Insights into the salt tolerance mechanism in barley (Hordeum vulgare) from comparisons of cultivars that differ in salt sensitivity

Although barley (Hordeum vulgare L.) is a salt-tolerant crop, the underlying physiological and molecular mechanisms of salt tolerance remain to be elucidated. Therefore, we investigated the response of salt-tolerant (K305) and salt-sensitive (I743) cultivars to salt stress at both physiological and molecular levels. Salt treatment increased xylem sap osmolarity, which was attributed primarily to a rise in Na⁺ and Cl^- concentration; enhanced accumulation of the ions in shoots; and reduced plant growth more severely in I743 than K305. The concentration of K⁺ in roots and shoots decreased during 8 h of salt treatment in both cultivars but with no marked difference between cultivars. Hence, the severe growth reduction in I743 is attributed to the elevated levels of (mainly) Na⁺ in shoots. Analysis of gene expression using quantitative RT-PCR showed that transcripts of K⁺-transporters (HvHAK1 and HvAKT1), vacuolar H⁺-ATPase and inorganic pyrophosphatase (HvHVA/68 and HvHVP1) were more abundant in shoots of K305 than in shoots of I743. Expression of HvHAK1 and Na⁺/H⁺ antiporters (HvNHX1, HvNHX3 and HvNHX4) was higher in roots of K305 than in I743 with prolonged exposure to salt. Taken together, these results suggest that the better performance of K305 compared to I743 during salt stress may be related to its greater ability to sequester Na⁺ into sub-cellular compartments and/or maintain K⁺ homeostasis.