Water scarcity is of major concern in the southwestern United States. Given this worry, it is remarkable that 62% of all freshwater is used for irrigation (Kenny et al., 2009). Alternative water sources such as brackish water, gray water, and reclaimed municipal water are becoming important resources for “new water” in arid to semiarid areas in the United States. Urban landscape irrigation with alternative waters to conserve potable water is common in water-scarce cities such as San Diego, CA, and Tucson, AZ (Grieve, 2011). However, this practice is still underused because high concentrations of salts in alternative waters can potentially stress, damage, and reduce plant growth if not managed properly (Niu and Cabrera, 2010). Water quality, specifically EC and harmful ions such as sodium (Na) and chloride (Cl), of alternative water also varies largely with the source of water and treatment method. The EC of reclaimed municipal water is usually two to three times higher than that of tap water (Niu and Cabrera, 2010). Maintaining a leaching fraction, monitoring salt concentration in alternative waters, improving drainage, and using salt-tolerant species are ways to avoid salt damage (Niu and Cabrera, 2010). More research on the salt tolerance of landscape plants is needed to prevent salt damage and maintain aesthetically appealing landscapes. To date, Wu and Dodge (2005) evaluated the salt tolerance of 86 tree species, 65 shrub species, 58 groundcover and vine species, and 57 grass species. More than 150 species and cultivars of landscape plants have been screened in greenhouse conditions at the Texas A&M AgriLife Research and Extension Center in El Paso, TX. These studies demonstrate that landscape species and cultivars exhibited considerable differences in salt tolerance (Niu and Cabrera, 2010; Niu et al., 2011). More studies are essential to evaluate plant responses to salinity and to identify salt-tolerant plants for landscapes that are irrigated with alternative waters.
Marigold, native to North and South America, is a genus of annual or perennial plants in the sunflower family (Asteraceae). Many marigold species have become naturalized around the world and are considered one of the most popular ornamental plants. Most marigold species have fine-textured, pinnate dark green leaves with golden, orange, yellow, and white flowers. French marigold is a short-stature plant, whereas african marigold is a relatively taller plant (Valdez-Aguilar et al., 2009a). Many cultivars of both species are used as container plants in landscapes and as bedding plants. In addition, some cultivars of african marigold are excellent candidates for cut and dried flowers for the florist market (Valdez-Aguilar et al., 2009a). In the United States, an estimated $82 million worth of marigold plants are sold annually as annual bedding and garden plants (U.S. Department of Agriculture, 2018). Numerous cultivars have been bred to offer different flower colors (golden, orange, yellow, and white flowers) and forms (erect growth or dwarf size) (PanAmerican Seed Co., 2018). For example, ‘Discovery Orange’ and ‘Discovery Yellow’ are dwarf african marigolds with a compact and tidy habit. ‘Hot Pak Gold’, ‘Hot Pak Orange’, and ‘Hot Pak Yellow’ are the first french marigold series with a mounded habit. They are heat-tolerant extra-dwarf plants bred to bloom all summer, even in the highest temperatures and humidity. ‘Taishan Gold’, ‘Taishan Orange’, and ‘Taishan Yellow’ are dwarf african marigolds and have a mounded and upright habit with 20% shorter peduncles and 15% thicker stems. ‘Taishan Gold’, ‘Taishan Orange’, and ‘Taishan Yellow’ african marigolds were featured with spectacular performance in landscapes at the 2008 Olympic Games in Beijing, China (PanAmerican Seed Co., 2018).
‘First Lady’ african marigolds exhibited salt damage in a peat–perlite medium when the EC of a sodium chloride (NaCl) and calcium chloride (CaCl2) solution exceeded 7.9 dS·m−1; therefore, it is rated as moderately tolerant to salinity (Huang and Cox, 1988). Villarino and Mattson (2011) also rated ‘Crested Bonanza Bolero’ french marigold as a moderately tolerant plant to salinity. Valdez-Aguilar et al. (2009a, 2009b) reported that the irrigation water EC(ECw) lower than 8 dS·m−1 had minimal effects on the plant quality of ‘Flagstaff’ and ‘Yellow Climax’ african marigolds, and ‘French Vanilla’ french marigolds that are recommended for specialty cut flower production and for use in landscape sites as bedding plants. The fact that marigold plant species and cultivars with diversified salt tolerance are used in urban landscapes warrants further research. Additional, suitable cultivars should be identified for breeding and selecting salt-tolerant marigolds for landscape use in high soil salinity regions. The present study was designed to compare the growth of eight popular marigold cultivars when irrigated with saline solutions supplemented with NaCl and CaCl2.
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