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

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