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

Abstract 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;5ₗike and HvSOS1ₗike genes in the root stele were observed to decrease after salt stress, while there was an increase in HvSKORₗike gene, indicating that these ion transporters are involved in primary Na + /K + movement into/out of xylem.