Department of Plant Sciences

Salinity, a priori, affects water availability in the soil rather than plant capacity to take up water. In the first phase of salt stress, osmoregulation can even enhance the plant capacity to take up water. Additionally, plasma membrane H+-ATPase can be affected, thus lessening proton extrusion into the apoplast necessary for leaf growth. In this study, salt-induced changes in growth rate were investigated and the pH dependence of acid-induced growth in relation to changes of plasma membrane H+-ATPase activity in two maize (Zea mays L.) genotypes differing in salt resistance (Pioneer 3906 and SR 03) were monitored. Leaf growth was reduced in the presence of 100 mM NaCl and effects were more pronounced for the more salt-sensitive Pioneer 3906. By using inside-out plasma membrane vesicles, it was shown that H+-pumping was reduced in the salt-sensitive genotype while it remained unchanged in the resistant genotype. Accordingly, changes in apoplastic pH were detected by ratiometric fluorescence microscopy and pH-sensitive microelectrodes. While Pioneer 3906 responded with a significant increase of up to 0.5 pH units, no apoplastic alkalization was found in SR 03. With respect to the hypothesis that the apoplastic pH is influenced by salinity, it is suggested that salt resistance is achieved due to efficient H+-ATPase proton pumping to acidify the apoplast.