Water shortage and poor water quality are critical issues in the Southwest and many other regions of the world. With a rapidly increasing population and diminishing water supplies, the competition for fresh water among agriculture, industry, urban, and recreational users has become intense. Use of municipal reclaimed water (also called recycled water in some states) to irrigate landscapes can conserve a substantial amount of potable water because ≈50% of the total municipal water consumption from May through October in the Southwest is used for landscape irrigation (Kjelgren et al., 2000). Currently, reclaimed water has been used mainly for irrigating golf courses and some horticultural production such as cut flower production in coastal areas of California. The application of reclaimed water for residential and commercial landscapes containing multiple plant species is limited, partly as a result of salt damage on sensitive plants and public perceptions of reclaimed water. As competition for water supplies intensifies, the use and acceptance of reclaimed water for landscape and agricultural irrigation will increase, leading to a need for dual water systems that are integrated fully into community and rural water supplies (Dobrowolski et al., 2008).
Reclaimed water contains nutrients such as nitrogen, phosphorus, and other minerals that plants can absorb and use; therefore, use of reclaimed water can reduce fertilizer costs. Many researchers have reported positive effects of elevated salinity on plant morphology, growth, and quality. Cabrera and Perdomo (2003) observed a positive correlation between relatively high leaf chlorine (Cl) concentration and dry weight of container-grown Rosa ×hybrida ‘Bridal Pink’ (on R. manetti rootstock). A number of herbaceous perennials, groundcovers, floricultural crops, and landscape woody shrubs became more compact with little visual damage when irrigated with low to moderate salinity compared with nonsaline control (Carter and Grieve, 2006; Gerhart et al., 2006; Grieve et al., 2006; Niu and Rodriguez, 2006a, 2006b; Wu et al., 2001). For landscape plants, maximizing growth is not essential and, indeed, excessive shoot vigor is often undesirable. To maintain a compact growth habit, ornamental plants traditionally have either been pruned or treated with growth regulators (Cameron et al., 2004). Therefore, reclaimed water may play an important role for landscape irrigation where the supply of high-quality water is limited. Nevertheless, the salinity tolerance range or threshold for optimal growth and quality of landscape plants needs to be identified for efficient and sustainable use of reclaimed water.
Salt tolerance of plants depends on species, climatic conditions, type of substrate or soil, and irrigation method. Although salinity levels of irrigation water were kept constant, root zone salinity increased with time, especially when peat-based substrate or clay soil was used (Miyamoto et al., 2005; Niu and Rodriguez, 2006a, 2006b). Salt accumulation in the root zone is affected by the physical and chemical properties of the substrate, plant size, and environmental conditions because these factors influence the substrate moisture content and cation exchange capacity in the root zone. Plants were more susceptible to salinity stress under sprinkler irrigation than drip irrigation as a result of direct contact with salts (Wu et al., 2001). Foliar salt injuries of a number of landscape plants irrigated with saline water were most affected during the hottest and driest periods of summer (Fox et al., 2005; Niu et al., 2007).
As more low-quality water is used for landscape irrigation, demand for salt-tolerant bedding plants will increase in arid and semiarid regions. Little information is available on salt tolerance of bedding plants. The objective of this study was to quantify the relative salt tolerance of 10 selected bedding plant species and cultivars, whose performance ranged from acceptable to excellent under limited irrigation conditions in a semiarid desert environment (Niu and Rodriguez, 2007). In the current study, the same 10 species and cultivars were irrigated with water at various salinity levels in two experiments conducted in a shadehouse (summer) or a greenhouse (winter).
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