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 ~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.