Deposition of ammonium and nitrate in the roots of maize seedlings supplied with different nitrogen salts

This study measured total osmolarity and concentrations of NH4+, NO3-, K+, soluble carbohydrates, and organic acids in maize seminal roots as a function of distance from the apex, and NH4+ and NO3- in xylem sap for plants receiving NH4+ or NO3- as a sole N-source, NH4+ plus NO3-, or no nitrogen at all. The disparity between net deposition rates and net exogenous influx of NH4+ indicated that growing cells imported NH4+ from more mature tissue, whereas more mature root tissues assimilated or translocated a portion of the NH4+ absorbed. Net root NO3- influx under Ca(NO3)(2) nutrition was adequate to account for pools found in the growth zone and provided twice as much as was deposited locally throughout the non-growing tissue. In contrast, net root NO3- influx under NH4NO3 was less than the local deposition rate in the growth zone, indicating that additional NO3- was imported or metabolically produced. The profile of NO3- deposition rate in the growth zone, however, was similar for the plants receiving Ca(NO3)(2) or NH4NO3. These results suggest that NO3- may serve a major role as an osmoticant for supporting root elongation in the basal part of the growth zone and maintaining root function in the young mature tissues.