The second mechanism of salinity tolerance we
considered was tissue tolerance of high internal Na+
concentrations. Tissue tolerance cannot be measured
directly, and is difficult to quantify. Yet it is clearly
important; overexpression of the vacuolar Na+/H+
antiporter that sequesters Na+ in vacuoles (NHX1)
improved the salinity tolerance of Arabidopsis, tomato
and brassica (Aharon et al., 2003). Variation in salinity
tolerance not associated with Na+ exclusion was
observed for a small number of accessions of the diploid
wheat ancestor Triticum tauschii (syn. Aegilops
squarrosa) accessions (Schachtman andMunns, 1992;
Schachtman et al., 1991), so it is likely that variation
exists within the tetraploid wheat ancestors. To assess
genetic variation in the tetraploid lines, we looked for
leaf longevity, lack of necrosis, and prolonged growth
despite very high accumulation of Na+.