The apoplastic pH and its significance in adaptation to salinity in maize (Zea mays L.): Comparison of fluorescence microscopy and pH-sensitive microelectrodes

The apoplastic ionic milieu contains essential determinants for cell expansion and plant growth. Since pH is a multifunctional basic component of this extracellular space, the knowledge of its behaviour during stress situations is of major importance. In detached leaves of maize (Zea mays L. cvs. Pioneer 3906 and SR 03) the effect of salinity on apoplastic pH was measured to investigate its adaptive role to salt stress applying two different methods: an optical approach using pH-sensitive fluorescent dyes (fluorescein isothiocyanate-dextran (FITC), fluorescein tetramethylrhodamine-dextran (FTMR) and Oregon Green® 488), and an electrophysiological technique, pH-sensitive microelectrodes. Both approaches yielded similar results. In the presence of 100 mM NaCl, which was added to the growth medium, apoplastic pH of the salt-sensitive maize genotype Pioneer 3906 leaves increased in maximum by 0.4 units (pH microelectrodes) and by 0.3 units (fluorescent dyes); the salt-resistant SR 03 hardly responded. The same treatment reduced leaf growth by 60% in Pioneer 3906, but only by 40% in SR 03. Since according to acid growth considerations apoplastic pH is an important factor in elongation growth, we suggest that this pH increase is a main cause for reduced leaf growth under salt stress conditions.