Salinity negatively affects plants’ morphological development and physiological processes. Salinity can cause foliar damage including necrosis, burn, scorch, and premature defoliation (Munns, 2002). Stunted growth, biomass reduction, and inhibited bud formation also happen when plants are grown under salinity stress (Taiz et al., 2015). More resource inputs such as seeds, water, and fertilizers are needed to make up for the loss of plant quality under salinity stress. Physiologically, high concentrations of soluble salts in the soil disturb nutrient uptake, cause protein denaturation, and inhibit plant photosynthesis, stomatal conductance, and biosynthetic processes (Munns and Tester, 2008; Taiz et al., 2015). Specific ions could also accumulate to toxic levels in plant cells.
Salinity tolerance varies among plant species, and selecting salt-tolerant plants and using them in landscapes can be sustainable. Salinity-tolerant plants are usually identified by manually irrigating plants with saline solutions (Liu et al., 2017; Sun et al., 2015). However, this protocol is time-consuming and only applicable to a limited number of plants. Using automatic irrigation systems, such as a drip injector irrigation system (Aragues et al., 1999), triple-line source sprinkler system (Aragues et al., 1992), and double-emitter source (DES) system (De Malach et al., 1996), can screen multiple plant species for salinity tolerance at a wide range of salinity levels while reducing labor costs. Hawks et al. (2009) modified the DES system and created an NCGD system with more flexibility, adaptability, and accuracy in delivery capacity. Multiple drip emitters are coupled to provide each plant with nutrient solution and saline solution at a designated ratio but the same total volume. Using the NCGD system to study the responses of different plants to a specific range of salt concentrations can help define the salinity threshold for plant species.
Rose of sharon, ninebark, and japanese spirea are commonly used in urban landscapes in Utah and the Intermountain West United States. According to Liu et al. (2017), the shoot dry weight and leaf area of ‘ILVOPS’ (Purple Satin®) rose of sharon was reduced when irrigated with saline solutions at an EC of 5.0 and 10.0 dS·m–1, and plants suffered serious foliar damage when supplied with saline solutions at an EC of 10.0 dS·m–1. The chlorophyll content of ‘ILVOPS’ rose of sharon also decreased significantly under irrigation of saline solutions at an EC of 10.0 dS·m–1. Curtis and Läuchli (1987) found the leaf area of kenaf (Hibiscus cannabinus) reduced when irrigated with saline solutions at an EC of 5.0 and 9.0 dS·m–1. Ninebark showed a poor growth rate when supplied with nutrient solutions at an EC of 2.4 and 2.6 dS·m–1 (Gils et al., 2005). Jull (2009) reported that japanese spirea were moderately tolerant to saline spray. However, in a study conducted by Wang et al. (2019), japanese spirea were moderately sensitive to saline irrigation water with an EC of 6.0 dS·m–1. The thresholds for their salinity tolerance have not been identified. To this end, a greenhouse study was conducted to investigate the responses of these three landscape plants to saline irrigation water using an NCGD system to determine their salinity thresholds.
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