SODIUM ACCUMULATION IN LEAVES OF TRITICUM SPECIES THAT DIFFER IN SALT TOLERANCE

The ability to limit the accumulation of Na+ in leaves may be an important mechanism in salt tolerance because the excessive accumulation of Na+ causes the premature senescence of leaves. The objectives of this study were to test the importance of Na+ accumulation rates in determining salt tolerance and to determine whether observed genotypic differences in leaf Na+ accumulation are linked to rates of leaf expansion. Six salt-sensitive and salt-tolerant genotypes of Triticum tauschii (Coss.) Schmal., T. aestivum L. and T. turgidum L. were grown in 150 mol m-3 NaCl and harvested at regular intervals over approximately 3 weeks. Na+ concentrations and leaf growth were measured in individual leaf blades over this time. The salt-tolerant accessions all had lower rates of Na+ accumulation than the salt-sensitive. This was not due to genotypic differences in growth rates: it was independent of the overall growth rate (vigour) of the genotypes, and the growth phase of individual leaves. In the growing leaf, the rate of Na+ accumulation was lower in salt-tolerant genotypes both during and after the phase of expansion. Leaf longevity was greater in the salt-tolerant genotypes. In one salt-sensitive genotype, the maximum Na+ concentration was much lower than that of all the other genotypes. Two mechanisms of salt tolerance appear to be operating in Triticum genotypes. One is a lower rate of Na+ accumulation which is independent of the growth of individual leaves and therefore probably regulated by some root process. The second is ion compartmentation within leaves, which enhances the ability to tolerate high concentrations of Na+ in leaves.