Soil salinity is a common problem encountered in arid and semiarid regions and is generally caused by insufficient drainage, low rainfall, and inappropriate irrigation management, although irrigation water is seldom saline (Boland, 2008; Pasternak and Malach, 1994). More than 800 million ha of land throughout the world are affected by salinity and sodic conditions, accounting for more than 10% of the world’s total arable land area (Munns and Tester, 2008; Tanji, 2002). As high-quality water supply becomes limited in many areas of the world, alternative waters such as municipal reclaimed water, which is treated for other beneficial uses, is being used for irrigating landscapes, mostly golf courses, in the United States and other countries (Dobrowolski et al., 2008; Duncan et al., 2009; Tanwar, 2003). Reclaimed water contains some nutrients essential for plant growth and therefore it may be possible to reduce fertilizer application when reclaimed water is used (Devitt et al., 2005). A potential problem of using reclaimed water is elevated salt levels, which are detrimental to sensitive plants if not managed properly. Therefore, it is imperative to screen salt tolerance of commonly used landscape plants so that recommendations on plant selection can be made appropriately.
Salt tolerance of landscape plants varies greatly with species and even cultivars within a species. Thousands of plant species are being used in urban landscapes. In the past decades, many studies of salt tolerance of landscape plants have been carried out on woody trees and shrubs and herbaceous perennials (Fox et al., 2005; Niu and Cabrera, 2010; Niu and Rodriguez, 2006a, 2006b; Tanji et al., 2008; Wu et al., 2001; Zollinger et al., 2007). However, information on salt tolerance of herbaceous annuals and perennials is limited. These are the group of plants in which new cultivars are frequently available in the market and are often used as bedding plants that are replaced annually or even seasonally in landscapes.
Zinnia marylandica is a hybrid between Z. angustifolia and Z. violacea with bright colors and prolific bloom and is resistant to disease, heat, and drought stresses (Spooner et al., 1991). Z. maritima ‘Solcito’, another popular plant for landscapes, stays vigorous and healthy all season with numerous small, golden-yellow blooms and tolerates heat and drought well in semiarid climate (Niu, unpublished data). However, no information is available on salt tolerance of these species. Villarino and Mattson (2011) reported that Z. angustifolia ‘Star Gold’ was sensitive to salt stress and plants did not survive at EC of 14.0 dS·m−1. However, Carter and Grieve (2010) reported that marketable flowers of two cultivars of Z. elegans ‘Benary’s Giant Salmon Rose’ and ‘Benary’s Giant Golden Yellow’ were produced when irrigated with saline solution at EC as high as 10 dS·m−1. These results indicate that salt tolerance in zinnia varied with species. The objective of this study was to determine the relative salt tolerance of six cultivars of Z. marylandica and one of Z. maritima by examining their growth and physiological responses to irrigation with saline solutions in a range of salinity.
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