Salinity of soil and irrigation water is a growing problem for agricultural production in the world. It is estimated that salt-affected soils impact on nearly 10% of the land surface and 50% of irrigated land in the world (Ruan et al., 2010). Global warming promotes water transpiration from plants and evaporation from soil, thereby increasing salt accumulation in soil. Salinity effects on plants include cellular water deficit, ion toxicity, nutrient deficiencies, and oxidative stress, which can lead to growth inhibition, molecular damage, and even plant death (Orcutt and Nilsen, 2000). The global annual losses in agricultural production from salt-affected land are in excess of U.S. $12 billion and rising (Flowers et al., 2010; Qadir et al., 2008).
Lettuce (Lactuca sativa L.) is one of the most important salad vegetables in the United States, and contains important phytochemicals, including vitamins, carotenoids, and other antioxidants (Humphries and Khachik, 2003; Nicolle et al., 2004). It is considered to be a relatively salt-sensitive vegetable (Shannon and Grieve, 1999). Salt stress reduced lettuce germination, leaf water content, photosynthesis rate, chlorophyll content, root and shoot growth, and increased sodium and chloride ion concentration and lipid peroxidation (Barassi et al., 2006; Eraslan et al., 2007; Kaya et al., 2002; Mohammadi and Khoshgoftarmanesh, 2014; Mota-Cadenas et al., 2010; Pérez-López et al., 2013; Shannon et al., 1983).
One approach to enhance lettuce production in saline soils and water is to develop salt-tolerant cultivars, which may be achieved by exploiting intraspecific variability (Dewey, 1962; Wei et al., 2014). Considerable variability in salt tolerance was found among 85 lettuce cultivars and breeding lines in one experiment (Shannon et al., 1983), although there was little variability in salt tolerance among 6 lettuce cultivars in another (Ayers et al., 1951). Furthermore, Shannon and McCreight (1984) screened 115 PIs of lettuce for salt tolerance and found that PIs had a wider range of salt tolerance and a higher mean average salt tolerance than standard cultivars. Several wild relatives of cultivated lettuce, Lactuca serriola, Lactuca Vignata, and Lactuca saligna, had an even higher range of tolerance than the introductions (Shannon and Grieve, 1999). However, they noted that their tolerance ratings were measured in terms of absolute growth under high salinity, and therefore did not account for natural difference in total growth potential that may exist between cultivars or PIs (Shannon and McCreight, 1984). Moreover, most lettuce cultivars and PIs have not been screened for salt tolerance yet. The objectives of this study were to screen lettuce germplasm and cultivars for their salinity tolerance in comparison with plants grown under control conditions, and identify traits related to salt tolerance.
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