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

Abstract

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.

Fig. 1.

(A) Fresh and dry mass of 1-mm long radial sections sampled at various distances from the apex of a maize seminal root. Nitrogen source did not influence these parameters significantly and so the data was pooled for the different N sources. Values are means ± SE (10 roots per treatment, three treatments). (B) Osmolarity of radial sections sampled at various distances from the apex of a maize seminal root for plants receiving nutrient solutions that were nitrogen free or contained 100 mmol m⁻³ NH₄NO₃, 100 mmol m⁻³ NH₄H₂PO₄, or 50 mmol m⁻³ Ca(NO₃)₂. Values are means ± SE (n = 3–4).

Fig. 2.

Tissue concentrations of (A) K⁺ and (B) malate at various distances from the apex of a maize seminal root for plants receiving nutrient solutions that contained 100 mmol m⁻³ NH₄NO₃, 100 mmol m⁻³ NH₄H₂PO₄, or 50 mmol m⁻³ Ca(NO₃)₂ or, for malate, were nitrogen free. Values for K⁺ are means (n = 2).

Fig. 3.

Tissue concentrations of soluble carbohydrates, (A) glucose and (B) fructose, at various distances from the apex of a maize seminal root for plants receiving nutrient solutions that contained 100 mmol m⁻³ NH₄NO₃, 100 mmol m⁻³ NH₄H₂PO₄, or 50 mmol m⁻³ Ca(NO₃)₂. Values are means (n = 2).

Fig. 4.

Tissue concentrations of NH₄⁺ at various distances from the apex of a maize seminal root for plants receiving nutrient solutions that were nitrogen free or contained 100 mmol m⁻³ NH₄NO₃, 100 mmol m⁻³ NH₄H₂PO₄, or 50 mmol m⁻³ Ca(NO₃)₂. (A) All data. (B) Data for locations close to the apex. Values are means ± SE (n = 3–6).

Fig. 5.

Tissue concentrations of NO₃^– at various distances from the apex of a maize seminal root for plants receiving nutrient solutions that were nitrogen free or contained either 100 mmol m⁻³ NH₄NO₃ or 50 mmol m⁻³ Ca(NO₃)₂. (A) All data. (B) Data for the locations close to the apex. Values are means ± SE (n = 3–6).

Fig. 6.

Influx [based on Taylor and Bloom (1998)] and deposition of (A) NH₄⁺ and (B) NO₃^– at various distances from the apex of a maize seminal root for plants receiving nutrient solutions that contained either 100 mmol m⁻³ NH₄NO₃ (n = 6) or 50 mmol m⁻³ Ca(NO₃)₂ (n = 9).

Fig. 7.

Concentrations of (A) NH₄⁺ and (B) NO₃^– in the xylem sap of maize plants receiving NH₄NO₃, Ca(NO₃)₂, or NH₄H₂PO₄ as nitrogen sources. Values are mean ± SE (n = 7–13).

Fig. 8.

The maize root apex. At location A, deposition of a substance (e.g., NH₄⁺ or NO₃^–) exceeds influx and so the tissue is importing the substance. At location B, influx exceeds deposition and so the tissue is exporting the substance.

Fig. 9.

Models of NO₃^– content (nmol mm⁻¹) and relative element length as a function of the time (bottom axis) or distance (top axis) that a root tissue element is displaced from the base of the growth zone. (A) Model results of potential NO₃^– uptake assuming influx with no translocation or assimilation. This is a material specification in which a material (real) tissue element is followed through time and space. The case of influx restricted to the apical 3 mm is given by the dotted line, and the case of influx throughout the root is shown by the dashed line. The solid line shows the length of the tissue element during the time of its displacement through and beyond the growth zone. (B) Material specification in which the potential uptake and observed content of NO₃^– in a material (real) tissue element is followed through time and space.