Decline in leaf growth under salt stress is due to an inhibition of H+-pumping activity and increase in apoplastic pH of maize leaves

In this study, salt-induced changes in the growth rate of maize (Zea mays L.) were investigated during the first phase of salt stress. Leaf growth was reduced in the presence of 100 mM NaCl, and effects were more pronounced for the salt-sensitive cv. Pioneer 3906 in comparison to the hybrid SR03. While hydrolytic activity of plasma membrane remained unaffected, H+-pumping activity was reduced by 47% in Pioneer 3906, but was unchanged in SR03. Changes in apoplastic pH were detected by ratiometric fluorescence microscopy using the fluorescent dye fluorescein isothiocyanate-dextran (50 mM). Pioneer 3906 responded with an increase of 0.2 pH units in contrast to SR03 for which no apoplastic alkalization was found. With respect to the hypothesis that the apoplastic pH is influenced by salinity, it is suggested that salt resistance is partly achieved due to efficient H+-ATPase proton pumping, which results in cell-wall acidification and loosening.