exhibited 8% and 23% reductions in height compared with the control when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively. Saline solution irrigation had a significant impact on plant growth rate when considering the interaction between plant species and salt treatment at the first harvest (P = 0.03) (Table 1). The plant growth rate of H. syriacus significantly decreased due to salt treatment, with reductions of 62% and 47% when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, compared with the control, respectively. In addition, V. carlesii irrigated with saline solution at ECs of 5.0 and 10.0 dS·m⁻¹ exhibited 14% and 40% reductions in plant growth rate, respectively, compared with the control.

Table 3. Height and plant growth rate of H. syriacus ‘JWNWOOD4’, V. carlesii ‘Spiro’, and V. agnus-castus ‘SMVACBD’ irrigated with a nutrient solution at an electrical conductivity (EC) of 0.9 dS·m⁻¹ (control) or saline solution at an EC of 5.0 dS·m⁻¹ (EC 5.0) or 10.0 dS·m⁻¹ (EC 10.0) after the fourth (first harvest) and the eighth (second harvest) irrigation event in a research greenhouse.ⁱ

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At the second harvest, significant differences were observed in plant height (P = 0.01) and plant growth rate (P = 0.0007) when plants were irrigated with saline solutions (Table 1). H. syriacus showed 20% and 32% reductions in height when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively (Table 3). The plant height of V. carlesii significantly decreased by 20% and 46% compared with the control when irrigated with saline solution at ECs of 5.0 and 10.0 dS·m⁻¹, respectively. The plant growth rate of H. syriacus significantly decreased by 64% and 68% when irrigated with saline solution at ECs of 5.0 and 10.0 dS·m⁻¹, respectively. Meanwhile, V. carlesii showed significant reductions of 37% and 67% when receiving saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively. In contrast, no reduction in height and plant growth rate was observed for V. agnus-castus in response to saline solution irrigation at both harvests.

Salt stress induces ionic imbalance within plant cells, reducing water uptake and impairing photosynthesis (Munns and Tester 2008). Therefore, increased salinity limits plant growth and development, reduces productivity, and, in extreme cases, leads to plant death (Krasensky and Jonak 2012; Munns and Tester 2008). According to our observations, saline solution irrigation influenced the height and plant growth rate of H. syriacus and V. carlesii. Birhanie et al. (2022) revealed that the plant growth of H. cannabinus (kenaf) significantly decreased due to salt treatment, with reductions of 15%, 23%, 30%, and 45%, respectively, when treated with 100, 150, 200, and 250 mM NaCl (∼7.3, 11.0, 14.6, and 18.3 dS·m⁻¹, respectively), compared with the control (0 dS·m⁻¹). In Viburnum lucidum (arrow-wood viburnum), the addition of NaCl or CaCl₂ significantly reduced plant height and relative growth rate (Cirillo et al. 2019). Our study aligns with these findings. However, a significant decrease in stem relative growth rate was reported by Yin et al. (2021) when V. trifolia var. simplicifolia (simple leaf chaste tree) was treated for 30 d with saline solution at EC levels from ∼6.6 to 19.7 dS·m⁻¹, compared with the control (0 dS·m⁻¹). In our study, V. agnus-castus did not show growth reduction under saline solution irrigation. Previous studies indicated that Vitex is a halophyte species capable of surviving under salinity conditions (∼21.9 to 29.2 dS·m⁻¹) due to salt tolerance developed during their phylogenetic adaptation (Carillo et al. 2011; Yin et al. 2021). Furthermore, the growing medium of the plants irrigated with nutrient solution might dry faster than those under saline solution irrigation, which could cause a cofounding effect from drought, potentially resulting in a growth reduction (Nepal et al. 2024).

At the first harvest, as EC levels in the saline solution increased from 0.9 to 10.0 dS·m⁻¹, the number of shoots decreased from 12 to 8 for V. carlesii; however, no statistical difference was observed for the other two species (Table 4). At the second harvest, a statistical difference (P = 0.03) was observed in the number of shoots as EC levels elevated in saline solutions (Table 1). The number of shoots decreased in H. syriacus by 5% to 13%, in V. carlesii by 25% to 32%, and in V. agnus-castus by 21% to 23%, when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, compared with the control.

Table 4. Number of shoots and flowers of H. syriacus ‘JWNWOOD4’, V. carlesii ‘Spiro’, and V. agnus-castus ‘SMVACBD’ irrigated with a nutrient solution at an electrical conductivity (EC) of 0.9 dS·m⁻¹ (control) or saline solution at an EC of 5.0 dS·m⁻¹ (EC 5.0) or 10.0 dS·m⁻¹ (EC 10.0) after the fourth (first harvest) and the eighth (second harvest) irrigation event in a research greenhouse.ⁱ

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At the first harvest, saline solution at an EC of 10.0 dS·m⁻¹ reduced the number of flowers by 5% and 32% for H. syriacus and V. agnus-castus, respectively, although these differences were not statistically significant (Table 4). However, increasing salt levels in the irrigation solution significantly decreased the number of flowers at the second harvest (P = 0.04). Specifically, for H. syriacus, there were reductions of 74% and 84% when irrigated with saline solution at ECs of 5.0 and 10.0 dS·m⁻¹, respectively.

Plant growth inhibition is recognized as one of the most obvious symptoms of salt stress (Das et al. 2024; Munns and Tester 2008). Galal (2017) demonstrated that different NaCl concentrations had a significant adverse impact on the seedling length of Hibiscus sabdariffa (roselle). Previous investigations have reported that flowering is associated with biosynthetic changes, with salinity being identified as one of the most critical problems (Lag-Brotons et al. 2013). Similar results were reported by Trivellini et al. (2014), showing that salinity significantly decreased the flower fresh weight of Hibiscus rosa-sinensis (Chinese hibiscus) when grown under salt stress (∼14.6 dS·m⁻¹) for 28 d. In addition, the number of branches on Vitex negundo (chaste tree) reduced by 23% and 51% at EC levels of 4 and 12 dS·m⁻¹, respectively, 150 d after treatment application (Das et al. 2024).

A significant difference in leaf area was found at the first harvest when considering the interaction between saline solutions and plant species (P = 0.001) (Table 1). The leaf area of V. carlesii reduced by 55% when treated with a saline solution at an EC of 10.0 dS·m⁻¹ compared with the control (Table 5). Meanwhile, the other two species did not exhibit a reduced leaf area. The saline solution irrigation had a significant effect on leaf area at the second harvest (P < 0.0001). Compared with the control, the leaf area of H. syriacus decreased by 22% and 32%, respectively, when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹. V. carlesii showed a reduction in leaf area by 78% and 94% when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively, compared with the control. Furthermore, although not statistically significant, V. agnus-castus exhibited a decrease in leaf area by 15% and 6% when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively, compared with the control.

Table 5. Leaf area, shoot dry weight, and root-to-shoot ratio of H. syriacus ‘JWNWOOD4’, V. carlesii ‘Spiro’, and V. agnus-castus ‘SMVACBD’ irrigated with a nutrient solution at an electrical conductivity (EC) of 0.9 dS·m⁻¹ (control) or saline solution at an EC of 5.0 dS·m⁻¹ (EC 5.0) or 10.0 dS·m⁻¹ (EC 10.0) after the fourth (first harvest) and the eighth (second harvest) irrigation event in a research greenhouse.ⁱ

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Salinity adversely affects leaf expansion through two primary mechanisms. Firstly, the osmotic effect occurs within minutes after exposure to salinity, altering the osmotic balance around the root zone and reducing the plant’s ability to absorb water, consequently decreasing leaf growth (Läuchli and Epstein 1990; Munns and Tester 2008). The second effect, known as specific-ion toxicity, results from excessive salt accumulation in leaves, leading to foliage death and a reduction in the photosynthetic leaf area (Läuchli and Grattan 2007). Our findings align with these previous studies, because plants exposed to salinity stress exhibited a reduction in leaf area.

Salinity negatively affected shoot dry weight with variations among species. At the first harvest, V. carlesii exhibited a 39% reduction in shoot dry weight when irrigated with a saline solution at an EC of 10.0 dS·m⁻¹, compared with the control (Table 5). However, there was no significant change in the shoot dry weight of the other two species. At the second harvest, shoot dry weight decreased with increasing salinity levels (P < 0.0001), with an interactive effect between salt treatment and species (P = 0.0002) (Table 1). Compared with the control, saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹ reduced the shoot dry weight of H. syriacus by 16% and 33%, respectively, although no statistical difference was found. Moreover, compared with the control, the shoot dry weight for V. carlesii showed significant reductions of 67% and 82% at ECs of 5.0 and 10.0 dS·m⁻¹, respectively. Interestingly, the shoot dry weight of V. agnus-castus remained unchanged when the EC of saline solutions elevated to 10.0 dS·m⁻¹.

Under salt stress, sodium accumulation and its transfer to the leaves can lead to a decrease in turgor pressure, negatively affecting plant growth and development (Kautz et al. 2014). Our observations highlighted that saline solution irrigation influenced the shoot dry weight of V. carlesii, which aligns with findings by Ali et al. (2012), who observed a reduction in shoot dry weight for H. sabdariffa when exposed to saline solution. In a previous study conducted by Liu et al. (2017), the shoot dry weight of H. syriacus ‘ILVOPS’ was significantly reduced by 26% and 61% when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively, compared with the control. In addition, Sun et al. (2020) conducted an experiment comparing the salt tolerance of 12 viburnum taxa, showing that 10 taxa exhibited decreased shoot dry weight when treated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹. In contrast, Das et al. (2024) reported that seawater with moderate (8 dS·m⁻¹) and high (12 dS·m⁻¹) salinity levels resulted in reductions of 39% and 76%, respectively, in the shoot dry weight of V. negundo 150 d after the initial treatment.

Elevated salinity stress affected the root-to-shoot ratio of the three species, with varying responses among the plants. At the first harvest, compared with the control, H. syriacus and V. carlesii exhibited 28% and 47% increases in the root-to-shoot ratio when irrigated with saline solution at an EC of 10.0 dS·m⁻¹, respectively (Table 5). However, V. angus-castus experienced a 31% reduction in the root-to-shoot ratio when irrigated with saline solutions at an EC of 10.0 dS·m⁻¹, compared with the control. At the second harvest, an interaction in the root-to-shoot ratio between salt treatment and species was observed (P = 0.001) (Table 1). H. syriacus did not exhibit a significant difference in the root-to-shoot ratio (Table 5). Meanwhile, V. carlesii showed 34% and 78% increases in the root-to-shoot ratio when irrigated with saline solution at ECs of 5.0 and 10.0 dS·m⁻¹, respectively, compared with the control. In contrast, V. agnus-castus displayed 27% and 45% reductions in the root-to-shoot ratio, respectively, when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹.

Under salinity stress, root-to-shoot signaling occurs to regulate the growth of the entire plant (Schmidt et al. 2014). The presence of salt in the soil reduces the plant roots’ ability to take up water, leading to a reduction in shoot growth rate, a phenomenon known as the salinity-induced water-deficit effect (Parihar et al. 2015). Conversely, during water deficit, the density of root system is greater under stress than under nonstressed conditions, as surface soil becomes depleted of moisture (Ober and Sharp 2007). Our findings for H. syriacus and V. carlesii align with these studies, showing an increasing trend of root-to-shoot ratio with elevated salinity levels in the irrigation solution. Regarding V. agnus-castus, our results are not consistent with a previous study by Yin et al. (2021), which reported that V. trifolia (simpleleaf chaste tree) exhibited a markedly increased root-to-shoot growth rate when treated with NaCl concentrations ranging from 90 to 270 mM (∼6.9 to 19.7 dS·m⁻¹), possibly due to the enhanced absorption and maintenance of K⁺ in the roots.

Leaf greenness and maximal photochemical efficiency of photosystem II.

At the first harvest, saline solution irrigation significantly influenced SPAD readings (P < 0.0001), an indicator of leaf greenness. Interactive effects between salt treatments and plant species were also observed (P = 0.0006) (Table 1). Specifically, H. syriacus showed an 11% decrease in the SPAD reading when irrigated with saline solutions at an EC of 10.0 dS·m⁻¹, compared with the control (Table 6). SPAD readings for V. carlesii decreased by 8% and 17% when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively, compared with the control. Meanwhile, V. angus-castus exhibited consistent SPAD readings among different EC levels, indicating that increased salinity did not affect its leaf greenness. At the second harvest, there was a significant difference in SPAD readings (P = 0.001), with interactive effects observed between salt treatments and species (P < 0.0001). SPAD reading of V. carlesii irrigated with saline solution at ECs of 5.0 and 10.0 dS·m⁻¹ decreased by 31% and 36%, respectively, compared with the control. In contrast, SPAD readings for H. syriacus and V. angus-castus remained unchanged.

Table 6. Leaf greenness [Soil Plant Analysis Development (SPAD) reading] and maximal photochemical efficiency of photosystem II (F_v/F_m) of H. syriacus ‘JWNWOOD4’, V. carlesii ‘Spiro’, and V. agnus-castus ‘SMVACBD’ irrigated with a nutrient solution at an electrical conductivity (EC) of 0.9 dS·m⁻¹ (control) or saline solution at an EC of 5.0 dS·m⁻¹ (EC 5.0) or 10.0 dS·m⁻¹ (EC 10.0) after the fourth (first harvest) and the eighth (second harvest) irrigation event in a research greenhouse.ⁱ

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Salinity stress reduces chlorophyll content even at low salt concentrations, primarily through increased chlorophyll degradation or decreased synthesis (Santos 2004). SPAD readings are used to estimate leaf chlorophyll content, making it a valuable tool for screening plant salt tolerance and an early indicator of salinity stress (Markwell et al. 1995; Niu and Cabrera 2010). Our study corroborates these findings, because SPAD readings decreased with increasing EC levels of the saline solution. Zhang et al. (2023) reported significant decreases in chlorophyll content in H. syriacus under 300 mM NaCl (∼21.9 dS·m⁻¹), and Percival (2005) found similar reductions in Viburnum ×carlcephalum (fragrant snowball). However, Kim et al. (2023) observed higher chlorophyll contents in potted Vitex rotundifolia (beach vitex) compared with those in coastal sand.

Saline solution irrigation significantly affected F_v/F_m at both the first (P = 0.03) and second (P = 0.01) harvests (Table 1). Interactive effects between salt treatment and species were also observed at both harvests (P = 0.005 and P = 0.02, respectively). At the first harvest, V. carlesii irrigated with saline solution at an EC of 10.0 dS·m⁻¹ exhibited reduced F_v/F_m (Table 6). Similarly, at the second harvest, decreased F_v/F_m values were observed in V. carlesii when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹. In contrast, H. syriacus and V. angus-castus showed similar F_v/F_m values at both harvests.

The decrease in F_v/F_m values in our study likely resulted from reduced SPAD readings, which reflects a loss of chlorophyll content. Similar findings were reported by Percival (2005), who observed significant reductions in the F_v/F_m of Viburnum ×carlcephalum under salt treatment. In our study, H. syriacus and V. angus-castus did not exhibit significant variance in F_v/F_m under saline solution irrigation. However, a significant decrease in F_v/F_m in H. syriacus ‘ILVOPS’ started to emerge from the fifth week after treatment initiation, when irrigated with saline solution at an EC of 10.0 dS·m⁻¹ (Liu et al. 2017). Moreover, the study by Zhang et al. (2023) reported a significant decrease in F_v/F_m in the leaves of H. syriacus when subjected to treatment with 300 mM NaCl (∼21.9 dS·m⁻¹). This salinity level likely induced severe stress on the plants, leading to substantial reductions in chlorophyll contents and photosynthetic efficiency. In contrast, Kim et al. (2023) observed higher F_v/F_m in potted V. rotundifolia plants, compared with those grown in coastal sand, which experienced salt stress. This discrepancy could stem from several factors, including the duration and intensity of salinity stress. The beach vitex plants in the study of Kim et al. (2023) might have been exposed to lower or more intermittent levels of salinity stress compared with the severe and prolonged stress conditions imposed on H. syriacus in the study of Zhang et al. (2023).

Gas exchange.

Stomatal conductance decreased with increasing salinity levels at both the first (P < 0.0001) and second (P < 0.0001) harvests (Table 1). Moreover, at the first harvest, significant interactive effects were observed between salt treatment and species (P = 0.04). At the first harvest, H. syriacus, V. carlesii, and V. angus-castus had 48%, 55%, and 61% reductions in stomatal conductance, respectively, when they were irrigated with a saline solution at an EC of 5.0 dS·m⁻¹ compared with the control (Table 7). When the saline solution at an EC of 10.0 dS·m⁻¹ was applied, stomatal conductance was further reduced by 55%, 81%, and 74% in H. syriacus, V. carlesii, and V. angus-castus, respectively, compared with the control. Similarly, at the second harvest, stomatal conductance was reduced by 54% and 82% for H. syriacus, 78% and 98% for V. carlesii, and 77% and 91% for V. angus-castus, respectively, when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, compared with the control.

Table 7. Stomatal conductance (g_s), net photosynthetic rate (P_n), and leaf vapor pressure deficit (VPD) of H. syriacus ‘JWNWOOD4’, V. carlesii ‘Spiro’, and V. agnus-castus ‘SMVACBD’ irrigated with a nutrient solution at an electrical conductivity (EC) of 0.9 dS·m⁻¹ (control) or saline solution at an EC of 5.0 dS·m⁻¹ (EC 5.0) or 10.0 dS·m⁻¹ (EC 10.0) after the fourth (first harvest) and the eighth (second harvest) irrigation event in a research greenhouse.ⁱ

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Stomatal conductance is the most readily measurable parameter in response to salinity stress and has been verified as an indicator of osmotic stress tolerance (Jiang et al. 2006; Munns and Tester 2008; Niu and Cabrera 2010; Rahnama et al. 2010). H. syriacus grown under 300 mM NaCl (∼21.9 dS·m⁻¹) exhibited a noticeable decrease in stomatal conductance compared with the control (0 dS·m⁻¹), which aligns with our findings (Zhang et al. 2023). A study by Chen et al. (2020) reported a reduction in stomatal conductance across 12 viburnum taxa in response to saline exposure.

Saline solution irrigation significantly affected the net photosynthetic rate (P_n) at the first harvest (P < 0.0001), with an interaction effect between salt treatment and species (P = 0.01) (Table 1). At the first harvest, P_n decreased by 45% for H. syriacus and 42% for V. angus-castus when irrigated with saline solutions at an EC of 10.0 dS·m⁻¹; however, no statistical significance was observed (Table 7). Meanwhile, V. carlesii experienced significant reductions of 42% and 84% in the P_n when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively. At the second harvest, there was a significant decrease in the P_n with increasing EC levels in saline solutions (P = 0.0002), but no interaction between salt treatment and species was found. Salt treatment did not have a significant impact on H. syriacus, although saline solution at an EC of 10.0 dS·m⁻¹ reduced its P_n by 36% compared with the control. In contrast, P_n of V. carlesii significantly decreased by 50% and 77%, respectively, when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, compared with the control. In addition, V. angus-castus showed reductions of 27% and 51% in P_n when saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹ were applied, respectively.

At the first harvest, leaf VPD was significantly influenced by saline solution irrigation (P = 0.01), with an interactive effect observed between salt treatment and plant species (P = 0.01) (Table 1). For V. carlesii, the leaf VPD was significantly greater when irrigated with a saline solution at an EC of 10.0 dS·m⁻¹, compared with plants in the control and those at an EC of 5.0 dS·m⁻¹ (Table 7). In contrast, similar leaf VPD was observed for H. syriacus and V. angus-castus under saline solution irrigation compared with the control. However, at the second harvest, no significant difference in leaf VPD was observed for all three species in response to the saline solution irrigation.

Elevated salinity levels induce stomatal closure, which minimizes water loss by reducing the transpiration rate but also compromises photosynthesis by limiting intercellular CO₂ concentration (Sim et al. 2021). Furthermore, F_v/F_m, which typically ranges from 0.80 to 0.86 under nonstressed conditions, is a key parameter for assessing the maximum quantum efficiency of photosystem II (Ranjbarfordoei et al. 2006). The reduction in the photosynthetic rate in our study was likely due to the decreased F_v/F_m (Table 6). As NaCl concentration increased, the stomatal conductance and net photosynthetic rate of H. syriacus ‘Duede Brabaul’ and ‘Blueberry Smoothie’ decreased (Lu et al. 2022). Net photosynthetic rate became increasingly pronounced over time between H. syriacus ‘ILVOPS’ plants in the control and those irrigated with saline solution irrigation (Liu et al. 2017). Cirillo et al. (2019) revealed that V. lucidum exhibited similar reductions in stomatal conductance and net photosynthetic rate in response to salt treatment. V. tinus also showed decreased stomatal conductance and net photosynthetic rate when irrigated with reclaimed water at an EC of 6 dS·m⁻¹ (Gómez-Bellot et al. 2015). Ben-Asher et al. (2006) and Bongi and Loreto (1989) reported an increasing trend in leaf VPD in response to salinity stress in several plant species, which is consistent with our results.

Canopy temperature.

Saline solution irrigation significantly influenced the canopy temperature of the three ornamental species at both the first (P < 0.0001) and second (P < 0.0001) harvests (Table 1). At the first harvest, a significant difference in canopy temperature was observed when considering the interaction between salt treatment and plant species (P = 0.004). The canopy temperature of H. syriacus increased by 0.4 and 0.5 °C, compared with the control, when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively (Table 8). Meanwhile, compared with the control, canopy temperature significantly increased by 0.4 and 1.2 °C for V. carlesii and 0.8 and 0.3 °C for V. angus-castus, respectively, when plants were irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹. At the second harvest, saline solution at ECs of 5.0 and 10.0 dS·m⁻¹ increased the canopy temperature of H. syriacus by 1.6 and 2.6 °C, respectively, compared with the control. The canopy temperature of V. carlesii increased by 1.1 and 1.2 °C as the EC levels of saline solutions increased; however, no statistical significance was observed. In addition, compared with the control, canopy temperature significantly rose by 1.2 and 1.7 °C for V. angus-castus when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively.

Table 8. Canopy temperature of H. syriacus ‘JWNWOOD4’, V. carlesii ‘Spiro’, and V. agnus-castus ‘SMVACBD’ irrigated with a nutrient solution at an electrical conductivity (EC) of 0.9 dS·m⁻¹ (control) or saline solution at an EC of 5.0 dS·m⁻¹ (EC 5.0) or 10.0 dS·m⁻¹ (EC 10.0) after the fourth (first harvest) and the eighth (second harvest) irrigation event in a research greenhouse.ⁱ

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Partial stomatal closure under elevated salinity levels limited transpiration, resulting in increased canopy temperatures. Canopy temperature serves as a valuable indicator of salinity stress (Acosta-Motos et al. 2017; Costa et al. 2013; Khasanov et al. 2023). A previous study by Tian et al. (2020) reported that the canopy temperature of Medicago sativa ‘Golden Empress’ (alfalfa) increased by 0.88 to 1.19 °C as soil salinity increased from slight (1 to 2 dS·m⁻¹) to severe levels (5 to 13 dS·m⁻¹). In addition, an increase in the canopy temperature of Hordeum vulgare (barley) was observed when soil salinity increased from 0.50 to 2.00 dS·m⁻¹ (Isla et al. 1998).

Mineral analysis.

Na⁺ content in the leaf tissue of three ornamental species was significantly affected by salt treatment (P < 0.0001) and plant species (P < 0.0001) and the interactive effects between salt treatment and plant species (P = 0.03) (Table 9). Compared with the control, the leaf Na⁺ content increased by 4.8 times for H. syriacus, 15.5 times for V. carlesii, and 9.0 times for V. angus-castus when irrigated with a saline solution at an EC of 5.0 dS·m⁻¹. At an EC of 10.0 dS·m⁻¹, leaf Na⁺ content of H. syriacus, V. carlesii, and V. angus-castus was 9.4, 19.6, and 95.4 times greater than the control, respectively. Among the species, V. carlesii had the highest Na⁺ content (6.69 mg·g⁻¹) under irrigation with a saline solution at an EC of 10.0 dS·m⁻¹, which was over 9 times greater than that in H. syriacus (0.62 mg·g⁻¹). Leaf Na⁺ content exhibited negative correlations with visual score, plant growth rate, leaf area, and shoot dry weight (P < 0.0001, P = 0.004, P = 0.01, and P = 0.004, respectively). In addition, leaf Na⁺ content demonstrated negative correlations with stomatal conductance and net photosynthetic rate (P = 0.004 and P = 0.0002, respectively).

Table 9. Mineral elements, the ratio of K⁺ and Na⁺ (K⁺/Na⁺), and the ratio of Ca²⁺ and Na⁺ (Ca²⁺/Na⁺) of H. syriacus ‘JWNWOOD4’, V. carlesii ‘Spiro’, and V. agnus-castus ‘SMVACBD’ irrigated with a nutrient solution at an electrical conductivity (EC) of 0.9 dS·m⁻¹ (control) or saline solution at an EC of 5.0 dS·m⁻¹ (EC 5.0) or 10.0 dS·m⁻¹ (EC 10.0) in a research greenhouse.ⁱ

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Sodium, abundant in most agricultural soils and highly soluble, is not an essential mineral for the growth and development of most terrestrial plants. However, moderate to high Na⁺ levels can be detrimental to many plants (Coca et al. 2023; Maathuis 2014). High Na⁺ content causes leaf discoloration and necrosis, affecting the aesthetic quality. Moreover, high Na⁺ uptake restricts the transport processes of K⁺ in xylem tissues, thereby impairing osmotic adjustment and maintenance and inhibiting metabolic activities (Coca et al. 2023). In our study, a notable increase in Na⁺ content was observed in V. carlesii irrigated with saline solution at an EC of 5.0 dS·m⁻¹ (5.35 mg·g⁻¹). Furthermore, at an EC of 10.0 dS·m⁻¹, the Na⁺ content in the three species (0.62, 6.69, and 3.81 mg·g⁻¹ for H. syriacus, V. carlesii, and V. angus-castus, respectively) exceeded the typical plant levels, which are generally less than 0.5 mg·g⁻¹ (Pandey 2015).

The Cl⁻ content in leaf tissues showed a significant increase in response to elevated EC levels of saline solutions (P < 0.0001) (Table 9). A significant interaction between salt treatment and plant species (P < 0.0001), with variations among species (P < 0.0001), was also observed. In H. syriacus, the Cl⁻ content increased by 3.8 and 6.9 times when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively, compared with the control. Similarly, V. angus-castus accumulated 2.0 and 7.3 times more Cl⁻ ions irrigated with saline solution at ECs of 5.0 and 10.0 dS·m⁻¹, respectively, compared with the control. V. carlesii exhibited the most substantial increase in leaf Cl⁻ content, with 20.7 and 22.5 times greater leaf Cl⁻ content than the control when irrigated with saline solution at ECs of 5.0 and 10.0 dS·m⁻¹, respectively. Excessive Cl⁻ contents in leaves led to foliage damage, exhibiting a negative correlation with visual score (P < 0.0001). Elevated Cl⁻ content inhibited growth parameters, including plant growth rate, leaf area, and shoot dry weight (P = 0.01, P < 0.0001, and P < 0.0001, respectively). In addition, there was a decreasing trend in stomatal conductance with increasing Cl⁻ content (P = 0.04), although no significant correlation was observed on the net photosynthetic rate.

Geilfus (2018) reported typical Cl⁻ contents in plant shoots ranging from 1 to 20 mg·g⁻¹. In our study, H. syriacus irrigated with saline solution at an EC of 10.0 dS·m⁻¹ (29.33 mg·g⁻¹) and V. carlesii irrigated with saline solution at ECs of 5.0 and 10 dS·m⁻¹ (26.05 and 28.25 mg·g⁻¹, respectively) exhibited excessively high Cl⁻ contents. Chloride toxicity manifests as yellowing and bronzing of leaf tips and margins, followed by leaf death and abscission (Saeed et al. 2020). Another adverse effect of high Cl⁻ uptake includes impairing enzymes, damaging photosystem II reaction centers, and inhibiting CO₂ fixation, thereby reducing the net photosynthetic rate (Geilfus 2018).

Salinization, primarily caused by NaCl, can negatively affect various physiological, biochemical, and molecular processes, prompting numerous studies to focus on mineral analysis under saline conditions (Coca et al. 2023). Similar results were found by Feng et al. (2021), in whose study H. moscheutos (swamp rose mallow) grown under 200 mM NaCl (∼14.6 dS·m⁻¹) solution for 60 d exhibited Na⁺ and Cl⁻ contents 2.1 and 0.8 times greater than the control, respectively. V. tinus var. lucidum (laurustinus) irrigated with 80 mM NaCl (∼5.9 dS·m⁻¹) solution for 112 d accumulated Na⁺ and Cl⁻ contents 7.2 times and 0.5 times greater the control, respectively (De Micco et al. 2022). These findings suggest that viburnum does not rely on ion exclusion mechanisms in response to salinity stress. When exposed to 100% simulated seawater (∼23.9 dS·m⁻¹) for 10 d, V. trifolia var. simplicifolia (simpleleaf chaste tree) exhibited Na⁺ and Cl⁻ contents 0.7 and 1.8 times greater than the control, respectively (Yin et al. 2018). However, our results indicated even greater Na⁺ and Cl⁻ contents in plants than those in previous studies, likely attributable to the extended exposure time to saline conditions.

The leaf Ca²⁺ content was significantly influenced by saline solution irrigation (P < 0.0001) (Table 9). In this study, Ca²⁺ contents in plants irrigated with saline solution at ECs of 5.0 and 10.0 dS·m⁻¹ increased by 33% and 44% for H. syriacus and 82% and 55% for V. carlesii, respectively, compared with the control. However, no significant change in the leaf Ca²⁺ content was observed in V. angus-castus when subjected to saline solution irrigation. Furthermore, the Ca²⁺/Na⁺ ratio declined with increasing salinity levels (P < 0.0001). The Ca²⁺/Na⁺ ratios of H. syriacus, V. carlesii, and V. agnus-castus in the control were 3.4, 8.0, and 7.5 times greater than that under saline solution at an EC of 5.0 dS·m⁻¹, respectively. The Ca²⁺/Na⁺ ratios of H. syriacus, V. carlesii, and V. agnus-castus in the control were 6.2, 12.3, and 83.0 times greater than those irrigated with saline solution at an EC of 10.0 dS·m⁻¹, respectively. Furthermore, the Ca²⁺/Na⁺ ratio showed a positive correlation with visual score (P = 0.03). Our study also revealed a positive correlation between the Ca²⁺/Na⁺ ratio and stomatal conductance (P = 0.001).

Calcium is an essential nutrient for plants, playing crucial structural roles in cell walls and membranes and functioning as an intracellular messenger within the cytosol (White and Broadley 2003). In addition, Ca²⁺ acts as a mediator that regulates cellular responses to abiotic stresses (Thor 2019). For example, in response to salinity stress, plants employ a complex calcium response network involving Ca²⁺-integrated proteins, phytohormones, and other signaling factors (Bachani et al. 2022). The Ca²⁺ content varies with environmental availability and plant requirements, typically ranging from 1 to 50 mg·g⁻¹ (Thor 2019; White and Broadley 2003). Läuchli and Bieleski (2012) and Pandey (2015) noted that an average Ca²⁺ content of 5 mg·g⁻¹ is required for optimal plant growth. In our research, the Ca²⁺ contents ranged from 8.94 to 38.49 mg·g⁻¹, indicating that plants might not experience Ca²⁺ deficiency. In addition, an increase in Ca²⁺ content and a decrease in Ca²⁺/Na⁺ ratio were observed. In contrast, Ahmed (2017) reported that Ca²⁺ contents of H. rosa-sinensis (Chinese hibiscus) reduced as salinity levels elevated from 0 to 6000 ppm (∼0 to 7.5 dS·m⁻¹), with values ranging from 3.19 to 9.24 mg·g⁻¹. Chen et al. (2020) reported that saline solution at ECs of 5.0 and 10.0 dS·m⁻¹ increased Ca²⁺ contents in the leaves of 12 viburnum taxa while decreasing the Ca²⁺/Na⁺ ratio. Conversely, Ca²⁺ content in V. trifolia ‘Purpurea’ (Arabian lilac) leaves was negatively affected by salinity stress (Ashour et al. 2021). Although Ca²⁺ uptake was inhibited by sodium-induced salinity stress (Bachani et al. 2022), in our study, the increase in Ca²⁺ content is likely due to the addition of CaCl₂ to the saline solution.

The K⁺ content in the leaves of ornamental plants was affected by saline solution irrigation (P = 0.002) and varied among species (P < 0.0001) (Table 9). There was also a significant interactive effect between salt treatment and plant species (P = 0.02). Potassium contents decreased by 4% to 12% for H. syriacus and V. carlesii irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹; however, these differences were not statistically different from the control. In contrast, V. angus-castus exhibited a significant 35% decrease in K⁺ content when irrigated with a saline solution at an EC of 10.0 dS·m⁻¹, compared with the control. Moreover, our study revealed a decline in the K⁺/Na⁺ ratio with increasing salinity levels (P < 0.0001). Specifically, the K⁺/Na⁺ ratio of H. syriacus in the control was 5.7 and 10.3 times greater than those irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively. For V. carlesii, the K⁺/Na⁺ ratio in the control was 16.2 and 20.9 times greater than that at ECs of 5.0 and 10.0 dS·m⁻¹, respectively. Similar results were found for V. angus-castus, plants in the control had 10 and 140 times greater K⁺/Na⁺ ratios compared with plants at ECs of 5.0 and 10.0 dS·m⁻¹, respectively. However, the K⁺/Na⁺ ratio positively correlated with stomatal conductance (P = 0.02) only.

Potassium is essential for various plant growth processes, including enzyme activation, stomatal activity, photosynthesis, and transpiration (Prajapati and Mod 2012). However, Na⁺ accumulation can inhibit K⁺ uptake because they share the same binding sites on enzymes, disrupting plant metabolism (Kumari et al. 2021; Pandey and Mahiwal 2020). Thus, the retention of K⁺ and a high K⁺/Na⁺ ratio are recognized as beneficial traits for plant salt tolerance (Kumari et al. 2021). Läuchli and Bieleski (2012) and Pandey (2015) reported that the average K⁺ content in shoots is around 10 mg·g⁻¹. In our study, the average K⁺ content of V. angus-castus was 8.83 mg·g⁻¹ when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. Moreover, saline solution irrigation significantly reduced the K⁺/Na⁺ ratio in three ornamental species. Azooz (2009) found that K⁺ content and K⁺/Na⁺ ratio of H. sabdariffa (roselle) significantly decreased with increased salinity levels, which is in line with our findings. In contrast, Cassaniti et al. (2009) observed that the K⁺/Na⁺ ratio of V. lucidum did not significantly decrease as NaCl concentrations in the irrigation solution increased from 0 to 40 mM (∼0 to 3.7 dS·m⁻¹). Similar findings were also reported for V. negundo, which showed a significant decrease in K⁺ content and K⁺/Na⁺ ratio as salinity levels increased from 4 to 12 dS·m⁻¹ (Das et al. 2024).

The Mg²⁺ content exhibited significant differences as the EC levels increased (P = 0.002) (Table 9). Compared with the control, Mg²⁺ contents in H. syriacus increased by 25% when irrigated with a saline solution at an EC of 5.0 dS·m⁻¹. However, the Mg²⁺ contents of V. carlesii and V. angus-castus did not increase with increasing salinity levels of the irrigation solution. Magnesium, the second most abundant cation in plants, is one of the most important nutrients (Chen et al. 2018; Guo et al. 2016). It plays essential roles in chlorophyll pigments, photosynthesis, enzyme activation, and protein synthesis (Chen et al. 2018). Mg²⁺ deficiency greatly limits plant growth and development, with the content for optimal plant growth typically ranging from 1.5 to 3.5 mg·g⁻¹ (Guo et al. 2016; Läuchli and Bieleski 2012; Verbruggen and Hermans 2013). Mg²⁺ content in our research ranged from 2.00 to 8.27 mg·g⁻¹, and no Mg²⁺ deficiency was observed.

In addition, Mn²⁺ content in the leaf tissue of ornamental plants increased along with elevated salinity levels in the irrigation solution (P < 0.0001) (Table 9). When irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, H. syriacus showed 65% and 67% increases in Mn²⁺ content, respectively, although no significant difference was observed. Meanwhile, Mn²⁺ contents showed a significant increment for V. carlesii and V. angus-castus as the EC levels of the irrigation solution increased. Saline solution irrigation significantly increased the Mn²⁺ content by 126% and 108% for V. carlesii and by 76% and 102% for V. angus-castus when irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively. Manganese is an important micronutrient in plant growth and development, involved in numerous enzyme activations (Schmidt and Husted 2019). Läuchli and Bieleski (2012) and Pandey (2015) found that Mn²⁺ content in plant shoots is around 0.05 mg·g⁻¹ for optimal plant growth. In our study, Mn²⁺ contents increased with rising salinity levels, with values ranging from 0.05 to 0.31 mg·g⁻¹.

Elevated salinity levels affected Zn²⁺ content in the leaves of the ornamental plants (P < 0.0001) (Table 9). Zn²⁺ content in H. syriacus was not statistically different among treatments, although saline solution at ECs of 5.0 and 10.0 dS·m⁻¹ increased Zn²⁺ content by 48% and 20%, respectively, compared with the control. In contrast, there were 85% and 60% increases in Zn²⁺ content of V. carlesii and V. angus-castus, respectively, when irrigated with a saline solution at an EC of 10.0 dS·m⁻¹, compared with the control. Zinc is a critical element in plants, interacting with thousands of proteins (Broadley et al. 2007). In addition, Zn²⁺ serves as an essential nutrient in alleviating the adverse effects of abiotic stresses, by enhancing plant growth, chlorophyll synthesis, and membrane protection (Shao et al. 2023). The typical Zn²⁺ content for adequate plant growth ranges from 0.015 to 0.02 mg·g⁻¹ (Läuchli and Bieleski 2012; Pandey 2015; Shao et al. 2023). In our research, the Zn²⁺ contents in plant leaves were 0.02 to 0.06 mg·g⁻¹, which falls above the healthy range.

No significant difference in Fe³⁺ content was found for all species, except for V. carlesii (Table 9). Fe³⁺ content increased by 41% for V. carlesii when saline solution at an EC of 5.0 dS·m⁻¹ was applied; however, it was not statistically different from the control. Iron is the fourth most abundant element in the Earth’s crust and an important micronutrient for plants, involving ribonucleotides and molecular nitrogen reductions, electron transfer, and photosynthesis (Lemanceau et al. 2009). From a survey conducted by Ancuceanu et al. (2015), the Fe³⁺ content of 1228 species has a median value of 0.51 mg·g⁻¹, whereas Läuchli and Bieleski (2012) and Pandey (2015) reported the average Fe³⁺ content of higher plants around 0.1 mg·g⁻¹. In our research, the Fe³⁺ content of H. syriacus varied from 0.32 to 0.40 mg·g⁻¹, which is within the health range. However, V. carlesii and V. angus-castus might experience iron deficiency, with content ranging from 0.06 to 0.14 mg·g⁻¹.

Salt treatment significantly affected P content in the leaf tissue of ornamental plants (P = 0.02) and varied among species (P < 0.0001) (Table 9). When irrigated with saline solution at an EC of 10.0 dS·m⁻¹, V. carlesii showed a 32% decrease in P content, whereas elevating EC levels did not affect P contents of the other two species. Phosphorus is one of the major nutrients for plants, involving respiration, the photosynthesis process, and the synthesis of DNA, phospholipids, and ATP (Pang et al. 2024). Previous investigation has reported that P deficiency limits plant CO₂ assimilation capacity, induces possible photosynthetic inhibition, and causes damage to photosystem II (Pang et al. 2024; Xu et al. 2007). The typical P content for adequate growth is around 1.9 mg·g⁻¹. In our study, P contents ranged from 2.28 to 4.75, indicating no P deficiency (Läuchli and Bieleski 2012; Pandey 2015).

Furthermore, this study revealed a remarkable decrease in S content with rising EC levels, with an interaction between salt treatment and plant species (both P < 0.0001) (Table 9). V. carlesii irrigated with saline solution at ECs of 5.0 and 10.0 dS·m⁻¹ exhibited decreases in S content by 37% and 41%, respectively, compared with the control. However, S contents of H. syriacus and V. angus-castus remained unchanged when receiving saline solution irrigation, with less than a 13% decrease compared with the control. Sulfur is essential for plant growth and development, contributing to chlorophyll formation, protein production, nutrient uptake, and enzyme activation (Narayan et al. 2023). The typical healthy S content in plants is around 1.0 mg·g⁻¹, as reported by Läuchli and Bieleski (2012) and Pandey (2015). From our observations, the S contents of plants ranged from 1.04 to 2.29 mg·g⁻¹, which is above the typical healthy range.