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 NO3alone. 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.