As urban population increases and fresh water supply is diminishing in the southwestern United States, many municipalities have encouraged using reclaimed water (treated municipal effluent, also called recycled water) for landscape irrigation. In the past decades, reclaimed water has been used for landscape irrigation, mostly gulf courses, school yards, and parks, in California and the other southwestern states (Dobrowolski et al., 2008) and in other countries (Gori et al., 2000; Marosz, 2004). However, use of reclaimed water for irrigating landscapes with multiple plant species has not been widely practiced partly as a result of salt damage on sensitive plant species. Reclaimed water contains beneficial nutrients for plant growth, but also contains an elevated salt load (Devitt et al., 2005). Therefore, investigating the salinity tolerance of native landscape plants, which are gaining popularity, is of increasing importance.
High salinity in soil decreases soil water potential, which makes water less available to plants. This could be aggravated by low rainfall. Typical plant responses to salinity include reduced shoot and root growth and reduced size of whole plants (Munns, 2002). As salinity stress becomes more severe, foliar damage such as leaf burn, scorch, necrosis, and premature defoliation could occur. High salinity levels may also reduce leaf photosynthesis and stomatal conductance (gS) (Munns and Tester, 2008; Zollinger et al., 2007). Salt tolerance of plants depends on not only species, but also climatic conditions, type of substrate or soil, and irrigation method. Although salinity levels of irrigation water were kept constant, root zone salinity increases with time, especially in peat-based substrate (Marosz, 2004; Niu and Rodriguez, 2006a, 2006b) or in clay soil (Miyamoto et al., 2005). Salt injury on a number of landscape plants irrigated with low-quality water was most noticeable during the hottest and driest period of summer (Fox et al., 2005; Niu et al., 2007).
Texas mountain laurel is an evergreen shrub or small tree native to Texas, New Mexico, and northeastern Mexico. Mexican redbud is a native, deciduous small tree. It is heat- and drought-tolerant and thrives in well-drained soil (Gilman and Watson, 1993). Both species are considered excellent landscapes plants in Texas and the other southwestern states (Gilman and Watson, 1993, 1994; Ruter and Ingram, 1991). Despite their ornamental values, little information is available on their salt tolerance. The objective of this study was to examine the relative salt tolerance of these two native woody species by growing them in two substrates irrigated with saline solutions at elevated salinity levels.
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