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 K⁺ represents an important and essentially overlooked component of a salinity tolerance mechanism. The strong positive correlation between mesophyll K⁺ retention ability under saline conditions (quantified by the magnitude of NaCl-induced K⁺ 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 K⁺ content under control conditions were found to possess better K⁺ retention ability under salinity and, hence, overall higher tolerance. Contrary to previous reports for barley roots, K⁺ retention in mesophyll was not associated with an increased H⁺-pumping in tolerant varieties but instead correlated negatively with this trait. These findings are explained by the fact that increased H⁺ extrusion may be needed to charge balance the activity and provide the driving force for the high affinity HAK/KUP K⁺ transporters required to restore cytosolic K⁺ homeostasis in salt-sensitive genotypes.