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

This study measured total osmolarity and concentrations of NH(4)(+), NO(3)(–), K(+), soluble carbohydrates, and organic acids in maize seminal roots as a function of distance from the apex, and NH(4)(+) and NO(3)(–) in xylem sap for plants receiving NH(4)(+) or NO(3)(–) as a sole N-source, NH(4)(+) plus NO(3)(–), or no nitrogen at all. The disparity between net deposition rates and net exogenous influx of NH(4)(+) indicated that growing cells imported NH(4)(+) from more mature tissue, whereas more mature root tissues assimilated or translocated a portion of the NH(4)(+) absorbed. Net root NO(3)(–) influx under Ca(NO(3))(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 NO(3)(–) influx under NH(4)NO(3) was less than the local deposition rate in the growth zone, indicating that additional NO(3)(–) was imported or metabolically produced. The profile of NO(3)(–) deposition rate in the growth zone, however, was similar for the plants receiving Ca(NO(3))(2) or NH(4)NO(3). These results suggest that NO(3)(–) 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.