As the demand for and cost of potable water has increased, recycled and secondary water have become attractive alternatives for landscape irrigation (Kjelgren et al., 2000). However, the use of recycled and secondary water in landscape applications is limited because salinity levels are often higher than in primary or culinary sources. Salinity levels for some recycled water may be as low as 2 dS·m−1 (Lindsey et al., 1998), whereas salinity levels for secondary water can exceed 5 dS·m−1 (James and Jurinak, 1986).
Relatively little research has been conducted to assess the soil salinity tolerance of ornamental plants. However, for some crop plants, damage can occur with irrigation salinity levels more than 1.0 to 2.0 dS·m−1 (Maas and Grattan, 1999). Salinity stress in soil-based systems is a result of both ion toxicities and the combined osmotic effects of salts accumulating in the soil and matric effects of soil drying (Bernstein, 1975; Glenn and Brown, 1998), and thus can produce different symptoms than those seen in hydroponic culture. Typical plant responses to soil salinity include reduced shoot and root growth rates (Munns, 2002), reduced leaf or shoot number (Munns, 2002), declines in stomatal conductance (g S) and photosynthesis rates (Kerstiens et al., 2002; Rivelli et al., 2002; Wang and Nii, 2000), and damage or death of leaves (Munns, 2002). Because many of the common responses to salinity decrease the visual quality of the plant, salinity may limit the use of specific plants in greenhouse or landscape settings where saline irrigation water is used.
Plant responses to salinity, particularly visual responses, are typically more severe when leaves are exposed directly to salinity (Simini and Leone, 1986; Wu et al., 2001). Thus, although salt spray studies (Deeter, 2001; Wu et al., 2001) may be valid for a general comparison of species tolerance to salinity, the results may not be readily extrapolated to soil-based systems in which irrigation water does not come in contact with leaves.
A limited number of commonly grown herbaceous perennials have native distributions in saline environments and are thought to have some tolerance to salinity. For example, Leucanthemum ×superbum grows along seashores (Phillips and Burrell, 1993), and Lavandula species are frequently found in harsh Mediterranean climates where they are exposed to sea spray (Upson and Andrews, 2004). These plants are well suited for use in landscapes where saline irrigation water is used. However, to maintain high visual appeal and ecological diversity within these landscapes, more salt-tolerant selections need to be identified. Because a large proportion of the soils in the intermountain western United States (IMW; the biogeographical region between the Sierra Nevada and Rocky Mountain ranges) are saline, this region may be a source of potentially salt-tolerant herbaceous perennials (Mee et al., 2003).
Quantitative knowledge of the salinity tolerance of specific herbaceous perennials is important if these plants are to have a place in landscapes irrigated with saline water. Visual quality is an important factor in screening for salt tolerance of herbaceous perennials for ornamental landscapes. Because plants respond differently to salinity, visual quality may or may not be related to biomass production and photosynthetic response. The primary objective of this study was to evaluate the relative salinity tolerance of four commonly grown and four IMW native ornamental herbaceous perennials based primarily on visual quality, survival, and plant biomass. A secondary objective was to assess the effect of saline irrigation on gas exchange properties of these species.
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