Durum wheat, Triticum turgidum L. (2n= 4x=28, genome formula AABB) is inferior to bread wheat, T. aestivum L. (2n=6x=42, genome formula AABBDD), in the ability to exclude Na(+) under salt strees, in the ratio of the accumulated K(+) to Na(+) in the leaves under salt stress, and in tolerance of salt stress. Previous work showed that chromosome 4D has a major effect on Na(+) and K(+) accumulation in the leaves of bread wheat. The 4D chromosome was recombined with chromosome 4B in the genetic background of durum wheat. The recombinants showed that Na(+) exclusion and enhanced K(+)/Na(+) ratio in the shoots were controlled by a single locus, Kna1, in the long arm of chromosome 4D. The recombinant families were grown in the field under non-saline conditions and two levels of salinity to determine whether Kna1 confers salt tolerance. Under salt stress, the Kna1 families had higher K(+)/Na(+) ratios in the flag leaves and higher yields of grain and biomass than the Kna1 (-) families and the parental cultivars. Kna1 is, therefore, one of the factors responsible for the higher salt tolerance of bread wheat relative to durum wheat. The present work provides conceptual evidence that tolerance of salt stress can be transferred between species in the tribe Triticeae.