In the arid and semi-arid southwestern United States, salinity is becoming one of the most serious problems threatening the quality of urban landscapes. High salinity in the soil and irrigation water affects plant growth, development, and survival (Munns and Tester, 2008). As salinity stress becomes severe, plants experience foliar damage, such as leaf burn, scorch, necrosis, and premature defoliation (Munns and Tester, 2008). As a result of excessive Na+ and Cl‒ absorption, plants usually exhibit reduced photosynthesis and stomatal conductance (gS) and nutrient imbalance, especially calcium deficiency (Munns and Tester, 2008). Salt-tolerant plants have a better ability to adjust internally to the osmotic effects of high salinity levels than salt-sensitive plants. It is noted that substantial differences in salt tolerance exist among various landscape plant species (Navarro et al., 2008). It is possible to identify salt-tolerant landscape plants by investigating their specific morphological and physiological responses to salinity stress. Using salt-tolerant plants would prevent salt damage and maintain aesthetically appealing landscapes in salt-prone regions. Additional research-based information regarding salinity tolerance is needed for widely used landscape plants.
Spiraea is the largest genus in Rosaceae (rose family), with more than 100 species of shrubs native to the northern temperate hemisphere and great diversity in eastern Asia (Khan et al., 2016). Many species in the genus Spiraea are used extensively as ornamental plants in temperate climates, particularly for their showy clusters of dense flowers. In the United States, an estimated $29 million of spirea plants are sold annually for garden and landscape uses, making them the fourth-best-selling deciduous shrubs (U.S. Department of Agriculture, 2015). Numerous spirea species, hybrids, and cultivars with various flower colors (white, pink, or dark red) and foliage colors (yellow, blue, green, or variegated foliage) are selected from natural populations and breeding efforts in gardens for landscape use (Dirr, 2009).
Efforts have been made to investigate the responses of spirea plants to abiotic stresses. For example, Stanton and Mickelbart (2014) evaluated the water stress responses of Spiraea alba (meadowsweet) and Spiraea tomentosa (steeplebush) under greenhouse conditions and found that both species had reduced growth, transpiration, and predawn water potential, but that they exhibited the ability to osmotically adjust to low soil water content to maintain water uptake and reduce water loss. Salt tolerance of spirea species has also been reported, but the results are inconsistent. For example, S. japonica and Spiraea ×bumalda (bumald spirea) were considered to be tolerant, moderately tolerant, or sensitive to saline soils or salt spray in different publications (Appleton et al., 2015; Beckerman and Lerner, 2009; Jull, 2009). These reports have been made on the basis of anecdotal observations without scientific research approaches. Therefore, a systematic approach is needed to evaluate different spirea species for salinity tolerance. Wang et al. (2019a) recently evaluated seven Japanese spirea cultivars in a greenhouse and reported that all Japanese spirea cultivars were moderately sensitive to the salinity levels at an EC of 3.0 and 6.0 dS·m−1. The fact that many spirea species with diversified salt tolerance are planted in gardens and landscapes merits further investigation. This study aimed to quantify the growth, morphological, and physiological responses of five spirea species to a range of salinity levels in a greenhouse setting.
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