Water quantity and quality are increasingly becoming critical global issues, especially in arid and semiarid regions, as well as in some coastal regions (Niu et al., 2012a). As municipalities are struggling to meet the demand for high-quality water to supply the growing urban population, treated effluent (reclaimed water or recycled water) is an alternative water source for irrigating urban landscapes and agricultural and nursery crops in suburban areas (Niu and Rodriguez, 2006a). Many places have switched to use reclaimed water for irrigating landscapes and golf courses, such as many cities in California (San Diego County Water Authority, 2015; Wu and Dodge, 2005) and in Israel (Shillo et al., 2002). However, high salinity levels of recycled water may cause damage or even death to sensitive plants if not managed properly (Niu et al., 2012b).
Salinity can reduce growth and cause foliar salt damage through physiological drought, ion toxicity, and nutrient deficiency (Munns, 2002; Niu and Cabrera, 2010; Veatch-Blohm et al., 2014). Most landscape plant species are nonhalophytes; therefore, assessment of salt tolerance is necessary for landscapes where poor-quality water may be used for irrigation. The salt tolerance of landscape plants should be based primarily on aesthetic appearance rather than maximizing growth rate (Niu and Cabrera, 2010; Shillo et al., 2002; Veatch-Blohm et al., 2014). In the past decades, many landscape plants have been investigated for salt tolerance. For example, salt tolerance of 86 tree and palm species, 65 shrub species, 58 groundcover and vine species, and 57 grass species has been screened by Wu and Dodge (2005). The salt tolerance of more than 100 species/cultivars of herbaceous landscape plants has been determined by Niu group since 2006 (Niu and Rodriguez, 2006a, 2006b; Niu et al., 2007, 2010, 2012a, 2012b). Considering the huge number of plant species potentially available for landscapes, there are still thousands of plant species and cultivars that have not been investigated for salt tolerance. The objectives of this study were to compare the relative salt tolerance of nine ornamental species, which are widely used in landscapes, based on their visual quality, growth, number of flowers, gas exchange, chlorophyll content, and shoot ion concentration when irrigated with saline solution in a range of salinities.
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