Ratiometric monitoring of transient apoplastic alkalinizations in the leaf apoplast of living V icia faba plants: chloride primes and PM – H + ‐ ATP ase shapes N a C l‐induced systemic alkalinizations

Summary Transient apoplastic alkalinization has been discussed as a general stress factor, and is thought to represent a root‐to‐shoot signal that transmits information regarding an ongoing N a C l stress event from the site of the trigger to the distant plant tissue. Surprisingly, despite this importance, a number of gaps exist in our knowledge of N a C l‐induced apoplastic p H alkalinization. This study was designed in order to shed light onto the mechanisms responsible for the initiation and transiency of leaf apoplastic alkalinization under conditions of N a C l stress as supplied to roots. An H + ‐sensitive fluorescence probe, in combination with ratiometric microscopy imaging, was used for in planta live recording of leaf apoplastic p H . The use of a nonionic solute demonstrated that the alkalinization is induced in response to ionic, and not osmotic, components of N a C l stress. Tests with C l − ‐ or N a + ‐accompanying counter‐ions strengthened the idea that the stress factor itself, namely C l − , is transferred from root to shoot and elicits the p H alterations. Investigations with a plasma membrane ATP ase inhibitor suggest that ATP ase activity influences the course of the alkalinization by having a shaping re‐acidifying effect on the alkalinization.