Uncoupling of sodium and chloride to assist breeding for salinity tolerance in crops

The separation of toxic effects of sodium (Na+) and chloride (Cl-) by the current methods of mixed salts and subsequent determination of their relevance to breeding has been problematic. We report a novel method (Na+ humate) to study the ionic effects of Na+ toxicity without interference from Cl-, and ionic and osmotic effects when combined with salinity (NaCl). Three cereal species (Hordeum vulgare, Triticum aestivum and Triticum turgidum ssp. durum with and without the Na+ exclusion gene Nax2) differing in Na+ exclusion were grown in a potting mix under sodicity (Na+ humate) and salinity (NaCl), and water use, leaf nutrient profiles and yield were determined. Under sodicity, Na+-excluding bread wheat and durum wheat with the Nax2 gene had higher yield than Na+-accumulating barley and durum wheat without the Nax2 gene. However, under salinity, despite a 100-fold difference in leaf Na+, all species yielded similarly, indicating that osmotic stress negated the benefits of Na+ exclusion. In conclusion, Na+ exclusion can be an effective mechanism for sodicity tolerance, while osmoregulation and tissue tolerance to Na+ and/or Cl- should be the main foci for further improvement of salinity tolerance in cereals. This represents a paradigm shift for breeding cereals with salinity tolerance.