Ammonium, nitrate, and proton fluxes along the maize root

Abstract

Ion‐selective microelectrodes were used to measure NH4+, NO3–and H+fluxes along the primary root of maize seedlings. Plants were exposed to nutrient solutions containing NH4+, NO3–or both ions. Nitrogen fluxes along the root varied substantially among the different treatments. Net NH4+and NO3–uptake and H+extrusion were low at the very apex of the root and generally increased in the more basal regions. In the absence of nitrogen or in the presence of NO3–alone, net H+uptake (and root surface alkalinization) occurred at the root tip (0–1 mm), whereas net H+extrusion occurred in all other regions. In the presence of NH4+alone, a dramatic increase in net H+extrusion was detected in all regions except for the region 6–11 mm from the apex. In contrast, when NO3–alone was supplied, net H+extrusion was depressed at all locations except for the tip (0–1 mm). When both NH4+and NO3–were supplied, NO3–uptake was suppressed at all locations while net H+extrusion was increased relative to NO3–alone. The capacities to absorb NH4+and NO3–at the tip were similar, as indicated by flux rates when NH4+or NO3–were supplied as sole sources, but when supplied together, net NO3–uptake was half that of net NH4+uptake, indicating that NH4+may satisfy the nitrogen requirements of the poorly vascularized apical tissue in the most energy‐efficient way. The high spatial resolution of the measurements enabled us to establish that acidification in the root expansion zone is maintained regardless of nitrogen source.