Microscopic and macroscopic monitoring of adaxial–abaxial pH gradients in the leaf apoplast of Vicia faba L. as primed by NaCl stress at the roots

•NaCl-induced transient leaf apoplastic alkalizations do not emerge uniformly.•We firstly report about large adaxial–abaxial pH gradients that were established.•Root system is necessary to shape and ensure the transiency of the alkalinization.•This is attributable to a limited root uptake of the alkalizing factor chloride.

The pH is a basic chemical requirement in cellular and apoplastic compartments that influences various physiological processes in plants. Apoplastic pH shifts can modulate the apoplastic and symplastic distribution of plant hormones or influence proton motive force-driven uptake processes over the plasma-membrane. Changing environments are known to effect on the apoplastic H+-concentration in leaves and roots. The onset of NaCl-stress at the roots for instance primes a chloride-specific systemic alkalinization of the leaf apoplast. By means of microscopy- and macroscopy-based in planta ratio-imaging we surprisingly found that large adaxial–abaxial pH gradients were established throughout the leaf apoplast during the formation of the NaCl-induced alkalinization. Moreover, the root system is necessary to ensure the transient nature of the leaf apoplastic alkalinization.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slide