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Albizia julibrissin (mimosa tree) and Sophora japonica (Japanese pagoda tree) are drought-tolerant landscape plants; however, salinity responses of these two species are not well documented. The objective of this study was to investigate the morphological and physiological responses of these two species to three salinity levels in greenhouse conditions. Two studies were conducted in the summer/early fall of 2020 and the spring of 2021. In 2020, uniform plants were irrigated weekly for the first 2 weeks and every other day for the following 3 weeks with a nutrient solution at an electrical conductivity (EC) of 1.2 dS·m⁻¹ as a control or saline solution at ECs of 5.0 or 10.0 dS·m⁻¹. In 2021, plants were irrigated weekly for 8 weeks with the same treatment solutions as described previously. Albizia julibrissin and S. japonica survived in both experiments with minimal foliar salt damage (leaf burn or necrosis). Irrigation water at ECs of 5.0 and 10.0 dS·m⁻¹ reduced plant height and dry weight (DW) of both species. In the fall experiment, A. julibrissin irrigated with a saline solution at an EC of 10.0 dS·m⁻¹ had the highest reduction in plant height (61%) compared with the control. Albizia julibrissin and S. japonica irrigated with a saline solution at an EC of 10.0 dS·m⁻¹ had 52% and 47% reductions in shoot DW compared with the control, respectively. In the spring experiment, compared with the control, there were 72% and 45% reductions in height of A. julibrissin and S. japonica, respectively, when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. In addition, compared with the control, A. julibrissin and S. japonica had 58% and 64% reductions in shoot DW, respectively, when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. Increasing salinity levels in the irrigation water also reduced leaf greenness [Soil Plant Analysis Development (SPAD)], leaf net photosynthesis rate (P_n), stomatal conductance (g _S), and transpiration rate (E) of both species. Furthermore, sodium (Na⁺) and chloride (Cl⁻) concentrations in leaves were affected by elevated salinity levels in the irrigation water. Visual score, P_n, g _S, and E negatively correlated to Na⁺ and Cl⁻ concentrations in leaves. But Cl⁻ accumulation had more impact on the growth of A. julibrissin and S. japonica. In summary, both species were tolerant to saline solution irrigation up to 5.0 dS⋅m⁻¹ and moderately tolerant to saline solution irrigation up to 10.0 dS⋅m⁻¹.

Native plants are of great value in landscape maintenance. Despite their importance in the landscape, the salt tolerance of most native plants has received little attention. The present research was designed to assess morphological, physiological, and biochemical responses of four Utah-native plants [Arctostaphylos uva-ursi (kinnikinnick), Cercocarpus ledifolius (curl-leaf mountain mahogany), Cercocarpus montanus ‘Coy’ (alder-leaf mountain mahogany), and Shepherdia ×utahensis ‘Torrey’ (hybrid buffaloberry)] at different salinity levels. Each species was irrigated with a nutrient solution at an electrical conductivity (EC) of 1.2 dS·m⁻¹ (control) or saline solutions at ECs of 5.0 or 10.0 dS·m⁻¹ for 8 weeks. The experiment was a randomized complete block design with 10 replications. At 8 weeks after the initiation of the experiment, A. uva-ursi and C. montanus ‘Coy’ had slight foliar salt damage with an average visual score of 3.7 (0 = dead, 5 = excellent with no sign of foliar salt damage) when irrigated with saline solution at an EC of 5.0 dS·m⁻¹ and were dead at an EC of 10.0 dS·m⁻¹. Similarly, C. ledifolius had an average visual score of 3.2 when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. However, almost no foliar salt damage was observed on S. ×utahensis ‘Torrey’ during the experimental period. In addition, the shoot dry weight of all species was reduced with elevated salinity levels in the irrigation water. Salinity stress also reduced gas exchange rates of plants and affected their mineral content. Proline accumulated in the leaves of native plants but was species-dependent. In conclusion, S. ×utahensis ‘Torrey’ was tolerant to salinity stress followed by C. ledifolius; A. uva-ursi and C. montanus ‘Coy’ were sensitive to salinity stress.

Sego Supreme^TM is a designated plant breeding and introduction program at the Utah State University Botanical Center and the Center for Water Efficient Landscaping. This plant selection program introduces native and adapted plants to the arid West for aesthetic landscaping and water conservation. The plants are evaluated for characteristics such as color, flowering, ease of propagation, market demand, disease/pest resistance, and drought tolerance. However, salt tolerance has not been considered during the evaluation processes. Four Sego Supreme^TM plants [Aquilegia barnebyi (oil shale columbine), Clematis fruticosa (Mongolian gold clematis), Epilobium septentrionale (northern willowherb), and Tetraneuris acaulis var. arizonica (Arizona four-nerve daisy)] were evaluated for salt tolerance in a greenhouse. Uniform plants were irrigated weekly with a nutrient solution at an electrical conductivity (EC) of 1.25 dS·m⁻¹ as control or a saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dS·m⁻¹ for 8 weeks. After 8 weeks of irrigation, A. barnebyi irrigated with saline solution at an EC of 5.0 dS·m⁻¹ had slight foliar salt damage with an average visual score of 3.7 (0 = dead; 5 = excellent), and more than 50% of the plants were dead when irrigated with saline solutions at an EC of 7.5 and 10.0 dS·m⁻¹. However, C. fruticosa, E. septentrionale, and T. acaulis had no or minimal foliar salt damage with visual scores of 4.2, 4.1, and 4.3, respectively, when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. As the salinity levels of treatment solutions increased, plant height, leaf area, and shoot dry weight of C. fruticosa and T. acaulis decreased linearly; plant height of A. barnebyi and E. septentrionale also declined linearly, but their leaf area and shoot dry weight decreased quadratically. Compared with the control, the shoot dry weights of A. barnebyi, C. fruticosa, E. septentrionale, and T. acaulis decreased by 71.3%, 56.3%, 69.7%, and 48.1%, respectively, when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. Aquilegia barnebyi and C. fruticosa did not bloom during the experiment at all treatments. Elevated salinity reduced the number of flowers in E. septentrionale and T. acaulis. Elevated salinity also reduced the number of shoots in all four species. Among the four species, sodium (Na⁺) and chloride (Cl^–) concentration increased the most in A. barnebyi by 53 and 48 times, respectively, when irrigated with saline solution at an EC of 10.0 dS·m⁻¹. In this study, C. fruticosa and T. acaulis had minimal foliar salt damage and less reduction in shoot dry weight, indicating that they are more tolerant to salinity. Epilobium septentrionale was moderately tolerant to saline solution irrigation with less foliar damage, although it had more reduction in shoot dry weight. On the other hand, A. barnebyi was the least tolerant with severe foliar damage, more reduction in shoot dry weight, and a greater concentration of Na⁺ and Cl^–.

Screening salinity-tolerant plants is usually time intensive and only applicable to a limited number of salinity levels. A near-continuous gradient dosing (NCGD) system allows researchers to evaluate a large number of plants for salinity tolerance with multiple treatments, more flexibility, and reduced efforts of irrigation. Rose of sharon (Hibiscus syriacus), ninebark (Physocarpus opulifolius), and japanese spirea (Spiraea japonica) were irrigated using an NCGD system with eight electrical conductivity (EC) levels ranging from 0.9 to 6.5 dS·m^–1. At 11 weeks after irrigation was initiated, there were no significant differences among EC levels in terms of visual score, growth index [(Height + Width 1 + Width 2)/3], stem diameter, number of inflorescences, and shoot dry weight (DW) of rose of sharon. However, the root DW, relative chlorophyll content (SPAD), and net photosynthesis rate (P_n) of rose of sharon decreased linearly as EC levels increased. Ninebark and japanese spirea had increased foliar salt damage with increasing EC levels. The growth index, stem diameter, number of inflorescences, shoot and root DW, SPAD, and P_n of ninebark decreased linearly as EC levels increased. The growth index and SPAD of japanese spirea decreased quadratically with increasing EC levels, but its stem diameter, number of inflorescences, shoot and root DW, and P_n decreased linearly with increasing EC levels. The salinity threshold (50% loss of shoot DW) was 5.4 and 4.6 dS·m^–1, respectively, for ninebark and japanese spirea. We were not able to define the salinity threshold for rose of sharon in this study. However, rose of sharon was the most salinity-tolerant species among the three landscape plants.

Reclaimed water provides a reliable and economical alternative source of irrigation water for landscape use but may have elevated levels of salts that are detrimental to sensitive landscape plants. Landscape professionals must use salt-tolerant plants in regions where reclaimed water is used. Ornamental grasses are commonly used as landscape plants in the Intermountain West of the United States due to low maintenance input, drought tolerance, and unique texture. Six ornamental grass species, including Acorus gramineus (Japanese rush), Andropogon ternarius (silver bluestem), Calamagrostis ×acutiflora (feather reed grass), Carex morrowii (Japanese sedge), Festuca glauca (blue fescue), and Sporobolus heterolepis (prairie dropseed), were evaluated for salinity tolerance. Plants were irrigated every 4 days with a fertilizer solution at an electrical conductivity (EC) of 1.2 dS·m^–1 (control) or with a saline solution at an EC of 5.0 dS·m^–1 (EC 5) or 10.0 dS·m^–1 (EC 10). At 47 days, most species in EC 5 exhibited good visual quality with averaged visual scores greater than 4.6 (0 = dead, 5 = excellent). In EC 10, most A. gramineus plants died, but C. ×acutiflora, F. glauca, and S. heterolepis had no foliar salt damage. At 95 days, C. ×acutiflora, F. glauca, and S. heterolepis in EC 5 had good visual quality with averaged visual scores greater than 4.5. Acorus gramineus, A. ternarius, and C. morrowii showed foliar salt damage with averaged visual scores of 2.7, 3.2, and 3.4, respectively. In EC 10, A. gramineus died, and other grass species exhibited moderate to severe foliar salt damage, except C. ×acutiflora, which retained good visual quality. Plant height, leaf area, number of tillers, shoot dry weight, and/or gas exchange parameters also decreased depending on plant species, salinity level, and the duration of exposure to salinity stress. In conclusion, A. gramineus was the most salt-sensitive species, whereas C. ×acutiflora was the most salt-tolerant species. Festuca glauca and S. heterolepis were more tolerant to salinity than A. ternarius and C. morrowii. Calamagrostis ×acutiflora, F. glauca, and S. heterolepis appear to be more suitable for landscapes in which reclaimed water is used for irrigation. Plant responses to saline water irrigation in this research could also be applied to landscapes in salt-prone areas and coastal regions with saltwater intrusion into aquifers and landscapes affected by maritime salt spray.

Viburnums are widely used in gardens and landscapes throughout the United States. Although salinity tolerance varies among plant species, research-based information is limited on the relative salt tolerance of viburnum species. The morphological and growth responses of 12 viburnum taxa to saline solution irrigation were evaluated under greenhouse conditions. Viburnum taxa included Viburnum ×burkwoodii, V. cassinoides ‘SMNVCDD’, V. dentatum ‘Christom’, V. dentatum var. deamii ‘SMVDLS’, V. dilatatum ‘Henneke’, V. בNCVX1’, V. nudum ‘Bulk’, V. opulus ‘Roseum’, V. plicatum var. tomentosum ‘Summer Snowflake’, V. pragense ‘Decker’, V. ×rhytidophylloides ‘Redell’, and V. trilobum. A nutrient solution at an electrical conductivity (EC) of 1.3 dS·m⁻¹ (control) or saline solutions at ECs of 5.0 and 10.0 dS·m⁻¹ were applied eight times over a 9-week period. Growth, visual quality, and morphological characteristics were quantified at the 4th week and 8th–9th week to assess the impact of salinity stress on the viburnum taxa. Saline solution irrigation imposed detrimental salinity stress on viburnum plant growth and visual quality, and the degree of salt damage was dependent on the salinity levels of irrigation solution and the length of exposure to salinity stress as well as viburnum taxa. Viburnum ×burkwoodii and V. בNCVX1’ had little foliar salt damage during the entire experiment, except those irrigated with saline solution at an EC of 10.0 dS·m⁻¹ exhibited slight to moderate foliar salt damage at the eighth week. Viburnum dilatatum ‘Henneke’, V. plicatum var. tomentosum ‘Summer Snowflake’, and V. trilobum irrigated with saline solution at an EC of 5.0 dS·m⁻¹ had slight and severe foliar salt damage at the 4th and 8th week, respectively. Plants irrigated with saline solution at an EC of 10.0 dS·m⁻¹ exhibited severe foliar salt damage at the 4th week, and all died by the 8th week. Other viburnum taxa also showed various foliar salt damage, especially at an EC of 10.0 dS·m⁻¹. The shoot dry weights of V. ×burkwoodii and V. בNCVX1’ irrigated with saline solution at ECs of 5.0 and 10.0 dS·m⁻¹ were similar to those in the control at both harvest dates. However, the shoot dry weight of other tested viburnum taxa decreased to some extent at the 9th week. A cluster analysis concluded that V. ×burkwoodii and V. בNCVX1’ were considered the most salt-tolerant viburnum taxa, whereas V. dilatatum ‘Henneke’, V. plicatum var. tomentosum ‘Summer Snowflake’, and V. trilobum were sensitive to salinity levels used in this study. This research may guide the green industry to choose relatively tolerant viburnum taxa for landscape use and nursery production where low-quality water is used for irrigation.

More than half of residential water in Utah is used for landscape irrigation. Reclaimed water has been used to irrigate urban landscapes to conserve municipal water. High salt levels in reclaimed water may pose osmotic stress and ion toxicity to salt-sensitive plants. Viburnums are commonly used landscape plants, but salinity tolerance of species and cultivars is unclear. The objective of this study was to characterize gas exchanges and mineral nutrition responses of 12 viburnum taxa subjected to salinity stress in a greenhouse study. Plants were irrigated with a nutrient solution at an electrical conductivity (EC) of 1.3 dS·m^–1 or saline solution at an EC of 5.0 dS·m^–1 or 10.0 dS·m^–1. The net photosynthesis rate (P_n), stomatal conductance (g _S), and transpiration rate (E) of all viburnum taxa, except for Viburnum ×burkwoodii and V. בNCVX1’, decreased to various degrees with increasing salinity levels. The P_n, g _S, and E of V. ×burkwoodii and V. בNCVX1’ were unaffected by saline solutions of 5.0 dS·m^–1 at the 4th and 9th week after treatment initiation, with the exception of the P_n of V. ×burkwoodii, which decreased at the 9th week. Leaf sodium (Na⁺) and chloride (Cl^–) concentrations of all viburnum taxa increased as salinity levels increased. Viburnum ×burkwoodii had relatively low leaf Na⁺ and Cl^– when irrigated with saline solutions of 10.0 dS·m^–1. Plant growth and gas exchange parameters, including visual score, plant height, P_n, g _S, E, and water use efficiency (WUE) correlated negatively with leaf Na⁺ and Cl^– concentrations. The ratio of potassium (K⁺) to Na⁺ (K⁺/Na⁺) and ratio of calcium (Ca²⁺) to Na⁺ (Ca²⁺/Na⁺) decreased when salinity levels increased. Visual score, plant height, P_n, g _S, E, and WUE correlated positively with the K⁺/Na⁺ and Ca²⁺/Na⁺ ratios. These results suggest excessive Na⁺ and Cl^– accumulation inhibited plant photosynthesis and growth, and affected K⁺ and Ca²⁺ uptake negatively.