Ammonium, nitrate, and proton fluxes along the maize root

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 ro ot varied substantially among the different treatments. Net NH4+ and NO3- u ptake 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 p resence of NO3- alone, net H+ uptake (and root surface alkalinization) occu rred at the root tip (0-1 mm), whereas net H+ extrusion occurred in all oth er regions. In the presence of NH4+ alone, a dramatic increase in net H+ ex trusion 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 depre ssed 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+ ext rusion was increased relative to NO,alone. The capacities to absorb NH4+ an d 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- uptak e was half that of net NH4+ uptake, indicating that NH4+ may satisfy the ni trogen requirements of the poorly vascularized apical tissue in the most en ergy-efficient way. The high spatial resolution of the measurements enabled us to establish that acidification in the root expansion zone is maintaine d regardless of nitrogen source.