Impact of magnesium - sodium supplementation on liveweight gains of young sheep grazing dual-purpose cereal or canola crops

K/Na ratios have been determined in the leaves of salt-treated plants of 14 disomic substitution lines in which each of the D-genome chromosomes replaces the homoeologous A- or B-genome chromosome in the tetraploid wheat variety Langdon (AABB genome). Aneuploid lines of hexaploid bread wheat (cv Chinese Spring) having a reduced or an enhanced complement of chromosome 4D have also been examined. These investigations show that the gene(s) determining K/Na ratios in the leaves of wheat plants grown in the presence of salt is located on the long arm of chromosome 4D.

Genes that enable crops to limit Na+ accumulation in shoot tissues represent potential sources of salinity tolerance for breeding. In barley, the HvNax4 locus lowered shoot Na+ content by between 12% and 59% (g−1 DW), or not at all, depending on the growth conditions in hydroponics and a range of soil types, indicating a strong influence of environment on expression. HvNax4 was fine-mapped on the long arm of barley chromosome 1H. Corresponding intervals of ∼200 kb, containing a total of 34 predicted genes, were defined in the sequenced rice and Brachypodium genomes. HvCBL4, a close barley homologue of the SOS3 salinity tolerance gene of Arabidopsis, co-segregated with HvNax4. No difference in HvCBL4 mRNA expression was detected between the mapping parents. However, genomic and cDNA sequences of the HvCBL4 alleles were obtained, revealing a single Ala111Thr amino acid substitution difference in the encoded proteins. The known crystal structure of SOS3 was used as a template to obtain molecular models of the barley proteins, resulting in structures very similar to that of SOS3. The position in SOS3 corresponding to the barley substitution does not participate directly in Ca2+ binding, post-translational modifications or interaction with the SOS2 signalling partner. However, Thr111 but not Ala111 forms a predicted hydrogen bond with a neighbouring α-helix, which has potential implications for the overall structure and function of the barley protein. HvCBL4 therefore represents a candidate for HvNax4 that warrants further investigation.