K＋ retention in leaf mesophyll, an overlooked component of salinity tolerance mechanism： A case study for barley

Plant salinity tolerance is a physiologically complex trait, with numerous mechanisms contributing to it. In this work,we show that the ability of leaf mesophyll to retain Kt represents an important and essentially overlooked component of a salinity tolerance mechanism. The strong positive correlation between mesophyll Ktretention ability under saline conditions(quantified by the magnitude of Na Cl-induced Kt efflux from mesophyll) and the overall salinity tolerance(relative fresh weight and/or survival or damage under salinity stress) was found while screening 46 barley(Hordeum vulgare L.) genotypes contrasting in their salinity tolerance. Genotypes with intrinsically higher leaf Ktcontent under control conditions were found to possess better Ktretention ability under salinity and, hence, overall higher tolerance. Contrary to previous reports for barley roots, Ktretention in mesophyll was not associated with an increased Ht-pumping in tolerant varieties but instead correlated negatively with this trait. These findingsare explained by the fact that increased Htextrusion may be needed to charge balance the activity and provide the driving force for the high affinity HAK/KUP Kttransporters required to restore cytosolic Kthomeostasis in salt-sensitive genotypes.