Plant materials and culture.

The study was conducted in a research greenhouse at USU in Logan, UT (lat. 41°45′28″N, long. 111°48′48″W, elevation 1409 m). On 24 Aug. 2018, 1-year-old Sego Supreme^TM plants produced from cuttings were received in a square pot (10.5 × 10.5 × 12.3 cm) from the USU Botanical Center (Kaysville, UT). On 27 Aug. 2018, plants were transplanted into 3.8-L injection-molded, polypropylene container (PC1D-4; Nursery Supplies, Orange, CA) filled with a soilless growing substrate consisting of 75% peatmoss (Canadian sphagnum peatmoss; SunGro Horticulture, Agawam, MA), 25% vermiculite (Therm-O-Rock West, Chandler, AZ), and 24.3 g·ft⁻³ white athletic field marking gypsum (92% calcium sulfate dihydrate, 21% calcium, 17% sulfur; Western Mining and Minerals, Bakersfield, CA). All plants were watered with tap water (EC = 0.344 dS·m⁻¹; pH = 7.65). During the experiment, aphids (Aphidoidea) and whiteflies (Aleyrodidae) were observed on C. fruticosa. To control aphids and whiteflies, all plants were sprayed with abamectin (Avid^® 0.15 EC; Syngenta Crop Protection, Greensboro, NC) at a rate of 0.1 mL per gallon as needed. During the experimental period, the average air temperature in the greenhouse was 24.9 ± 1.2 °C during the day and 22.0 ± 2.4 °C at night. The average daily light integral in the greenhouse was 24.8 ± 12.5 mol·m⁻²·d⁻¹. Supplemental light at 160.4 µmol·m⁻²·s⁻¹ was provided using 1000-W high-pressure sodium lamps (Hydrofarm, Petaluma, CA) from 600 to 2200 HR when light intensity inside the greenhouse was less than 544 µmol·m⁻²·d⁻¹ from 10 Oct. to 14 Dec. 2018. Supplemental light was measured using a full-spectrum quantum meter (MQ-500; Apogee Instrument, Logan, UT).

Treatments.

Plants were pruned to uniform height (≈13 cm), and flowers were removed before the experiment was initiated. From 15 Oct. to 3 Dec. 2018, treatment solutions were applied on a weekly basis up to 8 weeks. Plants were irrigated with 1 L of nutrient solution (control) at an EC of 1.25 dS·m⁻¹ or saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹ (Table 1) with a leaching fraction of 15.1% ± 6.4%. In addition, plants were watered with 500 mL of nutrient solution whenever the substrate surface became dry. The nutrient solution was prepared in a 100-L tank by adding 0.8 g·L⁻¹ of 15N–2.2P–12.5K water-soluble fertilizer (Peters Excel 15–5–15 Cal-Mag Special; ICL Specialty Fertilizers, Dublin, OH) to tap water and used as a control. The saline solution was prepared with an addition of sodium chloride (NaCl; Fisher Chemical, Logan, UT) and calcium chloride (CaCl₂; Fisher Chemical, Logan, UT) at a molar ratio of 2:1 in the nutrient solution. The ECs of the solutions, measured using an EC meter (LAQUA Twin; Horiba, Kyoto, Japan), were 1.2 ± 0.1 (control, nutrient solution), 2.5 ± 0.2 (EC 2.5), 4.7 ± 0.1 (EC 5), 7.5 ± 0.2 (EC 7.5), and 9.8 ± 0.2 dS·m⁻¹ (EC 10) (mean ± sd) during the experiment. The pH of the solutions was adjusted to 6.7 ± 0.4 using 1 m nitric acid.

Table 1.

The chemical compositions of tap water, nutrient solution (control), and saline solutions used in the experiment.^z

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Data collection.

Leachate solution was collected during each application of treatment solutions. The EC of the leachate solution was determined on one plant per species per treatment using the pour-through method described by Cavins et al. (2008). Plant heights, the length of the longest shoot, were measured at the start and end of the experiment. A visual score of 0 to 5 was assigned to each plant where 0 = dead, 1 = severe foliar damage (>90% leaves with burn, necrosis, and discoloration), 2 = moderate foliar damage (90% to 50%), 3 = slight foliar damage (50% to 10%), 4 = minimal foliar damage (<10%), and 5 = excellent without foliar damage (Sun et al., 2015). Plant size was not considered while assigning the visual score. Leaf greenness of each plant was measured 4 d before harvest using a handheld chlorophyll meter [soil-plant analysis development (SPAD) 502 Plus; Minolta Camera Co., Osaka, Japan]. Four fully expanded leaves from each plant were chosen for the measurements. The number of flowers of E. septentrionale and T. acaulis were counted, whereas A. barnebyi and C. fruticosa did not flower during the experimental period. The number of shoots were counted, and leaf area was measured using an area meter (LI-3100; LI-COR^® Biosciences, Lincoln, NE). In addition, shoot dry weight was taken after plants were dried in an oven at 70 °C for 3 d.

Mineral analyses.

Three Sego Supreme^TM plants per species per treatment were selected randomly and ground with a stainless Wiley mill (Thomas Scientific, Swedesboro, NJ) and allowed to pass through a 1-mm mesh screen. Powder samples were sent to the USU Analytical Laboratories for mineral analyses. In brief, the powder samples were extracted using 2% acetic acid (Fisher Scientific, Fair Lawn, NJ) following the protocol described in Miller et al. (2013). The Cl^– concentration was determined using a flow injection analysis and ion chromatography system (QuikChem 8000; Lachat Instrument, Loveland, CO) and reported on a dry plant basis (mg·g⁻¹). For Na⁺, calcium (Ca²⁺), and potassium (K⁺), 0.5 g of powder samples and 8 mL of nitric acid (HNO₃) were added into a digestion tube that was then placed in a digestion block (Environmental Express, Charleston, SC) at 95 °C for 1 h and subsequently cooled for 15 to 20 min. A total of 4 mL of 30% hydrogen peroxide (H₂O₂) was added into the digestion tube that was placed again in the digestion block at 95 °C for 30 min. The H₂O₂ addition was repeated two more times, and the tube was cooled for 15 to 20 min between H₂O₂ additions. Then the digestion tube was cooled at a room temperature, and deionized water was added to bring the final volume up to 25 mL. The digest was analyzed using an inductively coupled plasma-optical emission spectrometry (iCAP 6300 ICP-AES; Thermo Scientific, Waltham, MA) and reported on a dry plant basis (mg·g⁻¹).

Experimental design and data analyses.

The experiment was conducted using a randomized complete block design with six blocks. Each block consisted of 20 plants (four species and five treatments). A two-way analysis of variance was conducted to test the effects of salinity and species on plant growth and mineral nutrient data. Linear and quadratic trend analysis was performed for all data. Correlation analysis was also carried out between mineral concentration and visual quality. All statistical analyses were performed using a generalized linear model in JMP (Version 13.2; SAS Institute, Cary, NC).

Leachate and substrate EC.

As Sego Supreme^TM plants were irrigated with saline solution, the salinity levels of leachate solution gradually increased (Fig. 1). When irrigated with saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹, the ECs of the leachate solutions increased from 2.3 to 5.5 dS·m⁻¹, 3.6 to 12.4 dS·m⁻¹, 4.9 to 16.5 dS·m⁻¹, and 6.4 to 18.8 dS·m⁻¹, respectively. These values were greater than the EC of the leachate solution (1.7 ± 0.2 dS·m⁻¹) when the nutrient solution was used. Leachate solution EC increased over time with increasing salinity of irrigation water, which verifies that salts accumulate in plant rhizosphere (Wu et al., 2016a).

Electrical conductivity (EC) of leachate solution collected after irrigating the Sego Supreme^TM plants with a nutrient solution at an EC of 1.25 dS·m⁻¹ or a saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹. A nutrient solution at an EC of 1.25 dS·m⁻¹ was prepared by adding 0.8 g·L⁻¹ 15 N–2.2P–12.5K water-soluble fertilizer (Peters Excel 15–5–15 Cal-Mag Special) to tap water. Sodium chloride (NaCl) and calcium chloride (CaCl₂) were added to the nutrient solution to make the saline solutions. Vertical bars represent standard errors of four measurements, one plant per treatment per species.

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Electrical conductivity (EC) of leachate solution collected after irrigating the Sego Supreme^TM plants with a nutrient solution at an EC of 1.25 dS·m⁻¹ or a saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹. A nutrient solution at an EC of 1.25 dS·m⁻¹ was prepared by adding 0.8 g·L⁻¹ 15 N–2.2P–12.5K water-soluble fertilizer (Peters Excel 15–5–15 Cal-Mag Special) to tap water. Sodium chloride (NaCl) and calcium chloride (CaCl₂) were added to the nutrient solution to make the saline solutions. Vertical bars represent standard errors of four measurements, one plant per treatment per species.

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Electrical conductivity (EC) of leachate solution collected after irrigating the Sego Supreme^TM plants with a nutrient solution at an EC of 1.25 dS·m⁻¹ or a saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹. A nutrient solution at an EC of 1.25 dS·m⁻¹ was prepared by adding 0.8 g·L⁻¹ 15 N–2.2P–12.5K water-soluble fertilizer (Peters Excel 15–5–15 Cal-Mag Special) to tap water. Sodium chloride (NaCl) and calcium chloride (CaCl₂) were added to the nutrient solution to make the saline solutions. Vertical bars represent standard errors of four measurements, one plant per treatment per species.

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Visual quality.

Saline solution had different effects on the visual quality of the four Sego Supreme^TM plant species (Fig. 2A). The visual score of A. barnebyi and E. septentrionale decreased linearly [r² = 0.95 (P < 0.0001) and r² = 0.94 (P < 0.0001), respectively] with increasing EC levels in the treatment solution. However, the visual score of C. fruticosa and T. acaulis decreased quadratically [R² = 0.998 (P = 0.008) and R² = 0.995 (P = 0.0002), respectively] as EC levels increased. Aquilegia barnebyi showed slight foliar salt damage when irrigated with saline solution at an EC of 5.0 dS·m⁻¹ with an average visual score of 3.7, but more than 50% of plants died when irrigated with saline solution at an EC of 7.5 or 10.0 dS·m⁻¹. Similarly, Aquilegia canadensis (eastern red columbine) plants had moderate foliar salt damage when irrigated weekly with saline solution at an EC of 5.0 dS·m⁻¹ for 8 weeks, and all plants died at an EC of 10.0 dS·m⁻¹ (Wu et al., 2016a). Gerber et al. (2011) also reported that Aquilegia × cultorum ‘Crimson Star’ plants started to show foliar damage (marginal necrosis, chlorosis, and purpling) at the second weeks when they were watered with 0.05, 0.15, and 0.25 m NaCl solution (corresponding to about EC of 4.6, 11.0, and 18.3 dS·m⁻¹, respectively). Clematis fruticosa, E. septentrionale, and T. acaulis irrigated with saline solution at an EC of 10.0 dS·m⁻¹ had no or minimal foliar salt damage with averaged visual score of 4.2, 4.1, and 4.3, respectively. These results demonstrated that C. fruticosa, E. septentrionale, and T. acaulis had a strong tolerance to the salinity levels tested in this study. Likewise, in a 3-year field study, it is reported that all Clematis ispahanica plants survived without any foliar damage and their visual quality was still acceptable up to an EC of 16.3 dS·m⁻¹ in the treatment solution (Razmjoo and Aslani, 2018). Conversely, Wu et al. (2016b) reported that Tetraneuris scaposa (four-nerve daisy) showed moderate to severe foliar salt damage when irrigated with saline solution at an EC of 5.0 dS·m⁻¹ for 5 weeks and were almost all dead at an EC of 10.0 dS·m⁻¹.

Visual score (A), height (B), leaf area (C), and number of shoots (D) of Sego Supreme^TM plants after irrigating a nutrient solution at an EC of 1.25 dS·m⁻¹ or a saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹. Visual score reference scale: 0 = dead; 1 = severe foliar damage (>90% leaves with burn, necrosis, and discoloration); 2 = moderate foliar damage (50% to 90%); 3 = slight foliar damage (50% to 10%); 4 = good quality with minimal foliar damage (<10%); 5 = excellent without foliar damage. A nutrient solution at an EC of 1.25 dS·m⁻¹ was prepared by adding 0.8 g·L⁻¹ 15 N–2.2P–12.5K water-soluble fertilizer (Peters Excel 15–5–15 Cal-Mag Special) to tap water. Sodium chloride (NaCl) and calcium chloride (CaCl₂) were added to the nutrient solution to make the saline solutions. Vertical bars represent standard errors of six measurements.

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Visual score (A), height (B), leaf area (C), and number of shoots (D) of Sego Supreme^TM plants after irrigating a nutrient solution at an EC of 1.25 dS·m⁻¹ or a saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹. Visual score reference scale: 0 = dead; 1 = severe foliar damage (>90% leaves with burn, necrosis, and discoloration); 2 = moderate foliar damage (50% to 90%); 3 = slight foliar damage (50% to 10%); 4 = good quality with minimal foliar damage (<10%); 5 = excellent without foliar damage. A nutrient solution at an EC of 1.25 dS·m⁻¹ was prepared by adding 0.8 g·L⁻¹ 15 N–2.2P–12.5K water-soluble fertilizer (Peters Excel 15–5–15 Cal-Mag Special) to tap water. Sodium chloride (NaCl) and calcium chloride (CaCl₂) were added to the nutrient solution to make the saline solutions. Vertical bars represent standard errors of six measurements.

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Visual score (A), height (B), leaf area (C), and number of shoots (D) of Sego Supreme^TM plants after irrigating a nutrient solution at an EC of 1.25 dS·m⁻¹ or a saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹. Visual score reference scale: 0 = dead; 1 = severe foliar damage (>90% leaves with burn, necrosis, and discoloration); 2 = moderate foliar damage (50% to 90%); 3 = slight foliar damage (50% to 10%); 4 = good quality with minimal foliar damage (<10%); 5 = excellent without foliar damage. A nutrient solution at an EC of 1.25 dS·m⁻¹ was prepared by adding 0.8 g·L⁻¹ 15 N–2.2P–12.5K water-soluble fertilizer (Peters Excel 15–5–15 Cal-Mag Special) to tap water. Sodium chloride (NaCl) and calcium chloride (CaCl₂) were added to the nutrient solution to make the saline solutions. Vertical bars represent standard errors of six measurements.

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Plant height.

Height of A. barnebyi, C. fruticosa, E. septentrionale, and T. acaulis decreased linearly [r² = 0.96 (P < 0.0001), r² = 0.94 (P = 0.0001), r² = 0.98 (P < 0.0001), and r² = 0.98 (P < 0.0001), respectively] as EC levels increased (Fig. 2B). Height of the four Sego Supreme^TM plants irrigated with saline solution at an EC of 2.5 dS·m⁻¹ were similar to that of the control. Saline solution at an EC of 5.0 dS·m⁻¹ also did not affect the growth of C. fruticosa, but plants irrigated with saline solutions at an EC of 7.5 and 10.0 dS·m⁻¹ were 13.4% and 19.6% shorter than the control, respectively. When plants were irrigated with saline solutions at an EC of 5.0, 7.5, and 10.0 dS·m⁻¹, plants were 36.7% to 63.7% shorter for A. barnebyi, 12.6% to 35.4% shorter for E. septentrionale, and 18.0% to 33.6% shorter for T. acaulis, respectively, compared with the respective control. Similarly, Wu et al. (2016a) observed a 75% reduction in a growth index (average of height and two perpendicular crown diameter) of A. canadensis when they were irrigated weekly with saline solution at an EC of 10.0 dS·m⁻¹ for 8 weeks. Razmjoo and Aslani (2018) also found a 15% reduction in height of C. ispahanica when irrigated with saline solution at an EC of 16.3 dS·m⁻¹. In a 4-week study by Hadi et al. (2014), there was a significant reduction in growth of Epilobium laxum (lax willowherb) at 6000 ppm NaCl solution (EC of ≈7.5 dS·m⁻¹). Tetraneuris scaposa also had a 37.2% reduction in plant height when they were irrigated weekly with saline solution at an EC of 10.0 dS·m⁻¹ for 5 weeks (Wu et al., 2016b).

Leaf area.

Leaf area decreased linearly for C. fruticosa and T. acaulis [r² = 0.98 (P < 0.0001), and r² = 0.91 (P < 0.0001), respectively], but quadratically for A. barnebyi and E. septentrionale [R² = 0.98 (P = 0.0005) and R² = 0.98 (P = 0.0004), respectively] with increasing EC levels (Fig. 2C). Compared with the control, the leaf area of A. barnebyi, C. fruticosa, E. septentrionale, and T. acaulis decreased by 82.5%, 56.2%, 82.5%, and 65.1%, respectively, when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. Even at an EC of 5.0 dS·m⁻¹, A. barnebyi and E. septentrionale had a high reduction in leaf area of 71.2% and 55.7%, respectively. Similar results regarding the reduction of leaf area with increasing concentration of saline solution were observed in previous studies on A. canadensis and Tagetes sp. (marigold) (Sun et al., 2018; Wu et al., 2016a).

Number of shoots and flowers.

Saline solution irrigation affected the number of shoots, but there were no interactive effects between salt treatment and Sego Supreme^TM plant species. With increasing EC levels, the number of shoots of C. fruticosa and T. acaulis decreased linearly [r² = 0.88 (P = 0.0003) and r² = 0.84 (P = 0.004), respectively], but the number of shoots of E. septentrionale decreased quadratically [R² = 0.98 (P = 0.0007)] (Fig. 2D). Similarly, Sun et al. (2015) reported that the number of shoots of M. arboreus, S. leucantha, and V. ×hybrida decreased significantly when irrigated with saline solution at an EC of 10.0 dS·m⁻¹ for 8 weeks, whereas the number of shoots of S. farinacea was not reduced. Although the number of shoots of T. scaposa tended to decrease with saline solution irrigation at an EC of 10.0 dS·m⁻¹ for 5 weeks but still was not significant (Wu et al., 2016b). In addition, A. barnebyi had no significant reduction in the number of shoots in this study.

In the same way, increasing salt levels of the irrigation water reduced the number of flowers linearly in E. septentrionale and T. acaulis [r² = 0.74 (P = 0.0001) and r² = 0.97 (P = 0.001), respectively] (Fig. 3A). Compared with the control, the number of flowers of E. septentrionale and T. acaulis decreased by 46.5% and 50.3%, respectively, when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. According to the study by Wu et al. (2016b), T. scaposa had a 50% reduction in the number of flowers when irrigated weekly with saline solution at an EC of 5.0 dS·m⁻¹ for 5 weeks. High salinity levels can affect the development of flower buds, which fail to open or grow in flowering woody shrubs or trees (Azza-Mazher et al., 2007). Aquilegia barnebyi and C. fruticosa did not produce any flowers during the experimental period. However, lower numbers of shoots may help to predict a reduction in the number of flowers with increasing salt concentrations in the irrigation water. Razmjoo and Aslani (2018) actually observed a 20% reduction in the number of flowers of C. ispahanica when irrigated with saline solution at an EC of 16.3 dS·m⁻¹.

Number of flowers (A), shoot dry weight (B), and relative chlorophyll content [soil-plant analysis development (SPAD)] (C) of Sego Supreme^TM plants after irrigating with a nutrient solution at an electrical conductivity of 1.25 dS·m⁻¹ or a saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹. A nutrient solution at an EC of 1.25 dS·m⁻¹ was prepared by adding 0.8 g·L⁻¹ 15N–2.2P–12.5K water-soluble fertilizer (Peters Excel 15–5–15 Cal-Mag Special) to tap water. Sodium chloride (NaCl) and calcium chloride (CaCl₂) were added to the nutrient solution to make the saline solutions. No flowers were observed in A. barnebyi and C. fruticosa during the experimental period. Clematis fruticosa and E. septentrionale had no significant reduction in SPAD readings with increasing EC levels. Vertical bars represent standard errors of six measurements.

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Number of flowers (A), shoot dry weight (B), and relative chlorophyll content [soil-plant analysis development (SPAD)] (C) of Sego Supreme^TM plants after irrigating with a nutrient solution at an electrical conductivity of 1.25 dS·m⁻¹ or a saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹. A nutrient solution at an EC of 1.25 dS·m⁻¹ was prepared by adding 0.8 g·L⁻¹ 15N–2.2P–12.5K water-soluble fertilizer (Peters Excel 15–5–15 Cal-Mag Special) to tap water. Sodium chloride (NaCl) and calcium chloride (CaCl₂) were added to the nutrient solution to make the saline solutions. No flowers were observed in A. barnebyi and C. fruticosa during the experimental period. Clematis fruticosa and E. septentrionale had no significant reduction in SPAD readings with increasing EC levels. Vertical bars represent standard errors of six measurements.

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Number of flowers (A), shoot dry weight (B), and relative chlorophyll content [soil-plant analysis development (SPAD)] (C) of Sego Supreme^TM plants after irrigating with a nutrient solution at an electrical conductivity of 1.25 dS·m⁻¹ or a saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹. A nutrient solution at an EC of 1.25 dS·m⁻¹ was prepared by adding 0.8 g·L⁻¹ 15N–2.2P–12.5K water-soluble fertilizer (Peters Excel 15–5–15 Cal-Mag Special) to tap water. Sodium chloride (NaCl) and calcium chloride (CaCl₂) were added to the nutrient solution to make the saline solutions. No flowers were observed in A. barnebyi and C. fruticosa during the experimental period. Clematis fruticosa and E. septentrionale had no significant reduction in SPAD readings with increasing EC levels. Vertical bars represent standard errors of six measurements.

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Shoot dry weight.

Shoot dry weight decreased quadratically for A. barnebyi and E. septentrionale [R² = 0.95 (P = 0.001) and R² = 0.99 (P = 0.002), respectively] with increasing EC levels, but decreased linearly for C. fruticosa and T. acaulis [r² = 0.92 (P < 0.0001) and r² = 0.88 (P = 0.0017), respectively] (Fig. 3B). Compared with controls, the shoot dry weight of A. barnebyi, C. fruticosa, E. septentrionale, and T. acaulis irrigated with saline solution at an EC of 10.0 dS·m⁻¹ decreased by 71.3%, 56.3%, 69.7%, and 48.1%, respectively. Elevated salinity has been reported to reduce plant shoot dry weight in various ornamental plants such as A. canadensis, aster perennials, E. laxum, and rose rootstocks (Hadi et al., 2014; Niu et al., 2008; Wu et al., 2016a, 2016b). This is a consequence of salinity-induced water deficit, resulting in stunted growth and reduced leaf size described earlier.

Relative chlorophyll content (SPAD).

Elevated salinity affected the SPAD readings of the four Sego Supreme^TM plants with varying responses among species. Clematis fruticosa and T. acaulis had similar SPAD readings among different EC levels, which indicated that increased salinity level did not affect their relative chlorophyll content, but the SPAD readings decreased linearly [r² = 0.42 (P = 0.05)] for A. barnebyi and quadratically [R² = 0.75 (P = 0.02)] for E. septentrionale (Fig. 3C). Saline solution at an EC of 10.0 dS·m⁻¹ decreased the SPAD readings of A. barnebyi and E. septentrionale by 17.7% and 6.2%, respectively. Similarly, Sun et al. (2015) documented that the SPAD readings of M. arboreus, R. brittoniana, and V. ×hybrida did not change, but S. farinacea and S. leucantha decreased significantly when irrigated weekly with saline solution at an EC of 10.0 dS·m⁻¹ for 8 weeks. In a study of rose rootstocks by Niu et al. (2008), Rosa ×fortuniana had greater SPAD readings compared with R. ×multiflora and R. ×odorata when irrigated with saline solution at an EC of 6.0 dS·m⁻¹ for 15 weeks. These results consistently indicated that salinity stress negatively affected the chlorophyll content and the effect is dependent on plant species.

Mineral analyses.

Sodium and Cl^– concentrations in the leaf tissue of Sego Supreme^TM plants were significantly affected by both elevated salinity and plant species (Table 2). Sodium concentration of A. barnebyi, C. fruticosa, E. septentrionale, and T. acaulis increased linearly [all r² values ≥0.93 (all P values < 0.0001)] with increasing salt levels in the irrigation water. Compared with the respective control, A. barnebyi, C. fruticosa, E. septentrionale, and T. acaulis had 4 to 7 times increased Na⁺ concentration when irrigated with saline solution at an EC of 2.5 dS·m⁻¹. Similarly, the Na⁺ concentrations were further increased by 17, 37, and 53 times for A. barnebyi; 13, 25, and 33 times for C. fruticosa; 9, 26, and 38 times for E. septentrionale; and 8, 18, and 35 times for T. acaulis when plants irrigated with saline solutions at ECs of 5.0, 7.5, and 10.0 dS·m⁻¹, respectively. Chloride concentration of A. barnebyi, C. fruticosa, E. septentrionale, and T. acaulis increased linearly [all r² values ≥0.93 (all P values < 0.0001)] with increasing amount of salts in the irrigation water (Table 2). Compared with the respective control, the Cl^– concentration increased by 7, 24, 41, and 48 times for A. barnebyi, 6, 10, 13, and 16 times for C. fruticosa, 2, 6, 15, and 17 times for E. septentrionale, and 4, 5, 14, and 17 times for T. acaulis when plants were irrigated with saline solutions at ECs of 2.5, 5.0, 7.5, and 10.0 dS·m⁻¹, respectively. Wu et al. (2016b) observed that T. scaposa irrigated weekly with saline solutions at an EC of 5.0 and 10.0 dS·m⁻¹ for 5 weeks had 29 and 58 times more Na⁺ concentration and 9 and 21 times more Cl^– concentration in leaves than the control. These results indicate that T. scaposa accumulated much more Na⁺ concentration in the leaves than T. acaulis in the study, but similar Cl^– concentrations are observed in the leaves of both species. In another similar study, A. canadensis also accumulated a significant amount of Na⁺ and Cl^– ions in the leaves (Wu et al., 2016a), which were 6 and 35 times more for saline irrigation water at an EC of 5.0 dS·m⁻¹, respectively, and 70 and 62 times more at an EC of 10.0 dS·m⁻¹, respectively, compared with their respective control.

Table 2.

Leaf ion concentrations of Sego Supreme^TM plants irrigated with a nutrient solution at an electrical conductivity (EC) of 1.25 dS·m⁻¹ or a saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹ for 8 weeks.^z

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Sodium and Cl^– are common ions present in reclaimed water and cause leaf injury to plants (Wahome et al., 2001). Generally, salt-tolerant ornamental plants accumulate less Na⁺ and Cl^– ions in their leaves when compared with salt-sensitive plants (Wu et al., 2016a). In this study, Na⁺ and Cl^– concentrations significantly increased in A. barnebyi compared with the other three species, which is in line with the lower visual quality of A. barnebyi with increasing salt levels in the irrigation water. There was a significant correlation of visual score to Na⁺ [r = –0.94 (P < 0.0001)] and Cl^– [r = –0.96 (P < 0.0001)] concentration. These results point out that, like A. canadensis, A. barnebyi has low tolerance of Na⁺ and Cl^– accumulation and poor potential to exclude these ions from plant tissue. On the other hand, some plants can tolerate accumulated Na⁺ and Cl^– in shoot tissue (Munns and Tester, 2008). In this study, T. acaulis and C. fruticosa accumulated high levels of Na⁺ and Cl^- ions in the leaves and still had good visual quality. These results indicate that T. acaulis and C. fruticosa can tolerate high accumulation of Na⁺ and Cl^– ions in the tissue. Krupenikov (1946) also reported that Clematis orientalis (chinese clematis) was tolerant over 2.0% of total salts, which is calculated on the basis of solid residue. Similarly, Wu et al. (2016a) indicated that Scabiosa columbaria (butterfly blue), Lobelia cardinalis (cardinal flower), A. canadensis, and Cuphea hyssopifolia (mexican false heather) plants had severe foliar salt damage with increasing Na⁺ and Cl^– concentration in shoots, whereas Dicliptera suberecta (Mexican hummingbird bush) and Cestrum diurnum × C. nocturnum ‘Orange Peel’ (orange peel jessamine) plants had the ability to tolerate high Na⁺ and Cl^– accumulation without a reduction in visual quality.

Calcium concentration in the leaf tissue was affected by both salt treatment and plant species interactively (Table 2). The Ca²⁺ concentration in the leaves of A. barnebyi, C. fruticosa, E. septentrionale, and T. acaulis increased linearly [r² = 0.96 (P < 0.0001), r² = 0.83 (P = 0.008), r² = 0.91 (P < 0.0001), and r² = 0.91 (P = 0.0004), respectively] with elevated salt levels in the irrigation water. There was only a slight increase in the Ca²⁺ concentration compared with the increment in Na⁺ and Cl^– concentration, although CaCl₂ was used to prepare the saline solution. Compared with the control, leaf Ca²⁺ concentration of A. barnebyi irrigated with saline solution at an EC of 10.0 dS·m⁻¹ increased by 175.3%, which was the greatest increment observed among all tested plants in all treatments. In addition, there was a significant correlation of visual score and Ca²⁺ concentration of A. barnebyi [r = –0.89 (P < 0.0001)]. Nazrul-Islam (1986) reported that Epilobium hirsutum (great willowherb) grows well in calcareous soil and is susceptible to lower calcium level. Similarly, in the present study calcium level may not be sufficient for E. septentrionale at higher EC levels and they showed more reduction in shoot dry weight when compared with C. fruticosa and T. acaulis. Moreover, plant requirements for Ca²⁺ ions increase with increasing concentration of salts in the root zone. However, Ca²⁺ transport and mobility to growing parts of plant is reduced by Na⁺ dominated salinity stress (Grattan and Grieve, 1999).

Increasing salt concentration in the irrigation water linearly decreased the amount of K⁺ ions in the leaves of C. fruticosa, E. septentrionale, and T. acaulis (Table 2). However, elevated salinity did not affect the K⁺ concentration of A. barnebyi. Compared with the control, the leaf K⁺ concentration of C. fruticosa, E. septentrionale, and T. acaulis irrigated with saline solution at an EC of 10.0 dS·m⁻¹ declined by 25.1%, 32.7%, and 43.8%, respectively. When plants are exposed to salinity stress induced by high NaCl levels, Na⁺ accumulation increases and causes a reduction in K⁺ concentration (Hasegawa et al., 2000). Similar observations are made on Chrysactinia mexicana (damianita), T. scaposa, Santolina chamaecyparissus (lavender cotton), Leucanthemum ×superbum (Shasta daisy), and Viguiera stenoloba (skeletonleaf goldeneye) (Wu et al., 2016b), S. columbaria and C. diurnum × C. nocturnum ‘Orange Peel’, but not for A. Canadensis (Wu et al., 2016a).

In summary, Sego Supreme^TM plants had different responses to saline water irrigation. Clematis fruticosa and T. acaulis were the most tolerant perennials with the least reduction in visual score and growth when irrigated with saline solution. Epilobium septentrionale was moderately tolerant, and A. barnebyi was the least tolerant perennial to saline solution irrigation with the most reduced visual quality and growth while showing the greatest increase in leaf Na⁺ and Cl^– concentrations. These results are helpful for selecting salt tolerant Sego Supreme^TM plant species for use in the landscape where reclaimed water is used for irrigation.