Water availability and water quality have become critical local and global issues of concern, particularly in arid and semiarid regions such as the Southwest United States (Kjelgren et al., 2000; McKenney et al., 2016; Niu and Rodriguez, 2006a). Numerous factors such as natural deficiencies in rainfall, rapid population growth, expanding urbanization, and increased freshwater demand by agricultural, industrial, and municipal sectors have contributed to water quality and quantity problems (Ganjegunte et al., 2017; Niu and Cabrera, 2010). Considering water is the most limited—and poorly managed—natural resource, there is an urgent need to adopt proper water conservation practices (Niu and Rodriguez, 2006a). Water management strategies include implementing more efficient irrigation systems, using alternative water sources, and promoting water-wise landscaping (Kjelgren et al., 2000; Wang et al., 2019).
As the supply of freshwater becomes increasingly limited, use of alternative water sources (such as reclaimed water) for landscape irrigation may become necessary (McKenney et al., 2016; Niu and Rodriguez, 2006a; Qian et al., 2005). Moreover, reclaimed water availability increases with urban population growth (Qian et al., 2005). Use of reclaimed water can save a tremendous volume of potable water, and help with wastewater disposal (Niu and Rodriguez, 2006a). In addition, use of reclaimed water can be cost-efficient, is available year-round, may supply essential plant nutrients, and is considered to be sustainable (Hamilton et al., 2007; Qian et al., 2005). However, a concern of using reclaimed water is elevated salinity levels, which may affect plant performance (Niu et al., 2012b; Paudel et al., 2019).
Salinity, in conjunction with drought, represents one of the major threats to crop production around the world (Liu et al., 2020; Paudel et al., 2019; Veatch-Blohm et al., 2014). Salinity issues are especially critical in arid and semiarid regions, where evapotranspiration is high and precipitation is low (Yadav et al., 2011). Irrigating with low-quality water, inappropriate irrigation management strategies, high fertilization rates, and poor drainage may also exacerbate soil salinity issues (Niu et al., 2012a, 2012b). Salinity-related problems can be managed through various strategies, including leaching salts below the plant root zone, improving soil drainage, applying chemical amendments to soil, reducing soil evaporation, conducting proper irrigation management, planting salt-tolerant species, using phytoremediation, or a combination of these methods (Jesus et al., 2015; Kalantari et al., 2018).
Salt-tolerant plant species are characterized by the ability to resist the effects of elevated salinity levels without serious salt injury symptoms, such as reduction in growth, yield, or visual quality (Grieve et al., 2008; Liu et al., 2020). Degradation of visual quality is particularly problematic for ornamental plants (Veatch-Blohm et al., 2014). As internal salt concentrations increase, salt-induced stress may lead to plant death (Munns and Tester, 2008; Niu and Cabrera, 2010). Detrimental effects caused by salinity stress on plants include osmotic stress, ion toxicity, and ion imbalance (Munns, 2005). These effects are observed at the cellular, organ, and whole-plant level (Munns and Tester, 2008). To cope with effects of salinity stress, plants have evolved different salt tolerance mechanisms. Primary mechanisms for salinity resistance include osmotic adjustment, ion exclusion from root uptake, tissue tolerance by compartmentalization of ions into vacuoles, salt excretion by salt glands, and increased succulence to dilute salts within plant cells (Deeter, 2002; Munns, 2005; Yensen and Biel, 2006). In fact, a combination of salinity tolerance mechanisms has been used by many plant species (Deeter, 2002). In addition, a wide range of salt tolerance among landscape plant species, and even cultivars within the same species, has been reported (Niu et al., 2007; Veatch-Blohm et al., 2014). However, environmental conditions, dominant salt type, type of substrate or soil, irrigation management, and plant growth stage may affect plant responses to salinity stress (Niu and Cabrera, 2010; Zollinger et al., 2007).
With increased demand, and greater costs for high-quality water, implementation of water-wise landscapes is critical (Kjelgren et al., 2000; Paudel et al., 2019). Native wildflowers are considered to be excellent species in sustainable landscapes (Niu et al., 2012a). Indigenous wildflower species are well adapted to local climates, soils, insects, and diseases (Gioannini et al., 2018; Lee-Mäder et al., 2013). Once established, most wildflower species are known to be drought tolerant and typically do not require supplemental irrigation (Lee-Mäder et al., 2013; Niu et al., 2012a). An additional benefit of planting native wildflowers is cost efficiency associated with labor, maintenance, and use of resources. The specific benefits are a decrease in mowing frequency, noxious weed suppression, reduced water runoff, control of soil erosion, and minimization of fertilizer and pesticide application (Hopwood, 2013; Lee-Mäder et al., 2013). Planting indigenous wildflower species also ensures valuable wildlife habitat, and often serves as a reliable source of nectar and pollen for native pollinators (Gioannini et al., 2018; Hopwood, 2013). Although native wildflowers are often ignored in landscape gardening, interest in native plant species has recently risen as a result of the use of wildflowers in site restoration, improvement of local environments, and enhanced aesthetics (Cassaniti et al., 2012). For these reasons, use of native plants in arid and semiarid ecosystems with elevated soil salinity levels has gained attention (Kotzen, 2004).
For sustainable landscapes, there is a pressing need to identify salt-tolerant native wildflower species with desirable ornamental qualities (Kjelgren et al., 2009). In recent years, numerous researchers have conducted studies to assess salinity tolerance of landscape plants (Acosta-Motos et al., 2015; Cassaniti et al., 2012; Niu et al., 2010; Paudel et al., 2019; Veatch-Blohm et al., 2014; Wang et al., 2019; Wu et al., 2016). However, information on salinity tolerance of native wildflower species remains scarce (Niu et al., 2012a). Therefore, the objectives of this study are to identify the growth response and salt tolerance mechanisms of three wildflower species exposed to a range of salinity treatments. Results will provide baseline information on the relative salt tolerance of tested species, and will enhance planting recommendations for salt-tolerant wildflower species in areas with poor water quality or soil salinity concerns. Selected wildflower species for this study were Gaura villosa (gaura), Xanthisma texanum (sleepy daisy), and Ipomopsis rubra (standing cypress) (Fig. 1). Each species is native to North America, requires partial shade to full sun, thrives in sandy to gravely poor soils, and is thought to exhibit potential as an ornamental landscape species (Diggs et al., 1999; U.S. Department of Agriculture, 2018; Wasowski and Wasowski, 2000).
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