Much of semiarid western North America is salt affected (Szabolcs, 1989) due to alkaline water and insufficient leaching that would otherwise remove salts from weathering of minerals and groundwater (Pitman and Lauchli, 2002). Irrigation contributes to increased soil salinity in semiarid climates at times of high evapotranspiration coupled with inadequate leaching, low-quality irrigation water, or rising water tables that receive salts leached from the plant root zone (Carrow and Duncan, 1998). Other sources of landscape salinity include salts used for roadside deicing (Hutchinson, 1970) and recycled wastewater for landscape irrigation (Lockett et al., 2008; Marcum, 2006).
Kentucky bluegrass is a widely used cool-season species in cool, semiarid climates; however, it has relatively poor salt tolerance (Alshammary, 2013; Marcum, 2008). Many studies have attempted to assess the potential for salt tolerance among kentucky bluegrass cultivars through many varying methods (Alshammary et al., 2004; Horst and Taylor, 1983; Koch and Bonos, 2010; Koch et al., 2011; Liu et al., 2011; Poss et al., 2010; Qian et al., 2001; Robins et al., 2009; Rose-Fricker and Wipff, 2001; Suplick-Ploense et al., 2002; Torello and Rice, 1986). Most studies found that TQ, a measure of turf color, density, and texture, showed significant variation among cultivars under salt stress. Although various mechanisms have been suggested for the variation in salt tolerance, it was found that kentucky bluegrass did not have the ability to exclude salt outside of the leaves, nor was it effective in excluding ions in the root zones (Alshammary, 2013; Alshammary et al., 2004; Xu and Fujiyama, 2013). Compartmentalization or sequestration of Na ions into the vacuole or apoplast was presented as a potential mechanism by which the species could tolerate higher levels of salt (Alshammary, 2013; Marcum, 2008). This sequestration is hypothesized to be accompanied by antioxidant mechanisms to prevent cell damage (Hasegawa et al., 2000; Munns, 2005; Puyang et al., 2015), and physiological mechanisms to maintain turgor and water potential. Consistent with these findings, Bushman et al. (2016) identified genes involved in cellular ion and water transport [tonoplast intrinsic protein (TIP4-1), V-type ATPase subunit b, and H-type ATPase] and genes involved in maintenance of cellular redox state [thioredoxin h-type, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and glutathione S-transferase (GST)] in a transcriptome survey between a salt-tolerant and susceptible kentucky bluegrass accession.
Although substantial variation has been detected in assessments of kentucky bluegrass salt tolerance, cultivars often show changes of rank in relative salt tolerance, depending on the study methods. The cultivars Midnight and Baron, ubiquitous kentucky bluegrass cultivars often included in studies as checks, have shown a range of responses to salt stress (Koch and Bonos, 2010; Koch et al., 2011; Puyang et al., 2015); such that their relative salt tolerance is unclear. Robins et al. (2009) used a different approach to find kentucky bluegrass entries with putative salt tolerance. Although previous quantitative measures were intensive, which thereby limited the entry numbers, Robins et al. (2009) submersed multiple plants from many cultivars in increasingly concentrated saline solutions over time and measured the concentration of salts and number of days treated when a lethal dose, whereupon 50% of the plants were dead (LD50), was reached for each entry. Rather than a mechanistic approach on relatively few entries, that method focused on turf color of multiple genotypes per entry and a higher number of entries. Although that method included the cultivar Midnight as a susceptible control, whether tolerant entries detected by Robins et al. (2009) would be tolerant under mechanistic approaches is unknown.
One of the purposes of this study was to confirm the salt tolerance of two putatively tolerant kentucky bluegrass accessions reported in Robins et al. (2009), and to investigate mechanisms by which those accessions respond to salt stress. The two accessions, PI 371768 and PI 440603, and the cultivars Baron and Midnight are measured in multiple greenhouse trials for TQ, gS, ψLEAF, EL, and the accumulation of inorganic ions on control and salt treatments. In addition, the expression profiles of genes involved in cellular ion sequestration and antioxidant activities previously reported in Bushman et al. (2016) are characterized for variation between the tolerant accessions and susceptible cultivars. These results identify new germplasm sources of salt tolerance in kentucky bluegrass and pinpoint mechanisms by which those lines tolerate salt stress that may be useful for selection of other salt-tolerant kentucky bluegrass germplasm.
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Primer sequences used for quantitative real-time reverse-transcription PCR of transport and antioxidant genes in kentucky bluegrass.