Mechanisms Mediating Xylem Na⁺ Loading in Barley under Salinity Stress

Among the multiple physiological mechanisms conferring salinity tolerance, control of xylem ion loading and regulation of ionic exchange at the xylem-parenchyma boundary have often been named as central to salinity tolerance. The main objective of this work was to answer the following questions: (i) is there a difference in the kinetics of xylem ion loading under salinity stress between salt-tolerant and salt-sensitive genotypes? (ii) How is this loading controlled at the physiological level? (iii) What is the molecular identity of the transport systems behind this process, and to what extent changes in their expression pattern contribute to this process? In this work time-dependent kinetics of xylem Na⁺ loading was investigated using a large number of barley genotypes contrasting in their salinity tolerance. We found that 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 Ca²⁺, K⁺ and Na⁺ were measured from the xylem parenchyma tissue in response to H₂O₂ and ABA; both of them associated with salinity stress signalling. Our results indicate that NADPH oxidase-mediated apoplastic H₂O₂ production acts upstream of the xylem Na⁺ loading and is causally related to ROS inducible Ca²⁺ 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.