Bread Wheat With High Salinity and Sodicity Tolerance

Abstract

Soil salinity and sodicity are major constraints to global cereal production, but breeding for tolerance has been slow. Narrow gene pools, over-emphasis on the sodium (Na⁺) exclusion mechanism, little attention to osmotic stress/tissue tolerance mechanism(s) in which accumulation of inorganic ions such as Na⁺ is implicated, and lack of a suitable screening method have impaired progress. The aims of this study were to discover novel genes for Na⁺ accumulation using genome-wide association studies, compare growth responses to salinity and sodicity in low-Na⁺ bread Westonia with Nax1 and Nax2 genes and high-Na⁺ bread wheat Baart-46, and evaluate growth responses to salinity and sodicity in bread wheats with varying leaf Na⁺ concentrations. The novel high-Na⁺ bread wheat germplasm, MW#293, had higher grain yield under salinity and sodicity, in absolute and relative terms, than the other bread wheat entries tested. Genes associated with high Na⁺ accumulation in bread wheat were identified, which may be involved in tissue tolerance/osmotic adjustment. As most modern bread wheats are efficient at excluding Na⁺, further reduction in plant Na⁺ is unlikely to provide agronomic benefit. The salinity and sodicity tolerant germplasm MW#293 provides an opportunity for the development of future salinity/sodicity tolerant bread wheat.

Keywords: chloride; ionic; osmotic; salinity; sodicity; sodium; sodium humate; tolerance.

Figure 1

Boxplots of leaf Na⁺ (back-transformed), Cl^-, K⁺, Ca²⁺ and Mg²⁺ concentrations (mg kg^-1 DW) at heading in 100 bread wheat entries, 12 durum wheat entries and a barley cultivar grown under sodicity (8 g kg^-1 Na⁺-humate) in Experiment 1 (n = 4). See Table S3 for individual responses. The box represents the middle 50% of the distribution (the median is drawn as the solid line within the box) with whiskers extending to the lowest/highest value within 1.5* IQR (Inter-Quartile Range). Values outside this range are plotted separately.

Figure 2

Manhattan plot showing the association signals for leaf Na⁺ concentration using 100 bread wheat entries grown under sodicity (8 g kg^-1 Na⁺-humate) in Experiment 1 (n = 4). The x-axis indicates the physical location of SNP markers along each wheat chromosome; the y-axis shows the P-value of SNP markers for the association test on a log scale. The horizontal red line indicates the significance threshold (P-value = 8.91e-5). The marker names of SNPs above the threshold are shown.

Figure 3

Relative grain yield (%) (salinity or sodicity tolerance), and best linear unbiased estimates for leaf Na⁺ and Cl^- concentrations in wheat cv. Westonia, Westonia-Nax1, Westonia-Nax2 and Baart-46 under different levels of salinity (left panels) and sodicity applied as Na⁺ humate (right panels) in Experiment 2 (n = 4). The vertical bars indicate Least Significant Difference test value at P = 0.05 for variety x treatment interaction. See Table S8 for back-transformed Na⁺ and Cl^-concentrations for comparisons with published data.

Figure 4

Relationships of Na⁺, Cl^-, K⁺, Ca²⁺ and Ma²⁺ concentrations (mg kg^-1 DW), and grain yield (g plant^-1) under control, salinity (100 mM NaCl) and sodicity (8 g kg^-1 Na⁺-humate) in 20 bread wheat entries in Experiment 3 (n = 4) (r = 0.561,df = 18, P < 0.01). MW#293 is shown as an empty circle.

Figure 5

Best linear unbiased estimates for grain yield, and tolerance (grain yield under sodicity or salinity as a percentage of grain yield under control) of 20 bread wheat entries (Triticum aestivum L.), three durum wheat entries (Triticum turgidum subsp durum cv. Tamaroi, Tamaroi-Nax2 and Yawa) and one barley (Hordeum vulgare L. cv. Clipper) in Experiment 3 (n = 4). The vertical bars indicate Least Significant Difference test value at P = 0.05 for variety x treatment interaction. Entries are ordered in ascending order of leaf Na⁺ concentration.

Figure 6

Best linear unbiased estimates for grain yield, and leaf Na⁺ and Cl^- concentrations at heading in bread wheat cv. Mace, and two low-Na⁺ (MW#28 and MW#491) and two-high Na⁺ (MW#293 and MW#451) doubled-haploid lines selected from a cross between Mace and high-Na⁺ germplasm W4909 (n = 5). Sodium concentration data were transformed to natural logarithms. The vertical bars indicate Least Significant Difference test value at P = 0.05 for variety x treatment interaction. Wheat lines are ordered in ascending order of salinity tolerance (ratio of grain yield under salinity to grain yield under control, expressed as percent).

Figure 7

Representative pots of bread wheat (Triticum aestivum) cv. Mace and doubled-haploid line MW#293 grown under control and salinity (100 mM NaCl) in Experiment 4.