Physiological and molecular mechanisms mediating xylem Na + loading in barley in the context of salinity stress tolerance

Time‐dependent kinetics of xylem Na + loading was investigated using a large number of barley genotypes contrasting in their salinity tolerance. Salt‐sensitive varieties were less efficient in controlling xylem Na + loading and showed a gradual increase in the xylem Na + content over the time. To understand underlying ionic and molecular mechanisms, net fluxes of Ca 2+ , K + and Na + were measured from the xylem parenchyma tissue in response to H 2 O 2 and ABA; both of them associated with salinity stress signalling. Our results indicate that NADPH oxidase‐mediated apoplastic H 2 O 2 production acts upstream of the xylem Na + loading and is causally related to ROS‐inducible Ca 2+ uptake systems in the root stelar tissue. It was also found that ABA regulates (directly or indirectly) the process of Na + retrieval from the xylem and the significant reduction of Na + and K + fluxes induced by bumetanide are indicative of a major role of chloride cation co‐transporter (CCC) on xylem ion loading. Transcript levels of HvHKT1;5_like and HvSOS1_like genes in the root stele were observed to decrease after salt stress, while there was an increase in HvSKOR_like gene, indicating that these ion transporters are involved in primary Na + /K + movement into/out of xylem.