Plant materials and culture.

Two experiments were conducted in this study: from 3 Aug. to 16 Sept. 2020 and 10 Mar. to 3 May 2021. Experiments were 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). For simplicity, the two experiments are referred to as fall and spring experiments. Seeds of A. julibrissin and S. japonica were scarified by dipping in 98.1% sulfuric acid (Fisher Chemical, Ottawa, ON) for 30 min to break their exogenous physical dormancy. Seeds after scarification were germinated in trays with moist perlite (Expanded Perlite, Malad City, ID) and sphagnum peatmoss (SunGro Horticulture, Agawam, MA) at a volumetric ratio of 2:1. Trays were placed in the greenhouse and covered with a plastic cover until seeds germinated. The temperature of the greenhouse was maintained at 20 °C. Seedlings were transplanted into 3.9-L injection-molded, polypropylene containers (PC1D-4; Nursery Supplies, Orange, CA) filled with Metro-Mix 820 (Canadian Sphagnum peatmoss, 35% to 45% composted pine bark, coir, coarse perlite, and dolomitic limestone; SunGro Horticulture, Agawam, MA). Seedlings of A. julibrissin and S. japonica were 16.0 ± 3.6 cm (mean ± sd) and 56.1 ± 11.2 cm tall in the fall experiment and 7.6 ± 1.3 cm and 27.8 ± 6.6 cm tall in the spring experiment, respectively. Seedlings in the fall experiment were ≈8 months old and those in the spring experiment were 2 to 4 months old before transplanting. Plants were kept in the research greenhouse, and tap water was applied. During the fall experiment, greenhouse temperature was maintained at 25.5 ± 0.5 °C (mean ± sd) during the day and 24.1 ± 1.0 °C at night. During the spring experiment, greenhouse temperature was maintained at 25.0 ± 0.5 °C during the day and 21.5 ± 0.4 °C at night. Daily light integrals (DLI) inside the greenhouse were 32.6 ± 5.0 and 27.4 ± 8.7 mol·m⁻²·d⁻¹, during the fall and spring experiments, respectively. Light intensities were recorded using a heated silicon chip pyranometer (SP-230; Apogee Instruments, Logan UT) mounted to a weather station at the Greenville Research Farm, nearly 1000 m away from the research greenhouse. A light transmission rate of 68% was used to calculate the DLI inside the greenhouse. Supplemental light at 211 ± 67.7 µmol⋅m⁻²⋅s⁻¹, measured with a Quantum flux meter (MQ-200X, serial #1006; Apogee Instruments, Logan, UT), was provided using 1000-W high-pressure sodium lamps at plant canopy level from 600 to 2200 hr when light intensity inside the greenhouse was less than 500 µmol⋅m⁻²⋅s⁻¹.

Salinity treatments.

Two salinity treatments were tested on A. julibrissin and S. japonica that included irrigation water at ECs of 5.0 and 10.0 dS·m⁻¹. The control group received only a nutrient solution at an EC of 1.2 dS·m⁻¹. Uniform plants were selected and randomly assigned to the treatments. The nutrient (control) solution was prepared in a 100-L tank by adding 0.8 g·L⁻¹ 15N–2.2P–12.5K water-soluble fertilizer (Peters Excel 15-5-15 Cal-Mag Special; ICL Specialty Fertilizers, Dublin, OH) to the tap water. The saline solutions of EC of 5.0 and 10.0 dS·m⁻¹ were prepared using sodium chloride (NaCl; Fisher Scientific, Waltham, MA) and dihydrate calcium chloride (CaCl₂·2H₂O; Hi Valley Chemical, Centerville, UT) at a molar ratio of 2:1 to the nutrient solution (Table 1). The initial pH of treatment solutions was adjusted to 6.0 to 6.5 using 1 mol·L⁻¹ nitric acid (Fisher Chemical, Fair Lawn, NJ) as needed. The sodium adsorption ratio and elemental analysis were confirmed by the USU Analytical Laboratory, and these values are presented in Table 1. For the fall experiment, a 5-week study, 1000 mL treatment solutions per pot were applied manually once per week for the first 2 weeks and every other day thereafter (12 irrigation events). Plants in this study were growing vigorously and consumed more water; therefore, irrigation frequency increased. For the spring experiment, an 8-week study, 1000 mL of treatment solutions were applied manually to each plant weekly (eight irrigation events). The leaching fraction was targeted to ≈25%. In-between treatments, plants were watered with an additional 250 to 500 mL of distilled water, as necessary, to avoid drought conditions.

Table 1.

The mineral contents, sodium adsorption ratio (SAR), and electrical conductivity (EC) of nutrient and saline solution used in the study.

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Leachate and substrate EC.

Leachate EC was determined using the pour-through method described by Cavins et al. (2008) using an EC meter (LAQUA Twin; Horiba, Kyoto, Japan). In brief, at least 30 min after every irrigation, a saucer was placed under the container and 100 mL of distilled water was poured from the top surface. Afterward, EC was measured from the leachate. One plant per treatment per species was chosen for measurement. Substrate EC was measured using the saturated paste method explained by Gavlak et al. (2005) with some modifications. In brief, the pots containing soilless media were left to dry in the greenhouse for 2 weeks after harvest. A sample (10 g) was taken from the substrate at the top 5-cm surface as salts moved upward during the drying process. Then, 100 mL of deionized water was added to the substrate sample in a flask to make a paste. All samples were stored overnight at room temperature after covering the flasks with parafilm (American National Can, Menasha, WI) and EC measurements were taken.

Visual quality.

A visual score of 0 to 5 was assigned to each plant at the end of the experiment to assess foliar salt damage. Visual score was assigned as 0 = dead, 1 = severe foliar damage (>90% leaves with burnt edges or necrosis), 2 = moderate foliar damage (90% to 50%), 3 = slight foliar damage (50% to 10%), 4 = good quality with minimal foliar damage (<10%), and 5 = excellent without foliar damage (Sun et al., 2015). Plant growth parameters were not considered while assigning the visual score.

Growth parameters.

Plant heights (centimeters) were recorded at the beginning and end of the experiment. Height was recorded from the surface of the growing medium to the top of the plants. Increase in plant height was calculated as the difference between the initial height and final height. At harvest, leaf area (square centimeters) was measured using a leaf area meter (LI-3100; LI-COR Biosciences, Lincoln, NE). In addition, shoot dry weight (DW) (stem DW + leaf DW) and root DW of plants were measured after being dried in an oven at 60 °C for 1 week.

Chlorophyll content and gas exchange.

Relative chlorophyll content (or leaf greenness) of all plants was recorded using a chlorophyll meter [Soil Plant Analysis Development (SPAD)-502; Minolta Camera, Osaka, Japan] before harvest. Eight mature leaves from each plant were measured, and the averaged value was recorded. Leaf P_n, g_S, and E of plants in each treatment were measured 4 d before harvest using a portable photosynthesis system with an automatic universal PLC3 universal leaf cuvette (CIRAS-3; PP Systems, Amesbury, MA) or LI-6800 photosynthesis system (LI-COR Biosciences) for the fall and spring experiment, respectively. Fully expanded, healthy leaves without damage were used for the gas exchange measurements. Environmental conditions in the cuvette were controlled at 25 °C, 1000 µmol·m⁻²·s⁻¹ photosynthetic photon flux and 400 µmol·mol⁻¹ carbon dioxide concentration. Data were recorded once environmental conditions and gas exchange parameters in the cuvette became stable. All plants were watered 1 d before measurements to avoid water stress.

Mineral analyses.

In the spring experiment, four dried plants per species per treatment were selected randomly and each was ground with a stainless Wiley mill (Thomas Scientific, Swedesboro, NJ) and allowed to pass through 1-mm-mesh screen. The powder samples were analyzed at the USU Analytical Laboratories for mineral contents. In brief, the concentration of chloride (Cl⁻) was quantified using 2% acetic acid, and sodium (Na⁺), calcium (Ca²⁺), potassium (K⁺), magnesium (Mg²⁺), sulfur (S), zinc (Zn²⁺), and manganese (Mn²⁺) using nitric/hydrogen peroxide following the protocol described in Gavlak et al. (2005). The Cl⁻ concentration was determined by ion-selective electrode using a Flow Injection Analysis and Ion Chromatograph System (QuikChem 8000; Lachat Instrument, Loveland, CO) and reported on a dry plant basis (mg·g⁻¹). For Na⁺, Ca²⁺, K⁺, Mg²⁺, S, Zn²⁺, and Mn²⁺, 0.5 g of powder samples and 6 mL of nitric acid (HNO₃) were added into a digestion tube that was then placed in a digestion block for 10 min at 80 °C and subsequently cooled for 2 min. A total of 2 mL of 30% hydrogen peroxide (H₂O₂) was added into the digestion tube that was placed again in the digestion block at 130 °C for 1 h. Mixing using a vortex stirrer was performed followed by cooling and diluting. Then the digestion tube was cooled at room temperature, and the contents of the digestion tube were 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) and reported on a dry plant basis (mg·g⁻¹).

Experimental design and data analyses.

The experiment was a randomized complete block design with two species, three treatments, and 10 replicates. An experimental unit consisted of one pot containing one plant. Analysis of variance was conducted to test the effects of saline solution irrigation on plant growth, gas exchange parameters, and mineral nutrient concentrations. Log transformation was done for all data except for mineral analysis, substrate EC, and visual score data. Means separation among treatments was adjusted using Tukey’s method for multiplicity at α = 0.05. Correlation analyses were carried out for Na⁺, Cl⁻ concentrations, and the K⁺:Na⁺ ratio in plant tissue compared with visual scores and gas exchange parameters. All statistical analyses were conducted using SAS (Version 14.1; SAS Institute, Cary, NC) with PROC MIXED procedure.

Leachate and substrate EC.

Leachate EC increased over the time of saline solutions irrigation (Fig. 1). In the fall experiment, leachate EC ranged from 0.7 to 1.2 dS⋅m⁻¹ when a nutrient solution at an EC of 1.2 dS·m⁻¹ was applied. Irrigation with saline solutions at ECs of 5.0 and 10.0 dS⋅m⁻¹ increased leachate EC from 2.2 to 7.1 and 4.4 to 12.9 dS·m⁻¹, respectively. In the spring experiment, leachate EC ranged from 1.2 to 2.1, 3.7 to 11.0, and 5.7 to 19.0 dS⋅m⁻¹ when irrigated with a nutrient solution at an EC of 1.2 dS⋅m⁻¹ and saline solutions at ECs of 5.0 and 10.0 dS⋅m⁻¹, respectively. In both experiments, substrate EC increased with increasing salinity levels of irrigation water (Fig. 2). In the fall experiment, substrate EC was 3.5 dS⋅m⁻¹ for both A. julibrissin and S. japonica after irrigation with saline solution at an EC of 5.0 dS⋅m⁻¹ for 5 weeks; saline solution at an EC of 10 dS⋅m⁻¹ further increased the substrate EC to 8.5 and 10.5 dS⋅m⁻¹ for A. julibrissin and S. japonica, respectively (Fig. 2). In the spring experiment, substrate EC was 3.8 and 4.6 dS⋅m⁻¹ for A. julibrissin and S. japonica, respectively, after irrigation with saline solution at an EC of 5.0 dS⋅m⁻¹ for 8 weeks. Saline solution at an EC of 10 dS·m⁻¹ further increased the substrate EC to 9.7 and 10.6 dS·m⁻¹ for A. julibrissin and S. japonica, respectively (Fig. 2). Evaluation of leachate and substrate EC from both experiments indicated the effects of saline solution irrigation on the EC of the root zone. Similarly, Wu et al. (2016) and Xing et al. (2021) reported that leachate and substrate EC increased with saline water irrigation over time as salts accumulated in the substrate. Salt accumulation is a potential problem when poor-quality saline water is used for landscape irrigation. Therefore, best management practices including monitoring water quality, increasing leachate fraction, and using tolerant species should be adopted to limit salinity stress in plants.

Electrical conductivity (EC) of leachate solution collected after irrigating Albizia julibrissin and Sophora japonica with a nutrient solution (EC = 1.2 dS·m⁻¹; control) or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] over the course of experiments in Fall 2020 and Spring 2021. Saline solution was created by adding sodium chloride (NaCl) and dihydrate calcium chloride (CaCl₂·2H₂O) to the nutrient solution. Treatment solutions were applied from 3 Aug. to 5 Sept. 2020 (12 irrigation events) and 10 Mar. to 28 Apr. 2021 (eight irrigation events) in Fall 2020 and Spring 2021, respectively. Vertical bars represent standard errors of two measurements.

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Electrical conductivity (EC) of leachate solution collected after irrigating Albizia julibrissin and Sophora japonica with a nutrient solution (EC = 1.2 dS·m⁻¹; control) or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] over the course of experiments in Fall 2020 and Spring 2021. Saline solution was created by adding sodium chloride (NaCl) and dihydrate calcium chloride (CaCl₂·2H₂O) to the nutrient solution. Treatment solutions were applied from 3 Aug. to 5 Sept. 2020 (12 irrigation events) and 10 Mar. to 28 Apr. 2021 (eight irrigation events) in Fall 2020 and Spring 2021, respectively. Vertical bars represent standard errors of two measurements.

Citation: HortScience 57, 8; 10.21273/HORTSCI16479-21

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Electrical conductivity (EC) of leachate solution collected after irrigating Albizia julibrissin and Sophora japonica with a nutrient solution (EC = 1.2 dS·m⁻¹; control) or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] over the course of experiments in Fall 2020 and Spring 2021. Saline solution was created by adding sodium chloride (NaCl) and dihydrate calcium chloride (CaCl₂·2H₂O) to the nutrient solution. Treatment solutions were applied from 3 Aug. to 5 Sept. 2020 (12 irrigation events) and 10 Mar. to 28 Apr. 2021 (eight irrigation events) in Fall 2020 and Spring 2021, respectively. Vertical bars represent standard errors of two measurements.

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Electrical conductivity (EC) of soil extraction for Albizia julibrissin and Sophora japonica irrigated with a nutrient solution (EC = 1.2 dS·m⁻¹; control) or a saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] over the course of experiments in Fall 2020 and Spring 2021. Saline solution was created by adding sodium chloride (NaCl) and dihydrate calcium chloride (CaCl₂·2H₂O) to the nutrient solution. Treatment solutions were applied from 3 Aug. to 5 Sept. 2020 (12 irrigation events) and 10 Mar. to 28 Apr. 2021 (eight irrigation events) in Fall 2020 and Spring 2021, respectively. Vertical bars represent standard errors of five measurements. The same letters above column bars within species represent no significance among treatments as determined by Tukey’s method for multiplicity at α = 0.05.

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Electrical conductivity (EC) of soil extraction for Albizia julibrissin and Sophora japonica irrigated with a nutrient solution (EC = 1.2 dS·m⁻¹; control) or a saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] over the course of experiments in Fall 2020 and Spring 2021. Saline solution was created by adding sodium chloride (NaCl) and dihydrate calcium chloride (CaCl₂·2H₂O) to the nutrient solution. Treatment solutions were applied from 3 Aug. to 5 Sept. 2020 (12 irrigation events) and 10 Mar. to 28 Apr. 2021 (eight irrigation events) in Fall 2020 and Spring 2021, respectively. Vertical bars represent standard errors of five measurements. The same letters above column bars within species represent no significance among treatments as determined by Tukey’s method for multiplicity at α = 0.05.

Citation: HortScience 57, 8; 10.21273/HORTSCI16479-21

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Electrical conductivity (EC) of soil extraction for Albizia julibrissin and Sophora japonica irrigated with a nutrient solution (EC = 1.2 dS·m⁻¹; control) or a saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] over the course of experiments in Fall 2020 and Spring 2021. Saline solution was created by adding sodium chloride (NaCl) and dihydrate calcium chloride (CaCl₂·2H₂O) to the nutrient solution. Treatment solutions were applied from 3 Aug. to 5 Sept. 2020 (12 irrigation events) and 10 Mar. to 28 Apr. 2021 (eight irrigation events) in Fall 2020 and Spring 2021, respectively. Vertical bars represent standard errors of five measurements. The same letters above column bars within species represent no significance among treatments as determined by Tukey’s method for multiplicity at α = 0.05.

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Visual quality.

Salinity stress causes plant foliar damage, like leaf burn, necrosis, and/or discoloration (Paudel et al., 2019; Sun and Palmer, 2018). Saline solution irrigation had significant effects on the visual score of both species in the fall and spring experiments (P = 0.006 and P < 0.0001, respectively, Table 2). There were no interactive effects for visual scores between salinity treatment and species in the fall experiment (P = 0.06), but significant interactive effects were observed in the spring experiment (P = 0.02, Table 2). All A. julibrissin and S. japonica plants survived regardless of treatment (data not shown). No foliar damage was observed on plants irrigated with saline solution at an EC of 5.0 dS·m⁻¹ in both experiments, but both species had minimal foliar salt damage when exposed to the saline solution at an EC of 10.0 dS·m⁻¹ in the spring experiment (Fig. 3). The visual score of A. julibrissin was 4.7 and 3.9 when irrigated with saline solution at an EC of 10.0 dS·m⁻¹ in the fall and spring experiments, respectively. S. japonica exhibited no foliar salt damage in the fall experiment and had a visual score of 4.3 in the spring experiment when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. In this study, the experiment conducted in the fall had relatively less foliar damage than in the spring. Although the number of irrigation events was greater in the fall experiment, plants were treated for a longer time in the spring experiment. In addition, seedlings used in the fall experiment were older compared with those in the spring experiment. Salt injury was observed on old leaves in the lower canopy only; however, new leaves in the upper canopy were unaffected.

Visual score of Albizia julibrissin and Sophora japonica irrigated with a nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] over the course of experiments in Fall 2020 and Spring 2021. Saline solution was created by adding sodium chloride (NaCl) and dihydrate calcium chloride (CaCl₂·2H₂O) to the nutrient solution. Visual score reference scale: 0 = dead; 1 = severe foliar damage (>90% leaves with burnt edges or necrosis); 2 = moderate foliar damage (90% to 50%); 3 = slight foliar damage (50% to 10%); 4 = good quality with minimal foliar damage (< 10%); 5 = excellent without foliar damage. Treatment solutions were applied from 3 Aug. to 5 Sept. 2020 (12 irrigation events) and 10 Mar. to 28 Apr. 2021 (eight irrigation events) in Fall 2020 and Spring 2021, respectively. Vertical bars represent standard errors of 10 measurements. The same letters above column bars within species represent no significance among treatments as determined by Tukey’s method for multiplicity at α = 0.05.

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Visual score of Albizia julibrissin and Sophora japonica irrigated with a nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] over the course of experiments in Fall 2020 and Spring 2021. Saline solution was created by adding sodium chloride (NaCl) and dihydrate calcium chloride (CaCl₂·2H₂O) to the nutrient solution. Visual score reference scale: 0 = dead; 1 = severe foliar damage (>90% leaves with burnt edges or necrosis); 2 = moderate foliar damage (90% to 50%); 3 = slight foliar damage (50% to 10%); 4 = good quality with minimal foliar damage (< 10%); 5 = excellent without foliar damage. Treatment solutions were applied from 3 Aug. to 5 Sept. 2020 (12 irrigation events) and 10 Mar. to 28 Apr. 2021 (eight irrigation events) in Fall 2020 and Spring 2021, respectively. Vertical bars represent standard errors of 10 measurements. The same letters above column bars within species represent no significance among treatments as determined by Tukey’s method for multiplicity at α = 0.05.

Citation: HortScience 57, 8; 10.21273/HORTSCI16479-21

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Visual score of Albizia julibrissin and Sophora japonica irrigated with a nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] over the course of experiments in Fall 2020 and Spring 2021. Saline solution was created by adding sodium chloride (NaCl) and dihydrate calcium chloride (CaCl₂·2H₂O) to the nutrient solution. Visual score reference scale: 0 = dead; 1 = severe foliar damage (>90% leaves with burnt edges or necrosis); 2 = moderate foliar damage (90% to 50%); 3 = slight foliar damage (50% to 10%); 4 = good quality with minimal foliar damage (< 10%); 5 = excellent without foliar damage. Treatment solutions were applied from 3 Aug. to 5 Sept. 2020 (12 irrigation events) and 10 Mar. to 28 Apr. 2021 (eight irrigation events) in Fall 2020 and Spring 2021, respectively. Vertical bars represent standard errors of 10 measurements. The same letters above column bars within species represent no significance among treatments as determined by Tukey’s method for multiplicity at α = 0.05.

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Table 2.

A summary of analysis of variance for the effects of salinity treatments and their interactions with species on visual score (VS), plant height (Ht), leaf area (LA), leaf dry weight (DW), stem DW, shoot DW (leaf DW + stem DW), root DW, leaf greenness [Soil Plant Analysis Development (SPAD) reading], net photosynthesis rate (P_n), stomatal conductance (g_S), and transpiration rate (E) of Albizia julibrissin and Sophora japonica irrigated with a nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] in a greenhouse.^z

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Foliar salt damage is problematic for landscape plants (Veatch-Blohm et al., 2014); therefore, the aesthetic appearance of plants is one of the primary focuses when screening landscape plants for salt tolerance (Niu and Cabrera, 2010; Veatch-Blohm et al., 2014). It is important to select plants that maintain good visual quality in the landscape that are affected by salinity (Niu and Rodriguez, 2006; Wahome et al., 2001). In this study, A. julibrissin and S. japonica exhibited no foliar damage on plants irrigated with saline solution at an EC of 5.0 dS·m⁻¹ but minimal foliar salt damage when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. In line with our results, Niu et al. (2010) reported that S. secundiflora had no foliar salt injury when irrigated with saline solutions at ECs of 3.0 and 6.0 dS·m⁻¹. Based on visual quality alone, A. julibrissin and S. japonica can be good candidates for growing in salt-prone landscapes.

Growth parameters.

Plant height and leaf area were affected by saline solution irrigation in both experiments (P < 0.0001, Table 2). In the fall experiment, the plant height of A. julibrissin decreased by 38% compared with the control when irrigated with saline solution at an EC of 5.0 dS·m⁻¹ (Table 3). A. julibrissin and S. japonica were 61% and 50% shorter than those in control, respectively, when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. Similarly, in the spring experiment, A. julibrissin irrigated with saline solution at an EC of 10.0 dS·m⁻¹ was 72% shorter than those in control. Sophora japonica irrigated with saline solutions at ECs of 5.0 and 10 dS·m⁻¹ were 30% and 45% shorter than those in control. On the other hand, it has been reported that the plant height of S. secundiflora was unaffected when irrigated with a saline solution containing NaCl, magnesium sulfate heptahydrate (MgSO₄·7H₂O), and calcium chloride (CaCl₂) at an EC of 6.0 dS·m⁻¹ for the first 4 months (Niu et al., 2010). However, S. secundiflora plants were shorter after irrigation for 6 months. This contrast may be due to the application of saline solution at different concentrations and compositions and different duration of saline solution irrigation. In the current research, saline solutions containing NaCl and CaCl₂ were applied for 5 and 8 weeks in the fall and spring experiments, respectively. In addition, plant species may have different responses to salinity stress.

Table 3.

Plant height and leaf area of Albizia julibrissin and Sophora japonica irrigated with a nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] in a greenhouse.^z

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Albizia julibrissin and S. japonica had no significant reduction in leaf area when irrigated with saline solution at an EC of 5.0 dS·m⁻¹ in both experiments (Table 3). In the fall experiment, A. julibrissin and S. japonica had 41% and 36% reductions in leaf area, respectively, compared with the control when irrigated with saline solution at an EC of 10.0 dS·m⁻¹ (Table 3). In the spring experiment, there were 35% and 44% reductions in leaf area for A. julibrissin and S. japonica, respectively, when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. In addition, leaf area varied with species in both fall and spring experiments (Tables 2 and 3). Previous studies have also reported reduced leaf area with increasing salt concentrations in irrigation solution (Niu et al., 2012; Paudel et al., 2019; Sun et al., 2018a). Likewise, the leaf area of pomegranate cultivars decreased when irrigated with saline groundwater at an EC of 6.0 dS·m⁻¹ (El-Khawaga et al., 2013). This is because salinity-induced water deficit causes leaf senescence and reduces leaf expansion, thereby leading to decreased leaf area (Muchate et al., 2016; Munns and Tester, 2008).

Furthermore, leaf DW, stem DW, and shoot DW of both species decreased with increasing salinity levels in the irrigation water in both experiments (P < 0.0001, Table 2, Fig. 4). Compared with the control, the leaf DW and stem DW decreased by 37% to 48% and 56% to 60%, respectively, for both species irrigated with saline solution at an EC of 10.0 dS·m⁻¹ in the fall experiment. In addition, there were 53% to 58% and 68% to 71% leaf DW and stem DW reductions, respectively, for both species in the spring experiment. Likewise, shoot DW of S. secundiflora was 25% and 46% less when irrigated with saline solutions at ECs of 3.0 and 6.0 dS·m⁻¹ for 6 months, respectively, compared with the control (Niu et al., 2010). In addition, root DW of A. julibrissin and S. japonica decreased significantly with increasing salinity levels in the irrigation water in the fall (P = 0.002) and spring experiments (P < 0.0001, Table 2, Fig. 4). Root is the first tissue to perceive salinity stress, therefore, plays an important role in plant development. However, the effect of salinity stress on roots of A. julibrissin and S. japonica has not been reported previously. Salinity stress appears to stimulate the transition from cell division to elongation and suppress root meristem activity (West et al., 2004). In line with our results, it has been reported that declining root DW is common for landscape plants when exposed to salinity stress (Acosta-Motos et al., 2015; Hooks and Niu, 2019). For example, root DW of rose (Rosa ×fortuniana, Rosa multiflora, and Rosa odorata) rootstocks when irrigated with saline solutions at ECs of 1.6, 3.0, 6.0, and 9.0 dS·m⁻¹ decreased linearly with increasing salinity levels in the irrigation water (Niu et al., 2008).

Leaf dry weight (DW), stem DW, shoot DW (stem + leaf DW) and root DW of Albizia julibrissin and Sophora japonica irrigated with a nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] over the course of experiments in Fall 2020 and Spring 2021. Saline solution was created by adding sodium chloride (NaCl) and dihydrate calcium chloride (CaCl₂·2H₂O) to the nutrient solution. Treatment solutions were applied from 3 Aug. to 5 Sept. 2020 (12 irrigation events) and 10 Mar. to 28 Apr. 2021 (eight irrigation events) in Fall 2020 and Spring 2021, respectively. Vertical bars represent standard errors of 10 measurements for shoot DW and leaf DW and five measurements for root DW. The same letters above column bars within species represent no significance among treatments as determined by Tukey’s method for multiplicity at α = 0.05.

Citation: HortScience 57, 8; 10.21273/HORTSCI16479-21

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Leaf dry weight (DW), stem DW, shoot DW (stem + leaf DW) and root DW of Albizia julibrissin and Sophora japonica irrigated with a nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] over the course of experiments in Fall 2020 and Spring 2021. Saline solution was created by adding sodium chloride (NaCl) and dihydrate calcium chloride (CaCl₂·2H₂O) to the nutrient solution. Treatment solutions were applied from 3 Aug. to 5 Sept. 2020 (12 irrigation events) and 10 Mar. to 28 Apr. 2021 (eight irrigation events) in Fall 2020 and Spring 2021, respectively. Vertical bars represent standard errors of 10 measurements for shoot DW and leaf DW and five measurements for root DW. The same letters above column bars within species represent no significance among treatments as determined by Tukey’s method for multiplicity at α = 0.05.

Citation: HortScience 57, 8; 10.21273/HORTSCI16479-21

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Leaf dry weight (DW), stem DW, shoot DW (stem + leaf DW) and root DW of Albizia julibrissin and Sophora japonica irrigated with a nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] over the course of experiments in Fall 2020 and Spring 2021. Saline solution was created by adding sodium chloride (NaCl) and dihydrate calcium chloride (CaCl₂·2H₂O) to the nutrient solution. Treatment solutions were applied from 3 Aug. to 5 Sept. 2020 (12 irrigation events) and 10 Mar. to 28 Apr. 2021 (eight irrigation events) in Fall 2020 and Spring 2021, respectively. Vertical bars represent standard errors of 10 measurements for shoot DW and leaf DW and five measurements for root DW. The same letters above column bars within species represent no significance among treatments as determined by Tukey’s method for multiplicity at α = 0.05.

Citation: HortScience 57, 8; 10.21273/HORTSCI16479-21

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Leaf greenness (SPAD reading) and gas exchange.

Saline solution irrigation affected SPAD readings of A. julibrissin and S. japonica in the fall and spring experiments (P = 0.01 and P < 0.0001, respectively, Table 2). There were no interactive effects between salinity treatment and species (Table 2). In addition, SPAD readings varied with species in both fall (P < 0.0001) and spring (P = 0.0001) experiments. In the fall experiment, there was a 15% reduction in the SPAD reading of A. julibrissin irrigated with saline solution at an EC of 5.0 dS·m⁻¹ compared with the control, but not statistically different. However, saline solution at an EC of 10.0 dS·m⁻¹ reduced the SPAD reading of A. julibrissin by 17%. Saline solution at an EC of 5.0 or 10.0 dS·m⁻¹ did not impact SPAD readings of S. japonica (Table 4). In the spring experiment, compared with the control, saline solution at an EC of 5.0 dS·m⁻¹ reduced the SPAD reading of S. japonica by 19%. In addition, SPAD readings of both species decreased by 16% to 22% when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. Similarly, various studies reported that SPAD readings reduced with increasing salt concentrations in irrigation water (Chen et al., 2019; Liu et al., 2017; Sun et al., 2015). For example, SPAD readings of Physocarpus opulifolius (ninebark) were reduced by 19% when irrigated with saline solution at an EC of 6.5 dS·m⁻¹ compared with the control (Chen et al., 2019). These results consistently indicate that salinity stress causes chlorophyll degradation and decreases chlorophyll content (Santos, 2004).

Table 4.

Leaf greenness [Soil Plant Analysis Development (SPAD) reading] and net photosynthesis rate (P_n) of Albizia julibrissin and Sophora japonica irrigated with a nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] in a greenhouse.^z

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In the fall experiment, saline solution irrigation had significant effects on P_n (P < 0.0001), g_S (P = 0.02), and E (P = 0.05) (Table 2). P_n of A. julibrissin irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹ decreased by 38% and 45% compared with the control, respectively (Table 4). In A. julibrissin, compared with the control, saline solution at an EC of 10.0 dS·m⁻¹ reduced g_S by 56% (Table 5). Although E of A. julibrissin irrigated with saline solution at an EC of 10.0 dS·m⁻¹ decreased by 31% compared with control, it was not statistically significant. Likewise, compared with the control, S. japonica irrigated with saline solution at an EC of 10.0 dS·m⁻¹ had a 58% reduction in P_n (Table 4). However, there was no significant change in the g_S and E of S. japonica irrigated with saline solution at an EC of 10.0 dS·m⁻¹ compared with the control (Table 5).

Table 5.

Stomatal conductance (g_S) and transpiration rate (E) of Albizia julibrissin and Sophora japonica irrigated with a nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] in a greenhouse.^z

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In the spring experiment, saline solution irrigation significantly affected P_n (P < 0.0001), g_S (P < 0.0001), and E (P < 0.0001) (Table 2). In A. julibrissin, compared with the control, saline solution at an EC of 5.0 dS·m⁻¹ reduced P_n, g_S, and E by 44%, 53%, and 48%, respectively (Tables 4 and 5). Similarly, saline solution at an EC of 10.0 dS·m⁻¹ reduced P_n, g_S, and E of A. julibrissin by 72%, 73%, and 70%, respectively. Sophora japonica irrigated with saline solution at an EC of 5.0 dS·m⁻¹ had 49% reduction in P_n compared with the control. Likewise, compared with the control, S. japonica irrigated with saline solution at an EC of 10.0 dS·m⁻¹ had 66%, 75%, and 71% reductions in P_n, g_S, and E, respectively. In both experiments, gas exchange parameters were reduced at higher salinity levels. This is similar to the report by Niu et al. (2010) that leaf P_n and g_S of S. secundiflora were lower when irrigated with 3.0 or 6.0 dS·m⁻¹ compared with the control. However, different results were observed in the fall and spring experiments. These results may indicate that plant photosynthetic parameters depend on the environmental factors and plant growth stage. In addition, the use of two different instruments may have some influence on those parameters.

Salinity stress may harm plants’ photosynthetic apparatus and reduce photosystem II efficiency, which inhibits plant photosynthesis (Sharma et al., 2012; Taiz et al., 2015). Saline conditions also create water deficits that lead to stomatal closure and ultimately decrease transpiration (Wang et al., 2019). Salinity-induced stomatal closure reduces internal CO₂ concentration and decreases enzyme activity involved in carboxylation, such as Ribulose-1,5-biphosphate carboxylase oxygenase (Chaves et al., 2009), thus reducing net photosynthetic rate. In addition, a decrease in leaf area and chlorophyll content may reduce photosynthesis under high salinity stress (Sharma et al., 2012). Reduced photosynthesis eventually impairs plant growth (Menezes et al., 2017; Odjegba and Chukwunwike, 2012). Furthermore, a plant can experience growth reduction due to the diversion of energy from growth to the homeostasis of salinity stress (Atkin and Macherel, 2009).

Mineral nutrients.

Sodium and Cl⁻ concentrations in the leaf tissue of A. julibrissin and S. japonica were significantly affected by both elevated salinity levels and plant species interactively (Table 6). In this study, the Na⁺ concentration of A. julibrissin increased almost six times, from 0.04 to 0.26 mg·g⁻¹, as the EC of saline solutions increased from 1.2 to 10.0 dS·m⁻¹ (Table 6). However, there was no difference in the Na⁺ concentration among treatments for S. japonica. Furthermore, Na⁺ concentrations in the leaf tissue of A. julibrissin and S. japonica were less than 1 mg·g⁻¹. Salt-tolerant ornamental plants may accumulate less Na⁺ in their leaves compared with salt-sensitive plants (Wu et al., 2016) because sodium uptake is reduced, and/or restricted transport of Na⁺ from roots to shoots occurs for salt-tolerant plants (Munns, 2002). Similarly, the Na⁺ concentration of Punica granatum (pomegranate) was also less than 1 mg·g⁻¹ when irrigated with saline solutions up to EC of 15.0 dS·m⁻¹ for 7 weeks (Sun et al., 2018b). On the other hand, S. secundiflora irrigated for 6 months with saline solution at an EC of 6.0 dS·m⁻¹ had 8.5 mg·g⁻¹ Na⁺ ions in leaves, which is 15 times greater than the control (Niu et al., 2010). This contrast may be due to longer irrigation time and different species. Compared with the control, Cl⁻ concentration increased by 17 and 32 times for A. julibrissin and 14 and 25 times for S. japonica when plants were irrigated with saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹, respectively. Similarly, S. secundiflora accumulated ≈18.0 mg·g⁻¹ of Cl⁻ in its leaves when irrigated for 6 months with saline solution at an EC of 6.0 dS·m⁻¹, which was increased by three times compared with the control (Niu et al., 2010). These results indicate that A. julibrissin and S. japonica accumulated many fewer Na⁺ ions than Cl⁻ ions in their leaf tissue.

Table 6.

Leaf mineral ion concentrations and potassium-to-sodium (K⁺:Na⁺) ratio of Albizia julibrissin and Sophora japonica irrigated with a nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] in a greenhouse.^z

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In this study, minimal foliar salt damage was observed, but significant negative correlations between visual score and Na⁺ and Cl⁻ concentrations were obtained (Fig. 5). Similarly, it has been reported that Rosa chinensis ‘Major’ (China rose) and Rosa rubiginosa (sweet brier) accumulated Cl⁻ ions to toxic levels that are responsible for leaf necrosis (Wahome et al., 2001). More importantly, Cl⁻ is a beneficial micronutrient ion that helps in photosynthesis, osmoregulation, turgor regulation, and plant growth (Chen et al., 2016; Flowers, 1988; Homann, 1987). However, high concentrations in plant tissue can turn Cl⁻ from nutrient to toxicant (Geilfus, 2018). In addition, increasing Na⁺ and Cl⁻ concentrations in plant leaves can cause ion toxicity and reduce photosynthesis (Taiz et al., 2015). In this study, there were more Cl⁻ ions than Na⁺ ions in leaf tissue. Negative correlations between photosynthesis and Na⁺ (P = 0.01; r² = 0.25) and Cl⁻ (P < 0.0001; r² = 0.62) were observed (Fig. 5). Therefore, the inhibition of photosynthesis may be more related to Cl⁻ accumulation. Similarly, negative correlations between Na⁺ and Cl⁻ concentrations and g_S and E were also observed (Fig. 5).

Linear correlation analyses of sodium (Na⁺), chloride (Cl⁻), potassium-to-sodium ratio (K⁺:Na⁺) levels in plant tissue compared with the visual score, net photosynthesis rate (P_n), stomatal conductance (g_S), and transpiration rate (E) of Albizia julibrissin and Sophora japonica. Visual score reference scale: 0 = dead; 1 = severe foliar damage (>90% leaves with burnt edges or necrosis); 2 = moderate foliar damage (90% to 50%); 3 = slight foliar damage (50% to 10%); 4 = good quality with minimal foliar damage (< 10%); 5 = excellent without foliar damage.

Citation: HortScience 57, 8; 10.21273/HORTSCI16479-21

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Linear correlation analyses of sodium (Na⁺), chloride (Cl⁻), potassium-to-sodium ratio (K⁺:Na⁺) levels in plant tissue compared with the visual score, net photosynthesis rate (P_n), stomatal conductance (g_S), and transpiration rate (E) of Albizia julibrissin and Sophora japonica. Visual score reference scale: 0 = dead; 1 = severe foliar damage (>90% leaves with burnt edges or necrosis); 2 = moderate foliar damage (90% to 50%); 3 = slight foliar damage (50% to 10%); 4 = good quality with minimal foliar damage (< 10%); 5 = excellent without foliar damage.

Citation: HortScience 57, 8; 10.21273/HORTSCI16479-21

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Linear correlation analyses of sodium (Na⁺), chloride (Cl⁻), potassium-to-sodium ratio (K⁺:Na⁺) levels in plant tissue compared with the visual score, net photosynthesis rate (P_n), stomatal conductance (g_S), and transpiration rate (E) of Albizia julibrissin and Sophora japonica. Visual score reference scale: 0 = dead; 1 = severe foliar damage (>90% leaves with burnt edges or necrosis); 2 = moderate foliar damage (90% to 50%); 3 = slight foliar damage (50% to 10%); 4 = good quality with minimal foliar damage (< 10%); 5 = excellent without foliar damage.

Citation: HortScience 57, 8; 10.21273/HORTSCI16479-21

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Calcium concentration in the leaf tissue was affected by both salt treatment and plant species interactively (Table 6). In the present study, CaCl₂ was used to reduce the deficiency of Ca²⁺ ions and provide osmoprotection by its additive role with NaCl (Jaleel et al., 2007). Although CaCl₂ was added to prepare the saline solution, compared with the control, only an ≈2-time increment of the Ca²⁺ concentration in leaf tissue was observed. It has previously been reported that calcium transport and mobility to plant parts are reduced by salinity stress (Grattan and Grieve, 1999). Compared with the control, there was a 21% increment in the K⁺ concentration of A. julibrissin when irrigated with saline solutions at an EC of 10.0 dS·m⁻¹ (Table 6). However, researchers have reported that there is a decline in K⁺ concentration in plant tissue when plants are exposed to salinity stress (Grattan and Grieve, 1999). There was also an increase in K⁺ content in the tissue of Acacia auriculiformis (northern black wattle) with increasing soil salinity (Patel et al., 2010). A. julibrissin might have the ability to transport K⁺ against the Na⁺ gradient, which leads to an increase in leaf K⁺ concentration (Grattan and Grieve, 1999). On the other hand, there was no change in the leaf K⁺ concentration of S. japonica. As salinity levels increased in the irrigation water, the K⁺:Na⁺ ratio in the leaf tissue decreased in both species (P = 0.0003, Table 6). In line with our results, it has been reported that salinity stress decreased the K⁺:Na⁺ ratio in plants (Gómez-Bellot et al., 2015; Guo et al., 2020). External Na⁺ often inhibits K⁺ uptake and hence high cytosolic K⁺:Na⁺ ratios are the key salt tolerance trait in plants (Assaha et al., 2017; Shabala and Pottosin, 2014). In addition, the K⁺:Na⁺ ratio had positive correlations with the visual score, P_n, g_S, and E (Fig. 5). In the present study, Mg²⁺ and S content in the leaf tissue of A. julibrissin increased in response to elevated salinity levels in irrigation water (Table 6). In addition, Zn²⁺and Mn²⁺ contents in the leaf tissue of A. julibrissin and S. japonica increased in response to elevated salinity levels in irrigation water. Similarly, salinity stress increased Zn²⁺ and Mn²⁺ concentrations in A. auriculiformis (Patel et al., 2010). Magnesium and Mn²⁺ play important roles in the biosynthesis of chlorophyll and successful growth of plants under normal or stressful conditions (Farhangi-Abriz and Ghassemi-Golezani, 2021). Similarly, S is an integral part of several important compounds in plants, such as vitamins, coenzymes, and phytohormones (Li et al., 2020). In addition, Zn²⁺ reduces excessive Na⁺ uptake under saline conditions by affecting the structural integrity and permeability of cell membrane (Tolay, 2021). However, Yildiz et al. (2020) reported that Na⁺ and Cl⁻ ions compete with nutrients and lead to nutrient deficiency in plants. Therefore, an increase in the concentration of these nutrients with increasing salinity levels might be the strategy of these species to survive in saline conditions.

In conclusion, A. julibrissin had only minimal foliar salt damage at higher EC levels and S. japonica had no foliar salt damage. Saline solution irrigation reduced plant growth of A. julibrissin and S. japonica as indicated by plant height, leaf area, and DW in both experiments. Salinity stress also reduced plant photosynthesis and caused Cl⁻ uptake and accumulation. However, Na⁺ uptake and accumulation are less pronounced compared with Cl⁻. Albizia julibrissin and S. japonica are probably capable of restricting either the uptake or transport of Na⁺ and tolerating high concentrations of Cl⁻ in the leaf tissue while maintaining good aesthetic quality. Therefore, both species are suitable for landscape use in salt-affected areas.