Ionic relations and osmotic adjustment in durum and bread wheat under saline conditions

Wheat breeding for salinity tolerance has traditionally focussed on Na+ exclusion from the shoot, but its association with salinity tolerance remains tenuous. Accordingly, the physiological significance of shoot Na+ exclusion and maintenance of an optimal K+:Na+ ratio was re-evaluated by studying NaCl-induced responses in 50 genotypes of bread wheat (Triticum aestivum L.) and durum wheat (Triticum turgidum L. ssp. durum) treated with 150 mM NaCl. Overall, Na+ exclusion from the shoot correlated with salinity tolerance in both species and this exclusion was more efficient in bread compared with durum wheat. Interestingly, shoot sap K+ increased significantly in nearly all durum and bread wheat genotypes. Conversely, the total shoot K+ content declined. We argue that this increase in shoot sap K+ is needed to provide efficient osmotic adjustment under saline conditions. Durum wheat was able to completely adjust shoot sap osmolality using K+, Na+ and Cl-; it had intrinsically higher levels of these solutes. In bread wheat, organic osmolytes must contribute similar to 13% of the total shoot osmolality. In contrast to barley (Hordeum vulgare L.), NaCl-induced K+ efflux from seedling roots did not predict salinity tolerance in wheat, implying that shoot, not root K+ retention is important in this species.