Apoplastic pH and growth in expanding leaves of Vicia faba under salinity

Salinity affects water availability in the soil and subsequently the plant uptake capacity. Upon exposure to salt stress, leaf growth in monocot plants has been shown to be reduced instantaneously, followed by a gradual acclimation. The growth reactions are caused by an initial water deficit and an accompanied osmotic effect, followed by an IAA-induced sequestration of protons into the apoplast that increases leaf growth again as explained by the acid growth theory. In this study, we investigated the dynamics of growth reactions and apoplastic pH in leaves of the dicot Vicia faba in the presence of NaCl during the initiation of salt stress. Concurrent changes in apoplastic pH were detected by ratiometric fluorescence microscopy using the fluorescent dye fluorescein tetramethylrhodamine dextran. To elucidate the possible relation between the dynamics of leaf growth and apoplastic pH, results of the ratio imaging technique were combined with an in vivo growth analysis imaging approach. Leaf growth rate of V. faba was highest in the dusk and the early night phase; at this time a concomitant decrease of the apoplastic pH was observed. Under salinity, the apoplastic pH in leaves of V. faba increased with a simultaneous decrease of leaf growth towards increasing developmental stages, but with complex aberrations in the 24-h-leaf-growth pattern compared to control leaves. In conclusion, these results show that salt stress leads to an increase in apoplastic pH and to a declined leaf growth activity with complex 24-h-interactions of growth and pH in V. faba.