Net fluxes of ammonium and nitrate in association with H+ fluxes in fine roots of Populus popularis

Poplar plants are cultivated as woody crops, which are often fertilized by addition of ammonium (NH4 (+)) and/or nitrate (NO3 (-)) to improve yields. However, little is known about net NH4 (+)/NO3 (-) fluxes and their relation with H+ fluxes in poplar roots. In this study, net NH4 (+)/NO3 (-) fluxes in association with H+ fluxes were measured non-invasively using scanning ion-selective electrode technique in fine roots of Populus popularis. Spatial variability of NH4 (+) and NO3 (-) fluxes was found along root tips of P. popularis. The maximal net uptake of NH4 (+) and NO3 (-) occurred, respectively, at 10 and 15 mm from poplar root tips. Net NH4 (+) uptake was induced by ca. 48 % with provision of NO3 (-) together, but net NO3 (-) uptake was inhibited by ca. 39 % with the presence of NH4 (+) in poplar roots. Furthermore, inactivation of plasma membrane (PM) H+-ATPases by orthovanadate markedly inhibited net NH4 (+)/NO3 (-) uptake and even led to net NH4 (+) release with NO3 (-) co-provision. Linear correlations were observed between net NH4 (+)/NO3 (-) and H+ fluxes in poplar roots except that no correlation was found between net NH4 (+) and H+ fluxes in roots exposed to NH4Cl and 0 mM vanadate. These results indicate that root tips play a key role in NH4 (+)/NO3 (-) uptake and that net NH4 (+)/NO3 (-) fluxes and the interaction of net fluxes of both ions are tightly associated with H+ fluxes in poplar roots.